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.     

镁钙对酿酒酵母发酵的影响

在酿酒酵母发酵产乙醇的代谢途径中，金属离子镁钙会对整个代谢过程产生影响，就镁钙对酵母菌产乙醇的影响进行了简单比较，结果表明镁对酵母发酵产乙醇有重要促进作用，而钙却对其有拮抗作用。     

THE EFFECT OF MAGNESIUM AND CALCIUM ON YEAST GROWTH

When inoculated into a defined minimal medium, yeast could not absorb all the magnesium, even if this element was limiting. The cells tended to absorb a constant amount of magnesium, ca 1·1 pg per cell, so long as the content of the medium was sufficient, rather than absorb magnesium in proportion to its initial concentration. Calcium competed with magnesium and prevented yeast growth almost entirely when present at a ten times excess. At a two to three times excess the effect was more on the length of the lag phase and mean generation time than on the total amount of 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.     

德国啤酒酵母和国内啤酒酵母发酵性能的研究

酵母菌种是啤酒酿造的关键，不同的菌种可用来酿造不同类型的啤酒。本试验对德国酵母和国内酵母的发酵性能进行了对比研究，总结出了二者之间的差异。     

CAMBRIDGE PRIZE LECTURE IMPROVING YEAST FERMENTATION PERFORMANCE

A number of factors affecting yeast fermentation performance have been investigated. These include the mode of substrate feeding, nutrient supplementation, temperature, osmotic pressure, oxygen, intracellular ethanol accumulation, and yeast ethanol tolerance. Nutrient supplementation was observed to play a key role in yeast fermentations employing high gravity media and at high temperatures. Furthermore, complete attenuation of high gravity wort (25°P) could be achieved by optimizing the yeast pitching rate, fermentation temperature, and level of wort oxygenation. Genetic manipulation techniques, such as spheroplast fusion, were successfully employed to obtain ethanol and osmotolerant yeast strains. In addition, a number of stable 2-deoxy-D-glucose resistant mutants, isolated from brewing and non-brewing yeast strains, were observed to rapidly utilize maltose and maltotriose in the presence of high concentrations of glucose. Fermentation and ethanol production rates were increased in these strains. Therefore, employing strategies of optimized fermentation conditions and strain development have resulted in improvements in yeast fermentation performance.     

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.     

EFFECTS OF AERATION DURING THE CULTIVATION OF PITCHING YEAST ON ITS CHARACTERISTICS DURING THE SUBSEQUENT FERMENTATION OF WORT

The characteristics of cultivated yeast were studied after continuous aeration, aeration for the first hour and without aeration. The fatty acid content increased under aerobic conditions, while the level of storage sugars increased under anaerobic conditions. In a fermentation test, total number of cells and their viability and also the intracellular level of storage sugars and fatty acids were higher in yeast cultivated under aerobic conditions than in yeast cultivated under anaerobic conditions. Since there were no significant differences between the performance of yeast cultivated under continuous aeration and those with aeration for the first hour, the latter offer the best economic choice.     

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.     

THE INFLUENCE OF A COLIFORM BACTERIUM ON FERMENTATION BY YEAST

Hopped wort (20°C) was inoculated with brewing yeast alone (10⁷ cells/ml), or with Citrobacter freundii (10⁵ cells/ml) or with both organisms at the concentrations stated. The mixture grew somewhat more quickly than the pure yeast culture, the oxidation-reduction potential and S.G. fell more quickly but the pH less rapidly. The mixed culture produced more ethanol and various organic acids but less volatile carbon. Dimethyl sulphide was produced in the mixed culture but not in the pure yeast culture. It is concluded that C. freundii can have a profound influence on yeast fermentation, consistent with its pure culture characteristics.     

