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There are economic and other advantages if the fermentable sugar concentration in industrial brewery fermentations can be increased from that of currently used high-gravity (ca. 14 to 17°P [degrees Plato]) worts into the very-high-gravity (VHG; 18 to 25°P) range. Many industrial strains of brewer's yeast perform poorly in VHG worts, exhibiting decreased growth, slow and incomplete fermentations, and low viability of the yeast cropped for recycling into subsequent fermentations. A new and efficient method for selecting variant cells with improved performance in VHG worts is described. In this new method, mutagenized industrial yeast was put through a VHG wort fermentation and then incubated anaerobically in the resulting beer while maintaining the α-glucoside concentration at about 10 to 20 g·liter−1 by slowly feeding the yeast maltose or maltotriose until most of the cells had died. When survival rates fell to 1 to 10 cells per 106 original cells, a high proportion (up to 30%) of survivors fermented VHG worts 10 to 30% faster and more completely (residual sugars lower by 2 to 8 g·liter−1) than the parent strains, but the sedimentation behavior and profiles of yeast-derived flavor compounds of the survivors were similar to those of the parent strains.
Beer is traditionally produced by fermentation of brewer's wort with an original extract of about 10 to 12°P (degrees Plato; see Materials and Methods for definition of brewery terms). A 12°P wort contains about 90 g·liter−1 of fermentable sugars (mainly maltose, maltotriose, and glucose) and 25 g·liter−1 of nonfermentable polysaccharides (dextrins, etc.). Fermentation of a 12°P wort yields about 35 to 40 g of ethanol·liter−1, which is normal drinking strength. Since the 1960s, many large breweries use so-called high-gravity worts (about 16°P), producing beers containing higher concentrations of ethanol, which are then diluted to drinking strength. These high-gravity worts often contain adjunct carbohydrates in addition to those from the barley malt. Because the adjunct carbohydrates contain a higher proportion of fermentable sugars, relatively more ethanol is produced from each degree Plato of extract. Fermenting at high gravity provides increased production capacity from the same size brewhouse and fermentation facilities and thus decreases capital investment costs. Other advantages are decreases in energy consumption and in labor, cleaning, and effluent costs (26). These advantages would be greater if very-high-gravity (VHG) worts could be used. Above about 18°P, however, fermentation rates decrease disproportionately (i.e., it takes longer to ferment the same mass of sugar in the more concentrated wort), fermentations do not go to completion (i.e., unacceptably large concentrations of sugars, especially maltotriose, remain in the final beers, causing low ethanol yields and undesirable flavor), and the physiological condition of the yeast at the end of fermentation is poor (4, 7, 17, 27, 35). It is standard practice to crop (harvest) yeast at the end of brewery fermentations and to use this cropped yeast to pitch (inoculate) subsequent fermentations. Thus, it is important that the viability of the cropped yeast is high (>90%). Other problems in high-gravity and VHG wort fermentations include poor foam stability, possibly caused by the release of proteolytic enzymes from highly stressed yeast cells (6), haze or turbidity caused by release of glycogen from lysed yeast (18), and alterations in aroma profile. In particular, the concentration of some aroma esters in beers depends on the original gravity of the wort (20) as well as on the sugar composition (16, 33) and the ratio of total sugars to free amino nitrogen (FAN) (1, 14, 22).
The efficiency of VHG fermentations can be improved by changing process conditions, e.g., by raising fermentation temperature or using more yeast. However, the levels of aroma compounds, especially esters, in beer vary with fermentation temperature (20, 30). Increasing the yeast inoculum also alters the aroma profile and increases fermentation rate but only up to a limit (22, 28, 29).
Traditionally, brewers recycled yeast indefinitely from fermentation to fermentation, so that present brewer's yeast strains (Saccharomyces cerevisiae and Saccharomyces pastorianus) are the result of hundreds of years of selection in traditional brewer's worts. However, this selection stopped before the introduction of high-gravity fermentation some 40 years ago, because brewers had started to store their strains as pure cultures. Nowadays, yeast is usually recycled 5 to 20 times, depending on the particular brewery. A sample of stored pure culture is then propagated (i.e., cultivated through a series of steps with increasing scale) to produce the large yeast mass (often a ton) required to pitch an industrial fermentation. Thus, present-day strains have evolved at about 11°P but are already used at about 16°P and are required to perform well at 18 to 25°P in VHG fermentations. Because industrial selection stopped before high-gravity worts were introduced, an evolutionary engineering approach is likely to be successful. Each brewery has its own yeast strains, which are considered to have a major impact on brand character. As well as fermenting maltose and maltotriose (the sugars remaining toward the end of wort fermentations) rapidly and completely and maintaining high viability in beers containing at least 90 g ethanol·liter−1, a new strain must also produce the same aroma profile as a brewery's present strain. This suggests the approach of improving the performance of individual brewer's strains by searching for VHG-tolerant variants. Evolutionary engineering (23) has proved to be useful both as a complementary strategy to genetic engineering and in cases, such as the food and beverage industry, where genetically modified organisms (GMOs) are not desired. However, evolutionary engineering usually relies upon the ability of more suitable genetic variants to grow faster than the parent strain under specified conditions. The conditions (e.g., high ethanol and the absence of oxygen and some other nutrients) during the second half of an industrial wort fermentation do not permit yeast growth, so that any selection procedure based upon growth is probably selecting for the wrong character and under the wrong conditions. The physiological condition of yeast cropped from brewery fermentations, in particular its lipid composition (2, 9), differs markedly from that of yeast propagated with even limited access to oxygen. Lipid composition is known to influence ethanol tolerance (34) and sugar transport (12). We found that the ability of yeast cells to remain alive in strong beers could be increased by feeding them maltose or maltotriose. We designed a selection procedure based on this observation and applied it to yeast cells that had just completed a VHG fermentation. The objective was to find variants with improved ability to ferment α-glucosides under these harsh conditions but with the same organoleptic properties as the parent strain. Such variants have immediate application for commercial beer production using VHG fermentation.
