The Science of Heritage Grain Nutrition and Flavor

A wheat kernel is a complex, layered wonder known as the caryopsis, or one-seeded fruit, of the common wheat plant, Triticum aestivum. Botanists consider wheat a fruit because the grain consists of a seed within a protective pericarp. This particular type of fruit is termed a caryopsis but is more commonly known as a wheat grain kernel or berry. To scientists, however, these latter terms have other specific meanings: a kernel is the soft interior of a nut, a berry is a succulent fruit with many seeds. But the terms “wheat berry” and “kernel” are widely used by farmers and bakers.

The composition of a caryopsis varies more widely in wheat than in barley, corn, or other cereal grains with wheat protein levels commonly ranging from 8 to 18%. The variation is due to genetic differences among varieties as well as soil composition, rainfall, temperature, and cultural influences from field operations. The principal parts of a kernel are the thin outer pericarp (approximately 5-7%), aleurone layer (6-7%), endosperm (80-85%), and scutellum (1.5-2%). The pericarp can be subdivided into the protective epidermis and the inner seed coat, or testa, which unites with the aleurone. In red wheat varieties, the testa contains reddish-brown pigment which is much less concentrated or absent in white wheats. Red pigmentations is also linked genetically to sprout resistance which enables red wheats to be raised more often than whites in damper climates.

The aleurone layer is only a single cell in thickness and structurally is the outermost part of the starchy endosperm. During the milling process, the pericarp and aleurone coats are removed and constitute the fibrous bran. Of the carbohydrates in bran, approximately 35% is cellulose, 45% is hemicellulose (primarily arabinoxylan), 8% are sugars, and the rest is starch. Most of the bran’s vitamins, lipids, proteins, and amino acids are located in the aleurone as well as almost all of the phosphorus, which is about 1% of the bran’s weight.

The endosperm within the aleurone layer consists of cell walls tightly packed with starch granules that are embedded in a matrix of proteins. About one-fourth of the starch is amylose which forms in linear chains while the rest is branch-chained amylopectin. As starch accumulates during plant growth, cell division within the endosperm decreases and the kernel “fills” along the longitudinal crease of the grain. The starch accumulates in the form of relatively large lenticular granules which adhere to the proteins in ways that distinguish between soft and hard wheat texture. When the endosperm of a hard red kernel is fractured, the starch granules remain within the protein matrix, though cleavage in soft wheats releases the granules. These qualities are inherited characteristics and important for the intended use of milled flour as hard red varieties tend to be more suitable for leavened breads, while soft wheats are generally preferred for flatbreads, pastries, and pancakes.  

Cereal Grain Nutrition
Endosperm texture is one of the most characteristic qualities of the various wheat classes and varieties. Durum, or macaroni, wheats are hard and generally have translucent endosperm, while it is soft, whitish, and opaque in soft red varieties. Hard red wheat endosperms may be either hard or semi-hard. As shown in the table below, composition of these various kernel parts varies considerably and relates to the various milled grain products. Flour composition, in turn, varies considerably depending on the milling operations used to separate out the various grain tissues.

The outermost part of the endosperm forms exceptionally thick cell walls composed of 16-20% protein in hard red bread wheats, while inner endosperm cells generally have about 8-15%. Starch is the principal carbohydrate in wheat kernels and is almost entirely found in the endosperm. The exterior is usually vitreous or translucent while the interior is opaque, or floury. In 19th century terms, this outer coat was the “flinty” portion of the grain, while the inside was “mealy.” Starches are composed of granules and classified into three groups based on size and shape—lenticular-shaped A-type and B- and smaller C-type spherical granules. Starch content plays an important role in flour quality as the ratio of amylose (25-28%) to amylopectin (72-75%) influences physicochemical properties like texture and viscosity of breads and other processed foods.

