Effect of grape stem inclusion fermentation on Pinot noir wine composition

Abstract

Purpose/ Aim. Whole bunch fermentation with grape stems is believed by some winemakers to increase complexity, freshness, aromatic expression and textural smoothness in resultant wine, but may also impart green, herbal and earthy characters. However, there is limited published research focused on the impact of including stems during fermentation on Pinot noir wine composition. In this study, different proportions of whole bunches were included in the winemaking, and the resultant wines were analysed for general oenological parameters, phenolic composition and volatile aromatics with the aim to investigate the effect of stem inclusion fermentation on Pinot noir wine composition. Results of this study can be used to aid winemakers to produce Pinot noir wines in different styles. Methodology. Five treatments were examined in this study: 100% destemmed and crushed grapes (DS), 100% destemmed and crushed grapes with stems added back (DS100), 30% whole bunches (WB30), 60% whole bunches (WB60), and 100% whole bunches (WB100). Triplicate ferments of each treatment were done using a standard winemaking protocol, which included 5 days cold maceration at 4 0C and 4 days post-fermentation maceration at room temperature. Total soluble solids (TSS), pH, titratable acidity (TA), alcohol content, and residual sugar content were analysed. A range of phenolic and tannin assays was performed: total tannin was determined using the 1 mL assay of the methylcellulose precipitation (MCP) method, Folin-Ciocalteau colourimetry for total phenolic content, modified Somers method for total anthocyanin and colour evaluation (Mercurio et al., 2007), and solid-phase extraction (SPE) was used to separate monomeric phenolics and polymeric phenols before HPLC analysis Jeffery et al. (2008). The monomeric phenolics fraction was analysed using the method described by Gómez-Alonso et al. (2007). Headspace solid-phase microextraction gas chromatography mass spectrometry (HS-SPME-GC-MS) was used to determine selected aroma compounds in prepared wine samples as per the method described by Tomasino et al. (2015). In all cases, ANOVA was performed with the Tukey comparison test to compare means with a 5% level for rejection of the null hypothesis. Results. Stems were included into treatments either by adding stems (DS100) or whole bunches (WB30, WB60, WB100). Both stem inclusion and whole bunch addition caused a decrease in alcohol concentration compared to DS treatment. The pH values were significantly higher in whole bunches/stems added treatments compared to DS treatment. The effect of stem inclusion was evaluated by comparing DS100 and DS treatments, since the presence of stems in the DS100 treatment was the only difference. Stem inclusion increased tannin and total phenolic concentrations, but when comparing DS100 treatment with WB100 treatment (both contain the same amount of stems), total phenolics and tannin concentrations were higher in WB100. Tannin and phenolic concentration were increased with the rate of whole bunch addition (WB30, WB60, and WB100) compared to DS treatment. Stem inclusion and whole bunch addition caused a decrease in anthocyanin concentrations with the DS treatment being highest and the DS100 lowest. Whole bunch/stem inclusion caused an increase in the degree of ionisation of anthocyanin, and hue compared to DS treatment. Lowest SO2 resistant pigments was recorded in DS100 treatment and highest recorded in WB60 and WB100 treatments. It has been shown that anthocyanins can adsorb into stems during cold maceration as well as extract tannins from stems that may react with free anthocyanin to form pigmented polymers (Suriano et al., 2015). However, in most of the analysis results from oenological parameters and colour assessment results, WB30 was not sufficient to result in a significant difference compared to DS treatment. Concentrations of catechin, gallic acid, caftaric acid, cis-coutaric acid, caffeic acid, and resveratrol in resultant wines were increased with incremental whole bunch addition treatments compared to DS treatment respectively. However, in most cases, WB30 was not enough to result in a significant increase compared to DS treatment. When examining the contribution of stems, DS100 had values between WB60 and WB100 treatments, indicating that stems alone did not account for the increases. Ethyl 2-methylbutyrate, diethyl succinate, ethyl cinnamate, phenol, eugenol concentrations were increased with the rate of whole bunch/stem addition and decreased the concentrations of 2-methylbutyl acetate and hexyl acetate in resultant wines. Aroma compounds of cinnamon, vanilla, woody, fusel, spiritous characteristics were prominent in WB100 and DS100 treatments and fatty, cheesy, fruity and floral aroma compounds were prominent in DS, WB30 and WB60 treatments. Conclusion. Stem inclusion and whole bunch addition affected oenological parameters, monomeric phenol composition, and aroma profiles of resultant wines. The effect of stem inclusion evaluated by comparing DS and DS100 treatments, stem inclusion did significantly reduce alcohol, and total anthocyanin concentrations and significantly increased pH, tannin, phenols and hue. Wines fermented with higher proportion of whole bunches/stems showed lower alcohol content, and higher pH, MCP tannin, and total phenols compared to DS treatment. In most cases, 30WB showed no change to basic oenological parameters, wine colour and monomeric phenol composition to DS, but did significantly increase pH, and total anthocyanin compared to DS treatment. Both whole bunch addition and stem inclusion treatments resulted in cinnamon, vanilla, woody, fusel, and spiritous compounds.