UNIVERSIDADE ESTADUAL DE CAMPINAS

FACULDADE DE ENGENHARIA DE ALIMENTOS

ARIEL ANTONIO CAMPOS TOLEDO HIJO

PHASE EQUILIBRIUM OF SYSTEMS COMPOSED BY IONIC LIQUIDS BASED ON LIPIDIC

COMPOUNDS: USE IN THE FOOD INDUSTRY

EQUILBRIO DE FASES DE SISTEMAS COMPOSTOS POR LQUIDOS INICOS LIPDICOS: USO

NA INDSTRIA DE ALIMENTOS

Campinas

2016

ARIEL ANTONIO CAMPOS TOLEDO HIJO

PHASE EQUILIBRIUM OF SYSTEMS COMPOSED BY IONIC LIQUIDS BASED ON LIPIDIC

COMPOUNDS: USE IN THE FOOD INDUSTRY

EQUILBRIO DE FASES DE SISTEMAS COMPOSTOS POR LQUIDOS INICOS LIPDICOS: USO

NA INDSTRIA DE ALIMENTOS

Dissertao apresentada Faculdade de Engenharia

de Alimentos da Universidade Estadual de Campinas como

parte dos requisitos exigidos para a obteno do ttulo de

Mestre em Engenharia de Alimentos

Dissertation presented to the Faculty of Food Engineering of the

University of Campinas in partial fulfillment of the

requirements for the degree of Master in Food Engineering

Supervisor/Orientador: Prof. Dr. Antonio Jos de Almeida Meirelles

Co-supervisor/Co-orientador: Prof. Dr. Guilherme Jos Maximo

Co-supervisor/Co-orientador: Prof. Dr. Eduardo Augusto Caldas Batista

ESTE EXEMPLAR CORRESPONDE VERSO FINAL DE DISSERTAO

DEFENDIDA PELO ALUNO ARIEL ANTONIO CAMPOS TOLEDO HIJO, E

ORIENTADA PELO PROF. DR. ANTONIO JOS DE ALMEIDA MEIRELLES

Campinas

2016

Agncia(s) de fomento e n(s) de processo(s): CNPq, 133152/2014-6

Ficha catalogrficaUniversidade Estadual de Campinas

Biblioteca da Faculdade de Engenharia de AlimentosClaudia Aparecida Romano - CRB 8/5816

Toledo Hijo, Ariel Antonio Campos, 1990- T575e TolEquilbrio de fases de sistemas compostos por lquidos inicos lipdicos :

uso na indstria de alimentos / Ariel Antonio Campos Toledo Hijo. Campinas,SP : [s.n.], 2016.

TolOrientador: Antonio Jos de Almeida Meirelles. TolCoorientadores: Guilherme Jos Maximo e Eduardo Augusto Caldas

Batista. TolDissertao (mestrado) Universidade Estadual de Campinas, Faculdade

de Engenharia de Alimentos.

Tol1. Lquido inico. 2. Equilbrio slido-lquido. 3. cidos graxos. 4.

Etanolaminas. 5. Cristal lquido. I. Meirelles, Antonio Jos de Almeida. II.Maximo, Guilherme Jos. III. Batista, Eduardo Augusto Caldas. IV.Universidade Estadual de Campinas. Faculdade de Engenharia de Alimentos.V. Ttulo.

Informaes para Biblioteca Digital

Ttulo em outro idioma: Phase equilibrium of systems composed by ionic liquids based onlipidic compounds : use in the food industryPalavras-chave em ingls:Ionic liquidSolid-liquid equilibriumFatty acidsEthanolaminesLiquid crystalrea de concentrao: Engenharia de AlimentosTitulao: Mestre em Engenharia de AlimentosBanca examinadora:Antonio Jos de Almeida Meirelles [Orientador]Jorge Fernando Brando PereiraRosiane Lopes da CunhaData de defesa: 28-03-2016Programa de Ps-Graduao: Engenharia de Alimentos

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BANCA EXAMINADORA

Prof. Dr. Antonio Jos de Almeida Meirelles

(ORIENTADOR) DEA/FEA/UNICAMP

Prof. Dr. Jorge Fernando Brando Pereira

(MEMBRO) UNESP

Profa. Dr

a. Rosiane Lopes da Cunha

(MEMBRO) DEA/FEA/UNICAMP

Profa. Dr

a. Ana Paula Badan Ribeiro

(SUPLENTE) DTA/FEA/UNICAMP

Prof. Dr. Pedro de Alcntara Pessa Filho

(SUPLENTE) USP

A ata da Defesa, assinada pelos membros da Comisso examinadora, consta no processo de

vida acadmica do aluno

DEDICATRIA

Aos meus pais Ariel Antonio Campos Toledo e Leopoldina Maria Dias Campos e irm Erika

Dias Campos pelo amor incondicional, educao, apoio e motivao. Vocs tiveram um papel

fundamental para que eu atingisse os meus objetivos.

AGRADECIMENTOS

Deus, pelas benes recebidas no decorrer da vida, por sempre iluminar o meu caminho e

me dar sabedoria para tomar decises importantes.

Ao meu orientador, Antonio Jos de Almeida Meirelles, pela oportunidade, orientao,

sugestes e ensinamentos.

Ao meu co-orientador, Guilherme Jos Maximo, pela orientao, apoio, pacincia e

ensinamentos.

Ao meu co-orientador, Eduardo Augusto Caldas Batista, pelas dicas e apoio.

professora Mariana Conceio da Costa, pelo apoio.

Ao programa de Ps-Graduao em Engenharia de Alimentos da Faculdade de Engenharia de

Alimentos da Universidade Estadual de Campinas (UNICAMP).

Ao CNPq pelo financiamento deste trabalho atravs da concesso da bolsa de estudos de

mestrado e atravs dos projetos 133152/2014-6, 483340/2012-0, 305870/2014-9,

309780/2014-4.

FAPESP pelo apoio financeiro para a realizao deste trabalho atravs dos projetos

2014/03992-7, 2012/05027-1.

todos os colegas do grupo de pesquisa EXTRAE, pelo companheirismo, discusses e apoio

tcnico-cientfico.

Patrcia e Vanessa, tcnicas do EXTRAE e LEP, respectivamente, pelo apoio, ateno e

disponibilidade e aos secretrios da Ps-Graduao e do departamento de Engenharia de

Alimentos, sempre muito solcitos.

Aos meus amigos e colegas, Aureliano, Abel, Daniel, Eric, Fernan, talo, Lilian, Luiz, Paulo.

Obrigado pela amizade sincera e companheirismo.

Marina, pelo apoio, compreenso, pacincia e companheirismo.

todos aqueles que de alguma forma contriburam para o meu trabalho.

RESUMO

Lquidos inicos baseados em cidos graxos tm sido considerados compostos promissores e

atrado o interesse da indstria e da comunidade acadmica pelas suas propriedades nicas

e sua facilidade de sntese. Devido procura por aditivos renovveis por parte do mercado

consumidor, o uso de biocompostos, tais como os cidos graxos obtidos a partir de leos

vegetais, apresenta-se como uma possvel alternativa para produzir lquidos inicos com

baixa ou sem toxicidade e, consequentemente, expandir sua aplicao em bioprocessos, no

desenvolvimento de bioprodutos ou, ainda, no processamento de alimentos. Os lquidos

inicos baseados em cidos graxos podem se apresentar no estado lquido cristalino sob

determinadas condies de temperatura devido s longas cadeias alqulicas presentes na

sua estrutura, o que os torna compostos interessantes e com alto potencial de uso

tecnolgico e industrial. O presente trabalho teve dois objetivos principais. O primeiro foi

proporcionar uma viso geral das aplicaes dos lquidos inicos na indstria de alimentos

ou em bioprocessos relatados at o presente momento, da sntese de lquidos inicos

baseados em compostos naturais e propor novas aplicaes na rea. O segundo teve como

foco o entendimento das propriedades de equilbrio e fsico-qumicas de lquidos inicos

baseados em cidos graxos e de suas misturas binrias. Considerando a demanda de

trabalhos sobre as propriedades de fuso dos lquidos inicos prticos, este trabalho

proporcionou uma base para uma melhor compreenso do complexo equilbrio de fases de

tais compostos e das suas misturas, com formao de solues slidas e mesofases lquidas

cristalinas, o que relevante para a formulao de produtos e desenho de processos. Vinte

novos lquidos inicos baseados em cidos graxos e etanolaminas foram sintetizados e

investigados. As peculiares caractersticas destes compostos apresentadas neste trabalho,

tais como alta viscosidade, predominante perfil reolgico no-Newtoniano e habilidade de

estruturao, fazem deles interessantes biocompostos com potencial de aplicao como

surfactantes, emulsificantes e lubrificantes. As misturas avaliadas compreenderam vinte e

sete novos lquidos inicos e apresentaram um interessante perfil de fuso no ideal com

formao de solues slidas. Alm disso, com a mistura destes compostos obteve-se um

incremento na faixa de temperatura de cristal lquido, assim como uma melhoria

significativa do seu comportamento reolgico. O aprimoramento das propriedades fsico-

qumicas dos lquidos inicos baseados em cidos graxos proporciona novas possibilidades

no sentido de ampliar sua aplicao industrial, como em aplicaes tribolgicas, na

estruturao fsica de produtos e no desenvolvimento de processos.

