Water in Foods. Fundamental Aspects and Their Significance in Relation to Processing of Foods

535 Pages·1994·10.329 MB·English

by  Fito, Mulet and McKenna (Eds.)

Proceedings of the Fifth International Symposium on the Properties of
Water in Foods (ISOPOW-V), held in Peniscola, Valencia, Spain, 8-14
November 1992, organized by ISOPOW/IUFOST, Universidad Politécnica
de Valencia and Consejo Superior de Investigaciones Cientificas, under the
auspices of the Institute of Food Technologists and the Food Working
Party of the Federation of Chemical Engineering
WATER IN FOODS
Fundamental Aspects and Their Significance
in Relation to Processing of Foods
Edited by
PEDRO FITO
ANTONIO MULET
Departamento de Tecnologia de Alimentes,
Universidad Politécnica de Valencia, Spain
and
BRIAN McKENNA
Reprinted from
Journal of Food Engineering
Vol. 22 Nos 1-4
ELSEVIER APPLIED SCIENCE
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ELSEVIER SCIENCE LIMITED
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© 1994 ELSEVIER SCIENCE LIMITED
British Library Cataloguing in Publication Data
Water in Foods: Fundamental Aspects and
Their Significance in Relation to
Processing of Foods
I. Fito, Pedro
664
ISBN 1-85861-037-0
Library of Congress Cataloging-in-Publication Data
Water in foods: fundamental aspects and their significance in
relation to processing of foods / edited by Pedro Fito, Antonio
Mulet, and Brian McKenna.
p. cm.
"Reprinted from Journal of Food Engineering, vol. 22, nos. 1^."
"Proceedings of the Fifth International Symposium on the
Properties oof Water in Foods (ISOPOW-V), Peniscola, Spain, November
1992"-P. preceding t.p.
Includes bibliographical references and index.
ISBN 1-85861-037-0
1. Food-Water activity-Congresses. 2. Food industry and trade-
-Congresses. I. Fito Maupoei, Pedro. IL Mulet, Antonio.
III. McKenna, Β. M. IV. International Symposium on the Properties
of Water in Foods (5th : 1992 : Peniscola, Spain)
TX553.W3W367 1993
664-dc20 93-1321
CIP
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Preface
The ISOPOW-V meeting was held in Peniscola, Valencia, Spain, from
8-14 November 1992, with more than one hundred and seventy scien-
tists from all over the world attending. 'Water in foods: fundamental
aspects and their significance in relation to processing of foods' was the
main idea around which outstanding contributions and discussions took
place. This rapidly moving field of knowledge was approached from
different points of view and, consequently, an enriching multidisciplinary
environment was achieved.
There was a high degree of interaction between participants which
created a favourable atmosphere for international exchanges. As a result,
the main aim of the meeting was successfully accomplished.
The scientific quality of the contributions was of the highest level. It is
rare to have such a blend of diversity and excellence all in one meeting.
The work of the International Coordinating Committee, as well as the
involvement of the chairman of the plenary sessions in the preparation of
the scientific programme, greatly contributed to this success.
The expectations raised by the meeting were fulfilled, and the ISO-
POW-V organizing committees are proud to present this volume with the
text of the plenary lectures, representing the main contributions to the
meeting. Besides these lectures there were fifty nine posters that allowed
fruitful exchanges between participants. Professor Marcus Karel co-
ordinated a Committee that summarized the main developments during
the meeting.
The support of the Universidad Politécnica de Valencia and the
Consejo Superior de Investigaciones Cientificas as well as the express
encouragement from ISOPOW members should be acknowledged as
important factors for the success of this meeting.
Pedro Fito
Chairman
International Coordinating Committee
Antonio Mulet
Chairman
Local Coordinating Committee
ν
Acknowledgements
The ISOPOW-V Symposium was organized jointly by the ISOPOW/
IUFOST Executive Committee, Consejo Superior de Investigaciones
Cientificas and the Universidad Politécnica de Valencia. The organizers
acknowledge the support from Comision Internacional de Ciencia y
Tecnologia (CICYT), Instituto de la Mediana y Pequena Industria
Valenciana (IMPIVA), Ciencia y Tecnologia para el Desarrollo
(CYTED-D), Conselleria de Cultura Generalitat de la Valenciana,
Exoma, Diputacion de Castellon.
