CAMBRIDGE PRIZE LECTURE IMPROVING YEAST FERMENTATION PERFORMANCE

Cellular toxicity of the metabolic inhibitor 2-deoxyglucose and associated resistance mechanisms

Most malignant cells display increased glucose absorption and metabolism compared to surrounding tissues. This well-described phenomenon results from a metabolic reprogramming occurring during transformation, that provides the building blocks and supports the high energetic cost of proliferation by increasing glycolysis. These features led to the idea that drugs targeting glycolysis might prove efficient in the context of cancer treatment. One of these drugs, 2-deoxyglucose (2-DG), is a synthetic glucose analog that can be imported into cells and interfere with glycolysis and ATP generation. Its preferential targeting to sites of cell proliferation is supported by the observation that a derived molecule, 2-fluoro-2-deoxyglucose (FDG) accumulates in tumors and is used for cancer imaging. Here, we review the toxicity mechanisms of this drug, from the early-described effects on glycolysis to its other cellular consequences, including inhibition of protein glycosylation and endoplasmic reticulum stress, and its interference with signaling pathways. Then, we summarize the current data on the use of 2-DG as an anti-cancer agent, especially in the context of combination therapies, as novel 2-DG-derived drugs are being developed. We also show how the use of 2-DG helped to decipher glucose-signaling pathways in yeast and favored their engineering for biotechnologies. Finally, we discuss the resistance strategies to this inhibitor that have been identified in the course of these studies and which may have important implications regarding a medical use of this drug.

Impact of changes in fermentation time, volume of yeast, and mass of plantain pseudo-stem substrate on the simultaneous saccharification and fermentation potentials of African land snail digestive juice and yeast

This study was carried out to investigate the effect of variations in mass of plantain pseudo-stem waste, volume of yeast used, and fermentation time on the product yield resulting from simultaneous saccharification and fermentation using digestive juice of African land snail and yeast. The experiment was divided into three stages which included a total of fifty seven (57) experimental setups containing sixteen (19) different combinations of the varied substrates. The results show that by varying the mass of plantain pseudo-stem waste, the production of ethanol was optimized at a mass of 250 g, which yielded 125.6 ml ± 3.5 of distillate and a percentage ethanol composition of 25.0 ± 3.6. While varying the volume of yeast used between 50 and 250 ml, with 250 g of plantain pseudo-stem waste, 250 ml of snail digestive juice and 4 g garlic for 24 h, acetic acid was detected in the setup containing 200 ml of yeast, but was not detected in similar experimental setups containing 6 g garlic. The optimum ethanol production while varying the volume of yeast slurry was recorded to be 182.3 ml ± 4.9 of distillate with 28.0% ± 1.0 ethanol composition. Variations in fermentation periods had the greatest impact on the percentage composition of ethanol and the volume of ethanol produced showing the best fermentation period for obtaining optimal ethanol production to be at 96 h. These findings show that the best specifications for the optimum production of ethanol from a 250 g of plantain pseudo-stem waste using 250 ml snail digestive, are 200 ml of yeast slurry, 6 g of garlic to ferment for a period of 96 h.

Effects of soy protein hydrolysates on the growth and fermentation performances of brewer's yeast

Soy protein isolate was hydrolysed with Alcalase, Papain, Flavorzyme and Protemax, respectively, and further fractioned by ultrafiltration. The resulting soy protein hydrolysates (SPH) and their ultrafiltration fractions were used to examine their effects on the growth and fermentation performances of brewer's yeast. Results showed that degree of hydrolysis, molecular weight distribution and amino acid composition of SPH significantly affected the growth, viability and fermentation performance of brewer's yeast. The SPH prepared from different proteolytic enzymes exhibited distinct growth‐ and fermentation‐promoting activity for brewer's yeast. The SPH treated with Protemax for 9 h and with the molecular weight below 3 kDa showed the highest growth‐promoting activity and induced more rapidly reducing sugar consumption and higher ethanol production. The relatively lower molecular weight and the hydrophilic and electropositive amino acid residues (Lys, His, Arg and Ile) in SPH might be responsible for its functionality, promoting the growth and fermentation of brewer's yeast.

Selection from industrial lager yeast strains of variants with improved fermentation performance in very-high-gravity worts

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 degrees P [degrees Plato]) worts into the very-high-gravity (VHG; 18 to 25 degrees 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 alpha-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 10(6) 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.

