SUGAR UPTAKE AND SUBSEQUENT ESTER AND HIGHER ALCOHOL PRODUCTION BY SACCHAROMYCES CEREVISIAE

The production of a number of esters and higher alcohols by brewing strains of Saccharomyces cerevisiae, in synthetic media containing only glucose, fructose or maltose as sole carbohydrate source, has been investigated. Results indicated that production of most volatiles was generally lower when maltose was the sugar being fermented, despite maltose-grown cells having higher viabilities and vitalities than glucose or fructose-grown cells. There was no significant difference in the levels of esters and higher alcohols produced during fermentation when glucose or fructose was metabolised, although strain variation was observed. Similar results were obtained when wort was supplemented with either glucose, fructose or maltose. Wort supplemented with maltose produced fewer volatiles, especially higher alcohols, than that which had a hexose sugar added. The activity of estersynthesising enzymes present in glucose or maltose PYN grown cells was also examined. Similar levels of ethyl acetate and isoamyl acetate were obtained when cells grown in either glucose or maltose PYN were disrupted and ester production monitored. The implications of these results for the fermentation of high-gravity worts are discussed.     

THE EFFECTS OF GLUCOSE ADJUNCT IN HIGH GRAVITY FERMENTATION BY SACCHAROMYCES CEREVISIAE 2036

Laboratory fermentations of 16°Plato glucose adjunct worts by Saccharomyces cerevisiae 2036 demonstrated the absence of “glucose repression” of maltose and maltotriose uptake. However, when compared to worts in which maltose syrup was employed as an adjunct, residual glucose was present at the end of fermentation, maltose and maltotriose uptake rates were enhanced, fructose uptake was blocked and the sequence of sugar uptake was changed. These findings partially explain residual glucose and fructose that sporadically appear in commercial beers. Further research suggests that the physiological quality of the yeast is of prime importance in carbohydrate metabolism, and that critical concentrations of glucose vary with different physiological conditions for this brewing strain in 16°P wort .     

ON THE DIFFERING RATES OF FRUCTOSE AND GLUCOSE UTILISATION IN SACCHAROMYCES CEREVISIAE

Fructose is utilised slower than glucose when the two sugars are fermented separately. This phenomenon occurs in a growth promoting medium as well as in brewers wort when using the brewing yeast Saccharomyces cerevisiae. The use of a fructose adjunct in wort at concentrations of 2% w/v and above, may result in residual concentrations of fructose to remain at the end of fermentation and consequently taint the beer with a sweet off-flavour. Glucose and fructose have no effect on maltose utilisation. Thus they do not exert catabolite repression on the maltose membrane transport system in the particular brewing strain of S. cerevisiae under investigation, when fermented in brewers wort.     

Sugar content of swedes for stockfeeding

The sugar content was determined of 19 cultivars of swede (Brassica napus ssp rapifera L) grown in rows 50 and 75 cm apart. Sugar concentration ranged from 54.3 to 78.7 g kg^?1 fresh weight and was positively correlated with dry matter content which ranged from 90.4 to 138.7 g kg^?1 fresh weight. The average concentration of fructose, glucose and sucrose in the dry matter was 225.0, 328.4 and 34.5 g kg^?1 DM, respectively. The high-dry-matter cultivars tended to have less fructose and more sucrose than the low-dry-matter ones, but total sugar concentration in the dry matter was not associated with dry-matter content. Row width had little effect on sugar content compared with the differences between cultivars. The implications for the feeding value of swedes are discussed.     

FERMENTATIVE CAPACITY OF YEASTS WHICH METABOLIZE MALTOTRIOSE RAPIDLY

A strain of Saccharomyces carlsbergensis which removes maltotriose particularly rapidly from wort exhibits a very high rate of fermentation of this sugar in isolation. This rate is quickly established after pitching and is maintained throughout the greater part of a batch fermentation despite a rapid decline in the ability of the cells to ferment glucose. This same strain and certain others of the same species which also remove maltotriose readily from wort are able to ferment the trisaccharide as fast as or even faster than maltose after growth on the disaccharide as sole carbon source; this property is usually accompanied by repression of glucose fermentation. Evidence is presented which indicates the presence, in these strains, of two or more α-glucosidases having different relative affinities for the oligosaccharides.     