The contribution of the following private companies is also acknowledged:
The Coca-Cola Espana, Defensor AG, DECAGON Devices Inc, Unilever,
Pillsbury, Sistema de Ensayo de Materiales, S.A. (SEM).
Finally, the editors would like to highlight the relevant contributions
of the ISOPOW Executive Committee to the organization of the Sym-
posium, especially those of Dr R. B. Duckworth and Dr D. Simatos.
vi
Journal of Food Engineering 22(1994) 1-12
© 1994 Elsevier Science Limited
Printed in Great Britain. All rights reserved
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ELSEVIER
Summary of Advances Reported at the ISOPOW-V
Conference
Dr Marcus Karel for Physico-Chemical Aspects
Dr John Christian for Biological Aspects
Dr Pedro Fito for Technological Aspects
1 PHYSICOCHEMICAL ASPECTS
The conference started, appropriately, with a review of the theoretical
aspects of hydration. The first problem reviewed was the nature of water
activity depression in solutions. Water activity depends on water concen-
tration as shown in eq 1.
a = f(Xj=f(l-X) (1)
w s
where A^ is the mole fraction of all the solutes. It was recognised that the
s
function / often depends on the nature and concentration of individual
solutes, and their functional groups. Solute size, polarity, solute-solute
interactions and cooperative solvation may all contribute to this func-
tion. A thermodynamic analysis using the AH for individual group trans-
fer from gas to solution, assuming additivity, was found useful in
calculating water activity.
The importance of hydration in affecting, or even controlling,
solute-solute interaction energetics was reviewed. The MacMillan
Mayer model is often successful in predicting solubility (e.g. in the
sucrose-mannitol system). During the discussion, attention was given to
the problem of accessibility of individual groups in polymeric and con-
centrated systems, and to hydration and its effects on polymer conforma-
tion in solution. The minimum energy conformation is expected in the
crystalline state. In solution, conformation depends on molecular weight,
molecular size, solvent, pH and other factors. Important behavioural
features may be related to some conformational parameters (e.g. end-to-
end length, and persistent length). The nature of the solvent is important;
water may unfold a polymer chain, or make the polymer structure more
compact (e.g. cellulose). Ionisation may also cause drastic changes (e.g.
pectin in solution).
1
2 Dr Marcus Karel, Dr John Christian, Dr Pedro Fito
Molecular dynamics is capable of providing structural information
based on knowledge, or assumption of potential energy functions of
groups and molecules. It provides , i4n Dr Brady's words, an opportunity
to see molecules and their motion as God sees them'. Of particular value
to food science is the capability to obtain the location, conformation, and
exchangeability parameters for water molecules near specific groups of
the solute. It may be used to explain solubility of isomers, and, perhaps,
some of the effects which are observed in concentrated solutions result-
ing from formation of ice crystals. The discussion of the situation in solu-
tions was followed by a review of the 'oil'-water interface, with a focus
on location, conformation and surface concentrations of proteins at
oil-water interfaces which exist in emulsions. Monte Carlo simulations
were useful in predicting protein behaviour, and neutron reflectance was
used to confirm results experimentally. Studies on milk proteins were
used to analyse behaviour of two different protein classes, ^-casein (C)
and /Mactoglobulin (L ), which give films with different natures. Films of
C are mobile, disordered and exchange readily with bulk solution while
films of L give a rigid, packed layer of compact globular molecules with
little exchange. Effect of individual groups (ionic, S-S, etc.) must be con-
sidered in calculating 'stickiness' of the molecule to the interface. Fractal
analysis was also found of value in analysing conformation. It was
pointed out that the calculations based on equilibrium conformation fail
when one protein is adsorbed on the interface and its equilibrium-driven
replacement by another protein is studied. The process is often
controlled by process DYNAMICS. Complications also arise from for-
mation of complexes between proteins and polysaccharides. In these
situations, the nature and concentration of polysaccharides are import-
ant. In general, the formation of a complex at the interface promotes
emulsion stability, but complexes formed in the solution bulk may be
unfavourable if they hinder adsorption at the interface.