Palavras-chave: lquido inico, equilbrio slido-lquido, cido graxo, etanolamina, cristal

lquido, reologia.

ABSTRACT

Ionic liquids based on fatty acids have emerged as promising compounds and attracted the

interest of the industry and the academy community, due to their easy preparation and

unique properties. In the context of green chemistry, the use of biocompounds, such as fatty

acids derived from vegetable oils could disclose a possible alternative way to produce ionic

liquids with low or non-toxic effect and consequently, expanding their applicability in bio-

based processes or in the development of bio-products, or even in food processing. Under

certain temperature conditions, such ionic liquids are liquid crystals, due to the long alkyl

chain in their chemical structure, which makes them interesting compounds with high

potential for technological and industrial use, since they exhibit a plethora of thermophysical

properties. The present work was focused on two main objectives. The first was aimed at

providing an overview of applications of ionic liquids in the food industry reported up to date

in the literature, disclosing on possible lacunas related to their synthesis with natural

biocompounds, and proposing new applications in such a field. The second was focused on

the understanding of the thermodynamic and physicochemical properties of ionic liquids

based on fatty acids and the binary mixtures of them. Considering the lack of works on the

melting properties of protic ionic liquids, this work addressed efforts to a better

comprehension of the complex phase equilibrium of such compounds and their mixtures,

which is relevant for products and processes design. Twenty new ILs based on fatty acids and

ethanolamines were here synthesized and investigated. The peculiar characteristics of fatty

acids based ionic liquids here disclosed, such as high viscosity, marked non-Newtonian

rheological profiles, and self-structuration ability, make them interesting biocompounds with

application potential as surfactants, emulsifiers and lubricants. The mixtures here evaluated

comprehended twenty seven new ionic liquids, and presented an interesting nonideal

melting profile with the formation of solid solutions. Considering the interesting properties

of these compounds, the increasing of the ionic liquid crystal temperature domain and the

enhancing of their non-Newtonian profile were achieved by mixing them. The improvement

of the physicochemical properties of ionic liquids based on fatty acids, here disclosed by

such an approach leads to enlarge new possibilities on their industrial application, such as in

tribological application, in the physical structuration of products and development of

processes.

Keywords: ionic liquid, solid-liquid equilibrium, fatty acid, ethanolamine, liquid crystal,

rheology.

SUMRIO

CAPTULO 1 INTRODUO GERAL E OBJETIVOS .................................................................

1. INTRODUO .....................................................................................................................

2. OBJETIVOS ........................................................................................................................

2.1. Objetivo geral ............................................................................................................

2.2. Objetivos especficos .................................................................................................

CAPTULO 2 APPLICATIONS OF IONIC LIQUIDS IN FOOD PROCESSES AND

ANALYSIS....................................................

1. Introduction ............

2. Ionic liquids: overview ....

3. Ionic liquids in food processes: ongoing steps .........

4. Prospects .........

5. Possible edible-ILs from natural sources ..........

6. Ionic liquids tunability ...........

7. Final remarks ...................

CAPTULO 3 NEW PROTIC IONIC LIQUID CRYSTALS BASED ON FATTY ACIDS: SYNTHESIS AND

PHASE EQUILIBRIUM ........

1. Introduction .................

2. Material and methods ....

2.1. Materials ......

2.2. Synthesis of the ionic liquids and their characterization ....

2.3. Differential Scanning Calorimetry (DSC) .......

2.4. Light-Polarized Optical Microscopy (POM) ......

2.5. Critical Micellar Concentration (CMC) measurements ....

2.6. Viscosity and rheological measurements .....

3. Results and discussion ...

3.1. Phase Behavior and ionic liquid crystal characterization ...

3.2. Self-assembling ability of PILs ......

3.3. Rheological characterization of the PILs .

4. Conclusion

CAPTULO 4 ENHANCING PHYSICAL PROPERTIES OF IONIC LIQUID CRYSTALS FROM IONIC

LIQUID MIXTURES ......

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26

37

41

48

52

65

67

68

68

68

69

70

71

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92

95

1. Introduction .....

2. Material and methods ......

2.1. Materials ..

2.2. Synthesis of protic ionic liquids and binary mixtures of them

2.3. Melting profile of the mixtures .........

2.4. Modeling assessment .....

2.5. Self-assembling and rheological profiles ......

3. Results and discussion ......

3.1. Phase behavior and ionic liquid crystal characterization ......

3.2. Self-assembling ability of PILs binary mixtures .......

3.3. Rheological characterization of PILs binary mixtures ........

4. Conclusion ......................................................

CAPTULO 5 DISCUSSO GERAL ...............................................

1. Discusso geral ..................................................

CAPTULO 6 CONCLUSES GERAIS ....................................................................................

1. Concluses gerais .............................................................................................................

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- CAPTULO 1

Introduo Geral e Objetivos

14

1. INTRODUO

Nas duas ltimas dcadas, os lquidos inicos (LIs) tm despertado o interesse da

comunidade acadmica e industrial devido ao seu potencial de aplicao na industria

qumica, farmacutica e de alimentos. O papel promissor dos LIs nos processos industriais e

no desenvolvimento de produtos atribudo facilidade de sntese e a suas propriedades

nicas, como baixa presso de vapor, alta solubilidade, baixa inflamabilidade, estabilidade

trmica, viscosidade varivel, alta condutividade e ampla janela de fase lquida 2, 22. Alm

disso, as suas propriedades podem ser intencionalmente ajustadas pela escolha certa de

ons 25, 26, o que viabiliza a produo de novos LIs para determinados processos, levando em

considerao condies e restries especficas.

Lquidos inicos so sais formados, geralmente, por um nion e um ction

orgnico e/ou inorgnico com grande tamanho de cadeia e possuem, por conveno, ponto

de fuso inferior a 100 C, em referncia temperatura de ebulio da gua 1, 2. Os LIs

podem ser classificados em dois grupos: aprticos e prticos. Os lquidos inicos aprticos

(LIAs) representam os LIs clssicos comumente utilizados e podem ser formados por ctions

orgnicos (alkyl-piridinium, -imidazolium, -pirolidinium ou amnia quaternria) e nions

inorgnicos (cloreto [Cl-], brometo [Br-], tetrafluoroborato [BF4-] ou hexafluorofosfato [PF6

-])

3. Os lquidos inicos prticos (LIPs) so considerados uma nova gerao, com menos

trabalhos desenvolvidos na literatura, obtidos atravs da reao cido-base de Brnsted, na

qual um prton transferido de um acido de Brnsted para uma base de Brnsted.

Exemplos de LIPs, referentes ao estudo deste trabalho so os formados por cidos graxos

(cido mirstico, esterico e olico) como nions e aminas polissubstitudas (mono-, di- e

trietanolamina) como ctions.

A aplicabilidade dos LIs no setor de alimentos tem se concentrado

principalmente nas tcnicas de anlise 4, 5 e processos de extrao 6-8. Entretanto, as suas

propriedades e os recentes avanos em estudos sobre a compreenso das suas misturas 9, 10

os tornam aditivos interessantes em vrias aplicaes na indstria de alimentos, levando em

considerao seu potencial como lubrificante 11, solvente 12 e surfactante 13, potencialmente

til na formulao de produtos, assim como no desenho de processos. Porm, as aplicaes

dos LIs na indstria de alimentos ainda so poucas. Entre os principais motivos para as

poucas inovaes nesta rea est a divergncia na literatura relacionada sua toxicidade 14-

17, o que seria um impedimento para seu uso na rea de alimentos. Portanto, recentemente

15

estudos sobre o uso de compostos renovveis para sntese de LIs, tais como aminocidos 18,

compostos lipdicos 19, 20 e outros cidos de origem natural 21, 22 tm sido desenvolvidos para

tentar sobrepor a questo da toxicidade.

Por esta razo, uma nova gerao de LIs baseados em ons de origem natural

est se tornando uma tendncia. Este o caso de LIPs baseados em cidos graxos,

considerados promissores devido sua facilidade de sntese e propriedades especficas, tais

como habilidade de self-assembling, estruturao, viscoelasticidade, perfil reolgico no

Newtoniano, capacidade de formao de mesofases estveis e ampla janela de fase lquida.

Tais compostos possuem apelo renovvel, j que os cidos graxos so compostos lipdicos

obtidos a partir de leos vegetais como componentes livres da hidrlise de triacilgliceris.

Alm disso, LIs prticos baseados em cidos orgnicos tm apresentado baixo impacto

ambiental 23.

A sntese de LIs com nions e ctions de origem natural tem se revelado uma

forma interessante de produzir LIs com menor toxicidade, o que aumenta a possibilidade de

superar as limitaes em relao ao seu uso no processamento de alimentos. De fato, a

viabilizao do uso dos LIs como aditivos ou qumicos na indstria de alimentos requer

estudos toxicolgicos, fsico-qumicos e das suas propriedades termodinmicas. Neste

contexto, a avaliao do equilbrio de fases termodinmico tem sido empregada como

ferramenta til na formulao de produtos com caractersticas especficas, considerando

temperaturas de fuso 24, perfis reolgicos 25, ou a formao de estruturas cristalinas 26, que

impactam diretamente as propriedades fsico-qumicas dos produtos.

