Thermal processing of food

1.0 Introduction

The problem of finding and keeping food from day to day and from season to season necessitated the need to find ways of prolonging its storage life1. Methods were developed under natural conditions such as drying smoking, salting chilling and freezing for the preservation and storage of food and food products. With increasing scientific knowledge new methods of preservation have been developed and still newer ones are fast coming into existence.

Some processing methods change the raw material into a different form or change the nature of the product. A large determinant of this change is the texture of the food-that is the sensation the food imparts to the nerves and muscles in the mouth when it is bitten, chewed and swallowed. A key factor influencing food texture is food structure. The basic biochemical components forming the structure of food products consist of the natural materials assembled in relationships that can be altered by the presence of additives, ingredients and processing or handling2. This sparked studies on the relationship between food texture and processing methods to understand the significance of food structure for optimal food quality.

The most obvious and widespread forms of deterioration are due to microbial attack. Bacteria, yeasts and fungi will compete with man for his food if permitted, and the most elementary forms of preservation have been, without knowledge of their nature, methods of dealing with this problem

This subject looks at the processes that develop the structure of food through cooling and heating. Theoretical aspects of cooling (freezing) and heating (baking) processes will be reviewed as well as the effect of these processes on the structure of food will be discussed.

2.0 EFFECTS OF COOLING

2.1 Freezing

Freezing is one of the oldest and most widely used methods of food preservation, which allows preservation of taste, texture, and nutritional value in foods better than any other method1. The freezing process is a combination of the effects of low temperatures at which microorganisms cannot grow, chemical reactions are reduced, and cellular metabolic reactions are delayed3.

2.2 The Freezing Process

The freezing process mainly consists of thermodynamic and kinetic factors, which can dominate each other at a particular stage in the freezing process4. Major thermal events are accompanied by reduction in heat content of the material during the freezing process as shown in Figure 1.

Figure 1 A schematic illustrating the overall freezing process

During the freezing process pure step goes through a two-step (nucleation and propagation) crystallization process to form ice. The material to be frozen first cools down to the temperature at which nucleation starts. Before ice can form, a nucleus, or a seed, is required upon which the crystal can grow; the process of producing this seed is defined as nucleation. Once the first crystal appears in the solution, a phase change occurs from liquid to solid with further crystal growth. Therefore, nucleation serves as the initial process of freezing, and can be considered as the critical step that results in a complete phase change5.

There are several methods of food freezing, but regardless the method chosen, the main principle behind all freezing processes is the same in terms of process parameters. The International Institute of Refrigeration (IIR) has divided the freezing process into three stages based on major temperature changes in a particular location in the product, as shown in Figures 2 and 3 for pure water and food respectively6.

Beginning with the pre-freezing stage, the food is subjected to the freezing process until the appearance of the first crystal. For pure water, the freezing temperature will be 0 °C and, up to this temperature, there will be a sub-cooling until ice formation begins. In the case of foods during this stage, the temperature decreases to below freezing temperature and, with the formation of the first ice crystal, increases to freezing temperature. The second stage is the freezing period; a phase change occurs, transforming water into ice. For pure water, temperature at this stage is constant; however, it decreases slightly in foods, due to the increasing concentration of solutes in the unfrozen water portion. The last stage starts when the product temperature reaches the point where most freezable water has been converted to ice, and ends when the temperature is reduced to storage temperature7.

Figure 2. Practical definition of the freezing process for pure water6

Figure 3 Practical definition of the freezing process for foods6 (Mallett, 1993)

2.3 Freezing-point of Foods

Freezing point is defined as the temperature at which the first ice crystal appears and the liquid at that temperature is in equilibrium with the solid6. For pure water this temperature is 0 °C (273°K). However, when food systems are frozen, the process becomes more complex due to the existence of both free and bound water. Bound water does not freeze even at very low temperatures. Unfreezable water contains soluble solids, which cause a decrease in the freezing point of water lower than 0 °C. During the freezing process, the concentration of soluble solids increases in the unfrozen water, resulting in a variation in freezing temperature. Therefore, the temperature at which the first ice crystal appears is commonly regarded as the initial freezing temperature.

