The Survival of Starter Organisms in Concentrated Suspensions

Abstract

Summary: Two representative lactic acid bacteria were grown in a rich organic medium, the cells were harvested by centrifugation, and the organisms were suspended in skimmilk at concentrations of 25–55 × 109/ml. The pH was adjusted to different values and the suspensions were stored, with and without the addition of 20% (v/v) glycerol, at 4° and ‐ 20°.Both organisms survived better at pH 7°0 than at pH 5°0. Glycerol protected the cells at the lower pH value but offered no benefit at neutrality. At 4° about half the cells died within 5 or 6 weeks. At ‐ 20°, however, there was no appreciable loss of viability or acid producing ability up to 9 months at pH 7°0. Suspensions stored for 9 months at ‐ 20° produced excellent cultured buttermilk within the normal incubation period from an inoculum comparable to that used for a fresh culture. Cells harvested early in the maximum stationary phase of the growth cycle were more active than older cells, and they survived better than older cells during storage in the frozen state.