Virgin-Olive-Oil Color in Relation to Sample Thickness and the Measurement Method

Color changes of virgin-olive-oil samples contained in cells with different thicknesses were analyzed. Ten different commercial virgin olive oils were measured at different sample thicknesses by two methods: conventional spectrophotometry (5.0, 10.0, and 50.0 mm path length cells), and spectroradiometry (cylindrical cells with 8.0, 11.2, 15.6, 19.6, 23.2, 27.2, 36.8 and 46.4 mm internal diameters) with samples placed on the floor of a commercial cabinet using a standard daylight source. The illumination in these two methods was different, resulting in notable color differences in the samples. Color variations of virgin olive oils depending on thickness do not follow the same trend for all samples. Neither the Bouguer-Lambert-Beer law nor the Kubelka-Munk theory provided successful color predictions in the whole range of thicknesses considered here. We can conclude that for precise and reproducible color measurements of virgin-olive-oil color, it is necessary to fix both the thickness of the sample and the illumination geometry. To achieve an easier communication between industries and/or consumers, we propose that virgin-olive-oil color be measured using a spectrophotometer with 5.0-mm path length cells, for three reasons: conventional olive-oil laboratories have spectrophotometers more often than spectroradiometers; with virgin olive oils the cleaning of 5.0 mm cells is easier than for 1.0 mm cells and it does not consume a large amount of oil; spectrophotometric signals for 5.0-mm path length cells allow reliable measurements of even the darkest virgin-olive-oil samples.