Characterisation of liquid food colour from digital images

Figures & data

Figure 1. Picture of the developed acrylic cell.

Figure 2. Image of the apple juice sample opened in the GUI. The 20 patches selected to measure colour versus depth are shown; the initial and final patches are indicated. Predicted RGB colours of these patches are also depicted.

Table 1. L*a*b* colour values obtained from image processing and from spectrophotometer measurements.

	Image processing			Spectrophotometer
Sample	L*∞	a*∞	b*∞	L*	a*	b*
Grape juice	82.71	−14.74	32.35	94.41	−2.33	7.99
Pear juice	84.87	−5.24	27.39	99.53	−0.48	2.10
Multi-fruit juice	68.49	30.58	46.37	80.99	9.17	17.88
Apple juice	59.67	25.31	61.25	96.77	−0.81	9.69
Grapefruit energy drink	82.81	−1.42	32.70	83.58	2.49	19.34
Blue energy drink	56.45	−39.98	−26.75	74.55	−17.87	3.48
Lager beer	47.18	37.18	56.62	93.70	−2.47	21.71
White wine	69.75	11.74	59.34	98.02	−1.40	9.03
Rosé wine	22.07	29.81	11.21	61.76	35.68	33.25
Red wine	12.53	3.54	−2.89	25.02	51.07	39.94
Green cleaner	20.82	−34.58	23.93	89.58	−27.02	30.76
Violet cleaner	25.73	38.24	−24.60	79.64	10.69	−15.59
Blue cleaner	33.28	−21.40	−32.42	93.17	−4.63	−3.80

Figure 3. Colour variation versus depth for apple juice sample. Symbols: (

) L*; (

) a*; (

) b*. Dashed line represents Eq. (1) fitting.

Table 2. Colour appearance obtained from image processing (colour at infinite depth) and spectrophotometer (1 cm cell) measurements for some selected samples. Also a picture of the sample into a test tube is shown, using 10 cm liquid depth and a white background.

	Image analysis	Spectrophotometer	Picture of the sample
Grape juice
Apple juice
Blue energy drink

Figure 4. Characteristic colour of the multi-fruit juice sample, at different concentrations. Symbols: (

) L*; (

) a*; (

) b*. Patches with the predicted RGB colours are shown.

Figure 5. Characteristic colour of the blue energy drink, at different concentrations. Symbols: (

) L*; (

) a*; (

) b*. Patches with the predicted RGB colours are shown.

Table 3. Characteristic colour predicted using different background, for cool blue energy drink sample.

	Background colour
	Yellow	Green	White	Black
L*	55.89 ± 0.21^a	54.63 ± 0.26^b	55.86 ± 1.23^a	56.77 ± 1.43^c
a*	−41.54 ± 0.34^a	−39.74 ± 0.26^b	−41.29 ± 0.33^c	−39.86 ± 1.05^b
b*	−27.03 ± 0.29^a	−27.17 ± 0.96^a	−26.83 ± 0.73^b	−27.49 ± 0.82^c

Same letter in the same row indicates no significant differences (confidence level α = 0.05).

Figure 6. Effect of the background colour on liquid colour prediction. a) L*; b) a*; c) b*. Background colour: (

) yellow; (

) green; (

) white; (

) black. Dashed line represents Eq. (1) fitting.

Figure 7. Predicted colour variation with depth for the blue energy drink sample, using different background colours.

Figure 8. Total colour difference ΔE versus depth for different background colours: (

) yellow; (

) green; (

) white; (

) black. Dashed line represents Eq. (6) fitting.