BLUK139-Evans February 6, 2008 16:7
Frozen Food Science and Technology
Frozen Food Science and Technology. Edited by Judith A. Evans
© 2008 Blackwell Publishing Ltd, ISBN: 978-1-4051-5478-9
BLUK139-Evans February 6, 2008 16:7
Frozen Food Science and Technology
Edited by
Judith A. Evans
Food Refrigeration and Process Engineering Research Centre (FRPERC)
University of Bristol, UK
BLUK139-Evans February 6, 2008 16:7
©
2008 by Blackwell Publishing Ltd
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First published 2008 by Blackwell Publishing Ltd
ISBN: 978-1-4051-5478-9
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Frozen food science and technology / edited by Judith A. Evans.
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Includes bibliographical references and index.
ISBN-13: 978-1-4051-5478-9 (hardback : acid-free paper)
ISBN-10: 1-4051-5478-0 (hardback : acid-free paper) 1. Frozen foods.
I. Evans, Judith A. (Judith Anne), 1962-
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BLUK139-Evans February 6, 2008 16:7
Contents
Contributors vii
Preface ix
1 Thermal Properties and Ice Crystal Development in Frozen Foods 1
Paul Nesvadba
2 Effects of Freezing on Nutritional and Microbiological Properties of Foods 26
Mark Berry, John Fletcher, Peter McClure, Joy Wilkinson
3 Modelling of Freezing Processes 51
Q. Tuan Pham
4 Specifying and Selecting Refrigeration and Freezer Plant 81
Andy Pearson
5 Emerging and Novel Freezing Processes 101
Kostadin Fikiin
6 Freezing of Meat 124
Steve James
7 Freezing of Fish 151
Ola M. Magnussen, Anne K. T. Hemmingsen, Vidar Hardarsson,
Tom S. Nordtvedt, Trygve M. Eikevik
8 Freezing of Fruits and Vegetables 165
Cristina L.M. Silva, Elsa M. Gonçalves,
Teresa R. S. Brandćo
9 Freezing of Bakery and Dessert Products 184
Alain LeBail, H. Douglas Goff
10 Developing Frozen Products for the Market and the Freezing of
Ready-Prepared Meals 205
Ronan Gormley
11 Frozen Storage 224
Noemi E. Zaritzky
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vi Contents
12 Freeze Drying 248
Andy Stapley
13 Frozen Food Transport 276
Girolamo Panozzo
14 Frozen Retail Display 303
Giovanni Cortella
15 Consumer Handling of Frozen Foods 325
Onrawee Laguerre
Index 347
BLUK139-Evans February 6, 2008 16:7
Contributors
Mark Berry Ronan Gormley
Unilever Plc, Sharnbrook Ashtown Food Research Centre
Bedfordshire, United Kingdom (Teagasc) Ashtown, Dublin
Ireland
Teresa R.S. Brandćo
Escola Superior de Biotecnologia Vidar Hardarsson
Universidade Católica Portuguesa SINTEF Energy Research
Porto, Portugal Trondheim, Norway
Giovanni Cortella Anne K.T. Hemmingsen
Department of Energy Technologies SINTEF Energy Research
University of Udine Trondheim, Norway
Udine, Italy
Steve James
Trygve M. Eikevik Food Refrigeration and Process
Norwegian University of Science Engineering Research Centre
and Technology, Trondheim, Norway (FRPERC), Langford
North Somerset, United Kingdom
Kostadin Fikiin
Refrigeration Science and Technology Onrawee Laguerre
Technical University of Sofia Refrigerating Process Research Unit
Bulgaria Cemagref,
Antony, France
John Fletcher
Unilever Plc, Sharnbrook Alain LeBail
Bedfordshire, United Kingdom ENITIAA (École Nationale
D Ingénieurs des
H. Douglas Goff Techniques des Industries Agricoles et
Department of Food Science Alimentaires), UMR GEPEA,
University of Guelph Nantes, France
Guelph, Ontario, Canada
Ola M. Magnussen
Elsa M. Gonçalves SINTEF Energy Research
Departamento de Tecnologia das Trondheim, Norway
Indśstrias Alimentares
Instituto Nacional de Engenharia, Peter McClure
Tecnologia e Inovaçćo Unilever Plc, Sharnbrook
Lisboa, Portugal Bedfordshire, United Kingdom
BLUK139-Evans February 6, 2008 16:7
viii Contributors
Paul Nesvadba Cristina L.M. Silva