Barley syrup (Ohrasiirappi OS80) and maltose syrup (Cerestar C Sweet M 015S8) were both from Suomen Sokeri (Jokioinen, Finland). Nucleotides, enzymes, antimycin A, and ethyl methanesulfonate (EMS) were from Sigma-Aldrich (Helsinki, Finland) or Roche (Espoo, Finland).
Brewers monitor wort fermentations by measuring the decrease in specific gravity as fermentable sugars are converted into ethanol. Specific gravities can be converted into “extracts” and “apparent extracts” by, e.g., Analytica-EBC, method 9.4 (11). The extract is a measure of the sum of fermentable sugars and nonfermentable carbohydrate (mainly dextrins): a solution with an extract of x°P (x degrees Plato) has the same specific gravity as a solution containing x g of sucrose in 100 g of solution. For the worts in Table Table1,1, fermentable sugars accounted for 67% of the extract of the 18°P all-malt wort but 78% of the extract of the 25°P worts, which contained adjuncts with relatively low contents of nonfermentable polysaccharides. The “apparent extracts” that are routinely measured during fermentation have not been corrected for the effect of ethanol on the specific gravity. They can be corrected to so-called “real extracts” if the ethanol concentration is separately determined. “Attenuation” measures the proportion of carbohydrate that has been consumed: apparent attenuations are the difference between the original extract of the wort (OE) and the current apparent extract (CE) divided by the original extract ([OE − CE]/OE). “Limit attenuation” is an estimate of the maximum possible fermentative conversion of wort carbohydrate into ethanol determined by exhaustive fermentation of a wort sample with a large excess of yeast (Analytica-EBC, method 8.6.1 ). Limit apparent attenuations are typically 80 to 90%, depending on the concentration of nonfermentable carbohydrate in the wort.
High- and low-glucose, 25°P worts were made at the VTT Technical Research Centre of Finland from malt with barley syrup and high-maltose syrup, respectively, as the adjunct (accounting for 40% of the total extract). The all-malt 18°P wort was made at Heineken Supply Chain. The compositions of these worts are shown in Table Table1.1. Worts of about 16°P (15 to 17°P) were made at the VTT Technical Research Centre of Finland by dilution of 25°P wort with water containing 0.2 ppm Zn added as ZnSO4. The 28°P worts used in some experiments (see Table Table5)5) were made by the addition of solid maltose or glucose to 25°P worts. Before fermentation, the Zn content of 25°P worts was usually increased by 0.1 ppm and on one occasion by 2 ppm. Zn concentrations in worts are often suboptimal (8), and adding more Zn is standard industrial practice in countries where this is allowed (not Germany). Worts for laboratory-scale experiments at the VTT Technical Research Centre of Finland were collected hot (>90°C) in stainless steel kegs and stored at 0°C for at least 1 day and sometimes up to 3 months. Microbiological contamination was never observed. Stored worts were mixed before use to resuspend settled solids evenly.
VTT-A63015 (herein called A15) is a production lager strain from the VTT Culture Collection. GT344 is a UV-induced variant with improved VHG performance derived from the former production lager strain CMBS33 and described by Blieck et al. (3).
Viabilities above 10% were routinely determined by staining with methylene blue in phosphate buffer (pH 4.6) (10). Staining with fluorescein diacetate (FDA) (19) at pH 7.2 was used where stated. These methods record as alive metabolically active cells with intact membranes, which remain unstained by methylene blue and are stained green by FDA. Viabilities below 10% were determined by spreading suitably diluted cell suspensions onto agar plates containing YP (10 g yeast extract and 20 g peptone·liter−1) and 20 g maltose·liter−1. Colonies were counted after 3 days of incubation at 24°C.
A15 yeast was grown to stationary phase in YP containing 20 g maltose·liter−1. The yeast was harvested by centrifugation, washed, suspended in 0.1 M sodium phosphate (pH 7.0) to 25 mg fresh yeast·ml−1, and mutagenized with ethyl methanesulfonate (EMS) essentially as described previously (25). EMS (60 μl) was added to 3.0 ml of the yeast suspension (about 3 × 108 cells), and the mixture was shaken at room temperature (ca. 20°C) for 60 min. The EMS reaction was quenched by adding 20 ml of sodium thiosulfate (50 g·liter−1). Mutagenized yeast was collected by centrifugation, washed twice with sodium thiosulfate (50 g·liter−1), and suspended in sterile saline (9 g NaCl·liter−1). Dilutions were spread onto agar plates containing YP plus 20 g maltose·liter−1 to determine the proportion of dead cells, which was close to zero (0% ± 15%). The remaining yeast, a pool of about 3 × 108 mutagenized cells, was inoculated into 1 liter of YP plus 40 g maltose·liter−1 and grown to stationary phase, giving about 25 g fresh yeast.
