Dr. Art Hill
Department of Food Science
University of Guelph,
Guelph, ON, N1G 2W1
Part 4 in this series, Winter 07, discussed pH as a process control tool in cheese making. Here we consider several principles of cheese safety based on temperature history of the milk and cheese. Survival and growth of pathogenic bacteria, spoilage microorganisms and desirable microorganisms, including cheese cultures, are all dependent on temperature history. The principal temperature treatments and their outcomes for each stage of cheese making are described below.Milk storage and transportation
Temperature effects on cheese making properties of milk commence as soon as the milk leaves the udder. Cooling encourages low temperature tolerant bacteria and inhibits natural fermentation by lactic acid bacteria (see Figure 1 in Part 4 of this series). So, cheese making by natural fermentation (without culture) is not practical with cooled milk.Heat treatments of cheese milk
Raw cooled milk requires a large amount of culture to compete with cold tolerant bacteria and overcome bacterial inhibitors that are naturally present in raw milk. Accordingly, even for aged varieties where a diverse microflora is desirable, sub-pasteurization heat treatment is usually applied. Heat treatments range from 55 to 65°C for 16 s. Full legal pasteurization is 72°C for 16 s. As for raw milk, cheese made from heat treated milk must be stored at temperatures greater than 4°C for 60 days before retail sale. With respect to safety, properly pasteurized milk contains no illness causing bacteria (pathogens). However, as described in Cheese Safety Part 2, cheese makers must assume that raw milk contains pathogens that must eliminated during cheese making and ripening. The same is true for sub-pasteurization treated milk, although the probability of infectious doses of pathogenic bacteria in heat treated milk is much lower than in raw milk. Johnson et al., 1990a and b recommended heat treatment of 64°C for 16 s to reduce the risk of surving pathogenic bacteria to near zero.
Note that pasteurized milk may be contaminated after pasteurization, so, it is not recommended to produce pasteurized milk cheese in the same facility as raw milk cheese. However, experience has shown that pasteurized milk and heat treated milk can be made safely in the same facility, provided that heat treated milk is processed after pasteurized milk at the end of the daysâ€™ production just before plant clean up.Inoculation, Gelation and Cutting
Cheese is most vulnerable to growth of pathogenic bacteria during the early stages of cheese making when the milk is warm. So, an active culture is essential to control growth of spoilage and pathogenic bacteria. For varieties such as some Hispanic cheeses, which have pH near 6.5 and do not require culture, extra rigor in hygiene is required. I recommend a small amount of culture in these varieties to increase cheese safety. Cooking (nonmelting) properties will be preserved as long as the pH does not fall below 5.8.
Cooking after cutting is applied or not as required to reduce moisture. Maximum cooking temperature for most mesophilic varieties is 39°C because higher cooking temperatures destroy the mesophilic cultures. So, for mesophilic varieties, cooking treatments importantly ensure optimum rates of acid development and moisture release, but are otherwise not critical to safety. An exception is high temperature cooking (55ºC) of some fresh cheese such as cottage cheese, which destroys the mesophilic culture along with other organisms. The result is a product with few microorganisms that is prone to contamination and spoilage by yeasts and moulds, but relatively safe due to pH near 5.0. Thermophilic cheeses such as many Italian and Swiss varieties employ high temperature cultures and depend on high cooking temperatures, generally 50-55°C, for development of typical texture and ripening agents. These varieties are rarely associated with food borne illness because the extended high temperature cook dramatically reduces spoilage and pathogenic bacteria.Ripening
During stages subsequent to cooking (draining, forming, pressing etc), temperature control is important primarily to effect the optimum pH versus time profile for the particular variety. During ripening, temperatures greater than 4°C, ideally at least 5°C, are legally mandated for raw milk cheese to ensure that ripening agents (bacteria and enzymes) create an increasingly inhospitable environment for pathogenic bacteria, which gradually decrease to noninfectious levels. An exception is bloomy varieties where the acidity on the surface is neutralized by yeasts and moulds. In this case warmer ripening temperatures of up to 12 - 15°C probably encourage the growth of cold tolerant pathogens such as E. coli O157 H7 and Listeria monocytogenes.Summary
With respect to safety, temperature versus stage of manufacture has three important roles in cheese making. First, heat treatment of milk before cheese making and high cooking temperatures in thermophilic cheese varieties dramatically reduce spoilage and pathogenic bacteria. In the case of thermophilic varieties, cooking, which takes place late in the process, explains why thermophilic varieties are rarely the source of food borne illness. Second, temperature versus time control throughout the manufacturing process is critical to obtain the optimum pH versus time profile, which in turn is critical to safety as described in Part 4 of this series. Finally, temperature, especially during cooking, is a principal determinant of cheese moisture, which along with salt content is also an important safety factor. The role of salt in cheese safety is the topic of Cheese Safety 101 Part 6.References
Johnson EA, Nelson JH, Johnson M. 1990a. Microbiological safety of cheese made from heat-treated milk. III. Technology, discussion, recommendations, bibliography. J Food Prot 53:610-23.
Johnson EA, Nelson JH, Johnson M. 1990b. Microbiological safety of cheese made from heat-treated milk. III. Technology, discussion, recommendations, bibliography. J Food Prot 53:610-23.