LECTURE 8 LECTURE 8 THE EFFECT OF IONISING IRRADIATION ON THE PQSTHARVEST QUALITY OF VEGETABLES S, MORRIS THE EFFECT OF IONIZING IRRADIATION ON THE POSTHARVEST QUALITY OF VEGETABLES Stephen C. Morris Research Scientist, Gosford Horticultural Postharvest Laboratory, Gosford, NSW. The ultimate aim of all postharvest research and marketing is to minimize quality deterioration and present the fruits and vegetables to the consumer in as close to a freshly picked condition as possible. Postharuest deterioration of fruits and vegetables is associated with three factors. !• Physiological - internal changes associated with ripening and senescence. 2. Pathological - attacks by pathogens, fungi, bacteria and insects.. 3. Physical - mainly mechanical injury and dehydration. Currently used methods to minimize these changes can be summarized as: 1. To minimize physiological deterioration - refrigeration (almost all fruits and vegetables), control atmospheres (apples, stone fruit, bananas), growth regulators (sprouting and delay ripening), irradiation (prevent sprouting on potatoes, onions). 2. To minimize pathological deterioration - refrigeration (almost all fruits and vegetables), fungicides and sterilants (many fruits and vegetables), heat treatments (cantaloupes, citrus), growth regulators (eg: 2,4-0 against Alternaria on citrus), irradiation (strawberries), fumigants (eg: EDB against fruit fly). 3. M ijni_mi_z_e physical deteriorat ion - refrigeration (to reduce water loss), use of cartons, punnets, trays, palletization etc. (to reduce water loss and physical damage), waxing (to reduce water loss). The potential benefits of irradiation apply only to reducing physiological and pathological deterioration. Research to date has indicated irradiation does not reduce physical deterioration. In fact in most instances physical deterioration is actually increased by irradiation (Maxie and Abdel-Kader 1966). The overall benefit of an irradiation treatment is determined by balancing the benefits of any reductions in physiological and pathological deterioration against any increase in physical deterioration. It should be noted from the number of treatments listed that irradiation does not produce results that are unattainable by any other treatment, irradiation therefore is in direct competition to other currently used treatments. Its widespread adoption as a postharvest treatment must depend on superior performances with regard to product quality, economics, safety both during application and of the final product, and finally consumer acceptance. DEFINITION OF TERMS A variety of terms are used to describe the effects of irradiation, some of which are encountered elsewhere and some of which are unique. These terms I feel need defining before I proceed in order to clarify the concepts are involved. The terms in alphabetical order are: Decontamination; (Doses up to 1 mRad). The term is generally used in connection with spices (see radicidation). Disinfestation; (see Insect Disinfestation). Delayed Ripening; (up to 100 kRad). Ripening is delayed because the climateric of climateric fruit is retarded, along with subsequent senescence. Insect DisinFestation; (up to 100 kRad). The treatment of produce to kill insect larvae or prevent emergence of adult insects. Growth Inhibition; (up to 250 kRad). Describes the retarded growth that occurs in irradiated mushrooms and asparagus. Raciappertization; (greater than 2 mRad). Exposure to irradiation in sealed packaging to kill all food spoilage or disease organisms. Radicidation; (Doses generally 300-1000 kRad). Exposure to irradiation at doses necessary to kill non-sporeforming pathogens. R a d j. o P a s t e u rizatipn; A term used to describe the effect of pathogen reduction by irradiation, it includes both radicidation and radurization. Radurization; (generally 50-300 kRad). Exposure of food to irradiation at doses which reduce pathogen levels sufficiently to delay onset of spoilage. Sprout Inhibition; (up to 15 kRad). Describes the control or inhibition of sprouting which occurs after exposure to low doses or irradiation. REQUIREMENTS FOR IRRADIATION TO BECOME A PRACTICAL PROCESS Having outlined the basic causes of postharvest deterioration and the currently used methods to minimize this deterioration, I would like to summarize the points which must be demonstrated for irradiation to be adopted as a practical process with any fruit or vegetable. These points are based on those first outlined over twenty years ago by Maxie and Sommer (1964) and frequently quoted since. Food Safety 1. There must be no radiation-induced substances harmful to humans. Appearance and Quality 2. There must be no major loss in nutritional quality of the produce. 3. Radiation-induced flavours and odors must not be objectionable. 