Rubislaw Consulting Ltd Escola Superior de Biotecnologia
Angusfield Avenue Universidade Católica Portuguesa
Aberdeen, United Kingdom Porto, Portugal
Tom S. Nordtvedt Andy Stapley
SINTEF Energy Research Department of Chemical
Trondheim, Norway Engineering
Loughborough University
Girolamo Panozzo United Kingdom
Construction Technologies Institute Italian
National Research Council (ITC-CNR) Joy Wilkinson
Padova, Italy Unilever Plc, Sharnbrook
Bedfordshire,
Andy Pearson United Kingdom
Star Refrigeration, Glasgow
United Kingdom Noemi E. Zaritzky
CIDCA (Centro de Investigación y
Q. Tuan Pham Desarrollo en Criotecnología
School of Chemical Sciences and de Alimentos),
Engineering Universidad Nacional
University of New South Wales de La Plata,
Sydney, Australia La Plata, Argentina
BLUK139-Evans February 6, 2008 16:7
Preface
Freezing is one of the oldest and most commonly used means of food preservation. It has
been known to be an extremely effective means of preserving food for extended periods
since Paleolithic and Neolithic times, when man used ice and snow to cool food. The cooling
effect of salt and ice was first publicly discussed in 1662 by the chemist Robert Boyle, but
this technology was certainly known in Spain, Italy and India in the sixteenth century. The
manufacture of ice in shallow lakes using radiant night cooling and the preservation of ice
and snow in ice houses was a common practice in large country houses in the Victorian times.
Ice was a product only for the privileged, and iced desserts were extremely fashionable and
a sign of great wealth.
In more temperate climates the preservation of ice and snow was obviously difficult, and
it was only with artificial cooling that frozen food became available more widely. In 1755
William Cullen first made ice without any natural form of cooling by vapourising water at low
pressure. This was followed by Jacob Perkins in 1834 who made the first ice-making machine
operating on ethyl ether. In the following 30 years refrigeration technology developed rapidly,
spearheaded by the likes of Joule and Kelvin, and the first patents related to freezing of food
were filed. In 1865 the first cold storage warehouse in New York was built which used brine
for cooling. In 1868 a ship s cold air machine was used on board the Anchor line s Circassian
and Strathlevan ships that transported meat from New York to Glasgow. This was rapidly
followed in the 1880s by the transport of meat from Australia and New Zealand to London.
In the late nineteenth century, refrigeration and the freezing of food underwent rapid
developments in terms of the freezing processes and the refrigerants used. In 1880 ammonia
was first used as a refrigerant and in 1882 the first plate freezer was developed. Although
freezing was an extremely important technology, and a vital means of exporting meat for
the troops in World War I, it was only after the war that refrigeration machinery underwent
massive developments to improve reliability and efficiency.
In 1928 refrigeration was changed forever when Thomas Midgley invented CFCs (Freons).
These were hailed as wonder chemicals and were claimed at the time to be efficient and
environmentally harmless. Around the same time (1929) Clarence Birdseye began developing
frozen meals. His original intention (that another inventor, a Frenchman called Charles Tellier,
had in 1869) was to use freezing to dry foods that would have long-term stability and could be
reconstituted by the housewife. When this method was found to produce poor quality results,
Birdseye reverted to the fast freezing of food. Uniquely, he understood the beneficial impact
of fast freezing on the quality of foods that had until that time often been frozen at slow rates.
