Impact of steam pressure treatment on rough rice and its physicochemical properties

The impact of steam-pressuring and tempering (SP) on the physicochemical properties of waxy (WR), normal (NR), and high-amylose (HAR) Japonica rice were investigated. Water-soaked rough rice in a sealed pouch was retorted and incubated at a constant temperature before milling. SP increased the head rice yield, amylose content, protein content, and degree of starch gelatinization (≥85.4%). Differential scanning calorimetry and X-ray diffraction patterns revealed a distinct amylose-lipid complex for SP-NR and SP-HAR, and the degree of retrogradation decreased for all SP-rice samples. Swelling power and pasting viscosity significantly increased for SP-WR, but pasting viscosity decreased for SP-NR and SP-HAR. This result was attributed to the microstructure from SEM micrographs of SP-WR, which was more disrupted and exposed to water than NR and HAR. Reduced breakdown viscosity under acidic conditions (pH 3.0 and 4.5) indicated the acid tolerance of SP-NR and SP-HAR pastes. Overall, SP effectively altered rice flour properties, while the kernel remained intact. [Display omitted] •Rough rice varying in amylose content were steam-pressured and tempered.•Steam-pressure increased amylose and protein contents in rice flour and retarded retrogradation.•Steam-pressure disrupted the microstructure of waxy rice more than normal and high-amylose rice.•Swelling power and pasting viscosity significantly increased for steam-pressured waxy rice.•Steam-pressured normal and high-amylose rice pastes exhibited an acid-tolerant viscosity in RVA.