Este trabalho est dividido em trs partes, redigidas em formato de artigo

cientfico, visando sua publicao em peridicos. O Captulo 2 apresenta uma Reviso

Bibliogrfica de modo a investigar e analisar as aplicaes dos LIs na indstria de alimentos

ou em bioprocessos encontrados na literatura. Esta reviso bibliogrfica tem como objetivo

preencher lacunas encontradas na literatura em relao sntese de LIs baseados em

compostos naturais, considerando aspectos toxicolgicos e ajuste das suas propriedades

fsico-qumicas, e portanto, propor novas aplicaes na rea. O Captulo 3 apresenta a

sntese de LIs derivados de cidos graxos e etanolaminas, inditos na literatura, assim como,

o estudo do comportamento de fases slido-lquido cristalino-lquido e a caracterizao do

seu comportamento reolgico e concentrao crtica micelar. No Capitulo 4, misturas

binrias de LIPs apresentados no Captulo 3, foram avaliadas no desenvolvimento de novos

16

LIs com propriedades fsicas especficas. Neste captulo foi caracterizado o equilbrio slido-

slido-lquido cristalino-lquido (SSLcL) de 3 misturas binrias de LIs prticos derivados de

cidos graxos, assim como o perfil reolgico e concentrao crtica micelar.

2. OBJETIVOS

2.1. Objetivo Geral

O objetivo geral deste trabalho o estudo da sntese de lquidos inicos prticos

derivados de cidos graxos e etanolaminas, e das suas misturas binrias, assim como a

caracterizao do equilbrio slido-slido-lquido cristalino-lquido (SSLcL), do perfil reolgico

e da concentrao crtica micelar.

2.2. Objetivos Especficos

1. Avaliar o estado da arte da sntese e aplicao de lquidos inicos na indstria

de alimentos;

2. Avaliar o efeito do tamanho da cadeia alqulica da estrutura molecular dos

cidos graxos (cidos caprlico, cprico, lurico, mirstico, palmtico e esterico)

na determinao experimental do SSLcL dos LIs puros e das seguintes misturas

binrias de LIs baseados nos cidos mirstico, palmtico e esterico: miristato de

dietanolamina ([DEA][C14OO]) + palmitato de dietanolamina ([DEA][C16OO]),

miristato de dietanolamina ([DEA][C14OO]) + estearato de dietanolamina

([DEA][C18OO]);

3. Avaliar a influncia das insaturaes presentes na estrutura molecular dos

cidos olico e linolico na determinao experimental do SSLcL dos LIs;

4. Avaliar o efeito da simetria e tamanho da estrutura molecular das

etanolaminas (mono-, di- e trietanolamina) na determinao experimental do

SSLcL dos LIs puros e da seguinte misturas binrias de LIs derivados de mono- e

dietanolaminas: miristato de dietanolamina ([DEA][C14OO]) + miristato de

monoetanolamina ([MEA][C14OO]);

5. Sntese e caracterizao de cristais lquidos inicos prticos.

17

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- CAPTULO 2

Applications of ionic liquids in food processes and analysis

Artigo submetido ao peridico RSC Advances

20

APPLICATIONS OF IONIC LIQUIDS IN FOOD PROCESSES AND ANALYSIS

Ariel A. C. Toledo Hijo1, Guilherme J. Maximo1, Mariana C. Costa2, Eduardo A. C. Batista1,

Antonio J. A. Meirelles1

1 Laboratory of Extraction, Applied Thermodynamics and Equilibrium, School of Food

Engineering, University of Campinas, R. Monteiro Lobato 80, 13083-862, Campinas, So

Paulo, Brazil

2 School of Applied Sciences, University of Campinas, 13484-350, Limeira, So Paulo, Brazil

*tomze@unicamp.br Tel.: (+55 19 3521 4056)

Artigo submetido ao peridico RSC Advances

21

ABSTRACT

Ionic liquids (ILs) have been extensively used in many researches and industry fields,

especially for chemical and pharmaceutical applications. Nevertheless, during past years,

some works revealed that those green solvents in fact could present certain toxicity levels.

This is the reason why the use of biocompounds from natural sources, such as those present

in lipid or protein matrices, for synthesis of ILs has become a trend. Moreover, this has been

shown to be an interesting way to produce ILs with low or non-toxic effects, possibly

overcoming the major drawback of using them in the food industry. Despite of this

development, the applications of these compounds known as third generation ILs in food

processes are still mainly focused on extraction processes and food-compounds analysis.

However, due to their ionic character, together with specific physical properties required for

food additives, ILs has shown to be interesting chemicals for a plethora of applications,

taking into account the design of food-based products and processes. In this context, the

present review is aimed at providing an overview of applications of ILs in food industry

reported up to date in the literature, disclosing on possible lacunas related to their synthesis

with natural biocompounds, and proposing new applications in such a field.

22

1. INTRODUCTION

Ionic liquids (ILs) are chemical compounds that have attracted the interest of the

industrial and academic community due to their potential for application in chemical,

pharmaceutical, and food industries. The main reasons are their easy preparation and

unique properties, such as, self-assembling ability, low vapor pressure, non-flammability,

thermal stability and wide liquid phase range 1.

In the last two decades, the literature has revealed several applications of ionic

liquids in the pharmaceutical and biotechnological industry, among several other interests in

the chemical engineering field, such as biosseparation processes of active compounds 2-4,

development of drug delivery systems 5-7, electrochemistry 8-10, and enzymatic processes 11-

13. In particular, imidazolium based ILs have been widely employed in the formulation of

emulsions for drug delivery systems 5, solubility of gases 14, and surfactants 15, due to their

tuning solubility properties. However, few innovations related to food industry have been

revealed in comparison to other academic or industrial fields.

Literature on food related processes involving ILs has been mainly focused on

extraction processes 16-19 and food analysis 20-22. However, the ongoing access of their

properties, and the recent advances on the comprehension of their mixtures 23-26 make them

interesting additives for many applications in food industry, taking into account their

surfactant 27, lubricant 28, solvence ability 29, potentially usable for designing food-based

products and processes. Nevertheless, applications of ILs in food industry processes are still

scarce.

In fact, the viability of using ILs as additives or chemicals in food-related

industrial applications requires studies on their toxicological, physicochemical, and

thermodynamic properties. As well known, the right choice of anions and cations promotes

the tunning of ILs properties for the synthesis of additives/chemicals with specific

characteristics for a given process or product. Also, in this context, the evaluation of the

thermodynamic phase equilibrium has been applied as a useful tool for such a property

design, taking into account specific melting temperatures 30, rheological profiles 31, or

crystalline structures behavior 26 that directly impact products physicochemical properties

or sensory quality.

However, one of the main reasons of the lack of application of ILs in food

industry is the divergence in literature on their toxicity 32-35, and gradually several studies on

23

the use of renewable compounds for the synthesis of ILs, such as aminoacids 36, 37, lipidic

compounds 38 or other acids from natural origin 39-41 have emerged.

In this context, the present review attempts to summarize the classical definition

of ionic liquids in order to understand the already evaluated applications of ILs, with

emphasis in their application in the food industry or in bio-based processes, highlights the

tunability of their physicochemical properties, especially in the synthesis of ILs based on

natural compounds, also taking into account the interest in overcoming toxicity aspects, and

finally proposes potential new applications in such a field.

2. IONIC LIQUIDS: OVERVIEW

Definitions. Historically, some authors propose that the well-known work of Paul

Walden42, which investigated the synthesis and physicochemical properties of

monoethylammonium nitrate ([EtNH3][NO3]) 43, emblematically establishes the start-point of

the IL science field. After that, and until mid-90s, the basic concepts of such components

were already well-known by the academic community, especially within electrochemical

experts 43, 44. However, the technological importance of ILs has only emerged in the last two

decades along the search for new green solvents in the replacement of conventional ones

used in extraction processes 1, 45, 46. In fact, the number of patents in the area exceeded 900,

in 2007 47. However, despite the increasing rate of works published in the recent decades,

the applications of ILs in the food industry have been little explored a.

As classically claimed, ILs has been described as molten salts, i.e. liquids in a

thermodynamic definition, but also liquid crystals, depending on structural interactions or

temperature/pressure conditions, and remaining in such state in a wide temperature range.

Also, they have been commonly defined as salts with solid phase melting temperature

bellow 373.15 K, in reference to water boiling temperature 1, 45, 48, such as several well-

known ILs, especially those composed with imidazolium cations that present melting

temperatures lower than that fictional limit 49, 50. In fact, among ILs thermophysical

properties, melting temperature is probably one of the main targets taking into account

products and processes design.