EFFECT OF SUGARS ON THE THERMAL AND RETROGRADATION PROPERTIES OF OAT STARCHES

The thermal and retrogradation properties of oat starches from two cultivars (NO 753-2 and AC Stewart) were characterized in the presence of glucose, fructose and sucrose at a concentration of 36% w/v. In both oat starches, amylose leaching (AML) and swelling factor (SF) decreased in the presence of sugars (sucrose > glucose > fructose). These decreases were more pronounced in AC Stewart starch. The decrease in AML showed that sugars interact with amylose chains within the amorphous regions of the starch granule. The gelatinization transition temperature and the enthalpy of gelatinization increased in the presence of sugars (sucrose > glucose > fructose). The above increase was also more pronounced in AC Stewart starch. The decrease in SF and the increase in gelatinization parameters indicated that these changes were influenced by the interplay of factors such as starch-sugar interaction, changes in water structure in the presence of sugars, and the antiplastizing properties of sugars relative to water. The retrogradation enthalpy and the X-ray diffraction intensities of NO 753-2 and AC Stewart starch gels (stored at 4C) increased in the presence of sugars (glucose > fructose > sucrose). These changes were more pronounced in NO 753-2 starch. The results showed that interaction (during storage) between leached amylopectin and sugar molecules was the main causative factor influencing oat starch retrogradation.     

Determination of sugars in tropical root crops using 13C N.m.r. spectroscopy: Comparison with the h.p.l.c. method

The ¹³C nuclear magnetic resonance (n.m.r.) spectrum of sugars normally present in foods, viz. glucose, fructose, sucrose, maltose and raffinose contains resonances that are diagnostic of each sugar. The quantity of each sugar is determined by measurement of the ratios of the peak heights of the sugar resonances compared with peak height of an internal standard, present at constant concentration in all solutions. A linear calibration relationship between the peak height ratio and concentration is established for single resonances of glucose and fructose and for two resonances of each of sucrose, maltose and raffinose. Samples of sweet potato and taro flour dried at 40°C, were blended with 80% ethanol, the sugars were extracted and analysed by ¹³C n.m.r. and h.p.l.c. Both methods gave similar results and have similar degrees of reproducibility. The n.m.r. method is useful for quantitation of sugars and identification of other molecules extracted from the food, but requires more material than does h.p.l.c. Drying at 40°C causes a decrease in sucrose content and increases in glucose, fructose and maltose which partly compensate for the loss in sucrose. There is considerable variability of the sugar content between different sweet potato tubers and between different taro corms of the same cultivar as well as across different cultivars. For sweet potato the total sugar content is 0.6-3.6% fresh weight, with considerably more sucrose present than glucose, fructose and maltose. With taro total sugar content is 0.2-0.8% fresh weight. Raffinose is present in one sample only.     

Hydration properties and proton exchange in aqueous sugar solutions studied by time domain nuclear magnetic resonance

Proton NMR relaxation rates (R₁ and R₂) were measured in aqueous solutions of sucrose, d-glucose and d-fructose with increasing concentration. The measurements were carried out using Bruker PC 20 NMR Process Analyzer. Inversion recovery and CPMG pulse sequences are used for the measurement of relaxation rates. Results show that the values of relaxation rate increase as the concentration of the sugar is increased. The relaxation rate appears to be higher for sucrose solutions as compared to glucose or d-fructose solutions. These results were discussed on the basis of molecular association between sugar and water molecules through hydrogen bonding. The water self diffusion coefficient was measured in these sugar solutions by using pulse field gradient NMR method. As expected, the water self-diffusion coefficient was reduced with increased sugar concentrations. The results of translational mobility exhibited a higher mobility for fructose than glucose or sucrose in aqueous solutions.     

糖浓度对酒精酵母GJ2008果糖与葡萄糖利用差异性的影响

在不同糖浓度条件下,以YPDF培养基模拟甘蔗汁进行酒精发酵,测定过程中各参数的变化,并对果糖与葡萄糖消耗过程进行曲线拟合,以拟合方程计算出果糖与葡萄糖代谢一半和代谢完全所需时间.结果表明,糖浓度为90gL～270g/L,酵母GJ2008始终会偏用葡萄糖,果糖利用一直受到葡萄糖的竞争性抑制.糖浓度为90g/L时,细胞生长受糖浓度抑制程度最小,但乙醇产率较低;糖浓度为230g/L,发酵液中葡萄糖含量较低时,果糖受葡萄糖的竞争性抑制得到了解除,果糖的利用急剧加快;糖浓度为250g/L和270g/L,发酵液中葡萄糖含量较低时,果糖受葡萄糖的竞争性抑制得到了解除,但果糖的利用并没有加快,表现为后期酵母数维持值较低.糖浓度对果糖代谢的影响要大于对葡萄糖代谢的影响,较低糖浓度有利于后期果糖与葡萄糖利用差异性的缩小.     

