Classification of sheep category using chemical analysis and statistical classification algorithms

Peter Watkins

In Australia, dentition (eruption of permanent incisors) is used as a proxy for age to define sheep meat quality. Lamb is defined as having no permanent incisors, hogget as having at least one incisor and mutton is defined as having two or more incisors. Classification of the carcase is done at the abattoir prior to the removal of an animal’s head. Recently, an Australian Senate inquiry into meat marketing reported that there was concern that substitution of hogget and mutton for lamb may be occurring in the industry. At present, no objective method is available that can be used for classifying sheep category. The general aims of this thesis were to i) evaluate whether chemical analysis of branched chain fatty acid (BCFA) content could be used as an objective tool to determine sheep age, ii) understand the effect that some production factors had on BCFA concentrations in Australian sheep and iii) develop new approaches (whether chemical and/or statistical) for determining sheep category (age). BCFAs are implicated as the main contributors to “mutton flavour”, often associated with the cooked meat of older animals. BCFAs are reported to increase with age, which suggests that chemical analysis of these compounds could be used as an objective method. Concentrations of three BCFAs (4-methyloctanoic (MOA), 4-ethyloctanoic (EOA) and 4- methylnonanoic (MNA) acids) were measured in a survey of fat samples taken from 533 sheep carcases at abattoirs in New South Wales, Victoria and Western Australia. This thesis shows that, on its own, chemical analysis of the BCFAs is not sufficient to discriminate lamb from hogget and mutton as pre-slaughter nutrition is a significant factor in classifying sheep using this approach. Uncertainty at the BCFA concentration ranges found in Australian sheep was determined to be high making it difficult to discriminate between sheep carcases of different ages based on the BCFA level. Fast gas chromatography was evaluated as the basis for a high throughput chemical technique but was not sufficiently sensitive for BCFA measurements. Solid-phase microextraction (SPME) was also found to be suitable for sampling 3-methylindole and p-cresol, compounds responsible for diet-related “pastoral flavour” in sheep fat, but further work is needed to validate this approach for measurement of these compounds in sheep fat. Statistical classification algorithms, when applied to the chromatograms measured for the 533 carcasses, showed great promise for predicting sheep category. Specifically, the random forests algorithm, when applied to mean-centred data, gave 100% predictive accuracy when differentiating between lamb, hogget and mutton. This approach could be used for the development of an objective method for determining sheep age and category, suitable for use by the Australian sheep meat industry.

Classification of sheep category using chemical analysis and statistical classification algorithms

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