Development of a new commercial phenolic analysis method for red grapes

Abstract

Grape phenolic content is an important quality factor that influences the appearance and mouthfeel of premium red wines. The wine industry uses a variety of commercial phenolic assays to determine phenolic maturity in red grapes. Some of these assays, however, are dated more than 20 years ago and do not always accurately reflect wine phenolic content from grape phenolic extracts. The aim of this study is to develop an adjusted phenolic sample preparation and extraction protocol so it can be used in commercial wineries and/or laboratories.In this study, six grape cultivars (Pinotage, Cabernet-Sauvignon, Merlot, Shiraz, Cinsualt and Pertit Verdot) were collected from 42 different vineyards from across 15 different farms. Representative samples were taken from the grapes of each block. Grape extractions were done in duplicates using four different methods namely Glories, Iland, Modified Iland and a custom made Machine crushed method. The Glories, Iland and modified Iland methods produces homogenized grapes, while the machine crushed method uses grape samples where only the skins were crushed. The modified Iland and machine crushed extraction methods were exposed to microwave treatment and extracted in a 50% alcohol solution for 30 min and 1 hour and 3h, 24h and 40h, respectively.Wines were made from every grape samples. Phenolic analyses were done for anthocyanins, tannins, total phenols index and colour density on the grapes and wines. Variation in the phenolic composition of the grapes where the different extraction methods were observed. Correlations between grapes and wines phenolic data with the different grape extraction methods will also be shown.