CHANGES IN CYTOCHROME LEVELS DURING THE STORAGE OF BAKER'S YEAST AT DIFFERENT TEMPERATURES

The content of cytochromes determined per g fresh yeast was found to stay constant during nearly 3 months' storage of baker's yeast at 5°C. When stored at 24°C and 35°C the content of cytochromes apparently increased during the first 14 and 4 days of storage, respectively, and then decreased continuously.     

ESTERASES OF BAKER'S YEAST. II. SUBSTRATE SPECIFICITIES TOWARDS ESTERS FORMED DURING SUGAR FERMENTATIONS

Ethyl esters of fatty acids (C₂–C₁₂), isoamyl acetate and 2-phenethyl acetate were studied as substrates for yeast esterases and compared with the synthetic substrates, p-nitrophenyl esters and β-naphthyl esters. Intact yeast, the 55% and 55–75% ammonium sulphate precipitate of centrifuged yeast homogenate, and partly purified esterases were used for the determination of the hydrolysation activity towards the esters. The results showed that the yeast esterases prefer to hydrolyse the ethyl esters with acyl chain length of C₅ to C₁₂. The acetate esters, ethyl acetate, isoamyl acetate and 2-phenethyl acetate are only very slowly hydrolysed or remain unaffected. The substrate specificity of different esterases varies and can be used for their characterisation. Investigating pH optimum curves using intact yeast and a crude esterase preparation and different substrates confirmed the earlier result that there are esterases on both sides of the plasma membrane. The specificity of intracellular and periplasmically located esterases is, however, different.     

RESERVE CARBOHYDRATE METABOLISM AND CELL SURVIVAL IN AEROBICALLY STARVING BAKER'S YEAST

Changes in the level of reserve carbohydrates, cell number, viability and medium pH values were observed in aerobically starving baker's yeast. The starvation was carried out at 35°C in the physiological saline solution using two different initial pH values (4·4 or 5·5) of the medium. In addition, the formation of ethanol and acetate as well as actual and endogenous respirations were measured at a pH of 4·4. The results revealed that the initial pH value of the medium affected the pattern of glycogen and trehalose degradation and consequently, also caused the loss of viability and cell lysis. Changes in the quantity of metabolic products and respiration activity are discussed in connection with the metabolism of the starving cells and the effect of environmental conditions.     

YEAST ESTERASES AND AROMA ESTERS IN ALCOHOLIC BEVERAGES

It has been shown that the proportion of fatty acid ethyl esters retained by the yeast cell increases with increasing acyl chain length as the ester becomes more lipid soluble. The distribution of esters depends on the yeast strain and on the fermentation temperature; larger amounts of esters were found to transfer from the cells into the medium at higher temperature. It was shown that esterase activity is located both inside and outside the yeast cell plasma membrane. Intact yeast was capable of hydrolysing the ethyl esters of caproic, caprylic and capric acid. Acetate esters, were hydrolysed only very slowly or not at all. The hydrolytic activity of baker's yeast was studied with ethyl caprylate as substrate. The hydrolysis was very fast at the beginning. The equilibrium attained depended not only on the concentration of ester and alcohol but also on the pH, a higher amount of ester remaining in solutions of lower pH. It was also shown that the esterases possess appreciable ester synthesizing ability and an equilibrium was attained by incubating yeast with caprylic acid and ethanol. The experiments described show that the ester level in an alcoholic beverage, such as beer, is not dependent solely on the ester concentration formed during fermentation: in the presence of yeast the level can be shifted in either direction by changing temperature, pH or alcohol concentration — or the amount and type of yeast.     

EFFECT OF INCORPORATING ADDITIONAL OLEIC ACID INTO THE PLASMA MEMBRANE OF BAKER'S YEAST ON THE PERMEATION OF PYRUVIC ACID

Addition of Tween 80 to the growth medium of baker's yeast (Saccharomyces cerevisiae) under anaerobic conditions resulted in an increased content of 9-cis-octadecenoic acid (oleic acid) in a membrane preparation consisting mainly of plasma membrane. There was no significant effect on the pyruvate decarboxylation capacity of disintegrated cells, but that of intact cells was significantly enhanced, presumably because of an oleate-induced increase in the permeability of the plasma membrane to pyruvate.     

