Milling sorghum

by Arvin Donley
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Gluten is a special type of protein commonly found in wheat, rye and barley that helps make bread elastic, providing it with its chewy texture, enabling it to rise before it is baked and helping the bread maintain its proper shape once it is cooked.

But for people with Celiac disease — studies in both the U.S. and Europe show the disease effects about 1% of the population in those countries — gluten is something to be avoided at all costs, as ingestion of this protein can cause inflammation in the upper small intestine, leading to a wide variety of undesirable symptoms.

Increasingly, people with Celiac disease — and even those who don’t suffer from the disease — are demanding products without gluten, which has led to numerous product launches during the past decade.

A September 2009 report from the Hartman Group, Bellevue, Washington, U.S., showed about 40 million U.S. consumers, or 13.2% of the population, were interested in gluten-free products. The report also said that 93% of the people interested in a gluten-free diet are not diagnosed as having celiac disease.

In a survey of U.S. food stores (excluding supercenters) with sales of $2 million or more, the Nielsen Co. found that gluten-free product sales totaled more than $306 million in the four weeks ended Feb. 20, 2010, which marked an 18% increase compared to the four weeks ended Feb. 21, 2009 ($258.5 million).

"It’s always been on our customers’ radar," Kyle Marinkovich, marketing manager for Cargill, Minneapolis, Minnesota, U.S. told World Grain’s sister publication Milling & Baking News. "Over the last couple of years, interest in it has increased significantly."

To meet this growing demand, some milling companies are ramping up production of flour from grains that don’t contain gluten, including sorghum.

The main function of sorghum, which makes up less than 5% of the world’s grain production, is as an animal feed, especially in the U.S., Mexico, South America and Australia. In the U.S., approximately 12% of sorghum production goes toward ethanol and its various co-products.

However, about 40% of the global sorghum crop is used for human consumption, especially in semi-arid regions of Asia and Africa.

With concerns about global warming and its potential impact on traditional grains such as corn (maize) and soybeans, sorghum’s propensity to grow in dry and hot weather conditions is causing researchers to look at it more in terms of its food potential.

With the exception of its much smaller size and slightly oval shape, the structure of the sorghum kernel is very similar to corn. It is composed of three parts: the pericarp (outer covering which makes up 8% of the kernel), germ (embryo which makes up 10%) and endosperm (storage tissue which makes up 82%). The germ of the sorghum kernel is located at the base of the seed and is tightly embedded with the kernel, causing difficulty for removal during dry and wet milling.

Sorghum tends to have higher oil content in comparison to wheat, thus sorghum flour typically has a higher fat content, leading to issues with oxidative rancidity. Also, because the kernel contains both hard and soft endosperm, milling sorghum can be challenging.


Some of these challenges were examined in a 2009 research paper by Kansas State University (KSU) student Emily Frederick. The paper, "Effect of Sorghum Flour Composition and Particle Size on Quality of Gluten-free Bread," noted that limited publications exist on sorghum milling techniques, and processes developed for individual sorghum milling operations remain proprietary. But as research regarding sorghum continues to expand, milling techniques are being developed that take advantage of the unique characteristics of the sorghum kernel.

For sorghum flour production, there are two general approaches to milling. The first involves the removal of the germ and the bran, a process called degermination, followed by reduction of the remaining endosperm. This method is utilized in maize milling. Alternatively, the kernel can be first broken open, allowing the endosperm to be scraped from the bran layer. This method is primarily used in wheat and rye milling.

With either method, the endosperm fraction should have as little bran and germ contamination as possible, as these products discolor the flour and affect the shelf-life of the product.

The paper noted that sorghum bran is more fragile than wheat bran and therefore tends to fracture more easily into small pieces that contaminate the endosperm fraction.

Additionally, due to the small, slightly flattened, round shape of the sorghum kernel, there is difficulty in making clean separation of components, including removal of the germ. As a result, the sorghum endosperm becomes fractured during the degermination process, and bran contamination occurs.

According to the paper, decortication is often the preferred method of bran and germ removal for sorghum, particularly for tannin-containing varieties, as it aids in the removal of the testa and thus reduces the tannin content of the grain and flour.

More recently, the paper noted, research efforts have returned focus to applying wheat roller milling technologies and equipment to the milling of sorghum. While this technique is able to produce a somewhat more bran- and germ-free product, the main advantage to roller milling is the incorporation of reduction rolls, which helps produce finer sorghum flour. For this process, the top pair of rollers are coarsely corrugated "break" rolls and the second pair are finer corrugated break rolls. If present, the third pair of rolls are smooth reduction rolls.

Studies have shown that with moderate pre-conditioning (tempering to 16% moisture), roller milling can consistently produce sorghum flour of higher extraction and lower ash and fat levels compared to sorghum flour produced by decortication and subsequent hammer milling.

The paper said yield of the flour produced by the break rolls ranges from 10% to 15% and contains most of the floury endosperm, which is low in protein.

The flour produced from the reduction rolls contains a greater amount of bran and germ particles, and it is "specky" and higher in oil content.

Some researchers regard separation of sorghum grain constituents in dry roller milling as poor. J.E. Cecil described a semi-wet roller milling process for sorghum that resulted in more effective separation of the bran and germ. Research showed that with the addition of 20% water to the grain and conditioning at 60 degrees C for six hours, sorghum could be efficiently milled on standard wheat roller mills.

This water addition is far more than is used in dry milling, but considerably less than the amount used in wet milling.

According to Cecil, this process yielded "high-quality, fine flour." The process was also able to effectively reduce tannin content in the flour when red or broomcorn varieties of sorghum were milled.

However, the disadvantages of this method may outweigh the benefits, the study noted, pointing to microbiological growth inside the mill, poor flour yield, necessary removal of residual moisture in flour and byproducts, and screen blockage due to poor flow characteristics as problems associated with semi-dry milling.


The main goal of the KSU study was to examine the effects of sorghum flour composition and particle size on functionality in gluten-free batter bread. White, food-grade sorghum was milled to flour of varying extraction rates (60%, 80% and 100%), and was subsequently pin milled at different speeds to create flours of both variable composition and particle size.

Two commercially milled sorghum flour samples were included in the study and subjected to the same pin milling treatments. Characterization of each flour included measurements of flour composition, total starch content, particle size distribution, damaged starch and water absorption.

Bread characterization included measurement of specific volume, crumb properties and crumb firmness through the use of digital imaging and texture profile analysis.

Significant differences were found in the composition of sorghum flours of varying extraction rates, most notably for fiber and total starch contents. Flour particle size and starch damage were significantly impacted by extraction rate and speed of pin milling. With the exception of the commercial flour samples, water absorption increased significantly with increasing extraction rate and the pin milling speed.

Within all treatments, breads produced from 60% extraction flour had significantly higher specific volumes, better crumb properties and lower crumb firmness when compared to all other extractions and flour types. These measured bread characteristics were significantly impacted by flour properties, specifically particle size, starch damage and fiber content.

The commercial flour studied produced breads of low specific volume, poor crumb properties and dense textures.

The study concluded that sorghum flour composition and particle size affect the quality of gluten-free bread. It also found that, to an extent, flours with lower amounts of fiber and a smaller particle size will produce breads with more acceptable characteristics including volume, crumb structure, color and texture.

"However, it is important to note," the study said, "that these flour characteristics do not exert their influences independently of one another. In fact, this research points to the importance in understanding the impact of starch damage on bread performance.