Kennedy
et al. highlighted that wine tannins are a very complex and
heterogeneous mixture and therefore reported that tannin
concentration values can vary considerably depending on the
analytical method applied (8). Regression analyses of the MCP
and A-H tannin assays (Figure 1) showed slopes of 0.36 and
0.32 for grape and wine samples, respectively, revealing a
systematic, almost 3-fold, difference in tannin concentration.
Importantly, this difference exists for both grape and wine tannin
and therefore cannot be due primarily to structural differences
between grape tannins and wine tannins. The intercepts of the
line seen in Figure 1 are also of relevant. The intercept of the
line seen for the grape extracts was close to zero, indicating
that although the two methods gave different tannin concentration
values, they were removing very similar amounts of tannin
material from the sample. This was not the case for wine
samples. The x-axis intercept of the line for wine samples was
relatively large, indicating that the MCP tannin assay was
removing more tannin material than the A-H tannin assay.
According to the equation of the line shown in Figure 1b, a
wine sample analyzed with the MCP tannin assay giving a result
of 802 mg/L epicatechin equivalents would have contained no
tannin material when analyzed by the A-H tannin assay