Developments in freezing and frozen foods technology developed rapidly in the later half
of the twentieth century. With changes in consumers lifestyles the need for convenience
food increased and, coupled with the development of low-cost refrigeration technologies,
all households could have access to a freezer to store food. At the end of the twentieth
century the market for frozen food was increasing at about 10% per year with approximately
25% of refrigerated food being frozen. This growth has since slowed slightly but sales of
BLUK139-Evans February 6, 2008 16:7
x Preface
certain frozen foods such as fish and seafood are growing. Growth of frozen fish in Russia is
reported to be 17% per year (Cold Chain Experts Newsletter, January, 2006) and the British
Frozen Food federation has recently reported that sales by value increased by 3% in 2005/6
(Refrigeration and Air Conditioning, November, 2006).
Successful freezing can now preserve food almost in its original form. This makes it
possible to preserve and transport food worldwide. As freezing prevents growth of microbes,
frozen food can be stored for long periods; there is no need to use preservatives or additives
to extend shelf life. Freezing allows flexibility in manufacture and supply and means that
food can be preserved at near its optimum quality for distribution and transportation.
This book describes the current technologies to preserve food and the best practices to
ensure production of safe, high-quality frozen food. It also points to some new technologies
that are already making waves and are likely to cast an even greater impact on the frozen
food industry in the future.
One of the largest upheavals in the refrigeration industry in the last 30 years was caused by
the realisation that the chemicals invented by Thomas Midgley are harmful to the environment.
The phasing out of CFCs (chlorofluorocarbons) and introducing their replacements HCFCs
(hydrofluorocarbons) as part of the Montreal and Kyoto protocols, have brought about a
paradigm shift in the chemicals used as refrigerants. Many older refrigerants with low ODP
(ozone depletion potential) and GWP (global warming potential) have been, or are being,
re-evaluated so as to raise their refrigeration potential making use of the modern machinery.
For example, the refrigeration technology used on board the first ships, that brought meat
to the UK from America and Australasia, was based on the use of air as the refrigerant.
This technology, although effective, was based on large and inefficient machinery that could
not compete once newer equipment came into the market. With modern compact, efficient
turbo-machinery these disadvantages were overcome and air could once again be used as a
competitive refrigerant.
As well as addressing these refrigeration issues, the book examines many interesting
new freezing technologies such as pressure shift freezing. Although not yet a commercial
reality for large-scale production, the possibility of a rapidly frozen product with minimal
cell disruption is an exciting prospect for the future.
I hope that you will find that this book provides a comprehensive source of information on
freezing and frozen storage of food. Our aim is to provide readers with in-depth knowledge
of current and emerging refrigeration technologies and how these technologies can be used
to optimise the quality of frozen food. An impressive group of authors, each an expert in their
particular field, have contributed to this book. I would like to thank each of them for their
help in developing a practical and comprehensive guide to freezing and frozen foods.
Judith Evans
BLUK139-Evans March 5, 2008 16:14
1 Thermal Properties and Ice Crystal
Development in Frozen Foods
Paul Nesvadba
1.1 INTRODUCTION WATER IN FOODS
This book deals with freezing of foods, a process in which the temperature of the food is
lowered so that some of its water crystallises as ice. This occurs in freeze-drying, freeze
concentration of juices, and firming up meat for slicing or grinding ( tempering ). However,
the greatest use of freezing of foods is to preserve them, or to extend their storage life.
This is the basis of a huge frozen foods sector, widely established and accepted by the food
consumers. Low temperatures (-18ć%C in domestic freezers, -28ć%C in primary wholesale
cold stores or as low as -60ć%C in some food cold stores) slow down the spoilage processes
(enzymic autolysis, oxidation, and bacterial spoilage) that would otherwise occur at room
temperature or even at chill temperatures.
1.1.1 Foods commonly preserved by freezing
Water is a facilitator of biochemical deterioration of foods. Dry foods are much more stable
than wet foods, because any water remaining in them has low activity, aw. Freezing removes
water from the food matrix by forming ice crystals. Although the ice crystals remain in the
food, the remaining water which is in contact with the food matrix becomes concentrated with
solutes and its aw becomes low. Freezing is therefore akin to drying and this is the rationale
for preserving food by freezing. Most micro-organisms cease functioning below the water
activity of about 0.7.
