Physical and Molecular Properties of Starch Acetates Extruded with

The samples with DS of 2 required less specific mechanical energy and had higher solid density and water absorption indices than starch with DS of 3...
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Ind. Eng. Chem. Res. 1999, 38, 3892-3897

Physical and Molecular Properties of Starch Acetates Extruded with Water and Ethanol† V. D. Miladinov and M. A. Hanna* Research Associate and Director, respectively, Industrial Agricultural Products Center, University of Nebraska, Lincoln, Nebraska 68583-0730

Starch acetate with degrees of substitution of 2 and 3 was extruded with either water or ethanol in a single screw extruder. Properties of the extrudates such as unit and solid density, specific mechanical energy, water absorption and water solubility indices, and molecular degradation and degree of substitution (DS) of the starch were measured. The samples extruded with water had lower spring indices, lower water absorption, and higher solid density than acetylated starch extruded with ethanol. The samples with DS of 2 required less specific mechanical energy and had higher solid density and water absorption indices than starch with DS of 3. Both independent variables influenced unit density and water solubility indices. A slight decrease in the average molecular weight was registered as a result of the extrusion processing. Introduction Each year approximately 200 million pounds of petrochemicals are used to produce plastics. A significant portion of these plastic products are used once and then discarded. In recent years there has been increasing concern about the disposal of single-use plastics. There are many biodegradable resins currently available on the market. However, most of them are too highly priced to compete with the currently used petroleum-based products. Starch has been used successfully as a polymer in the production of a loose-fill packaging. It has good mechanical properties, readily biodegrades in soil, and sells at competitive prices. The most significant problem with the starch-based loose-fill packaging material is that it collapses when it is in contact with water or in an atmosphere with high relative humidity. One possible solution to this problem is the use of a modified starch. Acetylated starches are a good alternative. Technologies for their production have been known for more than 100 years. Acetylated starches with high degrees of substitution have not been produced commercially because of a preference for cellulose acetate. The main concerns have centered around the strength and cost of starch acetate. Currently, high amylose starch is made in large quantities at prices lower than that of cellulose. High amylose starch acetates, which has been reported to have strength comparable with that of cellulose acetate derivatives,1,2 are available.3 The properties of starch esters are frequently misunderstood because the average molecular weights of the respective polymers are rarely reported. Many of the procedures used to produce starch esters use harsh pretreatments, treatments, or isolation procedures. Therefore, the reported properties are of polymers with substantially reduced molecular weight. Commercial packaging foam materials are extruded with a volatile solvent which increases the volume of † This manuscript has been assigned Journal Series 12574, Agricultural Research Division, University of Nebraska, Lincoln, NE. * Corresponding author. E-mail: [email protected].

the extrudate by evaporating. Two solvents were chosen for the current study. Water is largely abundant, does not pose a health threat, and is inexpensive. Therefore, it is naturally the first choice as a solvent. Ethanol is a more hydrophobic solvent than water and still is fairly inexpensive. Being more similar in terms of hydrophobicity to acetylated starch, ethanol would be expected to perform better than water as a solvent. From previous reports4-6 it is known that acetylated starches with degrees of substitution (DS) of 2 to 3 are water insoluble and have good mechanical properties. Therefore, those values were chosen for this study. The objectives of this research were to evaluate acetylated starch with two different degrees of substitution as a material for loose-fill packaging and to evaluate the effectiveness of extrusion as a tool for processing acetylated starch with two different solvents by measuring selected physical properties of the extrudates and the molecular changes in the starch. Materials and Methods Materials. Amylomaize VII (70% amylose starch) was procured from American Maize Products Co. (Hammond, IN). The chemicals used for starch modification (acetic anhydride and sodium hydroxide) were purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI). Starch Modification. The starch was modified as described by Mark and Mehltretter.3 Approximately 440 g of 70% amylose starch were placed in a 5-L threenecked flask equipped with thermometer, reflux condenser, and mechanical stirrer. Acetic anhydride (1600 g) was added to the starch with stirring, followed by the addition of 88 g of 50% aqueous solution of sodium hydroxide. The temperature was kept at 128-129 °C with an infrared stirring hot plate model IR 4100 (Fisher Scientific, Pittsburgh, PA). These conditions were maintained for 1.5 and 5 h to obtain starch acetates with DS of 2 and 3, respectively. The starch was rinsed with water, and air-dried. The starch was ground using a Burr Mill (Laboratory Construction Co., Kansas City, MO) to pass it through a 5-mesh (4 mm) size screen. The starch for each extrusion was modified separately to produce three batches of each DS.

