Construction of a sucrose-fermenting bakers' yeast incapable of hydrolysing fructooligosaccharides.

Fructooligosaccharides stimulate the growth of intestinal bifidobacteria which are related to the favorable health and nutrition of humans and other animals. Since the efficient amount of fructooligosaccharide for an adult human is relatively large (about 5 g per day), its addition to daily foods like bakery goods might be beneficial. However, commercial Bakers' yeast hydrolyses fructooligosaccharides by the action of invertase encoded in SUC genes and ferments the resulting monosaccharides. According to the findings that strains carrying the MAL-constitutive gene and lacking the SUC gene fermented sucrose and not fructooligosaccharide, we constructed a sucrose-fermenting strain, YOY920, incapable of hydrolysing fructooligosaccharide, by cross-breeding a baking strain and a laboratory strain. In a molasses medium, the cell yield of YOY920 was comparable to that of a baking strain FSC6001, and much higher than that of the non-sucrose-fermenting strains. Although fructooligosaccharide inhibited the dough leavening ability of YOY920, white bread containing fructooligosaccharide could be produced in the defined dough formula using the new strain.     

Disruption of the CAR1 gene encoding arginase enhances freeze tolerance of the commercial baker's yeast Saccharomyces cerevisiae

The effect of intracellular charged amino acids on freeze tolerance in dough was determined by constructing homozygous diploid arginase-deficient mutants of commercial baker's yeast. An arginase mutant accumulated higher levels of arginine and/or glutamate and showed increased leavening ability during the frozen-dough baking process, suggesting that disruption of the CAR1 gene enhances freeze tolerance.     

Construction from a single parent of baker's yeast strains with high freeze tolerance and fermentative activity in both lean and sweet doughs.

From a freeze-tolerant baker's yeast (Saccharomyces cerevisiae), 2,333 spore clones were obtained. To improve the leavening ability in lean dough of the parent strain, we selected 555 of the high-maltose-fermentative spore clones by using a method in which a soft agar solution containing maltose and bromocresol purple was overlaid on yeast colonies. By measuring the gassing power in the dough, we selected 66 spore clones with a good leavening ability in lean dough and a total of 694 hybrids were constructed by crossing them. Among these hybrids, we obtained 50 novel freeze-tolerant strains with good leavening ability in all lean, regular, and sweet doughs comparable to that of commercial baker's yeast. Hybrids with improved leavening ability or freeze tolerance compared with the parent yeast and commercial baker's yeasts were also obtained. These results suggest that hybridization between spore clones derived from a single parent strain is effective for improving the properties of baker's yeasts.     

Technological properties of bakers’ yeasts in durum wheat semolina dough

Properties of 13 Saccharomyces cerevisiae strains isolated from different sources (traditional sourdoughs, industrial baking yeasts etc.) were studied in dough produced with durum wheat (Sicilian semolina, variety Mongibello). Durum wheat semolina and durum wheat flour are products prepared from grain of durum wheat (Triticum durum Desf.) by grinding or milling processes in which the bran and germ are essentially removed and the remainder is comminuted to a suitable degree of fineness. Acidification and leavening properties of the dough were evaluated. Strains isolated from traditional sourdoughs (DSM PST18864, DSM PST18865 and DSM PST18866) showed higher leavening power, valuable after the first and second hours of fermentation, than commercial baking yeasts. In particular the strain DSM PST 18865 has also been successfully tested in bakery companies for the improvement of production processes. Baking and staling tests were carried out on five yeast strains to evaluate their fermentation ability directly and their resistance to the staling process. Amplified fragment length polymorphism (fAFLP) was used to investigate genetic variations in the yeast strains. This study showed an appreciable biodiversity in the microbial populations of both wild and commercial yeast strains.     

Genetic Analysis of Haploids from Industrial Strains of Baker's Yeast

Strains of baker's yeast conventionally used by the baking industry in Japan were tested for the ability to sporulate and produce viable haploid spores. Three isolates which possessed the properties of baker's yeasts were obtained from single spores. Each strain was a haploid, and one of these strains, YOY34, was characterized. YOY34 fermented maltose and sucrose, but did not utilize galactose, unlike its parental strain. Genetic analysis showed that YOY34 carried two MAL genes, one functional and one cryptic; two SUC genes; and one defective gal gene. The genotype of YOY34 was identified as MATalpha MAL1 MAL3g SUC2 SUC4 gall. The MAL1 gene from this haploid was constitutively expressed, was dominant over other wild-type MAL tester genes, and gave a weak sucrose fermentation. YOY34 was suitable for both bakery products, like conventional baker's yeasts, and for genetic analysis, like laboratory strains.     

A note on the leavening activity of yeasts isolated from Nigerian palm wine

The role of the yeast flora of Nigerian palm wine in the leavening activity of the beverage was investigated by subjecting organisms from the wine to dough-raising tests. Those with appreciable leavening activity were identified as Saccharomyces cerevisiae and Candida spp. They produced maximum dough volumes in 3–4 h at 37°C. The study has provided experimental evidence that yeasts contribute to the leavening activity of palm wine and has identified strains which have potential utility in commercial bread baking.     

