What are the Effects of Freezing and Frozen Storage on Food Quality?

Freezing Food Guide

According to a study, an American consumes on average 71 frozen foods a year, most of which are pre-cooked frozen meals. Freezing food preserves it from the time it is prepared to the time it is eaten. Since early times, farmers, fishermen, and trappers have preserved their game and produce in unheated buildings during the winter season. Freezing food slows down decomposition by turning residual moisture into ice, inhibiting the growth of most bacterial species. In the food commodity industry, there are two processes: mechanical and cryogenic (or flash freezing). The freezing kinetics is important to preserve the food quality and texture. Quicker freezing generates smaller ice crystals and maintains cellular structure. Cryogenic freezing is the quickest freezing technology available due to the ultra low liquid nitrogen temperature (-196 °C).

The ice crystallization that occurs during freezing processes, and along frozen storage, causes the major important physic and chemical modifications which decrease food quality. Freezing, or Solidification, is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point.

PHYSICAL CHANGES ON FROZEN FOOD

The main physical changes of foods verified during freezing processes are related to the risk of freeze cracking, moisture migration, recrystallization of ice crystals and drip loss during thawing.

1) Freeze Cracking

The small ice crystals formed with high freezing rates obtained with cryogenic freezers, allow preservation of food structure. However, products may crack under those conditions. This may happen when the internal stress of unfrozen food is higher than the frozen material strength at food surface. To avoid cracking, a previous cooling step should be applied prior to freezing. The reduction of the temperature gradients between the product and the freezing medium or a pre-cooling step decrease significantly the risk of freeze cracking.

2) Moisture Migration

During freezing processes, when cell contents are super cooled, moisture movements may occur by an osmotic mechanism. The occurrence of temperature fluctuations results in vapor pressure differences, which are responsible for moisture migration. If frozen products are stored without an adequate moisture barrier, the ice on the food surface sublimes, since ice water pressure is higher than the environment vapor pressure. An opaque dehydrated surface is formed (microscopic cavities previously occupied by ice crystals) with an unsightly white color. This leads to freezer burn. Learn how to How to Freeze Your Food Without Getting Freezer Burn?

If temperature increases, water moves from the product; ice sublimes and water diffuses through the packaging film. If temperature decreases, the ice on the wrap tends to diffuse back to the food surface, however, the water reabsorption to the original location is very improbable. To reduce moisture migration, temperature fluctuations and internal temperature gradients should be minimized and internal barriers within the product and within the packaging should be included.

3) Recrystallization

Modifications in the size, shape or orientation of the ice crystals are known as "recrystallization" and usually lead to quality losses in some products. Recrystallization reduces the advantages of fast freezing leading to physico-chemical changes of food products. This process may happen in three different ways:

(i) changes in surface shape or internal structure (isomass recrystallization);

(ii) linkage of two adjacent ice crystals to form a large crystal (accretive recrystallization), and

(iii) increase of the average size of the crystal (migratory recrystallization). Migratory recrystallization is the most important and it is mainly related to temperature fluctuations during storage. If temperature increases, the product's surface warms slightly, the ice crystals melt, moisture moves to regions of lower vapor pressure and some areas will be dehydrated. When temperature decreases, water vapor does not form new nuclei points and links to the existing ice crystals. This originates a reduction of the number of small crystals and an increase of large crystals, disrupting the cellular structure. The recrystallization during storage and transportation may lead to freeze-dried packaged product or to toughening of animal tissue.

4) Drip Loss

During ice formation, water is removed from the original location. However, during thawing, water may not be reabsorbed in the same regions, and usually drip loss is observed. Size and location of ice crystals, rate of thawing, the extent of water reabsorption, the status of the tissue before freezing, and the water-holding capacity of the tissue have a great influence on drip losses. The time required for thawing should be longer than the one used for freezing (for comparable temperature driving forces). In frozen meats, a slow thawing process at low temperatures will permit a better water diffusion in the thawed tissue and its relocation in the fibers. In vegetable tissues, the water is not reabsorbed.

CHEMICAL CHANGES ON FROZEN FOOD

During freezing, changes in temperature and concentration (due to ice formation) play an important role in enzymatic and nonenzymatic reactions rates. Ice crystals may release the enclosed contents of food tissues, such as enzymes and chemical substances, affecting the product quality during freezing and frozen storage. The main chemical changes verified during freezing and frozen storage are related with lipid oxidation, protein denaturation, enzymatic browning, and degradation of pigments and vitamins.

