面包酵母发酵力测定方法的研究

探讨了面包酵母发酵力的测定方法。结果表明,标准排水法测得的产气量已不能正确反映现代商用面包酵母的发酵力,面团发酵0～2h的产气量与面包酵母发酵力呈正比,可作为衡量面包酵母发酵力的指标。活性干酵母用量在一定范围内,与发酵0.5～1.5h所失去CO的重量呈正2相关,相关系数R=0.9976以发酵0.5～1.5h的CO的2,所2失重量也可作为衡量面包酵母发酵力的指标。失重法无需专用设备,具有重复性好、操作简便易行、可以同时测定大量样品的优点,可作为实验室检测面包酵母发酵力的简易方法。     

面包酵母无糖面团发酵力与麦芽糖发酵力相关性的探讨

探讨了面包酵母在无糖面团中的起发速度与麦芽糖发酵力之间的关系，包括葡萄糖阻遏作用对麦芽糖发酵力的影响。结果表明，在8株供试的面包酵母菌中，麦芽糖发酵力较强的菌株在无糖面团中的起发速度较快，反之则较慢。验证了麦芽糖发酵力是决定面包酵母快速发酵无糖面团的关键因素。     

面包酵母发酵力测定方法的探讨

研究面包酵母面团发酵力的测定方法,讨论在不加糖面团和高糖面团中培养时间与酵母加量的关系,确定了最佳的测定条件,同时对市售8株商品面包酵母进行比较.     

面包酵母发酵力与面粉淀粉酶活力关系的研究

为了研究面包酵母不加糖面团发酵力与面粉中淀粉酶活力的关系,用5种不同品牌的标准粉和4个同一品牌不同生产日期的标准粉测定了一种活性干酵母的发酵力,并测定了这些面粉的淀粉酶活力。结果表明测定的发酵力最高的可以达到625 mL,最低的只有325 mL。而面粉最高的淀粉酶活力为18.5 U,最低的为2.7 U,并与发酵力的结果一致,表明面粉中的淀粉酶活力与测定的面包酵母不加糖面团发酵力结果呈正相关。     

高适应性面包酵母菌株的杂交选育

为了改善耐高糖酵母在不同含糖量面团中的发酵力,首先制备了2 4 0株耐高糖酵母单倍体,通过高麦芽糖发酵筛选培养基筛选出了2 8株麦芽糖发酵性能良好的单倍体菌株,通过测定这些单倍体在无糖面团中的发酵力,发现9株单倍体菌株的发酵力优于或等于其亲本,其中4株为a型,5株为α型。通过它们之间的杂交得到2 0 0株杂交株,在无糖、中糖、高糖面团中测定这些杂交株的发酵力,获得4株在不同含糖量(0～30 % )面团中都具有高发酵力的高适应性面包酵母杂交株。研究表明,来自同一亲本单倍体之间的杂交有可能改善面包酵母的某些特征。     

海藻糖对面包酵母高糖耐性的影响

通过高温刺激使面包酵母积累较高的胞内海藻糖,考察胞内海藻糖含量对面包酵母高糖发酵力和高渗存活率的影响。研究结果表明,发酵结束前2h培养温度提高至40℃后,菌株BY-6胞内海藻糖含量由4.21%提高到9.76%,高糖发酵力提高了17.2%,高渗存活率也有一定提高。另外对3株面包酵母的海藻糖积累能力和面团发酵力的比较表明,海藻糖积累能力强的酵母菌株高糖耐性较好。     

国内市售面包酵母发酵特性及其适用性的研究

目前我国市场上销售的进口的及国产的面包酵母品种繁多,它们的发酵特性差别很大,故有不同的适用性。本项研究为全国面包行业正确选用面包酵母提供科学依据,同时也填补了国内在面包酵母发酵特性比较研究方面的空白。选取五种国内市场上的有代表性的酵母,采用量筒面团直观法和Brabender Fermentograph(发酵仪）相结合,分析各种酵母的发酵力、发酵耐力、耐盐性、耐糖性以及面包配方中的酵母、盐、糖、乳在发酵过程中的交互作用。     

利用蜜二糖的葡萄汁酵母菌种筛选

葡萄汁酵母能有效利用糖蜜中的蜜二糖,但是发酵能力差.在与面包酵母杂交后,则其在继承面包酵母高发酵力特性的同时,兼具有利用蜜二糖能力.本试验对实验室保藏的8株葡萄汁酵母进行比较,经杜氏管试验、比生长速率、生物量、面团发酵力和蜜二糖利用等试验后,筛选得到了1株能高效利用蜜二糖的葡萄汁酵母M3,作为下一步杂交试验的亲本菌株.     

