Chapter 4

Thermochemical Processes for Derivatization of Starches with Different Amylose Content

R. E. Wing and J. L. Willett

Plant Polymer Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, 1815 N. University

Street, Peoria, IL 61604

Thermochemical oxidation of starches of different amylose content (0-70%) was investigated to maximize carboxyl and carbonyl content, to increase water solubility, and to give products with new and novel properties. Thermochemical processes evaluated were self-induced exothermal initiation-drum drying, jet cooking-drum drying, and drum drying using hydrogen peroxide and a ferrous-cupric catalyst. Data was presented to show product preparation, properties, and analysis. Preliminary data was presented for end-use applications in the areas of: 1) sequestration of calcium, manganese, and iron and 2) extenders in phenol-formaldehyde adhesives for plywood. Other potential applications being researched are set retardation and water reduction in concrete admixes, builders for detergents, micronutrient chelators, textile sizers, metal and rust cleaners, and cotton mercerizers.

With the increasing cost of petroleum-based products, use of natural polymer substitutes have advantages of lower cost and biodegradability, making them environmentally friendly. Starches have been oxidized with numerous oxidizing reagents for over 100 years, mainly to reduce molecular weight and to increase solubility for paper and food applications. However, all commercial products are still water insoluble. Commercial oxidized starches (1-4) are batch-prepared utilizing room temperature conditions and low (<3%) concentrations of oxidant (usually hypochlorite). During product isolation (filtration and water washing), some of the product is solubilized (due to molecular breakdown) and lost. While hypochlorite has been the oldest and most frequently used oxidant, other oxidants, such as permanganate, hydrogen peroxide, persulfate, periodate, and dichromate have been used. The different oxidation procedures result in variations in molecular structure and properties. The yearly consumption of oxidized starches is about 880 million kg with a predicted growth rate of 2.4% over the next decade, mainly in the adhesive area (5).

U.S. government work. Published 1999 American Chemical Society 55

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Maurer (6) reported using hydrogen peroxide (0.5%) at pH 4.5 in jet cooking starch for solubilization and use in adhesive formulations. This processing did not promote oxidation, only viscosity reduction. Ewing (7) obtained similar results using pH values of 6-7. Wing (8) reported the thermochemical oxidation of several starches by jet cooking and drum drying with several oxidants. Parovuori (9) studied the oxidation of potato starch with hydrogen peroxide at 40°C with metal catalysis to yield carboxyl and carbonyl groups using acidic and alkaline reaction conditions, respectively. Kruger (70) obtained an oxidized insoluble starch with improved properties using manganese ion catalysis with hydrogen peroxide. Blattner and Ferrier (77) evaluated several metal catalysts on the oxidation of cellulose and Skibida (12) with several carbohydrate polymers. Floor (75) oxidized maltodextrins and potato starch using tungstate catalyst at low pH and high peroxide levels to yield highly degraded products of moderate carboxyl content. Whistler and Schweiger (14) oxidized amylopectin under ambient conditions with no catalyst over the pH range 3-11 with high peroxide levels. At pH >7, extensive carbohydrate degradation was observed, while at pH <5 little oxidation occurred. Reactive extrusion of starch has been used successfully to yield products (cationics, alkyl glucosides, carboxylates, oxidized starches, etc.) with improved reaction efficiency and solubility (15-25).

Thermochemical oxidation of starches at the National Center for Agricultural Utilization Research (NCAUR) at pH's of 7-11.5 with several oxidizing agents and no catalysis resulted in water soluble products with increased carboxyl content (8). A reactive extrusion process developed at NCAUR (25) of different starches with hydrogen peroxide and a ferrous-cupric catalyst gave products with increased carboxyl and carbonyl content. The present oxidation study using several thermochemical techniques evaluated starches of different amylose content with a ferrous-cupric catalyst and hydrogen peroxide to yield water soluble products of extremely high carboxyl and carbonyl content with unique properties useful in several industrial applications.

Experimental Methods

Materials. Pearl cornstarch (PCS) was supplied by Corn Products International, Engelwood Cliffs, NJ, and had an amylose content of 24%. Waxy starch (WS) was supplied by American Maize Products, Hammond, IN, and had an amylose content <1%. Amylomaize VII (AM7) was supplied by American Maize Products, Hammond, IN, and had an amylose content of 70%. Hydrogen peroxide, ferrous sulfate, and cupric sulfate were reagent grade.

