YEAST GROWTH IN RELATION TO THE DISSOLVED OXYGEN AND STEROL CONTENT OF WORT

The extent of the requirement for oxygen in cells of brewing yeast is determined by the availability of oxygen during propagation. Cells with no oxygen requirement ferment satisfactorily when added to either air-saturated or de-aerated wort. Cells produced during fermentation develop an oxygen-requirement and ferment poorly when added to de-aerated wort because of restriction of both rate and extent of exponential growth. The quantity of dissolved oxygen needed to ensure satisfactory growth varies greatly with yeast strain. In all cases examined, the oxygen requirement can be eliminated by addition to the growth medium of ergosterol and Tween 80 However Tween 80 alone is without effect. It seems likely that oxygen is required because it is essential for biosynthesis of sterols.     

STEROL BIOSYNTHESIS BY STRAINS OF SACCHAROMYCES CEREVISIAE IN THE PRESENCE AND ABSENCE OF DISSOLVED OXYGEN

The content of sterols in Saccharomyces cerevisiae which has been harvested after anaerobic growth and then added to a complex nutrient medium, rises rapidly from ca. 1 mg/g dry yeast to ca. 10 mg in the presence of dissolved oxygen. A range of sterols, present principally as sterol esters, is formed during this period. The concentration of free sterols does not rise above 3 mg/g and esters are thought to form a reserve sterol pool. Cyclization of squalene to lanosterol in the presence of oxygen seems not to be markedly affected by oxygen concentration in contrast to demethylation and desaturation reactions on the pathway to ergosterol. When oxygen concentration falls to zero, further metabolism of preformed sterols continues, with the accumulation of episterol and ergosterol and reduction in the concentration of zymosterol and 24(28)-dehydroergosterol. During anaerobic growth a marked hydrolysis of sterol esters occurs and free sterols eventually predominate.     

SUGAR UTILIZATION BY A BREWING YEAST IN RELATION TO THE GROWTH AND MAINTENANCE PHASES OF METABOLISM

In batch fermentations, carried out using a range of conditions, the specific rate of sugar utilization is shown to decline throughout. When yeast mass is increasing, the yield coefficient relating carbohydrate utilization to mass production remains approximately constant and the declining specific fermentation rate parallels declining specific growth rate. When mass production ceases, maintenance of the yeast population requires only a low rate of sugar utilization. Attenuation associated with maintenance phase activity is relatively insignificant compared with that associated with growth.     

INDUCTIVE EFFECTS OF OXYGEN ON YEAST FERMENTATION IN GLUCOSE- AND MALTOSE-MEDIA

The findings presented demonstrate that both growth yield and fermentation activity of yeast cells cultured in maltose-media are highly affected by pretreatment of the pitching yeast or by oxygen induction in the medium and by the presence of ergosterol and Tween 80. Variable responses to oxygen are reported from three different strains of brewer's yeast (Saccharomyces carlsbergensis). Yeast growth and fermentation were significantly reduced in media containing glucose above a certain threshold value. Previous reports concerning glucose repression are discussed. It is suggested that sucrose has a repressive effect similar to that of glucose. When nitrogen is a limiting factor, the fermentation velocity in the presence of maltose is reduced.     

THE SENSITIVITY OF DIFFERENT BREWING YEAST STRAINS TO CARBON DIOXIDE INHIBITION: FERMENTATION AND PRODUCTION OF FLAVOUR-ACTIVE VOLATILE COMPOUNDS

The sensitivity of brewing yeast strains, with different oxygen demands, to carbon dioxide inhibition was investigated. Laboratory fermentations were performed with, and without, protein-based “yeast foods” to lower dissolved CO₂ during fermentation. Differences were observed in yeast fermentative performance in the presence and absence of “yeast foods” for all yeast strains tested. Fermentation performance was improved with the addition of “yeast foods”. There was improved carbohydrate utilisation and amino acid uptake, while acetaldehyde levels at the end of fermentation were decreased. There was an increase in fusel oil production and acetate ester levels at the end of fermentation. Sulphur dioxide levels at the end of fermentation were unaffected by “yeast food” addition. Different yeast strains displayed differing sensitivity to CO₂ inhibition for all parameters tested. Sensitivity to CO₂ was not found to be related to oxygen demand of the yeast strains.     

EFFECT OF LIPIDS AND OXYGEN ON YEAST GROWTH AND THE BIOSYNTHESIS OF ACETOIN DURING FERMENTATION

Brewery yeast needs traces of oxygen for the biosynthesis of unsaturated fatty acids and ergosterol. Owing to the increase in cell mass during primary fermentation the concentrations of these essential lipids decrease and thereby affect the physiological condition of the yeast. When the sterol concentration of whole cells has decreased to 0.2 to 0.3 mg per 100 mg dry yeast, the yeast changes its metabolism. This metabolic change is revealed by a decrease in acetoin concentration. The absorption of wort nutrients and consequently the efficiency of growth is at this point also greatly reduced. The ratio between yeast growth and mole ethanol formed (i.e. the molar growth yield) decreases greatly during wort fermentation. A close correlation between molar growth yield and the change in acetoin metabolism can be observed. This metabolic change occurs when the ratio between yeast growth and ethanol formed is in the range of 8.3 to 9.1, averaging 8.7 g/mole.     