The commonly frozen foods are those which contain appreciable amounts of water
(Table 1.1).
Living cells, biological materials (plant and animal tissues) in the natural state are able to
hold typically 80% water by mass on wet basis. Therefore foods derived from them contain
similar high proportions of water. This also applies to engineered foods such as ice cream
where water/ice mixture is required to impart texture.
1.1.2 Influence of freezing and frozen storage on quality
of foods
Food products thawed after cold storage should ideally be indistinguishable from the fresh
product (this obviously does not apply to products such as ice cream that are consumed
in the frozen state). This requirement is easier to achieve in some foods than in others.
Foods with a delicate structure are more likely to suffer cell damage. However, for the main
food commodities (bread, meat, fish, vegetables) the quality of the thawed product is indeed
Frozen Food Science and Technology. Edited by Judith A. Evans
© 2008 Blackwell Publishing Ltd, ISBN: 978-1-4051-5478-9
BLUK139-Evans March 5, 2008 16:14
2 Frozen Food Science and Technology
Table 1.1 Water content ranges of commonly frozen foods.
Water content
Food commodity (% wet mass basis) Reference
Breads 28 46 Holland et al. (1991)
Doughs 5 20 Miller and Kaslow (1963)
Fisha 50 80 Love (1982)
Ice cream 59 62 Holland et al. (1991)
Meats 35 90 Holland et al. (1991)
Vegetables 55 90 Holland et al. (1991)
Fruit (strawberries, raspberries) 87 90 Holland et al. (1991)
Ready meals 50 85 Kim et al. (2007)
a
Note: Water content of fish is approximately (80% fat content), Love (1982).
comparable with the fresh product (and in some cases, applying certain criteria, for example,
vitamin content, enhances the quality of fresh food sold as chilled).
The formation of ice crystals can downgrade the quality of the food by one of the following
three mechanisms:
(a) Mechanical damage to the food structure. The specific volume of ice is greater than
that of water (greater by about 10%) and therefore the expanding ice crystals compress
the food matrix. Ice crystal expansion in some fruits such as strawberry damages them
severely, because of their delicate structure (the fruit becomes soggy on thawing). On
a macroscopic scale, during rapid cryogenic freezing, thermal stresses due to expansion
may crack the food.
(b) Cross-linking of proteins (in fish and meat). Decrease in the amount of liquid water
available to the proteins and increase of electrolyte concentration during freezing lead to
aggregation and denaturation of actomyosin (Connell, 1959; Buttkus, 1970).
(c) Limited re-absorption of water on thawing. This is connected with mechanism (b).
Again, we can take the example of animal tissue in which the muscle proteins, during
frozen storage, become denuded of their hydration water and cross-linked. On thawing,
the tissue may not re-absorb the melted ice crystals fully to the water content it had before
freezing. This leads to undesirable release of exudate drip loss and toughness of
texture in the thawed muscle, the main attributes determining quality (Mackie, 1993).
Mechanisms (b) and (c) are usually the main causes of deterioration of quality of frozen
foods, which means deterioration of quality is caused mainly by processes taking place in
frozen storage rather than during the initial freezing. Rapid freezing is possible only for
small samples, not commercial ones. The rate of freezing achievable for large commercial
samples is so small that the quality of foods would not be greatly affected by the freezing
rate (extracellular ice invariably forms for all samples other than those which are small and
frozen in a laboratory by special techniques).
Both damage to food and its consequences for consumer-assessed quality depend on the
type of food (its biological makeup and structure). For example, meat is less prone to damage
from freezing and frozen storage than fish is. This is because meat protein fibres are more
robust and, moreover, meat is cooked for longer than fish. Fish, a cold-blooded animal,
starts cooking at 35ć%C the body temperature of mammals whereas meat proteins are
more stable (there seems to be a correlation between the temperature of the living animal
and the stability of proteins, e.g. tropical sea fish as compared with North Sea fish). Adding
BLUK139-Evans March 5, 2008 16:14
Thermal Properties and Ice Crystal Development in Frozen Foods 3
cryoprotectants to food reduces deterioration in frozen storage. The section Glassy State
discusses this further.