Genetic Analysis of Haploids from Industrial Strains of Baker's Yeast

Strains of baker's yeast conventionally used by the baking industry in Japan were tested for the ability to sporulate and produce viable haploid spores. Three isolates which possessed the properties of baker's yeasts were obtained from single spores. Each strain was a haploid, and one of these strains, YOY34, was characterized. YOY34 fermented maltose and sucrose, but did not utilize galactose, unlike its parental strain. Genetic analysis showed that YOY34 carried two MAL genes, one functional and one cryptic; two SUC genes; and one defective gal gene. The genotype of YOY34 was identified as MATα MAL1 MAL3g SUC2 SUC4 gall. The MAL1 gene from this haploid was constitutively expressed, was dominant over other wild-type MAL tester genes, and gave a weak sucrose fermentation. YOY34 was suitable for both bakery products, like conventional baker's yeasts, and for genetic analysis, like laboratory strains.     

A novel feeding method in commercial Baker's yeast production

Aims:  The aim of this investigation was to determine a better leavening ability and shelf life for the same biomass yield of final product.
Methods and Results:  A commercial fed-batch bioreactor equipped with circulation loop was used to study the effect of carbon source, molasses, profile on dough-leavening ability, shelf life and biomass yield of Baker's yeast, Saccharomyces cerevisiae . A set of 32 commercial batches were performed to investigate the effect of sugar concentration and compare with 32 control experiments.
Conclusions:  Higher local sugar concentration in circulation loop resulted in a better leavening ability and shelf life for the same biomass yield of the final product. In addition, this method improved nitrogen assimilation which resulted in higher protein content. Increase in leavening ability and protein content could be a result of the higher levels of glycolytic enzymes.
Significance and Impact of the Study:  It was observed that this change resulted in considerable improvement in leavening ability and shelf life at a commercial scale. It must be emphasized that to improve product quality, it is not necessary to pursue classical mutagenesis and selection strategies. A high-quality product can be achieved only by optimizing the feeding profile and strategy.     

Leavening ability and freeze tolerance of yeasts isolated from traditional corn and rye bread doughs.

Strains of Saccharomyces cerevisiae and Torulaspora delbrueckii isolated from traditional bread doughs displayed dough-raising capacities similar to the ones found in baker's yeasts. During storage of frozen doughs, strains of T. delbrueckii (IGC 5321, IGC 5323, and IGC 4478) presented approximately the same leavening ability for 30 days. Cell viability was not significantly affected by freezing, but when the dough was submitted to a bulk fermentation before being stored at -20 degrees C, there was a decrease in the survival ratio which depended on the yeast strain. Furthermore, the leavening ability after 4 days of storage decreased as the prefermentation period of the dough before freezing increased, except for strains IGC 5321 and IGC 5323. These two strains retained their fermentative activity after 15 days of storage and 2.5 h of prefermentation, despite showing a reduction of viable cells under the same conditions. The intracellular trehalose content was higher than 20% (wt/wt) in four of the yeasts tested: the two commercial strains of baker's yeast (S. cerevisiae IGC 5325 and IGC 5326) and the two mentioned strains of T. delbrueckii (IGC 5321 and IGC 5323). However, the strains of S. cerevisiae were clearly more susceptible to freezing damages, indicating that other factors may contribute to the freeze tolerance of these yeasts.     

优良面包酵母菌株的杂交育种

本文对实验室保存的面包酵母进行筛选,以不加糖面团发酵力最高的菌株BY-14和高糖面团发酵力最高的菌株BY-6作为两杂交亲本.二倍体菌株经过单倍体制备、分离和筛选后,利用群体杂交方法,获得了一株兼备两亲本优良性能的面包酵母菌株,其不加糖面团发酵力达到菌株BY-14水平,且高糖面团发酵力比菌株BY-6提高了25%.     

优良面包酵母菌株的杂交育种

本文对实验室保存的面包酵母进行筛选, 以不加糖面团发酵力最高的菌株BY-14和高糖面团发酵力最高的菌株BY-6作为两杂交亲本。二倍体菌株经过单倍体制备、分离和筛选后, 利用群体杂交方法, 获得了一株兼备两亲本优良性能的面包酵母菌株, 其不加糖面团发酵力达到菌株BY-14水平, 且高糖面团发酵力比菌株BY-6提高了25％。     

Superior molasses assimilation, stress tolerance, and trehalose accumulation of baker's yeast isolated from dried sweet potatoes (hoshi-imo)

Yeast strains were isolated from dried sweet potatoes (hoshi-imo), a traditional preserved food in Japan. Dough fermentation ability, freeze tolerance, and growth rates in molasses, which are important characteristics of commercial baker's yeast, were compared between these yeast strains and a commercial yeast derivative that had typical characteristics of commercial strains. Classification tests including pulse-field gel electrophoresis and fermentation/assimilation ability of sugars showed that almost the stains isolated belonged to Saccharomyces cerevisiae. One strain, ONY1, accumulated intracellular trehalose at a higher level than commercial strain T128. Correlated with intracellular trehalose contents, the fermentation ability of high-sugar dough containing ONY1 was higher. ONY1 also showed higher freeze tolerance in both low-sugar and high-sugar doughs. The growth rate of ONY1 was significantly higher under batch and fed-batch cultivation conditions using either molasses or synthetic medium than that of strain T128. These results suggest that ONY1 has potential commercial use as baker's yeast for frozen dough and high-sugar dough.     

