Yeasts in botrytized wine making

Abstract

The fungus Botrytis cinerea attacks ripening grapes in humid conditions and usually causes devastating grey rot. If humidity fluctuates (e.g. humid nights alternate with dry sunny days), the infected grapes develop a different type of rot, the benevolent “noble rot” (“pourriture noble”, Edelfäule”). Proper humidity fluctuation requires specific microclimatic conditions that are characteristic of terroirs of specific geographical locations. Many of the world’s greatest sweet wines, the so-called botrytized or Botrytis-affected wines are crafted from shriveled, mold-covered nobly rotten grapes. Upon Botrytis invasion, the berries are usually co-colonized by bacteria and yeasts whose activities modify the chemical composition of the grape juice. These microorganisms commence fermentation within the berries before harvest. The pre-harvest grape microbiota is particularly rich in non-Saccharomyces yeasts. These yeasts form then the starting microflora of the fermenting must but are gradually overgrown by strains of S. cerevisiae and S. uvarum. Some of them can persist throughout the fermentation-vinification process up to the aging phase and thus can have significant impact on the quality of the wine.