Combined effects of pulsed electric field pre-treatment and partial osmotic dehydration on air drying behaviour of red bell pepper

Pulsed electric fields (PEF) using varying field strengths (1.0, 1.5 and 2.0 kV/cm with application of 20 pulses having duration of 400 μs each) and pulse numbers (10-80) at a constant field strength of 2.0 kV/cm were applied to bell peppers as a pre-treatment to study their influence on the products' air drying kinetics. Air drying characteristics of PEF pre-treated pepper samples immersed in sucrose/sodium chloride solutions for either 30 or 60 min were also evaluated. Air drying was carried out at 60 °C in a fluidised bed dryer with an air velocity of 1 m2/s. Cell membrane permeabilisation increased with increasing field strength and higher pulse number. However, the increase became marginal after application of more than 30 pulses. Pre-treating pepper with PEF enhanced initial drying rates significantly. The initial rates of drying of all the PEF pre-treated samples (0.18-0.052 kg/kgmin) were consistently higher than the untreated ones (0.13-0.051 kg/kgmin) until average moisture content of 3.12 kg/kg in untreated and approximately 2.2 kg/kg in the treated samples (75 min of drying) beyond which the rate of losing water was faster in the untreated than the treated ones. Effective water diffusivity (Deff) values for PEF pre-treated samples pre-concentrated in osmotic solution for 30 or 60 min ranged from 0.94 to 1.36×10-9 m2/s depending on the treatment conditions while untreated samples immersed in osmotic solution for 30 or 60 min had 0.87 and 0.99×10-9 m2/s respectively. Partial osmotic dehydration before air drying resulted in minimal decrease in Deff values in all cases studied. The results also showed that the air drying of untreated, PEF pre-treated and partially osmotically PEF pre-treated pepper samples occurred in the falling rate period having two different slopes. © 2003 Elsevier Science Ltd. All rights reserved.