Reconstituted Nonfat Milk Research Articles

Effects of Antibiotics on Proteinase-Positive and Proteinase-Negative Variants of Streptococcus cremoris

The growth and acid-producing ability of proteinase-positive and proteinase-negative starter cultures were studied in the presence of several antibiotics commonly used in treating bovine mastitis. Proteinase-positive and proteinase-negative variants of two different strains of Streptococcus cremoris were inoculated separately into heat-sterilized, reconstituted nonfat milk containing a range of concentrations of either dihydrostreptomycin, erythromycin, or penicillin G. Generation times and pH changes were determined for each sample following incubation at 38°C for 5h.At 2% inoculation, proteinase-positive cultures produced more pH change than their proteinase-negative counterparts at all concentrations of all three antibiotics. Proteinase-negative cultures at 8% inoculation produced more acid than proteinase-positives did when inoculated at 2%. Generation times for proteinase positives and proteinase negatives were the same when both cultures were inoculated at 2%, but 8% inoculation of proteinase negatives produced significantly shorter generation times. Increases in antibiotic concentrations affected proteinase-positives more than proteinase-negatives in both pH change and generation time. Penicillin affected both proteinase positives and negatives more than the other two antibiotics did but affected positives more than negatives.

Relationship between Titratable Acidity and pH during Lactic Acid Fermentation in Reconstituted Nonfat Milk

Summary Titratable acidity and pH measurements were made during lactic acid fermentation in reconstituted nonfat milk ranging from 9 to 20% solids. Measurements were made on the reconstituted products and also on the corresponding whey obtained therefrom. The results, which are presented in graphic form, show the relationship of the titratable acidity, pH, and solids concentration during fermentation down to pH 4.4. Before fermentation, an initial apparent acidity of 0.16% in nonfat milk containing 9% solids increased 0.02% acidity for each additional 1% increase in solids reconstituted, accompanied by a slight decrease in pH from 6.6 to 6.4. During lactic acid fermentation proportionately greater increases in titratable acidities of both the reconstituted products and whey were observed for each additional 1% solids increase. At pH 4.6 both the reconstituted solids and whey showed a 0.06% increase in titratable acidity for each 1% increase in the original solids concentration. Good quality cultured buttermilk was obtained, using reconstituted nonfat milk containing 11% solids and developing the titratable acidity to 0.90-0.95% at pH 4.5 to 4.4. Using the short-time method, satisfactory cottage cheese was obtained using nonfat milk containing 13% solids, cutting the curd at a clear whey titratable acidity of 0.62% at approximately pH 4.8, and permitting additional acid development to a final pH of 4.7 to 4.6.

The Nutritional Requirements of Lactic Streptococci Isolated from Starter Cultures. II. A Stimulatory Factor Required for Rapid Growth of Some Strains in Reconstituted Nonfat Milk Solids

Summary Liver fraction L, trypsinized skimmilk, or peptonized milk, when added in a low concentration to reconstituted nonfat milk solids, produced greater stimulation in the growth of mixed starter cultures and individual strains of S. lactis and S. cremoris isolated from them than did any individual amino acid or vitamin. Liver fraction L produced marked stimulation in the initial growth rate of many so-called slow strains. Some of the faster strains were inhibited or showed only a slightly increased response when this factor was added to the milk. Results suggested that the material responsible for such increased initial growth rate might be a peptide or peptide-like in nature, and that an enzymatic digestion of milk may tend to release these substances from the milk proteins in a readily available form for the organisms comprising a starter culture. When some slow strains were supplemented with such hydrolytic substances, the rate of growth approached that of faster strains.