Whole wheat flour milling, storage challenges
October 9, 2015
With the advent of modern roller mills during the industrial revolution, whole wheat flour production all but disappeared during much of the 20th century. In the United States, for instance, whole wheat flour production was about 2% of total wheat flour production in 2000 and only 7% of the population that year consumed at least three servings of whole grains per day, according to the U.S. Department of Agriculture.
But since then, food companies worldwide have responded to the mounting evidence supporting the benefits of whole grains with a massive increase in whole grain product launches. In the U.S., the increase in whole grain food production has nearly tripled whole wheat flour production from 2000 through 2014.
A report titled, “Key issues and challenges in whole wheat flour milling and storage,” authored by researchers at the University of Nebraska (Devin Rose and Jess Sweley), KU Leuven in Belgium (Andres Doblado-Maldonado) and Brigham Young University (Oscar Pike), discusses milling and shelf-life strategies to overcome the new challenges relative to increased whole
The authors noted that no standard methods are available for whole wheat flour milling, resulting in very different bran particle sizes. They said literature suggests moderate bran particle size is the best for bread production, while small particle size is better for non-gluten applications.
They also noted that shelf life for whole wheat flour is shorter compared to white flour due to the presence of lipids and lipid-degrading enzymes. Lipolytic degradation leads to reduction in functionality, palatability and nutritional properties. Strategies to stabilize whole wheat flour have focused on controlling lypolytic enzyme activity and have marginally succeeded.
On a genotypic level, selection of varieties of wheat for whole wheat flour production may pose some challenges, the report said. End use attributes, such as water absorption and gluten strength, are an important part of wheat selection, although these analyses are typically performed only on wheat flour. Data from these tests may not accurately predict the performance of a variety of wheat in a whole wheat flour application.
Because the outer portions of the wheat kernel affect baking quality by both physical and chemical means, quantifying these attributes may be important in selecting wheat varieties that are most appropriate for whole grain baking. For instance, bran friability varies among cultivars and cultivars with low friability produce higher quality bread. The outer portions of the kernel also contain various chemical compounds and enzymes that can affect baking properties. Because these constituents are concentrated in the outer portions of the wheat grain, a given wheat variety may be acceptable for use in traditional baking but may adversely affect whole wheat baking.
“Consumers of whole grain products are generally more health conscious than those who do not consume whole grain products,” the researchers noted. “Therefore, it may be worthwhile to select wheat genotypes for whole grain applications based on desirable nutritional properties such as phytochemical and dietary fiber content. This is already done for some other grains, such as oats, where millers often select particular varieties of grain with B-glucan content to support health claims for heart health and cholesterol reduction.
The report said differences in whole wheat milling practices are evident in a survey of five national brands of whole wheat flour (Figure 1). As shown, 43% of whole wheat flour from brand 1 passed through a sieve with a 0.230 mm opening, while less than 6% passed through this sieve in the other brands. Brand 2 contained more than 19% of particles greater than a 0.841 mm, while the other brands had 1% or less of this particle size.
Particle size of the bran fraction in whole wheat flour has an amazing influence on functional properties of the flour. In general, large wheat bran particles (mean particle size of more than about 500 µm) lead to higher water absorption and loaf volume compared with finer bran particle sizes. However, if bran particles are too coarse (greater than 600 µm), bread possesses a rough crust appearance and gritty texture. Small particles have a greater negative impact on bread quality because chemical components in the bran can interact more readily with gluten and inhibit development.
However, from a nutritional standpoint, smaller particles could help in the release of vitamins and other components from the outer cells of the kernel. Thus, a moderate particle size may be the most desirable in whole wheat flour for bread production, the report said.
Whole wheat flour storage
While not enough data exist to suggest a definitive shelf life for whole wheat flour, it is well accepted that its shelf life is considerably shorter than white flour. Flour millers stamp use-by dates of 3 to 9 months after milling on whole wheat flour packages, while the white flour use-by dates range from 9 to 15 months. Although these dates can be helpful, actual shelf life could be shorter or longer depending on temperature and humidity during storage and on failure endpoints, the report said.
It noted that whole grain flour storage is accompanied by a cascade of biochemical changes that lead to reduced flour functionality. The most unstable components in whole wheat flour are the lipids. Lipid degradation is the predominant cause of the loss of flour functionality during whole wheat flour storage.
Lipids begin to break down in whole wheat flour by hydrolytic rancidity, which can be followed by oxidative rancidity. These changes can occur enzymically or non-enzymically and affect flour quality, the researchers noted.
Wheat lipase activity is mostly located in the bran fraction of the grain. There is an enzyme termed “wheat germ lipase” that catalyzes deesterification of triacetin and other artificial water-soluble substrates and thus is technically an esterase. True lipase activity of wheat germ lipase is likely a result of contamination with lipase from wheat bran. Lipase exhibits maximum activity in wheat at about 17% moisture content. However, at moisture contents commonly observed in flour storage (10% to 14%), lipase activity continues at about 50% of maximum.
