Effects of hops (Humulus lupulus L.) beta-acids on short chain fatty acid production from complex carbohydrates by rumen microbiota

Authors: Flythe, Michael; Harlow, Brittany

Submitted to: Advances in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2019
Publication Date: 12/11/2019
Citation: Flythe, M.D., Harlow, B.E. 2019. Effects of hops (Humulus lupulus L.) beta-acids on short chain fatty acid production from complex carbohydrates by rumen microbiota. Advances in Microbiology. 9(12):983-992. https://doi.org/10.4236/aim.2019.912063.
DOI: https://doi.org/10.4236/aim.2019.912063

Interpretive Summary: The rumen of ruminant animals is colonized by hundreds of types of microorganisms that help the animal digest feed. Most of the microorganisms are beneficial, but some conduct wasteful processes. The rumen hyper ammonia producing bacteria convert feed amino acids into ammonia. The ammonia production decreases the amino acids available to the animal, but the ammonia is also lost in the urine as urea. This is one of the many sources of nitrogen loss from agricultural systems. Feed antibiotics and certain phytochemicals (plant compounds) can inhibit the hyper ammonia producing bacteria. Our previous research has shown that phytochemicals called beta-acids from the hops plant can inhibit these wasteful bacteria and decrease ammonia production. However, carbohydrate fermentation is also affected. The current experiments were conducted to determine how beta-acids impact digestion of particular carbohydrates by rumen microorganisms. The experiment was conducted in the laboratory, but pure bacterial cultures were not used. Instead, we harvested uncultivated, mixed rumen microorganisms from steers, washed those microbial cells and used them to ferment carbohydrates in the presence or absence of hops beta-acids. The results indicated that the digestion of starch and cellulose (fiber) were impacted, but sugars and fructans (chains of the sugar fructose) were not. Decreasing starch fermentation is a positive result. Starch fermentation can cause excessive acidity in the rumens of animals on some diets, and starch can be digested by the animal in the intestines. However, cellulose fermentation is critical in cattle on forage-based diets. If hops phytochemicals inhibit cellulose fermentation, then it would not be beneficial to include them in forage-based rations or in a supplement for pasture animals. The impact of this research is that supplements including hops beta-acids, including brewery co-products, might have more utility in grain-based finishing or dairy rations than for forage-animals.

Technical Abstract: The aim of this experiment was to determine the effects of beta-acids, prenylated phenolic compounds from the hops plant, on fermentation of individual carbohydrates by rumen microorganisms. Mixed, uncultivated rumen microbiota were harvested from rumen fistulated steers and washed to make cell suspensions. The suspensions were used to inoculate media with a glucan, fructan or constituent sugar, and fermentation was evaluated by production of short-chain fatty acids (SCFA). Hops beta-acid (30 ppm) was not universally inhibitory, but each of the SCFA (acetate, propionate or butyrate) were decreased (P < 0.05) in one or more of each cellulose or starch tested. The fermentation of sugars and fructans (short- or long-chain inulins) was not impacted by the phytochemicals. Previous results have shown that hops and hops extracts had the beneficial effects of reducing rumen ammonia and methane. The current results indicate that both starch and cellulose fermentation could be impacted. Because cellulose fermentation is nutritionally important on forage-based diets, hops phytochemicals might have more utility in cereal grain-based rations.