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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School

Dairy Science

First Advisor

Sanjeev Anand


Three complete studies were conducted in this thesis to determine the incidence of thermoduric bacteria (TDB) and spore-forming bacteria (SFB) on-farm and in milk powders in the Midwest along with exploring ultrasonication in a continuous process as a means to reduce their levels. The first was conducted to determine the prevalence and species of spores and thermoduric bacteria naturally present on 10 Midwestern dairy farms in bulk tank milk, corn silage, and milking cluster equipment samples. The sampling periods consisted of winter (Jan–March) and summer (May-July) seasons to compare the seasonal impact of their incidence. Each dairy was sampled 3 times per season for a total of 6 trials for each site. The bacterial isolates obtained from these samples were randomly selected and identified using matrix assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry along with other genetic techniques. The second study was conducted on commercial nonfat dry milk (NFDM) samples produced in Midwestern processing plants. The incidence of TDB, SFB, and highly heat resistant spores (HHRS) were enumerated in 39 NFDM samples obtained from 3 manufacturing companies and 5 different processing plants. Randomly selected bacterial isolates from each category were identified using MALDI-TOF techniques. Based on the bacterial identifications Bacillus licheniformis was identified as the most prevalent contaminant on the farm level and in the NFDM samples in each of the categories. There was also a significant seasonal impact upon the level of SFB and TDB present on dairies. The third study utilized B. licheniformis was the microorganism of interest and utilized ultrasonication (US), pasteurization (LP), and a combination (US+LP) to reduce the levels of vegetative cells in three specific strains. One of the B. licheniformis isolates was obtained from the farm study and the other from the powder samples with a standard strain from ATCC® also utilized for preliminary standardization purposes. On average the US+LP treatment was found to have the highest level of lethality.

Library of Congress Subject Headings

Dairy farms -- Middle West
Bacterial spores
Dried milk
Milk -- Microbiology
Milk contamination


Includes bibliographical references (pages 118-119)



Number of Pages



South Dakota State University


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