• No results found

Engineering Aspects of Milk and Dairy Products

N/A
N/A
Protected

Academic year: 2021

Share "Engineering Aspects of Milk and Dairy Products"

Copied!
288
0
0

Loading.... (view fulltext now)

Full text

(1)
(2)

Engineering

Aspects of

Milk and

(3)

Contemporary Food Engineering

Series Editor

Professor Da-Wen Sun, Director

Food Refrigeration & Computerized Food Technology National University of Ireland, Dublin

(University College Dublin) Dublin, Ireland http://www.ucd.ie/sun/

Engineering Aspects of Milk and Dairy Products, edited by Jane Sélia dos Reis Coimbra and José A. Teixeira (2009)

Processing Effects on Safety and Quality of Foods, edited by Enrique Ortega-Rivas (2009) Engineering Aspects of Thermal Food Processing, edited by Ricardo Simpson (2009) Ultraviolet Light in Food Technology: Principles and Applications, Tatiana N. Koutchma,

Larry J. Forney, and Carmen I. Moraru (2009)

Advances in Deep-Fat Frying of Foods, edited by Serpil Sahin and Servet Gülüm Sumnu (2009)

Extracting Bioactive Compounds for Food Products: Theory and Applications, edited by M. Angela A. Meireles (2009)

Advances in Food Dehydration, edited by Cristina Ratti (2009) Optimization in Food Engineering, edited by Ferruh Erdoˇgdu (2009) Optical Monitoring of Fresh and Processed Agricultural Crops, edited by

Manuela Zude (2009)

Food Engineering Aspects of Baking Sweet Goods, edited by Servet Gülüm Sumnu and Serpil Sahin (2008)

(4)

CRC Press is an imprint of the

Taylor & Francis Group, an informa business Boca Raton London New York

Engineering

Aspects of

Milk and

Dairy Products

Edited by

Jane Sélia dos Reis Coimbra

José A. Teixeira

(5)

CRC Press

Taylor & Francis Group

6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742

© 2010 by Taylor and Francis Group, LLC

CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works

Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1

International Standard Book Number: 978-1-4200-9022-2 (Hardback)

This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint.

Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmit-ted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter inventransmit-ted, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers.

For permission to photocopy or use material electronically from this work, please access www.copyright. com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.

Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used

only for identification and explanation without intent to infringe.

Library of Congress Cataloging-in-Publication Data

Engineering aspects of milk and dairy products / editors, Jane Selia dos Reis Coimbra, Jose A. Teixeira.

p. cm. -- (Contemporary food engineering) Includes bibliographical references and index. ISBN 978-1-4200-9022-2 (hardcover : alk. paper)

1. Dairy processing. 2. Milk. 3. Dairy products. I. Coimbra, Jane Selia dos Reis, 1962- II. Teixeira, José A. (José António),

1957-SF250.5.E54 2010

637--dc22 2009032737

Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

(6)

Dedication

(7)
(8)

vii

Contents

Series Editor’s Preface ...ix

Preface...xi

Series Editor ... xiii

The Editors ...xv

Acknowledgment ...xvii

Contributors ...xix

1 Chapter Physical Chemistry of Colloidal Systems Applied to Food Engineering ...1

Ana Clarissa dos Santos Pires, Maria do Carmo Hespanhol da Silva, and Luis Henrique Mendes da Silva* 2 Chapter Bioseparation Processes ...27

Jane Sélia dos Reis Coimbra and José Teixeira 3 Chapter Applications of Membrane Technologies in the Dairy Industry ... 33

Antonio Fernandes de Carvalho* and J.-L. Maubois 4 Chapter Aqueous Two-Phase Systems Applied to Whey Protein Separation ... 57

Abraham Damian Giraldo Zuniga, Jane Sélia dos Reis Coimbra,* José Teixeira, and Lígia Rodrigues 5 Chapter Techniques Applied to Chromatographic Product Manufacturing ... 81

Rafael da Costa Ilhéu Fontan,* António Augusto Vicente, Renata Cristina Ferreira Bonomo, and Jane Sélia dos Reis Coimbra 6 Chapter Crystallization of Lactose and Whey Protein ... 121

Everson Alves Miranda,* André Bernardo, Gisele Atsuko Medeiros Hirata, and Marco Giulietti 7 Chapter Novel Technologies for Milk Processing ... 155

(9)

viii Contents

8

Chapter Active and Intelligent Packaging for Milk and Milk Products ... 175 Nilda de Fátima Ferreira Soares,* Cleuber Antônio de Sá Silva,

Paula Santiago-Silva, Paula Judith Pérez Espitia, Maria Paula Junqueira Conceição Gonçalves, Maria José Galotto Lopez, Joseph Miltz, Miguel Ângelo Cerqueira, António Augusto Vicente, José Teixeira, Washington Azevedo da Silva, and Diego Alvarenga Botrel

9

Chapter Microcalorimetry: A Food Science and Engineering Approach ... 201 Ana Clarissa dos Santos Pires, Maria do Carmo Hespanhol da Silva, and Luis Henrique Mendes da Silva*

10.

Chapter Potential Applications of Whey Proteins in the Medical Field ... 221

Lígia Rodrigues* and José António Couto Teixeira

(10)

ix

Series Editor’s Preface

CONTEMPORARY FOOD ENGINEERING

Food engineering is the multidisciplinary field of applied physical sciences combined with the knowledge of product properties. Food engineers provide the technological knowledge transfer essential to the cost-effective production and commercialization of food products and services. In particular, food engineers develop and design pro-cesses and equipment in order to convert raw agricultural materials and ingredi-ents into safe, convenient, and nutritious consumer food products. However, food engineering topics are continuously undergoing changes to meet diverse consumer demands, and the subject is being rapidly developed to reflect market needs.

In the development of food engineering, one of the many challenges is to employ modern tools and knowledge, such as computational materials science and nano-technology, to develop new products and processes. Simultaneously, improving food quality, safety, and security remain critical issues in food engineering study. New packaging materials and techniques are being developed to provide more protection to foods, and novel preservation technologies are emerging to enhance food security and defense. Additionally, process control and automation regularly appear among the top priorities identified in food engineering. Advanced monitoring and control systems are developed to facilitate automation and flexible food manufacturing. Furthermore, energy saving and minimization of environmental problems continue to be an important food engineering issue and significant progress is being made in waste management, efficient utilization of energy, and reduction of effluents and emissions in food production.

The Contemporary Food Engineering series, consisting of edited books, attempts to address some of the recent developments in food engineering. Advances in classi-cal unit operations in engineering applied to food manufacturing are covered as well as such topics as progress in the transport and storage of liquid and solid foods; heat-ing, chillheat-ing, and freezing of foods; mass transfer in foods; chemical and biochemi-cal aspects of food engineering and the use of kinetic analysis; dehydration, thermal processing, nonthermal processing, extrusion, liquid food concentration, membrane processes, and applications of membranes in food processing; shelf-life, electronic indicators in inventory management, and sustainable technologies in food process-ing; and packaging, cleaning, and sanitation. The books are aimed at professional food scientists, academics researching food engineering problems, and graduate level students.

The books’ editors are leading engineers and scientists from many parts of the world. All the editors were asked to present their books to address the market need and pinpoint the cutting-edge technologies in food engineering.

(11)

Furthermore, all contributions are written by internationally renowned experts who have both academic and professional credentials. All authors have attempted to provide critical, comprehensive, and readily accessible information on the art and science of a relevant topic in each chapter, with reference lists for further informa-tion. Therefore, each book can serve as an essential reference source to students and researchers in universities and research institutions.

