Effect of the nitrogen and potassium fertilization on fatty acids composition and oxidative stability for 'Carrrasquenha' cultivar olive oil at different harvest periods - Preliminary study

'Carrasquenha' is a Portuguese olive tree cultivar widely cultivated in Alentejo for oil production. The aim of the present study is to examine the effect of nitrogen and potassium fertilization on the composition of fatty acids and oxidative stability for monovarietal olive oils obtained from fruits in different stages of ripening. We obtained the experimental material during the campaign 98/99 and harvest for the two experimental dates, respectively: November 17 to 18, and December 9 to 11. The samples of olive were collected and a small laboratory mill (Abencor) was used to prepare the olive oil samples. Classical methods of analysis were used to study the composition of olive oil: fatty acids in capillary gas chromatography, oxidative stability in 'Rancimat 619' and oxidative susceptibility by formula. The results were statistically interpreted by Factorial ANOVA, using Fisher's comparison test at 95% significance level. The factorial ANOVA for the whole sample period allows. exclusion of the null hypothesis for saturated fatty acids, at a significant level (p < 0.05) for all factors (harvests and treatments), but not for the remaining comparisons (monounsaturated and polyunsaturated acids, oxidative stability and oxidative susceptibility). From the first to the second harvest date, the composition in fatty acids suffered a significant decrease of saturated fatty acids causing an increase in the relations between unsaturated/saturated and polyunsaturated/saturated. For the comparisons between treatments, the results suggest that high levels of nitrogen fertilization had a negative influence in saturated fatty acids. The correlation analysis of the nitrogen and potassium fertilization with the values of fatty acids for the whole samples in both harvesting, revealed a significative negative level of correlation -0,344 for nitrogen fertilization with saturated fatty acids.