Changes in Cell Membrane Fatty Acid Composition of Streptococcus thermophilus in Response to Gradually Increasing Heat Temperature.

In this study, a method of heat adaptation was implemented in an attempt to increase the upper thermal threshold of two Streptococcus thermophilus found in South Korea and identified the alterations in membrane fatty acid composition to adaptive response to heat. In order to develop heat tolerant lactic acid bacteria, heat treatment was continuously applied to bacteria by increasing temperature from 60°C until the point that no surviving cell was detected. Our results indicated significant increase in heat tolerance of heat-adapted strains compared to the wild type (WT) strains. In particular, the survival ratio of basically low heat-tolerant strain increased even more. In addition, the strains with improved heat tolerance acquired cross protection, which improved their survival ratio in acid, bile salts and osmotic conditions. A relation between heat tolerance and membrane fatty acid composition was identified. As a result of heat adaptation, the ratio of unsaturated to saturated fatty acids (UFA/SFA) and C18:1 relative concentration were decreased. C6:0 in only heatadapted strains and C22:0 in only the naturally high heat tolerant strain were detected. These results support the hypothesis, that the consequent increase of SFA ratio is a cellular response to environmental stresses such as high temperatures, and it is able to protect the cells from acid, bile salts and osmotic conditions via cross protection. This study demonstrated that the increase in heat tolerance can be utilized as a mean to improve bacterial tolerance against various environmental stresses.

Mild heat stress limited the post-acidification caused by Lactobacillus rhamnosus hsryfm 1301 in fermented milk.

OBJECTIVE: Fermented milk is the optimal vehicle for delivering probiotic bacteria. However, the viable count of probiotic bacteria such as some lactic acid bacteria and the post-acidification of fermented milk are a contradiction. The objective of this study was to restrict the post-acidification of the fermented milk containing living Lactobacillus rhamnosus hsryfm 1301. RESULTS: Mild heat stress treatment (46 °C, 1 h) was chosen to help control the post-acidification caused by L. rhamnosus hsryfm 1301. When fermented milk was produced by single L. rhamnosus hsryfm 1301, the heat stress treatment reduced the post-acidification from 0.39 to 0.11% lactic acid, and the viable cells were maintained above 2.0 × 108 CFU mL-1 during 21 days of storage. Although the post-acidification limitation of heat treatment was relatively weak in fermented milk produced by L. rhamnosus hsryfm 1301 and S. thermophilus grx02 (from 0.26 to 0.10% lactic acid), this treatment was still effective. Furthermore, no whey separation in the fermented milk was caused by the treatment. CONCLUSIONS: Mild heat stress treatment could limit the post-acidification caused by L. rhamnosus hsryfm 1301 by decreasing its metabolism and proliferation. This treatment is a promising strategy to improve the shelf life of probiotic fermented milk.