Efficacy of gamma irradiation for protection against postharvest insect damage and microbial contamination of adlay

Abstract

This study examines the protective effects of gamma irradiation against postharvest insect damage and microbial contamination and its effect on the nutritive value of adlay ( Coix lacryma-jobi L.). Adlay was treated with doses of 0–20 kGy gamma irradiation and subsequently stored at ambient temperature. The number of insects, microbial quality and chemical properties of irradiated and non-irradiated adlay were evaluated immediately after treatment and after 6 months of storage. Before irradiation, total aerobic microbial counts ranged from 5.6 × 10 2 to 1.4 × 10 5 CFU g −1 and the mean total number of insects was 3.2 ± 2.1 per 100 g of polished kernel. Two kilograys was the lowest dose that provided 100% insect control. A radiation-resistant bacterium, Deinococcus radiodurans RC1, was found in 2 of 10 adlay samples. Four kilograys was a sufficient dose for Enterobacteriaceae inactivation, and 6 kGy was a sufficient dose for yeast and fungi inactivation. Twenty and 8 kGy sufficed for the inactivation of all mesophilic microbes in samples with and without D. radiodurans, separately. Moreover the moisture, ash, crude fiber, crude fat, crude protein and riboflavin content all remained constant. However, 8 kGy irradiation reduced vitamin B1 concentration by 24.2%, but did not measurably reduce the amount of amino acids, except methionine and cysteine. Fatty acid contents did not alter after 8 kGy irradiation, but changes were observed after the 6 months of storage. These changes caused by irradiation were no greater than those caused by the 6-month storage. Irradiation up to 8 kGy did not markedly increase the acid value, but did increase the peroxide value to 13% of the initial value right after irradiation. However, significant changes in acid value and peroxide value were noted after 6 months of storage both in non-irradiated and post-irradiated storage samples. Additionally, 8 kGy irradiation did not significantly change the adlay appearance. The improvement in the hygiene of this vital food source compensates for the small loss of some nutritional constituents. Hence 8 kGy of gamma irradiation can be used in cold decontamination of adlay to prolong shelf-life, to improve postharvest quality, and to reduce the risk of food-borne disease.