Da-Wen Sun Series Editor

(12)

xi

Preface

Nowadays, it is impossible to imagine a diet not incorporating dairy products. The dairy industry has been able to meet consumer needs by offering a wide range of products that go from the traditional milk to the new and high-value-added products. In addition to the products that consumers traditionally associate with milk, such as cheese, butter, and yogurts, several products contain milk as a source of nutrients with important and unique properties. This reinforces the importance of milk as a raw material in the food industry, and consequently, the relevance of several processing technologies used for milk transformation. The complex nature of this unique material as well as its biological properties are a major challenge for process engineers. The development of new dairy products and the improvement of their safety are due to the developments of food technology which have been able to reply successfully to the challenges of consumers and the industry. Separation processes also play a major role in the processing of milk products, going from the “conventional” defatting to the purification of active proteins, passing by the crys-tallization of lactose. More recently, evidence of therapeutic properties of several milk proteins available in small amounts reinforced the importance of the applica-tion of advanced separaapplica-tion processes in the dairy industry. This book focuses on engineering aspects of food manufacture using the integration of concepts, unit operations, and physical chemistry. Aspects of packaging are also presented. The processing of milk and milk-based products is used as a case study to illustrate what happens in the production chain and to present applications of the biosepara-tion process.

Jane Sélia dos Reis Coimbra José Teixeira

(13)
(14)

xiii

Series Editor

Born in Southern China, Professor Da-Wen Sun is a world authority in food engineering research and education. His main research activities include cooling, drying and refrig-eration processes and systems, quality and safety of food products, bioprocess simula-tion and optimizasimula-tion, and computer vision technology. Especially, his innovative studies on vacuum cooling of cooked meats, pizza quality inspection by computer vision, and edible films for shelf-life extension of fruit and vegetables have been widely reported in

national and international media. Results of his work have been published in over 200 peer-reviewed journal papers and more than 200 conference papers.

He received a first class BSc Honours and MSc in mechanical engineering and a PhD in chemical engineering in China before working in various universities in Europe. He became the first Chinese national to be permanently employed at an Irish University when he was appointed college lecturer at National University of Ireland, Dublin (University College Dublin) in 1995, and was then continuously promoted in the shortest possible time to senior lecturer, associate professor, and full profes-sor. Dr. Sun is now Professor of Food and Biosystems Engineering and director of the Food Refrigeration and Computerised Food Technology Research Group at University College Dublin.

As a leading educator in food engineering, Professor Sun has significantly con-tributed to the field of food engineering. He has trained many PhD students, who have made their own contributions to the industry and academia. He has also given lectures on advances in food engineering on a regular basis in academic institu-tions internationally and delivered keynote speeches at international conferences. As a recognized authority in food engineering, he has been conferred adjunct/ visiting/consulting professorships from ten top universities in China including Zhejiang University, Shanghai Jiaotong University, Harbin Institute of Technology, China Agricultural University, South China University of Technology, and Jiangnan University. In recognition of his significant contribution to food engineering world-wide and for his outstanding leadership in the field, the International Commission of Agricultural Engineering (CIGR) awarded him the CIGR Merit Award in 2000 and again in 2006, the Institution of Mechanical Engineers (IMechE), based in the United Kingdom, named him Food Engineer of the Year 2004; in 2008 he was awarded CIGR Recognition Award in honor of his distinguished achievements as the top one percent of agricultural engineering scientists in the world.

He is a Fellow of the Institution of Agricultural Engineers and a Fellow of Engineers Ireland. He has also received numerous awards for teaching and research

(15)

xiv Series Editor

excellence, including the President’s Research Fellowship, and has twice received the President’s Research Award of University College Dublin. He is a member of the CIGR executive board and honorary vice-president of CIGR, editor-in-chief of Food and Bioprocess Technology—An International Journal (Springer), series editor of the Contemporary Food Engineering book series (CRC Press/Taylor & Francis), for-mer editor of Journal of Food Engineering (Elsevier), and editorial board member of Journal of Food Engineering (Elsevier), Journal of Food Process Engineering (Blackwell), Sensing and Instrumentation for Food Quality and Safety (Springer), and Czech Journal of Food Sciences. He is also a registered chartered engineer.

(16)

xv

The Editors

Jane Sélia dos Reis Coimbra is an associate professor who teaches unit operations at undergraduate and graduate levels at the Food Technology Department, Federal University of Viçosa, Brazil. She earned her B.S. in chemical engineering at Federal University of Minas Gerais, Brazil, and her D.Sc. degree in food engineering at State University of Campinas, São Paulo, Brazil, and at Heinrich-Heine Universität, Düsseldorf, Germany. Dr. Coimbra earned her postdoctoral degree in nanotechnol-ogy at the University of Minho, Portugal, and in protein adsorption at the State University of Campinas, Brazil. Coimbra’s research interests are focused on unit operations, bioseparation, and the design of nanostructures to food applications. José Teixeira is a professor at the Biological Engineering Department, Universidade do Minho, Portugal. He graduated in chemical engineering at Porto University, where he also earned his Ph.D. in 1988. His research interests include nonconven-tional food processes, advanced bioreactors for food and biotechnology applications, bioreactor hydrodynamics, and medical applications of dairy proteins. Dr. Teixeira supervised 15 Ph.D. theses and several postdoctoral researchers, was the coordinator of 21 research projects, four of them international, and is the editor of two books and author/co-author of 200 peer-reviewed papers. He also has an extensive cooperation with the Portuguese food industry.

(17)
(18)

xvii

Acknowledgment

(19)
(20)

xix

Contributors

André Bernardo

Georgia-Pacific Resinas Internacionais Jundai, São Paulo, Brazil

Renata Cristina Ferreira Bonomo Universidade Estadual do Sudoeste da

Bahia

Itapetinga, Bahia, Brazil Diego Alvarenga Botrel Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Antonio Fernandes de Carvalho Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Miguel Ângelo Parente Ribeiro Cerqueira

IBB (Institute for Biotechnology and Bioengineering)

Universidade do Minho Braga, Portugal

Jane Sélia dos Reis Coimbra Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Paula Judith Pérez Espitia Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Rafael da Costa Ilhéu Fontan Universidade Estadual do Sudoeste da

Bahia

Itapetinga, Bahia, Brazil

Marco Giulietti

Instituto de Pesquisas Tecnológicas do Estado de São Paulo

Universidade Federal de São Carlos São Paulo, Brazil

Maria Paula Junqueira Conceição Gonçalves

Universidad de Santiago de Chile (USACH)

Estación Central, Santiago, Chile Gisele Atsuko Medeiros Hirata Universidade Estadual de Campinas Campinas, São Paulo, Brazil Maria José Galotto Lopez Universidad de Santiago de Chile

(USACH)

Estación Central, Santiago, Chile Jean-Louis Maubois

Dairy Research Laboratory INRA (Institut National de la

Recherche Agronomique) Rennes, France

Joseph Miltz

The Goldstein Packaging Laboratory Haifa, Israel

Everson Alves Miranda

Universidade Estadual de Campinas Campinas, São Paulo, Brazil

(21)

xx Contributors

Ricardo Nuno Correia Pereira IBB (Institute for Biotechnology and

Bioengineering) Universidade do Minho Braga, Portugal Lígia Rodrigues

IBB (Institute for Biotechnology and Bioengineering)

Universidade do Minho Braga, Portugal

Ana Clarissa dos Santos Pires Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Cleuber Antônio de Sá Silva Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil

