WINETECH Technical Yearbook 2019

wine enzymes can shorten the contact phase without negatively impacting wine quality. Commercial enzymes are complex crude and semi-purified extracts from sources, such as the Aspergillus fungus. A problem is that we do not always know what are in the enzyme mixes, as we do not have a detailed inventory of the components present. The standard scientific theory is that polygalacturonases (enzymes that break the link between two unmethylated galacturonic acid units in the pectin chain) and pectinmethylesterases (enzymes break the link between the methyl groups and the galacturonic acid units) are the most important enzymes that work together to degrade pectin. In theory the combination of these two types of enzymes should be sufficient to be able to degrade the pectin layers of wine grapes. However, grape pectin is highly methylesterified (methyl groups added to the galacturonic chains) and just these two enzymes cannot necessarily effectively break down the polymer chains (Gao et al ., 2015). EXPERIMENTAL RESULTS The grape skins contain much of the colour pigments and tannins, and it is these thickened cells that are also rich in complex methylated pectins (Gao et al ., 2015). The research conducted at the Institute

for Wine Biotechnology (Stellenbosch University) on Cabernet Sauvignon has shown that purified polygalacturonases and pectinmethylesterases on their own had very little to no effect on breaking down these skin layers in experimental vinifications (Gao et al ., 2016). Adding just these enzymes did not enhance juice (must) volume or improve polymeric pigment and tannin levels in experimental wines. This was highly unexpected and led us to think more deeply about how these enzymes work, re-evaluating this conventional hypothesis (Gao et al ., 2016). That these two enzymes did not enhance colour pigments or tannins compared to untreated fermentations suggested the hypothesis needed updating. We therefore suggested that the complex pectin and cellulose-rich layers block enzyme action on skin-rich caps in wine ferments. The research has shown that grapes contain two layers of pectin and the skin cells appear to be coated with highly methylesterified pectins (Gao et al ., 2015). The research found formulations rich in pectin-lyases (enzymes that break the link between two methylated galacturonic acid units) and rhamnogalacturon-lyases were able to degrade this wine grape pectin. From this research work it appears that pectin-lyase- rich containing commercial wine enzyme

mixtures are critical for the preparations to work properly (Gao et al ., 2016). It would be important to test for pectin- lyase activity, amongst others, in enzyme mixes before investing in particular products. One such future outcome would be the development of a convenient pectin-lyase screening test that could be used by winemakers. Pectin-lyase is believed to work by creating pores or holes in the grape pectin layers. These pores become larger, which allows the leaching out of more juice (must) volumes. The formation of pores also allows the release of larger molecules, such as grape skin tannins, into the fermentation. In order to produce stable colours and polymeric pigments it is necessary to access these larger tannins in the skins. Many of the key advantages to using enzymes is to increase must and final wine volumes; and enhancing filterability during wine production. However, more than this, it seems from this research work that enzymes access polymeric pigments and tannins that are not accessible without enzyme addition. CONCLUSION It is possible that enzymes have the potential to access larger tannins, pigmented or otherwise, that contribute to wine composition. An interesting possibility is

that enzymes could enhance body (from tannins) and mouthfeel (perhaps from polysaccharides) in wines and also improve their long-term ageing potential. What would be interesting to know is if pectin- lyase-rich enzyme mixes produce wines with different compositions of polyphenols and polysaccharides than those produced without enzymes? Furthermore, this work would benefit from wine sensory analysis to see if these altered compositions translate into sensory differences. What appears clear is that careful screening and choice of enzyme mixtures is needed to ensure that the additions have the desired effect of improving the colour, body, mouthfeel and complexity of South African red wines. REFERENCES Gao, Y., Fangel, J.U., Willats, W.G.T., Vivier, M.A. & Moore, J.P., 2015. Dissecting the polysaccharide-rich grape cell wall matrix during the winemaking process using high- throughput and fractionation methods. Carbohydrate Polymers 133, 567-577. Gao, Y., Fangel, J.U., Willats, W.G.T., Vivier, M.A. & Moore, J.P., 2016. Dissecting the polysaccharide-rich grape cell wall matrix using recombinant pectinases during winemaking. Carbohydrate Polymers , 510- 519. DOI: 10.1016/.

– For more information, contact John Moore at moorejp@sun.ac.za.