Changes in the yeast Saccaromyces cerivisae cell wall associated with autolysis in a simulated industrial process and study of the activity of related enzymes
Although production of yeast autolysates is of considerable commercial importance, to date little has appeared in the published literature on the autolysis process. Yeast autolysates are widely used in the food industry because of their flavouring and taste-enhancing potential and for their nutritional benefit. Compounds arising from breakdown of proteins and nucleic acids and their derivitive nucleotides and amino acids, and lipid degradation are of primary importance as flavourings. In developing countries vitamins, and proteins, are of significance in enhancing nutritional value of foodstuffs. However, the dissolution of the yeast cell wall is central to the properties of the final autolysate and, to date, little information on this process has been reported. In this study the changes in the yeast cell wall that take place during autolysis were examined. A model for industrial autolysis of yeast, simulating a thermal induction that is in widespread commercial usa ge, was studied in detail using chemical and enzymic analyses and fluorescence light microscopy. Analysis of yeast cells at different stages of autolysis was carried out using scanning and transmission electron microscopy. It was found that general erosion of the cell wall took place during autolysis with the chitinous bud scars becoming increasingly prominent as the process proceeded. Microscopic study of ruptured cells indicated that there were no preferred points of cell wall fracture. Compositional analysis showed that glucan and mannan components of the cell wall were depleted as autolysis progressed whereas depletion of chitin was not significant. Analysis of the enzymes in autolysing yeast cells showed that thermal induction of protease, glucanase, mannanase and chitinase took place in the model process. Two major classes of glucanases could be identified: β-1,3 glucanases had the highest specific activity of enzymes in autolysing cells. Study of the glucanases using iso-electric focusing followed by overlaying of gels with yeast glucan showed that multiple individual enzyme species were induced during autolysis. It was concluded that during the commercial yeast autolysis process, general erosion of the cell wall takes place with depletion of glucan and mannan that can be observed in microscopic studies as a reduction in intensity of staining. Rupture appeared to take place non-specifically at points where erosion had weakened the integrity of the cell wall. It was noted that intact chitin-rich bud scars were prominent in fragmented cell walls.