Freezing is one of the most popular technologies over the past two decades which uses for food preservation. Because this method has better functional attributes than any other techniques, especially taste, texture, and nutritional preservation, the popularity of freezing process grows continuously in the food industry. The initial purpose of freezing process is to maintain the quality of product in the long term. As Mallett (1994) has demonstrated, freezing is a process that removes heat from food until the temperature being below its freezing point. Owing to low temperature, liquid water in food transforms to solid ice. Since liquid water is a crucial factor of microbiological and chemical activities, limited water in cell can minimize the reaction rate in products which is the primary cause of food deterioration. The storage temperature of frozen food is generally colder than -10Â°C or -18Â°C which extends the shelf-life of product more than a year.
1.1 The History of Freezing Technology
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The usage of freezing to preserve food has been found for thousand years ago despite the differentiation between natural and artificial refrigeration being unclear. Nevertheless, in 1755 William Cullen, the Scottish professor, invented the first artificial equipment which used for make ice. Then, the invention of the first refrigerator occurred in the next 100 years and a large scale of freezing was first introduced in 1870s. Sixty years later, fish and butter became the main frozen products. The modern frozen food industry began in 1928 when Clarence Birdseye, the American, produced the double-belt contact freezer. After that, he also developed the multi-plate freezer which is still use in the industry. Another crucial development of food freezing apparatus is individual quick freezing (IQF), which made several modern equipments were introduced during the early 1950s, for example, fluidized bed freezer, and cryogenic freezer.
1.2 The transformation of freezing
Nucleation is the establishment of tiny ice nucleus which is the foundation of crystal growth. This state can be divided into two types which are homogeneous and heterogeneous nucleation. The homogeneous nucleation happens in pure substance, such as pure water, which is the only interesting example in academic research. The heterogeneous nucleation is simply found in frozen foods (Ahmed et al., 2010).
Crystal growth is always followed by nucleation and start at the temperature lower than freezing point. This process is an increase in the size of ice nucleus. It is interestingly to note that the lower the temperature is, the higher the crystal growth rate is. However, the nucleation rate is more than the rate of crystal growth at low temperature leading to the large number of small nuclei (Ahmed et al., 2010).
Generally, the beginning process of freezing products is a decrease in the food's temperature to its freezing point which called the precooling period, which shows in the freezing curve from A to B (fig. 1). This is followed by a continuous drop in the temperature below the freezing point at B without a phase change named supercooling or undercooling state. This state is considered as the initiation of ice nucleation. After supercooling, the temperature increases to the freezing point (C) followed by ice crystallization. The ice crystallization has two phases which are nucleation and crystal growth, which can occurs at the same period. The formation of ice nuclei begin at B, after the supercooling state, with the removal of the latent heat resulting in a rise in the temperature to the food's freezing point (C). Subsequently, the crystal growth is found between C and D which is a period that the temperature remains almost unchanged with an increase in the amount of ice. When the crystallization finishes, the temperature drops to E which is as same as the freezing temperature because of the sensible heat removal.
1.3 Freezing and thawing of plant tissues
Mechanisms of freezing
Generally, plant tissues have cell walls and cell membranes which act as ice barriers. During freezing process, ice forming and propagation will be found only external cells because of cell walls. While heat is removed continuously, the external ice will reach equilibrium situation resulting in an increase in solute concentration which causes osmotic gradient between unfrozen internal and external component to increase. Consequently, in order to achieve equilibrium, water in cell, which has less concentration, will migrate through cell walls and cell membranes to external matrix. The main factor which has influence on this is heat removal rate (Mallett, 1994).
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Slow freezing rate
Owing to the slow rate of heat removal, the unfrozen matrix concentration also changes gradually which leads to the high rate of water migration from internal cells to form external ice crystals. As a result of water lost, the cell will have dehydration and volume reduction problems which damage cell's structures. Furthermore, slow cooling also lead to the small proportion of ice crystals with larger size which cause the low quality of frozen food (ibid)
Fast freezing rate
On account of the unfrozen solution matrix concentration varying rapidly, this contributes to the slow rate of internal water migration. Therefore, internal cell's solution will be frozen in some critical low temperature. According to Mallett (1994), "The more rapid the freezing process, the better the texture and flavor quality". Besides, rapid cooling brings about the higher number of ice crystals with smaller size which is less damage to plant cells.