2.4 Freezing time

The freezing time is one of the most important parameters in the freezing process, defined as time required to lower product temperature from its initial temperature to a given temperature at its thermal center6. The thermal centre is generally taken as a reference because of the temperature differential within the product during the freezing process6. Freezing time depends on several factors, including the initial and final temperatures of the product and the quantity of heat removed, as well as dimensions (especially thickness) and shape of product, heat transfer process, and temperature7.

Calculation of freezing time in food systems is difficult in comparison to pure systems since the freezing temperature changes continuously during the process. However, several empirical equations have been developed to calculate the initial freezing temperature of certain foods as a function of their moisture content8. Plank’s equation (Eq.1) is commonly used for obtaining an approximation of freezing time due to assumptions involved in the calculation. The derivation of the equation starts with the assumption that the product being frozen is initially at freezing temperature. Therefore, the calculated freezing time represents only the freezing period. The equation can be further modified for different geometries including slab, cylinder, and sphere, where for each geometry, the coefficients are arranged in relation to the dimensions as given in Table 19.

Equation 1 - Planks Equation for Freezing Time

where l1 is the latent heat of frozen fraction, k and r are the thermal conductivity and density of the frozen layer, while h is the coefficient of heat transfer by convection to the exterior. Tf denotes the body temperature of the product when introduced into a freezer in which the external temperature is Te .The coefficients R and P given in Table 1 below denotes the geometry of the product frozen. where the letter e denotes the dimension (i.e. for infinite slab geometry, e is thickness of the slab and for infinite cylinder or sphere e is replaced by r which denotes the radius of the clylinder or sphere).

Table 1. Coefficients P and R of Equation 1

It is suggested that the food is usually at a temperature higher than freezing temperature. The real freezing time should therefore be the sum of time calculated from the equation of Plank and the time needed for the product’s surface to decrease from initial temperature to freezing10.

Another attempt to calculate real freezing time (Eq.2), calculates the amount of heat elimination required decreasing a product’s temperature from initial temperature to freezing temperature, as well as the amount of heat released during the phase change and the amount of heat eliminated to reach freezing temperature11.

Equation 2 Nagaoka equation for calculating freezing time

where Ti is the temperature of the food at the initiation of freezing, DH is the difference between the enthalpy of the food at initial temperature and end of freezing. Re and Pl are the dimensionless numbers, while k and h ...

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Equation 2 Nagaoka equation for calculating freezing time

For calculating freezing time of products with irregular shape, a common property of most food products - especially fruits and vegetables - a dimensionless factor has been employed in equations12a,b.

For fruits and vegetables, the amount of energy required for freezing is calculated based on the enthalpy change and the amount of product to be frozen. The following equation is reported by Riedel (1949) for calculation of refrigeration requirements for fruits and vegetables13:

Equation 3 Riedel Equation for calculation of refrigeration requirements

XSNJ: Percentage of the product solids different from juice (Dry matter fraction of the juice)
DHj: Enthalpy change during freezing of the juice fraction
DT: Temperature difference between initial and final temperature of the product

2.5 Freezing rate

The quality of frozen foods is largely dependent on the rate of freezing14. The freezing rate (°C/h) for a product or package is defined as the ratio of difference between initial and final temperature of product to freezing time. At a particular location within the product, a local freezing rate can be defined as the ratio of the difference between the initial temperature and desired temperature to the time elapsed in reaching the given final temperature7.

2.6 Freezing Methods

There are many methods of freezing each depending on the kind of product to be frozen, scale of production, type of packaging, availability and price of refrigerants15a,b. The most commonly methods used are:

1). Air blast freezing (e.g. ventilation, fluidization)

2) Indirect immersion (e.g. plate)

3) Direct immersion (e.g. cryogenic)

In air blast systems, the products to be frozen are placed on metal trays, either in a loose form or in packages. Using the ventilation technique the trays are loaded onto racks and cold air is blown over the foods by large fans. This method is applied most often due to its simplicity adaptability and universality. The freezing cycle lasts from 2 to 40 hours, depending on the type of equipment, product size and kind of packaging. The disadvantage of this method is high unit energy consumption (operation of ventilators, temperature of ammonia evaporation from -40 to –45 C). Also the phenomenon of lumping may be observed in the case of crumbled products1.