This yeast was pitched at 8.0 g·liter−1 into 200-ml portions of high-glucose 25°P wort (Table (Table1)1) in 1-liter bottles fitted with stirring magnets, glycerol-filled air locks, and tubing for withdrawing samples anaerobically and for supplying liquid feeds through a capillary (1-mm) tube. The wort contained 3 mg antimycin A·liter−1 to prevent respiration of adventitious oxygen. Fermentation at 20°C was followed by loss of mass. On day 3, when the fermentation was complete, the yeast concentration and viability and combined concentration of maltose and maltotriose (13) were measured. The yeast in the bottle was then fed intermittently with 180 g·liter−1 maltose via a peristaltic pump at about 0.2 ml·h−1 in an attempt to maintain maltose plus maltotriose at a concentration equivalent to 100 mM hexose. (This procedure was also used to study the effect of feeding maltose or maltotriose on the viability of yeast under these conditions; see Fig. Fig.1).1). On day 17, the composition of the feed was changed to 60 g maltose plus 390 g ethanol·liter−1 in order to increase ethanol stress. Every 1 or 2 days, samples were plated onto YP plus 20 g maltose·liter−1, and the number of CFU was compared to the concentration of live cells on day 3 to give the survival rate. When the survival rate was below 10−4 (i.e., >99.99% of the original cells were dead), colonies were picked and grown to stationary phase in YP plus 20 g maltose·liter−1. The yeast cells (potential variants) were harvested, suspended in 30% glycerol, and stored at −80°C.
For strain GT344, the procedure was modified as follows. After mutagenesis with EMS (killing about 20% of the cells) and growth of the mutagenized pool in YP plus 40 g maltose·liter−1, the yeast was further grown anaerobically in 16°P wort, so that it more closely resembled yeast cropped from an industrial fermentation. The selection was carried out in three 2-liter bottles, each containing 700 ml 25°P wort. Low-glucose 25°P wort (Table (Table1;1; the sugar composition of industrial worts depends on the nature of adjunct carbohydrates) was used. When the fermentations were almost complete, the yeast cells in one bottle were intermittently fed 180 g·liter−1 maltose, the yeast cells in another bottle were fed 180 g·liter−1 maltotriose, and the yeast cells in the third bottle were not fed. Between days 18 and 26, the yeast cells in the bottles were intermittently fed a solution containing 390 g ethanol·liter−1.
Pure cultures of each strain were propagated to obtain enough fresh yeast mass to pitch 2 liters of wort in tall-tube fermentors essentially as described earlier (21, 31). Stored yeast suspensions in 30% glycerol were thawed, and 500-μl portions were inoculated into 100 ml autoclaved YP plus 40 g maltose·liter−1 in 250-ml Erlenmeyer flasks and grown overnight at 24°C to an optical density at 600 nm (OD600) between 6 and 10. From these precultures, 50-ml portions were inoculated into 3-liter lots of 16°P wort in 5-liter Erlenmeyer flasks and shaken on an orbital shaker at 24°C for 2 days. The flasks were then stood at 0°C for 16 to 24 h. Most of the supernatant was decanted from each flask. Settled yeast was mixed into a smooth slurry. Samples of slurry (samples about 5 ml each) were weighed and then centrifuged (10 min at 9,000 × g). The pellets were weighed, and the slurry was diluted with decanted supernatant to 20 g centrifuged yeast mass/100 g slurry.
Static fermentations were carried out in the stainless steel tall tubes (6-cm diameter and 100-cm height) as previously described (21, 31). Worts were oxygenated immediately before use to 11 to 13 mg·liter−1 for 25°P worts and 9 to 11 mg·liter−1 for the 16°P worts (oxygen was measured with a model 26073 oxygen indicator from Orbisphere Laboratories, Geneva, Switzerland). Well-mixed yeast slurries were pitched by mass into weighed, about 2-liter portions of 24 and 16°P worts to give 8.0 and 5.0 g centrifuged yeast mass·liter−1, respectively, equivalent to about 32 × 106 and 20 × 106 original cells·ml−1. Fermentations were started at 10 to 13°C. For 25°P worts, the temperature was raised to between 15 and 21°C after 20 to 24 h (see Results). Samples (about 30 ml) were withdrawn daily and centrifuged. The pellets were washed with water, and the mass (dry weight) of each pellet was determined after drying the pellet overnight at 105°C. The densities of the supernatants were determined using an Anton Paar DMA58 density meter.
At the end of fermentations, the levels of residual sugars in beers were determined by high-performance liquid chromatography (HPLC) (Waters, Milford, MA), and the level of ethanol was determined either by gas chromatography (GC) or by quantitative distillation according to Analytica-EBC method 9.2.1 (11).