In this context, both molecular structure and charge profile plays a fundamental

role in the search for novel applications 50-53. ILs are classified in two main groups with

specific behaviors according to the ions of which they are composed: aprotic and protic. The

24

aprotic ILs (APILs) are probably the most commonly used, being, in general, synthesized by

organic and large cations, such as alkyl-piridinium, -imidazolium and -pirolidinium, or

quaternary ammonium groups, as well as inorganic anions such as chloride [Cl], bromide

[Br], tetrafluoroborate [BF4], hexafluorofosfate [PF6] groups 54. On the other hand, protic ILs

(PILs) can be formed by a wide class of compounds through a proton transfer reaction from a

Brnsted acid to a Brnsted base, such as organic aliphatic acids 38, 41, methylsulfonate

[CH3SO3] 55, triflate [CF3SO3]

56, as anions and ammonium 57, phosphonium 58, pyridinium 59,

imidazolium 50 as cations. Until the recent past, PILs were still considered a new class of

compounds, with a large lack of studies to be completed, considering the infinite possibility

of combinations for their synthesis.

Thermophysical properties and solubility profile. Together with their properties,

the wide temperature range in which ILs are at liquid state make them interesting

compounds for a plethora of industrial applications. According to some authors 48, 60, 61, their

relative low melting point, in comparison to inorganic salts, is related to structural and

charge effects. Basically, the asymmetry and long length of the cation could decrease the

intermolecular interactions and the corresponding energy of cohesion, consequently

diminishing their melting temperatures. This feature is important from the manufacturing

point of view, since in the liquid state the substance can be easily transported and handled.

Also, by presenting low volatility, i.e. low vapor pressure and non-flammability,

ILs can be safely used in closed or low pressure processes, having low environmental impact,

and for these reasons being well-claimed as environmentally friendly. Moreover, due to their

high stability from the chemical and thermal points of views, ILs have been used in processes

that require high temperature conditions, such as catalytic reactions for biomass

pretreatment in the second generation bioethanol production 62, 63, or in the

transesterification of triacylglycerols for biodiesel production 64, 65.

Solubility is probably one of the main focuses on ILs research. They are highly

efficient solvents for organic and inorganic compounds, widely used in separation processes

involving two or more phases. The tunability of the physicochemical properties of ILs can

result in IL based solvents designed for separation of specific biocompounds 66, which is the

case, for example, of imidazolium-based ILs used for separation of caffeine in food systems

67. In this context, ILs have been employed in aqueous biphasic systems (ABS), that originally

consist of two water soluble polymers or polymer-salt mixtures used for extracting and

25

purifying biomolecules 68, 69. The replacement of one salt or polymer by ILs modifies the

phase solubility properties and might improve the selectivity of such separation process 67.

The design of ILs with high solubility has been also an innovative solution in

relation to the bioavailability of active components. Being liquid at body temperature, ILs

have been used as solubility enhancer of poorly soluble pharmaceutical components, such as

transdermal systems products developed by MedRx 35, 70. Other examples of applications are

the recovery of metals from water and food samples 22, 71, and chromatography 72 and

spectroscopy methodologies 73 related not only to the ILs high solubility but also to their

chemical stability, wide electrochemical window and high ionic conductivity. They can also

be used in the absorption of gases, such as H2, CO and O2 74, or used as cossolvents in high

pressure extraction processes using supercritical CO2 75.

Phase behavior. Above solid phase melting temperatures, some ILs, especially

those with long alkyl chain in their chemical structure, tend to form crystalline structures

within a non-isotropic liquid state 76. These structures are called mesophases and are

considered a fourth state of matter, since they present flow properties similar to isotropic

liquid phases and contain oriented microstructures as solid phases. Due to the presence of

such microstructures, the so called ionic liquid crystals (ILCs) exhibit a plethora of

thermophysical properties with high potential for technological and industrial use.

The formation of liquid crystalline mesophases is directly related to the

molecular structure of ILs 77, 78 or to the presence of additional compounds. The first case is

called thermotropic behavior and is characterized by the formation of mesophases of pure

ILs obtained by cooling the isotropic liquid phase or by heating the solid state IL. The second

one is called lyotropic behavior and, in this case, the ILC is assembled with other compounds

added to the system as in a surfactant containing mixture. Studies in literature have

disclosed a wide variety of mesophases that can be formed in the liquid crystalline state

according to the shape of arrangement of ILs molecules 26, 77-79.

By showing mixed properties of liquid crystals and ILs, ILCs are interesting

additives. The presence of electrostatic interactions promotes the formation of stable

mesophases with direct influence on physicochemical properties 78, such as increasing of

thermal stability in comparison to conventional ILs 80, enhancing of non-Newtonian profiles

with viscoelasticity behavior potentially usable in emulsifying or tribological applications 31,

81, 82, and complex changes in melting temperatures behavior and conductivity 25, 38.

26

Several ILs with mesophases at temperatures above their melting temperature

show specific non-newtonian rheological and viscosity behavior in accordance with the

crystalline structure and the strong electrostatic interactions prevailing within the

anisotropic liquid phase. For example, the ILs monoethanolammonium oleate and

diethanolammonium oleate show an unique non-Newtonian rheological profile and viscosity

behavior, depending on the shear rate applied to the sample and temperature, due to the

type of mesophase structure formed under the evaluated conditions 38. According to Wang

et al. 26 the non-Newtonian viscosity behavior is a common feature of ILCs with long alkyl

chain length, which is the case of 1,3-didodecylimidazolium tetrafluoroborate

[C12C12im][BF4], for example. In the design of ILs for application in food processing, the

understanding of fluid rheological behavior is important, since such substances can be

submitted to shear stress during pumping for transportation or be mixed with other

ingredients to create specific sensorial profiles.

3. IONIC LIQUIDS IN FOOD PROCESSES: ONGOING STEPS

The review on the application of ILs in the food and related areas, as well as their

discussion described in the following paragraphs, is based on many works available up to

date in the literature, which are indicated in Table 1.

27

Table 1 Brief overview on the application of ionic liquids in food industry

Fields Applications Ionic Liquids References

Extraction

Extraction of phenolic compounds

1-dodecyl-3-methylimidazolium chloride [C12mim]Cl; 1-dodecyl-3-methylimidazolium bromide [C12mim]Br; 1-dodecyl-3-methylimidazolium trifluoromethanesulfonate [C12mim][CF3SO3]; 1-dodecyl-3-methylimidazolium nitrate [C12mim][NO3]; 1-dodecyl-3-methylimidazolium hydrogen sulfate [C12mim][HSO3]

83

1-ethyl-3-methylimidazolium proline [C2mim][Pro]; 1-ethyl-3-methylimidazolium alanine [C2mim][Ala]; 1-ethyl-3-methylimidazolium lysine [C2mim][Lys]; 1-ethyl-3-methylimidazolium glycine [C2mim][Gly]; 1-ethyl-3-methylimidazolium serine [C2mim][Ser]

84

1-dodecyl-3-methylimidazolium bromide [C12mim]Br; 1-butyl-3-methylimidazolium bromide [C4mim]Br

85

Extraction of polycyclic aromatic hydrocarbons

1-hexadecyl-3-butylimidazolium bromide [C16C4im]Br 86

Extraction of piperine from pepper

1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4]; 1-butyl-3-methylimidazolium hexafluorophosphate [C4mim][PF6]; 1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4]; 1-butyl-3-methylimidazolium bromide [C4mim]Br

87

Extraction of caffeine 1-butyl-3-methylimidazolium chloride [C4mim]Cl 88

28

Table 1 Continued

Fields Applications Ionic Liquids References

Extraction

Extraction and deterpenation of essential oils

1-ethyl-3-methylimidazolium acetate [C2mim][CH3COO]; 1-butyl-3-methylimidazolium acetate [C4mim][CH3COO]

29

1-butyl-3-methylimidazolium chloride [C4mim]Cl, 1-allyl-3-methylimidazolium chloride [Amim]Cl; 1-ethyl-3-methylimidazolium acetate [C2mim][CH3COO]

89

1-ethyl-3-methylimidazolium 2-(2-methoxyethoxy) ethylsulfate [C2mim][Meesu] 90

1-ethyl-3-methylimidazolium ethylsulfate [C2mim][EtSO4] 91

1-ethyl-3-methylimidazolium methanesulfonate [C2mim][OMs] 92

1-hexyl-3-methylimidazolium hexafluorophosphate [C6mim][PF6] 93

1-lauryl-3-methylimidazolium bromide [C12mim]Br 94

Extraction of free fatty acids from vegetable oils 1-butyl-3-methylimidazolium dicyanamide [C4mim][DCA] 95

Extraction of food additives from herbs

1-butyl-3-methylimidazolium bromide [C4mim]Br 96

1-butyl-3-methylimidazolium bromide [C4mim]Br; 1-butyl-3-methylimidazolium chloride [C4mim]Cl

97

Fat extraction from foodstuffs 1-(2-cyanoethyl)-3-methylimidazolium bromide [CyanoC2mim]Br; 1-propyl-3-methylimidazolium bromide [C3mim]Br

98

29

Table 1 Continued

Fields Applications Ionic Liquids References

Food Analysis

Determination of vitamins 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] 99

Determination of folic acid 1-butyl-3-methylimidazolium hexafluorophosphate [C4mim][PF6] 100

