Occurrence, growth and survival of yeasts in matured cheddar

[摘要] Yeasts play an important role in dairy products causing spoilage or contributepositively to the ripening of some cheeses due to their lipolytic and proteolyticactivities and the ability to grow at low temperatures, pH and water activity, andhigh salt concentrations. Despite various factors affecting the survival and growthof yeasts in cheese, the food commodity is considered as a potential habitat ofyeasts. Cheese renders an ideal medium for the survival and progression ofyeasts due to the availability of the necessary nutrients. A historical review of theincidence of yeasts, their interaction with bacteria and their properties, during themanufacturing and ripening of matured Cheddar cheese is given in Chapter 1.The history of Cheddar cheese making, the fermentation of milk during cheesemaking, different starter cultures and methods of application and occurrence ofyeasts are highlighted. In Chapter 2 a survey was undertaken with the objective of identifying thepredominant yeast contaminants associated with matured Cheddar cheeseproduction. A total of 168 yeasts strains, representing 12 different species,isolated from matured Cheddar cheese and the immediate environment wereisolated. Once the identity of the yeast flora associated with the cheese had beenestablished, the survey was extended to determine the sources of yeast infectionduring the processing of the matured Cheddar cheese. The results obtained,showed that yeasts contributed substantially to the deterioration as well as theripening of the matured cheese, especially when stored under environmentalconditions exhibiting, low temperature, water activity and pH values and high saltconcentrations. In Chapter 3 seven of the most dominant yeasts frequently associated with dairyproducts are screened for their resistance against nine commercial cleaningcompounds and sanitizers commonly used in the dairy industry. None of thecompounds used, however, were able to sufficiently kill the yeasts, within 60minutes.In Chapter 4 the influence of temperature and NaCI on the dominant yeastsisolated in a cheese factory, during the manufacturing and ripening of maturedCheddar cheese was examined. Relevant yeast species were screened forlipolytic and proteolytic activities, and their ability to ferment lactose. Thechemical and physical characterition including pH, fat content, water activity, saltand moisture were determined and its relevance to the growth and survival ofyeasts studied.The interaction between yeasts, coliforms and total bacteria during themanufacturing and ripening of matured Cheddar cheese, by using DVI- andMass- starters was studied in Chapter 5. Results obtained indicate that yeastcontaminants have to be controlled during the salting process to preventovergrowth of the starter cultures. During the salting stage, the curd is prone toyeast contamination and may result in excessive loads due to the competitivegrowth advantage of the species and the simultaneous bacterial inhibition. Thismay contribute to product spoilage or stimulate the growth of the starters addingto the formation of aroma components.

[发布机构] University of the Free State