The dry matter content was determined [31 ] by drying samples in an oven at 105 °C until a constant weight was obtained (method No. 950.46B; AOAC, 1990). The crude protein content was determined by the Kjeldahl method using the Kjeltec system 1002 apparatus (Foss-Tecator AB, Höganäs, Sweden), and a conversion factor of 6.25 was used to convert total nitrogen into crude protein (method No. 981.10; AOAC, 1990). Crude fat was determined by the Soxhlet extraction method (method No. 960.39; AOAC, 1990). Ash was determined by incineration in a muffle furnace at 550 °C for 24 h (method No. 920.153; AOAC, 1990). The samples were analysed in duplicate for all analytes. The content of protein, fat and ash were expressed as the weight percentage of yolk dry matter.
The cholesterol content in yolk was determined according to the extraction method described by Zhang et al. [32 (link)] and followed by HPLC separation and analysis on Shimadzu10 A HPLC system (Shimadzu Corp., Kyoto, Japan). The data collection and evaluation were performed by using LC Solution (Shimadzu Corp., Kyoto, Japan) operating system. The analytical column was LiChrospher 100 RP-18e, 150 × 4.6 mm, 5 µm (Alltech Associates Inc., Columbia, MD, USA) with a guard column (LiChrospher 100 RP-18, 7.5 × 4.6 mm). The cholesterol content was expressed as mg/g of wet yolk weight.
The cholesterol content in yolk was determined according to the extraction method described by Zhang et al. [32 (link)] and followed by HPLC separation and analysis on Shimadzu10 A HPLC system (Shimadzu Corp., Kyoto, Japan). The data collection and evaluation were performed by using LC Solution (Shimadzu Corp., Kyoto, Japan) operating system. The analytical column was LiChrospher 100 RP-18e, 150 × 4.6 mm, 5 µm (Alltech Associates Inc., Columbia, MD, USA) with a guard column (LiChrospher 100 RP-18, 7.5 × 4.6 mm). The cholesterol content was expressed as mg/g of wet yolk weight.