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.     

METABOLIC RESPONSES OF BAKER'S YEAST TO GLUCOSE ADDITION DURING AEROBIC STARVATION

The kinetics and extent of glucose uptake, ethanol and acetate production, glucose-induced medium acidification and endogenous respiration were studied in baker's yeast starved aerobically for 0, 28, 54 and 96 h in saline (9 g litre^?1 NaCI) at 35°C. Except for the last interval, prolonged starvation brought about an overall increase in most parameters; the transient increase in endogenous respiration observed after glucose addition was lowered with proceeding starvation. A 96 h starvation brought about a change in the kinetics of ethanol production, sharp increase in acetate production and a strong reduction of both the extent and the rate of glucose-induced acidification and of endogenous metabolism.     

外源海藻糖对啤酒酵母在热胁迫下的保护作用

为探究外源海藻糖对啤酒酵母在热胁迫下的保护作用,作者在3个热胁迫条件下检测外源海藻糖对啤酒酵母活力及活性的影响,并通过分析转录组数据对热胁迫下外源海藻糖对啤酒酵母的保护作用机理进行初步分析。结果表明,外源海藻糖可以提升啤酒酵母的耐热性,但具有一定限度,温度的升高和胁迫时间的延长导致不同浓度海藻糖的作用差别减小。另外,添加海藻糖可以增加酵母胞内海藻糖质量分数,添加海藻糖对啤酒酵母转录水平上的影响主要体现在核糖体的合成和相关功能上。结合本研究推测,外源海藻糖对啤酒酵母在热胁迫下的保护作用是通过增加胞内海藻糖质量分数实现的,上升的胞内海藻糖质量分数促进了核糖体合成及相关代谢,使核糖体功能增强,进而实现对啤酒酵母耐热性的增强。     

The effect of osmosis pretreatment on hot‐air drying and microwave drying characteristics of chili (Capsicum annuum L.) flesh

Chili flesh pretreated with or without osmotic dehydration (OD) was dried in the hot‐air drying (AD) oven at 50–80 °C or in the microwave drying (MD) oven at 60–180 W. Results showed that the samples osmotically treated in mixed solution (10% salt + 50% sucrose) had the best dehydration effect as compared with single salt or sugar solutions. During the drying process, osmotically treated samples had one falling‐rate period and their effective moisture diffusivities (D_eff) showed a rapidly linear increase with the decrease in moisture content, while directly drying samples showed a three‐phase falling‐rate period and their D_eff increased gradually at the initial period and then rapidly at the final period. When the moisture content decreased, the activation energy increased gradually; however, for AD after OD, it decreased. Among all the processes, MD at 60 W after OD presented the largest vitamin C retention rate and the best colour difference, needing less drying time.     

Fruit snacks from raspberries: influence of drying parameters on colour degradation and bioactive potential

Raspberries were dehydrated using air and freeze‐drying with wet and dry sugar infusion pretreatments. Product quality factors such as colour, bioactive compounds, antioxidant capacity and sensorial characteristics were analysed. Special emphasis was placed on the analysis of anthocyanin degradation and its relationship with colour deterioration and with polymeric compounds development and browning. Freeze‐dried raspberries presented a higher retention of bioactive compounds and a lower content of polymeric compounds than air‐dried ones. Dried samples without pretreatment (control) showed the highest retention of total phenolic content (freeze‐dried ≈82% and air‐dried ≈37% retention), but the lowest sensory acceptability. Although sugar infusion pretreatments caused an important loss of bioactive compounds (9–18% of TPC retention), a higher sensorial acceptability was obtained. Pretreatments with bisulphite and acid allowed obtaining the best quality attributes in terms of anthocyanin and polyphenol content, antiradical activity and colour retention. Polyphenol intake through pretreated dried raspberries (115–299 mg gallic ac./100 g intake) would be higher in some cases than that of usually consumed foods as vegetables, cereals and several fresh fruits.