If you are concerned about sustainability and the environment and most people aren't in my judgement you might be interested to know that a wood-eating yeast in cat food has been found to be highly palatable and digestible.  Torula yeast. Picture believed to be in the public domain. It's a fungus and it can be used as a pet food ingredient. The yeast is called torula yeast ( Cyberlindnera jadinii ). It feeds on woody biomass and that's useful because there's a lot of forests being cut down and therefore a lot of biomasses lying around facilities which can be used to grow this yeast. Apparently, it is easy to manipulate and extrude and therefore it can be converted into a pet food ingredient conveniently. It doesn't contain allergens or heavy metals, but it does provide amino acids. Unbeknown to many cat owners some cat foods contain heavy metals . Torula yeast is approved as an ingredient by the AAFCO in America. It has a safe history of use. A Kansas State Universi