Development of the challenge test for the susceptibility of wool to yellow discolouration : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Agricultural Science at Lincoln University

Abstract

Wool colour is an important characteristic determining wool price and influencing end product use. Yellow discolouration affects the production, marketing and processing of New Zealand wools. There is a need to improve the current methods of determining yellow discolouration to assist in the selection of sheep and the reduction in incidence of discolouration.

This thesis reports studies carried out to develop an accelerated Challenge test to differentiate wool on the basis of its propensity to develop yellow discolouration. Wool samples from Corriedale and Merino sheep were used to standardise and verify methodology for a challenge incubation procedure. The effects examined included sample presentation, water addition (frequency, volume and method), incubation temperature,incubation time, level of wool and sample mass in the incubator, door-opening, and drying process. The variability and precision of the original and modified Challenge colour tests were determined.

Incubation of wool samples in jars rather than petri dishes was the most significant change that resulted in an accelerated test with more consistent results. The optimum incubation time using jars was found to be 6 days, a marked reduction from the 14 day period previously used with petri dishes. An incubation temperature of 40°C resulted in higher yellowness, and allowed better differentiation of samples than 45°C, 50°C and 55°C. Development and differentiation of yellowness were optirnum when 6.2 ml of thymol saturated-water was added to the sample. A single application of water by spraying at the beginning of incubation resulted in higher yellowness and greater differentiation of wools than repeated applications. The addition of drying cycles to the incubation procedure gave no improvement in the development of colour or differentiation of samples.

As a result of this work the revised recommendations for the Challenge colour test are as follows: incubate 7 g of greasy wool in a jar of 200 cm³ capacity for 6 days at 40°C and 100% relative humidity. Add 6.2 ml of water, applied as a spray, immediately prior to incubation. The measurement of colour in incubated wool should remain in accordance with the New Zealand Standard method (NZS 8707: 1984).

The modified Challenge test procedure was faster, more efficient, and produced colour of lower variability and higher precision than the original Challenge test with better distinction between resistant and susceptible wools. The proposed procedure is a reliable method which may prove useful in the selection of sheep to reduce the degree of discolouration evident in the New Zealand wool clip.