DISTRIBUTION OF ESTERS PRODUCED DURING SUGAR FERMENTATION BETWEEN THE YEAST CELL AND THE MEDIUM

The distribution of the esters formed during sugar fermentations between the yeast cells and the medium was investigated in fermentations by 5 strains of Saccharomyces cerevisiae and 3 strains of S. uvarum (carlsbergensis). The esters studied included the acetates of isoamyl alcohol and phenethyl alcohol and the ethyl esters of the C₆-C₁₂ fatty acids. All of both acetates appeared in the medium. The proportion of the fatty acid ethyl esters transferred to the medium decreased with increasing chain length: all in the medium for ethyl caproate, 54–68% for ethyl caprylate, 8–17% for ethyl caprate, and all remaining in the yeast cell for ethyl laurate. A higher proportion of the esters formed appeared to remain in the cells of the S. uvarum strains than in cells of S. cerevisiae.     

FORMATION AND REMOVAL OF ACETOIN DURING YEAST FERMENTATION

The main yeast fermentation process can be characterized by two separate phases, the acetoin-producing phase (phase I) and the acetoin-reducing phase (phase II). The time at which yeast changes from production to removal is here designated as the ‘point of metabolic change’. The rise and fall in acetoin concentration was affected by composition of the wort, inoculum size, yeast strain, presence of traces of oxygen and addition of ergosterol, whereas temperature seemed to be of somewhat less importance. A high and positive correlation between the maximum level of acetoin and the concentration of ethanol measured at the ‘point of metabolic change’ has been demonstrated. The appearance of the ‘point of metabolic change’ is discussed in light of the level of acetoin in finished beer. The absolute threshold for acetoin was determined as 17 ppm.     

蜂窝陶瓷固定化酵母细胞啤酒连续后发酵工艺的研究

将蜂窝陶瓷用作酵母细胞固定化的载体，进行啤酒连续后发酵试验研究。应用均匀试验设计法来安排试验．对试验数据进行统计分析及最优化处理，并通过验证试验，获得优化的工艺条件为：发酵温度13～14℃，稀释率0．142h^-1。啤酒主要理化指标的测定和口味品评结果表明：与传统工艺相比，采用蜂窝陶瓷固定化酵母细胞啤酒连续后发酵工艺不会影响啤酒的品质，而后发酵时间可大为缩短。     

活性干酵母在淀粉质酒精发酵生产中的应用

本文利用市售活性干酵母代替培养酒母,分别进行淀粉质酒精间歇发酵、半连续发酵和连续发酵大型生产,均取得了发酵速度快、出酒率高的好效果。     

THE INFLUENCE OF LIPIDS DERIVED FROM MALT SPENT GRAINS ON YEAST METABOLISM AND FERMENTATION

The addition to wort of lipids derived from malt spent grains had a pronounced effect on yeast metabolism. The lipids allowed the fermentation of de-oxygenated wort and also stimulated yeast growth and the corresponding rate and extent of fermentation of air-saturated wort by yeast strains having a high oxygen requirement. The lipids increased the fusel alcohols content of beer and decreased the content of esters and medium chain-length fatty acids. The yeast incorporated sitosterol and unsaturated fatty acids from the spent grain lipids and the unsaturated fatty acids changed the pattern of fatty acids and sterols synthesized by the yeast. The fatty acids were present in the spent grain lipids mainly as triglycerides, free fatty acids and phospholipids. Using pure lipid compounds it was shown that the triglycerides were inactive and that the spent grain lipids exerted their effect on fermentation through the synergistic action of free unsaturated fatty acids, sitosterol and phospholipid. Phospholipid could be replaced by the detergent, Triton X-100. The effect of the lipids on the synthesis of esters, fusel alcohols and medium chain fatty acids could be explained solely by their content of unsaturated fatty acids.     

