Low energy electron irradiation of an apple

Abstract

The viability of pathogenic organisms on the surface of fresh fruits and vegetables can be significantly reduced by low energy electron beam irradiation. The most difficult technical challenge for surface irradiation of fruits and vegetable is the need to achieve a uniform dose over the entire surface of convoluted shapes. The main goal of this research was to calculate the dose distribution produced by low energy electron irradiation of a typical complex shape, an apple, using Monte Carlo simulation. A software package, MCNP (Monte Carlo N-Particle), was used to simulate an electron beam irradiation with a 1.0, 1.5 and 2.0 MeV sources on an apple modeled by interconnecting two spheres. The apple radii were 4.4 cm (perpendicular to its axis) and 3.6 cm (parallel to its axis) and the volume was divided into small segments for the purpose of calculating energy deposition as a function of angular position and radius. Irradiation of the apple by a beam 1 cm wide and 10 cm high, propagating horizontally was simulated. The axis of the apple was vertical or tilted in 15° steps towards and against the source. The dose distribution on the apple was calculated at each orientation, while taking in consideration the time that a point at each latitude on the surface of the apple was exposed to the beam while the apple rotated one revolution about its axis at a rate of 13°/s. A combination irradiation treatment with the axis of the apple tilted 30° against and 30° towards the source irradiation position after one full revolution resulted in the most uniform dose distribution for the three energies source tested. The average dose at the surface of the front of the apple was in the range from 1.2 to 1.6 kGy and 3.6 to 4.0 kGy at the surface of the top and bottom. The most uniform dose distribution within the simulated apple was correlated with microorganism survival fractions on the surface of the apple. The simple target theory and D₁₀ values for Salmonella obtained from the literature (0.50 kGy) were used. At least two logarithmic reductions in the number of Salmonella were obtained at all positions on the surface of the apple.