Date of Award:
Master of Science (MS)
Nutrition, Dietetics, and Food Sciences
Craig J. Oberg
Cheddar cheese is manufactured by fermenting milk with lactic acid bacteria added as Lactococcus lactis. The two major subspecies used are lactis and cremoris. Starter culture for this study was prepared and donated by Vivolac Cultures Corporation, Greenfield, Indiana. Cheddar cheese was made in duplicate using two strains of lactis and cremoris. Milled curd was salted with 2.0, 2.4, 2.8, 3.2 and 3.6% salt, pressed for 3 h and stored at 6¬∞C. The curd was sampled during cheese making and after 6d storage. Two different methods of enumeration were employed. One method involved plate counting which only enumerates those bacteria that can reproduce and form visible colonies. The other method was flow cytometry (FC) that counts the number of cells based upon their combination with fluorescent dyes, which could be divided into three groups: live, live-semipermeable, and dead cells.
None of the four strains tested were influenced by the amount of salt added to the curd. Acid production rates during cheesemaking (with cook temperature of 38¬∞C) was more dependent on the individual strain than whether it was a lactis or cremoris strain. Based on flow cytometry, ~5% of the total starter culture cells in the cheese were dead after 6 d of storage. Another 5 to 15% of the cells were designated as being alive but live-semi permeable, with cremoris strains having the higher percentage of such cells. Thus, it can be said that plate count does not fully describe the fate and activity of starter culture during cheese storage, and that flow cytometry provides additional information on microbiology of cheese and can help track living cells during cheese storage and flavor development.
Poudel, Rhitika, "Understanding the Microbial Population of Lactococcus lactis. subsp. Strains During Cheese Making and Initial Storage" (2020). All Graduate Theses and Dissertations. 7938.
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