I’ve been doing some research regarding inactivating yeast so that it can be used as animal feed, and I’ve basically come up with two possible methods that could work for us: heat inactivation (though we don’t really have a good way to do this at this point) and chemical inactivation using formic acid. I haven’t been able to find anything that might tell me what amount of Formic acid would have to be added to an amount of yeast to achieve the desired inactivation. Would you happen to know anything about this or be able to point me to a resource that might be able to help me with this?
Also, I’m curious, in all of your years in the brewing industry, what different methods of yeast disposal have you seen?
A: I have never heard of using formic acid as a method to kill yeast in order to feed it to cattle. Yeast are fairly acid tolerant so I am not sure this would be a good method to use. Heat is about the best method if you want to achieve a relatively quick and effective kill. However, it is not necessary, and actually even counter-productive, to kill spent yeast that is to be given as cattle food. Live yeast has positive probiotic effects in cattle and will help them digest and get more out of their feed.
We have all heard (or experienced) what can happen if cattle are allowed to gorge themselves on live yeast. Cows are ruminants and have essentially three stomach areas to digest the cellulose rich grasses, hay and spent grain that make up the bulk of their diet. The stomach areas rely on a complex mixture of bacteria and yeast to help breakdown the cellulose into simpler carbohydrates they can use for energy. If allowed to gorge on live yeast it will disrupt this delicate gut microbiology often producing a gas build up and such a bloat as to block their air passageway causing suffocation and death. This can be an expensive lesson in not letting the cows engorge themselves. But live yeast can be safely fed to cattle in smaller amounts, mixed in along with their other foods, and will actually help their digestive process and gut microflora. My former brewery has been doing this with dairy and beef cattle for several years without incident.
With proper care in using it, live yeast can be a healthy part of the cattle’s feed regimen and does not require any heat (or acid) treatment at the brewery. As the live yeast autolyzes in the cow’s digestive tract it provides superior nutrients to the animal including vitamins, proteins and minerals. The benefit of live yeast is recognized and many live yeast products are sold to cattle farmers because when mixed with dry feed, the animals put on quicker growth. Typical addition rates are 10-15% by weight. However, you as the brewer must be careful to protect your liability. Write up an agreement with your spent yeast end user to let them know that you are shipping live yeast and agreeing that the use of this material as cattle feed is their responsibility.