Degradation of spent craft brewer’s yeast by caprine rumen hyper ammonia-producing bacteria

Authors: HARLOW, BRITTANY - UNIVERSITY OF KENTUCKY; BRYANT, RUSTY - ASHEVILLE FLAVOR INNOVATIONS LLC; COHEN, SETH - APPALACHIAN STATE UNIVERSITY; O'CONNELL, SEAN - WESTERN CAROLINA UNIVERSITY; Flythe, Michael

Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2016
Publication Date: 8/28/2016
Citation: Harlow, B., Bryant, R., Cohen, S., O'Connell, S., Flythe, M.D. 2016. Degradation of spent craft brewer’s yeast by caprine rumen hyper ammonia-producing bacteria. Letters in Applied Microbiology. 63(4):307-312. doi:10.1111/lam.12623.

Interpretive Summary: Ruminants evolved to graze, but modern production systems employ supplemental feeds for additional carbohydrate and protein. Ruminants are unlike other mammals in that microorganisms control the first enzymatic steps in digestion. These microorganisms enable ruminants to digest fiber, but protein degradation also occurs. The protein is converted to amino acids, then to ammonia and much of it is lost in the urine. Thus, a costly supplemental feed can be converted to an environmental contaminant by rumen microorganisms. A variety of microorganisms destroy amino acids, but the hyper ammonia-producing bacteria (HAB) are the fastest at converting amino acids to ammonia. Hops and hops beta-acids have been shown to inhibit HAB from both bovines and goats. However, hops are expensive due to the high demand by the brewing industry. Recent results have shown that spent yeast from American craft breweries can contain significant concentrations of hops alpha- and beta-acids, which led us to speculate that this waste product might be an inexpensive source of hops acids for ruminants. Additionally, yeast is high in protein. A set of experiments was initiated to determine if the hops acids in spent craft yeast could protect protein from degradation by rumen bacteria from goats. Baker’s yeast with the same amount of protein, but no hops acids was used as a control. The yeasts were pasteurized, and several concentrations were fermented by a pure bacterial culture of a caprine HAB. The pure culture did not produce as much ammonia in the presence of the craft yeast, which indicated that more protein was spared. The craft and baker’s yeasts were given to mixed, uncultivated microorganisms collected from goats. Again, less ammonia was produced from craft yeast, but the number of HAB from the goats was also decreased. When hops beta-acid was added to the baker’s yeast, the results were similar to the craft yeast. These results indicated that the hops acids in the craft yeast protected the protein from destruction by HAB. Brewery waste products long have been fed to animals. However, the current results suggest that the spent yeast from American craft breweries could have additional value because of the hops acid content. The impact of the research is identification of a potential co-product from craft breweries, a rumen-protected protein supplement.

Technical Abstract: Spent brewer’s yeast has long been included in ruminant diets as a protein supplement. However, modern craft beers often include more hops (Humulus lupulus L.) compounds than traditional recipes. These compounds include alpha and beta-acids, which are antimicrobial to the rumen hyper ammonia-producing bacteria (HAB) that are major contributors to amino acid degradation. The objective was to determine fermentation characteristics of a craft brewer’s yeast (CY; ~ 3.5 mg/g combined hops acids) or a baker’s yeast (BY; no hops acids) by caprine rumen microorganisms and HAB. Mixed cell suspensions were prepared by harvesting rumen fluid from fistulated goats (n = 3), straining, and differential centrifugation. The cells were re-suspended in media with 40, 20, 10, or 5 mg mL-1 BY or CY. After 24 h (39 °C), HAB were enumerated and ammonia was measured. Data were analyzed by ANOVA with Tukey’s test. At least 10-fold fewer HAB were enumerated and ~60% less ammonia was produced from CY than from BY (P < 0.05). Pure culture experiments (n = 3) were conducted with Peptostreptococcus anaerobius BG1 (caprine HAB). Ammonia production by BG1 from BY was greater than from CY (P < 0.05). Furthermore, ammonia production was greater when Trypticase (15 mg mL-1) was included, but similar inhibition was observed in CY treatments (P < 0.05). These results indicate that rumen microorganisms deaminated the amino acids in CY to a lesser degree than BY and support further studies on the use of spent craft yeast as a rumen-protected protein supplement.