Determination of preservatives in soft drinks

1-butyl-2,3-dimethylimidazolium chloride [C4C1mim]Cl 20

Determination of heavy metal ions

1-hexyl-3-methylimmidazolium bis (trifluormethylsulfonyl)imid [C6mim][Tf2N] 22

1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4] 101

1-butyl-3-methylimidazole hexafluorophosphate [C4mim][PF6] 102

Determination of ascorbic acid n-octylpyridinum hexafluorophosphate [OP][PF6] 103

Determination of acidic food additives from soft drinks

1-aminopropyl-3-methylimidazolium chloride [APmim]Cl 18

Determination of spices 1-octyl-3-methylimidazolium chloride [C8mim]Cl 104

Determination of phenolic compounds 1-ethyl-3-methylimidazolium tetrafluoroborate [C2mim][BF4] 105

Detection of adulteration of vegetable oils tributylammonium -cyano-4-hydroxycinnamate [TBA][CHCA] 106

Determination of bisphenol A

1,3-dipropylimidazolium bromide [Dpim]Br 21

1-octyl-3-methylimidazole hexafluorophosphate [C8mim][PF6] 107

1-butyl-3-methylimidazole hexafluorophosphate [C4mim][PF6] 108

Determination of antibiotics

1-octyl-3-methylimidazolium bromide [C8mim]Br 109

1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4] 110

1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4] 111

Determination of acrylamide 1-butyl-3-methylimidazolium bromide [C4mim]Br

112

1-butyl-3-methylimidazole hexafluorophosphate [C4mim][PF6] 113

Determination of herbicides

1-hexyl-3-methylimidazole hexafluorophosphate [C6mim][PF6] 114

1-hexyl-3-methylimidazole hexafluorophosphate [C6mim][PF6] 115

1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4] 116

30

Table 1 Continued

Fields Applications Ionic Liquids References

Food Analysis

Determination of dyes

1-hexyl-3-methylimidazolium hexafluorophosphate [C6mim][PF6] 117

1-butyl-3-methylimidazoliun tetrafluoroborate [C4mim][BF4] 118

1-hexyl-3-methylimidazole hexafluorophosphate [C6mim][PF6] 119

1-butyl-3-methylimidazolium hexafluorophosphate [C4mim][PF6]; 1-octyl-3-methylimidazolium hexafluorophosphate [C8mim][PF6]

17

1-butyl-3-methylimidazolium bromide [C4mim]Br 120

1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4]; 1-hexyl-3-methylimidazolium tetrafluoroborate [C6mim][BF4]; 1-octyl-3-methylimidazolium tetrafluoroborate [C8mim][BF4]; ammonium hexafluorophosphate [NH4][PF6]

121

Biodiesel Synthesis of biodiesel

1-n-butyl-3-methylimidazolium tetrachloro-indate [BMI-InCl4] 122

1-butyl-3-methylimidazolium hydrogensulfate [C4mim][HSO4] 123

1-(4-sulfonic acid)butylpyridinium hydrogen sulfate [HSO3bPy][HSO4] 65

1-(propyl-3-sulfonate)-3-(3-trimethoxysilylpropyl) imidazolium hydrogen sulfate [SO3H-PIm-CPMS][HSO4]

124

1-butyl-3-methylimidazolium tosylate [C4mim][CH3SO3] 125

3-(N,N,N-triethylamino)-1-propanesulfonic hydrogen sulfate 64

Bioethanol

Purification of bioethanol

3-methyl-1-octylimidazolium tetrafluoroborate [Omim][BF4] 126

1,3-dimethylimidazolium tetrafluoroborate [Dmim][BF4] 127

3-methylimidazolium tetrafluoroborate [mim][BF4]; 3-methylimidazolium chloride [mim]Cl; 3-methylimidazolium trifluoromethanesulfonate [mim][CF3SO3]

128

Pretreatment of sugarcane bagasse

1,3-dimethylimidazolium dimethylphosphate [Dmim][DMP] 63

1,3-dimethylimidazolium diethylphosphate [Dmim][DEP] 129

1-butyl-3-methylimidazolium Chloride/FeCl3 ([C4mim]Cl/FeCl3) 130

cholinium glycine [Ch][Gly], cholinium alanine [Ch][Ala], cholinium serine [Ch][Ser], cholinium threonine [Ch][Thr], and cholinium proline [Ch][Pro]

131

1-allyl-3-methylimidazolium chloride [Amim]Cl 132

Pretreatment of corncob 1-ethyl-3-methylimidazolium acetate [C2mim][CH3COO] 62

Pretreatment of wheat and corn 1-ethyl-3-methylimidazolium chloride [C2mim]Cl 133

31

Extraction processes. The replacement of conventional organic solvents by ILs in

industrial processes requiring high temperatures or higher solvent ability have been, the

main focus of works on the uses of ILs for food control and processes, especially in the

design of chemical analytical techniques and extraction of active compounds. In fact, the

ability of ILs to integrate chemical reactions or processes at room temperature, due to their

low melting temperature, allows decreasing of temperatures, energy costs, loss of solvents

by evaporation, and especially thermal injuries, which is a fundamental target taking into

account sensorial profiles and activity of nutraceutical compounds.

From the point of view of food processing innovation, it is possible to obtain high

value-added compounds from natural raw materials, such as polyphenols, essential oils and

natural dyes. Nowadays, there are several methods for extraction of such compounds from

food samples, such as liquid-liquid, solid-liquid or vapor-liquid extraction techniques.

However, the use of ILs for optimizing or enhancing the extraction yield of natural

compounds of interest for the pharmaceutical and food industries has increased in recent

years 88, 89, 93. Despite of this, their large potential still offers several possibilities with respect

to the design of new extraction methods 89. This is the case of the recovering of essential

oils, extraction of phenolic compounds, caffeine and terpenes.

Essential oils are substances present in herbs, seeds and fruits, commonly

extracted through mechanical processes, by using solvents or hydrodistillation. Their

technological importance is based on the presence of active compounds, such as esters,

alcohols and phenols, with antioxidant and antimicrobial properties, as well as their activity

as promoters of fragrance and flavor. In this way, Bica et al. 89 reported a novel method for

orange essential oil isolation from orange peels using ILs. The new method is based on the

well-known low volatility profile of the 1-ethyl-3-methylimidazolium acetate

([C2mim][CH3COO]) and its ability to dissolve lignocellulosic material of the orange peel

biomass. This promotes the formation of a media for further distillation and separation of

the orange essential oil. The process allowed complete dissolution of the raw material,

reuses of IL, and a higher yield (5% w/w) when compared to solid-liquid extraction

techniques using dichloromethane and diethyl ether, reported in literature (closed to 1%

w/w) 89, 134. Zhai et al. 93 employed a microwave-assisted extraction method using 1-hexyl-3-

methylimidazolium hexafluorophosphate ([C6mim][PF6]) for extracting essential oils from

dried fruits, and obtained results in terms of essential oil constituents similar to those

32

generated with hydrodistillation. According to the authors the advantages of the process

compared with hydrodistillation are the simplicity, less extraction time and sample

consumption. Moreover, extractions techniques using ILs (e.g. 1-decyl-3-methylimidazolium

bromide [C12mim][Br]) have been also employed to obtain essential oils from other natural

sources, such as fruits 94, 135 and herbs 97, 136.

Phenolic compounds are bioactive substances formed by a hydroxyl groups

bonded directly to an aromatic hydrocarbon group, and can be obtained synthetically or

naturally from foods, being the natural source more beneficial to human health than

synthetic ones 84. The obtainment of phenolic compounds from natural sources has some

barriers due to the fact that they are mixed with other compounds, requiring efficient

extraction techniques, such as those using ILs 83-85. Tocopherols, for example, can be

extracted from methylated oil deodorizer distillate, a byproduct obtained from refining

processes of vegetable oils 84. Ni et al. 84 employed a liquid-liquid extraction method using

amino acid based ILs for extraction of -tocopherol from such a media. According to the

authors, the efficiency of the extraction was due to the hydrogen bonding interactions

between the hydroxyl group of the phenolic compound and the amino groups of the ILs.

Caffeine is a substance with bioactive properties commonly found in foods, such

as coffee, tea, chocolate and guarana. It has been used in the formulation of pharmaceutical

products due to their stimulant effect on the muscular and nervous system. It has been also

considered as bioactive product for replacement of pesticides, since it has antimicrobial and

antifungical properties 88, 137. However, their extraction from food systems is increasingly

demanded for particular caffeine-sensitive consumers, especially in the case of soft drinks or

decaffeinated products. Claudio et al. 88 developed a more efficient extraction method for

extraction of caffeine from guarana seeds using aqueous solutions of imidazolium or

pyrrolidinium based ILs. The results showed excellent extraction yields of caffeine at

moderate temperature (up to 9 % w/w) with possible recovering of such an ionic solvent.

Extraction techniques using ILs are also applied to eliminate undesired

compounds, since they can affect negatively the shelf-life and sensorial profile of foods, for

instance causing off-flavors. This is the case of terpenes present in citrus oils and responsible

for loss of their quality and stability, whose extraction process is called deterpenation 29, 90-92.