Self-induced Exothermal Initiation-Drum Drying (SE-DD). Starch (200 g) was slurried in water (280 ml), catalyst (1.0 g ferrous sulfate + 0.8 g cupric sulfate) was added, and hydrogen peroxide (0-96 ml - 30%) was added. Slurries were allowed to react under ambient conditions. During the first 30 min, the pH decreased (2.7 to 1.5) and the temperature gradually increased. As the oxidation progressed, excessive foaming occurred and the temperature reached 65°C at low peroxide concentrations and 98°C at high peroxide concentrations. Solution color stayed light tan at low peroxide

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concentrations, but turned green at high peroxide concentrations. The reaction was over after the slurry reached its highest temperature (65-98°C). The hot solutions were immediately poured onto a double-drum drier with drum spacing of 0.005 cm at the drying temperature. The drier had 30 x 45 cm drums with a total drying surface (both drums) of 8750 cm2. The drums were heated with 175 kg/cm2 steam and rotated at a speed of 4 rpm. Surface temperature of the drums was 160°C and the dry product came off in sheets, flakes, or powder, depending on the amount of hydrogen peroxide added. All samples were subsequently ground into a powder.

Jet Cooking-Drum Drying (JC-DD). Slurries were prepared as in SE-DD and immediately passed through a laboratory-model continuous steam-injection cooker (Penick and Ford, Ltd., Cedar Rapids, IA) at 145°C and a flow rate of 1.3 1/min. The amount of steam entering the cooker was used to regulate the dispersion temperature and the back pressure was kept constant. Final reaction mixtures were diluted -5% due to steam condensation. The hot solutions were processed on the drum drier as previously described.

Drum Drying (DD). Slurries were prepared as in SE-DD and immediately processed on the drum drier as previously described.

Carboxyl Determination. A dry sample (1.0 g) was slurried in water (100 ml) and 0.100 N NaOH was added to keep the pH above 10. After stirring for 1 hr, the mixture was back-titrated with 0.100 N HC1 to the phenolphthalein end-point. Conversion factors were determined using oxalic and citric acid as standards. Starches with no peroxide were used as a control. Samples were run in triplicate and the coefficient of variation was 1%.

Carbonyl Determination. A dry sample (1.0 g) was slurried in water (300 ml) and heated if necessary to boiling for complete solubilization. The cooled solutions were adjusted to pH 3.2 with 0.100 N HC1 and 60.0 ml of a hydroxylamine hydrochloride (HMH) solution was added (HMH, 25 g; 100 ml 0.5 N NaOH diluted to 500 ml). The solutions were heated to 40°C in an oven for 4 hr and titrated rapidly to pH 3.2 with 0.100 N HC1. A water sample was used as the control. Calculation: % (C=0) = 0.100 x 0.028 x (ml control-ml sample) x 100. Samples were run in triplicate and the coefficient of variation was 2%.

Solubility. Samples (10 g) were placed in water (300 ml) and the pH adjusted and maintained at 9.0 for 1 hr. After filtration, the recovered solids were oven dried and the weight determined.

Results and Discussion

Self-induced Exothermal Initiation - Drum Drying (SE-DD). The SE-DD oxidation data reported in Table I show that increasing peroxide concentration increased carboxyl

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and carbonyl content. The data also show as the amylose concentration is increased, slightly reduced carboxyl and carbonyl content resulted. As the amylose content increased, the data show 6 to 100% reduced solubility. Drum drying was a convenient recovery technique and didn't promote further oxidation. Also, drum drying appeared to increase product solubility, likely due to the heat (160°C) of the drums. While this oxidation method has potential to yield products for industrial application, initial foaming needs to be controlled for complete product recovery.

Jet Cooking - Drum Drying (JC-DD). The JC-DD oxidation data reported in Table II show increasing peroxide concentration increased the carboxyl and carbonyl content. The data also show increasing amylose concentration resulted in slightly increased carboxyl and carbonyl content, which is opposite to results from SE-DD data. All products were 100% water soluble, which show the increased initial temperature and turbulence from the steam caused immediate starch gelatinization and oxidation. Again, drum drying was used as a method of convenient recovery.

Table I. Effect of Peroxide Concentration on SE-DD Oxidation of Starches8

Starch 30%H2O2 COOH, c=a Solubility, Type ml meq/g meq/g % WS 31 0.7 2.66 100 PCS 31 0.7 1.77 81 AM7 31 0.6 1.56 50 WS 62 1.5 4.99 100 PCS 62 1.3 4.30 94 AM7 62 1.0 3.60 67 WS 96 1.8 6.64 100 PCS 96 1.7 6.38 100 AM7 96 1.5 5.95 75

aStarch - 200 g; H 2 0 - 280 ml; Catalyst -1.8 g.

Drum Drying (DD). Drum drying alone was the most efficient and economical oxidation method evaluated in this study. The DD oxidation data reported in Table III show that increasing peroxide concentration increased carboxyl and carbonyl content. Data in Table III show the DD oxidation method was not influenced by the amount of amylose in the starch. All samples using the DD oxidation procedure were recovered as a fine powder, 100% water soluble, and ready for use. A proposed scale-up for the DD oxidation procedure is shown in Figure 1. The SEM shown in Figure 2 depicts the reactive structure of a highly oxidized PCS by the DD technique.

