Refractance Window and Conduction Dryings of Mango Pulp

Abstract

Mango pulp (2 mm thickness) was dried by Refractance Window (RW) technique using Mylar sheet with water bath temperature fixed at 95 °C and the product temperature varying with time to obtain a popular Indian intermediate moisture product known as mango leather (pestil) through good manufacturing practice.  Conduction drying was the major mode for drying of mango pulp on Aluminium sheet and foil.  Drying kinetics, water activity and color change were determined and the results were compared between mango pulp dried on Mylar sheet (MS), Aluminium sheet (AS) and Aluminium foil (AF), all placed on the top surface of hot water. The recommended moisture content for intermediate moisture food like mango leather, which is 18-33% dry basis (db) was achieved within 12 minutes using RW dryer with Mylar and for conduction drying on Aluminium sheets and Aluminium foil the duration was only 10 minutes. A final moisture content of 2.5% (db) was achieved in RW drying after 35 minutes of drying for both sheets and foil, as well.  After 25 minutes of drying, water activity of the product was 0.4 with corresponding equilibrium moisture content (EMC) value of 0.25 kg water/kg dry matter at 30.2ºC. The maximum moisture diffusivity of RW dried sample on Mylar sheet was (9 ± 2) × 10 ^-9 m² s ^-1. The same values for drying on the Aluminium sheet and foil were (1.05 ± 0.25) ⤫ 10 ^-8 m² s ^-1 and (1.2 ± 0.4) ⤫ 10 ^-8 m² s ^-1, respectively.  Measurement of colour values L^*, a^* and b^* revealed that around 20 minutes time there is a distinct break in the ∆L^*, ∆a^* and ∆b^*values indicating the pulp layer going through darkening to reach mango leather properties.