Factors affecting growth of and acid production by starter cultures in ultrafiltered milk
Abstract
Growth of and acid production by lactic acid bacterial starter cultures in milk concentrated by ultrafiltration (UF) was studied. Raw milk was skimmed, pasteurized at 62.8$\sp\circ$C for 30 min, and ultrafiltered at 50$\sp\circ$C to 2-, 4-, and 5-fold. Chemical and microbiological analyses were done on the milk and retentates. Chemical analysis indicated that proteins and fat concentrated by approximately 2-, 4-, and 5-fold in the 2-, 4-, and 5-fold retentates, respectively. During UF to 5-fold Ca, Mg, Zn, Cu, Fe, and Mn, concentrated by 4.3, 3.0, 4.9, 4.7, 4.9, and 3.0, respectively; the B-vitamins, thiamin, riboflavin niacin, pantothenic acid, and biotin, decreased by 85, 71, 87, 82, and 84%, respectively. Most free amino acids decreased by greater than 50% except for threonine and serine which decreased by 95.3% and 98.0%, respectively. Lactose decreased by 53, 74, and 84% in the 2-, 4-, and 5-fold retentates. Bacteria concentrated proportionally to the concentration factor of the milk. A sporeforming contaminant producing a rennin-like clot in skim milk and retentates was isolated and tentatively identified as Bacillus cereus. Heat treatments between 72$\sp\circ$C and 90$\sp\circ$C for 15 s were ineffective in eliminating this contaminant but a heat treatment of 80$\sp\circ$C for 10 min followed by a 4 h incubation at 32$\sp\circ$C, and a second heat treatment of 72$\sp\circ$C for 15 s eliminated it. Growth of and lactic acid production by S. cremoris, S. lactis, S. thermophilus, and L. bulgaricus were higher in retentates than in skim milk but the pHs remained high due to the increased buffering capacity of the retentates. A 1% mixed culture (1:1) of S. thermophilus and L. bulgaricus showed improved lactic acid production than when each was grown individually. Supplementing retentates with B-vitamins did not improve lactic acid production. In studying the effect of inoculum size on starter culture activity in skim milk and retentates, the TA increased with a corresponding decrease in pH as the inoculum size increased from 0.5-5.0%. However, at each inoculum size used, both TA and the pH increased as the concentration factor of the milk increased from 2 to 5.
Degree
Ph.D.
Advisors
Cousin, Purdue University.
Subject Area
Food science
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