Proteases, grape

Contrary to expectations that fungal diseases would lead to elevated levels of PR proteins in berries, Marchal et al. (1998) observed that juice from berries infected by B. cinerea showed reduced protein levels, and suggested that proteolytic enzymes from B. cinerea were responsible for this. Secretion of proteases by B. cinerea has been observed in culture media and on fruits such as apple (Zalewska-Sobczak et al. 1981) and tomato (Brown and Adikaram 1983). Girbau et al. (2004) also examined the impact of infection of grapes with B. cinerea in the vineyard and showed that infection resulted in marked decreases in the levels of PR proteins in... 

J., Watson, B. et al. (1984) Recent developments in the application of ultrafiltration and protease enzymes to grape juice and wine processing. Proceedings of the International Symposium in Cool Climate Viticulture and Enology, June 1984, pp. 25-28. 

When the mycelial hypha reaches a microfissure, it penetrates the grape. Thns B. cinerea development occnrs mainly in the grape s superficial cell walls. More precisely, the mycelial filaments are located in the middle lamella of the pectoceUulosic ceU walls. The latter are degraded by the enzymes of the fnngns (pectinolytic, cellulasic complex, protease and phospholipase enzymes). 

The grape possesses a low and constant proteasic activity during its herbaceous growth phase. From viraison onwards, this activity strongly increases. In a ripe grape, the proteasic activity is essentially located in the pulp (Table 11.7). But the proteases are generally bound to cell structures. Healthy grape juice thus has relatively few proteases (30% of total proteasic activity). 

Table 11.7. Distribution of proteasic activity in the different areas of a healthy grape (expressed as % of total activity per berry) (Cordonnier and Dugal, 1968)...

All physical treatments of grapes (mechanical harvesting, stemming, crushing) increase the proportion of soluble proteases. The higher free amino acid concentration of these musts attest to this (Cantagrel et al., 1982). 

Grape proteases are acidic, with an optimum pH near 2.0. In the pH range of must, 40-60% of the potential proteasic activity exists. Protein hydrolysis activity during the pre-fermentation phase varies greatly, depending on grape maturity and harvest treatments. This certainly affects fermentation kinetics but the relationship has never been established. A slight sulfur dioxide additiou (around 25 mg/1), however, has been confirmed to stimulate proteasic activity. This explains, at least partially, its activation effect on fermentation (Section 8.7.3). 

Finally, botrytized grapes also contain fungal proteases. Contrary to grape proteases, these are soluble and pass entirely into the must. Botrytis cinerea aspartate proteinase has an optimum pH in the vicinity of 3.5. Whatever their origin, proteases are thermostable they increase soluble nitrogen, even during thermovinification. 

Enzymatic reactions, activated by grape enzymes, are involved in cell wall degradation. They favor the dissolution of their vacnolar contents (Section 11.7.3). Commercial enzymatic preparations have recently been developed to activate these phenomena they have pectinase, cellulase, hemi-cellulase and protease activities of diverse origins (Amrani-Joutei, 1993). These enzymes seem to favor the extraction of skin tannins over skin anthocyanins. They act on the tannins linked to the polysaccharides of the cell wall, giving the enzymatic wine a more full-bodied character than the control wine. 

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