Maize grits with or without starchy additives (native wheat starch, distarch phosphates, soluble fiber) were extruded through a single-screw laboratory extruder Brabender KE 19/25 with a controlled barrel temperature in three zones and in a head, die diameter 4 mm, comprising a compression ratio of the screw of 2:1. The purpose was to influence resistant starch (RS) content together with improving/saving key physicochemical properties by controlling extrusion cooking variables (water addition into feed of 5-20%, screw speed 80-120 rpm, temperature, mass fraction of the additive in the dry premix 0-50%). The influences of most starchy additives were negligible. Pre-gelatinized cross-linked starch and soluble fiber nutriose influenced significantly the process pressure, where a low value exhibited high values RS. Based on the data of 48 trials, 7 mathematical optimization models were computed. High RS content worsened the expansion ratio. To obtain a RS content of 1%, it was necessary to decrease expansion ratio to 1.7 where the breaking strength was acceptable. The result of minimizing resistant starch content created an expansion ratio from 2.6 to 2.7, a breaking strength 1 N/mm 2 and a RS content (maximum) of 0.2%.

The aim of the paper was to seek suitable conditions of extrusion cooking using a laboratory single-screw extruder to increase slowly digestible (SDS) and resistant starch (RS) content in extrudates saving sensory characteristics of the fi nal product prepared from corn grits with added native wheat starch, distarch phosphates, or soluble fi bre Nutriose (Roquette, France).In addition to the composition of input mixtures, process parameters of a single-screw extruder, such as speed of screw and barrel temperatures, were changed. The highest expansion ratio (4.14) was found for temperatures 44 °C, 90 °C, 120 °C, and 140 °C, die diameter 3 mm, compression ratio 3:1, rotation speed 140 r.p.m., and dosing 15 r.p.m. The addition of native wheat starch to corn grits resulted in values of the expansion ratio ranging from 1.17-2.38.A high concentration of RS -2.4% per starch -was observed for a 20% ratio of Nutriose with a 10% addition of water. The addition of 20-50% of native wheat starch resulted in values ranging from 0.2-0.4% per starch. The highest SDS content (72.5% per starch) in extrudates was obtained for the mixture of corn grits, 20% cross-linked starch, and water (10% addition).

Smrčková P., Saglamtas M., Hofmanová T., Koláček J., Chena D., Šárka E. (2014): Effect of process parameters on slowly digestible and resistant starch content in extrudates. Czech J. Food Sci., 32: 503-508.A laboratory single-screw Kompaktextruder KE 19/25 was used at speeds 120-140 rpm of the screw with either a 2 : 1 or 3 : 1 compression ratio; the head had a 3 or 4 mm die. The temperature of the head was set at 131-144°C. The applied raw materials were maize grits, and mixtures with wheat starch, pea flour or chemically modified starch. Input mixtures differed in water addition (50 or 100 g water/kg). The highest amount of resistant starch (3.1% per total starch) in extrudate was found for mixture of pea flour and maize grits (with addition of 100 g water/kg of dry mixture), the output(extrudate)/input(mixture) ratio of the resistant starch was 45.8%. The highest SDS content was obtained for the maize grits and addition of 100 g water/kg using a die with a 3 mm diameter and a screw with a 3 : 1 compression ratio; the temperature of the head was approximately 140°C. Transportation rates: screw 140 rpm, dosing 15 rpm.