Hydrolytic rancidity in whole wheat flour can lead to a decrease in sensory quality and functional properties. Products of hydrolytic rancidity have an effect on baking quality. At low concentrations, non-esterified polyunsaturated fatty acids have a positive effect on loaf volume through co-oxidations of gluten protein sulfhydryl groups during mixing. However, at high concentrations, non-esterified polyunsaturated fatty acids affect dough mixing by reducing lipid binding capacity of gluten. This reduces gas holding capacity and elasticity of gluten.
Enzymic lipid oxidation occurs through the action of lipoxengenase, which is located in the germ and bran portion of the wheat kernel. Lipooxygenase attacks the methylene group between two double bonds of polyunsaturated fatty acids, preferentially non-esterified polyunsaturated fatty acids.
The report said lipid oxidation during storage of whole wheat flour is a much slower process than lipid hydrolysis. This is because, unlike lipase, lipoxygenase exhibits very little activity at moisture contents typically found during storage, and because whole wheat flour contains high levels of protective antioxidants.
Despite being a slower process than lipid hydrolysis during storage, lipid oxidation can contribute substantially to loss of product quality, the researchers said. While minimally active in dry flour, lipoxygenase becomes active when stored flour is mixed with water and rapidly oxidizes non-esterified fatty acids present in the flour from the action of lipase.
The report noted that oxidation of lipids can lead to a decrease in nutritional quality and consumer acceptability of whole wheat flour and whole wheat flour-based products. Lipid oxidation reduces nutritional quality through loss of essential fatty acids, although more significantly, reduced nutritional quality is affected through co-oxidation of other flour components.
Strategies to stabilize lipids
The rate of lipid degradation can be reduced by cold storage. Compared with 20 degrees C, storage of whole wheat flour at -20 degrees C for 20 weeks has been shown to prevent the deleterious changes in lipids that accompany loss in whole wheat flour functionality. Unfortunately, refrigerated transport and storage is probably cost-prohibitive for the flour industry, the researchers said.
Other strategies to stabilize lipids in foods have included modified atmosphere and the addition of antioxidants. These strategies may be more cost effective, but would likely be minimally effective, the report said. Modifying the atmosphere would be less effective, because degradation of whole wheat flour lipids begins with hydrolytic rancidity, which is enzymatic and does not require oxygen. Addition of antioxidants would also likely be minimally effective because non-esterified fatty acids produced during storage are rapidly oxidized by lipoygenase when the flour is mixed with water as would occur in any baking situation, and antioxidants are only marginally effective against lipoxygenase-mediated oxidation.
Therefore, other strategies to stabilize lipids in whole wheat flour must be employed, the report noted. A number of heat processing approaches have been explored to inhibit lipolytic activity in whole wheat flour. Since the lipase activity is concentrated in the bran, this fraction can be heated separately and then added to wheat flour in the proper proportions to make whole wheat flour. This allows for inhibition of lipase without risking influencing the flour functional properties.
The challenges with heat treatments in inactive lipase is that they can easily promote autoxidation, which means that heat treatments that totally inactivate lipase have resulted in flour that oxidize more rapidly than control flour, the report said. As a result, other strategies that do not involve heat have been employed to inhibit lipase activity. Because the activity of lipase can be influenced by the presence of metal ions, the rice bran was treated with zncl, NiCl², FeCl³, or CuCl², by dissolving each salt in HCl or methanol and spraying the solution in a fine mist over the bran. The practical applicability of this approach may be limited, however, since NiCl² addition to whole wheat flour for food use would be unacceptable, and FeCl³, while perhaps more nutritionally acceptable, would probably promote lipid oxidation, the researchers said.
Besides lipase inactivation, other strategies involving new processing technologies for food applications could be used as solutions to increase shelf-life of whole wheat flour, the report said. One study tested gamma irradiation to extend shelf-life of whole wheat flour. After six months of storage, chaptis, an Indian unleavened bread, made with whole wheat flour that had been treated with 0.25 kGy irradiation were significantly preferred over those made with untreated whole wheat flour.
Wheat selection and milling technique may be different when producing whole wheat flour compared to wheat flour. The researchers said chemical components and physical properties of the outer portion of the wheat kernel influence baking properties.
“We are only beginning to understand these effects; more research is necessary to identify components with the greatest influence that can manipulate whole wheat flours with optimum functionality,” they said.
During whole wheat flour storage, the products of lipase and lipoxygenase activity are the major culprits in loss of sensory acceptability, nutritional value and functional quality, the report said. The strategy to control rancidity of whole wheat flour has been inhibition of lipase activity, thus halting or slowing the early steps of
“Unfortunately, these approaches have been met with marginal success,” the researchers said.
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