There are four distinct stages in the making of wine after the grapes are grown.
The first is the harvesting of the grapes when they have reached the proper stage of maturity, which is known as "wine-making ripeness." This stage of ripeness is determined by means of a must-scale or saccharometer. The wine-maker squeezes the juice from a number of bunches of grapes into a receptacle into which he drops the must-scale, whereupon the sugar-content of the juice is indicated on the scale, determining whether the proper stage of ripeness has been reached. Suitable varieties of grapes having been grown, it is necessary that they be permitted to hang on the vine until the proper degree of ripeness is developed, after which they are delivered at the winery as free as possible from injury or decay.
The second stage is the preparation of the grapes for fermentation. The grapes are weighed on arriving at the winery and are then conveyed either by hand or more often by a mechanical conveyor to the hopper or crusher. The ancient method of crushing, which still prevails in some parts of Europe, was to tramp the grapes with bare feet or wooden shoes. Tramping has been superseded by mechanical crushers which break the skin but do not crush the seeds. The best mechanical crushers consist of two-grooved revolving cylinders. As the grapes pass through the crusher, they fall into the stemmer, a machine which tears off the stems, discharging them at one end, while the seeds, skins, pulp and juice pass through the bottom to the presses usually on the floor below. There are several types of wine-presses, all of which, however, are modifications of screw, hydraulic or knuckle-joint power. In large wineries, the hydraulic press has almost driven out the other two forms of power and when great quantities of grapes must be handled a number of hydraulic presses are usually in operation. The grape pomace is built up into a "cheese" by the use of cloths and racks variously arranged. The "cheese" is then put under heavy pressure from which the juice or "must" is quickly extracted.
The third stage is fermentation. The "must" is carried from the press into open tanks or vats which hold from 500 to 5000 gallons or even more. The yeast cells which cause fermentation may be introduced naturally on the skins of the grapes; or in many modern wineries the "must" is sterilized to rid it of undesirable micro-organisms and a "starter" of "wine-yeast" is added to start the fermentation. Yeast organisms attack the sugar and must, breaking it up into alcohol and carbonic acid gas, the latter passing off as it is formed. When active fermentation ceases, the new wine is drawn from the pomace and is put into closed casks or tanks where it undergoes a secondary fermentation, much sediment settling at the bottom of the cask. To rid the new wine of this sediment, it must be drawn off into clean casks, an operation called "racking." The first racking usually takes place within a month or six weeks. A second racking is necessary at the end of the winter and a third is desirable in the summer or fall.
The fourth stage is the aging of the wine. Before aging begins, however, the wine usually must be rendered perfectly clear and bright by "fining." The materials used in fining are isinglass, white of egg or gelatine. These, introduced into the wine, cause undissolved matters to precipitate. The wine is now ready for bottling or consumption. Most wines acquire a more desirable flavor through "aging," a slow oxidation in the bottles.