2. The Problem of Frozen Fruits and Vegetables
2.1 Browning Reactions
The main reason of color changes in frozen fruits and vegetables is browning reaction which is a result of oxidation reaction.
"Enzymatic browning is due to polyphenol oxidase reacting directly with oxygen from the air. Most fruits contain polyphenolic substrates, which in the presence of enzymes and oxygen are first changed into quinines and later into brown pigments." (Jeremiah, 1995)
The solution to browning reaction
As Jeremiah (1995) states that the most effective method to deal with enzymatic browning is blanching process since polyphenol oxidase, which is the primary cause of darkening of the color in some fruits and vegetables, is high sensitive to heat. There are three enzymes in fruits and vegetables used for the determination of end-point of blanching which are peroxidase, catalase and lipoxygenase. Owing to these substances are heat stable enzymes, their destruction can imply that other enzymes are also completely inactivated (Mallett, 1994). Although heating the product in order to overcome enzymatic browning is an essential factor in vegetable freezing production, only some fruits, such as apples, pears, peaches and apricots need blanching. This is because blanched fruits may lose freshness property (Jeremiah, 1995).
The purposes of blanching are the destruction of microorganisms, crop cleaning, gas expulsion which is important for canning, crop's color fixing and the inactivation of enzymes (Mallett, 1994). In addition, water is generally used as blanching medium whose common temperature is between 75Â°C and 90Â°C for 1 and 10 minutes resting on the type and size of vegetables. However, the drawback of hot-water blanching is heavy leaching losses. Therefore, steam blanching which is an alternative medium for delicate crops is recommended.
The usage of cryoprotectants
As Erickson and Hung (1997) explained, "Cryoprotectants are compounds that improve the quality of frozen food. The term of cryoprotectant includes all compounds that help to prevent deleterious changes in foods caused by freezing and thawing processes or freezing storage"
Adding sugars and syrups are the crucial pretreatment in frozen fruits. The primary function of these sweeteners is oxygen exclusion which brings about color and appearance preservation and browning prevention. The addition of sugars and syrups behave like a water withdrawer causing plant cells to have higher concentration of solution. Moreover, the high concentration of solution in cell also reduces the freezing point which leads to a decrease in freezing in plant cells. The advantages of this is the fewer the ice crystals do not form in cells, the less the cell's structure are damaged. Therefore, the cell membranes are not destroyed which results in less opportunity for enzymes mixing substrates. Consequently, frozen fruits which dip in sugar before freezing will have natural flavor, odor and color. Nevertheless, this technique also has some problems. According to research, the appropriate percentage of sugar usage is approximately 30-60%. If the amount of sugar adding to products is insufficient, the internal issues will be damaged. This is because the middle of products is the slowest part of ice crystal formation. Besides, syrups are considered to be better choice than dry sugar because liquid can permeate to the center of products as long as the cells are not frozen. To produce the high quality of frozen fruits, manufacturers should add dry sugar for 1-2 hours before freezing which leads to more sugar transference to the internal tissues (Jeremiah, 1995).
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Moreover, the addition of ascorbic acid is also used with sweeteners to preserve the quality of frozen fruits. Ascorbic acid acts as a reducing agent which react with the phenolic substances, the intermediate chemical occurring in browning reaction, thus using this cryoprotectant can protect color deterioration in fruits.
2.2 Moisture Migration and Ice Recrystallization in Frozen Foods
Moisture migration is one of the main influences on properties of frozen foods. This results from temperature fluctuation which always occurs during storage. According to Erickson & Hung (1997), there are many forms of moisture migration which are moisture loss by sublimation, moisture absorption and redistribution in foods or food components, recrystallization of ice and drip loss during thawing.
The effects of moisture migration
Frozen food quality
Firstly, the migration of water causes frozen foods to desiccate, especially meat products leading to a decrease in the storage life of food. For instance, desiccation is the main reason for the shortening of the shelf life of frozen lamb. Owing to this, unpleasant odors which are unacceptable to customers are produced in this product (Winger (1984) in Erickson & Hung (1997)). Another result of moisture migration is freezer burn. As Jeremiah (1995) has demonstrated, freezer burn is the patches of light-colored tissue on the surface of frozen meat. On account of ice evaporation, tiny cavities are produced between meat fibers contributing to unattractive appearances. Moreover, if moisture loss occurs in wrapped frozen food, ice crystals may form inside the package. Besides, this incidence can also be seen during thawing which causes drip loss (Erickson & Hung, 1997).