Spent craft brewer’s yeast reduces production of methane and ammonia by bovine rumen microbes

Authors: BRYANT, R - Warren Wilson College; BURNS, E - Warren Wilson College; FEIDLER-CREE, C - Warren Wilson College; CARLTON, D - Warren Wilson College; Flythe, Michael; MARTIN, L - Warren Wilson College

Submitted to: Frontiers in Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/10/2021
Publication Date: 10/1/2021
Citation: Bryant, R.W., Burns, E.E., Feidler-Cree, C., Carlton, D., Flythe, M.D., Martin, L.J. 2021. Spent craft brewer’s yeast reduces production of methane and ammonia by bovine rumen microbes. Frontiers in Animal Science. 2. Article 720646. https://doi.org/10.3389/fanim.2021.720646.
DOI: https://doi.org/10.3389/fanim.2021.720646

Interpretive Summary: Methane and ammonia are byproducts of rumen fermentation that do not promote animal growth, and methane is a key contributor to anthropogenic climate disruption. Cows erusticate every few breaths and typically emit 250–500 L of methane gas daily. Significant research is focused on finding diets and additives that lower the production of methane and ammonia. Emerging research has shown that humulones and lupulones, molecules that are found in the cones of hops (Humulus lupulus), have potential in this regard. These molecules, which are also key flavor components in beer, are biologically active: they are known inhibitors of Gram-positive bacteria.

Technical Abstract: Spent craft yeast harvested from six different ale-style beers used to determine the contents of the biologically active hop acids. Anaerobic incubation of uncultivated, bovine rumen microbes were used to compare the effects of the spent yeasts to a baker’s yeast control and to the industry-standard antibiotic, monensin. There was a strong correlation between the amounts of hop acid present and the inhibition of methane and ammonia production. When craft yeast, obtained from a moderately-hopped ale-style beer, was used as an additive, headspace methane production was reduced from an average of 4760 ppm to 3530 ppm after a 24-hour incubation, a roughly 25% inhibition. This inhibition was comparable to the reduction in methane caused by adding 10 µM of the ionophore antibiotic monensin. The three yeasts with the highest concentrations of hop acids were more potent than the antibiotic monensin at inhibiting both methane and ammonia production. Petri plate bioassay indicated that craft yeast inhibited the growth of the rumen hyper ammonia producing bacterium, Acetoanaerobium sticklandii. Thus, the decrease in ammonia production appeared to result from an inhibition of the growth of the hyper ammonia-producing bacteria by the molecules extracted from hops into the spent yeast during the beer-brewing process. Yeast alone did not inhibit growth of hyper ammonia-producing bacteria, whereas yeast containing hop acids were inhibitory. These results indicate that spent craft brewers' yeast can modulate rumen fermentation due to the presence of antimicrobial hop acids.