Test and optimization of pomace storage conditions
In an industrial context, pomace needs to be transported and stored, before being extracted. The storage protocol to be designed should preserve polyphenols in the pomace for up to 6 months, be cost-effective, convenient for different winery type/size and ecologically acceptable.
The preservation of polyphenols, in particular tannins and anthocyanins, in pomace is challenging, because it contains yeast and bacteria that can live on the remaining sugars and degrade the valuable compounds. Moreover, at ambient temperature and aerobic conditions (exposition to oxygen) oxidation and acetic acid formation can occur, as well as the development of moulds, which in all cases will reduce the amount of qualitative compounds that could be extracted from the pomace.
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 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. However, the addition of SO2 alone could not prevent a strong reduction of the polyphenol concentration at ambient temperature. Indeed, under ambient temperature and without protection against oxidation (N2 or SO2) bacteria and yeasts were active, as measured by the reduction of sugar contents, and the transformation of alcohol in acetic acid, thus reducing the amount, quality and extractability of the valuable molecules.
Therefore in a second step, storage under aerobic (open containers) and anaerobic conditions (closed containers saturated with N2) were tested on larger size samples (700 l), inside and outside at ambient temperature. We also made the assumption that the core of the 700 l heap would be naturally protected from oxidation, thus tests have been carried out in a heap in open air.
The results from this up-scaling experiments confirmed that the best way to preserve high-added value compounds in large amount of red grape pomace 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. However, the assumption regarding the natural protection of the heap core against oxidation proved to be incorrect, as shown by the strong increase of temperature even deep in the core, indicating the occurrence of important fermentation processes.
Evaluation of pomace production volume in Switzerland
The yearly production of red grapes has been estimated to around 60'000 to and the pomaces production to around 7'000 to (fresh weight, without stems), composed of 5 to of skins and 2 to of grape seeds.
Impact of pruned shoot export on soil organic matter
The pruned shoot are also a potential source of valuable molecules to be extracted by the new green extraction methods tested in this project. The question to be answered was: would the organic matter content of the soil decreases, if these shoots were harvested for valorisation, and therefore taken out of the system? A four year experimentation was carried out in Prangins, to compare the soil organic matter content and composition (using a Rock-Eval analysis) of plots with or without restitution of shoots. No significant differences on the amount and composition of organic matter could be observed between the treatments. A Rock-Eval analysis was also carried out on soils from different vineyards in Canton de Vaud (Aigle, Vinzel). In each vineyard, half the studied plot has received pruned shoots for at least 15 years and half did not. No differences on the quantity and the quality of organic matter could be observed in the comparable vineyard plots (same location). It has been observed however that generally the organic matter analysed in the vineyards has a low degree of humification, which means that in this context, the decomposition of the pruned shoots are very low. The undecomposed pruned shoots that were not incorporated into the soil escape Rock-Eval Analysis and could also be exported out of the plot by erosion.
