Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Dairy Science

First Advisor

George S. Torrey


The advent of on-farm refrigerated storage for raw milk in the late 1940's and its subsequent adoption by North American dairyman 'during the next 20 years essentially eliminated spoilage of milk by lactic acid bacteria. However, refrigeration does not prevent growth of all microorganisms present in raw milk (9, 33, 51, 52, 54, 55). Many microorganisms capable of growth at refrigeration temperatures, termed psychrotrophic bacteria, are present as normal contaminants in raw milk supplies. For purposes of clarity, psychrotrophic microorganisms have been defined as microorganisms capable of growth at 7°C or below regardless of their optimum growth temperature (9). Milk may be held under refrigeration for 5 days or longer before processing, or pasteurization for fluid consumption. This delay in processing results from a combination of events which include alternate day pickup of bulk milk from the farm, long distance hauling of milk, and shorter work weeks for processing plants. Extended refrigerated storage of raw milk selects for growth of psychrotrophic bacteria. Presence of these bacteria decreases milk quality by producing enzymes that degrade various components of milk (1, 9, 15, 28, 33, 37, 49, 52, 54, 55). Although the majority of psychrotrophic bacteria are killed by pasteurization, their proteolytic and lipolytic enzymes often survive (9, 15, 28). These heat-stable enzymes continue to diminish the quality of milk or other dairy products after pasteurization. Growth and metabolic activity of psychrotrophic bacteria in refrigerated raw milk is one of the most significant quality problems facing the dairy industry today (9). As a result of this problem, novel methods for controlling psychrotrophic bacterial growth are receiving attention from researchers and industry (6, 7, 8, 12, 13, 19, 20, 23, 24, 27, 29, 31, 36, 38, 40, 45, 47, 48, 58, 59). One attractive method proposed for controlling growth of psychrotrophic bacteria involves applying low levels (10-30mM) of carbon dioxide (CO2 ) to refrigerated milk. Results of preliminary studies (12, 27, 29, 47, 48) indicate CO2 is an effective inhibitor of psychrotrophic bacterial growth in refrigerated raw milk. The purpose of the research presented in this thesis was to evaluate the effectiveness of CO2 as a growth inhibitor of selected pure cultures of proteolytic psychrotrophs. A method for studying growth of psychrotrophs in milk treated with CO2 was developed. In addition, a gas chromatographic method was devised for quantifying CO2 levels in milk.

Library of Congress Subject Headings

Milk contamination -- Control
Psychrotrophic organisms -- Control
Milk -- Microbiology


Includes bibliographies.



Number of Pages



South Dakota State University


In Copyright - Non-Commercial Use Permitted

Included in

Dairy Science Commons