Abstract
One property of oenological tannins, oxygen reactivity, is commonly exploited in winemaking. The reactivity is mediated by the presence of catalysts (i.e., transition metals and sulfur dioxide) and protects wine against oxidation. This work compares the oxygen consumption rate (OCR) of four commercial oenological tannins (two procyanidins from grape skin and seed, an ellagitannin from oak wood and a gallotannin from gallnut) in a model wine solution and Chianti red wine. All samples were subjected to consecutive cycles of air saturation at 20 °C to increase the total level of oxygen provided. After each cycle, the oxygen level was measured by means of a non-invasive luminescent sensor glued to a transparent surface (sensor dots) until there was no further change in substrate reactivity. The OCR followed first-order kinetics, regardless of the tannin. As expected, the ellagitannin showed the fastest OCR, followed by the two from grape seeds and skins and finally the gallotannin. The total O2 consumption in the red wine was almost double that of the model solution, due to the oxidation of wine substrates. The measurement of OCR is helpful for setting up an advanced winemaking protocol that makes use of tannins to reduce the use of sulfur dioxide.
Highlights
Managing oxygen concentration is a big challenge in winemaking because the oxygen level affects wine development in terms of sensory and chemical characteristics
Due to the complexity of factors involved it is difficult to set the best level of oxygen exposure; at the current state of art, the application of consecutive cycles of oxygen enrichment/consumption is considered a suitable approach to simulate winery conditions and forced oxidation levels [28–30]
Their composition was assessed with reliable analytical protocols and the little amount of tannins compared to total polyphenolics (Table 1) highlighted the large presence of low molecular weight phenolics, which is confirmed by literature, together with the low amount of tannins in the wood [31–33]
Summary
Managing oxygen concentration is a big challenge in winemaking because the oxygen level affects wine development in terms of sensory and chemical characteristics. Moderate, controlled exposure to oxygen by means of winemaking practices like micro-oxygenation [1,2] seems to enhance wine quality during aging, while uncontrolled oxygen exposure—too much or too little—can lead to the creation of reductive off-flavors or oxidative spoilage of wine, respectively [3–6]. Sulfur dioxide is the main antioxidant used to protect wine against the detrimental influence of oxygen. Concerns about its allergenic side effects have created a search for effective alternatives [7,8], such as oenological tannins. Tannins have various health benefits, including antioxidant, antitumor, cardioprotective, anti-inflammatory and antimicrobial activity [9]. Grape tannins in particular seem to be effective dietary supplements [10]
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