面包酵母耐冷冻性能的评价方法

讨论了面包酵母耐冷冻能力的评价方法,通过对酵母在普通面团中冷冻后的相对发酵力、在液体模拟面团中冷冻后的相对发酵力和酵母细胞冷冻存活率之间的比较,发现3者之间具有显著的线性正相关性。而应用液体模拟面团测定酵母耐冷冻能力的方法简便、易控制,可用于耐冷冻酵母菌种的选育和耐冷冻酵母产品的检测中。     

敲除REG1同时过表达SNF1基因对工业面包酵母不加糖面团发酵的影响

研究了蛋白磷酸酶PP1调节亚基编码基因REG1缺失同时蛋白激酶基因SNF1过表达对工业面包酵母麦芽糖代谢和不加糖面团发酵的影响。比较分析REG1缺失同时SNF1过表达转化子BYKPS-R和亲本菌株BY14α、REG1缺失突变株BYK-R的MAL基因m RNA水平、麦芽糖酶和麦芽糖通透酶活力、麦芽糖代谢水平、不加糖面团发酵力,以及基本生长性能,结果表明,敲除REG1同时过表达SNF1能够显著提高麦芽糖代谢相关基因转录和酶活力,有效减弱葡萄糖阻遏,从而使面包酵母的麦芽糖消耗速率(葡萄糖被完全消耗完之前)较BY14α和BYK-R分别提高了18.59%和4.40%,不加糖面团发酵力分别提高了12.51%和3.22%。在REG1基因缺失的基础上,过表达SNF1可以进一步提高面包酵母的麦芽糖代谢和不加糖面团发酵水平,同时不影响面包酵母菌株生长性能,因此转化子BYKPS-R具备潜在的工业应用价值,同时该研究为快速发酵面包酵母菌株的选育奠定基础。     

酵母发酵力测定新方法的探讨

利用排水法、面团体积法和面团浮水法测定不同酵母添加量下发酵力的变化,并判断彼此间相关性。结果显示:随着酵母添加量的增加,在2 h内,用排水法、面团体积法和面团浮水法测得酵母发酵力均为直线增长趋势,且三种方法彼此间存在较高的相关性,均可用于测定酵母发酵力。     

Proline accumulation in baker's yeast enhances high-sucrose stress tolerance and fermentation ability in sweet dough

During bread-making processes, yeast cells are exposed to various baking-associated stresses. High-sucrose concentrations exert severe osmotic stress that seriously damages cellular components by generation of reactive oxygen species (ROS). Previously, we found that the accumulation of proline conferred freeze-thaw stress tolerance and the baker's yeast strain that accumulated proline retained higher-level fermentation abilities in frozen doughs than the wild-type strain. In this study, we constructed self-cloning diploid baker's yeast strains that accumulate proline. These resultant strains showed higher cell viability and lower intracellular oxidation levels than that observed in the wild-type strain under high-sucrose stress condition. Proline accumulation also enhanced the fermentation ability in high-sucrose-containing dough. These results demonstrate the usefulness of proline-accumulating baker's yeast for sweet dough baking.     

The influence of a baker's yeast pre-inoculum on bread-dough leavening

In order to investigate the influence of a pre-inoculum on the fermentation performance of compressed baker's yeast used for bread-dough leavening, pre-inocula were made by adding compressed baker's yeast to a premix-sugar broth and cultivating it for 30, 60, 90 and 120 min. The pre-inocula were added separately to the dough and the gas production measured with the aid of a fermento-manometer. The pre-inoculum cultivated for 120 min presented the highest increase (43·6%) in maximum gas produced (P_max) after 90 min of dough fermentation. The possibility of reducing lag time during dough fermentation, by using a cost-effective pre-inoculum, was assessed and results showed a significant (P ≤ 0·05) increase in gas production which could be highly beneficial to the bread industry.     

不同酵母对面团发酵特性及馒头品质的影响

就不同酵母对面团发酵特性和馒头品质的影响进行了试验.通过比较不同酵母发酵面团的产气量和面团膨发状态,进而确定相同条件38℃,88%RH,酵母添加量0.8%,不同酵母的最适发酵时间,然后对其发酵的馒头进行感官评价和质构测定,结果显示:不同酵母发酵面团时的产气量和面团膨发状态不同,Y-1、Y-2、Y-3和Y-4的最适发酵时间依次为40、30、25和30 min.由感官评价和质构测定结果还得出结论:不同酵母显著影响馒头产品的感官评价总分,且感官评价和质构测定参数与其面团发酵性能相一致.     

