Viability of anti-nutritional factors and negative flavours reduction in lentil and chickpea with 50-ohm radio frequency heating system

Abstract

Pulses are good source of protein, carbohydrate, dietary fibre, vitamins, and minerals in human and animal feeds. However, pulses are also high in anti-nutritional factors (ANFs) and negative flavours (NFs). Anti-nutritional factors and negative flavours have impeded the direct application of pulse protein in the mainstream food industry, as well as the acceptability of pulses in human diets. A 50-ohm radio frequency (RF) heating system was used to research the viability of reduction of these bioactive compounds in red lentil and Kabuli chickpea at moisture content of 11% (w.b.), three RF power levels (3, 6, and 9 kW), and three end temperatures (55, 85, and 115℃). Prior to heating the samples with RF, their dielectric and thermal properties were determined to predict the behaviour of these materials during RF heating. Dielectric properties of the samples were measured with a computer-controlled precision LCR device over four moisture contents, seven temperatures, and seven frequencies. The dielectric properties of the samples increased with moisture content and temperature, but decreased as the frequency increased. Penetration depth was calculated from the measured dielectric constant (ε^') and dielectric loss factor (ε'') data, and it decreased with moisture content, temperature, and frequency. Thermal properties such as thermal conductivities (k), specific heat (cp), and densities (ρ) of the samples were determined experimentally and with predictive mechanistic model as functions of temperature and moisture content (four levels). Except for cp which was measured at a temperature range of 30 to 90℃, other properties were measured at room temperature. Specific heat of the samples increased linearly with MC and temperature, thermal conductivity increased with MC in all samples, and thermal diffusivity which was calculated from known values of k, cp, and ρ, decreased as MC levels increased. A vertical tubular-type applicator was designed to house the samples during RF heating. Temperature histories during RF heating of the samples were monitored to determine if temperature gradient exists in the applicator. Post-processing analysis of heated samples showed that in both pulses, trypsin inhibitor activities (TIA) decreased as the temperature and power level increased. Radio frequency heating caused significant reduction in lipoxygenase activities in both lentil and chickpea, while there was no effect on phytic acid in lentil for the power levels and end temperatures considered. However, there was a considerable reduction in phytic acid content in chickpea when it was heated at 7 and 9 kW powers and 115℃ end temperature. There was no raffinose in the lentil variety tested; however, there was an insignificant increment in the amount of stachyose and verbascose. The same trend was observed in chickpea where the result showed an increment in raffinose and stachyose. No verbascose was found in the chickpea variety tested. Samples were tested for colour changes after RF processing and the result showed that there were no significant colour changes to the samples. From the observations made in this study, it can be inferred that the 50-ohm RF heating system has the potential to reduce ANFs and NFs in pulses significantly.