Crush Pad Operations
Destemming and Crushing Grapes
The purpose of destemming is to separate the stems from the berries, as they contain very high levels of tannins, and may contribute a hard vegetal or green flavor to the wine if the stems are fermented with the must. The purpose of crushing is to break the berries - skins in order to release the pulp and the juice. The crushed fruit consisting of pulp, skin, and seed is called must. These two operations of destemming and crushing are generally done consecutively by one machine. In some machines, destemming is done first, and then the crushing. In other machines the order is reversed. It is worth noting that crushing and pressing are quite distinct processes, used for two different purposes. Pressing in winemaking is the process where the juice is extracted from the must/grapes with the aid of a wine press.
Destemming Grapes
Typically, destemming is undertaken before crushing with the purpose of lowering the development of tannins and vegetative flavors in the finished wine. Though it must be stated some winemakers add back dry stems at levels of 20 to 50 percent to provide some woody herbaceous aromas in addition to the tannins. If stems are included, it is important that they remain reasonably intact; otherwise, excessive green or stalky flavors will be absorbed.
Advantages and Disadvantages in Destemming
Fermentations in the presence of stems are always quicker and more complete. Stems facilitate fermentation not only by ensuring the presence of air but by enabling the heat generated by the alcoholic fermentation to dissipate more easily through the cap. During fermentation, yeast produce carbon dioxide, this carbon dioxide causes grape solids to rise creating what is referred to as a cap. Stems protect wine color from polyphenol oxidase (also called tyrosinase) and in the case of botrytized grapes, laccase activity.
Crushing Grapes
Crushing is the step in the winemaking used to break open the grape berry so that the juice is more easily available to the yeast for fermentation. The handling of grapes after destemming plays an important role in the onset of alcoholic fermentation. Vigorous crushing will cause the extraction of more astringent and bitter tannins increasing the concentration of insoluble solids and causing the damage or breaking of the seeds. Seeds add unnecessary tannins and astringency to the wine and create a vegetal green plant taste in a wine.
Destemming-Crushing Options
There are several alternative ways in which grapes can be prepared for fermentation and there are conditions under which the destemming and crushing can be modified.
White Wine Grapes
The current received wisdom is that gentle fruit handling leads to better wine. First and foremost, handling the fruit gently yields juice with lower fine suspended solids, primarily a concern for white wine production. Lower suspended solids mean that less time, energy and labor need to be expended in getting the juice clarified for a (white) fermentation. For red musts, of course, such concerns are not really relevant.
Red Wine Grapes
The red wine maker has similar options whether or not to destem or retain uncrushed berries before fermentation.
Rose Wines
In most cases, when making rose wine, the grapes are destemmed and crushed and, in this instance, skins and must are in free contact with each other.
Destemmer-Crushers
Destemming and crushing are two different, albeit related, operations that are usually combined in a single piece of machinery (Figure 2.7). Destemming is the separation of the berries from the stems. Crushers makes sure every berry is broken open, thereby releasing the juice from the grape, making it available for fermentation. Grapes can be destemmed, crushed, or both destemmed and crushed. Additionally, grapes can be crushed first then destemmed or destemmed first then crushed. Destemmer-crushers usually consist of a hopper with a variable speed feeding auger, variable speed destemming drum and shaft, and a removable crushing unit.
Destemmers
A destemmer consists of a perforated circular or semicircular cage, and a main axle, with attached paddles, mounted down the axis of the cage (Figure 2.8). When the paddles are rotated, the berries are pressed through the holes in the cage to the rollers of the crusher where the berries are broken. The stems, however, are moved down the length of the inside of the cylinder by the paddles and discharged out the end (Figure 2.9). The paddles are needed because the centripetal acceleration of the cage alone is not quite sufficient to detach the berries from the rachis.
Crushers
The crushing units are usually mounted on rails or hinges underneath destemmers so that they can be moved out of service if a winemaker wishes to destem without explicitly crushing. The berries that fall through the cage and are then crushed by one or more pairs of rollers, which turn in opposite directions bursting berries as they pass between. The rollers usually have spiral ribbing or contain grooves with interconnecting profiles to draw the grapes down and through the rollers.
New Technology in Grape Crushing
A new crusher, developed by Pellenc, makes it possible to crush all the berries, without adjusting the space between rollers (Theron, 2013a). The berries are also crushed according to the degree of ripeness, irrespective of the berry size. The grapes are fed into a hopper, which discharges it into a wheel fitted with blades. As the wheel rotates the berries are cast against the outer plate of the wheel, which causes the splitting of the berries, without crushing the seeds or stems.
Click on the following topics for more information on crush pad operations.
