Identification of sites of injury in Lactobacillus bulgaricus during heat stress

Abstract

Heat resistance of Lactobacillus bulgaricus in skimmed milk at 62°, 64°, 65° and 66°C was studied. The response to increasing temperatures in this range was not linear, with temperatures at 65°C and above giving a lower survival rate than would be predicted from experiments at lower temperatures. To identify sites of injury at these temperatures, chemical markers were used. Heating at 64°C and below resulted in damage to the cytoplasmic membrane. At temperatures of 65°C and above chemical markers also indicated damage in the cell wall and proteins. Using differential scanning calorimetry analysis of whole cells of Lact. bulgaricus seven main peaks were observed (l–51, m1–61, m2–73, n–80, p–89, q–100,r–112°C). Three of these peaks (lr, mr and pr) were the result of reversible reactions. Analysis of cell fractions identified the cell structure involved in giving rise to each of the three reversible peaks; lr, cell membrane lipids, mr, ribosomes, and pr, DNA. The evidence presented in this paper shows that irreversible reactions in the cell ribosomes are a critical site of damage in Lact. bulgaricus during heat stress in liquid media at 65°C and above.