10.1021/ie990255p CCC: $18.00 © 1999 American Chemical Society Published on Web 09/11/1999

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Sample Preparation. The starch was dried before sample preparation in a Precision Mechanical Convection Oven (GCA Corp., Chicago IL) at 105 °C for 1 h and then cooled in a desiccator for 1 h to ensure it was moisture free. To 200 g of dry acetylated starch 40 g of either distilled water or ethanol were added. The sample was mixed and placed in a airtight plastic container to equilibrate for 12 h at 25 °C and then extruded. Extrusion. Extrusion processing was carried out in a single screw C. W. Brabender (Model 2003 GR-8) laboratory extruder. The barrel diameter was 19 mm with L:D ratio of 20:1, the screw had a compression ratio of 3:1, and the cylindrical die nozzle had a diameter of 3 mm. The drive system was a Plasti-Corder with controlling units type FE-2000 and FE-2000A. This system automatically controlled and recorded torque, barrel temperature, and pressure in the barrel. Temperatures of the feeding, metering, and die sections were maintained at 50, 140, and 140 °C, respectively. Screw speed was maintained at 140 rpm. The feed liquid content (ethanol or water) was 20% on a dry basis (db). The sample was fed manually as much as the extruder would process. Volatile Liquid Content. Volatile liquid content of native starch was determined by an oven-drying method.7 Approximately 5 g of sample were placed in a tared aluminum dish and weighed on a Mettler PJ 3000 balance (Mettler Instrument Corp., Highstown, NJ) with an accuracy of (0.01 g. The samples were dried in a Precision Mechanical Convection Oven (GCA Corp., Chicago IL) at 105 °C for 24 h and then cooled to ambient temperature (25 °C) in a desiccator for 1 h. The volatile liquid content of samples before extrusion did not deviate more than 0.5% db from the specified value of 20% db. Specific Mechanical Energy. Specific mechanical energy (SME) was defined as a total input of mechanical energy per unit dry weight of extrudate. It was determined as described by Bhatnagar and Hanna.8 Extrudate was collected for 30 s and dried. SME was calculated as

SME )

2×π

(60n ) × τ m ˘

where n ) screw speed (rpm), τ ) average torque required to turn the extruder screw (N - m), and m ˘ ) mass flow rate (kg/h). Differential Scanning Calorimetry (DSC). A DuPont Differential Calorimeter with a 910 Cell Base (TA Instruments, Inc., New Castle, DE) was used to study the glass transition and melting temperatures on the samples before and after the extrusion. The instrument was calibrated using indium, and the purging gas was nitrogen. Before scanning the samples were ground and dried in a desiccator at room temperature for 7 days. Approximately 20 mg of sample were sealed in an aluminum pan, equilibrated at room temperature (25 °C), and scanned from 30 to 300 °C at a heating rate of 20 °C. Solid and Unit Densities. Extrudate solid density was measured using a Multivolume Pycnometer 1305 (Micromeritics, Norcross, GA). For unit density determination, the diameters of the extrudates were measured along a 30-cm-long section. An average of 10 diameters was used. A circular cross-sectional area of the extrudate was assumed and calculated from the