Maria do Carmo Hespanhol da Silva Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Luis Henrique Mendes da Silva Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil

Washington Azevedo da Silva Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Paula Santiago-Silva

Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil Nilda de Fátima Ferreira Soares Universidade Federal de Viçosa Viçosa, Minas Gerais, Brazil José Teixeira

IBB (Institute for Biotechnology and Bioengineering)

Universidade do Minho Braga, Portugal

António Augusto Martins de Oliveira Soares Vicente

IBB (Institute for Biotechnology and Bioengineering)

Universidade do Minho Braga, Portugal

Abraham Damian Giraldo Zuniga Universidade Federal do Tocantins Palmas, Tocantins, Brazil

(22)

1

1

Physical Chemistry of

Colloidal Systems Applied

to Food Engineering

Ana Clarissa dos Santos Pires, Maria

do Carmo Hespanhol da Silva, and

Luis Henrique Mendes da Silva

*

1.1 INTRODuCTION

Formal studies of interface and colloid science began in the early nineteenth century; however, humans observed and made use of such phenomena thousands of years ear-lier. For example, the preparation of inks and pigments, baked bread, butter, cheeses, glues, and other substances all represent interfacial and colloidal phenomena of great practical importance to ancient cultures (Myers, 1999).

The scientific approach of interfacial phenomena started in the second half of the eighteenth century. Later, in the nineteenth century, the first quantitative studies of the properties of monolayers of surface-active substances in liquid–air interfaces were realized (Norde, 2003).

Colloidal dispersions were first described by Selmi in 1845 as “pseudosolutions.” In 1861 the name colloids (from the Greek, meaning “glue”) was assigned to the

CONTENTs 1.1 Introduction ...1 1.2 General Concepts ...2 1.3 Capillarity ...6 1.4 Adsorption ...8 1.4.1 Monolayers ... 10 1.4.2 Factors Affecting Adsorption ... 14 1.5 Micellization ... 14 1.6 Stability of Colloidal Systems ... 17 1.7 Double Electrical Layer ... 19 1.8 Colloidal Systems in Food Engineering and Technology ... 21 1.9 Concluding Remarks ...22 Acknowledgments ...23 References ...23

(23)

2 Engineering Aspects of Milk and Dairy Products

particles in Selmi’s pseudosolution. By choosing this name, Graham intended to emphasize the low rate of diffusion indicating a particle size of, at least, a few nano-meters in diameter (Norde, 2003).

There is great interest in studying and understanding the colloidal systems. In addition, the presence of colloids in food either as ingredients or natural constituents, as well as their importance as cleaning agents increases the involvement of food engineering and technology researchers in this area.

In this chapter, an introduction to colloid science is presented, including basic concepts and definitions.

1.2 GENERAl CONCEPTs

A colloidal system can be defined as a heterogeneous system, wherein one phase is finely dispersed in another continuous phase, as can be observed in Figure 1.1. Because the dimensions of the dispersed phase are too small, colloidal systems show a large interfacial area (Norde, 2003; Vicent, 2005). It is important to emphasize that the colloidal state is not a physical state but is an aggregation state.

In many practical cases, the system can be more complex, presenting more than one dispersed phase, and each of the phases can be multicomponent. Table 1.1 lists some common examples of colloidal systems present in everyday life.

Traditionally, colloids are classified as suspension, emulsion, foam, sol, gel, and aerosol. Table 1.2 shows examples of each type of colloidal system.

Colloids are an important class of materials, intermediate between bulk and molecularly dispersed systems. The colloid particles may be present in spherical form, but sometimes, one dimension is larger than the other two, such as with a needle shape. Generally, the designation of colloid is applied to particles that are in the range 10–9 m < r < 10–6 m. Therefore, the colloid size cannot be determined by either the naked eye or optical microscope, with light scattering the main method used to investigate colloidal particles (Voets et al., 2008).

Phase β Phase α

FIGuRE 1.1 A colloidal dispersion, where a is the continuous phase and b is the dis-persed phase.

(24)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 3

A phase of a colloidal system can be defined as a region formed by volume ele-ments, dV, where the intensive thermodynamics properties are constants. In a system with more than one phase, there is a region where molecules of phase a go to phase b, and vice versa, interacting with each other. This boundary place is called the interface (Figure 1.2).

Interfaces are the boundaries between immiscible phases, wherein the intensive thermodynamic properties are intermediate between the properties of phases a and b. They can be formed between solid/liquid, solid/gas, liquid/gas, and liquid/liquid. The common thickness of an interface is around 3 to 4 × 10–10 m (three times more than the diameter of a molecule). Some authors call the interface of a surface when one of the immiscible phases is a gas or vacuum.

To define an interface physicochemically, it is necessary to think about energy and keep in mind that nature will always act to reach a condition of minimum free energy in a system. Therefore, if the presence of the interface increases the total free energy, this region will be spontaneously reduced; consequently, the two phases tend to separate (Myers, 1999). In an interface region, there is an excess of energy in relation to the two phases of the system, because in this area, the intermolecular interactions between a-phase molecules and b-phase molecules are unfavorable.

Interfaces are the region of excessive Gibbs energy, which occurs as a consequence of the unbalanced intermolecular interactions field between molecules of phases a

TAblE 1.1

Common Examples of Colloidal systems

Detergent Ice cream Wastewater Shampoo Butter Dust Aerosol spray Fruit juice Blood Cosmetic cream Milk Digestive fluid Mayonnaise Beer foam Smoke

TAblE 1.2

Classification of Colloidal Dispersions

Dispersed Phase

Continuous Phase solid liquid Gas

Solid Solid suspension (e.g., bone, wood)

Solid emulsion (e.g., pearl)

Solid foam (e.g., bread, loofah)

Liquid Sol, suspension (e.g., blood, ink)

Emulsion (e.g., milk, shampoo)

Foam (e.g., detergent foam, beer foam) Gas Aerosol (e.g., smoke,

dust)

(25)

4 Engineering Aspects of Milk and Dairy Products

and b. This excess of Gibbs free energy gives rise to various interfacial phenomena, such as interfacial tension (g), wetting, adsorption (Γ), and adhesion. The resulting interfacial properties govern the interactions between colloidal particles and there-with the macroscopic behavior and characteristics of a colloidal system, such as its rheological and optical properties and its stability against aggregation.

The interfacial tension, g, can be thermodynamically defined as the increment of Gibbs free energy when reversibly extending the interfacial area by one unit, at constant temperature, pressure, and composition of the system (Norde, 2003). To achieve this definition, some thermodynamics aspects must be considered. For a reversible change in a heterogeneous system, the energy change can be demonstrated as in Equation 1.1:

dG Vdp sdT= − +

µidnidA (1.1)

where G is the Gibbs energy of the system, T is the temperature (in K), S is the entropy, p is the pressure, V is the volume, m is the chemical potential of the compo-nent i, ni is the number of moles of i in the system, g is the interfacial tension, and A

is the interfacial area. The term TdS refers to the heat energy absorbed by the system from its surroundings, and the other terms are related to the work (mechanical and chemical) performed on the system (Norde, 2003).

In practice, p and T are constants, and the number of mols of components i between the phases does not vary. Therefore, the interfacial tension can be defined as shown in Equation 1.2:

γ = dGdAT P n, ,i

(1.2) A rigorous definition of g can be based on Figure 1.3 and Equations 1.3, 1.4, and 1.5. Consider the prism shown in Figure 1.3, which has edges perpendicular to the interface. This prism is formed by the phase a side, by the volume Va, and by the

Phase β Phase α

Interface

(26)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 5

region associated with phase b with Vb. A limit –d is defined, below which the

volu-metric density of Gibbs free energy at the interface, fint = (dG/dV)P,T, is equal to fa, being, fint ≠ fa, above –d. For regions below +d, fint ≠ fb, and above +d, fint = fb.

