The effect of CO2 concentration on yeast fermentation: rates, metabolic products, and yeast stress indicators

This research aimed to assess how the partial removal of carbon dioxide affects fermentations to provide a better understanding of how the manipulation of carbon dioxide concentration can be used to optimize industrial fermentations. To achieve this, fermentation kinetics, fermentation metabolic products, and yeast stress indicators were analyzed throughout ongoing brewing fermentations conducted under partial vacuum with atmospheric pressure controls. The partial vacuum reduced the solubility of carbon dioxide in the media and decreased the time necessary to reach carbon dioxide saturation. The effect was an increased rate of fermentation, and significantly more viable cells produced under vacuum pressure compared to controls. Ethanol, glycerol, and volatile organic compound concentrations were all significantly increased under partial vacuum, while indicators of yeast stress (trehalose) were reduced. Additionally, as the number of yeast cells was higher under partial vacuum, less sugar was consumed per volume of yeast cell. This study measured fermentation kinetics, metabolic products, and yeast health to holistically assess the effect of partial vacuum during a batch fermentation and found significant differences in each that can be individually exploited by researchers and industry.

SUMMARY

An exploration of batch yeast fermentation in a low-pressure environment, with a focus on the health and productivity of the yeast cells.

Infective larvae of Anisakis simplex (Nematoda) accumulate trehalose and glycogen in response to starvation and temperature stress

Anisakis simplex L3 larvae infect fish and other seafood species such as squid or octopi; therefore, humans consuming raw or undercooked fish may become accidental hosts for this parasite. These larvae are induced to enter hypometabolism by cold temperatures. It is assumed that sugars (in particular trehalose and glycogen) are instrumental for survival under environmental stress conditions. To elucidate the mechanisms of environmental stress response in A. simplex, we observed the effects of starvation and temperature on trehalose and glycogen content, the activity of enzymes metabolizing those sugars, and the relative expression of genes of trehalose and glycogen metabolic pathways. The L3 of A. simplex synthesize trehalose both in low (0°C) and high temperatures (45°C). The highest content of glycogen was observed at 45°C at 36 h of incubation. On the second day of incubation, tissue content of trehalose depended on the activity of the enzymes: TPS was more active at 45°C, and TPP was more active at 0°C. The changes in TPP activity were consistent with the transcript level changes of the TPP gene, and the trehalose level, while glycogen synthesis correlates with the expression of glycogen synthase gene at 45°C; this suggests that the synthesis of trehalose is more essential. These results show that trehalose plays a key role in providing energy during the thermotolerance and starvation processes through the molecular and biochemical regulation of trehalose and glycogen metabolism.

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.

Protective agents used to reverse the metabolic changes induced in wine yeasts by concomitant osmotic and thermal stress

AIMS

The reversion of the metabolic changes induced in wine yeasts by stressors.

METHODS

Six strains of Saccharomyces were inoculated in grape must containing over 400 g l-1 of sugar and incubated at 35 degrees C, both with and without the addition of 100 mg l-1 of catechin, inositol or SO2.

RESULTS

Significant correlations between addition of the stress-protectants and change in the metabolic behaviour of the wine yeasts were observed. Depending on strain and protectant, and expressing data as a percentage of increase or decrease compared to the control, fermentation vigour after 3 d increased by up to 10%, titratable acidity of the wines increased by up to 7%, ethanol content increased by up to 20%, unitary acetic acid production decreased by up to 35%, and unitary glycerol production decreased by up to 20%. IMPACT OF STUDY: By using protective agents it is possible to minimize the abnormal fermentation performance that wine yeasts exhibit under thermal and osmotic stress.