Effect of Intermittent Microwave Drying on Protein and Starch Quality of Non-glutinous Sorghum

In order to clarify the influence of microwave drying on the protein and starch quality of sorghum, non-glutinous sorghum grains (cv. Longza 10) were intermittently dried in a microwave dryer. The protein content of sorghum was tested before and after drying, as well as its starch quality characteristics, such as starch content, microscopic morphology, functional groups, aging properties and gelatinization properties. The changes in differential protein expression, functional classification and metabolic pathways were also analyzed. The results showed that under the experimental conditions of this study, the total protein content of sorghum was not significantly changed after microwave drying (by up to 0.4%). In total, 85 significantly differential expressed proteins were identified, 51 of which were up-regulated while the rest were down-regulated. The differentially expressed proteins were involved in metabolic pathways, such as carbon metabolism, glycolysis/gluconeogenesis, carbon fixation in photosynthetic organisms, biosynthesis of amino acids, amino sugar and nucleotide sugar metabolism, and the tricarboxylic acid cycle (P < 0.01). The contents of total starch and amylose increased by up to 2.55% and 1.06%, respectively with increasing time of single microwave cycle. Microwave drying did not result in the formation of new functional groups in the starch but it altered the microscopic morphology of starch granules. In addition, the maximum increase in retrogradation value was 267 mPa·s, suggesting that the starch was easier to retrogradate. The phase transition temperature of the starch was not obviously changed, while the gelatinization enthalpy dropped significantly. These results will provide theoretical and data support for the industrial application of sorghum microwave drying and for the deep processing of sorghum.