Quantitative Measurement of Extrusion-Induced Starch Fragmentation Products in Maize Flour Using Nonaqueous Automated Gel-Permeation Chromatography

Cereal Chem. 71(6):532-536 Automated gel-permeation chromatography (GPC), with application pectins with a molecular weight of 107_109, which yielded fragments of of the universal calibration concept, was used to investigate the mechanism 1041 07. Consistent with gravity-flow GPC with dimethyl sulfoxide, sepaof extrusion-induced starch fragmentation in corn. High-amylose and ration indicated that fragmentation was promoted at low temperatures. high-amylopectin corn flours were subjected to twin-screw extrusion. The However, a correlation between degree of fragmentation and specific effects of moisture content, die temperature, screw speed, mass flow rate, mechanical energy was not observed. Formulations containing high amyloand amylose-amylopectin ratio were investigated. Nonaqueous GPC, using pectin levels were most prone to fragmentation. Die temperature signifia refractive index monitor and viscometer, yielded quantitative size profiles cantly affected such textural properties as cohesiveness, springiness, gumof native and extruded starches, as well as information describing branchminess, and chewiness of the extruded flours. ing patterns of the starch. Fragmentation was most pronounced in amyloDuring extrusion, starch is subjected to high shear forces that physically cleave glycosidic linkages forming fragments of lower molecular weight. Wen et al (1990) used gravity-flow gel-permeation chromatography (GPC), with dimethyl sulfoxide (DMSO) as the mobile phase, to measure extrusion-induced starch fragmentation. The qualitative polysaccharide size distributions showed that the high molecular weight material decreased as moisture content and temperature were decreased and screw speed was raised. Timpa (1991) developed a GPC procedure to determine the molecular weight distribution (MWD) of cellulose using dimethyl acetamide/ lithium chloride (DMAC/ LiCl) for solubilization and as the mobile phase. The technique uses viscosity and refractive index detectors, and enables data to be processed using the universal calibration concept (Grubisic et al 1967). MWD is based upon calibration with well-characterized narrow distribution polystyrene standards (Timpa 1991). Wasserman and Timpa (1991) used this technique to quantify extrusion-induced starch fragmentation in corn meal extrudates. They found that the largest molecules (molecular weight values of 107 101) were prone to fragmentation, yielding fragments in the weight range of _05 107. Using a statistical design, the objective of this study was to apply nonaqueous GPC to make a comprehensive assessment of corn starch structural changes occurring as a result of twinscrew extrusion. Quantitative profiles of starch size distribution were obtained, and molecular size changes were correlated with extrusion operating conditions and textural properties. Results illustrate the applicability of nonaqueous GPC, with application of the universal calibration concept, to obtain rapid quantitative data showing processing-related modifications of polysaccharide structure. MATERIALS AND METHODS Materials Corn meal was obtained from Lauhoff Grain Co., Danville IL. Analysis data provided by suppliers indicated the proximate 'Department of Food Science, New Jersey Agricultural Experiment Station, Center for Advanced Food Technology, Cook College, Rutgers University, New Brunswick. Publication D-10544-17-93. 2 USDA-ARS, Southern Regional Research Center, New Orleans, LA. Names of companies or commercial products are given solely for the purpose of providing specific information. Their mention does not imply recommendation or endorsement by the USDA over others not mentioned. 