Technological confirmation that low doses of medium chain fatty acids can arrest alcoholic fermentation to produce sweet wines in milder conditions

Abstract

The usual technology for sweet white wine production requires the cessation of the alcoholic fermentation before its completion by lowering the temperature in the fermentation tank under 8-10 °C, racking the wine off the lees and adding high doses of sulphur dioxide, usually 150 mg/L or more. This process is energy-consuming and can end up introducing more sulphur dioxide in the wine than in the case of dry wine production. An alternative method for stopping the alcoholic fermentation and producing wines with natural residual sugar is the treatment of the fermenting must with medium chain fatty acids (MCFAs), immediately after a racking off the lees. In this industrial trial 10 mg/l MCFAs were used, in the form of octanoic acid, decanoic acid or 1:1 mixture, respectively, in combination with a low dose of SO2 (60 mg/L). The treatments were performed at the normal temperature used in white wines for a controlled alcoholic fermentation (15 ºC), without decreasing the temperature any further. A control wine variant was also produced by applying the classical technology with temperature reduction at 8 ºC and addition of 150 mg/L SO2. All the variants were prepared in triplicate, at industrial scale, in 1000 L stainless steel tanks. The must, obtained from Tămâioasă românească grapes with an initial sugar content of 261 g/L, was inoculated with ERSA 1376 yeast. The results showed that all the treatments applied when the concentration of alcohol reached about 11.0% v/v were able to stop the fermentative process in 50 hours and produce wines with about 55-57 g/L residual sugar and a final alcohol concentration of about 11.7% v/v. The microbiological analyses carried out on solid DRBCA medium to detect the viable yeast, expressed as colony forming units (CFU/mL), showed some differences between the classical technology for sweet wine production and the alternative treatments with MCFAs. At the moment of fermentation interruption, the active yeasts population in wine was on average of 1.98 ± 0.23 x 107 CFU/mL, in all tanks. After the fermentation stopped, no viable yeasts were detected in the limpid wines above the lees, irrespective of the treatment. In the wine sediments of MCFAs-treated variants a few viable yeasts/mL were still detected, while in the lees of control wines no viable yeast was present. Anyway, racking the wines with MCFAs-stopped fermentation from the lees will prevent any possibility of refermentation. The main advantages of these alternative treatments with MCFAs are that they do not require a reduction of temperature from 15 ºC to 8 ºC and, especially, that they necessitate a much lower dose of SO2 (in this case 60 mg/L instead of 150 mg/L).