Dairy and Food Engineering

Lesson 28. Changes undergone by the food components during Freezing

28.2 EFFECT OF FREEZING ON PRINCIPAL CONSTITUENTS OF FOODS

The effect of freezing on the food components is diverse, and some components are affected more than others. For example, protein can be irreversibly denatured by freezing, whereas carbohydrates are generally more stable. Other common chemical changes that can proceed during freezing and frozen storage are lipid oxidation, enzymatic browning, flavour deterioration, and the degradation of pigments and vitamins. The main goal of the freezing process is to extend the shelf life of a raw material or product beyond that achievable at temperatures above the initial freezing point of the material. Therefore, it is important to understand the modifications that can occur during freezing in food components and that can further lead to quality degradation. This chapter focuses on chemical and biochemical reactions that affect the quality of frozen food systems. These reactions and specific examples in food are summarized in Table 28.1.

Changes that occur in foods during freezing, storage and thawing can be both chemical and physical in nature. Various chemical, enzymatic and physical changes are promoted as a result of the concentration of components (concentration effects) in the unfrozen water phase within the frozen foods. For example:

• Chemical changes such as oxidative rancidity or oxidation of flavour components, pigments and vitamins.

• Enzymatic reactions such as enzymatic browning or lipolytic rancidity.

• Meats become tougher due to protein denaturation by chemical effects and cell breakage by ice crystals

In freezing foods, the objective is to promote the formation of tiny ice crystals rather than the formation of fewer but larger ice crystals that cause cellular damage. Ice crystal damage can lead to loss of water from the food product once it is thawed. The drip that is found in thawed strawberries or beef is due in part to ice crystal damage to the cells, leading to leakage of cellular fluids into extracellular spaces, and to the loss of water-holding capacity of food components as a result of concentration effects. 

Other undesirable changes include formation of package ice and freeze dehydration which is popularly called freezer burn and can produce unsightly food surfaces and loss of nutrients. "Freezer burn" is a misnomer since the food does not "burn" in the freezer but rather takes on an appearance of having been burned because of the moisture loss that occurs during this freeze dehydration. 

28.2.1 WATER

Water is an essential constituent of most foods. It is present in a very wide range, varying, for example, from 4% in milk powder up to 95% in tomato and lettuce. Water may exist as an intracellular or extracellular component in vegetable and animal products, as a dispersing medium or solvent in a variety of products, as the dispersed phase in some emulsified products such as butter and margarine, or as a minor constituent in other foods. The conversion of water into ice during freezing has the advantage of fixing the tissue structure and separating the water fraction in the form of ice crystals in such a way that water is not available as a solvent or cannot take part in deterioration reactions. On the other hand, ice crystals formed during freezing can affect quality parameters such as color, texture, and flavor. Meanwhile, in the remaining unfrozen portion, the concentration of dissolved substances increases, while the water activity of a product decreases. Usually, this part of water is non-freezable and, therefore, not available for chemical reactions or as plasticizers. The water that does not freeze is normally considered to be the critical water content above which deteriorative changes may occur. Critical water is a rather unusual substance having high boiling and low freezing points, high specific heat, high latent heats of fusion and vaporization, high surface tension, high polarity, and unusual density changes. The considerable difference in the densities of water and ice may result in structural damage to foods when they are frozen, being more likely in plant tissue with its rigid structure and poorly aligned cells than in muscle with its pliable consistency and the parallel arrangement of cells.

Table-28.1: Chemical Reaction of Food Components during Freezing that Affect Food Quality

Food Components	Mechanism of degradation	Effect on quality	Studies in food
Protein	Denaturation	Degradation of texture and functional properties	Toughening and functional changes, particularly loss of protein solubility in fish,  Loss of protein solubility, emulsifying capacity  Loss of water holding capacity of meat for processing
Lipid	Hydrolysis	Release of FFA—known to contribute to unpleasant flavours	Short chain FFA giving rancid odour in dairy product  Medium chain FFA caused a ‘‘sweaty’’ flavour in mutton  Toughening of muscles in frozen Indian sardine
	Oxidation Autoxidation Enzymatic oxidation	Unstable hydroperoxides break down to reactive compounds and interact with other components to produce off flavours, discoloration, and toughening of muscle protein	Off-flavours shelled oysters Detection of rancid flavour in silver pomfret
Carbohydrates	Hydrolysis	Increases the amount of smaller molecular weight components—leads to lower melting temperatures   Change of texture	Sucrose hydrolysis         Firmness of ice cream decreased as hydrolysis progressed
Colour pigments   (a)  Chlorophyll	Pheophytinization	Green chlorophyll forms olivebrown pheophytin in the presence of acid or heat	Greenness in Brussels sprouts decreased Stability of green colour in kiwifruit Frozen blanched spinach had a higher amount of pheophytin than fresh
(b) Anthocyanin	Enzymatic reaction               Structure of anthocyanin depends on pH value	Glucosidase hydrolyses glycosidic linkages and produces sugars and aglycone compounds     Depending on the pH of the food, different forms of anthocyanin exist, usually from red to blue as pH increases	Loss of anthocyanin in raspberry in the late cultivar was more severe than the early cultivars     Red colour hue of sour cherry weakened during frozen storage
(c) Carotenoids	Oxidation	The loss of pigments causes fading of colour and loss of nutritive value	Loss of carotenoid in salmon
Flavour compounds	Enzymatic degradation           Lipid oxidation           Leaching of components during the blanching process         Effect of heat	Change of flavour profile           Produces off-flavours         Weakens and changes the sensory Perception	Change in the composition of aromatic compounds of strawberries Change in aroma profile of frozen green peas   Green and fatty off-flavour notes in frozen trout due to breakdown products of unsaturated FFA   Decrease of organic acids in green beans and Padron peppers  Change in volatile composition of guava   Effect of heat Changes in concentration of odorants during heat treatment
Micronutrients (a)  Vitamins             (b) Minerals	Oxidation             Generally stable         Loss mainly through leaching	Loss of nutritional value because of the loss of vitamins	Loss of vitamins C and B6 due to blanching in french fries Oxidation of AA in peas, lima beans, corn and green beans  Unchanged mineral composition in artichokes, green beans, and peas after freezing  Mineral content of boiled fresh vegetables was not different from frozen vegetables

