OPTIMIZATION PROCEDURE OF OPEN VESSEL MICROWAVE DIGESTION FOR KJELDAHL NITROGEN DETERMINATION IN FOODS

Wet ashing conditions were optimized for a recently developed open vessel microwave digestion system for Kjeldahl nitrogen determination in various food matrices. Short digestion times in association with maximum nitrogen recovery were sought using methodology based on experimental designs and response surfaces. Microwave digestion was carried out in two complementary periods: decomposition of the organic matrix with sulphuric acid without catalyst and oxidation with hydrogen peroxide. According to the matrix type, each of these two mineralization periods was found to require one or several steps. In order to use response surface methodology, these steps were transposed as experimentally controlled factors of microwave power, time and reagent volumes. The results obtained are illustrated by time-power diagrams indicating the optimum level of each factor and the relative influence of the setting. Digestion by microwaves is shown to allow an appreciable reduction in sample preparation time compared with official methods.