Improvement of Lactic Acid Production in Ultrafiltered Milk by the Addition of Nutrients

Abstract

In 4h at 32°C, mesophilic lactic acid bacteria produced .74% acidity in 3.3:1 retentate of ultrafiltration diluted to 2:1 with permeate. When diluted with distilled water instead of permeate, .56% acidity was produced. In 2.4:1 retentate containing .5% yeast extract, .95% acidity was produced compared with .84% in the control (retentate without yeast extract). The B vitamins, lactose, and minerals (NaCl, MgSO4, FeSO4, and MnCl2) had little or no effect. With another lactic starter, 22 amino acids together at .04 mg/g each in 2:1 retentate improved acid production (.72% compared with .61% for the control). Nonessential amino acids as a group were more stimulatory than essential amino acids. Cysteine and cystine at .04 mg/g each were as stimulatory as all amino acids together and all amino acids minus cysteine and cystine. Reducing agent dithiothreitol at .05 mg/g was as stimulatory as cysteine and cystine. Sulfur-containing compounds (methionine and Na2SO4) had no effect, suggesting that the stimulatory effect of amino acids is via the reducing power of cysteine and cystine as well as nutritive value of other amino acids.