Spent Brewers' Grain
One very old application for a screw press is dewatering spent brewer's grain. In his April 1900 US Patent, Valerius Anderson mentions that his invention of the interrupted flight screw was "one capable of handling brewers' slops, slaughter-house refuse, and like material which oftentimes is so soft and mushy as to be handled only with difficulty".
Beer, of course, can be made from a wide range of grains. Most commonly wheat, barley, corn and rice are used. After the fermentation process, wet fiber, called spent brewer's grain, is a waste residue. It is used as animal feed.
As spent grain comes from the brewery it is high in moisture content, generally 78% to 86%. Sometimes the spent grain is pumped to a filter screen, like a sidehill, to drain off some of the free water.
At that point the grain can be hauled to a farm, for use as animal feed. Small breweries frequently make arrangements for a farmer to pick up their spent grain. There is little value in the product, and it may be given away for free.
Larger breweries used to dry their spent grain down to 10% moisture and sell it, usually in pellet form. This commodity is known as DDG, Dried Distillers' Grain. Production of DDG involves first pressing the waste and then drying the press cake in a rotary drum dryer. Both steam tube dryers and direct fire dryers (the old Vincent design) were used. However with the advent of high fuel costs following the oil embargo of October 1973, the use of dryers was largely discontinued, especially in the United States.
Typically today the spent grain is run through a screw press, and the press cake is sold in wet (moist) form. Usually a moisture content in the range of 68% to 70% is specified.
The moisture content achieved by the screw press depends greatly by the type, and mix, of grain that is used in the beer production process. In one famous case in 1994, Vincent ran tests with an entire trailer load of spent grain from the Busch Gardens Brewery in Tampa. On the basis of this testing, screw presses were sold with a guarantee of 68% moisture because values of 66% easily were achieved in the tests. When these presses were put in service at the Anheuser-Busch brewery in St. Louis, it proved very difficult to get below 70%. The difference was traced to the differences in grains used in the two breweries.
We have seen other spent brewer's grains which can be pressed to as low at 55% moisture. It is all up to the brew master.
Competitors known in the brewery industry include Ponndorf and Vetter. Features of the Vincent press which result in superior performance include tight pitch flighting in the inlet hopper, a tapered shaft screw design, flight notches, and the use of wing feeders.
Note: Distilleries where spirits like whisky are produced rarely use screw presses. At distilleries the grain is first milled into a flour-like consistency. These fine particles, the spent grain, are very difficult to dewater in a screw press.