Frozen foods are always packaged before being displayed and in the majority of cases the packaging obscures, and protects, the food on display. If packed in transparent film the surface of many frozen foods will discolour rapidly when illuminated.
Traditionally open-well cabinets were used to display frozen products, but increasingly multi-deck cabinets are used because of their increased display space and sales appeal. The rate of heat gain in a multi-deck cabinet, and consequently the energy consumption, is much higher than in a well cabinet. Owing to the increased costs of energy, multi-deck cabinets are now appearing on the market with double glazed doors that have to be opened to access the food on display.
Frozen food products have significant market share. The growth of the frozen food industry is influenced by socioeconomic changes and technological developments. One of the main advantages of freezing preservation is the convenience and ease of food preservation with minimal impact on food quality compared to other preservation techniques (e.g., thermal processing or drying). Like any other preservation technique, freezing affects the quality of food products. However, the advancement in freezing technology and adoption of novel techniques can reduce the impact of freezing on food quality and nutrition. Application of novel processing techniques and use of edible coating/films will result in higher-quality frozen products with extended shelf life. The future commercial application of novel processing techniques, such as high pressure and power ultrasound-assisted freezing will depend on the development of commercial scale equipment that is easy to operate and economically viable.
Frozen foods contain crystalline ice within freeze-concentrated solids. The amount of ice is dependent on temperature and the ability of the solutes to depress the melting temperature of ice. In frozen foods the amount of ice affects also mechanical properties by changing the effective glass transition temperature of partially freeze-concentrated solute matrices and by probable interconnections within the ice network.
Experimental data on the mechanical properties for frozen foods are almost non-existent. Studies on the rheological properties of ice cream have suggested that the material in the frozen state has viscoelastic characteristics. Most studies reporting mechanical properties for freeze-concentrated materials have used sugar solutions as model systems. The results showed that the freeze-concentrated solute matrix had a glass transition around −46°C, but significant softening and flow in the solutions was observed to occur only above −32°C due to ice melting. The mechanical tests included a compression test for samples with a large elastic component, an annular shearing test for viscous liquids, and a pumping shear test for viscous liquids. The results for 50% sucrose solutions showed a decrease in storage modulus and decrease in loss modulus above −45°C and increase in mobility above −32°C. A slow discontinuous increase of the phase angle associated with ice melting above −32°C was considered to indicate some reorganization of ice crystals.
The main cause for changes in mechanical properties of frozen foods is probably related to ice melting. The most dramatic changes in mechanical properties of frozen foods are likely to occur in the vicinity of the melting temperature as most of the ice is transformed into the liquid state.
No frozen food, with the possible exception of ice cream, should be unwrapped when in a retail display cabinet. Traditionally frozen food was displayed in a “well-type” cabinet with only the top faces of food packs being exposed. In many cases the cabinets were fitted with a see-through insulated lid to further reduce heat infiltration. There is marketing pressure to display an increasing amount of frozen food in open multi-deck display cabinets. Maintaining the temperature of products below set limits while they are on open display in a heated store will always be a difficult task. Radiant heat gain on the surfaces of exposed packs can result in the food thawing in extreme cases. During display, temperature, temperature fluctuations and packaging are the main display parameters that control quality.
Temperature fluctuations can increase the rate of weight loss from wrapped frozen food. Higher rates of dehydration have been measured in a retail cabinet operating at−15°C than in another cabinet operating at−8°C. Fluctuations in air temperature in the−15°C cabinet ranged from−5 to−21°C compared with±1.5°C in the−8°C cabinet. Successive evaporation and condensation (as frost) caused by such a wide temperature differential resulted in exaggerated in-package dehydration.
The extent of temperature fluctuations will be dependent upon the air temperature over the product, the product packaging and the level of radiant heat. Retail display packs have a relatively small thermal mass and respond relatively quickly to external temperature changes. These can be from store and display lighting, defrost cycles and heat infiltration from the store environment. In products where air gaps exist between the packaging and the food, sublimation of ice within the product leads to condensation on the inside of the packaging, resulting in a build-up of frost. This dehydration causes small fissures in the surface of the food, allowing the ingress of any packaging gases into the food. This can aid the acceleration of oxidative rancidity within the product. Minor product temperature fluctuations are generally considered to be unimportant, especially if the product is stored below−18°C and fluctuations do not exceed 2°C.
‘Frozen foods have an excellent overall record of safety and illnesses associated with frozen foods are rare. However, in addition to preserving the quality of foods, freezing also preserves the viability of some pathogenic microorganisms’ (Archer, 2004). The author provided some examples of outbreaks and illnesses associated with frozen foods.
In general, freezing results in a small reduction in the numbers of viable microorganisms contained on and within poultry and poultry products. Any organisms present are affected by the low temperature, the formation of ice and the reduction in available water. During the initial stages of the freezing process, as the temperature is reduced, the rate of microbial growth is also reduced and growth ceases below the minimum temperature. In a very long freezing process, however, microbial growth could occur.
MAP of frozen products in the form of vacuum packaging is often necessary. Although chemical reactions proceed very slowly under freezing temperatures, they may still cause significant quality changes over time. The oxidation of vitamins and pigments continues to take place, and results in the loss of color, flavor and nutritive value. Another common problem in frozen products, the so-called freezer burn, is due to water lost from the surface of the product to the surrounding environment. All of the above problems can be significantly reduced or completely eliminated by the application of vacuum packaging.
Stability of frozen foods is related to several factors that are dependent on temperature and composition. Food components that decrease ice melting temperature decrease also the glass transition temperature of freeze-concen-trated solids and affect the unfrozen water content at food storage temperatures. Shelf-life predictions of frozen foods need to consider the effect of freeze-concentration and temperature on deteriorative changes.
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