Harvest time is almost here for vineyards throughout Minnesota. Grape growers start looking toward harvest when their grapes go through veraison, the final stage in the ripening process in which the grape skins change color and soften and the vine diverts its energy to the grapes. Sampling and testing help determine the optimal time to harvest, but it is typically 45 days after 50% of the grapes have changed color. (More on veraison at the UofM's Enology Blog.)
The next step after the harvesting of red or white grapes is crushing, a process that simply breaks the skins so the juice can come out. This begins the winemaking process. Red grapes are crushed and in most cases the juice and skins are left in contact for a certain period of time so that the color and tannins are extracted from the skin. This is called maceration. When making rose', the time is short. When making other reds, the time is longer. White grapes, on the other had, are crushed and in most cases pressed immediately to extract the juice without extracting any attributes from the skin.
Crushing in most commercial winemaking operations is done by machine, typically a two-function device called a crusher/destemmer. Harvested grapes come to this machine with stems and some leaves still attached. These are moved through a perforated spinning cylinder which allows the grapes to pass through the perforations, but the stems and leaves remain and are forced out the end of the cylinder. The grapes drop into a hopper equipped with tumblers that break the skins.
In small operations crushing is done by hand, or rather by feet. Even some large volume producers still have some of their grapes crushed at grape stomp events at their wineries. This begs the question: is it sanitary? The answer is yes, the acid present in freshly crushed grape juice keeps any bacteria away. Besides, just like cooking, you should wash-up before you start.
White grapes, and red ones when maceration is complete, are pressed to extract the juice from the skin. In large operations, this is accomplished with a machine equipped with a large air bag or bladder. The grapes and skins are loaded into the press and then the bladder is inflated a series of times with increasing amounts of pressure to properly extract the juice and leave behind the now undesirable elements. Small operations will use a screw-down style press.
From there the winemaking process continues, but let's not get ahead of ourselves. It's almost harvest time, go find some grapes to stomp. Just Google minnesota grape stomp or visit the Minnesota Grape Growers Association website to find the winery nearest you and check their website about stomping opportunities. Some may also want your help harvesting!