The effect of extrusion cooking on resistant starch formation in waxy and regular barley flours

Hull-less barley flours from CDC-Candle (waxy) and Phoenix (regular) were extrusion cooked under various combinations of temperature (90, 100, 120, 140 and 160 °C), moisture content (20%, 25%, 30%, 35% and 40%) and screw speed (60, 80 and 100 rpm). The effect of processing on the formation of resistant starch (RS3) in the extruded flour was determined by a technique that involved a step in which the flour were heated at 100 °C in the presence of thermostable alpha-amylase which was expected to destroy all the RS1 and RS2. Literature on resistant starch has suggested that RS3 forms only during heat treatment of moist starchy materials. In contrast, when determined by using the same methodology, the native (unprocessed) flours that is supposed to be free of RS3, showed resistant starch value of 40-60 mg/100 g. This indicated that RS1 and RS2 are not totally destroyed by the heat treatment step of the methodology used in determining RS3. Extrusion cooking did not significantly (P<0.05) alter the RS3 content of native flours, the RS3 contents generally decreased. Refrigeration at 4 °C for 24 h before oven drying of extruded flour samples slightly increased the RS3 content. An attempt was made to isolate the RS3 from the extruded flour using enzymes. The methodology involved sequential enzymatic treatments to flours with lichenase, beta-glucosidase, protease, thermostable alpha-amylase and amyloglucosidase. The RS3 content in the isolates ranged between 6-34 mg/100 g which was lower than that of the raw materials (extruded samples) before enzymatic isolation. This was unexpected but indicated that the RS3 did not concentrate, suggesting that hydrolysis of the other grain components such as beta-glucan and proteins may have exposed the RS1 and RS2, that initially escaped hydrolysis to alpha-amylase.