The five basic endosperm proteins are highly significant to the special baking properties of wheat and include water-soluble albumins and globulins (approximately 20%), insoluble gliadins (35%) and glutenins (35%), and remaining protein residue (10%). Proteins are found in all parts of the caryopsis with the embryo containing about 26% and the aleurone 24%. Compared to other cereal grains, wheat has a relatively high level of the vitamins thiamine and niacin, although it is low in riboflavin and devoid of vitamin A. Niacin is principally concentrated in the aleurone layer where most of the pyridoxine and pantothenic acid is also found. Niacin is not destroyed in breadmaking since it is not affected by heat, and loss of riboflavin is insignificant. Thiamine, however, is sensitive to heat and as much as 20% of this important member of the B complex is lost.

An overall grain protein level of 12-13% is a preferred target by millers and bakers for quality breadmaking with good loaf volume and texture. Approximately 40% of a wheat kernel’s protein is found in the endosperm, the aleurone layer contains some 15%, with the remaining amount distributed about equally among the scutellum, embryo, and pericarp and testa. The amount of endosperm protein is influenced by environmental conditions, available soil nitrogen, and the wheat variety.   The quality of hard red wheat protein preferred for making bread differs from that in pastries and pasta soft white flours, and a wheat variety’s distinct protein-starch interaction is also of considerable importance. For this reason, blended or fractionated flours from different grains and market classes are sometimes used by bakers.   

When flour is mixed with water and yeast to form dough and left to rise for several hours or kneaded, the glutenin forms long, chainlike molecules which are bridged by the shorter gliadins. The resulting webs are known as gluten which consist of 80-85% protein with lesser amounts of lipid and starch compounds. Bread rises since the cohesive, elastic properties of gluten retain the bubbles of carbon dioxide released by yeast which expand through fermentation. The more yeast spores and bubbles produced during preparation of the dough results in more tender bread.  American chemist Thomas Osborne (1859-1920) devised the method still in use to distinguish levels of these key endosperm protein levels based on differential solubility. Osborne’s 1907 monograph, The Proteins of the Wheat Kernel, still guides much of 21st century cereal chemistry. Farmers have their own method to give a general indication of a grain variety’s gluten level at harvest time—pop a small handful of kernels into the mouth and see how long it takes to make gum!

The scutellum derives its name from Latin scutella, or “small shield,” since this small formation at the base of the kernel protects the germ, or embryo, which together with the aleurone are the enzyme-rich living tissues of a ripe grain kernel. The scutellum further serves as the critical absorptive organ for transfer of food reserves from the endosperm to the embryo, which develops into the seedling during germination. The embryo consists of the primary root (radicle), the part of the kernel to emerge upon germination, and the upward shoot (plumule) which is covered by the protective coleoptile.

Wheat germ has a higher protein, fat, and sugar (primarily sucrose and raffinose) content than bran but is lower in carbohydrates. Because it is also a rich source of vitamins, minerals, and fiber, wheat germ is marketed as a valuable byproduct from milling white flour. Wheat germ oil is used as a substitute to cod liver oil as it is rich in omega 3 fatty acids which have been shown to lower blood cholesterol and benefit the immune and endocrine systems.

Commercial flour millers require acceptable and uniform flour color in their products. Bread and cake flours generally adhere to consistent standards of minimum yellowness and maximum brightness. Products like pasta are generally made from semolina or flours containing high amounts of natural yellow and brown pigments with low levels of enzymes that destroy these compounds. Most American millers treat flour to achieve a high level of whiteness by bleaching out carotene (vitamin A). Other pigments include the flavon tricin and the xanthophylls lutein diester, lutein monester, and free lutein.  Wheat kernels also contain a multitude of enzymes necessary for healthy growth including α-amylase, β-amylase, proteases, liped esterases, phytase, catalase, and ascorbic acid oxidase. Farmers want to harvest dry grain in the field since moisture and cool temperatures cause the germ to release enzymes and break seed dormancy. The germination process causes “sprout damage” and degrades the quality of baking flour with elevated enzyme levels. As little as 5% sprouted grain mixed with high quality grain can render flour unacceptable for baking.  

Comparing Old and New Varieties
Depending on variety and growing conditions, important macro-minerals like calcium, magnesium, and potassium, and such micro-minerals as iron, copper, zinc, and selenium compose about 1.5-2.0% of the kernel. Also known as the ash content, these are distributed variably in the aleurone layer (55%), endosperm (20%), pericarp (10%), scutellum and testa (10%), and embryo (5%). Chemical analyses have shown that the more highly refined the flour, the lower the mineral content, and that modern hybrid varieties have lower mineral rates compared to older genotypes.