average diameter. The volume of the extrudate was calculated as the product of the cross-sectional area and length. The weight of the extrudate was measured, and the unit density was calculated by dividing the extrudate volume by extrudate mass. Spring Index and Compressibility. Spring index refers to the ability of a material to recover its original shape after it has been deformed. It was determined by using an Instron Universal Testing Machine (model 5566) as described by Altieri and Lacourse.9 A flat plate (10 × 10 cm) was used to compress five 5-cm-long pieces of extrudate at once to deform them to 80% of their original diameter at a loading rate of 1 cm/min. After releasing the load from the first compression, the sample was recompressed after 1 min. The recovery of the sample was determined by dividing the recompression force after 1 min by the original force of compression and was reported as a coefficient of 0 to 1. The force required for the first compression divided by the sample density was reported as compressibility. Each sample’s compressibility and spring index was measured five times and reported as an average of the five readings. Degree of Substitution. DS indicates the average number of substitutions per anhydroglucose unit in starch. The highest possible DS is 3 because there are three OH groups available per anhydroglucose unit. The DS of esterified starch was determined by hydrolyzing substituted groups with 1 N NaOH and then titrating back with 0.5 N HCl to the original pH before the NaOH addition. A 2-g sample was placed in a 250-mL conical flask covered with 25 mL of distilled water. The mixture was conditioned in a Tecator 1024 shaking water bath (Ho¨gana¨s, Sweden) for 1 h at 30 °C, and then the pH of the mixture was measured. To each flask, 10 mL of 1 N NaOH were added. The sample was then conditioned for 48 h at 50 °C to hydrolyze the fatty acid substitutes. Then the samples were titrated with 0.5 N HCl to the original pH. DS was calculated as

DS )

MFA × MWAN W - MFA × (MWFA - MWH2O)

where DS ) degree of substitution; W ) weight of the sample (g); MFA ) moles of titrated fatty acid; MWFA ) molecular weight of the fatty acid; MWH2O ) molecular weight of water (18); MWAN ) molecular weight of an anhydroglucose unit (162). Size Exclusion Chromatography. Molecular degradation was traced by size exclusion chromatography. The sample (0.1000 g) was placed in a test tube, covered with 1 mL of 1 N NaOH, and conditioned for 48 h in a water bath at 50 °C. To each test tube, 1 mL of 1 N HCl and 18 mL of dimethyl sulfoxide were added. The sample was conditioned for another hour under the same conditions and then filtered through a 5-µm membrane (Altech, Deerfield, IL). Then 20 µL of the solution was injected into a Shodex KS-806 HPLC column. A refractive index detector was used. The mobile phase was water, and the flow rate was 1 mL/ min. Molecular weight of the fraction soluble in water was monitored by using a procedure similar to the one described above. A 2-g ground sample was covered with 10 mL of water and conditioned for 24 h at 50 °C. Then it was centrifuged and filtered. The filtrate (20 µL) was injected into a Shodex KS-806 HPLC column. The molecular weight of the sample soluble in tetrahydro-

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Table 1. Physical Properties of Extruded Acetylated Starch sample extruded with

DS

unit density g/cm3

SME, Wh/kg

compressibility, kN‚kg-1‚m-3

spring index

ethanol ethanol water water

2 3 2 3

0.062 ( 0.015 b 0.098 ( 0.042 b 0.224 ( 0.133 b 0.909 ( 0.188 a

53.82 ( 21.32 b 82.35 ( 7.59 ab 67.97 ( 27.12 ab 96.59 ( 7.59 a

40.42 ( 22.63 a 51.66 ( 24.92 a 28.66 ( 12.68 a 28.88 ( 6.257 a

0.964 ( 0.047 a 0.952 ( 0.018 a 0.744 ( 0.121 b 0.797 ( 0.081 b

a

a, b: the data in the same column followed by the same letter are not significantly different.

