Reduction of Phytic Acid During Breadmaking of Whole-Meal Breads'

Cereal Chem. 69(3):266-270 Reduction of phytate was studied in model dough systems using whole(up to 550 C) phytic acid concentrations were less than 8 and 4% of original grain coarse meals or flours and during breadmaking of coarse meal concentrations for wheat and rye, respectively. Further increase of the breads from wheat and rye. It was found that pH was the most important temperature (above 550C) of the unfermented sponge resulted in smaller factor in reducing phytic acid content. In doughs with pHs of 4.3-4.6, reductions of phytic acid. In acidified unfermented sponges (pH 4.4-4.6), adjusted with citric or lactic acid, phytic acid content was more effectively the residual phytic acid was less than 10% of original concentrations. reduced than in doughs with higher pHs. Phytic acid was almost completely From dough mixing through baking, the content of phytic acid was hydrolyzed in doughs made from whole-grain flours at pH 4.5 and 301C reduced to about 20 and 33% for wheat and rye, respectively. Phytic after a 4-hr incubation. Reductions of phytic acid content in doughs acid contents ranged from 6.3 to 10.1 mg/g (dry matter) and from 0.6 made from coarse meals were small, but with increasing temperature to 2.7 mg/ g (dry matter) for wheat and rye meal breads, respectively. Because of increasing health consciousness in our society, whole-meal breads have become more popular. In addition to the nutritional benefits of higher vitamin, mineral, and fiber levels in whole-meal flours, concentrations of some undesirable substances such as phytic acid are also higher in whole meal than in white flours. Phytic acid can bind multivalent cations (such as calcium, copper, iron, and zinc) to form insoluble complexes, thus lowering their bioavailability. The German Nutrition Report (Deutsche Gesellschaft fur Ernahrung 1988) stated that reduced minerals in food should not be a general concern. However, there might be a marginal supply of essential minerals in certain sectors of the p6pulation, such as vegetarians, children, and seniors. Wheat and rye contain about 1% phytic acid, which is localized in the aleurone layer of the kernel as the magnesium-potassium salt. Phytate is important in physiological functions as an energy source and as a phosphorus and mineral reserve for the growing plant (Cosgrove 1980). In flour-milling technology, the phytic acid content of flour is significantly correlated with the ash content and the milling extraction rate (Fretzdorff and Weipert 1986). In food processing, phytic acid in flour can be hydrolyzed by the enzyme phytase, which also is localized in the aleurone, to yield myoinositol and orthophosphate. Optimum conditions for phytase activity are a pH range from 5.0 to 5.5 and a temperature range from 50 to 55°C (Rohrlich 1969). Whole-meal breads contain considerable amounts of phytic acid. However, only a few studies have described breadmaking procedures aimed at lowering the phytate content of whole-meal breads. For example, increasing the yeast or malt in whole-meal wheat breads reduced the phytate content less than 50% (Harland and Harland 1980, Faridi et al 1983, Chhabra and Sidhu 1988). Wu et al (1984) and Meuser and Meissner (1987) separated milled grain into bran and flour and reduced the content of phytate in the bran fraction by hydrolysis catalyzed by endogenous phytase. The dephytinized wet bran was mixed into the dough before it was baked into whole-meal breads. However, this procedure has not been commercialized. Reduction of phytic acid content during breadmaking depends on phytase action. As with other enzyme reactions, various factors contribute to phytate degradation in doughs, including phytase activity, particle size of meals, pH, temperature, water content, and fermentation time. Published reports on the fate of phytic acid during bread production were reviewed by Lasztity and Lasztity (1990). Some European publications provide further information (Blumenthal and Scheffeldt 1983, McKenzie-Parnell 'Contribution 6022, Federal Centre for Cereal, Potato, and Lipid Research, D-4930 Detmold, Germany. Presented at the AACC 75th Annual Meeting, Dallas, TX, October 1990. 