Oxidative stability and lipid composition of hen eggs laid in organic and conventional systems.

In the organic farming system of laying hens (Council Regulation 1804/99/CE) only the use of organic raw materials and mineral sources are allowed whereas the solvent extracted meals, synthetic supplements including aminoacids, growth promoters and antioxidants are forbidden. Moreover the Italian legislation in force does not permit to include synthetic vitamins and allied and the natural supplements have a poor stability and a reduced efficacy. These limitations might be responsible of nutrient imbalances and therefore to have a serious impact on the quality of animal products. Indeed, the reduced birds intake of vitamins, particularly those having antioxidant properties, might limit the transfer of these molecules in birds tissues or products. A study was carried out to assess the oxidative stability as well as the fatty acid composition of eggs laid by hens kept either in organic farms or in conventional systems with cages. The hens of both the rearing systems, Hyline brown, received the commercial diets normally adopted in practice respectively for organic and conventional systems. More than 1,400 eggs have been collected in 3 different phases of the hen laying cycle: at the beginning (24-32 weeks of hen age), in the middle (47-50 weeks) and at the end of the laying phase (70-73 weeks) respectively in three and two organic and conventional cage farms located in the North of Italy. On a total of 120 pools of 5 yolks each (40 pools per each laying phase) the fatty acid composition was determined by gaschromatographic techniques and the oxidative stability was analysed through the iron-ascorbate induced lipid oxidation method. The data were analysed by two-way ANOVA using the GLM procedure of the SAS (1989) with the rearing system and laying phase as main factors. The oxidative stability was lower in eggs obtained from the organic system being the concentration of malondialdehyde and of the other TBARS reactive substances of yolk significantly higher (P<0.01) in 3 out of 5 times of sample incubation. The organic eggs showed significantly higher content (P<0.05) of polyunsaturated fatty acids (79.8 vs 75.0 mg/g of yolk), particularly of ω-6 (73.8 vs 69.1 mg/g of yolk) and consequently a significantly higher ω-6:ω-3 ratio (12.3 vs 11.7; P<0.05). These differences in the fatty acid composition of eggs between organic and conventional systems are attributable to the different raw materials used in feed formulation and to the limitations in their use imposed by the organic legislation. The higher level of polyunsaturated fatty acid along with the poor concentration of vitamins with antioxidant activity in organic feeds might be responsible of the lower lipid stability of organic eggs in comparison with the conventional ones. Considering the laying phase, the yolk concentration of ω-3 fatty acids significantly decreases (P<0.01) in the eggs laid in the third phase of the laying cycle and this might explain the significantly higher oxidative stability of these eggs.