A Review of Thirteen Years of CTS Winery Laboratory Collaborative Data

Data from 13 years (78 wines) of wine industry laboratory proficiency testing were reviewed. After outlier removal, within-laboratory precision (repeatability) and across-laboratory precision (reproducibility) were determined for measurements of alcohol, titratable acidity, volatile acidity, total SO2, free SO2, malic acid, specific gravity, pH, residual sugar, glucose plus fructose, and absorbance at 420 and 520 nm. Reproducibility was 3.6 to 57.8 times higher than repeatability. Reproducibility was evaluated with Horwitz ratios (HorRat); only alcohol, titratable acid, and total SO2 had acceptable values (mean HorRat <2). Measurement z scores demonstrated non-normal distributions, particularly for specific gravity, likely due to confounding with density. Reproducibility did not vary significantly over time, with exceptions: imprecision of ethanol measurements decreased (improved) by 0.0084% v/v per year, while the imprecision of titratable acidity, pH, and malic acid measurements increased by 0.0089 g/L as tartaric, 0.0008 pH units, and 0.13 g malic acid/L per year, respectively. The imprecision of reproducibility and repeatability generally increased with analyte concentration, with notable exceptions for alcohol (both), volatile acidity (reproducibility), and total SO2 (repeatability). The methods or instruments used to determine alcohol, titratable acidity, free and total SO2, and volatile acidity changed significantly over time. Significant differences were observed among techniques for many analytes, which can be rationalized by attribution to well-known matrix effects manageable in a properly run method; e.g., higher apparent concentrations of alcohol by boiling point methods in high-sugar matrices. Evaluation of method accuracy was not possible due to the lack of wine reference materials with known true values. Results demonstrate the need for industry-wide improvement in analytical performance for some assays, and the potential benefit of adopting criteria guidelines for method performance.