Using microwaves as a pre-treatment for enhancing the extraction of polyphenols from grape stems

Abstract

Grape stems are a residual woody material from the vinification process. A waste that, if it is not treated in a correctly manner, could represent an environmental problem. Moreover, in the last years, special attention has been paid to this waste due to its high content of polyphenols, mainly stilbenes and flavonoids. Several studies have already revealed the potential and possibilities of these compounds in industries, such as alimentary, cosmetic and pharmaceutic thanks to their antioxidant, antimicrobial and/or anticarcinogenic properties. In this work, the effect of solid-liquid extraction parameters of polyphenols from grape stems have been studied. These parameters were: solid-liquid ratio (RS-L), type of solvent (variation of the percentage of ethanol in the hydroalcoholic mixture) and temperature. Parameter values selected as the best for polyphenol extraction in a conventional solid-liquid extraction were: a RS-L of 0.10 g/mL, a temperature of 75ºC and a hydroalcoholic mixture with a 50% vol. of ethanol. Furthermore, microwaves were applied to grape stems as a pre-treatment prior to the conventional extraction for the first time. In this case the parameters assessed were also solid-liquid ratio, type of solvent and, in addition, the time of the pre-treatment. For this purpose, a statistical surface design was employed to obtain the optimum conditions which maximize the final TPC of the extracts. In a first approach, extracts were characterized in terms of total polyphenol content and total flavonoid content. The main result was that, microwaves make it faster the extraction (until 4 times) but it is not clear their role on the extraction yield. Key-words. Grape stems, stilbenes, flavonoids, microwave