Federico Casassa,* Esteban Bolcato, Vanesa Garcia, Santiago Sari, Martin Fanzone, and Viviana Jofre
*Wine and Viticulture Department, Cal Poly San Luis Obispo, Cal Poly SLO, Bldg 11, Room 219, San Luis Obispo, CA 93407 (lcasassa@calpoly.edu)

Microwave-assisted extraction (MW) is scalable to industrial conditions and thus potentially could be adopted by the wine industry. Merlot grapes harvested at three different stages of ripeness (21, 24.5, and 28 Brix) were subjected to MW (seven min, peak temperature 50 ± 4°C, n = 48) and made into wine. Additional, untreated wines were produced as a control. All wines were produced in triplicate with a 12-day maceration and were followed for phenolics and color from pressing to five months of bottle aging (BA). MW had no effect on the basic composition of the wines (alcohol, residual sugars, glucose, fructose, titratable acidity, malic, citric, lactic and acetic acids, pH, and glycerol) with the exception of tartaric acid concentrations, which were lower in MW wines. Extraction patterns of anthocyanins, tannins, polymeric pigments, and wine color indicated that MW applied to unripe fruit (21 Brix) increased all of these parameters at pressing and the differences persisted after five months of bottle aging. In ripe (24.5 Brix) and overripe fruit (28 Brix), MW only enhanced tannins by ~11%, but in unripe fruit, MW enhanced tannins by 27% after five months. For the three ripeness stages, MW achieved a quick extraction of monomeric anthocyanins, averaging a 193% increase relative to their controls, but these differences were reduced and disappeared in wines made from ripe or overripe fruit. The wines and their replicates were also analyzed for monomeric anthocyanins, pyranoanthocyanins, and flavanol-anthocyanin adducts by HPLC-DAD-MS after malolactic fermentation. Total anthocyanins, total glucosides, coumaroyl-glucosides, cinnamoyl-glucosides, B-type vitisins, and total pyranthocyanins increased progressively in the wines as fruit ripeness increased from 21 to 28 Brix. MW had no effect on total anthocyanins but increased acetyl-glucosides, B-type vitisins, and flavanol-acetyl-anthocyanin adducts, the latter probably due to their higher tannin concentration. In Merlot, MW could be a valuable tool to treat unripe fruit to enhance phenolic extraction and color stability over time.

Funding Support: INTA