Because of the variation of density of free energy in the interface, in comparison with the values found in the phases, the free energy of the real system is bigger than the energy of the idealized system, where there were no interfaces—that is, Greal > Ga+ Gb = faVa + fbVb.

Therefore, it can be defined that Gint = Greal – (faVa + fbVb) = gS. With regard to the

continuous variation, in the z axis, the density of Gibbs free energy can be written as Equation 1.3: Gα+Gβ = f z dzα + f z dzβ         +∞ −∞

( ) ( ) 0 0 (1.3)

The Gibbs free energy of the surface can be expressed as Equation 1.4:

Greal− G +G = f zf z dz+ f zf −∞

( α β) ( int( ) α( )) ( int( ) β 0 (( ))z dz S S 0 +∞

        =γ (1.4) Considering that fint≠ fb and fint≠ fa only in the region between –d < z < + d, Equation

1.4 can be rewritten, changing the limits of integration (Equation 1.5):

Greal− G +G = f zf z dz+ f zf

( α β) ( int( ) α( )) ( int( ) δ β 0 (( ))z dz S S 0 +

        = δ γ (1.5) z y x Phase α fα Phase β fβ + – G´´ G´

FIGuRE 1.3 A binary system and its interface, where f is the energy density, and fa and fb

(27)

6 Engineering Aspects of Milk and Dairy Products

Dividing Equation 1.5 by the interface area gives the definition of g (Equation 1.6):

γ α δ β δ = − + − − +

(fint( )z f z dz( ))

(fint( )z f z dz( )) 0 0       (1.6) 1.3 CAPIllARITY

The term capillarity comes from the Latin “capillus” and describes the rise of liq-uids in fine glass tubes. Laplace showed that the rise of flliq-uids in a narrow capillary was related to the difference in pressure across the interface and the surface tension of the fluid (Birdi, 2003b).

There are a lot of phenomena where curved interfaces play an important role. Figure 1.4 illustrates a capillary rise and a capillary depression. The angle formed between the liquid and solid is called contact angle (q), which we will discuss in coming sections. The quantitative interpretation of the capillary events requires an introduction to capillary pressure—the pressure difference across a curved interface as a function of the interfacial tension (Myers, 1999).

Consider the formation of an air bubble in a liquid medium. To blow this bubble, some pressure should be applied. This excess pressure is called capillary pressure (Norde, 2003). To understand the relation between capillary pressure, interfacial ten-sion, and the size of the bubble, we will begin with a picture of a cross section of a bubble with radius (R) (Figure 1.5).

Any infinitesimal change in the bubble volume is described by Equation 1.7:

dV= 4πr dr2 (1.7)

and any change in the bubble area by Equation 1.8:

dA= 8πrdr (1.8)

(a) (b) (c)

θ

FIGuRE 1.4 Capillarity effects: (a) capillary rise, (b) capillary depression, and (c) contact angle (q) formed between the liquid and solid surface.

(28)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 7

There are two forces that control the bubble size. The first drives the bubble expan-sion (Equation 1.9) and another force is the contraction (Equation 1.10). At equilib-rium, both forces are equal, as demonstrated in Equation 1.11, describing the relation between capillary pressure and interfacial tension (Equation 1.12) (Adamson, 1990):

dw= − 4P πr dr2 (1.9) dG=γ π8 rdr (1.10) −Pr dr2 = −8πrdr (1.11) ∆P r =2γ (1.12)

Equation 1.12 enables formulation of the balances between interfacial forces (F1) and body forces (F2) (Norde, 2003), allowing the calculation of the height of a liquid in a capillary (Equations 1.13 through 1.16):

F r A 1 2 = γ (1.13) F2=mg (1.14) at equilibrium: 2γ ρ r A= ∆ Vg (1.15) Therefore, h r g = 2γ ρ ∆ (1.16) dR R

FIGuRE 1.5 Cross section of a bubble with radius R. dR corresponds to the change in the bubble radius. The bubble volume is V=4 r

(29)

8 Engineering Aspects of Milk and Dairy Products

1.4 ADsORPTION

Adsorption is particularly important in surface and colloid science, because it is one of the main ways in which high-energy interfaces can be altered to reduce the overall energy of a system (Myers, 1999).

Adsorption of molecules from solution on interfaces is important in controlling a variety of interfacial processes. Adsorption is a consequence of energetically favor-able interactions between the molecules at interface and the solute species and also of the interactions between the solute and the molecules solution, which reflects the chemical potential. Several interactions, such as electrostatic attraction, covalent bonding, hydrogen bonding, or nonpolar interactions between the adsorbate and the adsorbent species, as well as the lateral interaction between the adsorbed species, and their desolvation, can contribute to the adsorption process (Somasundaran et al., 2003).

The basic concepts behind the factors governing the adsorption of surface-active molecules at interfaces are often mentioned in terms of surface excess concentration of the adsorbed species, Γi, to the surface or interface of the system (Myers, 1999). Mathematically, Γi can be defined as Equation 1.17:

Γi i i i i i t i N N N S C z C z dz C z C =

(

− −

)

=

(

)

+ − −

α β α δ β ( ) ( ) ( ) 0 (( )z dz

(

)

        +

0 δ (1.17)

where Ni, Niα, and Niβ are the total amount of substance “i” in the system and in the

phases a and b, respectively.

Consider that the component “i” does not move spontaneously to phase b. Therefore, the second integration term of Equation 1.17 is equal to zero, and this equation can be written as Equation 1.18:

Γi=

(

C ziC z dzi

)

        −

( ) α( ) δ 0 (1.18)

Applying the Integral Mean Value Theorem to Equation 1.18, Equation 1.19 is obtained:

Γi=

(

Ciint−Ciα

)

δ (1.19)

If Ciintis much higher than Ciα, the amount of adsorbed material can be defined as

shown in Equation 1.20:

Γi=

( )

Ciint δ (1.20)

Equation 1.20 indicates that the amount of adsorbed material is not equal to the con-centration of the compound in the interface, but it is equal to the multiplication of the interface compound concentration and the interface thickness.

(30)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 9

If the interfacial tension of a liquid is reduced by the addition of a solute, the solute must be adsorbed at the interface (Prpich et al., 2008). Equation 1.21 shows the funda-mental Gibbs equation for the adsorption phenomena occurring in a binary system:

GT= +γ Γ1 1µ (1.21)

where GT is the energy required for adsorption to occur, g is the energy change in the

interface area, and the term Γ1m1 is related to the energy change associated with the chemical work of solute transfer from the solution to the interface.

Using Equation 1.21, it is possible to obtain an equation that enables us to calcu-late the amount of adsorbed molecules in a system (Equations 1.22 through 1.25):

dG d d d d d T µ γ µ µ µ 1 1 1 1 1 1 = +Γ + Γ (1.22) dG d dG d d d T T µ1 1 µ 1 1 = Γ Γ (1.23) Joining both equations and recognizing that dGT / dΓ1 = m1,

µ µ µγ µµ 1 1 1 1 1 1 1 1 d d d d d d Γ = +Γ + Γ (1.24) It is possible to obtain Γ1 1 = − d d γ µ (1.25)

where dm1 can be defined as follows (Equations 1.26 and 1.27):

dµi=dµ0+RTln ai (1.26)

where ai is the activity. It is possible to express Equation 1.26 in terms of concentration: dµi=dµ0+RTdln [ ] γ1C1 (1.27) where g1 is the activity coefficient, and C1 is the solute concentration.

