Thesis (MScAgric (Viticulture and Oenology))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: One of the most striking qualities of wine is its tart, sour taste. The sensory perception
of sourness is mainly attributed to the presence of hydrogen ions (protons) at high
concentrations. Large amounts of weak carboxylic acids (organic acids) are the main
sources of these ions within wine. Once wine enters a person's mouth, the dissociable
protons of the weak organic acids within wine are partially neutralized or, in other words,
titrated by the saliva secreted inside one's mouth. This explains why the duration and
intensity of a wine's sourness is related to its titratable acidity content. The sour taste of
wine is usually considered refreshing and it helps balance wine flavour. In fact, wines
become watery when its titratable acidity content is too low.
After alcoholic fermentation, the titratable acidity of wine will usually be less than
that of the grape juice from which was made due to ethanol-induced precipitation of
potassium bitartrate crystals and partial consumption of malic acid by fermenting
wine yeasts. Occasionally however, increases in titratable acidity are observed during
alcoholic fermentation. If wine is produced from grape juice with optimum levels of
titratable acidity, unforeseen increases in titratable acidity during alcoholic fermentation
can be detrimental to the quality of the final product.
Although the net production of malic acid by wine yeasts contributes to increases in
titratable acidity seen during grape juice fermentations, the production of succinic acid is
regarded as the primary contributor. In fact, succinic acid accounts for approximately
90% of the non-volatile acids produced during fermentation of grape juice. Between 0.5
and 1.5 g/L succinic acid is normally found in wine, but higher concentrations thereof
(up to 3.0 g/L) have been detected within certain red wines.
Acidity adjustments should preferably be carried out before the onset of alcoholic
fermentation to allow better integration of the added compound(s) and to ensure that
conditions during fermentation favour the quality and microbial stability of the final
product. In doing so unfortunately, winemakers run the risk of ending up with wines that
may taste too sour if they are unable to accurately predict and take into consideration
the amount of succinic acid produced during alcoholic fermentation. Knowledge with
regard to the factors involved in succinic acid's production by fermenting wine yeasts is
therefore required in order to manage the titratable acidity of wines more accurately.
Ever since Louis Pasteur first noticed succinic acid amongst the by-products of
alcoholic fermentation, attempts have been made to determine the metabolic pathways
and factors involved in its production by fermenting wine yeasts. Up until now however,
it remains unclear why wines sometimes end up with exceptionally high levels of
succinic acid.
For these reasons it was decided to investigate the possible causes of very high
succinic acid concentrations within wine. Due to complexity of grape juice's chemical
composition and the problems associated with sterilizing grape juice, fermentation
experiments were conducted within a chemically defined grape juice-like medium. Succinic acid production by nine different industrial wine yeast strains was studied
under various conditions with regard to the nutrient status of the synthetic grape juice,
temperature and availability of molecular oxygen during alcoholic fermentation.
The amount of succinic acid produced during alcoholic fermentation was found to
depend on the yeast strain, fermentation temperature and chemical composition of the
synthetic grape juice. Out of the nine commercial yeast strains selected for this study,
strain WE372 produced the largest amount of succinic acid in synthetic grape juice at
28°C. Strain WE372 produced significantly smaller amounts of acetic acid than the
other yeast strains of this study and very little acetic acid at 28°C, which indicated that
strain WE372 may have less acetaldehyde dehydroganase activity than the other yeast
strains of this study under the conditions tested. The effect this has on NAD: NADH
balance is the probable cause for its ability to form more glycerol, succinic and malic
acid than the other strains.
Results from our study show that succinic acid production is influenced primarily by the
metabolizable fraction of YAN, which we termed metabolically available nitrogen (MAN).
Succinic acid production by fermenting yeasts will be favoured by moderate to high
fermentation temperatures (20°C to 28°C) in grape juice with a nicotinic acid and/ or
nicotinamide deficiency, high sugar content (200 g/L to 240 g/L), moderate amounts of
metabolically available nitrogen (300 ± 50 mg/L MAN), the presence of flavonoids and
large supplies of unsaturated long-chain fatty acids. Even higher concentrations of
succinic acid were produced when oxygen was made available to fermenting yeasts by
aerating the fermenting grape juice. Fermentation temperatures below 18°C, too much
metabolizable nitrogen (> 450 mg/L MAN), very high concentrations of fermentable
sugar (> 240 g/L), lipid deficiencies and a lack of pantothenic acid, thiamine, biotin or
pyridoxine will decrease the amount of succinic acid produced fermenting yeasts. / No Afrikaans summary available.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/4228 |
| Date | 03 1900 |
| Creators | De Klerk, Jean-Louis |
| Contributors | Van Rensburg, P., University of Stellenbosch. Faculty of Agrisciences. Dept. of Viticulture and Oenology. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 156 p. : ill. |
| Rights | University of Stellenbosch |
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