1) Impacts on Food Texture, Color and Flavor

Lipid oxidation and protein denaturation are the major important causes of quality loss in frozen meat and fish. Flavor, appearance, nutritional content and protein functionality are usually degraded by lipid oxidation. The solutes concentration during freezing processes catalyzes the initiation of oxidative reactions, which disrupt and dehydrate cell membranes, exposing membrane phospholipids to the oxidation process.

Food products stored in contact with air, mainly fish and poultry that have significant amounts of polyunsaturated fatty acids, also are susceptible to oxidation. The decomposition of hydroperoxides of fatty acids in aldehydes and ketones results in the formation of volatile compounds that gives rise to the aroma and taste characterized as "rancid". Lipid oxidation also has an impact in terms of pigment degradation and color quality deterioration of the products. Freezing and thawing accelerate pigment oxidation. For example, the metmyoglobin formation in red meats (brown color) and the carotenoid bleaching in fish and poultry favor parallel fat oxidation. Hydrolytic rancidity, textural softening, and color loss are also direct consequences of hydrolytic enzyme activities, which can be inactivated by heat.

In relation to fruits and vegetables, the ice crystals formation leads to undesirable losses in texture, such as loss of turgor during thawing. The semi-rigid nature of the cells and the less orderly packaging of the cells are mainly responsible for the textural damage observed in frozen/thawed fruits and vegetables. Low storage temperatures and slow thawing should be guaranteed to minimize losses of membrane semi-permeability and cellular disruption. Also to avoid tissues softening, pretreatments can be applied. The most important chemical changes verified in frozen products are associated with the reactions that produce off-odors and off-flavors, pigment degradation, enzymatic browning and autoxidation of ascorbic acid.

Water that does not freeze, even at very low temperatures, is responsible for deteriorative and enzymatic  reactions, particularly during frozen storage. In non-blanched products, enzymatic oxidation of phenolic compounds by polyphenoloxidase, leads to discoloration (browning) of food products. However, ascorbic acid can be introduced as an inhibitor of enzymatic reactions. The salts precipitation in concentrated solutions conduces to changes of anthocyanins color. During frozen storage of green blanched products, chlorophylls and carotenes are also degraded, the rate of pigment degradation being dependent on the extent of tissues damaged prior to freezing. The action of lipases and lipoxygenases leads to flavor alterations due to the accumulation of volatile compounds (carbonyl compounds and ethanol) in vegetable tissues.

2) Impact on Nutritional Quality of Frozen Food

Commonly, freezing is considered less destructive than any other preservation process and frozen products have a nutritional quality comparable to fresh products. Several unsaturated fatty acids (nutritionally essential or beneficial) are one of the major substrates for lipid oxidation, but the losses are not limiting in most of the frozen foods. Protein denaturation is mainly due to ice crystals formation and recrystallization, dehydration, solutes concentration and oxidation. Thus, several losses in protein functionality are reported in frozen fish, meat, poultry and egg products, and some texture deterioration in frozen muscle tissues may be attributed to protein damage. However, protein denaturation in frozen products is considered minimal when compared to the total available protein.

In terms of nutritional value, vitamins (essentially B and C) are the compounds that suffer a major negative impact with freezing and frozen storage conditions. Ascorbic acid losses are attributed to oxidative mechanisms during frozen storage. Blanching also affects negatively this quality indicator and the rates of deterioration are extremely slow when compared to ambient or chilled storage.

Vitamin Content of Frozen Foods

Vitamin C: Usually lost in a higher concentration than any other vitamin. A study was performed on peas to determine the cause of vitamin C loss. A vitamin loss of ten percent occurred during the blanching phase with the rest of the loss occurring during the cooling and washing stages. The vitamin loss was not actually accredited to the freezing process. Another experiment was performed involving peas and lima beans. Frozen and canned vegetables were both used in the experiment. The frozen vegetables were stored at −10 °F (−23 °C) and the canned vegetables were stored at room temperature (75 °F). After 0, 3, 6, and 12 months of storage, the vegetables were analyzed with and without cooking. O'Hara, the scientist performing the experiment said, "From the view point of the vitamin content of the two vegetables when they were ready for the plate of the consumer, there did not appear to be any marked advantages attributable to method of preservation, frozen storage, processed in a tin, or processed in glass."

Vitamin B1 (Thiamin): A vitamin loss of 25 percent is normal. Thiamin is easily soluble in water and is destroyed by heat.

