Effect of physical attributes of Robusta coffee on phosphine residues.

Responses produced by high- and low-grade Robusta coffee of the 1989-1990 crop to phosphine (PH3) fumigation were studied. Low grade Robusta coffee (broken, holed beans with dark, rough surfaces) contained higher levels of PH3 residues (0.1820 mg/kg) than higher grade beans. PH3 residues in pre-aired and 1 day aired low grade samples were computed and found to be 0.1541 and 0.0675 mg/kg, respectively; these values were less than the determined free PH3 values (0.1820 and 0.1530 mg/kg, respectively). Computed levels of PH3 residues in pre-aired beans from higher quality coffee were greater than corresponding determined free PH3 residue values. After 24 days, both pre-aired and 1 day aired samples of low grade coffee contained no residues of PH3, compared to 27-33 days for the higher grade beans. Results indicate that low grade Robusta coffee absorbs larger amounts of PH3 during fumigation; residues are, however, desorbed faster during storage.