Why is formaldehyde not used to sterilize pig food? Probably because formaldehyde makes for a suboptimal food supplement...

What organic acids are these? Doubt they'll sterilize anything.

You could try formaldehyde. It's very reactive so might take a lot and might make the product unpalatable to pigs, if that's possible..

Typical commodity pig feed in general does not need to be sterilized unless there are certain animal products in it. Acidifying the brewers yeast halts its growth which can be problematic during processing and storage, especially if it is mixed with fermentable sugars such as might be found in various grains. Also, some brewers feel their strains are highly proprietary and would want to prevent them from being easily cultured.

Formaldehyde might not be an allowable feed additive. As Martin noted, it would reduce its nutritional value, likely by forming undesirable adducts with the proteins.

Commodity pig feed typically doesn't require sterilization unless it contains specific animal products. Acidifying brewers yeast can prevent its growth, which is important during processing and storage, especially when mixed with fermentable sugars. Some brewers might want to protect proprietary strains from easy cultivation. Formaldehyde may not be an allowable feed additive, as it could reduce nutritional value by forming unwanted compounds with proteins.

Formaldehydr is no longer used in the european brewing industry : forbidden by law