Exploring differential scanning calorimetry as a tool for evaluating freezing stress sensitivity in Baker's yeasts

Thermal transitions were analyzed after freezing and thawing of two commercial baker's yeast cells of Saccharomyces cerevisiae having high and low tolerance to sugar. Yeast in stationary phase of growth were exposed to two cycles of freezing (20 h and 4 h) at −20 °C after being exposed to various physical and chemical mild stresses and differential scanning calorimetry (DSC) runs were performed at 10 °C/min from 20 °C to 100 °C. Thermograms showed two peaks that were attributed to low and high protein onset denaturation temperatures: 47–58 °C and 67–77 °C, respectively. Apparent enthalpy changes showed that the sugar tolerant strain had a relatively higher resistance to freezing and thawing than the non-tolerant one. On the other hand, some pre-stress treatments (e.g., temperature, ethanol and NaCl) increased both enthalpy values and survival after freezing and thawing. The analysis of data clearly showed that DSC can be a very useful tool to analyze the behavior of yeast cells after freezing and thawing stress.