The four yeast strains for pilot-scale experiments (A15 and the variants T24.1, T24.9, and T24.201) were each serially propagated in duplicate from slants through 50-ml, 1-liter, and 10-liter scale fermentations in 15°P all-malt wort containing 0.5 mg Zn·liter−1 and then pitched into 200 liters of freshly prepared (same day) 18°P all-malt wort containing 0.5 mg Zn and 12 mg oxygen·liter−1 in a 250-liter cylindroconical fermentor. Fermentations were pitched at 10 × 106 cells per ml and 9°C. The temperature was then allowed to rise to 14°C. The fermentation tanks were purged of sedimented dead yeast cells and wort particulates 24 h after pitching by removing 5 liters from the bottom of the cylindroconical vessel. Fermentations were monitored by daily assays for wort attenuation, cell concentration (using a Coulter Counter, model Z1; Beckman Coulter Nederland, BV), yeast viability, pH, and haze (see below). When the fermentations approached completion (after 10 days for both lots of T24.1 and T24.9 and after 11 days for both lots of A15 and T24.201), the sedimented yeast was harvested from the fermentations by removing 20 liters of sedimented yeast slurry from the bottom of the cylindroconical vessel. This yeast was then used to pitch 500-liter lots of freshly prepared (same day) 18°P all-malt wort (0.5 mg Zn and 12 mg oxygen·liter−1) in 800-liter cylindroconical fermentors. The fermentations were conducted, and daily assays were performed as described above. When these 500-liter primary fermentations were completely attenuated, all eight beers were lagered, filtered, pasteurized, and bottled by using standard brewery procedures.
The turbidity (haze) assay mimics an industrial-scale beer clarification procedure. Yeast cells and other large particles were removed from 300-ml samples of fermenting wort or green beer by mixing with 1 g of diatomaceous earth (Dicalite 231; Dicalite NV, Ghent, Belgium) and filtration through a 3-μm membrane filter under vacuum. The 90° light scattering of the filtrate was measured in a Vos 4010 Hazemeter (Vos Instruments B.V., Zaltbommel, the Netherlands) and is a measure of the presence of small particles, including glycogen, that can cause haze in the final beer (18).
Samples of final beers for aroma analyses were clarified by centrifugation. Sulfur dioxide was measured by the ρ-rosanaline method (Analytica-EBC method 9.25.3 ). Dimethyl sulfide, alcohols, esters, and acetaldehyde were determined by headspace gas chromatography with a 60-m DBWaxETR column (inner diameter [ID] of 0.32 mm), hydrogen carrier gas, and flame ionization detection. Samples were injected at 150°C, and the temperature program was 14 min at 60°C, 10°C/min to 85°C, 10 min hold at 85°C, and 60°C/min to 150°C.
All 8 bottled beers (4 yeast strains in duplicate) from the pilot-scale fermentations were evaluated by a taste panel of nine Heineken staff trained to evaluate beers. Each panel member gave a numerical score for each of 58 descriptors for each of the 8 beers. The products were evaluated in replicate in two sessions and were offered one by one (semimonadic) using a balanced design.
Yeast samples for lipid analyses were collected by centrifugation and then handled as previously described (12). Briefly, the yeast was washed with ice-cold water, saponified with 3 M NaOH, and methylated, and the methyl esters were extracted into hexane-methyl-t-butyl ester and analyzed by gas chromatography.
Statistical significances (P values) were calculated by Student's t test (two-tailed, unpaired, equal variance assumed).
We tested whether the survival of yeast cells at the end of a VHG wort fermentation could be prolonged by feeding maltose or maltotriose to maintain the concentration of fermentable α-glucosides at about 10 to 20 g·liter−1. A 25°P wort was pitched with lager yeast at 8 g fresh yeast·liter−1 and fermented anaerobically in 3 identical stirred bottles. Mass losses showed that after 3 days the fermentations were nearly complete. Residual α-glucosides (maltose plus maltotriose) were then kept between 40 and 120 mM hexose equivalents by feeding maltose or maltotriose to each one of two bottles. The viability of lager strain GT344 in bottles of yeast that were fed and unfed is shown in Fig. Fig.1.1. The proportion of yeast cells still alive on day 18 was <1% with no sugar feed and 27 and 43%, respectively, when maltose or maltotriose were fed to the yeast. A similar protective effect of maltose has been reported during anaerobic incubation of lager strain A15 in a VHG beer (maltotriose was not tested) (15).
These results showed that survival of yeast at the end of anaerobic VHG wort fermentations was promoted by maltose and maltotriose. If these sugars promote survival because they can be fermented, then in a pool of mutagenized cells, the cells with the best ability to ferment maltose or maltotriose under these conditions may survive the longest. This suggested a method to select for variants with improved ability to ferment α-glucosides under VHG conditions.
During the selection from EMS-mutagenized lager strain A15, α-glucosides were kept between 40 and 160 mM hexose equivalents by feeding the yeast maltose. Yeast viability fell to 10% on day 11 and 0.1% on day 18. Selection pressure was then increased by raising the ethanol concentration to 106 g·liter−1 during day 18 and to 160 g·liter−1 during day 23. After the second increase, the viability fell to 6 (day 24) and 1.5 (day 37) live cells per 106 original live cells.
