Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)


Dairy Science

First Advisor

Sanjeev Anand


Cavitation, Hydrodynamic cavitation, Skim milk powder, Sporeformers


Sporeforming bacteria and their endospores have the ability to survive thermal treatments and proliferate in the milk and dairy products to cause spoilages. We applied the effect of hydrodynamic cavitation in controlling these bacteria and their spores at pilot scale level. Our aim was to investigate a technology that would improve the microbial quality of milk and dairy products, and is capable of being scaled up to be adapted at industrial level. Cavitation damages the cell walls of the bacteria and their endospores (releasing dipicolinic acid, responsible for endospore resistance to extreme conditions), thereby making them susceptible to thermal treatments such as pasteurization. A hydrodynamic cavitator was setup in-line to a plate heat exchanger (with an adjustable holding tube) to provide cavitation effect (3600 rpm cavitator rotor speed) immediately followed by HTST pasteurization for 73 ºC for 15 s to artificially inoculated milk samples. Vegetative cells and endospores of Bacillus coagulans (ATCC 12245), Bacillus licheniformis (ATCC 6634), and Geobacillus stearothermophilus (ATCC 15952) were studied for their survivability against HTST pasteurization alone versus the in-line process, which had an additional cavitation effect in combination with HTST pasteurization. Cavitated milk was also used to produce skim milk powder, in order to study the overall reduction in the sporeformers and spore counts of the final product. This helped us to understand the feasibility and effectiveness (in controlling sporeforming bacteria and their endospores) of the process in manufacturing skim milk powder, which is one of the largest exported product from the United States. Quality defects (microbial inconsistencies being one major reason) has affected the export in recent years. We observed that HTST pasteurization reduced (P< 0.05) vegetative cells of B. coagulans (ATCC 12245), B. licheniformis (ATCC 6634) and G. stearothermophilus (ATCC 15952) by 99.99, 99.999, and 99.9% (i.e. a 4 Log, 5 Log and 3 Log reduction respectively) from initial bacterial (vegetative cells) loads of 5.43 ± 0.02, 5.24 ± 0.18 and 3.50 ± 0.37 Log CFU/mL respectively. Bacillus coagulans was further investigated using the in-line process, since it showed some resistance (more than 1 Log survivors) to HTST pasteurization alone. The in-line process effectively (P< 0.05) reduced the bacteria by 99.99%, a 4 Log reduction again but more effectively reducing it below 1 Log CFU/mL (0.90 ± 0.04 Log CFU/mL from an initial load of 4.75 ± 0.03 Log CFU/mL). For the endospore challenge studies it was observed that at least 2C1P effectively (PB. licheniformis, 47.66% (from initial mean loads of 1.51 ± 0.03 Log spore/mL to 1.22 ± 0.10 Log spore/mL) in case of G. stearothermophilus and 76.04% (from initial mean loads of 2.13 ± 0.06 Log spore/mL to 1.52 ± 0.06 Log spore/mL) in B. coagulans. 3C1P were most effective (P< 0.05) in reducing 92.41% (from initial loads of 1.69 ± 0.12 Log spore/mL to 0.57 ± 0.14 Log spore/mL) endospores of B. licheniformis, 91.58 % (from initial mean loads of 1.51 ± 0.03 Log spore/mL to 0.42 ± 0.15 Log spore/mL) endospores of G. stearothermophilus and 63.52% (from initial mean loads of 2.13 ± 0.06 Log spore/mL to 1.70 ± 0.05 Log spore/mL) endospores of B. coagulans. While 2C1P was more effective against the endospores of thermoduric B. coagulans, 3C1P was more effective against the thermophilic strains B. licheniformis and G. stearothermophilus, suggesting that the process can be fine-tuned depending upon the type of strain. A pilot scale study was conducted to manufacture low spore skim milk powder by incorporating cavitation into the regular protocol, double cavitation (2C) passes prior to HTST pasteurization, evaporation and Niro drying was used, B. licheniformis (ATCC 14580) was artificially inoculated into raw milk. The process reduced 99.840 % (2.96 ± 0.10 Log spore/mL in skim milk to 1.49 ± 0.09 Log spore/g in SMP, PB. licheniformis (ATCC 14580) in the final SMP as compared to a typical industrial process which left the final counts of 2.74 ± 0.03 Log spore/g in SMP. The final cavitated powder had better bacteriological as well as comparable physicochemical properties to the powder produced from regular protocol.

Library of Congress Subject Headings

Dairy processing -- Quality control.
Sporeforming bacteria.
Bacterial spores.



Number of Pages



South Dakota State University


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