Disruption of the CAR1 Gene Encoding Arginase Enhances Freeze Tolerance of the Commercial Baker's Yeast Saccharomyces cerevisiae

The effect of intracellular charged amino acids on freeze tolerance in doughs was determined by constructing homozygous diploid arginase-deficient mutants of commercial baker's yeast. An arginase mutant accumulated higher levels of arginine and/or glutamate and showed increased leavening ability during the frozen-dough baking process, suggesting that disruption of the CAR1 gene enhances freeze tolerance.     

Deficiency in the glycerol channel Fps1p confers increased freeze tolerance to yeast cells: application of the <Emphasis Type="Italic">fps1</Emphasis>Δ mutant to frozen dough technology

Intracellular glycerol content affects the freeze-thaw stress tolerance of Saccharomyces cerevisiae. We have recently reported that intracellular-glycerol-enriched cells cultured in glycerol medium acquire tolerance to freeze stress and retain high leavening ability even in dough after frozen storage [Izawa et al. (2004) Appl Microbiol Biotechnol ]. A deletion mutant of the FPS1 gene, which encodes a glycerol channel, accumulates glycerol inside the cell without an exogenous supply of glycerol into the medium. We found that the fps1 cells acquired tolerance to freeze stress and retained high leavening ability in dough after frozen storage for 7 days. These results suggest that the fps1 mutant is a useful strain for developing better frozen-dough with a commercial advantage.     

Glycerol Production by Fermenting Yeast Cells Is Essential for Optimal Bread Dough Fermentation

Glycerol is the main compatible solute in yeast Saccharomyces cerevisiae. When faced with osmotic stress, for example during semi-solid state bread dough fermentation, yeast cells produce and accumulate glycerol in order to prevent dehydration by balancing the intracellular osmolarity with that of the environment. However, increased glycerol production also results in decreased CO₂ production, which may reduce dough leavening. We investigated the effect of yeast glycerol production level on bread dough fermentation capacity of a commercial bakery strain and a laboratory strain. We find that Δgpd1 mutants that show decreased glycerol production show impaired dough fermentation. In contrast, overexpression of GPD1 in the laboratory strain results in increased fermentation rates in high-sugar dough and improved gas retention in the fermenting bread dough. Together, our results reveal the crucial role of glycerol production level by fermenting yeast cells in dough fermentation efficiency as well as gas retention in dough, thereby opening up new routes for the selection of improved commercial bakery yeasts.     

Leavening ability of baker's yeast exposed to hyperosmotic media

To develop a simple and rapid method for enhancing the leavening ability of baker's yeast, we examined the fermentation ability of baker's yeast exposed to hyperosmotic media. When baker's yeast cells were incubated at 25 degrees C for 1 h in a hyperosmotic medium containing 0.5% yeast extract, 0.5% peptone and 20% sucrose, the cells showed a higher fermentation ability in the subsequent fermentation test than those untreated. The increased ratios were from 40 to 60% depending on the strains used. Glucose and fructose showed a similar effect to that of sucrose, but sorbitol was less effective. A high correlation between the intracellular glycerol content and fermentation ability after the osmotic treatment suggested that glycerol accumulated during the hyperosmotic treatment was used in the subsequent fermentation as a substrate, lessened the lag time, and consequently enhanced the fermentation ability. Various baker's yeasts also showed a high leavening ability in dough after the hyperosmotic treatment.     

Osmotolerance and leavening ability in sweet and frozen sweet dough. Comparative analysis between Torulaspora delbrueckii and Saccharomyces cerevisiae baker's yeast strains

The response of Saccharomyces cerevisiae and freeze-tolerant Torulaspora delbrueckii strains to osmotic stress and their CO₂ production capacity in sweet and frozen-sweet dough has been examined. T. delbrueckii strains, IGC5321 and IGC5323 showed higher leavening ability than Saccharomyces, specially after exposure to hyperosmotic stress of bread dough containing 20% sucrose and 2% salt added. In addition, Torulaspora and especially T. delbrueckii IGC5321 exhibited no loss of CO₂ production capacity during freeze-thaw stress. Overall, these results appeared to indicate that Torulaspora cells are more tolerant than Saccharomyces to osmotic stress of bread dough. This trait correlated with a low invertase activity, a slow rate of trehalose mobilisation and the ability to respond rapidly to osmotic stress. Growth behaviour on high osmotic synthetic media was also examined. Cells of the IGC5321 strain showed intrinsic osmotolerance and ion toxicity resistance. However,T. delbrueckii IGC5323 exhibited a clear phenotype of osmosensitivity. Hence, this characteristic may not be essential or the only determinant for leavening ability in salted high-sugar dough. This revised version was published online in August 2006 with corrections to the Cover Date.