During the selection from EMS-mutagenized lager strain GT344, α-glucosides were kept between 40 and 120 mM hexose equivalents from day 5 until day 38, when feeding was stopped, and the levels were 20 and 50 mM, respectively, in the bottles containing yeast fed maltose and maltotriose on day 59. Viabilities during the first 18 days are shown in Fig. Fig.1.1. The viability of yeast in the bottle containing yeast fed maltose then fell to 20% by day 27, when the ethanol concentration was 120 g·liter−1. The viability of yeast in the bottle containing yeast fed maltotriose, however, fell to below 2% on day 27 at an ethanol concentration of 106 g·liter−1. We wanted to avoid exposing this yeast strain to ethanol concentrations much greater than might be reached in an industrial VHG wort fermentation. Therefore, no more ethanol was added, and the cells died more slowly than in the experiment with strain A15 (there may also be a strain difference). By day 41, both bottles contained 400 live cells per 106 original cells. Viabilities then fell to 5 live cells/106 original cells by day 47 in the bottle of yeast fed maltose and by day 51 in the bottle of yeast fed maltotriose. Viabilities then fluctuated between 0.1 and 5 live cells/106 original cells until the end of the experiment (day 86), but the viability of yeast in the bottle fed maltose was usually lower than the bottle of yeast fed maltotriose.
Single-cell colonies (potential variants) isolated from strain A15 after 24 and 37 days selection (at viabilities of about 6 and 1.5 cells/106 original cells, respectively) and from strain GT344 after 47 and 59 days selection (at viabilities of, respectively, about 5 and 0.05 cell/106 original cells in the maltose-fed bottle and 200 and 0.1 cell/106 original cells in the maltotriose-fed bottle) were screened for improved VHG fermentation in tall tubes. For each screening test, several potential variants were propagated from pure cultures together with independent duplicate lots of the corresponding parent strain (A15 or GT344). The propagated yeast cells were pitched into 16 or 25°P worts in tall tubes, and the resulting fermentations were monitored. A typical set of screening fermentations is shown in Fig. Fig.2.2. When the different yeast strains were pitched at the standard concentration of 8.0 g fresh yeast·liter−1, their growth in early fermentation and subsequent flocculation and sedimentation in late fermentation (indicated by the profiles of mass [dry weight] in suspension shown in the top panel of Fig. Fig.2)2) were similar, except that T24.5 sedimented faster than A15. The middle panel shows that the attenuation profiles were also similar throughout most of the fermentation. However, fermentations by the variants T24.1, T24.2, T24.5, and T37.2 reached lower final apparent extracts than those by the parent strain (Fig. (Fig.2,2, bottom panel). For fermentations pitched with 8.0 g fresh yeast·liter−1, the final apparent extracts were between 3.87 and 4.05°P for the variants, compared to 4.36 and 4.50 for the duplicate fermentations by parent A15 (Table (Table2)2) . Increasing the pitch rate of the parent yeast to 10.0 g·liter−1 increased the early fermentation rate but did not drive the final apparent extract down to the values observed with variants. Decreasing the pitch rate of variant T24.1 to 6.0 g·liter−1 decreased the early fermentation rate, but the final apparent extract (4.06°P) was still lower than that observed with even 10.0 g·liter−1 of A15 (4.23°P; Table Table22).
When pitched at the same rate as strain A15 (8.0 g·liter−1), the variants caused the consumption during fermentation of an extra 0.38 to 0.56°P, corresponding to increases in apparent attenuation from 81.2% ± 0.3% (mean ± range) for A15 to between 83.4 and 83.6% for variants T24.1, T24.2, and T24.5 and 82.8% for variant T37.2. Thus, the variants approached closer to the limit of apparent attenuation (~86%) of this wort. For variants T24.1, T24.2, and T24.5, the increases in apparent attenuation were accompanied by increases in final ethanol concentration from about 105 to 108 g·liter−1 for A15 to 109 to 113 g·liter−1 (Table (Table22).
In normal brewing practice, the fermentation mixtures shown in Fig. Fig.22 would be chilled to below 5°C and sedimented yeast cropped at about 168 h, when the decreases in apparent extract were essentially complete. We kept these fermentations at 20°C for a further 4 days. During this time, little or no further changes in apparent extract occurred (Fig. (Fig.2,2, bottom panel), although there was still yeast in suspension in all fermentations (Fig. (Fig.2,2, top panel). The differences between the final apparent extracts achieved by parent and variant strains therefore represented not just differences in fermentation speed but an inability of the parent strain to continue fermenting wort sugars at low concentrations that the variants could still use. When sedimented yeast cells were eventually cropped from the bottom of the tall tubes after 263 h of fermentation, they had been exposed to over 100 g ethanol·liter−1 at 20°C for 4 days and their viabilities were very low. However, the viabilities of the variant yeast crops (about 25 to 50%) were higher than those of the parent crops (about 15%) (Table (Table22).
The fermentation performances of 21 potential variants of strain A15 were screened in high- or low-glucose VHG worts. Six variants (29%) performed better than the parent strain by showing, in at least two independent tests (i.e., with yeast propagated and fermented on two different occasions), faster fermentation, more extensive fermentation, and sometimes higher crop viability (data not shown). Eight potential variants of strain GT344 (four each from the bottles of yeast fed maltose and maltotriose) were screened in low-glucose VHG worts. One of these variants (LL2-47-1, isolated from the bottle of yeast fed maltotriose on day 47 at a viability of 200/106 original cells) showed improved VHG fermentation performance compared to the parent GT344 strain in two independent experiments. Fermentation characteristics of this variant and of the three most studied variants of strain A15 are shown in Table Table33.