Another technique of air blast freezing is the fluidization method. In this method a stream of cold air goes through the whole mass of a frozen layer of loose products placed on a conveyor belt. This avoids lumping, even of dump products. The higher intensity of heat exchange (30-40 times than the ventilation method) shortens the freezing process. The disadvantages of this method are relatively high investment costs and high consumption of power needed to drive compressors and ventilators. In both methods the refrigerating medium is ammonia, used in a closed cycle (minimum losses)1.

In plate freezing the food product (packaged or unpackaged) is brought into contact with metal plates, which are cooled down to the desired temperature by either cold brine or evaporating refrigerants. Operating on the principle of heat transfer by conduction, high values of thermal conductivity and low electric energy consumption are achieved15a,b. The disadvantages of this method are problems with work mechanization: it cannot be adapted to continuous running and it is difficult to apply it in the case of belt-system production. It is employed mainly for fish freezing on factory ships, as well as fish processing on land. However, the method discussed is becoming less and less popular in world refrigeration.

The method of immersion freezing involves direct contact between the food and a cold liquid medium such as cryogenic refrigerants (liquid nitrogen, liquid and solid CO2, liquid air and liquid NO). An immersion apparatus is extremely economical (heat losses up to 7% of the total cold requirement, compared with 20 – 30% observed in ventilation and liquidization tunnels)1.

There are three basic freezing methods:

Immersion – products are put into liquid nitrogen
Spraying – products are in direct contact with nitrogen sprayed on them
Freezing in a stream of cold nitrogen steam1.

Each of those methods has both advantages and disadvantages. The immersion method has been almost completely eliminated, mainly for two reasons:

The freezing process is very quick due to a great temperature difference, which causes micro- and macro- damages of tissues,
The method uses heat of nitrogen evaporation only. Therefore the spraying method is commonly applied where cold nitrogen steam is used for pre-cooling and freezing and the products are then sprayed with liquid nitrogen1.

2.7 The Effect of Freezing on the Structure of Food.

Freezing of food products causes changes in their quality, the effect varies depends on individual products, the rate of freezing and formation of ice crystals1. The most effect in fruit and vegetables and slighter in meat /meat products

2.7.1 The Effect of Freezing Rate on Meat/Meat Products.

Quick frozen meat/meat products exhibit minimal textural damage, release less thaw exudates and undergo less chemical deterioration than those frozen at slow rates15a,b. Slow freezing (at -150 C) of beef (both ground and cut) results to decrease in water holding capacity (WHC) due to the destruction of protein structures by formation of large crystals between the cells.16

Studies by on the ultrastructure of pig’s muscular tissue frozen using the ventilation method revealed muscle fiber breaking, (extensive in some cases), complete disintegration of the sarcoplasmic reticulum and mitochondria as well as glycogen structure ‘blurring’1. Also serious damage and deterioration were seen in the sarcoplasmic basement and nuclear membranes. Large infiltration zones were observed both within and beyond muscle fibers.

However,the ultrastructure of muscles frozen by means of liquid nitrogen showed better preservation of the sacroplasmic reticulum, structure and uninterrupted sequence of the sarcoplasmic and basement membranes. Most infiltration zones were found within muscle fibers, and only few beyond them1.

Figure 4 Structure of Meat

2.7.2 Effect of freezing rate on ice crystal size and location within food structure:

Slow freezing of food results in the production of large ice crystals located exclusively in extracellular spaces. Products ‘slowly freezed’ have a shrunken appearance in the frozen state due to extensive dislocation of water15a. Fast freezing on the other hand results in the formation of small ice crystals located uniformly throughout the product; high rate of freezing minimize the dislocation of water hence the quickly frozen cells give a normal un-shrunken appearance15b.

During cold storage of food, ‘recrystallization’ may occur due to temperature fluctuations –that is bigger ice crystals; distributed between muscle fibers, continue to increase in size, contrary to smaller ones located in fibers16. Some scholars have suggested that the process of recrystallization, shows no differences between the quality of products frozen quickly or slowly. Therefore argue that although quick freezing brings better results as concerns quality, the freezing process is not considered the most important factor among those influencing the final quality of frozen products. Storage conditions and ways of thawing are highlighted as playing a key role.