Lago et al. 29 evaluated the use of acetate based ILs for the extraction of oxyterpenes from a

model system based on citrus essential oil and concluded that the ILs tested are quite

33

adequate compared to common ionic or molecular solvents. Other studies evaluated the use

of ILs as solvents for deterpenation of citrus essential oils by measuring the corresponding

liquid-liquid equilibrium. In this case, citrus essential oil was simulated by a binary mixture of

their main components, limonene and linalool, and the liquid-liquid equilibria for ternary

systems containing ILs (e.g. 1-ethyl-3-methylimidazolium ethylsulfate, 1-ethyl-3-

methylimidazolium methanesulfonate and 1-ethyl-3-methylimidazolium 2-(2-

methoxyethoxy) ethylsulfate) were determined 90-92. Arce et al. 92 found higher extraction

selectivity using 1-ethyl-3-methylimidazolium methanesulfonate, compared with the use of

organic solvents, such as 2-butene-1,4-diol and ethylene glycol.

Food analysis. New food analysis methods with high efficiency, sensitivity and

low cost is still necessary for determination and quantification of compounds of interest and

those prohibited or limited by food legislation. In this way, food analysis using ILs has gained

importance, mainly for determination of non-desirable or amount-limited compounds, such

as preservatives, heavy metals, antibiotics, herbicides and dyes. Safranin-T, for example, is a

synthetic dye commonly used in the production of foods due to their red color and low cost,

but prohibited by legislation in many countries. Zhang et al. 117 employed a new method for

extraction of Safranin-T from food products using 1-hexyl-3-methylimidazolium

hexafluorophosphate [C6mim][PF6] based on the combination of dispersive liquid-liquid

microextraction, micro-solid phase extraction and fluorimetric determination. According to

the authors, the new method was sensitive, efficient and easy to operate exhibiting

advantages in terms of time consumption and simplicity of the process in comparison with

conventional methods for determination of this type of dye, such as high-performance liquid

chromatography of fluorescence (HPLC-FL) and ultra-high-performance liquid

chromatography tandem mass spectrometry (UHPLC-MS/MS).

The determination of the concentration of heavy metals is important for the

evaluation of the food quality, and customer health assurance. Khani and Shemirani 22

employed a new method for simultaneous determination of cobalt and nickel in water using

1-hexyl-3-methylimmidazolium bis-(trifluormethylsulfonyl)imide [C6mim][Tf2N]. This is quite

important because, although nickel is nutritionally essential in trace levels, it is considered

toxic at high levels. The results obtained by the authors were satisfactory compared with

other methods, such as solid-phase extraction (SPE) and cloud point extraction (CPE). Their

34

extraction procedure was simple without requirement of high ILs concentration, an

important feature taking into account cost, toxicological and environmental aspects.

Antibiotics and herbicides are chemicals prohibited or limited by food legislation

and sometimes unduly present in foods. Antibiotics are used for treatment of animal

infections or diseases, for instance for the treatment of mastitis in cows, leading to the risk

of milk contamination. This is the reason why such substances must be detected and

controlled in food samples before human consumption. Shao et al. 110 developed an

analytical method for extraction and determination of sulfonamides (antibiotics) from milk

samples. The method consisted of an ABS based on 1-butyl-3-methylimidazolium

tetrafluoroborate [C4mim][BF4] and trisodium citrate dehydrate followed by high-

performance liquid chromatography, as an alternative to common methods based on

spectrophotometry and gas chromatography, that are time-consuming or use volatile

solvents. According to the authors the method is feasible for determination of sulfonamides

in milk samples, since it was performed in one clean-up and preenrichment step, showing

good results for recovery of analytes (higher than 72.32% w/w) and accuracy (0.56 12.20%,

standard deviations). Herbicides has maximum residue limits in food samples imposed in

many countries 115. ILs have also been used in the design of methods for herbicides

determination in foods 114-116, such as the triazine herbicides. Zhang et al. 115 developed a

method for extraction and separation of triazine herbicides from soybeans using an IL foam-

based solvent flotation step using 1-hexyl-3-methylimidazole hexafluorophosphate

[C6mim][PF6] as a pretreatment for HPLC determination. The authors suggested that the

method could be successfully applied, since it showed satisfactory recovery values (higher

than 84.5% w/w) and accuracy (closed to 5.1%).

Preservatives are food additives used in pre-specified amounts, according to

food legislation, in order to increase the shelf-life of foods through the inhibition of the

growth of pathogens or spoilage microorganisms. Thus, the knowledge of their levels in

foods is of great importance for food-safety, either in case of growth of pathogens or for

human health assurance due to their high toxicity when above of the maximum limits. Sun et

al. 20 reported a method for determination and quantification of eight preservatives, such as

sorbic and benzoic acids, commonly present in soft drinks, fruit juices and vitamin

supplements. The technique employed capillary electrophoresis using 1-butyl-2,3-

dimethylimidazolium chloride ([C4C1mim]Cl) or other similar ILs as an electrolyte additive.

35

According to the authors, the proposed method was satisfactory with a higher recovery in

comparison with traditional capillary electrophoresis techniques.

Adulteration is another interesting focus for food analysis. It is by definition an

illegal practice, consisting in a modification of food products in which a substance is

intentionally added for reducing cost. In the same way, food consumers and suppliers rely on

efficient analysis for adulteration detection, which is the case of adulteration of extra-virgin

olive oil with cheaper vegetable oils that requests highly sensitive analytical techniques 106.

Calvano et al. 106 developed a method for detection of extra-virgin oil adulteration with

hazelnut oil through extraction using tributylammonium -cyano-4-hydroxycinnamate

[TBA][CHCA], followed by mass spectrometry. The proposed method was based on the

determination of phospholipids. The authors reported an efficient detection from 1% of

contamination, and showed that [TBA][CHCA] was an excellent solvent for phospholipids

extraction from vegetable oils and a suitable matrix for mass spectroscopy.

Besides prohibited additives, vitamins 99, such as folic acids 100 and other

biocompounds are ingredients or additives which demand analytical methods for

quantification, eventually using ILs. Zhu et al. 104 reported a novel method for determination

of two spicy biocompounds, ethyl vanillin and ethyl maltol, in biscuit, chocolate and milk

powder using an aqueous solution of 1-octyl-3-methylimmidazolium chloride ([C8mim]Cl) to

extract these compounds from the food matrices followed by ion chromatography. The

authors reported that the method was simple in terms of implementation and used small

amounts of samples and ILs for an efficient recovery of spicy vanillins (79.8% and 95.8%

w/w).

Biofuel production from vegetable oils and biomass. ILs have also an important

role in the development of more green efficient methods for biodiesel production from

vegetable oils and biethanol from sugar cane. Conventionally, biodiesel, i.e. fatty esters, are

produced by the transesterification of triacylglycerols, the major component of vegetable

oils, by using methanol or ethanol and catalysts, such as NaOH and H2SO4 138, 139. However,

the use of some catalysts may have drawbacks 140, such as formation of soaps in the case of

basic catalysts, when free fatty acids are present in excess in vegetable oils, or corrosion and

environmental pollution in case of acid catalysts. In this context, due to their catalytic

properties, easy preparation, high solubility and variable acidity, the use of ILs as catalysts

for biodiesel production has revealed interesting results 124, 125, 141. Guo et al. 125 found high

36

catalytic activity (93% esterification rate for oleic acid) for 1-butyl-3-methylimidazolium

tosylate ([C4mim][CH3SO3]) in biodiesel production from Jatropha oil. Guo et al. 64 used 3-

(N,N,N-triethylamino)-1-propanesulfonic hydrogen sulfate as a catalyst and also found high

values of conversion (93.2%) for biodiesel production from soybean oil.

ILs could also be used as a solvent during bioethanol production, either for

distillation or fermentation. The bioethanol industry presents a promising and sustainable

market appeal as one of the most valuable alternatives to fossil fuels, being successfully

employed in some countries, such as in Brazil. Therefore, optimization of its production

chain is always being demanded in order to reduce costs and environmental impacts. Several

works in literature reported that some ILs, such as 1,3-dimethylimidazolium

tetrafluoroborate ([C1mim][BF4]) 142, 3-methyl-1-octylimidazolium tetrafluoroborate

([C8mim][BF4]) 126, and 3-methylimidazolium tetrafluoroborate ([mim][BF4])

128 could be

employed in the replacement of pollutant and hazardous conventional solvents for breaking

the water+ethanol azeotropy during distillation of biethanol.

ILs could also be used as solvents in the pretreatment of sugarcane that is

performed before the production of second generation bioethanol. Such a pretreatment

consists in the obtainment of simple sugars from cellulose, such as mono- and disaccharides,

and highly volatile and toxic organic solvents are commonly employed. Zhu et al. 132

evaluated the performance of choline based ILs and reported that the advantages in

employing ILs in the pretreatment of biomass are the use of low temperatures, the increase

of enzymatic activity for the digestibility of lignocellulosic biomass, and reuse of the solvent

for sequential pretreatments. In comparison to other green solvents for the pretreatment of

biomass, such as supercritical CO2, the processes using ILs do not require high pressures,

which reduces the manufacturing costs 143.

37

4. PROSPECTS

Prospects of ILs application in the food industry, as well as their current, are

showed in Figure 1.

