Optimisation of methods for the determination of total and red-free glycosol glucose in black grape berries of Vitis vinifera

Abstract

The determination of glycosyl glucose (G-G) of black grape berries was found to be optimal, if measured on an aliquot taken from an extract made for 1 h with 50% ethanol of homogenates prepared from either fresh or stored (frozen -20°C) berries. The homogenisation of berries resulted in compounds, presumably phenolic, being extracted from the seeds, which interfered with the enzyme assay of glucose. This interference was eliminated by passing the control and hydrolysate solutions through a C18 RP solid phase extraction cartridge prior to analysis of glucose. Without this step, the concentrations of G-G of berries were underestimated. Subtraction from the measured G-G of the concentration of the glucose moiety of the anthocyanins of the berries, determined spectrophotometrically on the ethanol extract, gives a new measure, red-free G-G. Preliminary data on each of these variables in Shiraz and Pinot Noir grapes are presented.