WINE MICRO PART 1 Flashcards
(98 cards)
Explain why some Saccharomyces cerevisiae strains produce SO2 during fermentation. Include in your answer descriptions of any factors may influence the production of SO2 by S. cerevisiae.
The reason why some yeasts are high producers of SO2 is believed to be due to modified activities of enzymes associated with the SRS pathway. A deficiency in pantothenic acid which may cause low SO2-producing yeasts to increase formation of SO2.
Interactions between micro organisms and their biochemical products during alcoholic fermentation may affect yeast growth. One of the most important interactions is that between Botrytis cinerea and Saccharomyces cereviseae. Briefly describe two inhibitory effects of Botrytis infection of grapes and Saccharomyces growth.
Botrytis infection causes increased glycerol in finished wines partially due to increased sugar concentration of grapes. Botrytis cinerea also produces an antimicrobial substance called Botryticine during growth on grapes that alters yeast carbon flux.
List four organic acids produced by yeasts during fermentation.?
Succinic, acetic, malic, and pyruvic acids.
Describe how ethanol impairs yeast cell function.
Ethanol, a major product of the alcoholic fermentation, exerts a toxic effect on yeast principally through a reduction in sugar and nitrogen transport across the cell membrane.
Fusel alcohol production by selected yeast strains may vary according to the fermentation conditions. Briefly describe how the following factors impact fusel alcohol production.
During the alcoholic fermentation, yeasts will produce small concentrations of higher alcohols, principally n-propanol. Although higher alcohols may contribute partially to the flavour and body to wines, it is generally believed that when present in excess amounts they impair the flavour of wine.
How can the growth conditions of a yeast starter culture be influenced to maximise the quantity of sterols and fatty acids available to the yeasts during fermentation?
Two major groups chemical additions to the juice, and clarification procedures.
1. Added SO2 will increase lag phase. Added nutrient supplements such as (DAP) helps with limited nitrogen concentrations. Vitamins supplements and yeast hulls which absorb medium chain fatty acids and act as a source of essential lipids (Sterols) which play an important role in ethanol tolerance which may overcome stuck ferments.
How can the growth conditions of a yeast starter culture be influenced to maximise the quantity of sterols and fatty acids available to the yeasts during fermentation?
Two major groups chemical additions to the juice, and clarification procedures.
1. Added SO2 will increase lag phase. Added nutrient supplements such as (DAP) helps with limited nitrogen concentrations. Vitamins supplements and yeast hulls which absorb medium chain fatty acids and act as a source of essential lipids (Sterols) which play an important role in ethanol tolerance which may overcome stuck ferments.
Briefly discuss fermentation parameters that influence the quantity of SO2 production by yeasts.
SO2 produced will become bound to other yeast metabolites (such as acetaldehyde), thereby increasing the total SO2 content of the wine. For wines already containing high levels of total SO2, it is possible to actually exceed the legal limit. Wines with excessive levels of bound SO2 may also develop a bitter taste.
Outline the biochemical pathways important in the production of H2S by yeasts during fermentation. Include in you answer an explanation of how these pathways are regulated by the yeast and the role that yeast assimilable nitrogen has in H2S production.
Hydrogen sulfide is produced by yeasts during fermentation during synthesis of sulfur containing amino acids. The most studied biochemical pathway is the sulfate reduction sequence (SRS)
The SRS pathway is a tightly controlled process that is normally not active in yeast unless the concentration of cysteine and methionine in the fermentation becomes limiting.
A number of vitamins are essential enzyme cofactors that participate in the growth of yeasts during fermentation. Complete the following table that best describes the most important biochemical pathway or function and the essential vitamin/cofactor.
Biotin
Biotin is involved in a wide variety of metabolic processes including the metabolism of carbohydrates, amino acids, proteins, nucleic acids and lipids (Figure 5). Biotin also plays an essential role in nitrogen metabolism and in decarboxylation reactions.
Inositol
Inositol (meso-inositol, myo-inositol, i-inositol) is stimulatory or essential for growth of a number of yeasts (Figure 7). The actual role of inositol in cell metabolism is not clear, although it seems to be involved in the actual structure of the cell through its presence in phospholipids (particularly in cell membranes).
Folic Acid
Folic acid, (as tetrahydrofolate), is involved in the metabolism of one-carbon fragments, such as transmethylation. Myo-inositol serves a structural function in phospholipids.
p-Aminobenzoic Acid
The main function of PABA (Figure 11) appears to be in the synthesis of folic acid and its related compounds
Vitamin B6 Group
All vitamins needed by yeasts (with the exception of inositol) serve vital catalytic functions as part of co-enzymes in the metabolism of yeasts.
