Thesis - Open Access
Master of Science (MS)
George S. Torrey
In the last twenty years a number of membrane separation processes have evolved from laboratory scale, through pilot plant investigation, to full scale production units. One such process, ultrafiltration, has been found particularly useful in the dairy industry. Ultrafiltration uses porous polymeric membranes to separate molecules, principally on the basis of their molecular weight. In milk, fat, protein and associated substances are retained on the membrane. Permeate, which passes through the membrane contains mainly water, lactose and other low molecular weight substances. There is interest in ultrafiltration of milk because of it is potential importance in saving costs to both farmer and processor. The advantages of using ultrafiltration include incorporation of whey proteins in cheese, increased production capacity, reduced rennet requirements in cheesemaking, reduced energy requirements and overall low operating costs (52, 53, 67, 68, 93). Dairy farmers gain in reduced cooling and hauling costs, and can feed the permeate to cows (68,93). The technical and economic feasibility of ultrafiltration on the farm as well as at the plant has been successfully demonstrated (26, 67, 68, 86, 92, 93), and ultrafiltration of milk has already proven to be a desirable pretreatment step in the commercial manufacture of various dairy products (10, 27, 40). The basic applications for .ultrafiltration of whole milk are total concentration of .milk, preconcentration of cheese milk and standardization of protein. The major objective of using ultrafiltration in the dairy industry is to remove bulk water and reduce volume, subsequently reducing operating costs. Reduction in volume allows the farmer to save on refrigeration and transportation costs. There is a great possibility that adaption of ultrafiltration in the dairy industry may induce changes in milk collection frequencies from the farm and operating schedules at the plant which may lead to extended refrigerated storage of retentates. Extended refrigerated storage of milk and milk products is selective for the growth of psychrotrophic bacteria (5). Growth of psychrotrophic bacteria in these products is of major concern because most of these species produce extra-cellular enzymes, such as proteases and lipases, many of which are heat stable and even survive ultra-high temperature treatment. Thus, the living bacteria and/or their enzymes may cause spoilage in milk or heat sterilized dairy products (1, 5, 13, 14, 20, 35, 42, 85, 90, 91). The introduction of ultrafiltration may potentially compound the problem of psychrotrophs since ultrafiltration directly or indirectly facilitates long holding periods. Study of psychrotrophic bacterial growth in retentates is important because retentates as a microbial medium is entirely different from whole milk in chemical composition. Ultrafiltration of milk affects the relative as well as the absolute concentrations of milk constituents, because the membrane is permeable to low molecular weight materials. Much of the lactose and some water soluble minerals and vitamins pass through the membrane and therefore decrease in concentration or are only slightly concentrated in ultrafiltered milk (27,53,67,92). The membrane is completely impermeable to fat, protein, vitamin B12 and folic acid in milk, so that these components are concentrated in inverse proportion to the volume decreased (30). Details of changes in mineral, vitamin and trace elements composition of milk during ultrafiltration are described by several authors (7, 8, 9, 24, 25, 29, 30). Previous studies (1, 6, 73, 87) of refrigerated retentates with natural mixed microbial flora indicate that retentates support good psychrotrophic growth, but that bacterial multiplication is greater in whole milk than in retentates after ·2-3 days of incubation. Tayfour et al. (73) observed that growth and proteolytic activity of Pseudomonas fluorescens P2a was less in skim milk retentates concentrated fivefold than in skim milk, however, skim milk concentrated 2 or 3 times showed better growth and proteolytic activity than did skim milk. Characterization of growth of specific bacteria in whole milk retentate and data comparing changes in composition of whole milk and whole milk retentate caused by growth of specific bacteria is needed. The present study was undertaken to compare the growth and proteolytic activity of selected psychrotrophic bacteria in retentates to that in whole milk.
Library of Congress Subject Headings
Milk -- Microbiology
Number of Pages
South Dakota State University
Reddy, Ravinder, "Growth and Proteolytic Activity of Selected Psychrotrophic Bacteria in Whole Milk and Whole Milk Retentate" (1987). Theses and Dissertations. 1328.