Some approximations are usually done to make the calculation easier, as can be seen in Equations 1.28 and 1.29.

In very diluted solutions,

dµ1=RTdln[ ] C1 (1.28)

with C1 as the solute concentration. Therefore, Γ1 1 1 1 = − d = − RTd C C RT d dC γ γ ln[ ] (1.29)

(31)

10 Engineering Aspects of Milk and Dairy Products

This is a very important equation, as it allows us to obtain the amount of adsorbed molecules (Γi) in an interface in an easier way than with Equation 1.25. Measuring

the interfacial tension as a function of the total solute concentration, it is possible to construct a graphic of interfacial tension versus concentration (Figure 1.6). From the slope of this representation, the value of Γ can be obtained. The main advantage of using Equation 1.29 in comparison with Equation 1.25 is related to the difficulties in obtaining the chemical potential of the solute in the solution (m), which is necessary to calculate the Γ. However, it is important to emphasize that Equation 1.29 is only an approximation, and Equation 1.25 is the precise definition of Γ.

1.4.1 Monolayers

The term monolayer refers to a layer of amphiphilic molecules that adsorb in an inter-face. Monolayers are well-defined systems formed by only one layer of amphiphilic molecules (Shah and Moudgil, 2002). Amphiphilic molecules contain a polar head and an apolar tail (Figure 1.7); therefore, these types of molecules are able to interact either with hydrophilic or hydrophobic medium. Surfactants are common examples of amphiphilic molecules.

Adsorbed monolayers are formed by allowing the surfactant molecules to adsorb from either one of the adjoining phases. Spread monolayers are obtained with mol-ecules that, at least on the time scale of the experiment, do not or barely dissolve in

Y

C

FIGuRE 1.6 Interfacial tension versus concentration. The slope in any point of the curve allows for the calculation of the amount of adsorbed molecules in the interface.

Hydrophobic tail Hydrophilic head

(32)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 11

the adjoining phases. Such monolayers are called insoluble monolayers or Langmuir monolayers (Norde, 2003).

According to the fundamental Gibbs equation (Equation 1.30), a monolayer is formed if the adsorption of molecules on the interface reduces only the total free energy (Eastoe, 2005).

Considering the formation of a surfactant monolayer in the air–water interface (Figure 1.8), the interaction between the hydrophobic tails with the water is stronger than with the air. However, the energy involved in this interaction is not enough to break the hydrogen binding between the water molecules. Therefore, the hydropho-bic tails are outside the water, lowering the enthalpy and raising the entropy, because the water molecules are free to interact with each other; this means that this con-formation is enthalpic and entropic favorable, according to the fundamental Gibbs equation (Equation 1.30):

dG dH TdS= − (1.30)

where G is the free Gibbs energy, H is enthalpy, T is temperature, and S is entropy. Langmuir monolayers are formed by depositing amphiphilic molecules in an inter-face. The most common procedure is spreading (Yam et al., 2008). The amphiphilic molecules are dissolved in a solvent and then this solution is applied at the interface, in which it is not soluble. The most often used equipment to form and study monolay-ers is the Langmuir trough (Figure 1.9).

In a Langmuir trough, the amphiphilic molecules are spread on the subphase, and the solvent disappears by evaporating. Spreading molecules at one side of the barrier results in a difference between the interfacial tension at both sides. This difference

Aqueous sub-phase

FIGuRE 1.8 A surfactant monolayer.

Aqueous sub-phase (a) (b) Surface pressure sensor Aqueous sub-phase Mobile barrier

FIGuRE 1.9 (a) A Langmuir trough. (b) Cross section of a Langmuir trough, showing the amphiphilic molecules in the aqueous subphase.

(33)

12 Engineering Aspects of Milk and Dairy Products

exerts a force on the pressure sensor, which measures this force called superficial pressure (p) (Equation 1.31):

π γ= 0−γfilm (1.31)

where g0 is the interfacial tension of the pure solvent, and gfilm is the interfacial ten-sion of the film.

The mobile barrier moves in a controlled way, compressing the molecules in the available area. At very low values of p, the monolayers display gaseous behav-ior, because the amphiphilic molecules are far from each other and the interac-tion between them is weak. Inasmuch as the compression gradually increases, the monolayer changes from the gaseous (G) state to the liquid-expanded (LE) state. A further increase in the compression allows a new transition to a liquid-condensed (LC) state, wherein the interaction forces between the amphiphilic molecules become higher, because these molecules are near each other. With a higher compression, the available area between the molecules reduces, and the molecules become closer to each other, this being the state called solid (S) state (Adamson, 1990). If further compression occurs, the collapse pressure is reached, and the film is not in a molecular conformation (Ferreira et al., 2005). In Figure 1.10 and Figure 1.11, it is possible to observe the different aggregation states of molecules in a monolayer and the molecule conformations in these dif-ferent states, respectively.

Monolayers formed in an air–liquid interface can be transferred to a solid support. The transference can be carried out moving the support vertically (Langmuir–Blodgett

7000 6000 LC LE G Area/cm2 mg–1 4000 5000 3000 0 10 20 ∏/mN m –1 30 40 50 S Collapse pressure

FIGuRE 1.10 The different aggregation states of a Langmuir monolayer: (G) gaseous, (LE) liquid expanded, (LC) liquid condensed, and (S) solid. The collapse pressure can also be seen.

(34)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 13

technique) (Seto et al., 2007) or horizontally (Langmuir–Schaeffer technique) through the monolayer (Carpick et al., 2004; Miyano and Maeda, 1986). The last one may be done above or under the monolayer. Figure 1.12 shows the different tech-niques used for Langmuir monolayer transference.

There are many new developments involving the use of solid support containing monolayers in the food industry, such as their use as biosensors to identify microor-ganisms, toxins, antibiotic residues, and pesticides.

Aqueous sub-phase

Solid support

(a) (b)

(c) (d)

FIGuRE 1.12 The transference process of Langmuir monolayers to solid support: (a) monolayer in an air–water subphase before the transference process, (b) vertical transference (Langmuir–Blodgett technique), (c) horizontal transference (Langmuir–Schaeffer [LS] tech-nique) above the monolayer, and (d) LS technique under the monolayer. In the last case, the subphase is removed by aspirating.

(a) (b) (c) (d)

FIGuRE 1.11 Different conformations of molecules in the different aggregation states of a monolayer: (A) gaseous, (B) liquid expanded, (C) liquid condensed, and (D) solid.

(35)

14 Engineering Aspects of Milk and Dairy Products

1.4.2 Factors aFFecting adsorption

Several factors can affect the mechanisms of the adsorption phenomenon. The nature of the surface, for instance, determines the area available for adsorption, and the chemical nature drives the interaction that occurs between adsorbent and adsorbate (Das et al., 2006; Somasundaran et al., 2003).

Another important point is the chemical nature of the solute and the solvent, and the interaction between both. For example, according to Equation 1.25, it is possible to promote adsorption even if the interfacial tension is increasing. To reach this con-dition, the chemical potential of the solute in the solution must be reduced.

Temperature can also influence the adsorption process, because it may alter the properties of the solute, surface, and solvent, as well as their interactions (Karadag et al., 2007). In food engineering and technology, this is especially important, because thermal processes are broadly used.