SOME CONSIDERATIONS OF THE FLOCCULATION CHARACTERISTICS OF ALE AND LAGER YEAST STRAINS*

While some ale yeast strains are able to flocculate when cultured in a defined medium of glucose, ammonium salts, vitamins and ions, others require the presence of a nitrogen-containing inducer in the growth medium. On the other hand, all flocculent lager strains examined to date are able to flocculate after being cultured in a defined medium and do not appear to require the addition of inducer material to the growth medium. The inducer material present in wort has been identified as peptide. By the use of ion exchange chromatography the peptide fraction that induces flocculation has been found to contain a high level of acidic amino acid residues with a very similar structure to that reported for the α-factor involved in sexual agglutination of haploid α and a cells of Sacch. cerevisiae. Studies on the adsorption of Ca⁺⁺ ion by the cell wall failed to reveal any significant differences in total uptake between flocculent and non-flocculent cultures. It would appear that Ca⁺⁺ ions are bound less tightly by non-flocculent cells than by flocculent cells. The contribution of calcium to flocculation is not the absolute amount of this ion adsorbed by the yeast cell wall but rather the stereo-specific manner by which it is bound, i.e., its position relative to the three-dimensional structure of the yeast cell wall.     

EFFECT OF SOME MONOVALENT AND DIVALENT METAL IONS ON THE FLOCCULATION OF BREWERS YEAST STRAINS*

In agreement with previous reports, it has been found that both Mg⁺⁺ and Mn⁺⁺ ions can imitate Ca⁺⁺ as inducers of flocculation, though the intensity of the flocculation is considerably reduced. This reduction is not dependent upon the ionic concentration and a 10-fold increase in Mg⁺⁺ or Mn⁺⁺ from the normal concentration of 10 mg ion/litre fails to increase the flocculation intensity. Low concentrations (1–10 mg ion/litre) of Na⁺ or K⁺ induce flocculation in those strains displaying intense flocculation with Ca⁺⁺, but high concentrations of either Na⁺ or K⁺ (50–100 mg ion/litre) antagonize floc formation. It is suggested that divalent ions act by bridging cells through negative charges on the cell surface, whereas monovalent ions induce flocculation via a “counter ion” effect where the repellent forces of the negative charges on the cell surface are neutralized, thus allowing some floc formation due to hydrogen bonding or other types of non-ionic bonding between cells. The antagonism of high concentrations of monovalent ions towards flocculation may be due to the fact that all available cell surface charges are neutralized, resulting in insulation of the cells and thus preventing cell-to-cell hydrogen bonding.     

培养基各成分对酵母生长及富铬性能的影响

以活性干酵母为菌种，采用摇瓶培养，通过单因素试验和正交试验，分析了发酵培养基各成分对酵母生长和富铬性能的影响，确定了发酵培养基最优配方，即酵母浸出粉10g／L、蛋白胨10g／L、蔗糖90g／L、KH2PO4 2．0g／L、MgSO41．5g／L和微量元素1mL／L。在该营养条件下发酵生产富铬酵母，生物量可达10．308g／L，有机铬含量达到5190．68μg／g，有机铬总含量为53504．7μg／L，富铬效果较好且稳定。     

FLOCCULATION—SOME OBSERVATIONS ON THE SURFACE CHARGES OF YEAST CELLS

Surface charges on flocculent and non-flocculent yeast cells have been measured by micro-electrophoresis. Yeasts were grown both in calcium deficient and in complete medium and particular attention was paid to changes as cells passed from a logarithimic to a stationary phase of growth. Many of the ionogenic groups contributing to surface charge are situated well within the cell wall; charges on cells from the calcium-deficient medium were higher than on cells from the complete medium. A pH-dependent rearrangement of an underlying surface protein layer is postulated for the flocculent yeast Saccharomyces cerevisiae Strain NCYC 1109. No evidence of a similar rearrangement was found with other flocculent strains examined. The results are discussed in relation to ‘calcium bridge’ formation.     

YEAST AND THE VICTORIAN BREWERS: INCIDENTS AND PERSONALITIES IN THE SEARCH FOR THE TRUE FERMENT*

Events in the elucidation of the role of yeast in fermentation up to the end of the nineteenth century are described. It is concluded that, contrary to the popular view, the more enlightenend brewers had deduced the true nature of yeast prior to the time of Pasteur and were in advance of the dogma of the chemical establishment as represented by Liebig. The activities of scientists of the Victorian period employed in the Burton-on-Trent breweries are placed within this context.