Application of microwaves during the winemaking of Garnacha grapes grown in a warm climate: effects on the final wine

Garnacha variety is one of the most important red Mediterranean grape cultivars grown in the world. However, it may not reach optimum chromatic characteristics in warm climates such as Andalusia (Southern Spain). This study analyses the impact of the application of microwaves over the crushed grapes (MW) at laboratory scale over two vintages on the characteristics of Garnacha wines elaborated from grapes cultivated in a warm climate as an additional procedure to traditional red winemaking (CT) to obtain highly coloured wines. Oenological and colour parameters were measured to determine the effects of this treatment on wines throughout the winemaking process. Overall, wines made by MW had a higher phenolic content and colour intensity. The analysis of the detailed composition of the wine showed that the treatment with microwaves before the alcoholic fermentation leads to important differences in its colour properties. Thus, the treatment with microwaves induced easily perceptible changes in CIELab coordinates (lower L* and higher a* and C*_ab values than CT). Additionally, it also showed to be significantly effective to increase the extraction of phenolic compounds from grape solids into grape juice, both globally (total polyphenol index and Folin–Ciocalteu index), and particularly regarding flavanols and flavonols content. Most of the above-mentioned significant differences remained with ageing. The reported results are promising and the treatment with microwaves in the optimized conditions seems a feasible auxiliary alternative in warm climates, where wines produced from some red grape cultivars often show suboptimal colour characteristics.