2 Federal Centre for Cereal, Potato, and Lipid Research, D-4930 Detmold, Germany. o 1992 American Association of Cereal Chemists, Inc. and Davies 1986, Meuser and Meissner 1987, Bartnik and Florysiak 1988). This article reports results of a study of the effect of pH and temperature on phytic acid degradation in model dough systems. We also report on the reduction of phytic acid content in rye and wheat whole-meal breads. MATERIALS AND METHODS Meals and Flours Whole grain coarse meals (ground using a hammer mill until about 80% of the particles did not pass through a 1.4-mm sieve) and whole-grain flours (ground using a modified Buhler laboratory mill until about 90% passed through a 2 50 -/Am sieve and more than 50% through a -,Am sieve) were prepared from wheat (cv. Ralle) and rye (cv. Danko) by standard procedures of the Federal Centre for Cereal, Potato and Lipid Research, Detmold, Germany. The two products are hereafter referred to as meal and flour. Preparation of Model Doughs Doughs for the pH study were prepared by mixing for 3 min (in a household mixer with kneading hooks) 300 g of flour or meal with 270 ml of deionized water or acid solutions of various concentrations. The solutions were 0.5, 1.0, and 2.0% citric acid; 2.0 ml of lactic acid (90%o) (0.7%, v/v); and 1.0 and 2.0% tartaric acid. Doughs were kept for 2 and 4 hr at 300C and then freezedried and ground into a fine powder. Doughs for the temperature study were prepared by mixing for 3 min (as above) 300 g of meal with 300 ml of 0.9% citric acid solution. Doughs were kept at 25, 30, 40, 45, 50, 55, 65, or 75°C for 2 and 4 hr and then freeze-dried and ground into a fine powder. Breadmaking Unfermented sponges were made by manual mixing (2 min) 300 g of meal with 300 ml of water or the same volume of 0.9% citric acid. Three sets of water and acid sponges were prepared by mixing at cold (25° C), warm (50° C), or hot (620 C) TABLE I Formulas and Procedures for Wheat and Rye Breads Ingredient Wheat Rye Coarse meal 700 g 300 g Unfermented sponge 600 g 600 g Sourdough ... 800 g Water 300 ml 50 ml Yeast 15 g 15 g Salt 15g 15g Dough rest (23-260 C) 60 min 60 min Proofing (320C) 60 min 50 min Baking (2000 C) 60 min 70 min 266 CEREAL CHEMISTRY temperatures and then storing 18 hr at 220C, 4 hr at 400C, or were stored for 15-18 hr at 260C. Wheat breads were prepared 4 hr at 220C, respectively. by the unfermented sponge procedure (Table I). Rye breads were Sourdoughs were prepared by mixing 1 kg of meal, 100 g of produced by the Detmold one-stage sourdough process (Spicher basic sourdough, and 800 ml of cold tap water. The sourdoughs and Stephan 1987, Table I). Analytical Methods Determinations of phytic acid and phytase activity were according to Wheeler and Ferrel (1971) and Fretzdorff and o12 1 2 3 4 -4 E Weipert (1986), respectively. pH values of the doughs were E 71 measured after suspending (with magnetic stirring, 3 min) 19 g |m of dough in 91 ml of deionized water. -l 1 0| 1g |. =,|3 = RESULTS AND DISCUSSION E 88 IS { I 1@ 1 1: | I > Phytic Acid Content and Phytase Activity of Meals and Flours _ l4|-' Phytic acid contents were relatively high, almost 12 mg/g or //% //> 1.2% of dry matter (dm) in both the wheat and rye samples (Fig. 1). -2 However, phytase activity in rye was almost three times that in X| = wheat (Fig. 1). These results are in agreement with those of 'a 4-1 10 0 | 1 z l |Gil> Rohrlich (1969) and Harland and Harland (1980). 4-J 4 cX Effect of pH in Model Doughs +J , Q' ,A question arises whether the optimum pH (5.0-5.5) of phytase CL >. activity in solution is also the optimum pH for the hydrolysis 10 of phytate in a dough system. Figures 2 and 3 show the changes of phytate in wheat and rye model doughs, respectively. Organic acids were used in the given concentrations to adjust the pH. R W R N These acids can be used for food processing and are part of Fig. 1. Phytic acid contents (1 and 2) and phytase activities (3 and 4) acidification improvers. The lower pHs were in the range of in rye (R) and wheat (W) whole meals. Phytase activity was assayed sourdoughs. by incubation of extract with sodium phytate in citrate buffer at pH For wheat, the flour doughs (Fig. 2B) had slightly higher pH 5.5 and 550C; 1 U represents 1 ,umol of liberated inorganic phosphate than the meal doughs (Fig. 2A). This might have been due to per minute in 5 ml of assay solution. a higher buffering capacity in the flour doughs, which had smaller particle sizes with larger surface area so that buffering substances Wheat were more readily extracted. A~~~~~~ 1004~~~~~~~~~~~.. 2|i.............. .......... . .......... Ry 7510... .... 2h Rye _ 100.00-