For the VHG fermentations summarized in Table Table3,3, the parent strains completed their fermentations in about 10 days (range, 7 to 13 days). Conditions varied between fermentations, both because of deliberate variations (e.g., zinc concentrations between 0.2 and 2 ppm and final VHG fermentation temperatures between 15 and 21°C were used in different fermentation sets) and because of accidental variations (e.g., fermentation sets with worts stored at 0°C for several weeks appeared to be slower than those with freshly prepared worts). Fermentation rates were quantified in each set of fermentations by measuring the time required to reach a certain high apparent attenuation. Where possible, the target apparent attenuation was 80%, but in experiments where the parent did not reach 80% apparent attenuation or reached it very slowly, targets of 77 or 75% were used. All the variants described in Table Table33 consistently fermented wort faster than the corresponding parent strain, with average time savings between half a day and nearly 3 days. All these variants also fermented the worts more extensively, with average gains in apparent extract (ΔAE) of between 0.25 and 0.88°P, depending on the variant and the wort. The increased apparent attenuations were accompanied by corresponding decreases in residual sugars of between 2.1 and 7.8 g·liter−1. Differences in residual maltotriose accounted for most (at least 90%) of the changes in residual sugar (for the parent strains, residual glucose and maltose after VHG fermentations were usually below 0.5 g·liter−1, whereas residual maltotriose was 10 to 20 g·liter−1 for 25°P worts and 3 to 4 g·liter−1 for 16°P worts). All variants produced more ethanol than the corresponding parent. At least for the most studied mutant, T24.1, the changes in fermentation time, final extract, and residual sugars were statistically significant in both 25 and 16°P worts and the changes in final ethanol were significant in the 16°P wort (Table (Table33).
For variant strain T24.1 (Table (Table3)3) and some other variants (T24.2, T24.5, and T37.2; data not shown), the viability of yeast cropped from high-glucose VHG worts was higher than that of cropped strain A15. This improvement in crop viability was not seen when low-glucose worts were used. Variants and parent yeast were always cropped at the same time, when the parent fermentation was complete. Because the parent yeast cells were usually cropped promptly when fermentations stopped, their viabilities were often ≥90%. By this time, the variant yeast strains had stopped fermenting 1 or 2 days ago, and their cropped yeast viabilities were usually close to or less than those of the parent yeast (Table (Table33).
To test whether the advantages of the variants were maintained when the yeast cells were recycled and used in larger scale fermentations, closer to industrial practice, strain A15 and three promising variants were first pitched into 200 liters of 18°P all-malt wort and then yeast cells harvested from these fermentations (two independent lots per strain) were pitched into 500 liters of 18°P all-malt wort. Yeast growth and sedimentation in the 500-liter scale fermentations were similar for all strains, except that the variants reached their peak concentrations 1 to 2 days earlier than the parent strain did (Fig. (Fig.3,3, top panel). Fermentations were faster with all three variants and reached lower final apparent extracts (Fig. (Fig.3,3, bottom panel and inset). The proportion of dead cells in suspension increased during fermentation from near zero to about 15% for parent and variant strains, except that the percentage of dead T24.9 cells increased sharply after 10 days in both replicate fermentations to more than 30% at 14 days (Fig. (Fig.4,4, top panel). Turbidity or haze, which is often caused by the release of glycogen particles from dead cells, rose more rapidly for variants T24.9 and T24.201 than for strain A15, but reached a similar final value (about 1.5 European Brewery Convention [EBC] units) and was lower for T24.1 than the other strains (Fig. (Fig.4,4, bottom panel).
Table Table44 shows the levels of several aroma compounds found in the beers produced on the 500-liter pilot scale. In general, differences between the variants and the parent strain, A15, were small. Free amino nitrogen (FAN) compounds were 15 to 20% lower in beers produced by variants, which may indicate that the variant yeast strains grew more, even though the peak yeast concentrations were similar (Fig. (Fig.3).3). Total SO2 (i.e., free SO2 plus carbonyl-bound SO2) was 20 to 30% lower for beers made by variant strains T24.9 and T24.201. The branched-chain alcohols, 2- and 3-methyl-1-butanol, were roughly 15% higher for all variants, ethyl acetate was slightly higher, and 3-methyl-1-butyl acetate (important for its fruity, banana flavor) was roughly 30% higher in beers made by variant strains T24.1 and T24.9, but not T24.201. Similar increases in these compounds were previously reported (15) for 2-liter-scale-tall-tube fermentations of 15 and 25°P worts by laboratory-grown strains T24.1, T24.9, and T24.201.
The organoleptic properties and foam stability (which has esthetic appeal) of all 8 bottled beers from the pilot scale were evaluated. There were small differences between the organoleptic properties of individual beers, but when the scores for both beers made by each strain were combined, no sensory differences between the 4 yeast strains could be detected by a trained taste panel (n = 9). No statistically significant differences between the 8 beers were observed in foam stability, measured as the time required to collapse the foam head by 3 cm. Beer made by strain T24.1 exhibited the highest foam retention (269 ± 22 s compared to 245 ± 22, 245 ± 24, and 246 ± 30 for strains A15, T24.9, and T24.201, respectively).
