# Fatty Acid Composition and Volatile Profile of _M. longissimus thoracic_ from Commercial Lambs
## 4 Discussion
### Relationships between Production Systems, Fatty Acids and Lipid Volatiles  
Lamb's slaughtered at weaning clustered with C12:0 and C14:0 in the same quadrant of the PCA plot. The lipid profile of adipose tissue from suckling lambs may reflect the composition of the ingested milk lipids, which contain higher proportions of C12:0 (3.99%), C14:0 (10.17%) and C16:0 (25.10%), and a lower proportion of C18:0 (8.85%) [29], compared to meat lipids. This was in agreement with [PERSON] and [PERSON] [30], who observed that fatty acid composition of tissues from milk-fed lambs was characterised by lower percentages of C18:0 and higher percentages of C14:0 and C16:0. Lamb's slaughtered at weaning or that grazed on mixed pasture, red clover or chicory were associated with higher content of PUFA and higher abundance of compounds arising from lipid oxidation, such as 2-heptanone on the top side of the PCA plot. In contrasts, the meat from the 12-month-old Merino lambs had the lowest PUFA and highest SFA, BCFA and total fatty acids content compared with meat from the other animal groups. A negative association between PUFA proportion and total lipid content has been reported in various bovine and ovine studies [31]. The PUFA proportion in meat decreases as animals become older due to the relative increase in tissue deposition of neutral lipids [32]. In the PCA plot, the PUFA were clustered in the same quadrant as 1-penten-3-ol, 1-hexanol, 2-pentanone, 2-methylbutanal, heptanal, 2-heptanone, acetone and 2-butanone (Figure 2),  
Figure 2: Principle component analysis (PCA) biplot of fatty acids content (mg/100 g) and volatile compounds identified in raw lamb meat (_Longissimus thoracis_) as affected by lamb production system. indicating that PUFA are more susceptible to oxidation [15] and contributed to the generation of these aldehydes, ketones and alcohols associated with lipid oxidation volatiles [33]. Carboxylic acids in raw meat can be derived from several pathways, including enzymatic and chemical reactions and from action of spoilage bacteria on lipids, amino acids or carbohydrates [17]. The lower abundance of acetic, propanoic, butanoic and hexanoic acid in meat from 12-month-old Merino lambs is potentially a consequence of differences in their fatty acid profile and associated peroxidation capacity. Acetic acid is present in meat that has been stored under various conditions and it has a high odour threshold of 200 mg/L [34]. Thus, although acetic acid was the acid present in greatest abundance in lamb meat in this study, it was unlikely to have a significant influence on the flavour characteristics. Butanoic acid can be formed from fermentative metabolism during aging and frozen storage in vacuum-packed meat [35], has lower odour threshold (240 mg/L in water) compared to acetic acid and is associated with rancid, sharp and cheesy odours [34]. The lower relative abundance of butanoic acid in meat from 12-month-old Merino lambs suggests lower likelihood of rancid flavour compared to 4- to 8-month-old lambs at weaning or after a period on a forage finishing diet. Pentanoic and heptanoic acids are also associated with imparting fatty, gammy, cheesy and dairy odours and are highly correlated with rancid odour scores according to a trained sensory panel [34; 36]. However, these volatiles were present at similar abundance in meat from lambs across the production systems and therefore, were unlikely to contribute to flavour differences among the types of lamb in this study. The lower abundance of hexanal and octanal in Merino lambs than that in ewe and wether lambs fed on chicory, and wether lambs fed on mixed pasture or red clover, is a potential consequence of their lower PUFA content. Straight-chain aldehydes including hexanal, octanal and nonanal are lipid oxidation products and are characterised to have green-grass and soapy odours [33]. An hexanal to nonanal ratio equal to or lower than 2.45 has been proposed as an indicator of lamb meat freshness and overall quality during storage [33].