Stabilization effects on immobilized yeast: Effect of gel composition on tolerance to water miscible solvents

A working system for systematically studying stabilization effects on yeast immobilized in gels of defined composition was established. The system is based on entrapment in polyacrylamide-hydrazide (PAAH) crosslinked by glyoxal as the parent polymeric system. By chemical modification of PAAH or by copolymerization, series of derivatives and analogues are available. PAAH, a negatively charged derivative of PAAH and an hydrophobic analogue, were used for studying the effect of gel entrapment on yeast tolerance to a series of water miscible solvents. PAAH entrapment increased the tolerance to ethanol and ethyleneglycol, but not to other solvents such as methanol and dimethylformamide. Modification of PAAH backbone had no additional effect on this phenomenon in respect to ethyleneglycol, but affected the stabilization to ethanol: added hydrophobicity reduced the protective effect while added charges seemed to improve it. A complex mechanism seems to be involved in this protective effect, probably involving single cell encapsulation within a thin, membrane-like gel layer. This hypothesis was strongly supported by studies on freely suspended cells, individually coated by polyacrylamide layer, chemically grafted onto cell surface. These coated cells exhibited enhanced tolerance to ethanol with a pattern resembling the behaviour of the PAAH entrapped cells.