Simultaneous accumulation of proline and trehalose in industrial baker's yeast enhances fermentation ability in frozen dough

Freeze tolerance is a necessary characteristic for industrial baker's yeast because frozen-dough baking is one of the key technologies for supplying oven-fresh bakery products to consumers. Both proline and trehalose are known to function as cryoprotectants in yeast cells. In order to enhance the freeze tolerance of yeast cells, we constructed a self-cloning diploid baker's yeast strain with simultaneous accumulation of proline, by expressing the PRO1-I150T allele, encoding the proline-feedback inhibition-less sensitive γ-glutamyl kinase, and trehalose, by disrupting the NTH1 gene, encoding neutral trehalase. The resultant strain retained higher tolerance to oxidative and freezing stresses than did the single proline- or trehalose-accumulating strain. Interestingly, our results suggest that proline and trehalose protect yeast cells from short-term and long-term freezing, respectively. Simultaneous accumulation of proline and trehalose in industrial baker's yeast also enhanced the fermentation ability in the frozen dough compared with the single accumulation of proline or trehalose. These results indicate that baker's yeast that accumulates both proline and trehalose is applicable for frozen-dough baking.     

Use of selected sourdough lactic acid bacteria to hydrolyze wheat and rye proteins responsible for cereal allergy

This work was aimed at showing the capacity of selected sourdough lactic acid bacteria to hydrolyze wheat and rye allergens. Hydrolysis was investigated after wheat sourdough fermentation and after treatment of wheat and rye sourdough breads with pepsin, trypsin and pancreatin, which mimicked the digestive process. As shown by immunoblotting with sera from allergic patients, wheat sourdough fermentation caused the disappearance of some IgE-binding proteins (albumins/globulins and gliadins mainly) with respect to the chemically acidified dough used as the control. The IgE-binding protein profile of wheat and rye sourdough breads differed from those of baker's yeast breads. The signals of the IgE-binding proteins contained in the sourdough breads disappeared after in vitro digestion with pepsin, trypsin and pancreatin. The same effect by digestive enzymes was not found for baker's yeast breads which showed persistent IgE-binding proteins. As shown by ELISA inhibition assays, the presence of IgE-binding low molecular weight proteins/peptides in sourdough breads significantly decreased with respect to baker's yeast breads. Proteolytic activity by selected sourdough lactic acid bacteria may have an importance during food processing to produce pre-digested wheat and rye dough which contains IgE-binding proteins degradable by digestive enzymes.     

Characterization of a new set of mutants deficient in fermentation-induced loss of stress resistance for use in frozen dough applications

In frozen dough applications a prefermentation period during the preparation of the dough is unavoidable and might also be important to obtain bread with a good texture. A major disadvantage of the prefermentation period is that it is associated with a rapid loss of the freeze resistance of the yeast cells. A major goal for the development of new baker's yeast strains for use in frozen dough applications is the availability of strains that maintain a better freeze resistance during the prefermentation period. We have isolated mutants that retain a better stress resistance during the initiation of fermentation. Some of these showed the same growth rate and fermentation capacity as the wild type cells. These mutants are called 'fil', for deficient infermentation induced loss of stress resistance. First we used laboratory strains and heat stress treatment, given shortly after the initiation of fermentation, as the selection protocol. The first two mutants isolated in this way were affected in the glucose-activation mechanism of the Ras-cAMP pathway. The fil1 mutant had a partially inactivating point mutation in CYR1, the gene encoding adenylate cyclase, while fil2 contained a nonsense mutation in GPR1. GPR1 encodes a member of the G-protein coupled receptor family which acts as a putative glucose receptor for activation of the Ras-cAMP pathway. In a next step we isolated fil mutants directly in industrial strains using repetitive freeze treatment of doughs as selection protocol. Surviving yeast strains were tested individually for maintenance of fermentation capacity after freeze treatment in laboratory conditions and also for the best performing strains in frozen doughs prepared with yeast cultivated on a pilot scale. The most promising mutant, AT25, displayed under all conditions a better maintenance of gassing power during freeze-storage. It was not affected in other commercially important properties and will now be characterised extensively at the biochemical and molecular level.     

Bread Dough and Baker's Yeast: An Uplifting Synergy

Yeast‐mediated dough fermentation is an important phase in the bread making process. The fermentative performance of yeast cells during fermentation is of critical importance for final bread quality, since yeast cells produce CO₂ and other metabolites that have an influence on dough rheology and bread texture, volume, and taste. Different factors affect the fermentative performance of yeast cells during dough fermentation, including dough ingredients, fermentation conditions, the type of yeast strain used and yeast pregrowth conditions. Bread dough is a complex matrix that contains several ingredients that can affect the fermentation rate of yeast cells. Although the individual effects of sugar availability and salt level on the leavening ability of yeast have been studied extensively, a comprehensive overview of the relationship between bread dough constituents, fermentation conditions and yeast functionality is still lacking. Moreover, the dough environment is highly variable as several types of dough like lean, sweet or frozen doughs are currently produced by commercial bread producers. For optimal fermentation rates in different types of dough, the use of appropriate yeast strains with specific phenotypic traits is required. Therefore, many researchers have focused on the improvement of yeast strains for optimal fermentation in different types of dough like lean, sweet or frozen dough. Against this background, this review summarizes the current knowledge on the interaction between bread dough and baker's yeast and how to improve this interaction, thereby providing a useful background for further research concerning the functionality of yeast in bread dough.