Figure 1 Prospects of ILs applications and their current use in the food industry

Surfactants and lubricants. Several research studies about the micelle-forming

ability (amphiphilic characteristic) of ILs were published from 2004 on the investigation

about the use of ILs for modification of the properties of surfactants 144 being a field still

little explored. ILs based surfactants can present low values of critical micellar concentration

(CMC), which makes them more efficient for interfacial tension decreasing. Also, their ionic

profile can interfere in the balance between electrostatic and hydrophobic interactions,

leading to important changes in the micellar structure 145.

Due to their characteristics as surfactant, ILs can act as emulsifiers in food

processing and food products formulation. Emulsifiers are additives widely used to provide

physicochemical characteristics of interest and to improve texture, stability, softness,

aeration and shelf-life of foods. In this context, ILs could be used, for example, as modifiers

of oil and fat crystallization, influencing the stability of food products significantly. Also, ILs

38

could improve the thermodynamic stability of emulsions due to their electrostatic

properties, avoiding crystallization of oil droplets and influencing the coalescence process of

fat globules.

Besides their properties as surfactants, some ILs, such as imidazolium-based ILs

82, have friction reducing ability with high thermal stability. Thus, several ILs, such as 1-ethyl-

3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][TFSI]) and 1-butyl-3-

methylimidazolium iodide ([C4mim]I) have been studied as low-toxic lubricants for

substitution of conventional lubricants 28, especially those with low biodegradability, such as

petroleum based ones. This is quite interesting, taking into account food processes. In this

context, ILs could act in the replacement of conventional lubricants in sanitary plants

accessories, valve or pumps, being more efficient on friction reduction and/or leakage

retention due to the formation of protective films among surfaces.

In many cases, surfactant and lubricant properties are related to the formation of

liquid crystalline structures. When submitted to a determined shear stress/strain value, the

interactions and orientation of the molecules can be changed, influencing their rheological

characteristics. Thus, ILs, such as imidazolium-based ILs 82 or ammonium based ILs 38 can

reveal a complex rheological behavior, characterized by non-Newtonian flows. Although

there are many studies available in the literature about the physical properties of ILCs 49, 77,

79, 146, their rheological behavior is far from being fully evaluated, being thus essential to

determine their application in industrial processes.

Solubility of bioactive compounds. Another interesting application of ILs in the

food industry is the modification of the solubility of biocompounds, either in the case of

organic compounds with low solubility in aqueous foods, such as dyes, flavors and essential

oils or in the replacement of volatile organic solvents, in a similar manner as they are used

during the synthesis of pharmaceutical active products 35. Essential oils, for example, are

widely used in the formulation of flavoring and antimicrobial chemicals for food and

pharmaceutical products, due to the bioactive compounds present in their composition. This

is the case of limonene present in the orange oil and that has been gaining attention for the

production of cleansers, cosmetics or beverages 89. However, their use as an additive for

aqueous systems presents some technological drawbacks due to their low dispersion ability.

In this way, amphiphilic ILs could play an important role for enhancing the solubility of

essential oils in foods, or even for the design of more stable flavoring additives.

39

Treatment of food industry waste. The use of ILs in the treatment of food

industry waste is another possible application that is still little evaluated. The efficient

management of the waste produced by food processing is still a challenge for industries. The

reduction of the production costs with the reuse of waste has encouraged the development

of processes to recover bioactive compounds. This is the case of a process using 1-(2-

cyanoethyl)-3-methylimidazolium bromide ([CyanoC2mim]Br) and 1-propyl-3-

methylimidazolium bromide ([C3mim]Br) in the recovery of fats from discarded foods as raw

material for biodiesel production described by Lateef et al. 98. Treatment of fruit peels

generated in large scale in the production of juices and fruit pulps is another example. As an

important source of high value-added substances, as mentioned before, Bica et al. 89

explored the dissolution of such a biomass using 1-ethyl-3-methylimidazolium acetate

([C2mim][CH3COO]) for further extraction of biocompounds. This could be successfully

applied in case of several kinds of biomass produced by food industries.

Other target in such a field is the environmental responsibility. This is the case of

the recovering of heavy metals, antibiotics or herbicides from waste-water, as common

residues from industrial processes. The ILs applicability in such a field is quite evident, taking

into account their efficiency for identification of such compounds in food analysis 22, 101, 102,

109-111, 114-116 and their potential use, in a similar way, for the characterization of those

residues.

Antimicrobial and enzymatic activity. In the food industry field, there are

desirable (those of interest and used in food production) and undesirable microorganisms.

This is the case of pathogens, responsible for food-borne diseases or spoilage agents, which

cause changes in the physicochemical and sensorial characteristics. The antimicrobial

properties of some ILs were studied to inhibit or decrease the growth rate of pathogens and

spoilage microorganisms in foods, ensuring high quality and safe products production. Some

imidazolium- and pyridinium-based ILs, depending on their alkyl-chain length, presented

efficient antimicrobial activity for gram-negative and -positive bacteria as well as for fungi

147. According to the authors, these ILs are similar or better than cetyltrimethylammonium

chloride (CTAC), commonly used as antimicrobial agent against gram-positive

microorganisms. Pernak et al. 148 studied the antimicrobial activity of ILs with 3-

alkoxymethyl-1-methylimidazolium as cation, against cocci, bacilli and fungi. The authors

also showed that the increase of the alkyl-length of the cation increased their antimicrobial

40

activity. Being an unexplored field, more efficient sanitizers could be formulated using ILs

with good antimicrobial properties.

The increasing of the kinetic of enzymatic reactions, as well as the enzymatic

stability is another target of few studies in the literature involving ILs, but with promising

and relevant results. Process involving enzymes generally lead to high-cost production and

purification steps. Because of this, ILs surfactant ability have been evaluated for enhancing

enzymes activity and stability 149-153 while avoiding industry waste production due to the

possibility of recycling such catalysts 154. Ventura et al. 149, for example, developed an

aqueous solution formed by 1-decyl-3-methylimidazolium chloride ([C10mim]Cl), whose self-

aggregation ability were used to form microemulsions in order to increase the activity of

Candida antarctica lipase B enzyme. This work highlights the development of micellar

systems composed of ILs as promoters of enzymatic superactivity, taking into account the

wide numbers of ILs with surfactant character, especially those with long alkyl chain length.

Another example is the production of esters with flavor profile, and so used in the

production of food flavorings 12, 155-157. Flavor esters, as is well known, can be synthesized by

fermentative processes. This is the case of ethyl acetate, naturally formed during beer

manufacturing, and one of the responsible of its sensorial quality. However, their synthesis

could be also induced. Gubicza et al. 156 reported the good performance of 1-butyl-3-

methylimidazolium hexafluorophosphate [C4mim][PF6] as a reaction media of an enzyme

catalyzed esterification of acetic acid in the presence of ethanol. In this case, IL could avoid a

significant drawback of this process, in which the enzymatic activity could be impaired in

acidic media. Lozano et al. 12 reported a method for synthesis of geranyl acetate and neryl

propionate by using N,N,N,N-hexadecyltrimethylammonium

bis(trifluoromethylsulfonyl)imide [C16tma][NTf2] as reaction media. This process promoted

the production of pure products, and the possibility of recycling the ionic biocatalyst. The

method is based on a switchable solid-phase transition in which flavor esters are separated

from the biocatalyst by centrifugation and cooling below IL melting temperature. Taking into

account this results, the biocatalysis using ILs is still an unexplored field in order to build

more efficient methods and synthesis of specific flavor esters for the food industry.

41

5. POSSIBLE EDIBLE-ILS FROM NATURAL SOURCES

Due to the search of renewable additives, the demand for natural compounds for

synthesis of ionic liquids is becoming a trend. Although some works revealed toxicity level

for several ILs, authors have proven that there is a decrease of toxicity in some cases.

Researches on toxicity indicate that the toxicity of ILs depends directly on their structure, i.e.

their cation-anion profile 158, 159. The tunability of ILs allows the combination of a wide

plethora of anions and cations. It is believed that is possible to synthesize at least one trillion

ILs with different anions and cations known up to date 35, 160. Different cation-anion

structures promote different physical properties, as well as different toxicity levels. In this

context, the search of anions and cations from natural sources for the synthesis of ILs had

revealed an interesting way to produce ILs with lower toxicity, possibly overcoming the

drawback of using them for food processes. Table 2 shows anions and/or cations obtained

from natural sources used up to date for synthesis of such new generation ILs. They mainly

include amino-acids, carboxylic acids and choline derivatives, compounds that show both

lower environmental impacts and toxicity.