furan was monitored in a similar fashion. The sample (0.0300 g) was dissolved in 3 mL of tetrahydrofuran, centrifuged at 10 000g for 5 min, and filtered through a 5-µm membrane (Altech, Deerfield, IL). Then 20 µL of the solution was injected into a Jordi DVB 500 Å HPLC column. A refractive index detector was used. The mobile phase was tetrahydrofuran, and the flow rate was 1 mL/min. Water Absorption and Water Solubility Indices. Water absorption index (WAI) is the weight of gel obtained per gram of dry sample. It was determined by the method of Anderson et al.10 Water solubility index (WSI) is the amount of starch which stays permanently in the water phase when starch is submerged in water. It was measured as total carbohydrate in solution using the phenol sulfuric acid method of Dubois et al.11 Statistical Analysis. A randomized, complete block experimental design with full factorial treatment design was used. The experiment was replicated three times with a different day of extrusion representing a replicate. For each replication, acetylated starch of the same batch was used. Independent variables were DS and type of solvent. SAS Institute12 software was used to analyze the data for possible relationships and interaction between DS, type of solvent, and physical properties of extrudates using analysis of variance (ANOVA) and lease significant difference (LSD) with significance established at p e 0.05. The reported data are means of three readings. Results and Discussion Specific Mechanical Energy. SME requirements (Table 1) is an easy-to-monitor, real-time indicator of the processes inside the extruder. The dissipated mechanical energy in the extruder converts mostly into thermal energy but some of it is used to break or create new covalent bonds in the extrudates. Changes in the chemical structure of the polymer, especially reduction in the average molecular weight, during extrusion ordinarily are not desirable. Lower molecular weight polymers have less strength and lower melting points, which limits their application. Reducing the amount of SME is important not only to reduce the processing cost but to preserve the original molecular weight. The type of the solvent did not influence (P > F 0.18) the SME reqirements. The DS had a statistically significant impact (P > F 0.02) on the SME requirements. The samples with DS of 2 required less energy to extrude. The SME in the single-screw extruder depended on the barrel friction and the viscosity of the material.13 Viscosity of the sample depended on its molecular weight and intermolecular interactions. The fully substituted starch molecules (DS of 3) interacted via hydrophobic interactions, whereas the DS 2 starch had a significant number of hydroxy groups. The availability of hydroxy groups facilitated their participation in hydrogen bondings when they came in close proximity. However, a starch molecule is quite rigid and formation of a conformation whereby the unsubstituted

Figure 1. Differential scanning calorimetry thermographs of acetylated starch before and after extrusion. 2, nonextruded starch acetate DS 2; 2a, starch acetate DS 2, extruded with alcohol; 2w, starch acetate DS 3, extruded with alcohol; 3, nonextruded starch acetate DS 3; 3a, starch acetate DS 2, extruded with water; 3w, starch acetate DS 3, extruded with water.

hydroxy groups are in close proximity is not very likely. It is much more likely that the hydroxy groups had hydrophobic groups in their immediate vicinity. In that case, the polar hydroxy groups would have a disrupting effect on the hydrophobic interactions. The decrease in intermolecular interactions resulted in lower viscosity. Differential Scanning Calorimetry. The thermographs obtained for extruded and nonextruded samples are illustrated on Figure 1. The samples had glass transition temperatures (Tg) in the range of 163 to 175 °C and melting temperatures (Tm) in the range of 249 to 276 °C. Very broad ranges of Tm are published for high DS starch acetates, depending on the method of obtaining the acetates, the DS and the origin of the starch. The melting temperature of acetylated starch with DS of 2.5 to 3.0 was reported to be in the range of 195 to 292 °C.14,15 For the starch with DS of 2 the Tg and Tm decreased after extrusion. For samples with DS of 3 a decrease of Tg was registered after extrusion. The Tm did not change when the sample was extruded with water and slightly increased when the sample was extruded with ethanol. The samples with DS of 2 started thermal decomposition characterized with steep exotherm at 265 to 280 °C, whereas the samples with DS of 3 decomposed at 280 to 295 °C. The increase in crystallinity after the extrusion was more pronounced in the samples with DS of 3 and the samples extruded with alcohol. Most likely, the starch acetate molecules realigned during the extrusion, facilitating crystal formation. The higher crystal formation energy of the samples with DS of 3 can be attributed to the more uniform structure of the starch acetate molecules.

