Changes in grape-associated microbiome as a consequence of post-harvest withering

Abstract

AIM: Grape withering is an oenological post-harvest process used for production of reinforced and sweet wines. Drying can be carried out by keeping the ripe grape in traditional large, well-aired rooms (non-controlled environment) or, more and more often, in a warehouse under controlled conditions of airflow and relative humidity (controlled environment)[1]. The microbiome associated with withering grapes has been showed to be profoundly linked with the process and its results[2,3]. The main aims of this study were to (a) provide detailed information on bacterial and fungal communities evolution throughout the grapes withering process, and (b) perform a comparative study between two dehydration methods, regarding the associated microbiomes.

METHODS: Samples of withering grapes were collected in the Italian viticultural zone Valpolicella, where the renowned wine Amarone is produced using non-botrytized withered grapes of Corvina variety. Two different post-harvest conditions were analyzed (non-controlled and controlled withering environment); grapes coming from two vineyards (close but differing for soil characteristics) were considered, during 2 subsequent vintages. To map the microbiome during withering, Next-Generation Sequencing (NGS) was employed[4]: the progression of fungal and bacterial species was characterized through metabarcoding (ITS and 16s) at 4 different time points (from 0 to 30% of weight loss).

RESULTS: No significant differences, at biodiversity level, were found between the microbial communities of grapes from the two vineyards, nor between the two vintages. The evolution of microorganisms during drying was instead interestingly variable. Moreover, slight but significant differences were found between the two withering systems, although significant only for some taxa.

CONCLUSIONS: NGS metabarcoding showed to be an effective technique in the study of withering-grape microbiome and provided new information on the changes occurring in microbial communities because of the drying process. Indeed, to our knowledge, the present work is the first time-course study of both mycobiome and bacteriome throughout withering. The study also showed that changes of drying conditions can lead to significant modifications of the berry-skin microbiota.