4. Appearance of the produce must be attractive and typical of the species or variety. Phytotoxicity 5. Post-irradiation susceptibility of the produce to infection by decay organisms must not be enhanced. 6. Radiation-induced changes in texture must not make the commodity excessively susceptible to impact or vibration injuries during transport. Economics 7. Irradiation treatment must not entail extensive additional handling of the commodity. 8. The refrigeration requirement for the irradiated commodity must not be excessive. 9. The cost of the process must not exceed the benefits to be gained. Safety. All research work to date with irradiated produce has not found any harmful radiation—induced products (Maxie & Sommer 1964, Moy 1983). Therefore this first criteria has been adequately demonstrated for all fruits and vegetables. Appearance and Quality There has been shown to be reductions in levels of vitamins sensitive to radiation induced oxidixation such as Vitamin C at doses of less than or equal to 120 kRad. Because these losses are minor (generally below 15%) and no major losses of other nutritional compounds occur (Beyer e_t aJU 1979, Moy 1983, Matsuyama & Umeda 1983, Maxine & Abdel-Dader 1966) therefore one can say that this criteria has also been met. Demonstration of the remaining seven criteria, however depends on the particular horticultural produce and the type of detrioration that irradiation is being used to minimize. Therefore we must examine the efficiency of irradiation as a treatment for each vegetable. Need for objective assessment of results and benefits. Before 1 examine the experimental results obtained for each crop, 1 feel it is important to state the need for objectivity and for reasoned analysis of results obtained. A quote fom Maxie and Abdel-Kader (1966) should be carefully considered: "There is, perhaps, no area in applied biology where claims for the effectiveness of a protective process have been 'as extravagant as for the irradiation of fruits. Stories in the popular press have implied that irradiated fresh fruits may be kept for as long as two years with no loss in quality. This is a biological absurdity, for it ignores the living nature of the products and the endogenous degradative processes in them". While most research workers in this field do objectively analysis and present data that includes any observed phytoxic effects, there is a significant minority of workers whose results are presented in a very partial and unscientific manner. One striking example of this is in a paper by Cooper and Salunkhe (1963). In several tables they reported that strawberries irradiated with 300 kRad resulted in 100% marketable fruit (the controls had 0% marketable fruit). However, further comments in the text invalidate these results because strawberries irradiated with 300 kRad "developed a soft, spongy texture and had a water-soaked appearance" - strawberries with these characteristics can hardly be described as "marketable" fruit. RESULTS FOR INDIVIDUAL VEGETABLES There has not been nearly as much research on the effects of irradiation on vegetables as there has been on fruits. This is largely due to' fruit being more valuable on a per unit weight basis. However, it should be noted that the only commercial irradiators ever used have been dedicated almost solely for irradiation of vegetables, specifically preventing sprouting of potatoes and onions. The two commercially operated plants are the currently functioning plant in Shiroho, Japan and the food irradiation plant in Canada that is now unfortunately no longer operating. 1 would like in the following tables to consider virtually all published papers relating to the effects of irradiation on vegetables. Unfortunately, for the sake of brevity when the much more copious literature relating to spout inhibition of potatoes and onions was considered only some of the references relating to each effect was included . Table 1. The effects of irradiation on "above ground" vegetables Response uose knao nererence TOMATOES Physiological Effects Delayed ripening of green and 186 Salunkhe et al. (1959) pink tomatoes and shelf life extension. Reduce respiration and ripening 75 Mathur (1968) when combined with IAA. Delayed ripening of 2-4 days 500 Abdel-Kader e_t al. (1968b) at breaker. Pathological Effects Reduction of some storage rots 300 Bramlage & Lipton (1965) Reduce decay, extend storage 250 Abdel-Kader et al. (1968) life (4-12dr Phytotoxicity Softening of fruit and 200-300 Bramlage & Lipton (1965) mottled colouring Mature, green and breakers 250 Abdel-Kader et al.(1968b) very sensitive to irradiation, all tomatoes softening and slight off flavour. Uneven colouring and softening 166 Salunkhe et al. (1959) Abnormal colour development