By John Considine and Elizabeth Frankish (Auth.)

ISBN-10: 0124080812

ISBN-13: 9780124080812

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Extra resources for A Complete Guide to Quality in Small-Scale Wine Making

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Yellowing, then browning is not a simple chemical reaction. It requires the presence of an enzyme and/or a metal ion and oxygen and a substrate. In white and also in red wines, the phenols, caftaric and coutaric acids, are the primary substrates (Singleton, 1985). In grape juice, must, glutathione intervenes: PPO Caftaric acid 1 O2 ÀÀÀÀÀÀÀÀÀ! 1 Glutathione Caftaric quinone ÀÀÀÀÀÀÀÀÀ! S-Glutathionyl caftaric acid This product does not brown, leading Singleton (1985) to conclude that grape musts possess a built-in system to protect against browning, at least to a degree.

All terpenes share a common metabolic origin, although some are formed de novo while others are a product of senescence. Terpenes are common in nearly all aromatic plant products, otherwise known as essential oils. ). 19). As such, they are not volatile nor can the enzymes in saliva (α-amylase) or many yeasts (zero or very low levels of β-glycosidase) release the free, volatile, aroma compound. The natural hydrolases present within the grape have a low activity at wine pH, are inhibited by sugars in the juice and are not active with all sugar glycosides found naturally (reviewed by Maicas and Mateo, 2005).

Data from Francis and Newton, 2005). known series, and only through application of relatively recent advances in mass spectrometry and stable isotopebased techniques has the importance of this group been unveiled. 23). Descriptors for this group vary from the exquisite to the disgusting, presumably depending on concentration and matrix (Francis and Newton, 2005). Many of the unpleasant-smelling sulfur compounds originate from the degradation of the sulfur-containing amino acids, cysteine and cystine, which are important protein-forming amino acids.

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A Complete Guide to Quality in Small-Scale Wine Making by John Considine and Elizabeth Frankish (Auth.)

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