The ability to determine the quality of frozen foods rapidly in their frozen state, without
having to thaw the food for analysis, is of great significance. Kent et al. (2001, 2004, 2005)
developed a microwave method for this. If, in a certain situation, this instrumental method
cannot be used, a sensory assessment panel is used. The quality attributes of thawed foods
are sensory (appearance, odour, flavour, texture in cooked products). The attributes that are
directly connected with water in foods are water-holding capacity and drip loss.
In the UK, frozen thawed fish cannot legally be presented for sale as fresh for the quality
changes freezing causes. This raises the question of enforcement of the law. Apart from
the biochemical methods which are slow (Kitamikado et al., 1990; Salfi et al., 1986), it is
preferable to use rapid physical and, in particular, electrical methods that have been developed
for fish quality measurement but are also useful to check whether the fish had been frozen at
all (Jason and Richards, 1975; Rehbein, 1992).
Another legal issue is added water . During freezing of fish fillets, water sprayed on
their surface creates a layer of ice that provides some protection against oxidation in frozen
storage. On the other hand, the temptation may be to add too much of water because fish is
sold by weight. For this problem, rapid methods to detect the amount of water added have
been developed (Kent et al., 2001; Daschner and Knöchel, 2003).
Consumers often ask whether thawing and refreezing is detrimental to food quality. The
answer is that when done properly (hygienically, thus preventing microbial contamination
during thawing), the effect of multiple freezing on quality (e.g. increased drip) is usually not
very serious (Oosterhuis, 1981).
1.1.3 Water-binding capacity (or water-holding ability) of foods
Food holds water by several mechanisms. It may be cells holding the water either with
cell membranes or between cells and in pores by capillary forces. Such water could be
expressed (removed) by pressing. Water binds to hydrophilic components of foods (proteins,
carbohydrates, salts and micronutrients) by van der Waals forces including hydrogen bonding.
Interaction of water with fats (lipids) is small because fats are hydrophobic, not readily
soluble in water. On the cellular level, exclusion of water from cells is regulated by both
the permeability of cell (or micelle) lipid bilayers and osmotic mechanisms. The molecular
force in the hydration shell around proteins increases from the outer to the inner hydration
layer. The most tightly bound water may not be removed by freezing; this water is called
unfreezable water .
The methods to measure water-binding capacity of foods have great commercial and
scientific significance. Trout (1988) reviewed the following methods for measuring water-
holding capacity of foods: the press, centrifugal, capillary suction, filter paper, small-scale
cook yield test and NMR.
1.2 FREEZING OF FOODS
1.2.1 Freezing curves
Freezing of food starts when the food is placed in contact with a cold medium, which can be
solid (for example, heat exchanger plates at -30 to -40ć%C, solid carbon dioxide (dry ice) at
BLUK139-Evans March 5, 2008 16:14
4 Frozen Food Science and Technology
T0
B C
Tf
Ts A
Te
t1
Time
Fig. 1.1 A schematic plot of temperatures in food during freezing, showing the starting temperature,
T0, the initial freezing temperature, Tf, the temperature to which the food may supercool, Ts, the freezing
plateau B C and the equilibrium temperature, Te.
-78.5ć%C), liquid (immersion in a cooling mixture or cryogenic fluid such as liquid nitrogen
at -196ć%C) or gas (a stream of air, gaseous nitrogen or CO2). The surface of the food cools
faster than the centre of the food because the heat from the interior of the food has to reach
the surface by conduction.
