Malt Vinegar. A dilute solution of alcohol which is made from malt by fermentation with yeast contains the nutritive substances necessary for the growth of the vinegar plant, and can therefore be used as a starting-point for the manufacture of vinegar. Sprouted barley or malt is mixed with oats, barley, rice, or other starch-containing material. The mixture is mashed with warm water and then fermented with yeast, giving what is called "raw spirit." This is converted into vinegar by the "quick" process.
The vinegar generator (Fig. 10) is a large barrel divided into three compartments by two perforated partitions. The lower disc is fixed about one-third of the way up the barrel, and near it holes are bored to admit air. The upper disc, fixed near the top of the barrel, is perforated with a large number of small holes which are partially stopped up with short threads or wicks, which hang from the under side. The space between the two discs is packed with specially prepared beech shavings, which have been left to stand in strong vinegar until they are covered with the vinegar plant.
[Illustration: Fig. 10. QUICK VINEGAR PROCESS]
The weak spirit is delivered into the upper portion of the barrel and is distributed in very small drops by the threads; it then passes slowly over the vinegar plant, to which the air also has free access. When it reaches the bottom, it overflows into a reservoir and is again passed through the generator; this is repeated until the product contains the desired amount of acetic acid.
The principle of the quick vinegar process is the same as that employed in making wine vinegar. The speed of the reaction is, however, greatly increased by having the ferment spread over a very large surface and by the free circulation of air. It is possible to make wine vinegar by the quick process, but it is not done, because the product is inferior in taste and aroma to that made by the slow process.
Both wine vinegar and malt vinegar when freshly prepared have a stupefying and unpleasant odour. Before the product is ready for the market, it has to be matured in barrels. During this process, a small quantity of alcohol which still remains in the vinegar combines slowly with some of the acetic acid, producing acetic ester, a substance which has a pleasant fruity odour.