For more than 30 years, animal nutritionists have been puzzled as to why yeast cultures appear to be beneficial for farm livestock production. Slowly but surely, the veil has been lifted. Yeast’s ability to take up oxygen is what does the trick.
By Prof John Wallace, Rowett Institute of Nutrition and Health, University of Aberdeen, Scotland, UK
A yeast is defined by microbiologists as a single-cell fungus. The yeast that we usually mean in the context of food and feed is the species named Saccharomyces cerevisiae. Saccharomyces cerevisiae has been used by man for millennia to produce alcoholic beverages, including beer and most spirits, and to enable bread to rise during the baking process. In recent times, S. cerevisiae has been applied in the production of biofuels. Biofuel, like the production of the alcoholic drinks, depends on the ability of yeast to ferment starch to ethanol under the conditions of low oxygen that prevail in the large tanks used for industrial fermentation. Breadmaking utilises the ability of yeast to form abundant volumes of the gas, carbon dioxide, which makes the dough rise.
Why then would the use of yeast as a feed additive be expected to benefit animal production, particularly at the low addition rates generally recommended by their distributers? Additional carbon dioxide would certainly have no benefit. Some have claimed that the production of ethanol in the rumen might benefit ruminal fermentation. The vitamins that they produce might be stimulatory to ruminal micro-organisms. Their highly absorptive cell envelope might bind toxic materials. Thus, when ‘yeast culture’ was introduced in the 1980s, there was confusion about why, what do they do, are they worth the cost of supplementation? Trying to answer these questions has been a fascinating investigative story, particularly against a background of understandable cynicism that perhaps companies were simply disposing of a byproduct from other industries with aggressive marketing and at inflated cost.
Effects on ruminants
The 1980s brought forth a series of papers and reports that described effects of yeast culture on ruminant production and rumen microbial metabolism. Effects on production were always small, which led to many questioning their statistical validity. The reported effects on rumen metabolism often seemed unrelated: Stabilised pH, improved fibre digestion, lower lactate concentrations, altered fermentation product proportions in favour of propionic acid, lower methane emission, increased concentrations of cellulolytic bacteria, increased concentrations of cellulolytic bacteria, lower soluble sugar concentrations, decreased ammonia concentrations, all by the supplementation of a few grammes of yeast to a cow with a rumen volume of 100-150 litres.
The acidity of the fermentation products that they produce repeatedly causes problems to ruminal micro-organisms, almost all of which can only grow at neutral pH >6.0. Thus, if yeast could maintain a more stable, neutral pH, ruminal micro-organisms would be healthier: healthier ruminal micro-organisms lead to a more productive animal. But how could the primary effect be pH? Yeast is not a buffer. And so on: Effects all seemed to be in a positive direction, which seemed to many, including this author, to be too good to be true. Multiple modes of action seem so improbable.
Experiments targeting the mode of action then began to explain what was might be happening. Yeast extracts, not containing live yeast, did not replicate the effects of yeast culture on the ruminal micro-organisms. This indicated that the vitamins present in the yeast culture were not responsible.
The possibility remained, however, that the live yeast cells produced vitamins in the rumen itself. It was shown around the same time that yeast cells did not grow in rumen contents. They did not die, but neither did they reproduce, rendering it much less likely that vitamin supply could be at the root of the observed effects. It also explained why yeast had to be supplemented on a daily basis: They were washed out of the rumen continuously. And it explained too what rumen microbiologists already knew, that numbers of yeast normally found in the rumen were very low. It was additionally found that yeast ‘culture’, defined as a mixture of live yeast cells and culture constituents, was not required for the observed effect. Yeast cells alone could achieve the same stimulatory effect. Different strains of yeast had different effectiveness. Indeed the strains used commercially were sometimes not the most potent. Why?
Live yeast cells in ruminal digesta remained metabolically active to some degree. Was this the reason that yeast stimulated microbial activity? The mechanism behind such an effect became clear when small amounts were added to pure cultures of fibre-digesting ruminal bacteria, particularly Fibrobacter succinogenes. F. succinogenes is difficult to grow in the laboratory. It grows so slowly that often oxygen permeates into the culture solution. The culture usually contains an indicator, resazurin, that turns pink when the medium becomes ‘oxidised’ like this. It is only tiny amounts of oxygen that pass into the medium and turn it pink, but this is enough to poison F. succinogenes. With yeast added to the medium, the pink colour disappeared and F. succinogenes could grow.
This observation led us to think – could the same be happening in the rumen? The rumen was reputed to be anaerobic (oxygen absent). A few studies had shown that there were small but finite concentrations present. When this was confirmed, and it was shown that ruminal digesta from animals receiving yeast had lower prevailing concentrations of oxygen, the jigsaw began to become clear.
All the previously observed effects could be ascribed to the yeast stripping low concentrations of oxygen out of ruminal contents. Improved fibre digestion occurred because the highly oxygen-sensitive fibre-digesting bacteria were protected. The protection of other anaerobic species made it more difficult for the less oxygen-sensitive lactic acid-producing bacteria to compete. Lower lactic acid means higher pH, which means pH stabilisation, all to the benefit of the host animal. The different effectiveness of different yeast strains turned out to correspond to their ability to scavenge oxygen. And when the ability to utilise oxygen was deleted from active yeast strains by mutation, they became unable to stimulate ruminal activity.
Sensitivity to oxygen
||Prof John Wallace: "The different effectiveness of different yeast strains turned out to correspond to their ability to scavenge oxygen."
These observations led to teleological discussions concerning why ruminal bacteria should be so sensitive to oxygen. Had they not evolved for millions of years in this environment, where traces of oxygen enter the rumen in the feed and across the rumen wall? The answer, of course, was ‘yes’, but the point was made that ruminants had evolved as grazing animals, not under present-day conditions of concentrate feeding or supplementation. When Jamie Newbold and I wondered what other biological materials might replace yeast as oxygen scavenger, we found many, and demonstrated with some that they also stimulated ruminal bacteria. Crucially, we noticed that one of the biological materials with the greatest activity to consume oxygen (in the dark, as prevails in the rumen) was… grass.
These observations probably explain the variability in response to supplemental yeast in animal trials. One might expect a greater effectiveness of yeast culture supplementation with grain-based rations, which do not replicate the protective properties of grass. Over recent years, so-called ‘meta-analyses’ of the effectiveness of yeast under different production conditions have appeared. They confirm that the benefits to be obtained from yeast culture supplementation are modest and variable.
Yet, the majority of ruminant livestock in North America now receive yeast culture, as do many in Europe. Other companies have added yeast to their portfolio, citing oxygen-scavenging, or control of redox potential (a consequence of oxygen concentration) as the mode of action. The case for the effectiveness of yeast culture has been made, thanks to our understanding of its mode of action at the molecular and cellular level.