Colour dilution factor

In many ways, colour, which is a sensation, should be assessed visually. On analogy with the treatment of odorants, Hofmann73 has taken a crucial step in this area by defining a colour dilution factor (CD) and a colour activity value (CAV). CD is the factor required for any solution of a colorant x to be diluted to its colour threshold. CAVX is the ratio of the concentration of x (jug kg-1) to its threshold concentration (jug kg-1)- The colour contribution of a component colorant to the colour of a mixture can then be defined as... 

If the readings lie outside the colour range of the comparator, the water sample to be analyzed must be diluted appropriately with distilled water and the tests repeated. The dilution factor should be taken into consideration during conversion. 

An old method of measuring the degree of jaundice which consists of assessing the intensity of the yellow colour of the serum. The serum is diluted until it matches the colour of a 1 in 10 000 solution of potassium dichromate. The dilution factor is termed the icteric index. 

Tubes containing the culture medium are inoculated with a pure or diluted sample. The analyst evaluates the number of tubes which are positive at each dilution. Positive tubes usually change colour or gas is produced in the tube. The more tubes used, the better the statistical evaluation of the contamination level will be. The number of tubes rapidly becomes a limiting factor due to workload. With miniaturised tubes (e.g. Microtiter plates developed by the Institut Pasteur uses 96 tubes for 8 dilutions) the MPN method becomes economically affordable. When all tubes for a dilution are positive the result is deduced from the next dilution. For very contaminated samples or very clean samples the MPN method becomes a real qualitative method. 

The effects of some of these preparative factors - dilution, test temperature and time, presence or absence of a black cover - on the colour values of tomato pastes have been discussed by Brimelow (1987). As already noted, MacDougall... 

Finally, an exhaustive study of some of the variables affecting the measurement of tomato paste colour has been carried out as part of the COST 90 programme by Brimelow (1987). The effects of such factors as paste dilution, test duration, instrument aperture size and illumination area, sample temperature, standardisation hitching-post tile temperature, and the presence or absence of extraneous light were all examined. A standard colour measurement procedure was advocated for this important food commodity. As a parallel effort in this work, a new European standard tomato red tile was specified by the COST 90 colour group participants, to act as a hitching post in the measurement of tomato pastes and purees. 

Prepare in advance five 50-ml cylinders, four of which contain 49 ml, and the other 50 ml, of filtered surface sea water. Immediately after taking the 25.0-ml aliquots for standardization (above) remove four 1.00 ml aliquots of the dilute standard with a 1.00-ml bulb pipette and add these to the four cylinders containing 49 ml of sea water. Mix by gently inverting each cylinder once. Immediately (within 1-2 min) continue the method as described in F.1-4 above. Finish the titrations described in H.4 above, whilst waiting for the colour to develop. Calculate the factor F (1 cm) from the expression ... 

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