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Table II. Effect of Peroxide Concentration on JC-DD Oxidation of Starches' Starch 30% Hfi* COOH, c=o, Type ml meq/g meq/g

WS 31 1.0 2.94 PCS 31 1.0 2.97 AM7 31 1.1 3.47 WS 62 1.5 5.32 PCS 62 1.5 5.65 AM7 62 1.5 6.15 WS 96 1.8 6.80 PCS 96 1.9 7.14 AM7 96 2.2 7.43

"Starch - 200 g; H 2 0 - 280 ml; Catalyst - 1.8 g.

Table III. Effect of Peroxide Concentration on DD Oxidation of Starches' Starch 30%H2O2 COOH, c=o, Type ml meq/g meq/g WS 31 0.9 2.35 PCS 31 1.0 3.14 AM7 31 1.1 3.77 WS 62 1.6 5.82 PCS 62 1.6 6.00 AM7 62 1.6 6.12 WS 96 2.2 7.93 PCS 96 2.2 7.86 AM7 96 2.3 7.94

"Starch - 200 g; H 2 0 - 280 ml; Catalyst -1.8 g.

Sequestration of Metals. Metals (calcium, iron, magnesium, manganese, etc.) in hard water cause precipitation (metal hydroxides and oxides) problems in many industrial operations. The metals have to be kept soluble by chelation. Most industries use petrochemical-based chelants like EDTA, while others use silicates, zeolites, phosphates, etc. The afformentioned materials are not always environmentally friendly (biodegradable and non-polluting). Specifically, 1) the pulp and paper processing industry uses 450 thousand kg EDTA annually to keep iron and manganese in solution as precipitation can reduce fiber strength and discolor fiber; 2) the detergent industry uses 2 billion kg of phosphates, zeolites, silicates, etc. as detergent builders to keep calcium, magnesium, and iron from precipitating during cleaning operations; 3) the dietary fiber industry uses mainly sodium silicate for chelation of calcium, magnesium,

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Figure 2. SEM of Highly Active Oxidized Starch.

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and iron, but the FDA is imposing strict regulations on silicon-based materials in food related products; and 4) the micronutrient industry provides at least 8 metals in chelated form (mainly EDTA) to prevent metal precipitation in soil, so metals are more available to plants. Thermochemically produced oxidized starches are being evaluated for the above mentioned applications because they are water soluble, biodegradable, and have chelating potential. The data in Table IV show chelating values of an oxidized PCS compared to EDTA at several pH's.

Table IV. Metal Chelation of Oxidized Starch Versus EDTA Sample Calcium Chelation at pH Iron Chelation at pH Manganese Chelation at pH

—111 9 117 12.7 7 HI mg CaCO/z mgFe3Vg mgMrtVg

Ox. PCS 400 240 20 400 125 100 EDTA-38% <100 <100 <100 100 100 100

Adhesive Extenders. The most prominent wood adhesive is a petrochemical-based phenol-formaldehyde (P-F) resin (0.7 billion kg worldwide). Increasing cost of phenol and decreasing availability of non-environmentally friendly formaldehyde have resulted in companies searching for extenders and/or replacements for P-F resins. Some researchers have explored carbohydrates to extend P-F resins with some success and presendy 7-8% wheat flour is the only commercial extender used in P-F formulations. Water soluble oxidized starches were evaluated as P-F extenders and/or substitutes in plywood adhesives. Results to date indicate 5-10% loadings of water soluble oxidized starch yield comparable adhesive strength and wood failure as shown in Table V.

Table V. Dry Shear Strength/ Wood Failure Data Sample Shear Strength (psi) % Wood Failure P-F Resin 425 89 5% Ox. PCS 401 92 10% Ox. PCS 377 90

Safety. Safety in any chemical reaction is a prerequisite. During these reactions, safety glasses and gloves were worn. All reactions were well ventilated and protective shields were used around steam. NCAUR requires SOP's and two people were present during all oxidations.

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Conclusions

Several thermochemical oxidation techniques were evaluated with starches of variable amylose content to yield water soluble products possessing high carboxyl and carbonyl content. One technique (drum drying) yielded the highest activity, while being the most continuous and economical. Product evaluations for potential end-use applications as metal sequestrants and adhesive extenders were presented.

Acknowledgments

The authors wish to thank M. Redman for Figure 1, F.L. Baker for Figure 2, A.H. Conner and L.B. Dunn, Jr. for adhesive data and G. Whitehurst for chelation data.

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