Oak Container See Cask entries.

Off-Flavor Any taste in a wine which, as a result of infection or poor fermentative technique, destroys the true flavor of the wine. See Diagnosis of Faults, Appendix XI.

Oil, Fusel See Fusel Oil.

Oiliness See Lactobacilli ;

Organic Constituents of Wine See Constituents of Wine

Organism Correctly speaking, means any life form, but bacteriological it is taken only to include singe-celled life forces.

Original Gravity The S.G of the must before fermentation is begun. If exponential feeding is practiced, it is the S.G that the must would have had, had all the sugar been included at the start.

Osmosis One of the laws of nature is that substances in solution pass from a region of high concentration to one of low concentration until the mix fully. For example, a spoonful of sugar placed in a tumbler of water, unstirred, will gradually dissolve. If a barrier is placed between substances which can only let water (or equally small molecules) through, the law has to be observed in a different way. Since they cannot mix, the water has to pass through the barrier to dilute the sugar. If too much sugar is added to a must, this situation applies. Water leaves the yeast to dilute the solution, eventually killing the cells. This results in a stuck ferment. To avoid such a situation, exponentially feeding is recommended.

Over-Acid See Acid Reduction.

Over-Fining See Fining

Over-Pressing Care must be taken not to try to wring out the last drop of juice when pressing. To do so will result in excess tannin entering the must, which in turn will lead to an over astringent harsh wine.

Over-Sweet A wine which stops fermenting too soon, or one which has too much sweetening added will taste syrupy. Even with extra acid added to try to counteract this, the wine will still be cloying. To prevent this, careful fermentation is required and, when sweetening a wine, care must be taken to work out exactly the amount of sugar needed.

Ox Blood See Blood.

Oxalic Acid is present in rhubarb and, since it is toxic, care must be taken with the preparation of this fruit. There is no need, unless the acidity of the juice is too high, to lower the acidity routinely and then add further acid. Provided it is not pressed and the leaves are not used, the acidity is usually within acceptable levels. Contrary to the belief of many people, oxalic acid is not the principal acid of rhubarb stalks; malic acid is. Oxalic acid is mainly in the leaves.

Oxidation and Filtration As with any procedure carried out in the presence of air, there is the risk of oxidation. To prevent this, the wine should be sulphited and open funnels should not be used. Rather, use one of the closed systems such as the EuroFilter, Buon Vino or Harris filters

Oxidation and Sulphite See Sulphite.

Oxidation due to Enzymes See Casse, Oxidative.

Oxidation in Cask See Cask, Oxidation in.

Oxidation, Prevention of If all procedures involving the transfer of wine from one container to another are carried out as gently as possible to avoid splashing, the oxygen uptake will be kept to a minimum. It is possible to create an oxygen-free atmosphere by piping carbon dioxide from an active ferment into the receiving carboy. Other than this, the only methods open to the amateur are the uses of sulphite or ascorbic acid.

Oxidation/Reduction Reaction See Redox.

Oxidative Casse See Casse, Oxidative.

Oxidative Enzymes See Enzymes , See Oxidative ; See O-Polyphenoloxidase

Oxido-Reductase The enzyme which catalyses the reaction to change three carbon sugars to glycerol.

Oxygen is required to encourage a yeast colony to develop after pitching, during the aerobic phase of fermentation. After this, no oxygen is needed until the small amounts required for maturation. During secondary fermentation, oxygen must be excluded to avoid aerobic infection. Hence the use of an airlock and bung.

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