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Physicochemical Properties of Starch in Extruded Rice Flours

¹USDA, ARS, Southern Regional Research Center, New Orleans, LA 70179. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. ²Corresponding author. Phone: 504-286-4332. Fax: 504-286-4419. E-mail: rkadan@srrc.ars.usda.gov.

The effects of extruding temperatures and subsequent drying conditions on X-ray diffraction patterns (XRD) and differential scanning calorimetry (DSC) of long grain (LG) and short grain (SG) rice flours were investigated. The rice flours were extruded in a twin-screw extruder at 70–120°C and 22% moisture, and either dried at room temperature, transferred to 4°C for 60 hr, or frozen and then dried at room temperature until the moisture was 10–11%. The dried materials were milled without the temperature increasing above 32°C. XRD studies were conducted on pellets made from extruded and milled flours with particle sizes of 149–248 μm; DSC studies were conducted from the same material. DSC studies showed that frozen materials retrograded more than the flours dried at room temperature. The LG and SG samples had two distinct XRD patterns. The LG gradually lost its A pattern at >100°C, while acquiring V patterns at higher temperatures. SG gradually lost its A pattern at 100°C but stayed amorphous at the higher extruding temperatures. DSC analysis showed that retrograded flours did not produce any new XRD 2θ peaks, although a difference in 2θ peak intensities between the LG and SG rice flours was observed. DSC analysis may be very sensitive in detecting changes due to drying conditions, but XRD data showed gradual changes due to processing conditions. The gradual changes in XRD pattern and DSC data suggest that physicochemical properties of the extruded rice flours can be related to functional properties.