Irradiation and storage influence on bioactive components and quality of early and late season ‘Rio Red’ grapefruit (Citrus paradisi Macf.)

Abstract

Irradiation as an alternative quarantine treatment has been under consideration by the International Consultative Group on Food Irradiation. This study was conducted on early and late season ‘Rio Red’ grapefruit to investigate the effects of harvest date, storage, and low dose irradiation on functional constituents, and quality. Fruit was treated with 0, 70, 200, 400 and 700 Gy and then stored under simulated storage conditions by subjecting the fruit to 10 °C for 4 weeks followed by 1 week at 20 °C with 90–95% relative humidity. Flavanones (naringin and narirutin), terpenoids (limonin 17-β-d-glycopyranoside, β-carotene and lycopene) and quality (ascorbic acid content, soluble solids (%), titratable acidity) were evaluated immediately following irradiation treatment and storage. Results demonstrated that the response of fruit to irradiation depended on harvest time. Lower doses (at or below 200 Gy) of irradiation coupled with 35 days of storage were useful in enhancing health promoting compounds in early season grapefruit. Higher doses of irradiation (400 and 700 Gy) and 35 days of storage had detrimental effects on quality of early season grapefruit, however, no significant effect was observed on the quality of the late season fruit.

Introduction

Texas grapefruit exported to international markets such as Japan or domestic markets such as Florida, California and Arizona must be certified free of Mexican fruit fly, Anastrepha ludens (Loew). Currently, methyl bromide (MB) fumigation is one of the commercial quarantine treatments used in Texas to overcome trade barriers; however, it is toxic to humans and causes damage to the stratospheric ozone layer. Although exemptions to use methyl bromide for post-harvest treatments have been granted, it is anticipated to be phased-out by the year 2010 in developed countries. The international community has also strongly sensed that an alternative treatment must be developed before the total phase out of MB. The United Nations Environmental Programme (UNEP) established the Methyl Bromide Technical Options Committee (MTOC), which has identified irradiation as being a versatile and viable alternative for quarantine treatment in the fight against fruit flies.

A low dose of gamma irradiation as a quarantine treatment against Mexican fruit fly was recently developed for citrus fruit. Recent studies in our laboratory have shown that a minimum dose of 58 or 69 Gy was suggested for disinfestations; however, depending on the level of security required in commercial scale operations, fruit could receive up to three times the minimum absorbed dose for disinfestations (Hallman and Martinez, 2001).

Treatment of grapefruit with a dose of 300 Gy resulted in minimal injury to the fruit (Spalding and Davis, 1985, Miller and McDonald, 1996). Furthermore, our studies have demonstrated that ‘Rio Red’ grapefruit exposed to irradiation doses of up to 500 Gy did not affect soluble solids (%), titratable acidity, appearance, and organoleptic quality compared to untreated fruit (Hallman and Martinez, 2001).

Type and intensity of injury to grapefruit due to low dose irradiation (300–900 Gy) has been attributed to time of harvest. Early-season grapefruit, harvested from October to December, were more susceptible to scald and less susceptible to rind breakdown, while late-season fruit were more susceptible to rind breakdown after irradiation and storage (Hatton et al., 1982). Irradiation applications to improve bioactive components in fruit and vegetables has been reviewed (Patil, 2004).

Although studies have reported the effect of low dose irradiation on grapefruit quality parameters such as soluble solids, acidity and appearance, very little information is available on the effect of low dose irradiation on health promoting compounds in grapefruit such as flavanones (naringin and narirutin), limonin, carotenoids, lycopene and Vitamin C. The present study was undertaken to examine the effects of gamma irradiation and simulated storage conditions on functional constituents and quality of grapefruit harvested in different seasons.