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Unit Densities. Unit density is one of the most important characteristics of a loose-fill packaging. Low unit density is very desirable, because it results in lower manufacturing costs due to reduced polymer cost and more consumer appeal due to reduced freight costs. There were statistically significant interactions (P > F 0.0014) between both type of solvent and DS. The sample with the highest unit density was the one with DS of 3 extruded with water (Table 1). The water had a higher boiling point than the ethanol. At a given temperature, water would generate steam with lower pressure. Upon leaving the extruder the extrudate would be expected to expand less. In addition, the water was a more hydrophilic solvent, whereas the acetylated starch was hydrophobic. Thus, the water was not expected to mix intimately with the starch but to form pockets of water inside the extrudate. Upon leaving the extruder these water pockets evaporated more quickly forming channels in the extrudate instead of forming foam. Compressibility. Compressibility of the loose-fill packaging material is a significant characteristic, because it describes the cushioning ability of a foam. There was no statistically significant difference either between the different DS samples (P > F 0.584) or between the samples extruded with different solvents (P > F 0.12). The moisture content of the samples when compressibility and spring indices were determined was within the 3-5% db range. Spring Index. The spring index is an indirect measure of the ability of materials to absorb energy. Packaging material preferably should have a higher spring index. There was no difference in the spring indices of the extrudates with different DS (P > F 0.64). However, there was a strong main effect of type of solvent used (P > F 0.02). The samples extruded with alcohol had higher spring indices. Lower spring indices (more fragile samples) can be explained by the more hydrophilic solvent forming a two-phase system. As a result, the extrudate cell walls would have numerous channels, compromising the structural intergity of the extrudates. Degree of Substitution. There was no statistically significant (P > F 0.15) loss of substitutions. Apparently the extrusion conditions were not severe enough to cause noticeable hydrolysis of the acetic acid residues. The starch modification was carried out to impart water resistance in the starch foam. Significant loss of the substitutes would make the starch sensitive to water. Therefore, no significant loss of the substitutes was desirable. Size Exclusion Chromatography. The true molecular weight (MW) distribution pattern of the extruded samples was difficult to establish. No suitable solvent was found to dissolve the acetylated starch. Organic solvents such as ethyl acetate, tetrahydrofuran, and chlorinated organic solvents formed a colloid mixture with acetylated starch with DS of 3 with very little particulate matter left (50%) of the starch with DS of 2 did not dissolve in the above-mentioned solvents. After injection, similar graphs were obtained for all samples (Figure 2). No signal was detected in the amylopectin region (molecular weight, 1 to 2 million). As can be seen in Figure 3, an amylopectin fraction was present in the samples. The absence of the amylopectin fraction on Figure 2 can be attributed either to insolubility of the high molecular

Figure 2. Size exclusion chromatography of extrudates in tetrahydrofuran. 2, nonextruded starch acetate DS 2; 2a, starch acetate DS 2, extruded with alcohol; 2w, starch acetate DS 3, extruded with alcohol; 3, nonextruded starch acetate DS 3; 3a, starch acetate DS 2, extruded with water; 3w, starch acetate DS 3, extruded with water.