In recent studies at the Swedish University of Agricultural Sciences in Alarp, cereal researchers Abrar Hussain, Hans Larsson, Ramune Kuktaite, and Eva Johansson conducted chemical analysis using plasma mass and plasma atomic emission spectrometry on 321 spring and winter wheat genotypes. These were divided into six groups: primitive grains, spelts, landraces, old variety selections, old cultivars (1900-1960s releases/hybrids), and new cultivars (varieties released since 1970) to determine nutritional variations among the genotypes. (The researchers define old variety selections as wheats that have been selected for organic cultivation from such “old material” spelt, durum, and bread wheat breeding lines.)

The grains were grown at several locations in Sweden and under organic conditions in order to provide comparative results without influence of synthetic soil amendments, herbicides, pesticides, or other chemical inputs. Results of the study were published in “Mineral Composition of Organically Grown Wheat Genotypes” in the International Journal of Environmental Research and Public Health (September 2010) and indicate substantial variation among genotype groups.

Primitive grains and old variety selections were found to have the highest concentrations of the most minerals with selections highest in manganese, phosphorus, and selenium. The primitive grains einkorn, emmer, and durum showed significantly the highest level of zinc and elevated levels of potassium, boron, and copper compared to the other groups. Old variety selections showed the highest levels of magnesium, potassium, and selenium, and a high concentration of calcium. Landrace wheats showed the highest concentration of calcium and high levels of boron and iron. Spelts were highest in sulfur and high in copper, while old cultivars led in iron and were high in calcium.

The Alnarp researchers suggest that the negative correlation between recent cultivars and mineral density indicated in their study and similar investigations elsewhere is likely due to a dilution effect given the increased yield of most modern varieties. A related effect may explain the significantly higher levels in spring wheats of six minerals (B, Cu, Fe, Zn, Ca, S) compared to two in higher yielding winter varieties (Mo, P). Concentrations of three minerals (Se, Mg, Mn) did not differ significantly between spring and winter (fall seeded) wheats.

These studies indicate that mineral levels in whole grain kernels depend on absorption in the soil by the plant’s root system and subsequent redistribution to the kernels through vegetative tissues that are also influenced by photosynthesis. Higher chlorophyll content, for example, is positively correlated to iron concentration, as is availability of nitrogen which facilitates photosynthesis. The Alnarp study also indicates that grain genotype is more influential than location for mineral content in primitive grains. Finally, growing environments significantly contribute to variations for others, and high organic matter and increased soil pH also favor mineral concentration.

Wheat and other grains are important sources of minerals for maintaining human health, and 200 grams of flour per day from many wheat types can contribute nearly 100% of recommended daily requirements. Levels are dependent on the part of the grain that is used, with whole grain flour the most nutritious given the high concentration of minerals in the aleurone level. Microelement malnutrition is a significant global problem as some three billion people today suffer from such “hidden hunger” that can cause high premature infant mortality rates, permanent cognitive impairment, and lower workplace efficiency.

Wheat represents an important renewable resource to alleviate these problems and in many places in the United States and abroad is much more available and affordable than other mineral sources like fish, fruits, and vegetables. Some of these critical minerals and their important functions are summarized below.

Magnesium: Contributes to efficient metabolism as well as proper muscle and nerve functioning; also shown to reduce diabetes and metabolic ailments.
Calcium: An essential component for development of the musculoskeletal, cardiovascular, and nervous systems, and promotes overall physiological performance.
Phosphorus: Necessary for proper functioning of kidneys and heart muscle, contributes to bone and dental strength, and regulates protein reactions.
Potassium: Contributes to proper heart muscle contraction, neural impulse transmission, and fluid system balance.
Copper: Facilitates the functions of C-oxidase enzymes, promotes connective tissue development, and iron metabolism.
Selenium: Inhibits some types of cancer cell formation and promotes essential antioxidant reactions.
Iron: Synthesizes hemoglobin and produces energy.
Zinc: Regulates the function of many enzymes, glucose, and insulin, and synthesizes proteins.

