Microwave-assisted maceration and stems addition in Bonarda grapes: effects on wine chemical composition and sensory properties over two vintages

Abstract

AIM: Bonarda, the second red grape variety in Argentina, produces high yields per hectare generating, in several cases, wines with low levels of quality compounds. Microwave-assisted extraction (MW) is a novel extraction technique for winemaking, widely applied in other foods. Stems addition (S) during vinification can be a sustainable technology for phenolic and aroma contribution without additional cost. Therefore, this study aimed to evaluate the combined effect of MW application with stem additions in different conditions, before fermentation, on the chemical composition and sensory properties of Bonarda wines.

METHODS: During two consecutive vintages (2018-2019), 450 kg of grapes were harvested (≈24°Brix) from a commercial vineyard (Mendoza, Argentina), and made into wine in 25 L following a standard protocol. The experimental design consisted of ten treatments (two factors) by triplicate. Two maceration strategies were applied [control (C), and microwaved-assisted extraction after grape crushing (MW; 2450 MHz, 7600 W, 45-50°C)], combined with five stem-contact conditions [control without stems (WS), 50% stems addition (S50), 50% stems addition + MW of the stems (S50MW; 2450 MHz, 7600 W, 60°C), 100% stems addition (S100), 100% stems addition + MW (S100MW)]. Wines were analyzed for basic chemistry (1), phenolic composition and color parameters (2-5), polysaccharides (6), and aroma profiles (7). Additionally, a descriptive sensory analysis (QDA) was performed with 19 panelists in 8 sessions, and 22 attributes were established.

RESULTS: In both seasons, the application of microwaves significantly reduced microbial flora in musts (fungi, yeasts, and acetic acid bacteria), in addition to inhibiting enzymatic activity (cellulase and pectinase). Due to the significant difference of the vintage and its interaction with some of the studied factors, the chemical and sensory characterization of wines were evaluated separately for each season. The 2018 wines showed higher pH with stem additions and MW application in both matrices. Stem additions increased tannin content by 63% (S100) and by >35% for the other treatments; while MW consistently improved phenolic extraction (mainly, anthocyanins and derivatives), and polymeric pigments formation. Likewise, combined strategies increased polysaccharides extraction (FI, 165 kDa; FII, 45 kDa; FIII, 12 kDa), enhanced wine color (greater saturation), and intensified violet hue. Finally, the PCA including sensory variables described the MWS50 wines with higher color intensity and chocolate aroma, and 100% stems addition treatments with more astringency and violet hue. The behavior observed in 2019 was similar, with a more marked effect of MW on wine color (C*ab and polymeric pigments).

CONCLUSIONS:

The reported results are promising and are considered the first advance in the knowledge of the impact of the proposed technological strategies on the chemical and sensory quality of red wines.