1.5 MICEllIzATION

In addition to forming oriented interfacial monolayers, amphiphilic molecules can aggregate to form micelles (Figure 1.13). According to Eastoe (2005), micelles are clusters of around 50 to 200 molecules, whose size and shape are governed by geo-metric and energetic considerations.

Micelle formation occurs when the concentration of amphiphilic molecules in solution increases and overcomes the critical micelle concentration (CMC), which is an important parameter (Figure 1.14). Above the CMC, the amphiphilic molecules form micelles, whereas under the CMC, the molecules are in solution.

When amphiphilic molecules are added to a solution, they are able to reduce the interfacial tension, because they are adsorbing on the interface, as can be seen in part 1 of Figure 1.14. Inasmuch as the concentration increases, no more reduction in the interfacial tension occurs, and micelles are formed, as shown in part 2 of Figure 1.14. According to Holmberg et al. (2002), in addition to this, osmotic pressure takes on an approximately constant value, light scattering starts to increase, and self-diffusion starts to decrease. It is also important to highlight that the higher the amphiphilic mol-ecule concentration, the higher the number of micelles (not the size of the micelles).

But, why is the interfacial tension reduction stopped for concentrations above CMC? The answer to this question is related to the energetic saturation on the inter-face. In this sense, if one molecule above the CMC goes to the interface, there would

(36)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 15

be an increase of enthalpic content as a function of the repulsive forces occurring between the amphiphilic molecules. The entropy would be lower, and according to the fundamental Gibbs equation (Equation 1.30), an increase in energy content on the interface would occur, which is energetically unfavorable.

Another important question is “Why are micelles formed?” To answer this ques-tion, it is essential to know that the intermolecular interaction between the hydropho-bic tails is smaller than the one between the hydrophohydropho-bic tail and the water molecules. Therefore, it must be clear that the micelle formation is not only related to “protection” of hydrophobic tails from water. Actually, micelles form to liberate water molecules to interact between them, because this binding is more enthalpic favorable. In addi-tion, the system entropy rises even though there is a reduction in the entropic content of amphiphilic molecules. However, the water molecules are free to form different bindings between them, and consequently, the system entropy increases. These facts contribute to the reduction of Gibbs free energy of the total system (Equation 1.30).

To understand the thermodynamics of micelle formation, it is necessary to con-sider the micelle as a phase, presenting intensive thermodynamic properties different from the solvent phase and also from its hydrophilic interface—it means its hydro-philic part. Based on this, Equations 1.32 and 1.33 can be written:

µamphsol = ° +µamphsol RTlnaamphsol (1.32)

where µamphsol is the chemical potential of the amphiphilic molecule in the solution,

µamph° is the chemical potential of the amphiphilic molecule in a very diluted solu-sol

tion, and aamphsol is the activity of the amphiphilic molecule in a solution:

µamphmic = ° +µamphmic RTlnaamphmic (1.33)

where µamphmic is the chemical potential of the amphiphilic molecule in the micelle,

µamph° is the chemical potential of the pure amphiphilic molecule, and amic amphmic is the

activity of the amphiphilic molecule in a micelle.

CMC 2

[C] 1

Y

FIGuRE 1.14 The change of interfacial tension as a function of the concentration of amphiphilic molecules in the solution. At the critical micelle concentration, micelles start to form.

(37)

16 Engineering Aspects of Milk and Dairy Products

Amphiphilic molecules have different chemical potential when they are in solu-tion or in micelles, as different kinds of interacsolu-tions take place in the solusolu-tion and in the micelle. For example, in the solution, there are more water molecules solvating the hydrophilic and hydrophobic regions, and in the micelle, there are almost no water molecules solvating the hydrophobic tails of the amphiphilic molecules.

At the thermodynamic equilibrium, there is no difference between µamphsol and

µamphmic . Therefore, Equation 1.32 can be subtracted from Equation 1.33, resulting in

Equation 1.34:

0= ° − ° +µamphmic µamphsol

amphmic amphsol

RT a

a

ln (1.34)

By rearranging Equation 1.34, the required energy for 1 mol of amphiphilic mol-ecule to go from solution to micelle, the Gibbs free energy of micellization (DGmic) is obtained (Equations 1.35 and 1.36):

− ° − °

(

µamphmic µamphsol

)

=

amphmic amphsol

RT a

a

ln (1.35)

As µamphmic = µamph° at any temperature, In a = 0 (a = 1). Because the amphiphilic con-mic

centration is very low, aamphsol = [Camphsol ] can be considered. Hence,

−∆micG RT= ⇒∆mic =

CMC G RT CMC

ln 1 ln (1.36)

The CMC is a fundamental characteristic of an amphiphilic molecule (Liu et al., 2008), because by knowing this parameter, important thermodynamic properties, such as the Gibbs free energy, the entropy, and the enthalpy of micellization, can be calculated.

The relevance of this phenomenon in the food industry can be demonstrated if a simple and practical application is considered. In the cleaning step, detergents are used to remove the organic residues from food-contact surfaces. In order to solubi-lize the fat residues, for instance, the surfactants present in a detergent must be in a concentration above the CMC, because the fat globules are solubilized inside the micelles (Figure 1.15).

Fat

(38)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 17

Several factors can influence the CMC. For example, it varies according to the chemical composition of the molecule (Colafemmina et al., 2007). Inasmuch as the alkyl chain increases, the CMC decreases strongly. The temperature and presence of salts can also affect the CMC (LaRue et al., 2008).

It is important to emphasize that in a nonaqueous solution, amphiphilic molecules can associate with their polar head, exposing their apolar tails (Figure 1.16) and forming reverse micelles. The thermodynamic of inverted or reverse micelle forma-tion is similar to the micelle formaforma-tion.

1.6 sTAbIlITY OF COllOIDAl sYsTEMs

A colloidal dispersion is considered stable if the dispersion is able to resist aggrega-tion into larger entities that would then segregate from the medium (López-León et al., 2008).

A colloidal system to be considered as thermodynamically stable requires that the size and the size distribution of the system particles are not altered and cannot sediment or float. On the other hand, colloidal systems can also be classified as kinetically stable. These systems are stable for a period of time but will destabilize in the future.

Colloidal stability is a main issue in applications in food technology and engi-neering. In most cases in the food industry, stable dispersions are desired, as is the case of milk, fruit juices, and processed foodstuffs, such as butter, mayonnaise, and salad dressings; many times, the product shelf life is related to its colloidal stability (Jang et al., 2005). On the other hand, in some applications, such as wine clarifica-tion, aggregation is needed (Norde, 2003). Therefore, it is essential to understand the stability of colloidal systems and manipulate the state of the dispersions for specific applications (Cruz-Silva et al., 2007; Eastman, 2005).

Colloid systems can be classified as lyophilic and lyophobic. The first refers to systems that are thermodynamically stable, and the other is related to unstable sys-tems. Lyophobic particles tend to aggregate, because they try to minimize contact with the continuous phase.

(39)

18 Engineering Aspects of Milk and Dairy Products

There are many factors contributing to the instability of a colloidal system (Meyer et al., 2006; Zhang et al., 2008) which will be discussed in this section. First, the mechanisms of colloid formation will be presented.

There are two ways to form colloids. The first is related to breaking down large pieces to the size required, known as comminution, and the other refers to starting with a molecular dispersion and building the size by aggregation—that is, by condensation (Myers, 1999). Both colloid formation mechanisms are presented in Figure 1.17.

