Identification of Factors that Influence the Level of Tannins and Polymeric Pigments in Grapes and Wines
We measured tannin in five separate lots of Pinot noir fruit destined for commercial fermentations ranging from 1088 Kg to 517 Kg. All of the values we obtained for fruit tannin are similar to values we have seen for Pinot noir fruit in our previous studies, ranging from 3.6- 4.3 mg/gFW of fruit. We collected wine samples from each of the lots at pressing and recorded volume of each lot. We measured tannin in the wine from each lot and found that the values for wine tannin were similar to values we have seen from Pinot noir press wine in the past, from 441 to 863 mg/L. We measured tannin in the pomace, and the seeds from the pomace and the gross lees associated with each lot. Nevertheless, our calculations show that we can account for only 48-71%of the tannin we measured in the fruit. Thus 29-52%of the tannin in the fruit is designated as “missing tannin”.
We used a procedure referred to as “œthe acid butanol/ferrous sulfate” method in an attempt to detect tightly bound tannin in the insoluble material from the pomace. The samples used were ones that had previously been extracted with acetone to remove all of the easily extracted non-covalently bound tannin from the pomace. This material was suspended in an aliquot of 1-butanol with HCl and ferrous sulfate and heated for the time specified in the procedure (Waterman and Mole, 1994). The pomace from all of the lots tested gave a very strong reaction in the acid butanol system. The fact that the insoluble material from the pomace gave such a high absorbance after treatment in the acid butanol procedure suggests that there may be a considerable amount of tightly or covalently bound tannin in the insoluble matrix of the pomace. At this point we cannot estimate how much tannin this might be in terms of catechin equivalents, which are the units we use for the protein precipitation analysis of tannins. Thus we cannot estimate how much of the “missing tannin” this might represent.
We made cell wall preparations from skin and mesocarp tissue from fruit at harvest by extracting the material with acetone and then grinding to a fine slurry in a ground-glass homogenizer. We also made similar preparations from the pomace and the gross tank lees after fermentation. These samples will be used to compare the tannin binding capacity of cell wall material in the fruit to the binding capacity of the insoluble matrix after fermentation. This will give information as to how the tannin binding capacity of the insoluble matrix of the berry changes during fermentation.
