CLEANSING CONTAMINATED PITCHING YEAST WITH NISIN

Abstract

The ability of solutions of nisin to cleanse pitching yeast of contaminating gram-positive lactic bacteria (at levels of about 1% by cell numbers) was compared to that of three different acid-washing procedures: ammonium persulphate, acidified ammonium persulphate or phosphoricacid. 1000 international units ml−1 of nisin killed all the cells of more sensitive bacterial strains in less than 30 minutes, equivalent to the killing ability of the acid-washing procedures, and more than 99·9% of cells of a more resistant strain in 2 hours. Washing contaminated pitching yeast with nisin has several advantages over acid-washing methods. It has no effect on yeast viability and vitality, and, in subsequent fermentations, the yeast shows unchanged flocculation characteristics and fermentative performance.