There are three basic mechanisms for the destabilization of colloidal systems: isothermic distillation, coalescence, and coagulation.

The basic principle of isothermic distillation is that smaller particles transfer mol-ecules to bigger particles. Hence, smaller particles become increasingly smaller, and bigger particles become increasingly larger, destabilizing the colloidal system. This process occurs as a function of a transference process from a region with higher chem-ical potential to a region with smaller potential, reducing the free Gibbs energy.

The difference in chemical potential is related to the Gibbs energy excess on the inter-face, and this energy excess is a consequence of the closeness between molecules in the smaller particles, which promotes repulsive forces and reduction in entropy. To avoid the particle increase, it is possible to add a surfactant in the solution to reduce the interfacial tension, as when a stabilizer is added in a food formulation, improving the stability.

Coalescence is the collision phenomenon between two particles, producing just one particle. This mechanism promotes the diminution of the interfacial area (Figure 1.18) and, consequently, the free Gibbs energy. Food emulsions often undergo coalescence (Akartuna et al., 2008).

(b) (a)

FIGuRE 1.17 Colloid formation: (a) comminution and (b) condensation.

(a) (b)

FIGuRE 1.18 The coalescence process: (a) particles present smaller radius and bigger inter-facial area and (b) particles have bigger radius and smaller interinter-facial area.

(40)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 19

There are some strategies to stop the increase of the colloidal particles, such as using a surfactant to reduce the interfacial tension. In food systems, proteins are often used as an adsorbed layer to stabilize fat (Jang et al., 2005). There are other ways to avoid coalescence, such as diminishing the system temperature, because this action decreases particle movement and, consequently, the frequency of collisions; an increase in the system viscosity to reduce the speed of particles also results in a diminution of collisions.

Sherman (2007) studied the colloidal stability in ice cream and observed that the size of the oil globule, as well as the number of globules and variation in holding temperature, influences the coalescence process. The author found that globules of diameter greater than 0.95 m allow a sharp reduction in coalescence rate, because decreasing the interfacial area with increasing diameter of the globule leads to a more stable colloidal system.

Coagulation can be defined as the aggregation of particles that start moving together (Figure 1.19). This phenomenon occurs aiming to reduce the interfacial area, but this reduction is smaller than in the coalescence process.

Sometimes the coagulation phenomenon is desirable, as shown when a practical example in the dairy industry is considered. The milk stability is mainly attributed to the presence of casein. When rennin enzymes are added in the milk, the casein micelles are destroyed. Therefore, cheese formation is a coagulation process that results from the destabilization of a colloidal system, the milk. On the other hand, the acid coagulation of milk, as a result of removing calcium bound between casein micelles, causes destabilization of casein which aggregates and forms a curd, com-promising milk and yogurt shelf life (Shaker et al., 2000).

To avoid the coagulation process, similar procedures to those applied to avoid coalescence can be adopted.

1.7 DOublE ElECTRICAl lAYER

Interfaces in contact with water or an aqueous solution can develop small or large electrical charge (Nikolov et al., 2007). The presence or absence of charge in colloid particles is extremely important, as it implies significant features related to stability of the systems. The presence of charges in surfaces is essential to food technology.

A surface can acquire charge by different mechanisms, such as ionization of sur-face groups, dissolution of ionic solids, and preferential ion adsorption (Riley, 2005).

(a) (b)

FIGuRE 1.19 The coagulation process: (a) particles present bigger interfacial area and (b) particles together have smaller interfacial area.

(41)

20 Engineering Aspects of Milk and Dairy Products

The electrical charge generated at the interface gives rise to an electrical field around the interface that may modify the ion and molecule spatial distribution close to the interface (Figure 1.20).

The ionic distribution around the interface aims at reducing the Gibbs free energy of the system. When the thermodynamic equilibrium is achieved, the electrical-chemical potential of all ionic species is kept constant, as can be observed in Equation 1.37:

µ µ= +i z e x Ni ϕ( ) Aio+RTlnni+z e x Ni ϕ( ) A=Const (1.37) where µi is the electrical-chemical potential, Zi is the ion charge (positive for a cation and negative for an anion), e is the electron charge, j(x) is the difference in the electrical potential between the interface and a point P placed at a certain distance from the interface. miis the chemical potential,ni is the amount of ions

per m3, N

A is the Avogadro constant, R is the gas constant, and T is the temperature

in Kelvin.

Equation 1.37 shows that there are three main factors that drive the ion configura-tion around the interface. The first, ( ),µio is the energy of intermolecular interaction

between ions and molecules present in the interface. The second (RT ln ni) is associ-ated with the configurational entropy of the ions, determined mainly by the thermal movement of charged species. The third factor (Ziej(x)NA) is the energy due to the electrostatic interactions occurring between an ion with charge Z and the ionic envi-ronment that generates the electrical potential j(x).

The intermolecular ( )µio and electrostatic (Ziej(x)NA) interactions will mainly

determine the ion packaging in a dense layer formed by nonsolvated chemical spe-cies. This layer is closer to the interface, and it is called the Stern–Helmholtz layer. More distant from the interface, a diffuse layer, named the Gouy–Chapman layer, is formed where the ion distribution depends on the entropy (RT ln ni) and the

electro-static interactions (Figure 1.21).

The double electric layer is responsible for all electrical properties related to col-loidal systems: electrophoresis, electroosmosis, flow, and sedimentation potentials. FIGuRE 1.20 The double electrical layer.

(42)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 21

1.8 COllOIDAl sYsTEMs IN FOOD ENGINEERING AND TEChNOlOGY

Colloidal systems are often present in food processes. In this section, studies involving colloids in different areas of food engineering and technology will be presented.

Complexation between proteins and carbohydrates has been used to stabilize food emulsion and foams. In this context, Semenova et al. (2009) used static and dynamic light scattering to determine various structural and thermodynamic parameters of particles formed from sodium caseinate and dextran sulfate in aqueous solution and at interface, with different molar ratio. They observed that the structure formed in the bulk aqueous phase was able to provide a more effective stabilization of the mixed emulsions, as compared with the interfacial complexes.

Many studies have been done in edible coatings applications. Due to their hydro-phobicity, lipid compounds have been used as a moisture barrier to coat food prod-ucts. The influence of polymer (agar and cassava starch) on the structure and the functional properties of emulsified films were evaluated, with observation directed at the formation of an aggregate of lipids in the film formed by vegetable oil and cas-sava starch. There was no coalescence required to the formation of a continuous lipid phase necessary for the existence an effective barrier. The authors concluded that the application of agar is better suited for most applications (Phan The et al., 2009).

Many food products are made up of emulsions, and the stability of these emulsions is one of the key factors that determine the food shelf life. It is known that the inter-actions between emulsions and other ingredients present in the food may affect the emulsion stability. In this context, Chuah et al. (2009) evaluated the effect of chitosan (CHI) on the stability of monodisperse modified-lecithin (ML)-stabilized soybean oil-in-water emulsion. The stability of the ML-stabilized monodisperse emulsion droplets was investigated as a function of CHI addition at various concentration,

Gouy-Chapman layer Stern-Helmholtz layer

(43)

22 Engineering Aspects of Milk and Dairy Products

pH, ionic strength, thermal treatment, and freezing–thawing treatment by means of particle size and x-potential measurements as well as microscopic observation. The emulsion was stable in the presence of NaCl, and aggregation was observed in the presence of CHl. In the presence of CHl, the emulsion was more stable at higher temperatures, such as 70°C. These results demonstrate the importance of the food components for emulsion stability.