We tested whether there were differences in lipid composition between the parent strain and several successful variant strains. We could not detect reproducible differences between the sterol contents of strain A15 and tested variants. In contrast, there were strain-specific differences between their fatty acid contents. The top portion of Table Table55 shows the results of analyses of total fatty acids in strain A15 and two variants cropped at the end of a 25°P fermentation. The yeast strains contained considerable amounts of wort-derived linoleic acid (C18:2) and linolenic acid (C18:3) (S. cerevisiae cannot synthesize these polyunsaturated acids ). Fatty acid unsaturation indices and the ratios of total C18 and C16 fatty acids were also calculated both including and excluding these wort-derived C18 fatty acids. The unsaturation indices were greater for strain T24.1 than for A15, but smaller for T24.5. The ratio of C18/C16 fatty acids was greater for T24.1 than for A15 or T24.5. T24.1 contained a higher proportion of oleic acid (C18:1/total acids) than either A15 or T24.5. When 11 independent samples of each strain from six fermentation series were analyzed, only the difference in C18/C16 ratios between T24.1 and A15 was statistically significant (P < 0.01) (Table (Table5,5, bottom portion). The difference in the proportions of oleic acid between these strains approached significance (P = 0.06), but no significant difference was found between their unsaturation indices (P > 0.2) and no significant differences were found between T24.5 and A15.
We also examined the fatty acid compositions of strain A15 and three variants during shake-flask cultivations of YP plus 20 g·liter−1 maltose or glucose, where the yeast strains were not exposed to high-gravity stresses (Table (Table6).6). With maltose as the carbon source, the C18/C16 ratios were significantly higher for strain T24.1 than for strain A15 (P < 0.01 in growth and stationary phases), and the unsaturation index of T24.1 was significantly higher during growth (P < 0.01). The C18/C16 ratio was significantly higher also for T24.9 than A15 during growth on maltose (P < 0.05) but not in stationary phase (P > 0.5). Also with glucose as the carbon source, the C18/C16 ratios were markedly higher for T24.1 in both growth and stationary phases and for T24.9 in growth phase. No significant differences were observed between T24.201 and A15 (P > 0.1).
These results suggest that the high C18/C16 ratio of variant strain T24.1 is a specific characteristic of this variant, possibly shared by T24.9, but not by T24.201 nor T24.5. The results of Table Table66 show that the different C18/C16 ratios of T24.1 and A15 are not a consequence of exposure to high-gravity stress.
With the selection method described, a high proportion of the single-cell colonies isolated from the industrial lager strain A15 showed improved VHG fermentation performance compared to the parent. Six variants out of the 21 tested isolates showed faster fermentations, more complete attenuation (Table (Table3),3), and sometimes higher crop viability (15). For 3 of the variants (T24.1, T24.9, and T24.201), the improvements were observed both in 2-liter-tall-tube fermentations pitched with yeast propagated in the laboratory (Table (Table3)3) and in 500-liter fermentors pitched with recycled yeast (Fig. (Fig.3).3). This is important because nearly all industrial brewery fermentations are made with yeast recycled from earlier fermentations, and this recycled (cropped) yeast differs physiologically from yeast freshly propagated from pure culture (2, 9). The improvements in fermentation time and extent were obtained without significant changes in the profile of yeast-derived aroma compounds (Table (Table4),4), in sensory perception by a trained panel, or in foam stability. As well as showing improved performance in VHG worts (25°P) containing adjunct carbohydrates, variants also performed better than the parent A15 in about 16°P worts (Table (Table3)3) and in 18°P all-malt wort (Fig. (Fig.3).3). Variant T24.9 exhibited poor viability after the tenth day of the 18°P all-malt pilot fermentations (Fig. (Fig.4).4). This might cause problems with this strain, although on this occasion it did not affect the organoleptic properties of the beer and sedimented yeast might be cropped earlier at acceptable viability, as is standard practice in many breweries. Some of the other variants, especially T24.1, seem to be suitable for industrial use under current industrial high-gravity conditions and to offer potential to increase the gravity of industrially used worts into the VHG region of 18 to 25°P.
The selection method also yielded at least one improved variant (LL2-47-1) from strain GT344 (Table (Table3)3) out of the 8 tested isolates from this strain. GT344 is itself a variant with improved VHG performance, isolated from the former production lager strain, CMBS33 (3). Thus, compared to CMBS33, LL2-47-1 presumably contains at least two genetic changes that promote VHG performance. It is no surprise that VHG performance depends on more than one genetic locus.
The selection procedure involves first a VHG fermentation and then a long incubation of the resulting yeast in its own beer, during which the concentration of α-glucosides is maintained (with the objective of prolonging the life of variants best able to ferment these sugars) and the concentration of ethanol is increased (with the objective of increasing stress). Continuous monitoring of α-glucosides, ethanol, and cell viability is required, and the exact protocol is dependent on the strain because different brewer's yeast strains have different tolerances toward VHG stresses. One of the strains in the current work (GT344) exhibited large oscillations in viability toward the end of the selection (days 47 to 86). This yeast formed films on the surfaces of the bottles, so possibly occasional detachment of pieces of these films released viable cells into suspension. Another possibility is that nutrients released from the mass of dead yeast may have promoted growth of some of the very rare survivors. Such release of nutrients means that survivors were selected in a medium that is nutritionally richer than the beer at the end of a VHG fermentation. Some of the isolated colonies that did not perform well in VHG fermentations may be cells that exploited these nutrients.