CAMBRIDGE PRIZE LECTURE: STUDIES ON YEAST PHYSIOLOGY-IMPACT ON FERMENTATION PERFORMANCE AND PRODUCT QUALITY*

The physiology of brewing yeast is of pivotal importance in securing and maintaining beer quality. Fundamental studies on aspects of yeast physiology are critically evaluated, areas considered include alternative approaches to yeast propagation and methods for strain differentiation. The proposed role of glycogen in fuelling lipid synthesis and other early events in fermentation is discussed, as are methods for measurement of this reserve polysaccharide. As an extension of this work, a new and novel approach to the improved control of fermentation is described. The interaction between yeast physiology and beer flavour is considered. Evidence is presented that the synthesis of higher alcohols and volatile esters contribute to the regulation of intermediary metabolism in yeast during fermentation.     

ROLE OF WORT AERATION IN THE BREWING PROCESS PART 1: OXYGEN UPTAKE AND BIOSYNTHESIS OF LIPID BY THE FINAL YEAST

While yeast at the end of fermentation (final yeast) contained minimal amounts of lipid, no significant differences could be found in the lipid compositions of various strains of final yeast obtained from several practical brewing plants. Lipid content of the final yeast increased during storage in cold water until the 3rd or 4th day. External nutrients were found to be necessary for the final yeast to synthesize well-balanced lipid during pre-aeration. The final yeast required more than 30 mg of available oxygen/g of dry yeast synthesizing the sufficient amounts of lipid which are necessary for desirable fermentation.     

THE EFFECTS OF INCREASED MAGNESIUM AND CALCIUM CONCENTRATIONS ON YEAST FERMENTATION PERFORMANCE IN HIGH GRAVITY WORTS*

The response of e number of Saccharomyces cerevisiae (ale) and Saccharomyces uvarum (caris-bargenaia) (lager) strains to altered starting levels of magnesium and calcium in 12°P (1048 original gravity) or 20°P (1080 original gravity) wort were investigated. In general, the same trends were observed in all 6 strains, however the extent of the response to adjusted levels of magnesium and calcium were found to be strain dependent. The results indicate that an increased ratio of magnesium to calcium causes an increase in the initial fermentation rate, the rate and yield of ethanol produced and an increase in vitality at the end of fermentation, in all strains employed. Upon increasing the calcium to magnesium ratio it was found that the initial fermentation rate was decreased, resulting in an increased attenuation time in the case of the lager strains. It was also noted that increasing the calcium to magnesium ratio led to a decreased ethanol production, maltotriose uptake, and in the case of the lager strains, maltose uptake was also adversely affected under these conditions. Altering the calcium and magnesium levels had no effect on the viability of the yeast or on glycogen levels .     

THE METABOLISM OF PUTRESCINE,SPERMIDINE AND SPERMINE BY YEAST IN RELATION TO THE AVAILABILITY OF MAGNESIUM

During the lag period immediately after inoculation of yeast into a defined salts medium rapid changes in the internal concentrations of the oligoamines occurred when the medium was deficient in magnesium or high in calcium. Pronunced changes in the oligoamine content of yeast also occurred after inoculation into a range of worts and the extent of the effect appeared to depend on the magnesium content of the wort. The amount of magnesium in all the worts was much higher than was found to be saturating in a minimal medium and it therefore seems that the magnesium in worts is not freely available to the yeast cells. Putrescine was the only oligoamine found in wort and was present at about 1 mg/litre. The putrescine concentration in the wort always increased during fermentation and the period of most rapid increase corresponded to the period of decline in the internal oligoamine pools.     

STRAIN SPECIFIC RESPONSE OF BREWER'S YEAST STRAINS TO ZINC CONCENTRATIONS IN CONVENTIONAL AND HIGH GRAVITY WORTS*

The effect of zinc on a variety of yeast strains is extensively documented in the literature. However, due to the varied experimental protocols employed in each study there is little opportunity to directly compare the strain specificity of this ion. In the present study, the response of six yeast strains (three Saccharomyces cerevisiae (ale type) and three S.cerevisiae (lager type)) to altered zinc concentrations, in both high (1080 OG) and conventional (1048 OG) gravity worts, was investigated. Varying the initial wort zinc concentration in both gravities had an effect on the ethanol production, rate of fermentation, cell number and sugar uptake of all six strains studied. The extent of the response was found to be dependent upon the zinc concentration, the strain employed and wort gravity employed.     

TANNING PROPERTIES OF FLAVANOLS IN BARLEY AND HOPS MEASURED BY REACTION WITH CINCHONINE SULPHATE IN RELATION TO HAZE FORMATION IN BEER

The flavanoid polyphenol extracts from barley and hops were each separated into six fractions by adsorption chromatography on Sephadex LH20. These fractions were further characterised by several analytical methods, including high-performance liquid chromatography and a colorimetric measurement of polymerisation index. The tanning powers of the fractions were graded according to their reactivities with cinchonine sulphate solution in a standardised turbidometric test. Whereas, almost 75% of the flavanols from Ark Royal barley were non-tanning oligomers almost 96% of the flavanols from Bullion hops were polymeric tannins. Reactivity of most of the barley flavanols with cinchonine sulphate was increased greatly by oxidation with peroxidase and hydrogen peroxide. Some effects of polymerisation, caused by enzyme action or by exposure to air, on oxidisable polyphenols (non-tannins) were measured using (+)-catechin, procyanidin B3 and prodelphinidin B3 in model systems. These, and other measurements on experimental and commercial beers indicated that oxidation of simple flavanols from barley produced polymers with tanning properties. In contrast, the hop flavanols when extracted apparently in their native forms, were capable of co-precipitating with polypeptides in beer. Treatment of beers with different stabilising agents, such as Polyclar AT and silica hydrogel, retarded haze formation by restricting ‘protein-polyphenol’ interactions.