Beverage emulsions are often stabilized by arabic gum, xanthan gum, or hydro-phobically modified starch. The effects of different concentration levels of arabic gum, xanthan gum, and orange oil on physicochemical emulsion properties and flavor release from orange beverage emulsion were investigated (Mirhosseini et al, 2008b). In another work, Mirhosseini et al. (2008a) evaluated the effects of pectin and car-boxymethylcellulose on physical stability, turbidity loss rate, cloudiness, and flavor release of orange beverage emulsion stored for 6 months. It was observed that the stability of orange beverage emulsions decreased during the stored period and that pectin was generally more effective. In relation to flavor release, it was concluded that the type and concentration of hydrocolloid as well as the storage time were important factors. The results exhibited that a decrease in the release content of some volatile compounds appeared to be in parallel with the decrease in emulsion stability.

Mayonnaise is a much studied food colloidal system because of its stability issues. Iota-carrageenan (IC) and wheat protein (WP) were evaluated as emulsifier alterna-tives to egg yolk in a model mayonnaise system. According to the authors, the main motivation for this work was based on the need to replace egg yolk, due its choles-terol content. A 0.1% IC and 4% WP solution was prepared and used as an emulsifier in five different mayonnaise formulations. The obtained mayonnaises were analyzed for viscosity and stability at different temperatures. The authors concluded that the mayonnaise formulation containing a high proportion of IC and WP were stable at 4°C (Ghoush et al., 2008). This kind of study enables us to understand the impor-tance of different compounds on colloidal system stability.

1.9 CONCluDING REMARks

Colloidal systems are present in many areas, including the food sector. In this chap-ter, the most important concepts and issues involving colloids from the point of view of food engineering and technology were presented.

We cited some examples within the chapter, aiming to clarify some aspects of colloids in a food system. We also presented some of the numerous studies, including issues and developments in the colloid world, applied to food research.

Foods are complex matrices, containing a lot of different ingredients. Many of these ingredients are in the colloidal state, making it of fundamental importance to understand colloid properties in order to obtain a deep knowledge of food systems. In addition, as has been emphasized, one of the most important factors governing food shelf life is related to its colloidal stability.

Another important point is the increasing interest in fat replacement in food, as food researchers and technologists are asked to develop lighter and healthier products with the same quality and stability as their counterparts. This points out how relevant will be the knowledge of colloidal science and technology for food application.

(44)

Physical Chemistry of Colloidal Systems Applied to Food Engineering 23

ACkNOwlEDGMENTs

The authors wish to acknowledge the National Council of Technological and Scientific Development (CNPq) and the Foundation to Research Support of the Minas Gerais State (FAPEMIG) for their financial support.

REFERENCEs

Adamson, A.W. Physical Chemistry of Surfaces, 5th ed., John Wiley and Sons, Chichester, 1990, 777p.

Akartuna, I., Studart, A.R., Tervoort, E., Gonzenbach, U.T., Gauckler, L.J. (2008). Stabilization of oil-in-water emulsions by colloidal particles modified with short amphiphiles. Langmuir, 24 (14), 7161–7168.

Birdi, K.S. Introduction to surface and colloid chemistry. In: Birdi, K.S. (ed) Handbook of Surface and Colloid Chemistry, 2nd ed., CRC Press, Boca Raton, FL, 2003a, pp. 11–14. Birdi, K.S. Surface tension and interfacial tension of liquids. In: Birdi, K.S. (ed) Handbook

of Surface and Colloid Chemistry, 2nd ed., CRC Press, Boca Raton, FL, 2003b, pp. 76–125.

Chuah, A.M., Kuroiwa, T., Kobayashi, I., Nakajima, M. (2009). Effect of chitosan on the sta-bility and properties of modified lecithin stabilized oil-in-water monodisperse emulsion prepared by microchannel emulsification. Food Hydrocolloids, 23 (3), 600–610. Colafemmina, G., Fiorentino, D., Ceglie, A., Carretti, E., Fratini, E., Dei, L., Baglioni, P.,

Palazzo, G. (2007). Structure of SDS micelles with propylene carbonate as cosol-vent: a PGSE−NMR and SAXS study. Journal of Physical Chemistry B, 111 (25), 7184–7193.

Cruz-Silva, R., Arizmendi, L., Del-Angel, M., Romero-Garcia, J. (2007). pH- and thermosen-sitive polyaniline colloidal particles prepared by enzymatic polymerization. Langmuir, 23 (1), 8–12.

Das, S.K., Bhowal, J., Das, A.R., Guha, A.K. (2006). Adsorption behavior of rhodamine B on Rhizopus oryzae biomass. Langmuir, 22 (17), 7265–7272.

Eastman, J. Colloid stability. In: Cosgrove, T. (ed) Colloid Science: Principles, Methods and Applications, Blackwell, Ames, IA, 2005, pp. 36–49.

Eastoe, J. Surfactant aggregation and adsorption at interfaces. In: Cosgrove, T. (ed) Colloid Science: Principles, Methods and Applications, Blackwell, Ames, IA, 2005, pp. 50–76.

Ferreira, M., Caetano, W., Iltri, R., Tabak, M., Oliveira Junior, O.N. (2005). Técnicas de caracterização para investigar interações no nível molecular em filmes de Langmuir e Langmuir-Blodgett (LB). Química Nova, 28, 502–510.

Ghoush, M.A., Samhouri, M., Al-Holy, M., Herald, T. (2008). Formulation and fuzzy model-ing of emulsion stability and viscosity of a gum–protein emulsifier in a model mayon-naise system. Journal of Food Engineering, 84 (2), 348–357.

Holmberg, K., Jönsson, B., Kronberg, B., Lindman, B. Surfactants and Polymers in Aqueous Solution, John Wiley and Sons, Chichester, 2002, 545p.

Jang, W., Nikolov, A., Wasan, D.T., Chen, K., Campbell, B. (2005). Effect of protein on the texture of food emulsions under steady flow. Industrial and Engineering Chemistry Research, 44 (14), 4855–4862.

Karadag, D., Turan, M., Akgul, E., Tok, S., Faki, A. (2007). Adsorption equilibrium and kinetics of Reactive Black 5 and Reactive Red 239 in aqueous solution onto surfactant-modified zeolite. Journal of Chemical and Engineering Data, 52 (5), 1615–1620.

References

Related documents

1) Put into practice classes in management and information technology thanks to an action-based pedagogy. 2) Develop cooperation between manager students and engineer students

Bertambahnya komposisi campuran material Hab pada material filamen spesimen 80:20, nampak terlihat material Hab bertambah, ukuran butir semakin besar, dan menggumpal. Hasil

If a risk based view of defence industry capability were developed as advocated in the ABDI submission to DWP 2015, and as summarised in the preamble to this submission, the level

Beagle Ulradi's Magic Of Murphy's Law Gold Line's Party Officer Gentleman Gold Line's Magic Sound Of Blues Kennel Gold Line's. Dalmatiner Jilloc's Spendid Choice Dalmrosa's

Provided that the effort estimation methods presented are no appropriate to estimate the development effort of Web-based information systems in Chilean scenarios, in the next

If age-specific and age-duration- specific first birth rates of childless women had been fixed at their average values throughout the period, but full-time educational enrollment

Asian participants averaged lower than non-Asian participants in perceived satisfaction of autonomy, competence, and self-actualization needs and in most aspects of

It is very important for all of us to train conservatively and not overdo things. 1) Don't do any exercise that you aren't sure how to do. Always get personal instruction from