Influence of processing on the content of sugars in green Arabica coffee beans

Quantitative analyses of low molecular sugars in green coffees (Coffea arabica L. var. Acaià) that were processed in parallel either by the traditional wet or the traditional dry method, revealed a close correlation between the kind of post-harvest treatment and the contents of fructose and glucose. While in washed coffee beans (wet processing) only low amounts of these both hexoses were present, those in unwashed coffees (dry processing) were significantly higher. Model-processing experiments in the laboratory confirmed these findings. Moreover, a comparison with the untreated controls revealed that the low levels of both sugars are the consequence of a decrease in the case of wet processing, whereas they remained unchanged or even increased in the case of dry processing. Further minor sugars are also affected by post-harvest treatment. The amounts of galactose, arabinose and mannose show a similar arrangement as those for glucose and fructose, although their overall concentration is about 100-fold lower. Sucrose, the major low molecular sugar in green coffee beans, is not significantly affected by coffee processing. The influence of an active seed metabolism on the observed alterations of the sugar concentrations is discussed.     

Influence of freezing process on free sugars content of papaya and banana fruits

Changes in individual sugar content (fructose, glucose and sucrose) during freezing and frozen storage have been evaluated by HPLC on papaya (Carica papaya grupo Solo var Sunrise), fruits proceedings from female and hermaphrodite flowers, and banana (Musa cavendishii var enana). Effects of thermal treatments, such as blanching and microwaving, prior to freezing banana fruit were examined. Both an increase in total sugar levels of papaya samples during freezing and a decrease in sucrose, by invertase, were found followed by an increase in fructose and glucose. A slight decrease in total sugars of banana samples during freezing and storage has been observed, with no significant differences between the pretreatments applied. © 1998 SCI.     

Comparison of the effect of sugars on the viscoelastic properties of sweet potato starch pastes

Viscoelastic properties of sweet potato starch (SPS) pastes (5% w/w) were studied in the presence of various sugars (sucrose, glucose, fructose, and xylose) at different concentrations (0, 10 and 20%) by small‐deformation oscillatory measurements. Dynamic frequency sweeps at 20 °C indicated that all SPS–sugar mixtures were more elastic than viscous with storage moduli (G′) higher than loss moduli (G′′) at all values of frequency with a frequency dependency. Dynamic moduli (G′ and G′′) values increased with the increase in sugar concentration from 10 to 20%. Changes in the dynamic moduli were more pronounced with xylose in comparison to the control (no sugar) and other sugars. G′ values as a function of ageing time (10 h) at 4 °C continuously increased with time during ageing without a plateau region. In general, G′ values of SPS–sugar mixtures during ageing decreased in the following order: pentose (xylose) > hexose (glucose and fructose) > control > disaccharide (sucrose), indicating that the xylose had the greatest ability in retarding retrogradation of SPS.     

Monitoring of monosaccharides,oligosaccharides, ethanol and glycerol during wort fermentation by biosensors,HPLC and spectrophotometry

The aim of the present study was to analyze sugar levels (namely maltose, maltotriose, glucose and fructose) and alcohols (ethanol and glycerol) during the fermentation process in wort samples by amperometric enzymatic biosensors developed by our research group for industrial application, HPLC and spectrophotometry, and to compare the suitability of the presented methods for determination of individual analytes. We can conclude that for the specific monitoring of maltose or maltotriose only the HPLC method was suitable. On the other hand, biosensors and spectrophotometry reflected a decrease in total sugar concentration better and were able to detect both glucose and fructose in the later stages of fermentation, while HPLC was not. This can be attributed to the low detection limits and good sensitivity of the proposed methods. For the ethanol and glycerol analysis all methods proved to be suitable. However, concerning the cost expenses and time analysis, biosensors represented the best option.     

