Nutritional Profile of Whole-Grain Soft Wheat Flour

Cereal Chem. 88(5):473–479 Whole-grain wheat flour is used in baking to increase fiber content and to provide vitamins from the bran layers of the kernel. We surveyed whole-grain soft flour samples from North America to determine the nutritional profile using recently revised fiber quantification protocols, Codex 2009.1. Standard compositional and vitamin analyses were also included in the survey. Three separate studies were included in the survey: sampling of commercial whole-grain soft wheat flour, a controlled study of two cultivars across three years and two locations, and a regional study of soft white and soft red grain from commercial grain production. The Codex method for fiber measurement estimated total fiber concentration in the commercial sampling at 15.1 g/100 g, dry weight basis (dwb). In the controlled research trial, the largest source of variation in total fiber concentration was attributed to year effects, followed by genotype effects. For the two locations used in this study, location effects on fiber concentration were significant but an order of magnitude less important than the year and genotype effects. The third study of regional variation within North America found limited variation for total fiber, with the resistant oligosaccharide fraction having the greatest variation in concentration. When all three studies were combined into a meta-analysis, the average total fiber concentration was 14.8 g/100 g dwb. In the meta-analysis, concentrations of folate, thiamin, riboflavin, niacin, and pyridoxine were lower than in previous summary reports. Vitamin E and pantothenic acid were the exceptions, with concentrations that were nearly identical to previous standard reports. Several other recent studies also point to current cultivars and production systems as producing lower concentrations of the essential vitamins than previously reported. The results suggest that vitamin concentrations in diets of populations using grain-based diets from modern cereal-production systems may require review to determine if previous assumptions of vitamin consumption are accurate. The Codex Committee on Nutrition and Foods for Special Dietary Uses recently updated the dietary fiber definition (Codex Alimentarius Commission 2009). The new definition of dietary fiber, based on functionality in human digestion, is defined as carbohydrate polymers with ten or more monomeric units that are not hydrolyzed by the endogenous enzymes in the human small intestine. Dietary fiber encompasses the following categories: edible carbohydrate polymers naturally occurring in food as consumed; carbohydrate polymers that have been obtained from food raw material by physical, enzymatic, or chemical means and that have been shown to have a physiological effect beneficial to health as demonstrated to competent authorities by generally accepted scientific evidence; and synthetic carbohydrate polymers that have been shown to have a physiological effect beneficial to health as demonstrated to competent authorities by generally accepted scientific evidence. The decision on whether to include carbohydrates of three to nine monomeric units was left to national authorities. Until recently, AOAC procedures for measuring dietary fiber (985.29 and 991.43) did not quantitatively measure either resistant starch or digestion-resistant oligosaccharides (ROS). McCleary (2007) developed a method incorporating the digestion procedures of AOAC method 2002.02 (for measurement of resistant starch) with the isolation and gravimetric procedures of AOAC methods 985.29 and 991.43 to quantify high molecular weight dietary fiber (HMWDF). In the new method, the filtrate from the quantification of HMWDF subsequently is concentrated, deionized, concentrated again, and analyzed by liquid chromatography to quantify the low molecular weight soluble dietary fiber (all ROS with degree of polymerization  3). Total dietary fiber (TDF) then is calculated as the sum of ROS and HMWDF. This method for dietary fiber is known variously as the McCleary, all-in-one, or Codex fiber method and is standardized as AOAC method 2009.01. Whole-grain snack foods are made primarily of soft endosperm wheat flour from grain with either red or white seed coats. Nutritional labeling based on the U.S. Department of Agriculture National Nutrient Database for Standard Reference (USDA-SR) is used for dietary and commercial purposes. USDA-SR reports the TDF of whole-grain wheat flour as 10.7 g/100 g, based on seven samples at an average moisture concentration of 10.74 g/100 g (USDA 2010). The average protein concentration in the database samples is 13.21 g/100 g, significantly greater than is typically found in soft wheat and most hard winter wheat grain, which suggests that the samples of the USDA-SR derive from North American hard spring wheat, the predominant class with protein in this range. If the ratio of dietary fiber to total carbohydrates remains constant in wheat grain of varying protein concentrations, then as wheat protein concentration decreases, fiber concentration as measured after digestion of total carbohydrates would increase. Therefore, lower-protein grain, such as soft wheat, may have a greater fiber concentration than high-protein grain, with the consequence that low-protein grain could be targeted for high-fiber applications. A range of estimates of dietary fiber content in wheat grain can be found in the literature. Hard red winter wheat evaluated following the enzymatic-gravimetric method of Prosky et al (1988) had 10.2% TDF (8.9% insoluble and 1.3% soluble; Ranhotra et al 1990) at 15.9% moisture. Bach Knudsen et al (1995) reported that whole-kernel wheat had 11.8 g/100 g of dietary fiber and 2.8 g/100 g of soluble nonstarch polysaccharides (dwb). And Hernot et al (2008), also using the Prosky method, reported 14.2% TDF (12.3% insoluble and 1.9% soluble). In the HEALTHGRAIN survey of 151 wheat genotypes grown in one environment in Hungary, TDF in whole grain, as measured by a subtraction methodology, ranged from 11.5 to 18.3 g/100 g (Gebruers et al 2008). However, in a subsequent HEALTHGRAIN study using AOAC method 994.13 for determination of TDF and testing in four environments, 24 wheat genotypes had lower TDF concentrations, 9.6–14.4 g/100 g (Gebruers et al 2010). Refining of wheat grain into flour results in a substantial loss of B vitamins, vitamin E, and folate, as these nutrients are concentrated in the aleurone and pericarp of the kernel. Schroeder (1971) estimated that 50–86% of these vitamins are lost in producing white flour. As a consequence, in the United States and in most countries, wheat flour is enriched with thia1 United States Department of Agriculture, Agricultural Research Service, Soft Wheat Quality Laboratory, Wooster, OH 44691. 2 Corresponding author. E-mail: [email protected] 3 Ohio State University, Ohio Agricultural Research and Development Center,