Relationship between solubility of freeze-dried skim milk and death of freeze-dried Lactobacillus delbrueckii ssp. bulgaricus during storage

To elucidate the relationship between death of freeze-dried cells of Lactobacillus bulgaricus during storage and decrease in powder solubility, the effect of milk precipitation during rehydration of freeze-dried Lb. delbrueckii ssp. bulgaricus upon death of these cells throughout storage, via addition of various carbohydrates to the drying medium was studied. The degree of survival of freeze-dried Lb. bulgaricus was found to be directly proportional to the square root of the absorbance of the supernatant. A darker powder resulted when most of the microorganism had died. A tentative explanation for the insolubility observed when the viability of the cells decreased is based on an increase in the extent of nonenzymatic browning (Maillard reactions), as a result of cell death. Results obtained throughout this study might represent a helpful tool towards development of a novel, straightforward, shortcut method for monitoring the viability of freeze-dried samples during storage.