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Microbial Acid Production in and Subsequent Coagulation of Milk as Affected by Added Sodium Chloride

Research and Development Division, National Dairy Products Corporation, Glenview, Illinois

ABSTRACT

Skimmilks fortified with 1.0% NFDMS and 1.5% fat were steamed for 1 hr and cooled to incubation temperatures. Before inoculation, NaCl at levels of 0.00, 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50% was added to milks. A commercial lactic starter culture was added in amounts of 1.0 or 5.0% to skimmilks subsequently incubated at 22.2 or 30 C.

At 22.2 C and with a 1.0% inoculum, rate of acid production was: not affected by 0.25%, reduced slightly by 0.50%, and reduced appreciably by 0.75% NaCl. Presence of 1.00 to 1.50% NaCl further depressed rate of acid formation. Use of a 5.0% inoculum was associated with a reduction in rate of acid production when 1.00% or more NaCl was present. Incubation at 30 C and use of a 1.0% inoculum yielded results comparable to those obtained at 22.2 C with a 5.0% inoculum.

No appreciable difference in acid production rate was noted in milks with 0.75% or less NaCl, when a 5.0% inoculum and 30 C incubation were used, whereas 1.0 to 1.50% NaCl was accompanied by a slight reduction.

Coagulation of milk was frequently delayed by the presence of 0.75% or more NaCl and, sometimes, inhibited completely, even though titratable acidity was in the proper range. Determination of pH revealed that it may not have been low enough to permit coagulation of salted milks, even though they received sufficient heat for coagulation at higher pH values. The effect of salt on coagulation was greatest with a 1.0% inoculum or incubation at 22.2 C and least with a 5.0% inoculum or incubation at 30 C.