Yeast nutrient products
Please check back later for additional changes
Yeast extract is a water soluble autolysate derived from spent yeast and contains many of the nutrients required to maintain yeast metabolism. The nutritional value of yeast extract and the abundance of spent yeast available to the brewing industry make this a potentially valuable supplement for improving the nutritional status of higher gravity worts. Yeast extract, in particular, is a rich source of freely assimilable nitrogen and may be used to balance the high C:N ratio that occurs as a consequence of adjunct brewing. A number of commercial preparations termed yeast foods are available to improve the nutrient composition of wort. Yeast extract is typically a principal component of such preparations along with inorganic nutrients such as Mg (magnesium, or did they mean manganese?) and Zn. Yeast foods can improve yeast fermentation performance in traditional and higher gravity worts, but have not been universally adopted by brewers. This may relate to the often undefined nature of the products. An investigation by Ingledew et al. revealed that the chemical composition and nutritive value of yeast foods on the market at the time was highly variable. In many cases the products comprised high levels of non-utilizable protein and often low or undetectable levels of amino nitrogen. It was recommended that before using such a product, the brewer should be aware of its composition, the manufacturer's intended use, and any potential unwarranted or unwanted effects on fermentation performance or beer quality.[1]
Benefits of yeast extract use include greater yeast growth and higher viability, shorter times to attenuation, more complete sugar consumption, and greater ethanol production.[1] Use of yeast extract or yeast foods may not be beneficial in all cases and careful consideration must be given to the nutritional deficiencies of a given wort and the potential of the supplement to correct these deficiencies. the benefits of yeast extract supplementation may be confined to adjunct worts.
Improved fermentation performance with yeast foods is, as noted by Ingledew et al.92, not solely a result of increased nitrogen availability, and yeast foods can, in addition, contribute vitamins and metals to the fermentation.[1]
The brewer’s decision to include a yeast food in wort will depend not only on the influence this supplement has on fermentation characteristics, but also on the flavour profile of the resultant beer. Any gains in fermentation should not be at the expense of product quality. In this regard, the literature concerning the influence of yeast extracts/foods is limited to a small number of studies. The presence of yeast food in standard gravity fermentation wort caused an increase in higher alcohol and ester synthesis with all eight yeast strains tested109. Total higher alcohol production was in some cases doubled in the presence of yeast food. This result was attributed to an improvement in the uptake of amino acids from the wort109. Conversely, Casey et al.35 reported lower concentrations of most higher alcohols in beers tested when VHG wort fermentations contained yeast extract. Under the same conditions a general relative increase in ester concentration was observed, with ethyl acetate and isoamyl acetate increasing by 40% and 80%, respectively. Improved fermentation performance due to the presence of a proteinbased yeast food has been found to coincide with lower acetaldehyde concentrations in beers109. This result was observed with all eight strains tested, though the reduction was strain-dependent and varied between 10 and 70%.[1]
Evidence indicates that yeast extract supplementation may be particularly beneficial for worts containing adjuncts. Beer produced from a 40% rice adjunct wort rated poorly when evaluated by a taste panel, with diacetyl notes, wateriness and lack of mouthfeel cited as reasons for the poor evaluation. Beer produced from the same wort supplemented with yeast extract showed a significant improvement and was deemed comparable to an all-malt beer124. Contrary to this, McCaig et al.139 noted that the presence of yeast food in fermenting worts (18–24°P) resulted in beers which were considered less palatable than those without supplementation. This result was most likely influenced by the lack of improvement in fermentation performance with these supplemented fermentations, relative to non-supplemented fermentations. The use of yeast extract appears to reduce the levels of VDK in finished beers. Le Van et al.124 reported that the VDK levels of the beer produced from a 40% rice adjunct wort could be reduced from 0.26 mg L–1 to 0.14 mg L–1 when yeast extract was introduced to the wort, which was similar to levels found in all malt beers124. McCaig et al.139 also reported that where yeast foods stimulated fermentation performance, there was an increase in the levels of VDK in the undiluted beers. The compositional complexity of yeast extracts and yeast foods used to augment fermentation make interpretation of volatile and organoleptic profiles difficult. Flavour characteristics may be dependent, to a considerable extent, on the amino acid content of the supplement. Amino acid profiles of yeast extract based foods are known to vary and it is likely that this variability influences the production of esters, higher alcohols and SO2 218. In particular, increased availability of the branched chain amino acids is expected to lead to increased generation of higher alcohols with valine, leucine and isoleucine influencing the formation, respectively, of isobutanol, isoamyl alcohol and amyl alcohol12,66,180,185 and to some extent also the corresponding esters66. It is also likely that VDK generation is influenced by the amino acid composition of yeast extracts. VDK levels are known to be influenced by FAN concentration, with valine in particular being cited as an important factor due to its influence on production of the diacetyl precursor α-acetolactate167. Low initial levels of valine in wort stimulate the anabolic production of valine and a corresponding increase in α-acetolactate161. The result is a second diacetyl peak forming during fermentation, leading to a relatively high concentration of diacetyl in the final beer161. Increased wort valine added, for example, with a valine-rich yeast food, would be expected to eliminate this secondary diacetyl peak and result in beers which require less time to mature. Other amino acids may have a less direct effect on VDK level. For example, Lekkas and co-workers119 found reduced VDK levels with methionine supplementation and increased levels with lysine supplementation. In this case the differences in VDK concentration were due to the longer and shorter fermentation times for methionine-supplemented (103 hours) and lysine-supplemented (48 hours) worts when compared to control wort fermentations (96 hours)119. Amino acids may also have a direct effect on beer flavour with proline, for example, imparting a sweet taste204. Hydrogen sulphide generation during wort and must fermentation is known to increase with increased availability of the sulphur-containing amino acid cysteine63,100. Duan et al.63 measured H2S production by nine yeast strains (six lager and three ale) in 14°P wort with different additions of cysteine (50, 100 or 150 ppm). In each case cysteine supplementation resulted in increases in H2S, with concentrations increasing by between 1.7- fold and 6.7-fold depending on the strain involved. In the same study, methionine addition did not increase H2S levels and, in fact, lowered H2S production in cysteine-supplemented fermentations. The underlying causes of these phenomena are unclear.[1]