Technical Yearbook 2023

protected from light and either with or without the addition of 40 mg/L sulphur dioxide and/or 30 mg/L glutathione. Results and discussion Sulphur dioxide concentration: Light vs dark When stored in darkness, the wines retained their sulphur dioxide content much better than those exposed to light. After 30 days of storage, the wines exposed to light had a residual sulphur dioxide concentration of around 10 mg/L (starting concentration between 40 and 50 mg/L). When stored in darkness, the residual sulphur dioxide concentration in the wines was more than 20 mg/L after 30 days in the bottle. When protected from light, the wines only reached concentrations below 10 mg/L after 90 days in the bottle. Browning: Light vs dark The optical density at 420 nm is commonly used in the wine industry to measure yellow/brown colour development due to oxidation. Overall, a higher absorbance reading (more intense yellow/brown colour) was recorded for the wines stored in light compared to those stored in the dark. The higher optical density for the light-exposed wines was already evident seven days after bottling. In contrast, an increase in absorbance was only noticed after 30 days of storage for the wines stored in the dark. After 60 days of light exposure, the wine’s absorbance values were between 2.8 and 4.8 times higher than the values recorded at the beginning of the study. Further exposure to light (90 days) resulted in a significant decrease in the optical density of some of the wines. This decrease in the yellow/brown tint of the wine was accompanied by a visible brown precipitate at the bottom of the bottles and is likely due to the formation of insoluble phenolic condensation products. In contrast, the wines stored in darkness showed lower browning increments that, in most cases, were less than double the initial readings. It seems that light had a catalytic effect on the rate of oxidation of wines and the formation of light reflecting pigments in the yellow-brown colour range. Aroma: Light vs dark Sensory evaluation performed after 90 days of storage revealed that the light-exposed wines had a higher intensity of aroma attributes “earthy”, “solvent” and “honey” when compared to wines stored in the dark. On the other hand, wines stored in the dark had higher intensities of the sought-after attributes “herbaceous”, “citrus fruit”, “tropical fruit”, “stone fruit” and “floral”. Chemical analyses also showed higher concentrations of esters in the wines stored in the dark, while the wines exposed to light had higher concentrations of aroma compounds that typically contribute to the oxidised aroma

in wines. Interestingly, the wines exposed to light reported much higher concentrations of 1,1,6-trimethyl-1,2 dihydronaphthalene (TDN). This effect was particularly significant in the absence of sulphur dioxide and could result in increased aromatic intensities of attributes “kerosene” and “petrol” in the low sulphur dioxide light-exposed wines. The protective role of added antioxidants The effect of the added antioxidants (sulphur dioxide and glutathione) played a minor role in the study outcomes. Exposure to light was, by far, the most influential factor affecting the wines’ chemical and sensory composition and quality. Sensory analyses showed that adding sulphur dioxide was advantageous (higher intensities of favourable aroma attributes) when the wines were stored in darkness. However, no apparent differences or trends were observed between sulphur dioxide-added wines and no sulphur dioxide-added wines when the samples were exposed to light. In most cases, the addition of glutathione had no significant protective effect against oxidation when used alone or in combination with sulphur dioxide. Conclusion Protecting wines from light exposure remains key to limiting the chemical and sensory changes in wines over time. In this study, the wines protected from light and those receiving higher doses of sulphur dioxide showed fewer signs of oxidative decay compared to wines exposed to light or with less antioxidant content. However, the protective effect of the antioxidants was limited when the wines were exposed to light. Producers often choose specific packaging to increase marketability and brand name recognition. However, the packaging can play an important role in protecting the quality and integrity of the wines. For one, the type of packaging used will determine the amount and type of light reaching the wine. Packaging such as aluminium cans and Bag-in-Box ® will effectively prevent light exposure and reduce the risk of light-induced oxidation reactions. While glass bottles, on the other hand, will allow varying amounts and wavelengths of light to pass through the packaging, possibly affecting the wine. Abstract A recent study showed that exposure to light could potentially induce major changes to a wine’s composition. The addition of antioxidants resulted in favourable effects when the wine was protected from light but came up short when the wine was exposed to light.  References https://www.wineland.co.za/effect-of-light-exposure-on bottled-sauvignon-blanc-wine/

For more information, contact Carien Coetzee at carien@basicwine.co.za.

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TECHNICAL YEARBOOK 2023