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winemaking nones are largely formed by the action of PPO in wine that are formed from nonenzymatic oxidation reactions, unless laccase is present. When flavanols are oxidized by caftaric acid o-quinone, the corresponding quinones polymerize and precipitate as brown pigments.27 There is significant variation in the levels of both PPO and caftaric and coumaric acids in cultivars, and therefore the tendency to brown varies. In red musts and juices, PPO activity has been shown to lead to a loss of color,35 and browning may also occur. Browning of juices increases with the level of phenolic extraction. Winemaking processes that increase extraction, such as pomace contact, hard pressing and high temperature exposure during processing increase the tendency of a wine to brown.29 Once browning is initiated, it is autocatalytic, meaning "brown begets brown" due to creation of intermediates that are more reactive than their substrates in leading to formation of brown polymers.29 Sulfur dioxide is an inhibitor of PPO. Levels of 25 – 75 mg/L of SO2 in clarified juices led to inhibitions of 75% and 97% of polyphenol oxidase activity.5 It is suspected the sulfur dioxide acts by reacting with o-quinones or other intermediates in the browning reaction, preventing formation of colored complexes (Figure 6).1 It is also possible that sulfur dioxide interacts directly with the enzyme, inhibiting it.1 Oxidation of phenolic compounds to quinones can occur non-enzymatically and co-produce a strong oxidant, hydrogen peroxide or H2O2, which then oxidizes other components.34 H2O2 is not a direct product of enzymatic oxidation. Thus simple inhibition of PPO might not prevent oxidative damage from occurring in juice or must if there is significant oxygen exposure. Sulfite can interact with other oxidants and can impact non-enzymatic and enzymatic browning. Grape glutathione can also inhibit PPO-induced browning. Caftaric acid o-quinone can bind to glutathione, forming GRP or the "grape reaction product," thereby trapping the quinone and preventing it from reacting with other juice components.8,30 Microbial consumption of oxygen Energy generation in biological systems takes advantage of the energetic favorability that accompanies the movement of electrons from molecules with low affinity to those with high electron affinity. The process of respiration uses oxygen as a terminal electron acceptor, passing will use organic compounds as electron electrons along a chain and capturing acceptors. energy released in the form of adenosine During grape juice fermentation, sugtriphosphate (ATP). ars are converted to pyruvate, and pyruMany organisms are obligate aerobes, meaning oxygen must be present to serve as an electron acceptor. H2O O2 Other organisms, such PPO as many found in wine, can use oxygen in this role if it is avail- Figure 6: Oxygen and enzymatic oxidation of phenolics. able; if it is not, they © 2013 StaVin Inc. For over twenty years we've searched the world to source the very finest oak known to wine. Our hikes through numerous forests in countries on two continents have yielded findings some winemakers consider treasure. Such travels have shown us the rewards of perseverance, and the importance of variety. Because even the most delicious wines don't always taste the same. ® StaVın Inc, P.O.Box 1693, Sausalito,CA 94966 (415) 331-7849 f (415) 331-0516 stavin.com pr actica l win ery & vin eya r d O CTO B ER 20 13 69