Figure 1.1 shows a typical temperature record during freezing. The temperature at the
surface of the food may show supercooling (point A (t1, Ts)) before increasing momentarily to
approximately the initial freezing temperature Tf, and thereafter continuing along the thermal
arrest plateau (the B C part) as transfer of the latent heat of freezing of water (334 kJ/kg for
pure free water) from the food begins. The first ice crystals are formed between A and B and
further crystals are formed all the way to the final temperature Te where the temperature of
the food equilibrates to the temperature of the cooling medium. No further rapid increase in
the amount of ice occurs except for the slow accretion discussed in section 1.2.4.
1.2.2 Supercooling
Below its initial freezing point, a liquid is said to be supercooled. This is a metastable state of
the liquid; the liquid can continue to be in this state for a very long time, before nucleation of the
first crystal takes place. Following this the crystals grow and spread throughout the volume
rapidly. Pure water (free of impurities such as dust particles that would act as nucleation
centres) can be supercooled to around -40ć%C. At lower temperatures water freezes due
to homogeneous ice nucleation and growth. In foods the degree of supercooling is much
smaller than in pure water because of heterogeneous ice nucleation. Supercooling is important
in nature since this is one of the mechanisms by which living plants and animals cope
with sub-zero temperatures or minimize the damage of their tissue that ice formation can
cause.
Temperature
BLUK139-Evans March 5, 2008 16:14
Thermal Properties and Ice Crystal Development in Frozen Foods 5
1.2.3 Ice nucleation and growth
Ice crystals come to existence as nuclei (seeds) of a critical size that subsequently grow. The
critical size is that at which growth of the nucleus results in reduction of surface energy à as
compared with the increase in Gibbs free energy Å‚ due to increase in volume (for a spherical
ice crystal of radius r, this happens when Ãr2 <Å‚r3).
Nucleation can be homogeneous or heterogeneous. Homogeneous nucleation occurs only
in homogeneous particle-free liquids and happens due to random fluctuations of molecules
(the random clusters of molecules momentarily assume the configuration of ice and act
as seeds). In solid foods the nucleation is heterogeneous, with the cell surfaces acting as
nucleation sites. The probability of nucleation at a site is enhanced if the molecular structure
of the surface resembles that of ice, i.e. matches the lattice size of the ice crystal and acts
as a template. This happens notably with ice nucleation active (INA) proteins found in some
bacteria and plants (Govindarajan and Lindow, 1988).
1.2.4 Ice fraction frozen out
Pure water freezes at 0ć%C (save for the phenomenon of supercooling), but water solutions
(in food sodium chloride or other salt solutions) have a lower freezing point, the depression
being approximated by Raoult s equation (Miles et al., 1997). During cooling below Tf, the
extracellular region forms ice first and then the intracellular region begins to change state.
This can be attributed to the fact that the cell (typical diameter 50 µm) membrane prevents
growth of external ice into the region inside the cell (called intracellular region) making the
intracellular region supercooled (<"-8ć%C).
Figure 1.2 shows a schematic diagram of an aqueous binary solution. The equilibrium
between ice frozen out below Tf and the remaining solution requires the chemical potential
of the two to be the same (Pippard, 1961). This leads to a relation between the water activity
aw of the solution and the molecular masses of the components and their fractions. It is
possible to show from these thermodynamic considerations (for example, Miles, 1991) that
the amount of ice xi frozen out at each temperature T < Tf, is in the first approximation
Aqueous solution
0°C
Solute + solution
Ice + solution
E
TE
Ice + solid solute
0 25 50 75 100
Pure water Pure solute
Concentration of solute (%)
Fig. 1.2 A state diagram, showing schematically the behaviour of an aqueous binary solution with eutectic
point E and eutectic temperature TE.
Temperature
BLUK139-Evans March 5, 2008 16:14
6 Frozen Food Science and Technology
(assuming an ideal binary solution and small temperature differences Tf - T ) given by
xi = (xw - xu)(1 - Tf/T ) (1.1)
where Tf and T are in degrees Celsius, xw is the total water content of the food and xu is
the unfreezable water content. The last one is typically 5% and includes the so-called bound
water, so that xu > xb where xb is the content of bound water.
