Vermicomposting grape marc yields high quality organic biofertiliser and bioactive polyphenols

Grape is the largest fruit crop in the world, and most (80%) of the harvested fruit is used to make wine. The main by-product of the wine industry is called grape marc, which consists of the stalks, skin, pulp and seeds that remain after pressing the grapes. The aim of this study was to evaluate whether grape marc could be processed by vermicomposting on an industrial scale to yield both a high-quality organic, polyphenol-free fertiliser and grape seeds (as a source of bioactive polyphenols). Vermicomposting reduced the biomass of grape marc substantially (by 58%), mainly as a result of the loss of volatile solids. After 2 weeks, the process yielded a nutrient-rich, microbiologically active and stabilised peat-like material that was easily separated from the seeds by sieving. Although the polyphenol content of the seeds was considerably reduced, this disadvantage was outweighed by the ease of separation of the seeds. Separation of the seeds also eliminated the polyphenol-associated phytotoxicity from the vermicompost. The seeds still contained useful amounts of polyphenols, which could be directly extracted for use in the pharmaceutical, cosmetic and food industries. The procedure described is effective, simple and economical, and could easily be scaled up for industrial application.