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Content archived on 2024-04-16

REFERENCE TILES FOR THE CALIBRATION OF MEASUREMENT OF COLOUR OF TOMATO PASTE AND SIMILAR PRODUCTS

Objective


Subjective methods for assessing colour of tomato paste by visual comparison with colour cards are being replaced by objective methods employing colorimeters. These devices measure colour by mimicking the response of the human eye to the 3 primary colours, red, blue and green and allow any colour and shade to be expressed numerically in terms of tri-stimulus or colour values. Consultation with major users of tomato paste confirmed the need for reference tiles specifically for the calibration of colorimeters used for industrial purposes.

A batch of 1200 tiles having a colour very close to the target values and a narrow range of L, a and b values (26(+/-0.5) 32.8(+/-0.5) 14.4(+/-0.4)respectively) has been produced. Individual colour values have been assigned to each tile by one institute using an instrument whose calibration is directly traceable to a national metrology institute. The tile was shown in a study involving 52 European Laboratories to virtually eliminate calibration errors in the measurement of tomato colour.
The annual European production of tomato paste is valued at some 600 MECU, its principal usage being as an ingredient in food. Among several attributes, the colour of tomato paste is tightly specified by the purchasers who need to ensure that their food products have a consistent appearance. Increasingly, subjective methods for assessing colour of tomato paste by visual comparison with colour-cards are being replaced by objective methods employing colorimeters. These devices measure colour by mimicking the response of the human eye to the three primary colours, red, blue and green and allow any colour and shade to be expressed numerically in terms of "tri-stimulus" or colour values (e.g. Judd-Hunter L.a.b. values).
The colour sub-group of COST 90 Bis (Physical Properties of Foods) investigated sources of disagreement in measurement of food colour.
Consultation with major users of tomato paste confirmed the need for reference tiles specifically for the calibration of colorimeters used for industrial purposes.

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Ministry of Agriculture, Fisheries and Food (MAFF)
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