WINETECH Technical Yearbook 2019

BENCHMARKING OF COMMERCIAL TANNINS AND MANNOPROTEINS

KARIEN O’KENNEDY: Winetech KEYWORDS: Commercial tannins, mannoproteins. FEBRUARY 2019

Australian researchers did an in-depth study of the compositions of 14 commercial grape- derived tannins and eight mannoproteins obtained from five different suppliers in the Australian market (Li et al ., 2018). They found significant differences in the compositions of these commercial products and cautioned winemakers that significantly different effects can be obtained during winemaking, depending on the choice of product. The use of commercial grape derived tannins (oenotannins) and mannoproteins is widespread amongst winemakers around the world. Oenotannins are added to juice or fermenting must to promote colour stability, increase tannin content in final wines, prevent oxidation or to mask wine faults. Mannoproteins are usually added to finished wines to increase mouthfeel, reduce astringency or to improve tartrate stability. Generally suppliers of commercial tannins and mannoproteins place emphasis on the

proposed effect of these products and supply only limited compositional information on these products. EXPERIMENTAL LAYOUT Tann i n produc t s were l abe l l ed sk i n , seed, or skin and seed according to the information given by the suppliers. The total tannin concentration of all products was measured using the MCPT essay. All the oenotannin products were then purified to obtain only the tannin fractions in order to determine subunit composition, degree of polymerisation (mDP) and molecular mass. These three parameters determine the origin of the tannins, i.e. skin or seed. The unpurified oenotannin products were also analysed for monomeric phenolics (tannins are polymeric structures), such as catechin, epicathechin and gallic acid, in order to determine their contribution to the total product sold in the bag (or container in the case of the two liquid oenotannins analysed).

Finally anthocyanin concentration was also measured. Both powdered and liquid mannoproteins were reconstituted in a model wine solution at 1 g/L. After hydrolysis total polysaccharide composition was determined and the released monosaccharides were quantified. Total nitrogen content was also measured. RESULTS AND DISCUSSION There was a big difference in the actual tann i n content between commerc i a l products, ranging from as low as 16% to as high as 90%. This finding is in accordance

with a previous study that found oenotannin products to contain only 12-48% of actual tannin (Harbertson et al ., 2012). The monomeric phenolic content only accounted for 1-10 %, therefore not explaining what is filling the rest of the bags. One product contained 12% anthocyanin, most likely not declared by the supplier on the packaging. All the products analysed contained tannins from skins, or seeds, or both. Several products contained what was indicated on the packaging. In some cases the analytical methods could not determine whether the tannins were from skins or seeds, indicating