Search

Tannins are polyphenolic compounds extensively present in plants and used by food industry as processing aids. Due to the heterogeneity of plant sources, actions involved in food processing and tannin commercial costs can be different. In the last years different approaches aimed at correctly identifying the tannin botanical origin have been developed, in order to satisfy the industry's request to verify product labels. This work aimed to define the glycosidic simple phenolic profile of a large selection of monovarietal commercial tannins of different origin, using a high-resolution untargeted approach. Using accurate mass, isotopic pattern and MS/MS fragmentation, 167 precursors, 89 as monoglycosylated and 78 as diglycosylated derivatives were tentatively identified in tannins, validating the untargeted approach with 3 custom-synthesized glycosidic precursors. Almost all tannin botanical varieties were shown to be characterised by a specific glycosylated phenolic profile, providing possible tools for tannin classification in the case of glycosylphenol standard availability.

Commercial tannins from several botanical sources and with different chemical and technological characteristics are used in the food and winemaking industries. Different ways to check their botanical authenticity have been studied in the last few years, through investigation of different analytical parameters. This work proposes a new, effective approach based on the quantification of 6 carbohydrates, 7 polyalcohols, and 55 phenols. 87 tannins from 12 different botanical sources were analysed following a very simple sample preparation procedure. Using Forward Stepwise Discriminant Analysis, 3 statistical models were created based on sugars content, phenols concentration and combination of the two classes of compounds for the 8 most abundant categories (i.e. oak, grape seed, grape skin, gall, chestnut, quebracho, tea and acacia). The last approach provided good results in attributing tannins to the correct botanical origin. Validation, repeated 3 times on subsets of 10% of samples, confirmed the reliability of this model.

Grape pomace is a winemaking by-product that can be used to extract oenological tannins. Recently, some grape skin tannins were shown to contain very high amounts of two polyfunctional thiol precursors (3-S-glutathionylhexan-1-ol, 3-S-cysteinylhexan-1-ol) whose free forms are responsible for appreciated tropical-like flavours. This study shows that an oxidative treatment (no SO2) of white grape pomace and the presence of grape leaves and stems can increase the content of the above mentioned precursors. Moreover, it shows significant differences between Sauvignon Blanc, Gewuerztraminer and Mueller-Thurgau grape pomace for the 3-mercaptohexan-1-ol precursors and 4-S-cysteinyl-4-methylpentan-2-one. The grape cultivar is crucial, but the technological ability of enhancing the level of the volatile thiol precursors simply by treating the grape marc in different ways is a promising and powerful tool for the production of potentially flavouring tannins intended for food and beverage industry.

The recent finding that grape tannin may contain significant amount of S-glutathionylated (GSH-3MH) and S-cysteinylated (Cys-3MH) precursors of the varietal thiols 3-mercapto-1-hexanol and 3-mercaptohexyl acetate, characteristic of Sauvignon blanc wines, offers new opportunities for enhancing the tropical aroma in fermented beverages. In this study this new hypothesis was investigated: Muller Thurgau (17 samples) and Sauvignon blanc (15 samples) grapes were fermented with and without addition of a selected grape tannin. As expected, the tannin-added juices were higher in precursors, and they produced wines with increased free thiols. Preliminary informal sensory tests confirmed that in particular the Sauvignon wines produced with the tannin addition were often richer with increased “fruity/green” notes than the corresponding reference wines. This outcome confirms that grape tannin addition prior to fermentation can fortify the level of these compounds.

Different approaches to analysing the botanical origin of tannins have been proposed in the last fifteen years, but are generally time consuming and require the use of advanced instrumentation. This study aims to suggest an effective, easy, rapid and cheap method based on the acquisition of FT-IR spectra of 3g/L hydroalcoholic tannin solutions, overcoming possible disadvantages due to sample or particle size inhomogeneity. 114 commercial powder tannins from 7 different botanical sources (oak, chestnut, gall, quebracho, tea, grape skin and grape seed) were collected and the FT-IR spectra were acquired in the region 926-5011cm-1. Partial Least Squares regression, Discriminant Analysis and Artificial Neural Networks were applied to FT-IR spectra to investigate the possibility of differentiating the 7 botanical origins. The best results were obtained using Discriminant Analysis, with 95% correct re-classification, and 97% grouping of grape skin and seed in a single source.

The ability of cellulose acetate, cellulose acetate propionate (CAP), cellulose acetate butyrate and cellulose propionate (CP) fibres to reduce 4-ethylphenol and 4-ethylguaiacol, causing the off-flavour named “Brett character”, was studied. CAP and CP performed best in a preliminary comparative test. The former was chosen because of its more favourable FDA classification for food contact substances. CAP effectiveness was studied in relation to the reduction in volatile phenols with doses of up to 20 g/l and wine contact times of up to 60 min. Using 4 g/l, reduction of both phenols was, on average, 31–32% in defective red wines. Wine treatment affected neither colour nor total proanthocyanidins and catechins, and wines were judged to be better than the corresponding spoiled controls. CAP fibre can be regenerated by washing with ethanol or aqueous solution (pH 12), without notable changes in depletion efficiency. The technological characteristics of CAP and, to a lesser extent, CP offer interesting prospects for producing filtration beds, pads and membranes for the treatment of wines and beverages affected by “Brett character”.

Tannins are widely used in winemaking and food and beverage preparation for the many different contributions they can give to the overall characteristics of the product (e.g., colour stability, mouthfeel and aromatic composition). Varietal thiols and their precursors are one of the most interesting research areas in food science and a lot of effort has been put to further the current understanding on their formation and on the impact of different production strategies on their concentration in the final product. This paper reports the identification of two important thiol precursors (Cys-3MH and GSH-3MH) in commercial grape oenological tannins and, to the best of our knowledge, this information is reported here for the first time. This finding allows potential new perspectives in the winemaking and the food industry, offering the possibility of controlled additions of thiol precursors in pre-fermentative stages in order to tune the aroma profile of fermented products.