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 mus 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 degreesC in a fluidised bed dryer with an air velocity of 1 m(2)/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/kg min) were consistently higher than the untreated ones (0.13-0.051 kg/kg min) 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 (D-eff) values for PEF pre-treated samples pre-concentrated in osmotic solution for 30 or 60 min ranged from 0.94 to 1.36 x 10(-9) m(2)/s depending on the treatment conditions while untreated samples immersed in osmotic solution for 30 or 60 min had 0.87 and 0.99 x 10(-9) m(2)/s respectively. Partial osmotic dehydration before air drying resulted in minimal decrease in D-eff 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. (C) 2003 Elsevier Science Ltd. All rights reserved.