Fatty acids composition of layer fat from lamb's carcasses and its firmness and colour characteristics.

Fatty acid (F.A.) composition of subcutaneous fat sampled on Lacaune lamb's carcasses was determined by gas chromatography to evaluate the relationship with firmness and colour. Four classes of firmness from very hard layer fat up to very soft and oily fat (n = 4 x 20) and seven classes of colour adjusted with Munsell's toner (n = 7 x 16) were investigated. Very hard layer fat contains more even saturated chain F.A. (35.6 % versus 27.5 %, P < 0.05; eta = 0.78) than soft and oily fat. These are mostly long chain fats: C16 and C18. In the opposite the concentration of odd and/or branched chain F.A. is higher in soft fatty tissues (10.2 % versus 5.6 %, P < 0.05; eta = 0.72). In this category, IC13 and C15 and AIC17 are the most linked eta = 0.75, eta = 0.74, eta = 0.72 respectively). The group of monoenoic F.A, is less discriminating than former ones (eta = 0.53) except for one difference between very hard fat and others (53.5 % versus 55.7 %, P < 0.05). Therefore, oleic acid is like a long chain acid with its intermediate melting point (cis- and trans-forms) and it is more present in firmest fats (46 % versus 42 %, P < 0.05). In contrast C11 = 1 and C12 = 1 and C16 = 1 are better connected with soft oily tissues (eta = 0.75, eta = 0.80, eta = 0.72 respectively) but they are present in small amounts. In this way average concentrations are well balanced between the different classes of firmness (14.6 %, 12.2 %, 10.4 %, 7.6 % from very soft and oily up to very firm fat, P < 0.001). Colour is less dependant on fatty acid composition. The strongest difference for long chain F.A. is found between clear colours (white and cream white) and brown red colour (C18 + C18 = 1:55.1 % versus 48.5 %, P < 0.01; eta = 0.53). With odd number and/or branched chain F.A. the tendency is inverted (6.8 % Versus 11.8 %; eta = 0.61). The higher moisture of light swarthy carcasses (24.5 % versus 20.4 %, P < 0.05) acid their lower fattening grade will be a consequence of post mortem autoxidation during carcass shrinkage. But first of all, fatty acid composition is poorly determined and this supposes that other sources of explanation will need to be investigated. The observations realised on 570 carcasses show that white fats are very often associated with firm layer fat and that other colours are randomly distributed over the firmness classes. This strengthens this point of view. (C) Elsevier/Inra.