It is also assumed that the amount of drip during thawing under standard conditions may reflect the degree of tissue structure damage in the freezing process. For example, in the case of strawberries the drip amount may be limited considerably if freezing lasts for 10 – 12 minutes1.

With regards beef, there is only a slight correlation between the drip amount and freezing rate compared with fruits. If the freezing rate is very high (above 10 cm/h), considerable mechanical stresses may appear in the surface layer of the frozen products1, resulting to breaking and damaging of tissue structure. The phenomenon discussed is especially well visible while freezing food products which contain large amounts of water (e.g. tomatoes) and meat elements whose thickness exceeds 10 cm. Hence cryogenic methods of freezing should be quick enough to prevent the occurrence of undesirable cryo-biochemical and microbiological processes, particularly at the initial stage of freezing1.

Food products have therefore been divided into four groups with regards sensitivity to the rate of freezing1:

Products whose quality is not affected by the rate of freezing (green pea, blueberries),
Products not sensitive to changes in the freezing rate, except for the rate lower than 0.3 cm/h (e.g. fish),
Products whose quality improves significantly if the freezing rate increases to the level of 5 – 8 cm/h with the application of liquid gases (berries, portioned meat, some vegetables, mushrooms, forest fruits),
Products, which crack when subjected to very quick freezing – above the level of 10cm/h (animal carcasses, large-size products).

Effect of Freezing Rate on Food Colour

Colour is the feature of food products that is visible and easily evaluated as first indicators of quality change/deterioration24. Very quick freezing (e.g. with the use of liquid gases) causes the formation of tiny ice crystals on the product surface, producing an optical effect known as ‘whitening’17. From the consumers’ perspective, this effect may be both positive, e.g. in the case of poultry, and negative, e.g. in that of red meat. However, the ‘whitening’ of products frozen cryogenically is a physical, reversible phenomenon, which disappears in the process of thawing.

An extreme form of colour changes observed in frozen plant and animal tissues as well as in fresh juices or mushrooms is ‘frost scald’- a particular form of dehydration of the surface parts of frozen products caused by slow freezing and long cold storage.

Freezing by means of liquid gases has a positive influence on the sensory quality of frozen food. Systematic studies conducted on meat products of different composition, frozen applying the ventilation method, liquid nitrogen and carbon dioxide showed that products subjected to very quick freezing were characterized by significantly better quality, palatability and texture, both directly after freezing and after several months of cold storage18. Beef frozen in this way showed a lower degree of fat oxidation, brighter color and better water-holding capacity, compared with ventilation freezing.

2.7.4 Chemical Changes on Fruit and Vegetables during Freezing

Fresh fruits and vegetables, when harvested, continue to undergo chemical changes, which can cause spoilage and deterioration of the product19. This is why these products should be frozen immediately after harvest as possible and at their peak degree of ripeness.

Enzymes contained in the fresh produce causes the loss of color, nutrients and flavor changes in frozen fruits and vegetables. These enzymes must be inactivated to prevent such reactions from taking place. Thus the freezing process for vegetables begins with the blanching process to inactivate the enzymes. Blanching is done by exposing the vegetables to boiling water or steam for a brief period of time. Blanching also helps to destroy microorganisms on the surface of the vegetable and make some vegetables, such as broccoli and spinach, more compact.

The major problem associated with enzymes in fruits is the development of brown colors and loss of vitamin C19. Because fruits are usually served raw, they are not blanched like vegetables. Instead, using chemical compounds, which interfere with deteriorative chemical reactions, controls enzymes in frozen fruit. The most common and effective control chemical is ascorbic acid (vitamin C), which may be used, in its pure form or in commercial mixtures with sugars. Other temporary measures include soaking the fruit in dilute vinegar solutions or coating the fruit with sugar and lemon juice. However, these latter methods do not prevent browning as effectively as treatment with ascorbic acid.

Another group of chemical changes that can take place in frozen products is the development of rancid oxidative flavors through contact of the frozen product with air. Using a wrapping material, which does not permit air to pass into the product, can control this problem.

