Preparation and Pasting Properties of Wheat and Corn Starch Phosphates'

Cereal Chem. 70(2):137-144 Wheat and corn starches were phosphorylated in a semidry state at P from phospholipids), and its pasting properties showed optimum 1300C with 5% (based on dry starch) sodium tripolyphosphate (STPP) thickening power with shear stability when cooked at 950C. The best and/or 2% sodium trimetaphosphate (STMP). All phosphorylation corn starch phosphate, judging from its paste properties, was obtained reactions were done using 5% sodium sulfate and adjusting the initial at an initial pH of 11 with STPP and contained 0.16% P (including reaction pH by adding aqueous sodium hydroxide or hydrochloric acid 0.02% from lipid). When corn or wheat starch was heated with a mixture to the prereaction slurries. As reaction pH was increased from 6 to 11, of 5% STPP and 2% STMP, the best product when pasted at 950C the degree of phosphorylation decreased 40-50% with STPP, whereas was obtained at the initial reaction pH of 9.5. Paste clarity of the it increased 100% with STMP. At an initial reaction pH of 10 with STPP, phosphorylated starches indicated that cross-linking accelerated rapidly a wheat starch phosphate was obtained with 0.22% P (including 0.05% above pH 8 with STMP but above pH 10 with STPP. Native starches generally contain small amounts (<0.1%) of phosphorus (P). In root and tuber starches, P is covalently linked to the starch (Hodge et al 1948, Posternak 1951, Hizukuri et al 1970), whereas in cereal starches it occurs mostly as contaminating phospholipids (Schoch 1942, Tabata et al 1975, Meredith et al 1978, Morrison 1978). Starch phosphates, which are prepared by chemical methods, have been reported to give clear pastes of high consistency, with good freeze-thaw stability and emulsifying properties (Kerr and Cleveland 1959, 1962; Klostermann 1963; Nierle 1969; Lloyd 1970). The preparation, properties, and uses of phosphorylated starches were reviewed by Solarek (1986). Starch phosphates may be grouped into two classes: monostarch phosphates and distarch phosphates (cross-linked starches). In general, monoesters are introduced at a much higher level of substitution on starch than are diesters because even a very few cross-links can drastically alter paste and gel properties of starch. Formation of distarch phosphates is generally considered the most important reaction used to prepare modified food starches. In the United States, phosphorous oxychloride and three inorganic phosphate salts (sodium orthophosphate, sodium tripolyphosphate [STPP], and sodium trimetaphosphate [STMP]) may be used to prepare food starch phosphates (CFR 1991). During phosphorylation, pH plays a major role in determining the ratio of monoester bonds to diester bonds. The phosphorylation of starch with phosphate salts has been investigated by several authors (Kerr and Cleveland 1959, 1960, 1962; Paschall 1964; Nierle 1969; Lloyd 1970; Wurzburg et al 1979, 1980). The objectives of this study were 1) to compare the reaction of semidry wheat and corn starches with STPP and STMP and 2) to use STPP or a combination of STPP and STMP to prepare wheat and corn starch phosphates having nearly the maximum P (0.4%) allowed by regulation (CFR 1991), as well as white color, high paste consistency, short paste texture, and good shear stability upon cooking at atmospheric pressure. MATERIALS AND METHODS Materials Prime wheat starch was donated by Midwest Grain Products, Inc. (Atchison, KS) and corn starch by A. E. Staley Manufacturing Co. (Decatur, IL). Potato starch was purchased from Sigma Chemical Co. (St. Louis, MO). The wheat, corn, and potato starches contained 0.05, 0.02, and 0.06% P, respectively. Penta'Contribution 92-354-J, from the Kansas Agricultural Experiment Station, Manhattan 66502. 2 Research associate, Department of Food Science and Human Nutrition, Iowa State University, Ames 50011. 