Comparison between roasting by superheated steam and by convection on changes in colour, texture and microstructure of peanut (Arachis hypogaea)

Abstract

Colour, texture (hardness and fracturability) and microstructure were compared in peanuts (Arachis hypogaea) that were roasted by superheated steam oven (A-1500V, SHARP) in superheated steam mode and convection mode (normal without steam) operated at three different temperatures (250°C, 200°C and 150°C) for durations between 5 to 45 min. Different temperatures required different heating times for the roasting to be completed. Colour was expressed in terms of brightness (L*), whiteness (WI), redness (a*) and yellowness (b*) for each combination of roasting temperature and time. We found that L* and WI values decreased with increased roasting time and temperature, while a* and b* values increased when roasting time was extended. All the colour values (L*, a*, b*, WI) were more affected during superheated steam roasting than during convection roasting. Temperature increase by superheated steam was faster than in convection; however the roasting procedure was completed within a shorter time in convention. Hardness and fracturability, which represent the texture quality of peanuts after roasting, decreased with roasting time. This trend was, however, not statistically significant (p > 0.05). More oil was viewed to be extracted during superheated steam roasting than during convection roasting in scanning electron micrograph.