Irradiation of Tropical Fruits and Vegetables

Experiments have been carried out on delayed ripening, through the use of cobalt-60 gamma radiation of mangoes, showing a delay of up to six days by an optimum dose of 25 krad when irradiated in air, carbon dioxide or, preferably, nitrogen atmosphere. The effect of radiation on the skin of the fruit is more prominent in terms of inhibition in chlorophyll disappearance and carotenoid formation than in ripening changes in the meat of the fruit. These studies have been extended to mangoes skin-coated with an emulsion made of an acetylated mono-glyceride preparation. Skin-coated fruits show physiological damage due to the excessive inhibition of respiration, which is offset by a spurt in respiratory activity when irradiated in air or nitrogen. An added delay of six days in ripening is achieved by combined skin-coating and irradiation. Other fruits studied include guavas, sapotas (sapodillas) and tomatoes, with all of which a delay in ripening of about five days could be effected with a dose of 20-25 krad. Fruit like chillies, bananas and oranges do not show any delay in ripening on exposure to radiation, there being slight enhanced ripening with oranges. Semi-dried bananas (40% moisture), irradiated by 0.5 Mrad, keep well for at least three months, compared with the dehydrated (10% moisture) product which, besides being highly susceptible to mould infection, possesses poor attributes of colour, flavour, retention of nutrients and reconstitutability. Suitable combinations of heat treatment and irradiation have been successfully employed for the sterilization of mangoes, guavas, sapotas and apples where, usually, canned products with better texture, flavour and retention of nutritive qualities, could be obtained with treatment at 70°C for 10 min and 400 krad. Likewise, excellent-quality canned peas are obtainable with the combined use of 800 krad and 100°C for 5 min. Orange juice could be radiation-sterilized by a combination of 400 krad and subsequent mild heat treatment at 50°C for 15 min. Sprouting could be inhibited in potatoes (10 krad) and onions (6 krad), resulting in shelf-life extension of 32 and 24 weeks, respectively, over the 8 weeks for the unirradiated controls. Basic researches relating to radiation preservation of fruits and vegetables include studies on (a) sensitization to radiation of spoilage microorganisms (Micrococcus radiodurans, Streptococcus faecalis) by previous ultrasonic treatment, (b) sensitization to heat of spoilage organisms, such as Saccharomyces cerevisiae and spores of Bacillus cereus and Bacillus subtilis by previous radiation treatment; (c) effect of irradiation on changes in the pectic constituents of fruit, like the apple; and, (d) radiation sensitivity of carotenoids in oranges. (author)