The presence, or absence, of minerals also contributes significantly to food flavor since taste the tongue’s taste bud receptors are sensitive to many metal and hydrogen ions as well as protein compounds. Selection of grain variety and the milling process affects mineral content since it is reduced through heat, extended exposure to processing and bleaching, and removal of the aleurone level. As these factors are moderated a wider range and deeper intensity of flavors are sensed that has led some agronomist and artisan bakers to speak of grains in ways akin to noble wine grape varieties.

Acclaimed Wales & Johnson University baker-instructor Peter Reinhart writes in Bread Revolution: World Class Baking with Sprouted Grains, Heirloom Flours & Fresh Techniques (Crown Publishing Group, 2014) of “the full potential to unlock the flavor trapped in the grain” as a twin mission of craft and quality. He cites the work of Dr. Stephen Jones, director of the Washington State University Agricultural Research Center at Mt. Vernon, Washington, who has championed a national effort to breed a new generation of wheats with the distinct flavor and nutritional profiles of heritage varieties crossbred with other strains for disease resistance and adaptability to particular soil and climatic conditions.

This “evolutionary participatory breeding” approach to meet needs of local farmers, artisan bakers, and consumers has led to new hybrid wheats like hard white Edison and Bauermeister, extensive research on the potential of perennial grains, and reintroduction of the landraces soft Russian Red and White Lammas—the Northwest’s original frontier-era wheat. Jones also speaks of superior flavor profiles from these introductions: “If you plant wheat that’s designed for wet regions… it will produce chocolate, spicy, and pleasurable grassy flavor tones. But if you plant that same wheat in dry regions where so much commodity wheat is grown, it’s almost flavorless.” 

Jones introduced Victoria, British Columbia artisan baker Cliff Leir to breads made from the flour of Red Fife, a high quality eastern European hard red spring variety introduced to Canada in the 1840s. Most 19th century hard red wheat introductions like Red Fife and Turkey Red were not favored by millers in the U. S. and Canada in spite of favorable flavor profiles and desirable baking characteristics. The kernels were simply too hard to properly mill using the equipment of the time so many growers shifted to production of softer varieties. A recent if modest revival of heirloom Red Fife production encouraged by Jones has provided Leir’s Fol Èpi Bakery with flour he stone-mills on  site that he describes as “yellow crumb with an intense scent of herbs with a rich spicy flavor.”

The subtitle of Maine farmer David Buchanan’s recent book, Taste, Memory: Forgotten Foods, Lost Flavors, and Whey They Matter (Chelsea Green Publishing, 2012), suggests that healthy food systems involve a balance of culinary traditions and biodiversity with a context of sustainable and economically efficient agriculture. The Slow Food USA organizer explored industrial and craft milling operations across the country and tasted vegetables, fruits, and grains in foods from the Southwest to the Northeast. Through Glenn Roberts, founder of Anson Mills in Columbia, South Carolina, Buchanan experienced the unique flavors of such heritage wheats as soft winter Red May, soft white spring Sonora, and hard Turkey Red, and their distinctive suitability for various  foods. Turkey Red made dark, earthy yeast breads of “excellent flavor,” while the flatbreads and muffins of Sonora he found “light, nutty, and nearly free of gluten.” Sounding more like a sommelier, Roberts spoke of Sonora’s “lingering sweetness on the back palate… and a haunting minerality,” which has been characterized by others as “a rich buttery earthiness.”

James Beard Outstanding American Chef Dan Barber of Manhattan’s Blue Hill Restaurant is a grateful beneficiary of Jones’s research which he describes in The Third Plate: Field Notes on the Future of Food (Penguin Press, 2014). Barber points out the relevance of both older varieties and new selections not only for improved quality of breads and other bakery products, but also for malting barleys that impart distinctive tastes in a new generation of craft brews that can be paired with fine food like premium wines. For this work, Jones and Oregon State University crop breeder Pat Hayes have collaborated on participatory breeding introduction of Alba and Copeland barleys for Northwest coastal environments, and have worked with area growers to revive such flavorful hulless, colored heritage barleys as Purple Egyptian and Blue Himalayan.