2.7.5 Textural Changes during Freezing of Fruit and Vegetables

Water makes up over 90 percent of the weight of most fruits and vegetables. This water and other chemical substances are held within the fairly rigid cell walls, which give support structure, and texture to the fruit or vegetable. Freezing fruits and vegetables actually consists of freezing the water contained in the plant cells.

When the water freezes, it expands and the ice crystals cause the cell walls to rupture. Consequently, the texture of the produce, when thawed, will be much softer than it was when raw. This textural difference is especially noticeable in products, which are usually consumed raw. For example, when a frozen tomato is thawed, it becomes mushy and watery. This explains why celery, lettuce, and tomatoes are not usually frozen and is the reason for the suggestion that frozen fruits, usually consumed raw, be served before they have completely thawed. In the partially thawed state, the effect of freezing on the fruit tissue is less noticeable.

Textural changes due to freezing are not as apparent in products which are cooked before eating because cooking also softens cell walls. These changes are also less noticeable in high starch vegetables, such as peas, corn, and lima beans.

2.7.6 Rate of Freezing

Freezing produced as quickly as possible can control the extent of cell wall rupture. In rapid freezing, a large number of small ice crystals are formed. These small ice crystals produce less cell wall rupture than slow freezing which produces only a few large ice crystals.

2.7.7 Changes Caused by Fluctuating Temperature

To maintain top quality, frozen fruits and vegetables should be stored at 0° F or lower. Storing frozen foods at temperatures higher than 0° F increases the rate at which deteriorative reactions can take place and can shorten the shelf life of frozen foods. Fluctuating temperatures in the freezer can cause the migration of water vapor from the product to the surface of the container. This defect is sometimes found in commercially frozen foods, which have been improperly handled.

2.8 Moisture Loss

Moisture loss, or ice crystals evaporating from the surface area of a product, produces freezer burn—a grainy, brownish spot where the tissues become dry and tough. This surface freeze-dried area is very likely to develop off flavors. Packaging in heavyweight, moisture proof wrap will prevent freezer burn.

2.9 Microbial Growth in the Freezer

The freezing process does not actually destroy the microorganisms, which may be present on fruits and vegetables. While blanching destroys some microorganisms sufficient populations are still present to multiply in numbers and cause spoilage of the product when it thaws21. For this reason it is necessary to carefully inspect any frozen products which have accidentally thawed by the freezer going off or the freezer door being left open.

2.10 Nutrient Value of Frozen Foods

Freezing, when properly done, is the method of food preservation, which may potentially preserve the greatest quantity of nutrients22. To maintain top nutritional quality in frozen fruits and vegetables, it is essential to follow directions for pretreatment of the vegetables, to store the frozen product at 0° F and to use it within suggested storage times.

2.11 Storage Times for Frozen Foods and Vegetables

Fruits—Most frozen fruits maintain high quality for 8 to 12 months. Unsweetened fruits lose quality faster than those packed in sugar or sugar syrups.

Vegetables—Most vegetables will maintain high quality for 12 to 18 months at 0° F or lower. However, it is a good idea to plan to use your home frozen vegetables before the next year crop is ready for freezing.

Longer storage of fruits and vegetables than those recommended above will not make the food unfit for use, but will decrease its quality.

2.12 Selecting Freezer Containers

The use good quality freezer container is essential to maintain the quality of frozen fruits and vegetables. A high quality wrap should be both moisture and vapor proof so that moisture can be kept in the product and air kept away from it. They should be strong, pliable and adhere to the shape of the food item and can be sealed easily with heat or freezer tape. Tapes designated for freezer use should be used because other household tapes lose adhesive quality in the extremely cold freezer temperatures. Moisture and vapour resistant wraps include heavyweight aluminum foil, plastic coated freezer paper, saran, and other plastic films. These wraps are not as convenient for fruits and vegetables as plastic bags or rigid freezer containers.

2.13 Methods of Packing Fruits

There are three ways to pack fruits for freezing: sugar, syrup and unsweetened pack. Although some fruits may be packed without sweeteners, the flavor of many fruits is retained well with the use of sugar. Gooseberries, currants, cranberries, blueberries, and rhubarb give good quality packs without or with sugar.

