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Paired and Single Strain Protease Negative Lactic Streptococci for Cheese Manufacture¹

Department of Nutrition and Food Sciences, Utah State University, Logan 84322

ABSTRACT

After propagation in pH-controlled media, pairs of protease negative lactic streptococci produced acid better than the single component strains. Some protease positive pairs remained balanced with 1:1 ratios and others produced 9:1 ratios after one propagation.

Acid production by protease negative variants was unaffected by homologous phage (i.e., remained at 100% of uninoculated controls) either when paired or in single strain culture if the total numbers were about four times those of protease positive cells. Single strain cultures of protease positive cells lost significant activity with added bacteriophage. As bacteriophage concentration increased from 10¹ to 10⁹ plaque forming units per milliliter, the activity dropped from 72 to 1% of the controls. When paired, the rate of acid production improved and was 90 to 62% of protease negative cells as bacteriophage increased from 10¹ to 10⁹ plaque forming units per milliliter.

Antibiotics affected acid production of both cell types, but protease positives were more inhibited. Pairing did not improve performance of either culture for added antibiotics.

¹ Contribution number 2821 of the Utah Agricultural Experiment Station. Approved by the Director.

² Visiting scientist from University of Cairo, Cairo, Egypt.