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P. TEIXEIRA, H. CASTRO, C. MOHÁ CS I-F AR KA S AN D R . KI RB Y . 1997. 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, m 1 -61, m 2 -73, n-80, p-89, q-100, r-112°C). Three of these peaks (l r , m r and p r ) 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; l r , cell membrane lipids, m r , ribosomes, and p r , 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.

P. TEIXEIRA, H. CASTRO AND R. KIRBY. 1996. Membrane fatty acids of Lactobacillus bulgaricus were analysed by gas‐liquid chromatography before and after spray drying. The ratio unsaturated/saturated fatty acids decreased following spray drying, indicating the formation of lesions in cellular lipid‐containing structures. The same method was used to analyse membrane lipids of Lact. bulgaricus during storage. Similarly the ratio of unsaturated/saturated fatty acids in dried cells decreased further during storage in air, presenting evidence of lipid oxidation after prolonged storage. The mechanisms of cell death during storage in the dried state are still unknown, but from these results and those presented in the literature, it seems evident that lipid oxidation and survival during storage may be related.

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