If applied properly, freezing is the cheapest way of food preservation, compared to canning and drying. The most important point is if the temperature of the material begins at a temperature colder than room temperature or even close to freezing point, better-controlled crystallization occurs in the freezer. Different types of food systems are available due to the variety of food products and production characteristics. Freezer selection should match product requirements, reliable, sanitization requirements, and economic factors.
Freezing methods include plate contact, air blast, individual quick freezing systems, and cryogenic freezing. Heat transfer rates are about 5-2000 W/m2K for convective cooling and 0.5 to 1.5 W/mK for conductive cooling. Some new freezing techniques or combinations are being developed for their potential benefits, technical and economical advantages, and quality enhancements.
Freezing Dairy Products
Although freezing was applied to dairy product for preservation about 1930s, it was developed for general use after World War II. Some factors were identified, relating to the stability in storage of frozen dairy products. These factors are fat emulsion disruption, protein flocculation, and developing off-flavor and bacteriologic problems in defrost products. In common words there is no significant difference between fresh and frozen milk samples including some properties such as pH, acidity, acid degree value, peroxide value, protein sediment, apparent viscosity, or coliform counts. Furthermore, it was detected that bacterial counts in frozen milk is lower than fresh one.
Frozen dairy-based products are generally divided into two groups. Some frozen dairy products are preferred for their long shelf life and they are thawed for being component for further processing. Other groups are frozen for developing some properties of them, such as structure, texture, aeration, and so on. In this chapter, the second category is focused and it covers ice cream, related frozen, aerated desserts, ice milk, and frozen yogurt.
The composition of dairy products is unique compared to the other food materials such as plant and meat. Especially properties and amount of fat content makes dairy product controversy for freezing process. Freezing by slow cooling makes more damage to fat globule and degredates the aroma of the end product compared to faster one. Moisture content of dairy products vary around 87% to 91% involving significantly more physical changes during freezing phenomenon. For milk freezing, it is not economical due to its high water content and preconcentration required before freezing. If the freezing is applied properly to the dairy products, their aroma will be comparable to their original state. If the thawed product is pasteurized and homogenized for further processing for dairy or other food products, all side effects of freezing and frozen storage can be recovered.
The freezing process affects dairy components such as fat and nonfat solid. Fat is the most fragile to freezing phenomenon. Expanding water crystals cause mechanical damage to fat globules and make them coalescence and accumulate. Addition of sugar or homogenization before freezing can prohibit this side effect and make emulsion stable even after thawing.
The large effect of freezing can be observed on milk protein. During freezing, casein micelles lose their stability and precipitate, especially after thawing. It makes products thicken and casein flocks precipitate on the bottom of the package. Although this precipitation is unwanted, casein removal process called cryo-casein is applied by cooling to -10°C. Then the produce is thawed to remove casein from concentrated milk serum. Casein accumulation is reversible if adequate agitation is applied but becomes permanent after long-term storage. Reasons for casein flocculation are high-degree concentration, excessive heating (more than 77°C), storing under -23°C and higher than -18°C and long-term storage. State of lactose and serum proteins are the other factors. Lactose is responsible for about 55% of freezing point depression. Sun (2012) found that slow freezing is better than fast freezing for protein stability due to lactose nucleation procedure. When dissolved lactose limits the concentration of the salt in unfrozen phase and ensures high viscosity. Dialysis of the milk product before freezing contributes protein stability.
For aerated dairy desserts, components such as ice, air, and fat have different effects, but their interaction is determinant for the end product structure. Quality and shelf life of the product are affected by ice phase.
Dairy product freezing is quite problematic. First of all, milk is daily available all over the world. Also, there are many challenges due to its unique composition, and frozen dairy desserts are preferred to be eaten in frozen form, so there is no special problem related to thawing as observed in other frozen foods.