Sunlight exposure caused yellowing and increased mineral content in wool
M. R. Fleet A , K. R. Millington B C and A. L. King BA South Australian Research and Development Institute, Livestock Systems, Turretfield Research Centre, Rosedale, SA 5350, Australia.
B CSIRO Materials Science and Engineering, PO Box 21, Belmont, Vic. 3216, Australia.
C Corresponding author. Email: keith.millington@csiro.au
Animal Production Science 50(4) 300-308 https://doi.org/10.1071/AN09117
Submitted: 4 September 2009 Accepted: 15 March 2010 Published: 12 May 2010
Abstract
This study determines how levels of various trace metals in wool and the colour of the fibre change as a result of sunlight exposure and treatment with chelating compounds during wool growth. Twenty-four yearling Merino sheep were clipped on the shoulders and rumps and fitted with sheep coats modified with transparent patches. Patches over the shoulder wool (one per sheep) were either polyethylene (PE) that transmits ultraviolet light or polyvinyl chloride (PVC) that excludes ultraviolet light. The rump wool on each sheep was treated either with a copper chelator treatment (kojic acid or methyl gentisate in aqueous alcohol) or aqueous alcohol only. For 12 of the sheep the rumps were exposed to sunlight through PE patches while rump wool on the other sheep was covered by the sheep coat. Wool was harvested after 11 weeks’ growth with yellowness (Y-Z) and individual mineral contents measured using the same clean wool sample. Sunlight exposure through PE patches caused a mean increase in Y-Z to 9.1 (shoulder) or 9.5–10.1 (rump) from a base level of 7.1–7.2 (shoulder) or 7.0–7.6 (rump) in wool protected by the sheep coat. In contrast, there was no significant change in Y-Z for the PVC patch (shoulder). Therefore, it appears that ultraviolet light damage caused the increased Y-Z. Most of the trace metals analysed increased in the shoulder wool exposed to sunlight but the paired differences for PVC were lower than PE. It appears that changes in fibre caused by sunlight exposure (especially ultraviolet light) facilitate adsorption of minerals from the environment, including the animal’s own suint. Application of the chelating compounds to the rump wool caused pronounced yellowing of the wool with Y-Z increase being most pronounced for kojic acid. Copper levels in the wool were reduced by kojic acid and methyl gentisate while calcium levels were increased by kojic acid and reduced by methyl gentisate. It is not clear from these findings whether minerals and copper in particular contribute to yellowing of wool. However, the different effects of sunlight and chelation on mineral contents in wool shown may well relate to alternative mechanisms of discoloration (i.e. photoyellowing versus bacterial).
Additional keywords: chelation, color, copper, trace metals, ultraviolet light.
Acknowledgements
We thank farm staff at Turretfield (SARDI) for assistance with the sheep management and field operations, Dr Trevor Mahar, Jenny Marlin and operators at AWTA Ltd (Sydney Laboratory) for the clean wool colour measurements, and Cheryl McHugh (CSIRO Minerals, Clayton) and Michelina Del Giudice (CSIRO Materials Science and Engineering, Belmont) for the mineral analysis. Drs Ian Carmichael and Forbes Brien (SARDI) provided comments on the original manuscript. This project was supported by the CRC for Sheep Industry Innovation and by Australian wool producers and the Australian Government through Australian Wool Innovation Ltd.
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