Effects of pretreatments and moisture content on microstructure and physical properties of microwave expanded hull-less barley

•Hull-less barley can be puffed by microwave heating.•The significant changes occurred in microstructure of barley grains.•Changes in morphological features of grains were measured using image analysis.•The optimum initial moisture content needed depends on the pretreatment method.•Addition of infrared heating to preconditioning leads to improved product quality.

The aim of this study was to investigate the effects of the pretreatment methods and the initial moisture content on microstructure and physical properties of microwave expanded barley. Barley grains were first prepared by preconditioning, autoclaving and their combination with infrared heating and then expanded by microwave heating. Barley kernels were initially tempered to 14.6, 19.5 and 29.5% moisture for all treatments. SEM micrographs showed that microwave expansion caused porous structure. The degree of porosity and also the size and shape of pores varied with respect to the initial moisture content and type of the pretreatment method. Microwave puffing increased the water absorption by 220% with the pretreatment methods of preconditioning and combination of infrared heating at the initial moisture content of 29.5%. The pretreatment method and the initial moisture content had significant effect on size, expansion, bulk density, hardness and color of puffed grains. The maximum expansion with low bulk density and hardness was obtained at 29.5% moisture with the addition of infrared heating to preconditioning method, whereas the minimum expansion with high bulk density and hardness was found at 29.5% for the combination of autoclaving and infrared heating. The pretreatments, depending on the initial moisture content of grains, caused significant changes in starch structure and these changes had different impacts on the degree of gelatinization of starch molecules and expansion of grains during the microwave heating.