Precision and accuracy in the color specification of virgin olive oils from the bromthymol blue method

Twenty experienced observers with nondefective color vision judged 27 virgi n olive oil samples within an acceptable color range, using the bromthymol blue (BTB) method, under controlled observation conditions (daylight source with a correlated color temperature of 6500 K, and standard gray backgroun d). On the average, 44.8% of the observers agreed in their selections of th e BTB standard solution matching a given oil sample, and this percentage in creased to 88.2% considering +/- one step in the two dimensions (pH and con centration) of the BTB scale. On the average, the lowest color difference b etween oil samples and available BTB solutions was 6.6 Commission Internati onale de l'Eclairage 1976-(L*a*b*) (CIELHB) units, but this color differenc e was approximately two times greater for the color difference between oil samples and BTB solutions selected by our observers. The colors of the BTB standard solutions in the CIELAB space are not uniformly distributed, and t hus one step in pH or concentration is equivalent to CIELAB color differenc es varying in a wide range (1.7-13.5 and 1.7-26.3 CIELAB units, respectivel y). From these values, indicating low precision, accuracy, and uniformity, some suggestions are made for future improvements of the current BTB method .