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Growth and Activity of Mesophilic Lactic Acid Streptococci in Ultrafiltered Skim Milk and in Reconstituted Nonfat Dry Milk of Differing Total Solids Contents

Department of Food Science and Nutrition, University of Minnesota, St. Paul 55108

ABSTRACT

The activity of mesophilic starter cultures Streptococcus lactis C₂, Streptococcus cremoris SK₁₁, and Streptococcus diacetylactis DRC₃ in 8.3 and 15% total solids reconstituted skim milk was compared with growth of these organisms in 8.9% total solids skim milk (unconcentrated control) and in UF concentrated skim milk of 12 and 13% total solids. The starter cultures were inoculated into the different milk samples and incubated at 30°C for 15 to 16 h. Samples were taken periodically for determination of pH, percentage titratable acidity, and cell populations. Cultures grown in UF concentrated milk consistently exhibited higher cell numbers and developed greater acidity than when grown in reconstituted skim milk of similar solids content. Streptococcus cremoris SK₁₁ was the most active culture in acid development in the various media. Growth was not related to solids level or milk type, that is, retentate or reconstituted skim milk. A model for predicting developed acidity from solids content and from type of milk was determined by multiple regression analysis. The results suggest that UF concentrated skim milk is superior to reconstituted skim milk as a substrate for lactic acid bacteria and that skim milk retentates have potential for use as media for starter cultures.

¹ Present address; Oscar Mayer Food Corp., Quality Assurance Division, Madison, WI 53707.