The decreases in total fermentation time were about 1 day in fermentations that lasted about 10 days. This represents a significant (ca. 10%) increase in brewery fermentation capacity. The more complete apparent attenuation is still more important. The gains (0.3 to 0.9°P lower final apparent extract in fermentations where the total changes in apparent extract were about 12 and 20°P for high-gravity and VHG worts, respectively) correspond to 3 to 8 g·liter−1 less residual sugar (Table (Table3)3) and up to 4 g·liter−1 more ethanol. This has considerable economic impact when beers are adjusted to a standard final ethanol concentration and also lessens the possibility of undesired flavor changes caused by residual sugar. The extra apparent attenuation reached by the A15 variants was not reached by the parent strain when fermentation time was prolonged or fermentations were pitched with more yeast cells (Fig. (Fig.2).2). It is not yet known why the parent yeast (and other current industrial strains) stop fermenting VHG worts before all fermentable sugar (mostly maltotriose) is consumed.
The success, and limitations, of the present method are related to the selection conditions used. We looked for cells that could stay alive when supplied with maltose or maltotriose under the adverse conditions (high ethanol concentrations and low nutrient concentrations, including a complete lack of oxygen) at the end of VHG fermentations. The selection procedure was applied to yeast cells that were in the same physiological condition as yeast cells at the end of an anaerobic VHG fermentation. We obtained variants that showed improved performance at the end of high-gravity and VHG fermentations (Fig. (Fig.33 and and4).4). None of the variants obtained by this method fermented faster than the parent strain during the early stages of wort fermentation, presumably because the method applied no selection pressure for such a characteristic. It follows that to obtain variants with improved performance throughout the fermentation, it may be necessary to apply different selection pressures sequentially. Variants isolated at each step can be subjected to further mutagenesis and selection under different conditions, as was done here to obtain variant LL2-47-1 from the already improved (3) GT344 strain.
The physiological condition of Saccharomyces yeast strains varies greatly with growth conditions. It is known, e.g., that the sterol and unsaturated fatty acid contents of brewer's yeast cells cropped from brewery fermentations are much lower than those of yeast cells grown under normal laboratory conditions or with aeration during propagation in a brewery (2, 9), whereas the correct function of maltose transporters is dependent on the lipid composition of the yeast (12). The variants with improved survival under our selection conditions may have genetic alterations that increase the ability of anaerobically grown, sterol- and unsaturated fatty acid-deficient cells to ferment α-glucosides in the presence of high ethanol concentrations and other VHG stresses. The VHG fermentation performance of one of the parent lager strains (A15) used in the present work is known to be limited by its α-glucoside transport activity and to be increased by genetic engineering of the AGT1 gene that encodes a transporter of maltose and maltotriose (31).
We have not identified the mutations responsible for the improved performance of the variants. Such identification is of scientific interest and would also facilitate the directed improvement of strains by genetic engineering or directed evolution and facilitate the combination of different positive mutations identified in different variants. All 21 tested isolates from parent strain A15 were karyotyped using pulsed-field gel electrophoresis, and no alterations of chromosome structure were detected (L. Mulder and M. Walsh, unpublished data). At least one successful variant (T24.1; possibly also T24.9, but probably not T24.5 nor T24.201) had a greater average length of fatty acid carbon chains (higher ratios of C18 fatty acids to C16 fatty acids) than A15 under a variety of conditions, including conditions (YP plus 20 g·liter−1 glucose or maltose) where the yeast was not exposed to high-gravity stress (Tables (Tables55 and and6).6). This change in the fatty acid composition of T24.1 compared to its parent, A15, was not, therefore, a result of its improved performance in high-gravity fermentations. However, we do not know whether the change is a cause of the improved VHG performance of T24.1. Brewer's yeast usually contains about twice as much palmitoleic acid (C16:1) as oleic acid (C18:1) (5), but there is evidence that increase in, specifically, oleic acid, rather than palmitoleic acid, increases the ethanol tolerance of Saccharomyces cerevisiae (34). Identifying the mutations responsible for the improved performance presents considerable problems. Blieck et al. (3) isolated variants with improved VHG fermentation performance from the lager strain CMBS33 and detected differences in gene expression levels between CMBS33 and two variants, GT336 and GT344 (lower expression of HXK2 and increased expression of LEU1). They presented evidence that these differences might be causing the improved performance of the variants. However, there is, in general, no reason to suppose that the causative mutations will change the expression levels of the responsible genes. It is at least as likely that these mutations will be changes in coding sequences that alter the functional properties of enzymes or other proteins. A genomic library of variant T24.1 is now available (J. Dietvorst and Y. Steensma, unpublished work), which provides a possible route to identifying the causative mutation(s) in this variant.
We thank Merja Penttilä, Silja Home, and Esko Pajunen for their support and encouragement and Marilyn Wiebe for critically reading the manuscript. We thank Hannele Virtanen for aroma and lipid analyses, Arvi Wilpola for wort preparation and other help in the pilot brewery at the VTT Technical Research Centre of Finland, and Aila Siltala and Eero Mattila for skilled technical assistance. Lies Blieck and Johan Thevelein (Katholieke Universiteit, Leuven, the Netherlands) are thanked for kindly providing strain GT344.
This work was supported by the European Community (contract QLK1-CT-2001-01066).
Published ahead of print on 15 January 2010.