Browning of freeze-dried probiotic bacteria cultures in relation to loss of viability during storage.

Freeze-dried cultures of Lactobacillus acidophilus (La-5) showed visible brown discoloration even after a short storage at relatively mild conditions (a(w) = 0.22 and 30 degrees C), and the browning processes were found to coincide with bacteria inactivation. It was demonstrated, by using high-pressure treatment for obtaining bacteria samples with different ratios of live/dead bacteria, that death of bacteria is not a prerequisite for the browning processes. Furthermore, it was shown that hydroxymethylfurfural (HMF) (or condensation products of HMF) introduces accelerated viability loss when HMF is added to the freeze-drying medium. Discoloration of bacteria cultures containing only sucrose/maltodextrin or lactose/maltodextrin in the freeze-drying matrices is suggested to be related to various types of nonenzymatic browning reactions, including carbonyl-protein (or carbonyl-DNA) interactions and carbohydrate condensation/polymerization (without involvement of proteins), the latter proceeding at low a(w) following hydrolysis of the peptidoglycan layer in the bacteria cell wall. More than one single type of browning reaction is accordingly concluded to be related to bacteria death, and the loss of viability in freeze-dried bacteria seems to be influenced by oxidation reactions, browning reactions, and the physical instability of the bacteria membrane/cell wall.