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Proteinase System of Lactic Streptococci. IV. Relationship Between Proteinase Activity and Growth at 32 C

Department of Food Science, North Carolina State University, Raleigh 27607

ABSTRACT

The effect of aging at 3 C on cell viability, proteinase activity, ability to grow in casein medium and acid production at 32 C on Streptococcus lactis Number 3 was examined in milk (pH 7.0), sodium phosphate (0.05m, pH 7.0), and potassium phosphate buffer (0.05M, pH 8.0). In milk, cell viability remained constant, but acid production by stored cells subcultured at 32 C was moderately reduced. Residual proteinase activity was diminished considerably. Similar observations were made when the culture was stored in sodium phosphate buffer, pH 7.0. The ability of cells stored in buffer at pH 7.0 to initiate growth in casein medium following storage paralleled the decreased amount of acid formed in milk. It is suggested that cells stored at pH 7.0 may be capable of re-synthesizing damaged proteinase at 32 C. Storage of the culture in potassium phosphate buffer, pH 8.0, resulted in variable cell survival. Acid production by stored cells when cultured in milk at 32 C was diminished considerably. Stored cells became progressively less able to initiate growth in media containing protein as the primary source of nitrogen, or in media supplemented with a protein hydrolysate (Trypticase). The inability of stored cells to initiate rapid growth when subcultured at 32 C in casein medium or milk was related to a dependency on the need of a functional proteinase system.