Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Dairy Science

First Advisor

Sanjeev Anand


biofilm, hydrophobicity, sporeformers, spores, ultrasonication, zetapotential


Sporeformers are common dairy contaminants, which are of a great concern to the dairy industry. Quality of dairy products is affected by these sporeformers such as Bacillus species, which are a significant cause of spoilage of dairy products. Thermal processes such as pasteurization have been used to inactivate pathogens, but some of the sporeformers and their endospores are resistant to such heat treatments. Beside thermal treatments, various new approaches are being developed to improve the quality of dairy products. Amongst these, ultrasonication is a promising non-thermal technique for the inactivation of thermoduric sporeformers and their endospores. Current study was carried out using ultrasonication as a non-thermal technique to evaluate its effect on biofilm forming ability, injured cells recovery, zeta potential and hydrophobicity of common dairy sporeformers. Three different bacteria, Geobacillus stearothermophilus (ATCC®15952), Bacillus licheniformis (ATCC® 6634), and Bacillus sporothermodurans (DSM 10599) were selected for this study based on our previous experiments. Skim milk was inoculated with a specific amount of spores or vegetative cells followed by ultrasonication for 10 min at 80% amplitude in an ice bath. The ultrasonicated cells were tested for the properties as outlined above. Based on the results, ultrasonication was observed to affect the ability of biofilm formation on stainless steel surfaces of all three sporeformers. Bacillus sporothermodurans formed the significantly higher biofilms, while Geobacillus stearothermophilus formed the least biofilms. In addition, a greater attachment was observed for B. sporothermodurans as supported by the highest hydrophobicity with least zeta potential among the three bacteria. On the other hand, G. stearothermophilus was found to attach less with least hydrophobicity and highest zeta potential. Overall, a positive relationship was found between attachment and hydrophobicity, when hydrophobicity increased, the capability of spores to attach stainless steel surfaces also increased. In addition, it was observed that injured cells were capable to regain and recover under favorable conditions, within as short as 4 hours. The results thus indicate that post-ultrasonication, the injured cells can recover, and replicate under favorable conditions to form biofilms.

Library of Congress Subject Headings

Dairy processing -- Quality control.
Bacterial spores.
Ultrasonic waves -- Industrial applications.


Includes bibliographical references



Number of Pages



South Dakota State University



Rights Statement

In Copyright