Ingredients should not spoil inside a brewing barrel while it's brewing

I just dumped 100 wheat into a brewing barrel and ended up with only a few beers and over 70 spoiled food. Beer doesn't spoil while it's in the process of fermenting. Fermenting vegetables don't either - the process of fermenting is what is causing it to be preserved and protected from spoilage bacteria. The way it is now, I can't fill the brewing barrel to capacity to maximize production, I can only put 20 or so wheat in at a time or else the rest spoils.