“More food-scrap based composts are coming into our lab than before and we’re seeing a significant increase in presence of fatty acids”, says chemist Jon Dyer, Woods End Labs. That could easily be explained by successful mandates for organic waste diversion which nationwide are increasing the amount of food scraps going to composting. On the laboratory side, it can also explain the distinct signature of “volatile fatty acids”, also known as VOA’s. These are fermentation byproducts and include such familiar compounds as lactic acid and acetic acid and also malodorous butyric.
Woods End first developed a VOA detection program in the 90’s at a time when food waste composting was expected to grow in America. While many early pioneering food waste collection programs were scrapped (such as by Rudy Guiliani of NYC) many more have started recently and “with it the need to better measure and manage VOA’s present”, says Brinton.
The chemistry challenge is that VOA’s may accumulate with high loading rates of food-scraps. One thing is that they impart a characteristic odor that is different from all others. “One can compare the buildup of fatty acids in compost to lactic-acid accumulation in muscle-tissue during strenuous exercise – it’s a similar metabolic pathway”, says Brinton, who published a paper on compost VOA in a 1998 report Compost Science Utilization. If sufficient oxygen is not delivered to cells under heavy loads then the fatty acids are not completely oxidized, and accumulate. In muscle tissue, lactic acid is commonly associated with the burning sensation and fatigue. In compost the organic acids, which include propionate and butyrate, depress the pH which in turn slows the rate of degradation, a complex self-reinforcing spiral.
In early composting years, Brinton associated high VOA’s in compost with phytotoxicity or poor plant performance. “Now its more of an issue of slowing down the process”, says Brinton, “which is an economic loss”. The good news is that microbes easily consume VOA’s given sufficient air and time. Woods End has established compost QA standards and developed an odor-index linked to presence of 7 different organic acids.
There’s no reason increased food scrap composting rates should compromise the quality, but some fresh insights are needed. As in all such matters “laboratory measurement helps leads to proper management” – a slogan for the lab.