3 Professor, Department of Grain Science and Industry, Kansas State University, Manhattan 66506. © 1993 American Association of Cereal Chemists, Inc. sodium tripolyphosphate (STPP) was purchased from Fisher Scientific Company, Pittsburgh, PA, and trisodium trimetaphosphate (STMP) and blue dextran 2000 were from Sigma Chemical Co. All other chemicals were reagent grade unless specified. Preparation of Starch Phosphates Starch was phosphorylated by the procedure described by Kerr and Cleveland (1959), with two modifications. Sodium sulfate was used in all reactions, and starch that had been impregnated with an aqueous solution of sodium sulfate and phosphate salt(s) was not filtered or centrifuged to remove excess salt solution. Instead, the entire slurry was dried to less than 15% moisture content before reaction at high temperature. Fifteen grams of STPP and 6 g of STMP, separately or together, were dissolved in 300 ml of water containing 15 g of sodium sulfate. The pH of the solution was adjusted between 6 and 11 by adding 10% aqueous HCO or NaOH. Starch (300 g, db) was dispersed in the solution. Then the pH of the dispersion was readjusted with 5% aqueous HCl or NaOH, and the total weight was brought to 667 g by adding water. The starch solids in the flowable dispersion amounted to 45%. The slurry was stirred for 1 hr at room temperature and dried to 10-15% moisture at 40°C in a forced-air oven. To effect phosphorylation, the dried starch cake in the dish was heated for 2 hr at 130° C in a forced-convection oven. After being cooled to room temperature, the reaction mixture was dispersed in distilled water (350 ml), and the pH of the dispersion was recorded. The starch was recovered by centrifugation (1,500 X g, 10 min) and redispersed in 600 ml of distilled water. The dispersion was adjusted to pH 6.5 with aqueous 5% HCl or 5% NaOH solution, and after three washings with water (3 X 600 ml), the product was dried at 400 C. Duplicate runs of the phosphorylation procedure gave good reproducibility as judged by P incorporated into the starches and by the pasting properties. Determination of Phosphorus in Starch Phosphorus in starch was determined by the procedure of Smith and Caruso (1964). Unless otherwise stated, P levels are the sum of endogenous P in a starch plus that incorporated by chemical treatment. Water Uptake by Starch The amount of water imbibed by starch was measured by the procedure of BeMiller and Pratt (1981). Paste Consistency The pasting of starch samples was examined in a Brabender Viscograph-E (C. W. Brabender Instruments Inc., Hackensack, NJ) using 75 rpm and a torque of 700 cm-g equivalent to 1,000 BU. The starch slurry (400 ml at 7.5% starch solids) was adjusted to pH 6.5 with a few drops of 5% HCl or 5% NaOH solution, pasted at a heating rate of 1.5°C/min from 30 to 95°C, held Vol. 70, No. 2,1993 137 at 95C for 30 min, cooled at 1.5 C/min from 95 to 50'C, and finally held at 500 C for 30 min. To permit examination of the effects of sodium ions on paste consistency, aliquots (0.2-1.0 ml) of aqueous NaCl solution (4M) were added to aqueous suspensions of a wheat starch phosphate (degree of substitution [DS] = 0.01) prepared at pH 10 using STPP. The resulting sodium ion concentrations in the suspensions were 0.002-0.01 M. Light Transmittance of Starch Pastes The transmittance of a starch paste was measured by the procedure of Kerr and Cleveland (1959). A 1% aqueous suspension of starch near neutral pH was heated in a boiling water bath for 30 min with intermittent shaking. After the suspension was cooled for 1 hr at 250 C, transmittance was read at 650 nm. Freeze-Thaw Stability Freeze-thaw stability was determined as described by Takahashi et al (1989). RESULTS AND DISCUSSION Phosphorylation with STPP Unmodified wheat and corn starches used in this work contained 0.05 and 0.02% P (dwb), respectively, almost all of which occurred as lysophospholipids (Meredith et al 1978, Morrison 1978). Reaction of semidry wheat or corn starch with 5% STPP in the presence of 5% sodium sulfate at pH 6-11 and 1300C for 2 hr gave starch phosphates that contained 0.1-0.3% P (Fig. 1). In STPP phosphorylation, as the initial reaction pH was raised from 6 to 10, the P level in the product decreased gradually, but between pH 10 and 11, phosphorylation decreased rapidly (Fig. 1). At pH 11, corn and wheat starches showed the lowest incorporation of P (0.16 and 0.12%, respectively), which was roughly one half the incorporation at pH 6. Phosphorylation of starch with STPP would be expected to increase further below pH 6.0. However, low pH was not examined because acids catalyze hydrolysis and browning of starch (Kerr and Cleveland 1959). Our data are in conflict with those of Nierle (1969), who found that the degree of phosphorylation of corn starch increased from 0.12% P at pH 7.5 to 0.36% P at pH 9.5 upon reaction with STPP at 1200C for 1 hr. The highest degree of phosphorylation of starch using STPP was obtained at pH 6, where P levels of 0.31 and 0.28% for corn and wheat starch, respectively, were found. After subtraction of the P contributed by lipids (0.02 and 0.05% for corn and wheat, respectively), the corn and wheat starch phosphates prepared at pH 6 contained 0.29 and 0.23% P, respectively, which are equiva-