Influence of skin contact and different extractants on extraction of proteins and phenolic substances in Sauvignon Blanc grape skin

Background and Aims: Although most proteins and a significant proportion of phenolic substances are extracted from grape pulp, the extraction of grape skin components may contribute to and modulate the final concentration of these components in juice. This study investigated the influence of skin contact time and various extractants on the extraction of Sauvignon Blanc grape skin components. Methods and Results: Two trials were conducted. The first evaluated the impact of skin contact time on the extraction of phenolic substances and proteins from grapes (fresh vs chilled) into juice. The juice with 24 h skin contact showed a significantly higher concentration of phenolic substances and chitinases, but not of thaumatin-like proteins. No difference was observed between fresh and chilled berries. The second trial evaluated the extractability of phenolic substances and proteins from grape skin (ground vs peeled) using different extractants. More phenolic substances and tannin were extracted using ground skin powder as greater mechanical damage assisted in the extraction of skin components. Compared to mechanically crushed and pressed grape juice, hand squeezed juice showed a lower protein concentration and the presence of tannin. It is suggested that hand squeezing was more effective in extracting components (mainly phenolic substances) from grape skin, but mechanically crushing and pressing was more effective in extracting components (mainly proteins) from grape pulp. Extractants containing bovine serum albumin or protein resulted in a lower concentration of phenolic substances and tannin in the extracts, and the concentration of bovine serum albumin and protein was dramatically decreased after extraction. Conclusion: This study confirmed that longer skin contact increased the extraction of skin phenolic substances. Longer skin contact also increased the protein concentration in juice, particularly the pathogenesis-related proteins, but it can be modulated by the co-extraction of phenolic substances. Significance of the Study: This study improves our current understanding of interactions between phenolics and proteins during grape processing, which can be used as a tool for winemakers to manage the extraction of these compounds into juice

Time course of the colour of young red wines from Vitis vinifera L. during ageing in bottle

The colour characteristics of red wines from Vitis vinifera L. cv Tempranillo, Graciano and Cabernet Sauvignon (vintage 2000) from Navarra (Spain), was studied during 26 months of ageing in bottle through the evaluation of the wine visible spectrum and of several colorimetric indices (colour intensity, %red, %yellow, %blue, %dA and tint) and CIELAB variables (L*, C*, h, a* and b*). During ageing in bottle, the spectrum of Tempranillo wine (pH 4.3) mainly changed in the absorbance range between 420 and 500 nm, whereas Graciano (pH 3.5) and Cabernet Sauvignon (pH 3.6) wines registered a decrease in absorbance in the interval between 500 and 560 nm. The time course of the different wine colour parameters was found to fit either a second-order polynomium or a linear model, depending on the grape variety employed. CIELAB variables could be described in terms of their colorimetric index counterparts, showing b* and h relative greater errors. Although the wine total chromatic changes in CIELAB units registered after 26 months of ageing in bottle indicated changes perceivable by the human eye (ΔE* ≥ 2.7) for the three varieties studied, Graciano and Cabernet Sauvignon wines showed a more balanced colour evolution than Tempranillo wine.The authors also thank the Agencia Española de Cooperación International (AECI) for a MUTIS predoctoral scholarship to M.M. and the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) (Project AGL2003-07394-C02-02) for funding.Peer reviewe