The objective of this study was to evaluate the effect of microwave treatment of crushed grapes on the yeast population of the must and on the development of alcoholic fermentation, as well as on the extraction of different compounds from the grapes such as polysaccharides and amino acids that can affect the organoleptic quality and stability of the wine.
Experimental layout
Cabernet Sauvignon red grapes were harvested at 24.9 °Brix. After destemming and crushing, samples were divided into two batches (60% must and 40% skins for each one) and sulfited with 50 mg/L of potassium metabisulfite. One batch was immediately moved to a chamber at 22 °C (±2 °C) without any treatment and used as control. The other batches (MW) were microwave-macerated for 12 min at 700 W using a domestic LG microwave oven (LG electronics). The temperature of the batch prior to the treatment was 14 °C. To avoid an increment up to 50 °C, the microwave treatment was realized at 4 min intervals. Alcoholic fermentation was realized in 10 L glass containers that were not filled more than 60%, using Saccharomyces cerevisiae (CECT no. 10835) as the starter culture, at 22 °C.
Results
The application of microwaves in crushed grape maceration did not modify the population of yeasts and total bacteria in the must, although it had an important effect on the biodiversity of the grape biota, producing a reduction in the number of species identified. Additionally, a reduction in the lag phase and more efficient fermentation kinetics were observed in wines from microwave treatment. The basic composition of the treated wine was little affected, but samples from microwave treatment showed a higher total polyphenol index, and higher content of major amino acids and total assimilable nitrogen. Some volatile compounds that could influence the aroma of wines, such as 2-phenylethanol and isoamyl acetate, were also increased by MW treatment.
The effect of microwave grape treatment on polysaccharide composition was more evident in musts than in wines, mainly in the case of pectic monosaccharides. Likewise, the PRAGs, the main polysaccharides liberated from crushed grapes, showed higher amounts in musts when microwaves were applied before maceration.
According to the results obtained, the microwave treatment of crushed grapes is proposed as a very interesting technique to increase the extraction of compounds from the grape, such as amino acids and polysaccharides, which may play an important role in the organoleptic quality and stability of the wine. In addition, the effect of this treatment on the microbial population of the must and the development of fermentation must be considered. In any case, it would be necessary to carry out additional experiments with different grape varieties and degrees of maturation to corroborate these results.
Reference:
Muñoz García R, Oliver-Simancas R, Arévalo Villena M, Martínez-Lapuente L, Ayestarán B, Marchante-Cuevas L, Díaz-Maroto MC, Pérez-Coello MS. Use of Microwave Maceration in Red Winemaking: Effect on Fermentation and Chemical Composition of Red Wines. Molecules. 2022; 27(9):3018. https://doi.org/10.3390/molecules27093018
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