A discussion of gels followed. Starch gels are hydrated gels with a
complexity surprising in a polymer with a single hydrating group type
—OH. The fact that great differences in molecular weight, conformation,
and packing, as well as complexation with lipid, occur in this biopolymer
makes generalisations difficult. Swelling, gelatinisation and amylose loss
from starch granules were all shown to be complex kinetic processes.
The structure of amylose gels was proposed to involve oriented packing
of amylose helices.
The rather perplexing phenomenon previously reported at the
Nottingham Easter School on the 'Glassy State in Foods' was mentioned.
This so-called 'Gidley endotherm' (reported by Dr Gidley of Unilever) is
Summary of advances reported at the ISOPOW-V conference 3
an endotherm at 40° to 60°C observed in DSC heating of non-annealed
glassy polysaccharides, including starches. In re-scans it disappears, but
reappears on ageing below 7. Its temperature location is independent of
water content, but the size of the endotherm {AH) correlates with water
content. The present writer proposed in his review summary at
Nottingham that this 'apparition' deserves study. Unfortunately no new
hypotheses or results were available at the time of writing.
Protein gels can also have considerable complexity. Casein gels
formed by the action of rennin and those obtained by acidification are of
similar fractal nature and show similar sorption and hydration charac-
teristics immediately after gel formation, but their ultimate water holding
capacity is very different. The differences are attributable to the
DYNAMICS of gel aggregation, which in turn is dependent on the ionic
state of the polymer, which differs because acid causes dissolution of
amorphous calcium phosphate but the enzyme action does not.
During the past decade, it has become recognised by an increasing
number of food researchers that the mobility — and to some extent
reactivity — enhancing plasticising effect of water can often be explained
by considering glass-rubber transitions in food polymers, and
glass-'very viscous fluid' transitions in amorphous structures formed by
monomeric food components (notably sugars). The ISOPOW V confer-
ence took note of this trend by devoting considerable attention to this
subject.
The theories of glass transitions, and of the characteristic transition
temperature ( T) were reviewed. The transition event may be considered
from the point of view of solid state, and of the concentration of defects
in the solid state. The results of 'solid state' theories may be compared
with 'mode coupling' theories applied to the liquid state. The two
theories can be reconciled by invoking thermally activated processes
occurring between T„ and a higher temperature 7 , which is claimed by
C
some to be the true transition. Relaxation above T„ is of considerable
importance not only to physicists but to food technologists as well. Vis-
cosity, diffusivity, viscoelastic parameters G' and G", and 'apparent reac-
tion rate constants' of some chemical reactions change above 7 , and,
g
while each may have its own relaxation constant, it has often been
observed that many processes in food systems show similar relaxation
time dependence on temperature, often in accordance with the WLF
equation. It is of interest to note that theoreticians of the glass state tran-
sition have proposed a classification of liquids into 'strong' and 'fragile'.
The 'fragile' liquids (many polymer and monomer solutions of import-
ance in food are apparently 'fragile') show rapid relaxation time and vis-
4 Dr Marcus Karel, Dr John Christian, Dr Pedro Fito
cosity drop below T , consistent with WLF-like behaviour. 'Strong'
&
liquids (some electrolyte solutions, Si0 , glycerol) show more moderate
2
decrease in viscosity, and their temperature dependence is more 'Arr-
henius'-like. It was also proposed that some polyamines are 'super-
fragile' and that glass transition may occur within protein molecules
(Cytochrome C).
