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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)


Dairy Science

First Advisor

Sanjeev Anand


Studies were conducted to track the progression and control of thermoduric bacteria (mesophiles and thermophiles) during cheddar cheese manufacturing and whey processing in a commercial cheese manufacturing plant. Duplicate samples were drawn at four intervals; the start vat, the pre and post mid-day wash vats, and the last vat. The entire cheese making process was divided into three sampling stages; raw milk, pasteurized milk, and cheese blocks. The average counts of thermoduric thermophiles in raw milk were log cfu 1.44/mL. These counts increased by about 1.16 logs during pasteurization run of 9 to 10 h, indicating a build-up of thermoduric thermophiles during pasteurization. Mid-day wash reduced thermophilic counts by about 1.30 logs, as evident by pre and post mid-day wash counts. However, a thermophilic build up during post midday wash pasteurization was again noticed near the end of the 20h run. The final whey retentate contained higher thermoduric thermophiles as compared to mesophiles. Bacillus licheniformis was the most frequently isolated bacteria in this study, and was recovered at all production stages sampled during the cheese making and whey processing. We continued studying the profiling and progression of thermoduric bacteria (mesophilic and thermophiles) for a period of 6 and 2 months during normal and accelerated ripening, respectively. During normal ripening process, the thermoduric mesophiles ranged from log 2.77 to 3.47cfu/g. In this study we found that thermoduric thermophiles decreased during first 60 days followed by an increase until the end of 180 days. It was also found that accelerated ripening could also be used to evaluate the potential of thermoduric thermophiles for causing cheese defects in a shorter period of time. Bacillus licheniformis was found to be the dominant thermoduric species during normal and accelerated ripening. A study was performed to evaluate the prevalence of thermodurics on the product contact surfaces (cheese vats, stirred cheese belt, the cheese tower) and in the environment samples (raw milk silo, cheese vat, cheese belt, and cheese tower areas) during cheese making, and whey processing. The samples were taken in parallel while performing the first study. Rinse samples after the CIP were also analysed microbiologically to evaluate its efficiency. Thermoduric biofilms were found to be more prominent in the regeneration section of the pasteurizer. The thermoduric mesophiles, thermoduric thermophiles, and spore counts were observed to be 1.30, 1.30, 1.20 log cfu/cm2, respectively, in the regeneration section. Other product contact surfaces and environment showed lower levels of thermoduric biofilm (<1.01og/cm2). Bacillus licheniformis turned out again to be predominant bacterial isolate in this study. It was also observed that CIP rinse bacterial counts alone cannot be used as an absolute indicator to conclude its efficiency in cleaning biofilms in milk pasteurizer. Mid-day wash was able to reduce the biofilm population and break the cycle of bacterial build up. We also conducted a study to isolate the bacteria responsible for slit defects in some samples of under-graded cheddar cheese. These under-graded samples were obtained from a commercial cheese plant and were analysed for possible defect causing isolates. The Analytical profile index (API) strips were used for final identification of bacterial isolates. Results indicated Lactobacillus fermentum and Clostridium beijerinckii as the causative microflora for slits defects in the examined samples. Coliforms and yeasts and molds were not detected. A challenge study was also conducted to evaluate the effect of salt levels (low and high), cooling rate (slow and fast) on the growth of gas producing contaminants and the appearance of cheese slits during ripening. None of the samples showed any slits during the first month of ripening. However, all cheeses spiked with the cocktail of thermoduric contaminants showed gas production after 2 months, irrespective of the salt levels and cooling rates. A possible reason could be the predominance of added Lactobacillus fermentum, observed at a level of log 6.0-7.0/g. All these studies may have their own limitations. First three studies were conducted in only one dairy plant, which had its own unique processing operations, and thus may not be representative of the entire industry. Fourth study was limited to only a few samples of under-graded cheese having slits, hence it does not represent all different kinds of gas producing bacteria and their population distribution.

Library of Congress Subject Headings

Cheese -- Microbiology


Includes bibliographical references



Number of Pages



South Dakota State University


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