Figure 3. Size exclusion chromatography of extrudates after removing the substitute in water. 2, nonextruded starch acetate DS 2; 2a, starch acetate DS 2, extruded with alcohol; 2w, starch acetate DS 3, extruded with alcohol; 3, nonextruded starch acetate DS 3; 3a, starch acetate DS 2, extruded with water; 3w, starch acetate DS 3, extruded with water.

weight fraction or to a weak signal that was not detected because of a poor signal-to-noise ratio. The fraction that was not soluble in tetrahydrofuran was most likely not a fully substituted starch. In favor of this hypothesis is the fact that the insoluble portion was much larger in the acetylated starch with DS of 2 (>50% of the sample) and much smaller in the acetylated starch with DS of 3 ( F 0.0001) main effect of the type of solvent used for extrusion. The samples extruded with ethanol had higher WAI values (Table 2). The samples extruded with alcohol had thin flaky particles which when ground resulted in higher surface area per unit weight than the samples extruded with water. Water did not penetrate into the material to a great extent and interacted only on the surface because the acetylated starch was hydrophobic. Therefore, the samples with greater surface area were expected to have higher water absorption. The DS significantly (P > F 0.04) influenced the WAI. The samples with a lower degree of substitution had slightly lower WAI values. Because the DS of 2 starch was slightly more hydrophilic, it was expected to absorb a little more water. The statistically significant (P > F 0.02) difference between extruded and nonextruded samples can be explained by the difference in the porosity of the materials. When originally prepared, the acetylated starch was dissolved in a 3-fold quantity of acetic acid. This solution was poured in water to extract the acetic acid, forming numerous pores and channels. These channels were destroyed when starch was melted during the extrusion process. Formation of new channels, due to evaporation of the solvent, was limited because the solvent was in much lower quantity.

Figure 4. Size exclusion chromatography of water-soluble fraction of the modified starch with DS of substitution of 2 (2) and DS of 3 (3).

There were statistically significant (P > F 0.003) interactions between DS and type of solvent with respect to WSI. The highest WSI was observed in a sample with DS of 2 and extruded with water. Only small molecules, or molecules that were not substituted to a large extent, were sufficiently polar to be water soluble. Because the DS 2 starch was subjected to modification under the same conditions for a shorter time, the chances of finding small molecules that were not significantly substituted as to be soluble were much higher. This is well illustrated in Figure 4. The acetylated starch with DS of 2 even had an amylopectin portion that was water soluble. Starch acetylation up to DS of 1 improves the water solubility by preventing syneresis.15,16 When the average DS increased to 3, the chance of finding molecules that were not sufficiently substituted decreased. Therefore, the water-soluble portion was much smaller and consisted predominantly of lower weight molecules. The higher water-soluble portion of DS 2 samples before extrusion predetermined the higher WSI values of the extruded samples. Extrusion reduced the molecular weight of all extrudates to some extent. During this degradation it was more likely to release a water-soluble fragment from starch with DS of 2 than from starch with DS of 3. The newly released water-soluble fragments were found in the water phase when the sample was extruded with water. Upon evaporation of the water, the water-soluble fraction remained close to the surface and it was easy to

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reextract subsequently. When the samples were extruded with alcohol, a single-phase extrudate was formed and the distribution of the water-soluble fraction was expected to be random throughout the extrudates. Solid Density. There was no significant (P > F 0.52) difference between the extruded and nonextruded samples. There was a significant (0.04) main effect of the DS of the samples. The fully substituted sample had molecules that participated in weaker hydrophobic interactions. The samples with DS of 2 had a significant number of hydroxy groups that participated in stronger hydrogen bondings, thus bringing the molecules closer. Statements about the contribution of hydroxy groups cannot be made because the distribution of the hydrogen groups throughout the molecules was not clarified in this study. Depending on the distribution of the hydroxy groups throughout the molecule and their surroundings they may attract or repulse adjacent molecules. The distribution of the hydroxy groups on starch molecules would be an interesting topic for further research. There was a significant (P > F 0.003) main effect of the type of solvent used during the extrusion. The samples extruded with ethanol had lower solid density than the samples extruded with water. Conclusions Acetylated starch extruded with ethanol has promising properties as a loose-fill packaging material. It has low unit density, 0.062 and 0.098 g/cm3, and excellent spring indices of 0.964 and 0.952 for acetylated starch with DS of 2 and 3, respectively. Because there was little decrease in the average molecular weight and no noticeable change of the DS of the acetylated starch as a result of the extrusion processing, it seems that extrusion would be an appropriate technology to process acetylated starch into loose-fill packaging material. Water cannot be used successfully as a solvent to extrude acetylated starch. The resulting extrudates are brittle, with spring indices of 0.744 and 0.797 for acetylated starch with DS of 2 and 3, respectively. Extruded acetylated starch exhibited little water solubility, with water solubility indices ranging from 4.43 × 10-3 to 38.57 × 10-3.