Degradation of 5-hydroxymethylfurfural during yeast fermentation

5-Hydroxymethyl furfural (HMF) may occur in malt in high quantities depending on roasting conditions. However, the HMF content of different types of beers is relatively low, indicating its potential for degradation during fermentation. This study investigates the degradation kinetics of HMF in wort during fermentation by Saccharomyces cerevisiae. The results indicated that HMF decreased exponentially as fermentation progressed. The first-order degradation rate of HMF was 0.693?×?10^?2 and 1.397?×?10^–2?min^?1 for wort and sweet wort, respectively, indicating that sugar enhances the activity of yeasts. In wort, HMF was converted into hydroxymethyl furfuryl alcohol by yeasts with a high yield (79–84% conversion). Glucose and fructose were utilised more rapidly by the yeasts in dark roasted malt than in pale malt (p?<?0.05). The conversion of HMF into hydroxymethyl furfuryl alcohol seems to be a primary activity of yeast cells, and presence of sugars in the fermentation medium increases this activity.     

Effect of Amyloglucosidase on Wort Composition and Fermentable Carbohydrate Depletion in Sorghum Lager Beers

A three‐factorial experiment with a level of confidence of P < 0.05 was performed to study fermentable carbohydrate depletion and ethanol production during 144 h fermentations of lager beers produced with barley malt (BM), sorghum malt (SM), refined maize (MZ) or waxy sorghum (WXSOR) grits treated during mashing with or without amyloglucosidase (AMG). The percentage glucose, maltose and maltotriose, based on total fermentable carbohydrates for the BM wort was 20, 68 and 13% and for the SM wort 35, 48 and 17% respectively. Treatment with AMG increased wort glucose from 9.3 to 24.5 g/L wort and total fermentable sugar equivalents, expressed as g glucose/L, from 59.2 to 72.6 g/L wort. The SM worts had approximately 50% more glucose and 40% less initial maltose content respectively compared to the BM worts. The WXSOR grits produced worts and beers with similar properties to those produced from the MZ adjuncts. AMG addition led to a >2.5 fold increment in wort glucose and 23% in total fermentable carbohydrate content. Linear regression analysis determined that the consumption rate of fermentable carbohydrates during fermentation followed first order reaction kinetics. Depletion times to reach 50% of the initial concentrations of glucose, maltose and maltotriose were 49, 128 and 125 h, respectively, clearly indicating that the fermenting yeast preferred glucose. Maltose and maltotriose depletion times of the AMG treated worts were significantly faster and lower, respectively, when compared with the untreated worts. At the end of the fermentation, the BM beers contained higher ethanol levels (5.1% v/v) than the SM beers (3.9% v/v). For AMG treated beers, no significant differences in ethanol content were observed among samples mashed with BM and beers produced from SM and MZ grits. The results demonstrated that AMG could be used to increase the initial concentration of glucose and total fermentable carbohydrates thus decreasing dextrin levels, especially from sorghum mashes.     

Amino acid residues involved in ligand preference of the Snf3 transporter‐like sensor in Saccharomyces cerevisiae

Snf3 is a plasma membrane protein in Saccharomyces cerevisiae able to sense the presence of glucose. Although the Snf3 protein does not transport sugars, it shares sequence similarity with various glucose transporters from other organisms. We investigated the sugar specificity/preferences of Snf3. The ability of cells to sense sugars in vivo was monitored by following the degradation of the Mth1 protein, an early event in the signal pathway. Our study reveals that Snf3, in addition to glucose, also senses fructose and mannose, as well as the glucose analogues 2‐deoxyglucose, 3‐O‐methylglucoside and 6‐deoxyglucose. The signalling proficiency of a non‐phosphorylatable analogue strongly supports the notion that sensing through Snf3 does not require sugar phosphorylation. Sequence comparisons of Snf3 to glucose transporters indicated amino acid residues possibly involved in sensing of sugars other than glucose. By site‐specific mutagenesis of the structural gene, roles of specific residues in Snf3 could be established. Change of isoleucine‐374 to valine in transmembrane segment 7 of Snf3 partially abolished sensing of fructose and mannose, while mutagenesis causing a change of phenylalanine‐462 to tyrosine in transmembrane segment 10 of Snf3 abolished sensing of fructose. Neither of these amino acid changes affected the ability of Snf3 to sense glucose, nor did they permit Snf3 to sense galactose. These data indicate a similarity between a ligand binding site of the sensor Snf3 and binding sites used for facilitated hexose transport in the GLUT proteins. Copyright © 2009 John Wiley & Sons, Ltd.     

