Tannin composition is an important attribute in red wine quality, and it is therefore critical to understand the factors influencing tannin extraction during alcoholic fermentation. Tannins contribute to the mouthfeel of wines, but they also form pigmented polymers with anthocyanins to produce the stable pigments needed for red wine’s colour stability. The total amount of extractable phenolics in grapes is distributed as follows: 10% in the pulp, 28-35% in the skins, and 60-70% in the seeds. Grape tissues are disrupted prior to vinification which therefore makes it difficult to determine the relative contribution of berry tissues (seeds, skins, and pulp) to wine tannins. The main of this work was to assess the contribution of seeds to the tannin concentration in Shiraz and Merlot red wines.
One representative sample of two thousand berries for each cultivar was taken and thoroughly mixed. Three treatments were investigated: seedless, normal seeds and 2x seeds ratio. The berries were then manually deseeded, the skins and seeds were divided by weight and juice by volume. Three different grape phenolic methods, namely Iland, Glories and Machine crush were used to test the phenolic composition of each treatment. Each treatment was then fermented in 350 mL coffee plungers which facilitated the mixing of skins and must during alcoholic fermentation. Phenolic analyses were done using spectrophotometric methods.
Significant differences were observed on the phenolic composition of the wines for the different treatments. No significant differences were observed for anthocyanins and colour density on all treatments for both cultivars. The 2x seeds treatment had higher tannin levels than the other treatments for both cultivars. Shiraz seedless and normal seeds had no significant differences for tannins but were significantly different for Merlot with seedless having lower tannin levels than normal seeds. Total phenolic index for both the Merlot and Shiraz red wines were higher for the 2x seeds than the other treatments.  Seedless was lower than the normal seeds treatment for Merlot, but there were no significant differences for Shiraz. Phloroglucinolysis analysis was done for the Shiraz and seedless had higher mDP than the other treatments. Seedless also had higher Average molecular weight than normal seeds and 2x seeds. There were no significant differences on % Gallolyl units for all treatments. 

Authors: Makalisa Asiphe1, Tudo Jose Luis Aleixandre2 and Du Toit Wessel1

1Department of Viticulture and Oenology, South African Grape and Wine Research Institute, Stellenbosch University
2Universitat Politecnica de Valencia, Instituto de Ingenieria de Alimentos para el Desarrollo (IIAD), Departamento de Tecnología de Alimentos and Stellenbosch University, South African Grape and Wine Research Institute (SAGWRI), Department of Viticulture and Oenology

*corresponding author:

Keywords: tannins, red wines, seeds, tannin composition