When water freezes at atmospheric pressure, it expands nearly 9%. The degree of expansion varies considerably owing to the following factors:

• Moisture content: Higher moisture contents produce greater changes in volume.

• Cell arrangement: Intercellular air spaces, which are common in plant tissue. These spaces can probably accommodate growing crystals, and thereby minimize changes in the specimen exterior dimensions; for example, whole strawberries increase in volume by 3%, whereas coarsely ground strawberries increase by 8.2%, when both are frozen to −20°C

• Concentration of solutes: High concentrations reduce the freezing point and do not freeze or expand at commercial freezing temperatures.

• Freezer temperature: It determines the amount of unfrozen water and hence the degree of expansion.

• Crystallized components: It includes ice, fats, and solutes, which contract when they are cooled; this reduces the volume of food.

28.2.2 PROTEINS

Proteins may undergo changes during freezing and frozen storage, primarily because of denaturation. Denaturation can be defined as a major change in the native structure that does not involve alteration of the amino acid sequences and usually involves the loss of biological activity and significant changes in some physical or functional properties such as solubility. Oxidative processes during storage can also contribute to protein denaturation; oxidizing agents (e.g., enzymes, transition metals) can react with proteins via lipid and nonlipid radicals.  For example, the addition of malonaldehyde, a commonly occurring product of lipid oxidation, to trout myosin solutions during storage at −4°C was found to accelerate protein denaturation. Fish protein is particularly sensitive to denaturation where the protein develops cross links between adjacent protein molecules that effectively stop the thawed fish protein to reabsorb water to recreate the pre-frozen gel structure. This denatured protein has a much tougher and rubbery texture than the native protein. The textural changes that occur in fish proteins have been attributed to changes in the myofibrils. The rate at which fish or beef muscle is frozen also influences the degree of protein denaturation. Although rapid freezing generally results in less denaturation than slower freezing, intermediate freezing rates can be more detrimental than slow freezing, as judged by textural changes and the solubility of actomyosin. For example, cod fillets frozen at intermediate rates developed intracellular ice crystals large enough to damage the cellular membranes.

Freezing and frozen storage do not significantly affect the nutritional value of meat and fish proteins. However, on thawing frozen meat and fish, substantial amounts of intra- and extracellular fluids and their associated water-soluble proteins and other nutrients may be lost (the so-called drip loss). The volume of drip loss on thawing of meat and fish is highly variable, usually of the order of 2%–10% of net weight; however, in exceptional circumstances, up to 15% of the weight of the product may be lost. Nevertheless, it was observed for fish that if the product is stored for an appropriate short time and at a sufficiently low temperature, the subsequently thawed fish would rehydrate with the protein returning to its original gel condition. The caseinate micelles of milk, which are quite stable to heat, may also be destabilized by freezing. On frozen storage of milk, the stability of caseinate progressively decreases and this may lead to complete coagulation. Enzymes have also been linked to protein denaturation, as it is known that low temperature decreases the activity of enzymes in tissue, but does not inactivate them.

28.2.3 LIPIDS

Lipids in food exhibit unique physical and chemical properties. Their compositions, crystalline structure, melting properties, and ability to associate with water and other nonlipid molecules are especially important to their functional properties in many foods. During processing, storage, and handling of foods, lipids undergo complex chemical changes and react with other food constituents, producing numerous compounds both desirable and deleterious to food quality. The process of auto oxidation and the resulting deterioration in flavor of fats and fatty foods are often described by the term rancidity. In particular, the unsaturated bonds present in all fats and oils represent active centers that, among other things, may react with oxygen. This reaction leads to the formation of primary, secondary, and tertiary oxidation products that may make the fats or fat-containing foods unsuitable for consumption.