PITCHING RATES RELATED TO GLUCOSE AND FRUCTOSE UTILIZATION IN Saccharomyces cerevisiae

Pitching rates, in the range 1.5 g/litre to 24 g/litre of yeast in high gravity sucrose adjunct laboratory-scale wort fermentations, are directly proportional to the natural logarithm of glucose utilization throughout fermentation, whereas this relationship only holds for fructose utilization during the intermediate phases. The natural logarithmic rate of fructose utilization in the latter stages of fermentation remains constant irrespective of the pitching rate used. Adjustment of the pitching rate allows residual fructose concentrations at the end of fermentation to be regulated such that they remain below the fructose taste threshold in beer. The difference in utilization rates produced by the preferential utilization of glucose over fructose appears to remain constant irrespective of the pitching rate used. This differential may be ascribed to the competitive inhibition of fructose transport by glucose for their common hexose constitutive facilitated diffusion membrane transport system in S. cerevisiae.     

THE INFLUENCE OF MEDIA CONDITIONS ON THE UTILISATION OF MONOSACCHARIDES BY A STRAIN OF SACCHAROMYCES UVARUM (CARLSBERGENSIS)

The utilisation of glucose and fructose by the brewing yeast species Saccharomyces uvarum (carlsbergensis) was investigated. When sucrose was used as the substrate, one of the hydrolysis products, glucose was taken up preferentially over the other hydrolysis product fructose. However, when glucose and fructose were added separately to the media, the uptake profile for each sugar was very similar. Growth promoting media enhanced the differential uptake, whereas non-growth promoting media reduced it. Increase in osmotic pressure of the medium eliminated the preferential uptake of glucose over fructose. The results suggest the presence of a complex uptake system and could be explained by a combination of (1) molecular configuration of the sugars in aqueous solutions, (2) differential phosphorylation effect of glucose and fructose and (3) intracellular accumulation of ethanol in yeast.     

Pasting properties of Angelica dahurica starches in the presence of NaCl,Na2CO3, NaOH,glucose, fructose and sucrose

The pasting viscosities of starches from Angelica dahurica (three cultivars named Chuan Baizhi, Yu Baizhi and Qi Baizhi in Chinese, respectively), in the presence of NaCl, Na₂CO₃, NaOH, glucose, fructose and sucrose were determined with a rapid visco analyser. The pasting peak, trough or final viscosities and the pasting temperatures (Pt) of Baizhi starches increased with NaCl concentration increased from 0 to 3.0%. The peak viscosities of Baizhi starches increased at 0.2% Na₂CO₃ concentration and then decreased with Na₂CO₃ concentration up to 0.8%. The trough and final viscosities and the Pt decreased with Na₂CO₃ concentration up to 0.8%. The peak viscosities increased in 0.1%, but decreased in 0.2% NaOH concentration. The trough and final viscosities and the Pt decreased in the presence of NaOH. With the concentration of sugar (glucose, fructose and sucrose) increased from 0 to 20%, the pasting peak, trough and final viscosities of these starches increased significantly. For the same Baizhi starch, the elevation of peak viscosity was the most significant when fructose was added, and the elevation of final viscosity was the greatest when sucrose was added.     

Calorimetric study of the glass transition occurring in aqueous glucose: Fructose solutions

Glass transition temperatures (T_g) for dilute and concentrated glucose: fructose: water solutions have been determined by differential scanning calorimetry. The supplemented phase diagrams (glucose: fructose 1:4, 1:2, 1:1, 2:1, 4:1 (w/w)) are presented and from these the temperature (T_g') and concentration (C'_g) of the maximally freeze-concentrated glass have been determined. Theoretical predictions of T_g for binary (glucose: fructose) and tertiary (glucose: fructose: water) systems according to the Couchman–Karasz and Gordon-Taylor equations are given and discussed. Annealing vitrified glucose: fructose glasses above their T_g allows the glass concentration to approach C'_g as a result of ice formation. Viscosity measurements of fructose and fructose: glucose mixtures allow for a calculation of T_g of the non-aqueous solutions.