Comparison of various storage conditions to preserve polyphenols in red-grape pomace

Abstract

Red grape pomace, a waste from wine production, can be valorised by extracting polyphenols, high-added value compounds used in cosmetics or oenology. For use at an industrial level, using green extraction techniques, pomace need to be stored before being processed. The aim of this study is to test various storage conditions in order to maintain high level of polyphenols over 180 days, while keeping storage cost economically interesting. In a first step, different storage conditions (ambient temperature or cooled (4°C) temperature, anaerobic (saturation with N2) or aerobic conditions, and addition of sulphur dioxide (SO2)) were compared on small samples (1 kg) packed in plastic pockets. The quality of storage was assessed by following the optical density of the pomace extract at 280 nm (DO 280 expressed as mg/l eq gallic acid), which is an indication of the amount of remaining extractable polyphenols. The Colour Intensity (CI) (DO 420/520) was also measured as an indicator for oxidation. Organic acids (e.g. malic acid), sugars (e.g. glucose) alcohol, bacteria and yeasts were also quantified as an indicator for the occurrence of fermentation processes in the stored pomace. The results show that storage at 4°C under anaerobic conditions provides the best conditions to preserve polyphenols. Only little reduction of the polyphenol concentration, as well as little oxidation were observed. The addition of SO2 alone could not prevent a strong reduction of the polyphenol concentration in samples stored at ambient temperature. The preservation of pomace under ambient temperature, without protection against oxidation (N2 or SO2) induced a high activity of bacteria and yeasts measured by the reduction of sugar contents, and the transformation of alcohol in acetic acid. Even though refrigeration is efficient against polyphenol lost, at industrial scale it is costly. Therefore in a second step, storage under aerobic and anaerobic conditions (saturation with N2) at ambient temperature in opened or close containers were tested on larger size samples (700 l) to evaluate the effect of large volumes on storage. One assumption was that the core of the heap would be protected from oxidation. The results show that temperature measured in the pomace heap at a depth of 20 cm did not differ from temperature measured at 50 cm. It was confirmed by the monitoring of the fermentation processes which were identical at the surface and deep inside the heap. The assumption regarding protection of the heap core against oxidation was thus incorrect. A strong increase of temperature during the 60 first days of storage was observed under aerobic conditions indicating the occurrence of important fermentation processes. The best way to preserve high-added value compounds in large amount of red grape pomaces is to store it in a sealed tank saturated with N2. Under these conditions the fermentation processes were very limited and the polyphenols were protected from oxidation.