The technique of micro-oxygenation involves the deliberate addition of continuous, metered amounts of oxygen into a vessel of bulk wine during the maturation period (between the end of fermentation and bottling). The aim of the process is to improve the sensory properties of red wine, particularly the mouthfeel characteristics associated with the various polyphenol constituents. The success of the process appears to depend strongly on the ability to control the rate of oxygen dosage. The effect of dosage rate on the chemical and corresponding sensory changes of a red wine is the central theme of this thesis. A method of dosing oxygen (at typical micro-oxygenation rates) into small volumes of wine (<100 litres) was developed using a dense polymer membrane diffuser. It was clearly demonstrated that wine could be reliably oxygenated at very low rates using a coiled length of FEP as the diffuser material. Oxygen dosage was regulated by adjusting the oxygen pressure inside the tube. The advantage with a dense polymer diffuser is that no bubbles are generated and the oxygenation efficiency is 100%. The diffuser was fully modeled and characterised for use in the laboratory scale trials detailed in Chapters Four and Six. The small scale oxygenation equipment was used to conduct a fully replicated experiment to investigate the evolution of a Cabernet Sauvignon wine under four oxygenation treatments at dosage rates of 0, 10, 23 and 36 mg/L/mth. The total period of the trial was 105 days. HPLC analysis indicated that the rate change of low molecular weight polyphenols is directly related to the oxygen dosage rate. The concentration of the majority of the identifiable monomers, most notably the anthocyanins decreased throughout the course of the trial. The rate of decrease was directly related to oxygen dosage rate. Thiolysis results showed an increase in mDP for all treatments over the course of the trial until day 77 when they were observed to decrease for all treatments. The decrease in mDP coincided with an addition of SO2 which was investigated in a subsequent trial. Spectrophotometric results indicated that the rate of formation of non-bleachable pigments was directly related to the rate of oxygen dosage with significant differences between the high rates (23 and 36 mg/L/mth) and the low rates (0 and 10 mg/L/mth). The trend for all treatments was for increased levels of stable pigments. The sensory results show that the measured organoleptic temporal development exhibits a similar oscillatory behaviour compared to the anecdotally derived curve presented in figure 1-2. The distinction between the respective phases described in section 1.1.1 was, however less clear. The most significant factor in the model weighting was mouthfeel and astringency which correlates with the observed changes occurring in the wine polypenols during maturation. Overall the laboratory scale trial showed that the chemical polyphenol development was directly related to the oxygen dosage rate. The sensory evolution also appeared to be accelerated with higher oxygen dosage rates, although the oscillatory nature of the sensory response given a single linear input indicates a complex underlying mechanism driving the changes. The effect of SO2 on the development of wine polyphenols with and without oxygen was also investigated. The presence of SO2 was found to have a significant effect on both mDP and the concentration of non-bleachable pigments. mDP was observed to decrease over the six week trial period irrespective of whether oxygen had been added or not. The mDP for the treatments without SO2 increased steadily over the course of the trial. Similarly the formation of non-bleachable pigments was suppressed and even retarded with SO2 present whereas for the treatments without SO2 a steady increase was observed. The implication of these results is that SO2 may have a much larger effect on tannin development than oxygen. The use of electrochemical micro-oxidation (or ELMOX) was examined ostensibly to determine proof of concept and also compare the performance of glassy carbon and titanium as electrode materials against traditional micro-oxygenation. Notable transformations occurred with titanium showing higher levels of ethanal than the other treatments both chemically and by sensory measure. A greater rate of stable pigment formation was also observed for the titanium compared to the other treatments. The respective dosage rates for the glassy carbon ELMOX and traditional micro-oxygenation treatments were too low to be able to discriminate any significant differences compared to the control wine. / AGMARDT Doctoral Scholarship
| Identifer | oai:union.ndltd.org:ADTP/278410 |
| Date | January 2008 |
| Creators | Dykes, Stuart |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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