Impact of mild heat treatments on induction of thermotolerance in the biocontrol yeast Candida sake CPA-1 and viability after spray-drying

Abstract

The objective of this study was to examine the induction of thermotolerance in the biocontrol agent Candida sake CPA-1 cells by mild heat treatments to enhanced survival of formulations using spray-drying. The possible role of heat-shock proteins (HSPs) biosynthesis in induced thermotolerance and the role of sugars and sugar alcohols were also determined. Studies were conducted on C. sake cells grown in molasses medium and exposed to mild temperatures of 30 and 33 degrees C during mid- (16 h), late-exponential (24 h), early- (30 h) and mid-stationary (36 h) growth phases. The effect on viability was determined both before and after spray-drying. Cycloheximide and chloramphenicol were used to examine the role of HSPs and HPLC was used to analyse the accumulation of sugar and sugar alcohols. The results indicate that both temperatures induced thermotolerance in cells of C. sake. Mild heat-adapted cells at 33 degrees C in the early- or mid-stationary phases had survival values after spray-drying significantly higher (P <or= 0.05) than nonadapted cells. However, viabilities were not high enough to be considered for commercial use with values up to 17%. HSPs were not implicated in thermotolerance acquired by mild heat-adapted cells as similar viabilities were obtained in the presence of protein inhibitors. Little change was observed in sugar and sugar alcohols with an increase in glucose and arabitol in some treatments. This study suggests that it is possible to induce thermotolerance in biocontrol yeasts such as C. sake. However, this does not improve survival of cells exposed to spray-drying sufficiently to consider this a suitable formulation method for this biocontrol agent. HSPs, sugars and sugar polyols were not directly responsible for induced thermotolerance in yeast cells. This type of information can be effectively applied to improve the viability of cells in the process of formulation.