Vitamin B2 (Riboflavin): Not much research has been done to see how much freezing affects Riboflavin levels. Studies that have been performed are inconclusive; one study found an 18 percent vitamin loss in green vegetables, while another determined a 4 percent loss. It is commonly accepted that the loss of Riboflavin has to do with the preparation for freezing rather than the actual freezing process itself.

Vitamin A (Carotene): There is little loss of carotene during preparation for freezing and freezing of most vegetables. Much of the vitamin loss is incurred during the extended storage period.

3) Microbiological Aspects

During the pre-freezing stage, microorganisms can grow but very slowly (have a long generation time) when the temperature is approaching the minimum growth temperature. If the temperature is kept below the minimum temperature for growth, some microorganisms may die. However, above 0°C the loss of microorganisms' viability is limited and in practice, is negligible. When bacteria, in the exponential growth phase, are cooled quickly it is expected that microorganisms inactivation is more pronounced, but an abrupt temperature drop may lead bacteria to form cold shock proteins that protect them against other stresses such as heating, low pH or low water activity.

The freezing stage causes the apparent death of 10%-60% of the viable microorganisms and these values increase during frozen storage. Factors such as low temperature, extracellular ice formation, intracellular ice formation, concentration of solutes and internal pressure may be involved in the microbial inactivation. The sensitivity of microorganisms to the freezing process differs considerably

Thus, the main concern is related to the microorganisms that survive during the freezing step, and with the ones that can grow when the product is thawed. Usually, the less resistant microorganisms are the Gram-negative bacteria followed by the Gram-positive bacteria. Nonsporulating rods and spherical bacteria are the most resistant ones, and spores (such as Clostridium and Bacillus) remain unaffected by freezing. Bacteria in the stationary phase are more resistant than those in the exponential phase.

The freezing process causes damage mainly in the microorganisms membrane, which loses some barrier properties at temperatures below 15°C, leading to leakage of internal cell material. The dissociation of lipid-proteins may injure the cells during the freezing process. Cell membranes may also suffer mechanical damage due to ice crystals formation. After storage at different temperatures, it is common to observe higher microbial inactivation for warmer storage (e.g., -8°C) temperatures than for colder storage temperatures (e.g., -18°C or lower). Freezing and storage at very low temperatures (-150°C or even colder) seems to result in increased survival. The long-time exposure to concentrated solutions (both internal and external) may lead to microbial death in both conditions. However, the recrystallization of ice observed if temperature fluctuations occur, increases the solutes concentration and consequently provokes damage to microorganisms. Temperature fluctuations at lower storage temperatures generate smaller ice crystals than at higher storage temperatures.

Technology Used in Freezing

The freezing technique itself, just like the frozen food market, is developing to become faster, more efficient and more cost-effective. Mechanical freezers were the first to be used in the food industry and are used in the vast majority of freezing / refrigerating lines. They function by circulating a refrigerant, normally ammonia, around the system, which withdraws heat from the food product. This heat is then transferred to a condenser and dissipated into air or water. The refrigerant itself, now a high pressure, hot liquid, is directed into an evaporator. As it passes through an expansion valve, it is cooled and then vaporizes into a gaseous state. Now a low pressure, low temperature gas again, it can be reintroduced into the system.

Cryogenic or (flash freezing) of food is a more recent development, but is used by many leading food manufacturers all over the world. Cryogenic equipment uses very low temperature gases – usually liquid nitrogen or solid carbon dioxide – which are applied directly to the food product.

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Effectiveness of Freezing as Food Preservation

Freezing is an effective form of food preservation because the pathogens that cause food spoilage are killed or do not grow very rapidly at reduced temperatures. The process is less effective in food preservation than are thermal techniques, such as boiling, because pathogens are more likely to be able to survive cold temperatures rather than hot temperatures. One of the problems surrounding the use of freezing as a method of food preservation is the danger that pathogens deactivated (but not killed) by the process will once again become active when the frozen food thaws. Read... Food Storage Shelf Life Chart

Foods may be preserved for several months by freezing. Long-term frozen storage requires a constant temperature of -18 °C (0 °F) or less.

Freezing is one of the oldest and most common processes used in food preservation and one of the best methods available in the food  industry. There are several methods and types of equipment that can be used and adapted according to the different types of foods. Freezing usually retains the initial quality of the products. However, during freezing and frozen storage, some physical, chemical and nutritional changes may occur. To avoid this impact on fresh products, mainly in fruits and vegetables, some pretreatments may be required to inactivate enzymes and microorganisms.

Vacuum packing combined with freezing will increase the storage life of food. Learn the correct way to vacuum pack food at... How To Vacuum Pack Meat, Poultry and Sea Food Properly?

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