Water activity at the fruit surface: a potential indicator ofgrape berry susceptibility to Botrytis cinerea

Abstract: Water activity (Aw) is a physical property of a humid solid, which is of primeimportance in food microbiology. This parameter is an assessment of available water, unbound tonutrient molecules, which allows damaging bacteria, yeasts and fungi to grow in a food product.As most moulds which do not grow below an Aw level of ca. 0.8, Botrytis cinerea was unable togrow on artificial media at an Aw of 0.93 and below. In 2008, in an experimental vineyard (Vitisvinifera L. cv. Merlot noir) near Bordeaux, Botrytis bunch rot development was assessed and theevolution of water activity was investigated at the surface of healthy grape berries during fruitdevelopment. Our results showed that Aw decreased steadily from the berry herbaceous stage tomaturity and was correlated negatively and significantly (P = 0.01) with disease incidence.Furthermore, an experiment under controlled conditions showed the significant effect of relativehumidity (RH) on the water activity level at the berry surface. When healthy berries at two differentdevelopment stages (“herbaceous” and “beginning of colour change”) remained at 80 % RH for 6days, the Aw values decreased significantly (P = 0.05). On the basis of the close relationshipbetween water activity and Botrytis bunch rot development, the potential of Aw for use as a newdisease indicator is discussed as well as the influence of climatic conditions on water activitymeasured at the grape berry surface.