Fermented Milk Product
Freezing process can be applied to fermented foods, including beneficial cultures such as yogurt, while inhibiting spoiling activities. It is known that yogurt is a well-established dairy product and is protected by developed acidity (lactic acid) from fermentation of lactose by bacterial culture. Bifidobacteria has beneficial effects for human intestine, including antagonistic effects on enteropathogens and suppression of liver tumorigenesis. During acidification, casein micelles in the milk are destroyed and they establish the typical acid gel.
Frozen yogurt, therefore, should be much like the commercial yogurt and should be characterized also by developed acidity from fermentation. Because frozen yogurt is not standardized in regulations, a wide range of products exists, due to citric acid content, which acidity is not developed by bacterial culture.
In the frozen yogurt process, fermentation is the core phenomenon. For sweet mix, the cream sugar, stabilizer, skim milk powder, and skim milk are combined and pasteurized, homogenized, cooled, before mixing with yogurt . The completed frozen yogurt mix is then aged and prepared for flavoring and freezing.
Yogurt acidification cultures are freeze-resistant, such as strains of Lactobacillus acidophilus, and can be stored frozen at -80°C. Many strains of the culture may also survive at -30°C. The basic concern about viability is storage temperature compared to freezing speed. As an example, only 1 log cycle and 40%-70% fermentation activity lost was observed at -80°C for 1 year for culture of dairy products such as yogurt. However, fermentation activity was less than 10% when cultures were stored for 1 year at -30°C. The fermentation activity of Streptococcus thermophilus was similarly reduced to 10%- 60% after 1 year of storage at -30°C. Protective solutes can be used to improve survival rates.
Cheese is another group can be freezable. Properties of cheese such as meltability, elasticity, oil formation, and cohesiveness are changed after freezing and thawing. Due to the acceptability, although some types of cheeses such as cream cheese, unripened cammeembert, and brick cheese are suitable for freezing, gouda and cheddar cheese do have not good properties after freezing.
It is common knowledge that although high-moisture cheeses have short storage life at refrigerated temperatures, the shelf life of cheese during storage could be extended by freezing. Mozzarella is prevalently consumed due to being a high-moisture cheese used as a pizza component, and studies have focused on this cheese type to investigate different methodologies and analyzing the effect of different operating conditions on the main characteristics of the cheese. Several authors have studied the changes in texture, melted functional properties and proteolysis of frozen Mozzarella cheese (Bertola et al., 1996b; Califano and Bevilacqua, 1999; Ribero et al., 2007).
Califona et al. (1999) studied changes in organic acid content of frozen low-moisture Mozzarella cheese. It was detected that there were no important difference between fresh and frozen cheese for organic acid content even after freezing and stored at -20°C for 14 to 21 days before thawing.
The initial freezing point of mozeralla cheese was measured by Ribero et al. (2007). In their study, the influence of the water-soluble solids on the freezing point depression of unsalted Mozzarella cheese was analyzed. It was determined that both NaCl and other soluble solids found in the aqueous phase have drastic effects on freezing point depression of mozzarella cheese.
Bertola et al. (1996a; 1996b) studied characteristics of frozen low-moisture mozzarella. They investigated apparent viscosity, free oil formation, and meltability of cheeses after freezing and frozen storage at -20°C. They determined that there was no quality loss between frozen and unfrozen samples.
Van Hekken et al. (2005) studied the effect of frozen storage on the proteolytic and rheological properties of soft goat milk cheese. They showed that the rheology and proteolysis of soft cheese were not sensitive to freezing and long-term frozen storage. Soft cheeses had fragile textures that showed minimal change after freezing or over 28 days of aging at 4°C. Factors such as the formation and thawing of ice crystals in the cheese medium and the limited proteolysis of the caseins caused only minimal impact on cheese texture. It is advised that frozen storage of soft cheeses may be possible for year-round supply with minimal loss of textural quality.
Cheddar is the other investigated frozen cheese in literature. Researchers manufactured different cheddar cheeses varied in several compositions and frozen them, then thawed. They showed that the most acceptable body and texture in frozen and thawed cheeses were normal fat, normal moisture cheeses and high fat, normal moisture cheeses after 8 weeks of aging. The TCA-soluble N increased significantly during frozen storage and also after thawing (Kasprzak et al., 1994).