Activity-loss kinetics of freeze-dried lactic acid starter cultures

Abstract

Lactic acid starter cultures were freeze-dried in whey and nonfat skim milk (NFSM) suspending media with varying amounts of glycerol, adonitol, and sucrose as the additives. Generally additives had different cryoprotective effects in NFSM and whey media. There was also an optimum cryoprotective additive concentration where viability and activity were protected best. The best results were obtained with freeze-dried L. bulgaricus with 0.29 m sucrose, 0.75 m adonitol, or 0.75 m glycerol in whey medium. Similarly, the best protection was obtained with S. thermophilus with 1 m , 0.5 m , and 0.75 m adonitol in NFSM medium and 1 m adonitol in whey medium. Viability loss during freeze-dried storage was modeled with a first-order kinetic expression in viable cell concentration. Activity loss was modeled with a first-order kinetic expression in activity. The major cause of activity loss was death of the microorganisms. Activity loss and viability loss were parallell during storage; an occasional deviation from such a parallel trend occurred due to the presence of injured microorganisms that did not produce lactic acid.