e n t r a t i on s
previously reported for grape juice and must.30 −32 With the
exception of two Chardonnay samples, the isatin method gave
significantly lower proline concentrations than the HPLC
method. Similarity ranged from 31.2% to 83.1%. This was
partly attributed to interference resulting from the high sugar
concentrations, being greater than 119.5 g/L. Consistent results
were only obtained for juice samples with extremely high
proline levels (e.g., Chardonnay 1 and Chardonnay 3). These
samples required the greatest dilution prior to analysis (i.e., to
fall within the 0 −200 mg/L working range of the assay).
Through this process, the sugar content was also diluted,
thereby reducing interference. These data further highlight a
limitation of the isatin-based colorimetric method for proline
determination of grape juice.