Riboflavin
The importance of riboflavin (Figure 13) lies in the co-enzyme derivatives, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). Free riboflavin has no known co-enzymic function. FAD is[…]”
What is Biotin?
Biotin is involved in a wide variety of metabolic processes including the metabolism of carbohydrates, amino acids, proteins, nucleic acids and lipids. Biotin also plays an essential role in nitrogen metabolism and in decarboxylation reactions.
What is Inositol?
Inositol (meso-inositol, myo-inositol, i-inositol) is stimulatory or essential for growth of a number of yeasts. The actual role of inositol in cell metabolism is not clear, although it seems to be involved in the actual structure of the cell through its presence in phospholipids (particularly in cell
What is Folic Acid?
Folic acid, (as tetrahydrofolate), is involved in the metabolism of one-carbon fragments, such as transmethylation. Myo-inositol serves a structural function in phospholipids.
What is p-Aminobenzoic Acid?
The main function of PABA (Figure 11) appears to be in the synthesis of folic acid and its related compounds
What is the Vitamin B6 Group?
All vitamins needed by yeasts (with the exception of inositol) serve vital catalytic functions as part of co-enzymes in the metabolism of yeasts.
What is Riboflavin?
The importance of riboflavin (Figure 13) lies in the co-enzyme derivatives, flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN). Free riboflavin has no known co-enzymic function. FAD is important for the proper functioning of the respiratory chain, and is involved in the transfer of hydrogen or electrons from the reduced form of NAD to cytochromes.
During fermentation monitoring a winemaker notices a slowing of carbohydrate utilisation and decides to make an addition of DAP. A 70 ppm addition is made however no substantial increase in fermentation rate occurs. Explain the non-responsive behaviour of the yeast strain to the DAP addition.
Yeasts ferment at a slower rate under nitrogen-starvation conditions because they lack the necessary nitrogen with which they can synthesise new proteins for growth (e.g. enzymes). A major difficulty facing winemakers is that fermentations that become sluggish due to nitrogen-starvation may potentially become non-responsive to the addition of nitrogen supplements such as DAP.
A white grape juice must at 20°C is inoculated with 1% volume of a wet yeast starter culture which has a total yeast cell count of 4.2x108 cells/mL. Briefly describe the growth phases of the yeast inoculum and the chemical changes in the must in respect to alcohol and sugar concentration that will occur during growth. Assume the juice has been pasteurised and that no endogenous viable yeast is present prior to inoculation.
- Short lag phase 2- 5 days.
- Followed by a stationary phase where number of viable cells may decreaseFermentative activity (Ethanol production) reaches maximum during this stage.
- Composition of wine becomes inhibitory due to rising ethanol concentrations. Yeast enters a decline phase and consumption of sugars and ethanol production and carbon dioxide is slowed
Briefly describe two mechanisms for the production of fusel alcohols by yeasts during fermentation.
- The carbon skeletons of some amino acids are catabolised and converted into higher alcohols.
- Higher alcohols may also be produced via anabolic processes originating from pyruvic acid (sugar) metabolism .
Excessive clarification of juice may arise through employment of highly efficient technology such as centrifugation. Describe what nutrients are likely to be decreased by excessive clarification, how these specific nutrients impact fermentation if not present in sufficient quantities, and what wine production strategies may be employed to correct deficient nutrient levels.
Leads to low solids content in must like sterols and fatty acids which support yeast growth.
During fermentations in which high concentrations of sulphite are present greater concentrations of glycerol can be produced by Saccharomyces cereviseae. Briefly describe the biochemical mechanisms responsible for this observation.
Glycerol is also produced by Saccharomyces cerevisiae during biomass formation at commencement of fermentation to protect the cells from the high osmolar concentrations of sugars.
Describe the mechanism that allows sulfur dioxide to enter and accumulate in a yeast cell.
The presence of sulfur dioxide during fermentation will cause increased glycerol formation as sulphites bind to acetaldehyde preventing its reduction and subsequent regeneration of NAD+. Regeneration of NAD+ occurs by glycerol formation.
Explain the mechanism by which the addition of diammonium phosphate (DAP) decreases production of hydrogen sulphide by yeasts during primary fermentation.
The mechanism explaining the non-responsive behaviour of S. cerevisiae to nitrogen supplementation relates to the activity of transport systems responsible for bringing glucose into the cell. A lack of nitrogen appears to trigger an irreversible arrest of protein synthesis in the cell, which explains why the addition of DAP after this event has little effect in stimulating sugar catabolism in stuck fermentations.
List three organic acids produced by yeasts during fermentation besides succinic acid.
Malic, Acetic, Pyruvic