Literature Cited (1) Gros, A. I.; Feuge, R. O. Properties of Fatty Acid Esters of Amylose. J. Am. Oil Chem. Soc. 1962, 39, 19. (2) Kruger, L. H.; Rutenberg. Production and Uses of Starch Acetates. In Starch: Chemistry and Technology; Whistler, R. L., Pashal, E. F., Eds.; Academic Press: New York, 1967; Vol. 2, p 369. (3) Mark, A. M.; Mehltretter C. L. Facile Preparation of Starch Triacetates. Starch 1972, 24, 73. (4) Clarke, H.T.; Gillespie, H. B. The Action of Acetic Acid upon Certain Carbohydrates. J. Am. Chem. Soc. 1932, 54, 2083. (5) Paschall, E. F. Preparation of Starch Esters. U.S. Patent 2,914,526, 1959. (6) Jarowenko, W. Acetylated Starch and Miscellaneous Organic Esters. In Modified Starches: Properties and Uses; Wurzburg, O. B., Ed.; CRC Press Inc.: Boca Raton, FL; 1986, p 55. (7) Chinnaswamy, R.; Hanna, M. A. Optimum Extrusion Cooking Conditions for Maximum Expansion of Corn Starch. J. Food Sci. 1988, 53, 834. (8) Bhatnagar, S.; Hanna, M. A. Extrusion Processing Conditions for Amylose-lipid Complexing. Cereal Chem. 1994, 71, 587. (9) Altieri, P. A.; Lacourse, N. L. Starch-based Protective Loosefill Material. Proceedings of the Corn Utilization Conference, III, St. Louis, MO, June 20-21, 1990; National Corn Growers Association. (10) Anderson, R. A.; Conway, H. F.; Pfeifer, V. F.; Griffin, E. L. Gelatinization of Corn Grits by Roll and Extrusion-cooking. Cereal Sci. Today 1969, 14, 4. (11) Dubois, M.; Giles, K. A.; Hamilton, J. K.; Roberts, P. A.; Smith, F. Colorimetric Method for Determining Sugars and Related Substances. Anal. Chem. 1956, 28, 350. (12) SAS Institute, Inc. Proprietary Software Release 6.09; SAS Institute Inc.: Cary, NC, 1996. (13) Harper, J. M. Food Extruders and Their Applications. In Extrusion Cooking; Mercier, C., Linco, P. J., Harper, M., Eds.; American Association of Cereal Chemists, Inc.: St. Paul, MN, 1989; p 1. (14) Wolff, I. A.; Olds, D. W.; Hilbert, G. E. Triesters of Corn Starch, Amylose and Amylopectin. Ind. Eng. Chem. 1951, 43, 911. (15) Burkhard, C. A.; Degering, E. F. Derivatives of Starch. III Properties of Starch Acetates. Rayon Text. Mon. 1942, 23, 416. (16) Degering, E. F. Derivatives of Starch. In Chemistry and Industry of Starch, 2nd ed.; Kerr, R. W., Ed.; Academic Press: New York, 1950, p 259.

Received for review April 9, 1999 Revised manuscript received August 3, 1999 Accepted August 4, 1999 IE990255P