Thesis (PhD(Agric) (Viticulture and Oenology))--University of Stellenbosch, 2006. / The winemaking process involves different complex chemical and biochemical
reactions, which include those of oxygen (O2). Oxygen can come into contact with the
wine through various winemaking procedures and can be used by the winemaker to
enhance the quality of red wine. In wine, the main substrates for oxidation are
phenolic molecules, which form quinones. These can influence the sensory
characteristics of the wine. O2 can be used in fresh must to remove oxidisable
phenolic molecules through a process called hyper-oxidation and can also be added
to fermenting must to enhance the fermentation performance of yeast. Controlled O2
additions during ageing can lead to the wine’s colour being increased and the
astringency of the wine decreased. This is due to the formation of acetaldehyde from
the oxidation of ethanol, which induces the polymerisation of tannin and anthocyanin
molecules. The addition of too much O2 to wine can, however, lead to unwanted
over-oxidation, with certain off-odours being formed. It can also enhance the growth
of unwanted spoilage microorganisms, such as Brettanomyces and acetic acid
bacteria. Although research on O2 in wine was started many years ago, many
questions still remain. These include the general effect of O2 on the sensory and
phenolic profile of red wine especially and the microbiology of wine during ageing. An
effective way of measuring oxidation, especially in red wine must also be developed.
In the first part of this study, the effects of O2 and sulfur dioxide (SO2) additions
on a strain of Brettanomyces bruxellensis (also known as Dekkera bruxellensis) and
Acetobacter pasteurianus were investigated. Epifluorescence microscopy and plating
revealed that the A. pasteurianus strain went into a viable but non-culturable state in
the wine after prolonged storage under relative anaerobic conditions. This state,
however, could be negated with successive increases in culturability by the addition
of O2, as would happen during the transfer of wine when air is introduced. The A.
pasteurianus strain was also relatively resistant to SO2, but the B. bruxellensis strain
was more sensitive to SO2. A short exposure time to molecular SO2 drastically
decreased the culturability of the B. bruxellensis strain, but bound SO2 had no effect
on the culturability or viability of either of the two types of microorganisms. Oxygen
addition to the B. bruxellensis strain also led to a drastic increase in viability and
culturability. It is thus clear that SO2 and O2 management in the cellar is of critical
importance for the winemaker to produce wines that have not been spoiled by
Brettanomyces or acetic acid bacteria. This study should contribute to the
understanding of the factors responsible for the growth and survival of
Brettanomyces and acetic acid bacteria in wine, but it should be kept in mind that
only one strain of each microorganism was used. This should be expanded in future
to include more strains that occur in wine.
The second part of this study investigated the effect of micro-oxygenation on four
different South African red wines. It was found that the micro-oxygenation led to an
increase in the colour density and SO2 resistant pigments of the two wines in which micro-oxygenation was started just after the completion of malolactic fermentation. In
one of these wines, a tasting panel preferred the micro-oxygenation treated wines to
the control. In the other two red wines, in which the micro-oxygenation was started
seven months after the completion of malolactic fermentation, very little colour
increase was observed. One of these two wines was also matured in an oak barrel,
where the change in phenolic composition was on par with the treated wines. A
prolonged period of micro-oxygenation, however, led to this wine obtaining an
oxidised, over-aged character. Micro-oxygenation and maturation in an oak barrel
also enhanced the survival of acetic acid bacteria and Brettanomyces in this wine.
Micro-oxygenation can hence be used by the wine producer on young red wines to
enhance the quality of the wine, but should be applied with care in older red wines.
Future research into micro-oxygenation should focus on whether it can simulate an
oak barrel. More research into the effect of micro-oxygenation on the sensory profile
of the wine is needed.
As mentioned, the addition of O2 can lead to oxidative degradation of wine. The
brown colour in wine is often used as an indication of oxidation, but oxidative aromas
can be perceived before a drastic increase in the brown colour has been observed in
red wine.
The third part of this study was to assess the possible use of Fourier Transform
Infrared Spectroscopy (FTIR) to measure the progression of oxidation in Pinotage red
wines. Three wines were used in this study and clear separation between the control
and aerated wines was observed by using Principle Component Analysis (PCA).
Sensory analysis of these wines confirmed this observation, with a reduction
especially in berry fruit and coffee characters and an increase first in potato skin and
then acetaldehyde aroma characters as the oxidation progressed. PCA analysis also
revealed that in certain wines the visible spectrum of light did not indicate the
progression of oxidation as sensitively as with the use of FTIR. This also correlated
with the inability of the panel to observe a drastic colour change. FTIR should be
further investigated as a possible means of monitoring oxidation in wine and this
study should be expanded to wines made from other cultivars as well.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1145 |
| Date | 03 1900 |
| Creators | Du Toit, Wessel Johannes |
| Contributors | Du Toit, M., Marais. J., Pretorius, I. S., Lonvaud-Funel, A., 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 |
| Rights | University of Stellenbosch |
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