The term bound water is not understood well and not defined clearly. Fennema (1985)
defines it in practical terms as
. . . water which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced
molecular mobility and other significantly altered properties as compared with bulk water in the
same system, and does not freeze at -40ć%C.
This definition has two desirable attributes. One, it produces a conceptual picture of bound
water, and two, it provides a realistic approach to quantifying the bound water. Water un-
freezable at -40ć%C can be measured with equally satisfying results by either proton NMR
or calorimetric procedures.
Figure 1.3 shows the graph of xi for Tf =-1ć%C and xu = 5%. Riedel (1957, 1978)
made the first systematic experimental determination of the ice fraction xi by calorimetric
measurements. Other experimental investigations, for example by NMR, confirm that the
approximation of xi by equation (1.1) is acceptable for engineering purposes such as the
calculation of thermal properties of frozen food, requiring accuracy of about Ä…10% (Novikov,
1971).
Equation 1.1 is derived from thermodynamic considerations (see for example Miles
(1991)) that do not take into account the fact that even at constant temperature the fraction of
ice increases with time, as was observed, for example, by Kent (1975). The time dependence is
due to kinetically hindered mobility of the water molecules. Frozen food is not an equilibrium
80
70
60
50
40
30
20
10
0
-45 -40 -35 -30 -25 -20 -15 -10 -5 0
Temperature (°C )
Fig. 1.3 Proportion of water frozen out in food as a function of temperature, calculated for a food with
water content xw of 80% and unfreezable water content xu of 5%.
Ice fraction (%)
BLUK139-Evans March 5, 2008 16:14
Thermal Properties and Ice Crystal Development in Frozen Foods 7
Aqueous solution
Tm
Tg
0°C
Tg´
Ice + solution
Glass Cg´
0 25 50 75 100
Pure water Pure solute
Concentration of solute (%)
Fig. 1.4 A supplemented phase diagram showing schematically the behaviour of aqueous solution with
the melting line Tm, glass transition line, Tg, the concentration of the maximally concentrated solution, Cg
and the corresponding glass transition temperature, Tg .
system. The water that stays close to the food matrix may be in a glassy state. Then the simple
binary diagram in Fig. 1.2 is extended into a supplemented state diagram of foods (Roos,
1992, 1995; Rahman, 2006). This diagram (Fig. 1.4) can incorporate equilibrium melting
points, heterogeneous nucleation temperatures, homogeneous nucleation temperatures, glass
transition and devitrification temperatures, recrystallisation temperatures and, where appro-
priate, solute solubilities and eutectic temperatures (MacKenzie et al., 1977). So far only
simple binary systems such as water glucose have been investigated thoroughly enough.
1.2.5 Effect of freezing rate on ice crystal structure
Hayes et al. (1984) define the freezing rate in relation to the velocity of movement of the ice-
water freezing front. This has also been adopted by the International Institute of Refrigeration
in their Red book (BÅ‚gh-SÅ‚rensen et al., 2007).
The rates of freezing determine the type, size and distribution of ice formation. These
can be extracellular or intracellular ice, dendritic or spherulitic (in rapidly frozen aqueous
solutions; Hey et al., 1997), and may be partially constrained by the food matrix. Using very
high rates of cooling (up to 10,000ć%C/min) it is possible to avoid ice formation altogether
and instead achieve vitrification leading to glassy state.
Angell (1982), Franks (1982), Garside (1987) and Blanshard and Franks (1987), among
others, have reviewed crystallisation in foods. Because of the difficulties in interpreting the
results of measurement of ice formation in complex food matrices, most definitive studies
have started with simple systems based on aqueous solutions (Bald, 1991). A number of
studies of ice formation and its prevention by cryoprotectants or anti-freeze proteins have
also been carried out in the context of medical applications, preservation of biological tissue
for viability, notably by Mazur (1970, 1984). This clearly shows a considerable commonality
between researches in food and medical sciences.
Slow freezing produces fewer larger ice crystals, fast freezing produces a greater number
of smaller crystals. Whether large or small c
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