High gravity batch and continuous processes for beer production: Evaluation of fermentation performance and beer quality

This study deals with two innovative brewing processes, high gravity batch and complete continuous beer fermentation systems. The results show a significant influence of the variables such as concentration and temperature on the yield factor of the substrate into ethanol and consequently on the productivity of the high gravity batch process. The technological feasibility of continuous production of beer based on yeast immobilization on cheap alternative carriers was also demonstrated. The influence of process parameters on fermentation performance and quality of the obtained beers was studied by sensorial analysis. No significant difference in the degree of acceptance between the obtained products and some traditional market brands was found.

Yeast responses to stresses associated with industrial brewery handling: Figure 1

During brewery handling, production strains of yeast must respond to fluctuations in dissolved oxygen concentration, pH, osmolarity, ethanol concentration, nutrient supply and temperature. Fermentation performance of brewing yeast strains is dependent on their ability to adapt to these changes, particularly during batch brewery fermentation which involves the recycling (repitching) of a single yeast culture (slurry) over a number of fermentations (generations). Modern practices, such as the use of high-gravity worts and preparation of dried yeast for use as an inoculum, have increased the magnitude of the stresses to which the cell is subjected. The ability of yeast to respond effectively to these conditions is essential not only for beer production but also for maintaining the fermentation fitness of yeast for use in subsequent fermentations. During brewery handling, cells inhabit a complex environment and our understanding of stress responses under such conditions is limited. The advent of techniques capable of determining genomic and proteomic changes within the cell is likely vastly to improve our knowledge of yeast stress responses during industrial brewery handling.

Viability of yeast cells in well controlled propagating and standing ultrasonic plane waves

Recent studies have shown that there is no loss of cell viability when the cells are subjected to ultrasonic standing wave fields in acoustic cell retention systems. These systems are characterised by waves that spatially vary in pressure amplitude in the direction of sound propagation. In this work an anechoic 'one-dimensional' sonication chamber has been developed that produces propagating waves, which differ from standing waves in that the pressure amplitude remains constant as the wave travels in a medium with negligible attenuation. The viability of yeast cell suspensions as a function of treatment time was investigated during exposure to both standing and propagating wave fields with frequencies slightly above 2 MHz. The influence of 12% (vol/vol) of ethanol in water on the spatial arrangement of the cells in suspension was also studied. Changes in yeast cell morphology caused by the different types of suspension media and the ultrasonic treatment were examined by transmission electron microscopy (TEM). The agglomeration of yeast cells within the pressure nodal planes appears to minimise damaging effects due to ultrasonic fields.

Genome-wide transcriptional analysis of aerobic and anaerobic chemostat cultures of Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is unique among eukaryotes in exhibiting fast growth in both the presence and the complete absence of oxygen. Genome-wide transcriptional adaptation to aerobiosis and anaerobiosis was studied in assays using DNA microarrays. This technique was combined with chemostat cultivation, which allows controlled variation of a single growth parameter under defined conditions and at a fixed specific growth rate. Of the 6,171 open reading frames investigated, 5,738 (93%) yielded detectable transcript levels under either aerobic or anaerobic conditions; 140 genes showed a >3-fold-higher transcription level under anaerobic conditions. Under aerobic conditions, transcript levels of 219 genes were >3-fold higher than under anaerobic conditions.

Fungal interactions in food fermentations

Fermented foods are of importance worldwide. Most are prepared under nonsterile conditions using mixed cultures, either deliberately or unavoidably. Fungal mixed cultures show interactive relations at various levels. In this paper, inhibitory effects among fungi owing to competition, formation of organic acids, toxic proteins, and mycotoxins are discussed. In addition, fungi show inhibitory effects towards bacteria and vice versa, through pH changes, and excretion of organic acids, antibiotics, peptides, etc. Stimulatory interactions among fungi and between fungi and bacteria relate mainly to carbon and nitrogen metabolism, and they play an important role in the inherent stability of mixed-culture systems maintained by enrichment techniques. Better understanding of natural mixed-culture fermentations has evolved into the development of the concept of cocultivation employing compatible microbial strains of complementary metabolic ability. Especially in the area of direct conversion of complex carbohydrates (e.g., starch, inulin, or lignocellulosic matter into ethanol), cocultivation has much to offer. Genetic modification of starter organisms offers opportunities to improve, for example, their ability to degrade substrate with a minimum of catabolite repression, and produce final products of superior quality. This is illustrated by recent recombinant DNA constructs for alcoholic fermentations. Key words: food, fungi, interaction, inhibition, stimulation, cocultivation.