Effects of processing on the distribution of pirimiphos-methyl residues in milling fractions of durum wheat

Three hundred kilograms of durum wheat of two cultivars 'primadur' (small kernel) and 'ardente' (large kernel) treated with pirimiphos-methyl (PMM) at 10 mg kg-1 were processed into fractions under standard conditions of milling to evaluate the fate and distribution of the residues in the milling fractions. Half the quantity of each variety was processed 7 d after the treatment, and the second half was stored for 127 d in controlled conditions before milling. The residue content in treated grain just before milling as well as in the final 18 milling fractions was determined by GC-MS analysis carried out on crude methanol extracts. During the 127-d storage period, nearly 65% of the PMM initial content was degraded. With the first sub-sample (7 d after treatment), the amount of residue in the processed fractions compared to the total amount in grain prior to milling was reduced by 12.3% and 26.5% for primadur and ardente varieties, respectively. With the 2nd sub-sample (127 d after treatment), the residue losses induced by the milling operations were 21.8% and 24.6%, respectively. Between 79.5% and 74.5%, respectively, of the total amount of recovered residues after milling was concentrated in the bran layers 7 d after treatment and 79.5% and 80.2% after 127 d. The translocation of the residues during milling from the bran-coat to the inner endosperm was significant in all cases: a fraction from 14% to 20% of the total residue content was drawn into the semolina. An interaction between the milling operations and the residue translocation was observed. The greater translocation observed with the ardente cultivar was related to the specific characteristics of the bran-coat structure of this durum variety. With the grain batch held for the longer term, a lower rate of translocation was observed as the level of PMM fell to about a third of that at 7 d. A scenario of the behaviour of the residues during milling in relation to processing and grain conditions was proposed for the assessment of the maximum amount of PMM residues in the premium semolina fractions. The impact for consumer's health of this residual contamination of semolina used for food purposes is discussed.