| Three experiments were conducted to investigate the enrichment of dietary conjugated linoleic acid (CLA) in egg yolks, and its effects on yolk fatty acid profile, cholesterol content and egg quality. In addition, basing on the determination of stearoyl-CoA desaturase (SCD) activity and its gene mRNA abundance, the regulation mechanism of dietary CLA on saturated levels of fatty acid in yolks were also explored. In Exp. 1, adopting 2X3 double factors random blocks design, 216, 50-week-old, Brown Dwarf hens and White Leghorn hens, respectively, were fed corn-soybean meal based diets containing 0, 2.5, 5.0% CLA for 56 days to explore the effects of dietary CLA on yolk fatty acid profile, cholesterol content of different type laying hens. Four hens were placed in one cage and three cages were grouped as one replicate resulting in 6 replicates per treatment. With increased dietary CLA, increases (P < 0.01) in the concentration of myristic, palmitic and stearic acid and concurrent decreases (P < 0.01) in oleic, linoleic, linolenic and archidonic acid in egg yolk lipids were observed. Fatty acid profile was also affected by hen types. Compared with that of yolks from White Leghorn, there were much more linoleic, archidonic, and docosahexaenoic acid in yolk lipids from Brown Dwarf hens. Furthermore, cholesterol content of yolks from Brown Dwarf hens was significantly decreased by feeding 2.5% CLA (P < 0.01), which indicated that Brown Dwarf hens are much more adapted to produce health eggs. In exp. 2, adopting single factor random block design, 504, 40-week-old, Brown Dwarf hens were fed corn-soybean meal based diets containing 0, 1.0, 2.0, 3.0, 4.0, 5.0, or 6.0% CLA for 56 days to explore dietary CLA deposition in yolks and its effects on egg quality during refrigerated storage. Four hens were placed in one cage and three cages were grouped as one replicate resulting in 6 replicates per treatment either. With dietary CLA increasing, concentrations of CLA in the yolk lipids linearly increased at first. However, when the supplementation concentrations of CLA in diets was up to 5.0%, the enrichment of CLA in yolk lipids reach to peak, which was about 630 mg per egg, and continuing to increase the concentrations of CLA in diets hardly improved CLA deposition in yolk. The negative effects of CLA on flavor of yolks were observed if supplying with more than 4.0% CLA in diets, however, there was no effects of CLA on egg flavor when the supplementation concentrations of CLA in diets was controlled less than 3.0%, which indicated that the effect of dietary CLA on yolks flavor relied on its dietary concentrations. Days of storage and CLA concentration significantly (P < 0.01) increased yolk firmness. However, there was no effect of dietary CLA on egg yolk firmness were observed during the initial period (0~7 d) of egg storage (P > 0.05). We speculate that the hardening of egg yolks from CLA fed hens may be induced by alteration of vitelline membrane permeability, which results in more sodium, potassium, and magnesium moving from the albumen to the yolk during storage. These alkaline ions exchange with hydrogen ions in the yolk, which may cause yolk pH to increase and albumen pH to decrease. The change of pH induce denaturation of the protein in yolk and thereby increase yolk firmness. In exp. 3; 216, 60-week-old Brown Dwarf hens were fed corn-soybean meal baseddiets containing 0, 2.5, 5.0% CLA for 56 days to explore the regulation mechanism of dietary CLA on yolk fatty acid profile. Four hens were placed in one cage and three cages were grouped as one replicate resulting in 6 replicates per treatment. At the 56th d, six hens from per treatment were blooded, slaughtered to sample liver for the determination of fatty acid profile, SCD activity and its gene mRNA abundance. The activity of SCD and its gene expression were significantly downregulated by dietary CLA, which indicated that the inhibiting of translation from saturated to monounsaturated fatty acids may be by direct or indirect mechanism...
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