The theoretical reviews were followed by Dr Slade's excellent review
of the 'polymer science approach' to mobility in foods above T„. State
diagrams considering both the T„ versus Cor W (solute concentration or
water concentration, respectively) and the liquids line may be construed
as first proposed by Professor Franks. Other equilibrium lines may be
included (e.g. solubility) and iso-viscosity lines may be drawn using
appropriate relations (e.g. WLF). The use of state diagrams has become
a tool for food product development, and in this conference their use
was discussed in the case of products in which the glassy matrix is due to
sugar (cookies), and for processes in which a continuous matrix was
developed based on protein (extruded wheat and corn products dis-
cussed by Dr Kokini). The application of state diagrams, and in particu-
lar of events related to T and ice melting, to frozen food stability were
g
reviewed by Dr Reid who noted that stability could be described with
reference to a 'critical temperature' ( T ) but that the data do not allow a
C R
confirmation of the correspondence T to T (or 7^), and that kinetics
CR g
can often be described by either WLF or Arrhenius kinetics, since the
experimental range of temperatures is quite narrow. Our knowledge of
the transitions in sugar solutions has been extended by the excellent
posters presented by Dr Mclnnes, which showed the frequency depen-
dence of mechanical spectrometer measurements of glass transitions,
and by Dr Roos who reported his careful determinations of Τ and for
a range of sugars and sugar alcohols. In his poster, Dr Nesvadba com-
pared glass transitions in simple solutions and in cod muscle, and differ-
ences in behaviour. Dr Chinachotti considered transitions as a factor in
staleing behaviour of bread. Professor LeMeste's group presented a
poster on the important textural attributes of cereal food, and how they
are affected by glass transitions.
Many processes in food systems above Tj, are time-dependent, and
this is certainly true of diffusion and stress relaxation and flow. At the
ISOPOW III conference, Karel presented a schematic diagram relating
properties above T to the Debra number De, and this concept continues
u
to be referred to in many processes.
The Debra number figured very prominently in the review of diffusion
in polymers by Professor Peppas. He noted that the Fickian model of dif-
Summary of advances reported at the ISOPOW-V conference 5
fusion is often inadequate. In some classes of deviations from Fick's Law,
the diffusional behaviour may be described with reference to the co-
existence of swelling and dif fufscion. This behaviour varies with the magni-
tude of De, defined here as the ratio of relaxation time to diffusion time'.
Fick's law applies only for extremely small and extremely large values of
De. Applications to food systems were discussed, especially for con-
trolled release of flavours. In subsequent discussions, Dr Karel described
release of flavour compounds or of oil initially encapsulated in hydro-
philic glassy walls or pores in a porous dried food system. Swelling of the
walls causes simultaneous partial release and pore collapse, resulting in
Non-Fickian release.
The diffusion of water and other components, as well as some relaxa-
tion events, can be visualised by Magnetic Resonance Imaging (MRI),
and two of the world's experts in this field presented dramatic evidence
of progress. The technique is still expensive and its resolution relatively
coarse (1 mm according to Dr McCarthy), but it allows the determina-
tion of the progress of diffusional and relaxation processes non-destruc-
tively. These presentations were augmented by posters demonstrating
the usefulness of MRI in cakes and biscuits, various heterogeneous sys-
tems, potatoes, frozen courgettes and grains. The posters were prepared
by the staff at Cambridge University, the AFRC Institute of Food
Research and the University of Illinois, as well as various UK food
industries.
Ice nucleating proteins were studied by Dr Watanabe and her
colleagues as aids in freeze concentration. Xanthomonas species were
found effective in promoting nucleation which improved the quality of
freeze-concentrated products.
Proteins can also be effective in inhibiting crystallisation of ice, and
the mechanism of their action and their great commercial potential were
reviewed by Dr Lillford.
2 BIOLOGICAL ASPECTS
In the session dealing with hydration, enzyme activity and hydration and
membrane structures, Dr Klibanov considered enzyme action in non-
aqueous systems. It has been established that enzymes can function
vigorously in organic solvents containing little or no water. The amount
of water required for activity is considerably less than that needed to
establish a monolayer on the enzyme, and the enzyme activity falls with a
decrease in the amount of water bound to the enzyme. A hydrophilic sol-