To freeze fruits using sugar pack, the required amount of sugar is sprinkled over the fruit. Gently stirring id done until the pieces are coated with sugar and juice. For the sugar syrup, the amount of sugar needed is dissolved in cold water. The mixture is stirred and left to stand until the solution is clear.

2.14 Methods of Packing Vegetables

Two basic methods for packing vegetables for freezing are:

Dry pack- method used to pack blanched and drained vegetables into containers or freezer bags. The vegetables are tightly packed to cut down on the amount of air in the container. If the vegetables are packed in freezer bags, air is pressed out of the unfilled part of the bag. When packing broccoli, alternate the heads and stems.

Tray pack- method of freezing individual pieces of blanched and drained vegetables on a tray or shallow pan, then packing the frozen pieces into a freezer bag or container. This method produces a product similar to commercially frozen plastic bags of individual vegetable pieces and is particularly good for peas, corn, and beans. In this method it is most important to pack the individually frozen pieces into a bag or container as soon as they are frozen.

3.0 HEATING

According to the legend, man was introduced to the phenomenon of fire by an accidental lightening flash. Since then, he has quickly learned to control and exploit the beneficial effects of heat both as a means of providing warmth and of improving the palatability of his food.

3.1 Baking

In the commercial processing of starch-rich materials, practical regard has to be given to their properties so that the end product meets the need of the consumer. Probably the most advanced cooking techniques of this kind, is employed in the baking industries, using wheat flour to produce, most importantly bread, cakes, and biscuits.

Baking is the method of cooking food in an oven in which dry heat applied evenly throughout the product. The dry heat of baking changes the structures of starches in the food and causes its outer surfaces to brown giving it an attractive appearance and taste while partially sealing in the food’s moisture.

The following is a brief outline of some of the aspects of modern baking practices, with particular reference to the nature and behaviour of the starch granules in the wheat flour.

One of the general features of the baking process is that the predominantly fluid dough or batter is transformed into a predominantly solid baked product.

The properties of the starch granule that are of importance in baking involve:

The extent of granular disorganisation
The surface property of the granule and their modification
The interaction of the starch granules with other components in the baking process.

In summary it will appear that the role of the starch granule in all forms of baked goods is very important.

3.2 The Bread Making Process

Flour, yeast, salt and water are the basic ingredients of all bread products to which may be added supplements (fat, yeast food or ammonium chloride, soya flour, sugar, malt flour, and also gluten), flavouring and nutritional adjuncts (milk, sugar, fat, egg, and dried fruit)25.

Wheat flour contains proteins that harbour special properties, that when hydrated with water and mixed into a dough, they form a three-dimensional, viscoelastic matrix, known as ‘gluten’26. This matrix surrounds small air cells in the mixed dough. As a result of mixing, the air cells expand forming the basis of the characteristic loaf texture. In the protein matrix, there are starch granules, which absorb water contributing to the overall texture and structure. The starch and protein lose their granular characteristics during baking, enabling amylolytic enzymes in the wheat flow to be able to access the dough.

3.3 Structural Changes

Starch – During the baking process, starch gelatinization is probably the most clearly perceptible change in the dough or batter. It also helps in explaining why the viscous dough is transformed into a solid bake. This transformation occurs by:

Swelling, that is the absorption of water by the starch granules and the increase in volume of these granules.
Melting, whereby the crystalline structure of the granules is lost.
Disruption of the starch granules and the exudation of amylose.27

It must be noted that swelling and exudation increases viscosity. In wafers for example the original starch granules are destroyed and even enzymatically degraded28. The cause of these differences is due to the composition of the dough and in the temperature history; in addition, local shear conditions probably affect the later stages of gelatinisation (swelling and rupturing of starch granules due to heat and starch moisture), in which the starch granules are no longer as rigid as they were before. This ‘gelatinization’ phenomenon is of very great importance, for it is this change, which is the essence of the conversion of raw starch to metabolisable carbohydrate, i.e. the cooking process. Gelatinization does not progress far in biscuits but proceeds much further in wafers as it depends strongly on the quantity of water available.

