Abstract
Commercial-sized presses were used to press destemmed and crushed Koshu grapes with stems at different pressures. It was found that the composition of the juices was significantly affected by the type of press, pressing pressure, and presence or absence of stems. The free-run had the highest concentration of glutathione while pressing at moderate pressures yielded juice with very high concentrations of proteins and polyphenoloxidase (PPO). On the other hand, maximum concentrations of phenols including caffeoyl tartrate (caftaric acid), 2- S-glutathionyl caftaric acid (GRP), catechin and epicatechin were found in juices from high pressure pressing. The low concentration of glutathione, when compared to the amounts of caftaric acid and PPO, is one of the major reasons why Koshu juice is very susceptible to browning.
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