Lipids can degrade in frozen systems by means of hydrolysis and oxidation. Lipid oxidation is indeed one of the major causes of food spoilage. It is of great economic concern to the food industry because it leads to the development of various off flavors and off-odors. In addition, oxidative reactions can decrease the nutritional quality of foods. Lipids in foods can be oxidized by both enzymatic and non enzymatic mechanisms. One of the enzyme that is considered important in lipid oxidation is lipoxygenase, which has recognition for its off-flavor development in vegetable. Lipoxygenase is the main enzyme responsible for pigment bleaching and off-odors in frozen vegetables; if the enzyme is not inactivated before freezing by blanching, it can generate offensive flavors and loss of pigment color. At temperatures below −10°C, both enzymatic and non enzymatic reactions associated with lipid oxidation are decelerated. However, in the range from 0°C to −10°C, decreased oxidative stabilities have been noted. Unless the rate is very slow, the rate of freezing has been found to have little influence on the oxidative stability of frozen products. Instead, storage temperatures play a dominant role in dictating the stability of food products, including muscle foods. The order of time/temperature holding treatments, on the other hand, markedly influences the development of rancidity. The hydrolysis of lipids or lipolysis results in the release of free fatty acids. Freezing can facilitate lipid oxidation, partly because the competing reactions of microbiological spoilage are avoided and partly because of the concentration effects. Thus, lipid oxidation is relatively more important in frozen muscle tissue than in fresh tissue. Lipid degradation can be reduced in frozen foods by lowering the storage temperature, excluding oxygen (e.g., use of vacuum packaging), adding antioxidants (e.g., butylated hydroxytoluene or BHT as well as natural vitamin E), and supplementing the diet of animals with antioxidants.

28.2.4 VITAMINS

Freezing is considered as one of the best food preservation methods when judged on the basis of nutrients retention. However, it is well known that significant amounts of some vitamins can be lost from processing prior to freezing (e.g., peeling and trimming, leaching especially during blanching), chemical degradation, and thawing. The stability of vitamins in foods is generally influenced by pH and the presence of oxygen, light, metals, reducing agents, and heat. It has been reported that for some frozen foods such as strawberries, the total and biologically active ascorbic acid remain at essentially the same level for a year or longer if the foods are stored below −18°C, although vitamin C losses have also been found to occur at temperatures as low as −23°C. The conversion to the partially active dehydroascorbic acid and the totally inactive 2,3-diketogulonic acid increases with increasing storage temperature; complete conversion practically occurs in 8 months at −10°C and in less than 2 months at −2°C. Such findings were instrumental in establishing −18°C as the upper limit for frozen food storage and for using biologically active ascorbic acid as a general indicator of quality deterioration during frozen storage. For peaches and boysenberries, a 10°C rise in the temperature from −18°C to −7°C caused the rate of vitamin C degradation to increase by a factor of 30–70. Vitamin C and thiamine (vitamin B1) have been studied extensively since they are water soluble, highly susceptible to chemical degradation, and present in many foods; they are also required in the diet and are sometimes deficient in the diet. Therefore, it is generally assumed that if these vitamins are retained, all other nutrients would also be well retained.

28.2.5 CARBOHYDRATES AND MINERALS

Carbohydrates occur in plant and animal tissues in many different forms and levels. In animal organisms, the main sugar is glucose and the storage carbohydrate is glycogen; in milk, it is almost exclusively the disaccharide lactose. In plant organisms, approximately 75% of the solid matter is carbohydrate. The total carbohydrate content can be as low as 2% of the fresh weight in some fruits or nuts, more than 30% in starchy vegetables, and over 60% in some pulses and cereals. In plants, the storage carbohydrate is starch, while the structural polysaccharide is cellulose. The nutritive value of carbohydrates is not significantly affected during handling of fresh foods and the subsequent processing and distribution of frozen foods. In general, carbohydrates are susceptible to hydrolysis during frozen storage, which can still occur at temperatures as low as −22°C. Like B vitamins and proteins, carbohydrates are less affected by process and more by loss through drip following a freeze–thaw cycle. Sugar hydrolysis increases the number of solutes in the food matrix, resulting in a reduction in the amount of ice in the product, which may alter certain physical properties; for example, the firmness of ice cream was found to inversely relate to the degree of hydrolysis. Blanching and freezing can cause changes in texture and the pectic composition of certain foods. Both treatments produce a gradual breakdown in the protoplasmic structure organization, with a subsequent loss of turgor pressure, release of pectic substances, and final softening effect.

Minerals present in any form (e.g., chemical compounds, molecular complexes, and free ions) can dramatically affect the color, texture, flavor, and stability of foods. Minerals are chemically stable under typical conditions of handling and processing, and nutrient losses are negligible, provided that losses by physical means (e.g., leaching) are avoided. Nevertheless, no changes were observed in six mineral elements (Ca, Cu, Mg, Mn, Ni, and Zn) between fresh and frozen artichokes, green beans, and peas; boiled fresh vegetables and boiled frozen vegetables also exhibited similar mineral contents.

28.3 CONCLUSION

Freezing is complex process involving physical and chemical changes that might greatly affect the food quality. Further to minimize the changes in food components during freezing it is imperative to to freeze a product quickly to –18°C and store it at the same temperature throughout the cold chain.