Solubility change is another phenomenon occuring during heating of carbohydrates. Smaller carbohydrate molecules, such as sugars, are much more water-soluble than the larger molecules, such as starch. Heat treatment of starch can give rise to degraded starch products whose solubility in water is much greater whilst still possessing some of the useful properties of their starch precursor. The starch granule is not chemically homogeneous, and can be separated into at least two distinct fractions: amylose and amylopectin. On cooking rice, for example, these two components are released, and the type of rice and the precise conditions of the cooking process determines the ratio of the two, and hence the ‘stickiness’ of the cooked rice grains.

Proteins – Changes in the rheological properties of isolated wet gluten by heating at various temperatures have been investigated by oscillatory measurements. Heating at temeperatures above 60 oC, results in the increase of the storage modulus, which characterizes the elastic properties; the loss modulus, characteristic of the viscous properties, rises to a lesser degree29. These changes in the continuous gluten phase enhance the effect of starch gelatinisation i.e. the transformation of viscous dough into a predominantly elastic material.

Crust – Starch gelatinisation and evaporation of water cause the formation of a hard surface layer, the crust, which obstructs further expansion in the bread baking process.

4.0 CONCLUSION

I believe that freezing has and will continue to be successfully employed for the long-term preservation of many foods, providing a significantly extended shelf life. Freezing changes the physical state of food materials as energy is removed by cooling below freezing temperature. The extreme cold retards the growth of microorganisms and slows down the chemical changes that affect quality or cause food to spoil20.

Competing with new technologies of minimal processing of foods, I would say that industrial freezing is the most satisfactory method for preserving quality during long storage periods. Also when compared in terms of energy use, cost, and product quality, freezing requires the shortest processing time. Any other conventional method of preservation focused on fruits and vegetables, including dehydration and canning, requires less energy when compared with energy consumption in the freezing process and storage. However, when the overall cost is estimated, freezing costs can be kept as low (or lower) as any other method of food preservation22.

Also I feel that one of the largest determinants of the quality of a food is its texture. The textural sensations that are perceived when food is consumed are dependent on the food structure. Therefore safety and nutrition quality of frozen products are emphasized when high quality raw materials are used, good manufacturing practices are employed in the preservation process, and the products are kept in accordance with specified temperatures23.

Generally, rapid freezing results in better quality frozen products when compared with slow freezing. In contrast, if freezing is slow, the crystal growth will be slower with few nucleation sites resulting in larger ice crystals.

The freezing time and freezing rate are the most important parameters in designing freezing systems. The quality of the frozen product is mostly affected by the rate of freezing, while time of freezing is calculated according to the rate of freezing. For industrial applications, they are the most essential parameters in the process when comparing different types of freezing systems and equipment7.

The baking process transforms the predominantly fluid dough or batter into a predominantly solid product. I think that numerous problems in baking remain to be solved or improvements made. Prolongation of the shelf life of bread by preventing staling and mould growth can be explored. Improvements in the efficiency of ovens will no doubt follow from more detailed studies of heat and mass transfer during baking.

I feel that consumer acceptability of baked products and of the manner of their production, together with economic considerations, will be major factors dictating the direction of advances in baking technology.

5.0 REFERENCES

Kondratowicz, J. and Matusevicius, P. (2002). Use of Low Temperatures for Food Prservation: ISN 1392-2130.Veterinarya IR Zootechnika. T. 17 (39).

Ajlouni, S. (1995). Food Chemistry Biology and Nutrition: G.G. Anikumar (ed.), Ingredients Interactions (Effects on Food Quality), Marcel Dekker, Inc.

Delagado, A.E. and Sun, D.W. (2000). Heat and mass transfer for predicting freezing processes, a review. Journal of Food Engineering 47, pp.157-174.

Franks, F. (1985). Biophysics and biochemistry at low temperature, Cambridge University Press, Cambridge, UK, pp.21.

Sahagian, M.E and Goff, H.D. (1996). Effects of freezing rate on thermal, mechanical and physical aging properties of the glassy state in frozen sucrose solutions. Thermochemica Acta, pp. 246-271.

