Reflectance spectrum of adipose tissue to trace grass feeding in sheep: influence of measurement site and shrinkage time after slaughter

The signature of carotenoid pigments stored in the caudal fat has been used to discriminate carcasses of lambs raised on pasture from those fed concentrates. However, the literature reports higher concentration of carotenoids in perirenal than in caudal fat, and it is likely that carotenoid concentration in fat increases with shrinkage time. Our objectives were to investigate the effects of measurement site and shrinkage time after slaughter on the reliability of the discrimination between grass- and concentrate-fed lambs. Thirty-two lambs were fed concentrate-based diets (stall, n = 16) or grazed a natural pasture in France (grass, n = 16). The concentrate supply was regulated to have similar ADG between the two treatments. Periodically lambs weighing about 35 kg were slaughtered. Reflectance spectrum and lightness (L*), redness (a*), and yellowness (b*) were measured at slaughter and after 24 h of shrinkage in the subcutaneous caudal and in the perirenal fat. The spectra were translated to have a value of reflectance at 510 nm = 0 and the integral of the translated spectrum was calculated between 450 and 510 nm (i.e., the zone of light absorption by carotenoids). The integral value of subcutaneous caudal fat at slaughter differed between stall- and grass-fed lambs (P < 0.05) but was not reliable to discriminate between feeding treatments. After 24-h shrinkage, the discrimination was better but was uncertain for 50% of the lambs. The integral value of perirenal fat at slaughter allowed us to discriminate all the lambs fed concentrates (16) from all those fed pasture (16). After 24-h shrinkage, the discrimination based on perirenal fat was partially compromised; 21.9% of the animals were not correctly discriminated. The integral value of the translated spectrum was highly (P < 0.001) correlated with the fat yellowness (b*), but the b* value was not reliable to discriminate between feeding treatments. However, plotting yellowness (b*) against redness (a*) values allowed us to discriminate 31 out of 32 lambs. Measuring reflectance spectrum of perirenal fat at slaughter allowed for complete discrimination of grass-fed from stall-fed lambs. However, because the integral values were affected by shrinkage time, measurements should be rigorously made immediately after slaughter.