Table 2 Anions and/or ca

Ions Group

Anions

Fatty acids

Amino acids

Sweetener

Other acids

nions and/or cations obtained from natural sources

Source Precursor

Vegetable oil

Oleic acid

Propionic acid

Fruits

Glycine

Lysine

Anthranilic acid Saccharin

Camphor laurel (Cinnamomum camphora)

Camphorsulfonic acid

obtained from natural sources for synthesis of ionic liquids

Structure

Propionic acid

Camphorsulfonic

ionic liquids

Ionic liquid

diethanolammonium oleate[DEA][C18OOH]

2-hydroxydiethanolaminepropionate (2-HDEAPr)

1-ethyl-3-methylimidazoliumglycinate [C2mim][Gly]

cholinium lysine [Ch][Lys]

didecyldimethylammonium saccharinate [DDA][Sac]

alkylammonium salt of Dcamphorsulfonic acids D

42

References

diethanolammonium oleate 38, 161

hydroxydiethanolamine HDEAPr)

41

methylimidazolium mim][Gly]

84

lysine [Ch][Lys] 37

didecyldimethylammonium saccharinate [DDA][Sac]

162

alkylammonium salt of D-camphorsulfonic acids D-CnACS

39

43

Table 2 Continued

Ions Group Source Precursor Structure Ionic liquid References

Cations

Natural amine

Soybeans, eggs and peanuts

Choline

cholinium serine [Ch][Ser] 131

cholinium proline [Ch][Pro] 163

choline cyclopentanecarboxylate [Ch][CPC]

164

Choline chloride [Ch]Cl 165

cholinium alanine [Ch][Ala] 166

Mandelic acid

Bitter almonds Mandelic acid derivatives

methyl 2-(3,4-methylenedioxyphenyl)-2-pyridinium acetate, bromide salt

40

44

Amino acids are one of the classes of natural biocompounds recently used for

the synthesis of low-toxicity ILs. Glycine, for example, also known as amino acetic acid, is

mainly used as nutritional supplement in food additives and one of the amino acids most

found in some large-scale raw material, such as sugarcane and fruits. Amino acids based

ionic liquids have been recently synthesized due to their physical properties for extraction

processes 84, sweetish taste, and natural origin (Figure 2). The extractive ability of these ILs

with amino acids as the anion have been related to strong hydrogen-bonding interactions

with acidic compounds, making them good solvents for phenolic compounds extraction 84,

for example.

Figure 2 Ions obtained from natural sources and used for ILs synthesis

Choline is another biocompound widely evaluated for the synthesis of those low-

toxic ILs. Choline, also known as belonging to the B-complex vitamins, can be found in

several foods, such as eggs and peanuts, as well as soybeans (Figure 2), and is considered an

essential and non-toxic nutrient recommended on human diet for normal human health

assurance 167. ILs synthesized with choline and amino acids have been produced for biomass

pretreatment in order to replace toxic ILs, such as those containing imidazolium in their

structure 36, 37, 163, 166. Choline has been also used in the development of sweeteners, due to

45

their ability to increase the sweetening power 168. Nockemann et al. 169 synthesized and

studied the toxicity of choline saccharinate and choline acesulfamate toward crustaceans

and found low ecotoxicity, relating it to the natural origin of the ions. Frade et al. 170 also

found non-toxicity of choline saccharin and choline acesulfame towards human colon

carcinoma HT-29 cell lines.

Fatty acids have been used for the synthesis of ILs. During the refining of

vegetable oils, such as palm, soybean and sunflower oil, a significant amount of fatty acids

are generated as byproducts (Figure 2). Other natural compounds, such as fatty acid esters

and phospholipids, may also be produced from such a source and be used for the production

of surfactants, cosmetics and pharmaceutical products 171. Recently, renewable fatty acids

based PILs have been produced by some authors 38, 161. According to the authors, the

renewable aspect, easy preparation and low-cost reactants, makes them interesting ILs with

a wide range of industrial applications and low environmental impact. The interest of fatty

acids is evident, due to the increasing worldwide large scale production of vegetable oils.

Also, their physical and functional properties seem to be an important aspect to consider

when evaluating these PILs as relevant products. Peric et al. 41 studied the eco-toxicity and

biodegradability of PILs containing organic acids in their structures with aquatic toxicity tests

and concluded that they exhibit no toxicity compared with toxic AILs.

Toxicity. In recent years, toxicological studies have shown that there are a

number of ILs with certain level of toxicity 172-176, and as the interest in application of ILs

increase, the number of studies on the toxicity of ILs grows steadily. Although most of these

ILs have been used in processes not involving foods, toxicity is a concerning of researchers.

At the beginning, authors believed that ILs properties, especially those related to their low

volatility and low flammability, could be enough to consider them as environmentally

friendly. However, for such a claim, different toxicological studies started to be required.

The toxicological aspect of ILs can be understood in many perspectives as there

are different types of analysis on toxicity assessments. According to Zhao et al. 33 the main

toxicological analysis employed are based on aquatic ecosystems, microorganisms, animal

tests, cytotoxicology and enzyme inhibition. According to these different techniques, the

effect of the alkyl chain length on the toxicity is evident, as well as the effect of the type of

ion used to design ILs 177. For example, the work of Patern et al. 173 shows that, the ILs

ecotoxicity increases as consequence of the alkyl chain length of the hydrophobic ion. This

46

shows that the relationship between the structure, nature of the ions, and their toxicity is an

important subject, taking into account the well-claimed tunability of ILs properties, which

means that, ILs toxicological aspect can be reduced using proper chosen ions.

In order to investigate the toxicological effects of ILs on the aquatic environment

researchers have used organisms, such as the Vibrio fischeri bacteria 178, species of green

algae, such as Chlorella vulgaris and Pseudokirchneriella subcapitata 179-181, crustaceans 159,

172, 182 and aquatic plants, such as Lemna minor 173. In general, according to Stolte et al. 178,

the toxicological effects in aquatic organisms are more dependent on cation changes. Among

ILs, those with methylimidazolium cation have been widely studied. According to some

authors, the alkyl chain length of such ILs is a relevant factor for their toxicity levels 177, 180.

On the other hand, methylimidazolium and pyridinium ILs could present lower toxicity when

synthesized with substituted mandelic acid derivatives 40, naturally found in almonds

extracts (Figure 2).

The use of microorganisms for the study of ILs toxicity is mainly related to their

role on biodegradation and waste recycling. The microorganisms commonly employed for

this technique are bacteria, due to their short generation time 33. The bioluminescence

technique using the bacteria Vibrio fischeri is well-known and used due to its time- and cost-

effectiveness 40, 176, 181. Similarly, the inhibitory effect of ILs on the growth of microorganisms

of interest had been evaluated, such as in the case of Staphylococcus aureus and Escherichia

coli 148, some common pathogens of food-borne diseases.

Cytotoxicity, i.e. toxicity studies towards human and animal cells, is another

important technique for the evaluation of the potential risk of ILs in human health and

environment. Cytotoxicity of ILs have been performed using fish cell line 183, 184, rat cell line

174, 185-187 and human cell line 159, 170, 188-191. Nevertheless, there are still few studies using

human cell lines and most of them evaluating those ILs with imidazolium cation 184, 188.

Among some of these works, Chen et al. 192 evaluated the cytotoxicity of imidazolium based

ILs in human lung carcinoma A549 cell line and concluded that the toxicity effect depends on

the anion-cation association, as well as on the alkyl chain length of the structure.

On the other hand, Stepnowski et al. 190 compared the ILs toxicity towards

human cell line HeLa with organic solvents, which is quite interesting taking into account

their use in replacement of these conventional compounds. They found that 1-butyl-3-

methylimidazolium chloride ([C4mim]Cl), 1-butyl-3-methylimidazolium hexafluorophosphate

47

([C4mim][PF6]) and 1-hexyl-3-methylimidazolium tetrafluoroborate ([C6mim][BF4]) are less

toxic toward those human cell lines, in terms of concentration, than some classical solvents,

such as dichloromethane, phenol and xylene.

Nevertheless, toxicity studies of ILs to mammals are still scarce in the literature

162. Landry et al. 34 studied the oral toxicity of 1-butyl-3-methylimidazolium chloride

[C4mim][Cl] and concluded that its toxicological response in Fischer 344 rats depended on

concentration. Xu et al. 136 evaluated the oral toxicity of 1-alkyl-3-methylimidazolium

tetrafluoborate [Cnmim][BF4] toward mice and found medium levels of toxicity for

[C10mim][BF4], [C12mim][BF4], [C14mim][BF4] and [C18mim][BF4]. Over again, the main

conclusion took into account the alkyl chain length effect on the toxicological response.

Jodynis-Liebert et al. 162 studied rats oral exposure and cytotoxicity of

didecyldimethylammonium saccharinate, an IL based on a non-toxic anion. The authors

concluded that such a compound presented no toxic effect on warm-blooded organisms.

This is quite noteworthy, considering that this IL is an active agent for cosmetics, toothpaste

and antiseptics manufacturing.

A more complete toxicological assessment of ILs impact on human health and

environment is also found in literature, including all the perspectives described before.

Gouveia et al. 159 studied the toxicity of a group of ILs based on imidazolium, pyridinium and

choline as cations combined with amino acids as anions, towards crustacean Artemia salina,

HeLa cells (from cervical carcinoma), Bacillus subtilis and Escherichia coli. The authors

showed that the use of choline as cation, such as in cholinium methionine [Ch][Met], instead

of methylimidazolium cation, lead to a drastic decreasing on toxicity toward Artemia salina

and HeLa cells. In this way, the authors highlighted the use of choline and amino acids as

natural biocompounds, due to their possible lower toxicity on human health and

environment.

Considering that the toxicity of ILs depends on the model of the toxicological

assay and the nature and structure of the ions involved, there is still a l