Mallet, C.P. (1993)/ Frozen Food Technology. Chapman and Hall, London, UK.

Persson, P.O and Lohndal, G. (1993) Freezing Technology, Frozen Food Technology (Mallet CP ed), Chapman and Hall, London, UK.

Levy, F. (1979). Enthalpy and specific heat of meat and fish in the freezing range. J Food Technol, 14:549.

Plank, R. (1980). El Empleo del Frenla Industria de la Alimentacion, Barcelona: R Everte.

Barbosa-Canovas, G.V and Ibarz, A. (2002). Unit Operations in Food Engineering. CRC Press, BoCa Raton, FL, USA.

Nagoaka, J., Takigi, S. and Hotani, S. (1995). Experiments on the freezing of fish in air-blast freezer. Proc.9th Congr. Refrig. Paris France.

12a. Clealand, D. J and Clealand, A.C (1987a). Prediction of freezing and thawing times for multi-dimensional shapes by simple formulae. Part:1 Regular shapes. International Journal of Refrigeration. 10, pp.156-164.

12b. Clealand, D.J. and Clealand, A.C. (1987b). Prediction of freezing and thawing times for multi dimensional shapes by simple formulae. Part:2 Irregular shapes. International Journal of Refrigeration. 10, pp.234-240.

13. Riedal, V. L. (1949). Chemical Engineering Technology. 21, pp. 340.

14. Ramaswamy, H. S and Tung, M. A (1984). A review on predicting freezing times of foods. Journal of food Process Engineering. 7, pp. 169 -203.

15a. Fennema, O. (1996). An overall view of low temperature food preservation. Cryobiology 3:197 213.

15b. Fennema, O., (1968). General Principles of cryogenic processing. Proc. Meat Ind. Res. Conf. p.109.

16. Hoard, N. F. (1995). Food as cellular systems impact on quality and preservation. A review. J. Food Biochem. 1995. N 19. p.191-238.

17. Czapski J. Rola i kształtowanie barwy produktów spo ywczych w: Food product development – opracowanie nowych produktów ywnościowych. Wyd. AR Poznań 1995.

18. Kondratowicz J., Bąk T. Meller Z. Effect of enrichment and different methods of freezing on the weight losses and different methods of freezing on the weight losses and taste qualities of horsemeat during cold storage. Pol. J. Food Natur. Sci. 1999. N 2 P. 185-193.

19. Schafer, W., and Munson, S. T. (1990) Structural Changes in Fruit and Vegetables. (www.extension.umn.edu/topics.html?topic=1 accessed on 16/03/2007).

20. George, R.M. (1993). Freezing process used in food industry. Trends in food Science and Technology. 4, pp. 134.

21. Arthey, D. (1993). Freezing of vegetables and fruits. In: Mallet, C. P. ed., Frozen Food Technology Chapman and Hall, London, UK.

22. Harris, R. S. and Kramer, E. (1975) Nutrition Evaluation of food Processing, 2nd ed. Avi Publishing Co. Westport, USA.

23. Fennema, O. (1997). Loss of vitamins in fresh and frozen foods. Food Technology. 12:32-38.

24. Rahman, M. S. (1999). Handbook of Food Preservation, Marcel Dekker, NY, USA.

25. Spicer, A. (ed.) (1975) In: Bread. London: Applied Science.

26. Mcweeny, D. J. (1973) In: Molecular Structure and Function of Food Carbohydrate, eds. Birch, G. G. &Green, L. F., p.21. London: Applied Science.

27. Anderson, R. A. (1967) In: Starch: Chemistry &Technology, eds. Whistler, R. L. & Paschall, E. F. Vol II, p.53. News York: Academic Press.

28. C. T. Greenwood, in ‘Advances in Cereal Science and Technology’, ed. Y. Pomeranz, American Association of Cereal Chemists, St. Paul, MN 1976, p.119.

29. G. A. LeGrys, M.R. Booth, and S. M. Al-Baghdadi, in ‘Cereals, a Renewable Resource, Theory and Practice’, ed. Y. Pomeranz and L. Munck, American Association of Cereal Chemists, St. Paul, MN, 1981, p.243.