Brewer’s yeast used to make beer is typically discarded once it’s no longer needed. The yeast, known as Saccharomyces cerevisiae, absorb humolones, lupolones and other compounds from hops that contribute to beer’s flavor and aroma. Humolones and lupolones are both biologically active molecules that inhibit certain bacteria and other microbes, including those that trigger the cow’s release of methane and ammonia.
Methane from cows is a waste byproduct arising from the fermentation activity of certain kinds of microbes, called methanogens, in the first of the animal’s four stomach chambers, the rumen. Another group of rumen microbes, known as “hyper-ammonia-producing bacteria,” are behind the animal’s excretion of ammonia, a potential air and water quality concern. The microbes’ production of methane and ammonia from food the cow eats also robs the animal of amino acids needed for growth and milk production, explains Michael Flythe, a research microbiologist with the U.S. Department of Agriculture’s (USDA) Agricultural Research Service (ARS) Forage-Animal Production Research Unit in Lexington, Kentucky.
Flythe’s research is part of an ongoing effort to develop natural alternatives to using expensive protein supplements and monensin (a special class of ionophore antibiotic only approved for use in cattle) to keep the gas-producing microbes in check. To further explore the preventive potential of leftover brewer’s yeast, Flythe teamed with Rhys Burns, Christopher Feidler-Cree, Denia Carlton and Langdon Martin at Warren Wilson College (WWC), and Robert Bryant, both at WWC and Asheville Sustainability Research LLC. They took samples of spent brewer’s yeast used to make six different kinds of beer at a local brewery and added them to flasks containing live cultures of either methane- or ammonia-producing microbes. After a 24-hour incubation period, the gases were sampled and analyzed. The researchers observed a direct correlation between the concentration of hops compounds in the spent yeast and the amount of gas produced.
The team also used baker’s yeast and monensin as controls for comparison. Not surprisingly, the baker’s yeast, which had not been exposed to hops during the brewing process, failed to tamp down microbes’ production of the gases. However, the spent brewer’s yeast — flush with the hops compounds it had absorbed — curbed the microbes’ methane production by 25% percent on average, a reduction comparable to monensin. Although spent brewer’s yeast is sometimes used as a livestock feed additive, Flythe said cow feeding trials would still be necessary to fully assess its potential to reduce methane and ammonia on a farm scale. Findings of the study are reported in Frontiers In Animal Science.
In terms of the supply, the brewing process generates 15 to 18 tons of spent brewer’s yeast per 10,000 hectoliters (or approximately 2,641 gallons) of finished beer, making it the second largest byproduct next to spent brewer’s grains (SBGs). According to a 2019 study, brewers in the European Union generate 6 million tons of SBGs annually and 1 million tons of spent brewer’s yeast.