3Author to whom correspondence should be addressed. © 1994 American Association of Cereal Chemists, Inc. 532 CEREAL CHEMISTRY composition: 12% moisture, 7% protein, 0.7% fat, 0.5% fiber, 0.4% ash, and 79.4% N-free extract. The two corn flours (National Starch, Bridgewater, NJ) evaluated in this study were a highamylose flour (42% amylose and 18% amylopectin) and a highamylopectin flour (60% amylopectin). Amylose and amylopectin from corn were obtained from Sigma Chemical Co., St. Louis, MO. Extrusion Extrusion was performed in a corotating twin-screw extruder (model ZSK-30, Werner & Pfleiderer). The extrusion conditions were selected based upon one-half fraction of a 25 factorial design (Table I), where the five factors were die temperature, screw speed, mass flow rate, moisture content, and amylose-amylopectin ratio. Due to low moisture conditions and limitations of the extruder, there were three points in the design that could not be processed. Extruder specifications and screw configuration were: barrel bore diameter, 30.9 mm; screw length, 878 mm; maximum screw diameter 30.7 mm; kneading blocks at 440 mm (45/5/14), 480 mm (45/5/14), 538 mm (45/5/20), 592 mm (45/5/28), and 620 mm (45/5/14 LH); igels at 210 mm (42/42), 336 mm (42/42); die opening, 3.0 mm. TABLE I Factorial Design for Corn Flour Total Mass Screw Die Material % Moisture Flow Rate Speed Temperature Sample (w/w) (g/min) (rpm) (O C) High Amylopectin G33 20 400 500 180 G31 20 400 250 140 G32 20 200 500 140 G34 20 200 250 180 NAa 12.5 400 500 140 G36 12.5 400 250 180 G35 12.5 200 500 180 NA 12.5 200 250 140 High Amylose G25 20 400 500 140 G27 20 400 250 180 G28 20 200 500 180 G26 20 200 250 140 G24 12.5 400 500 180 NA 12.5 400 250 140 G30 12.5 200 500 140 G29 12.5 200 250 180 aNot available due to limitations of the extruder. Specific mechanical energy (SME) was calculated for each extruded sample. SME (Whr/kg) is defined as the product of the torque (N X m) times the rotational screw speed (sec-') divided by the mass flow rate (kg/hr) (van Lengrich 1990). GPC The methodology of Timpa (1991) was applied to the corn samples, with procedures for sample preparation, GPC, data acquisition, and analysis as described in Politz et al (1994). Mark-Houwink plots, showing the relationship between intrinsic viscosity and molecular weight (Yau et al 1979), were obtained for the corn amylose and amylopectin standards for comparison with the unprocessed flours. Texture Profile Measurements Texture profile analysis (TPA) parameters (hardness, cohesiveness, springiness, gumminess, and chewiness) were determined according to the methods of Halek et al (1989) and Bourne and Comstock (1981). Measurements were taken with an Instron universal testing machine (model TM, Instron Corp., Canton, MA) as described in Politz et al (1994). Differential Scanning Calorimetry and Polarized Light Microscopy Programs were obtained using a DSC-7 Perkin Elmer differential scanning calorimeter equipped with an intercooler (Politz et al 1994). Statistical Analysis Statistical results were obtained using the SAS program (SAS 1989). Parameters used for the analysis of variance (ANOVA) included extrusion conditions, MWD, and TPA values. Simple linear regression was conducted to study the relationship between SME and the MWD. Correlation analysis was performed on the MWD and TPA values. RESULTS AND DISCUSSION Extrusion-Induced Starch Fragmentation Corn meal and two varieties of corn flour were extruded under 13 different operating conditions (Table I). Operating variables studied included moisture content, screw speed, die temperature, mass flow rate, and amylose-amylopectin ratio. GPC profiles of native corn meal were similar to those of previous analyses (Wasserman and Timpa 1991). MWD of native and extruded starches is illustrated in Figures 1 and 2. Each chromatogram provides a graphic representation of weight fraction versus logarithm of molecular weight (Yau et al 1979). Starch size in unprocessed flours ranged from 18.8 X 106 to 2.3 X 106. These values are 5-10% higher than previously conducted characterizations of corn starch using aqueous GPC with refractive index and light-scattering detectors (Takeda et al 1988). All samples were solubilized in DMAC/ LiCl for comparison of peak areas. After extrusion, extensive fragmentation, characterized by downshifts in MWD, was observed in all samples (Figs. 1 and 2). Both MWD and cumulative molecular weight plots (Fig. 3) show that the largest starch molecules (in the range of 107 to 109) were most prone to fragmentation. The loss of high molecular weight (HMW) starch was accompanied by the production of fragments in the weight range of 104 108. Data taken from the cumulative