Sorghum milling study
July 9, 2013
by Arvin Donley
Sorghum is one of the oldest known grains originating in Africa and India, where it is commonly used in a variety of foods. Around the world, it is used in food products including porridges, breads, cookies, tortillas and extruded commercial products.
While sorghum traditionally has been used in many places for animal feed, nearly 40% of the global sorghum crop is now being used for human consumption.
The recent interest in a wide variety of antioxidants naturally found in sorghum has been driving new research on characterization of processing properties of different hybrids. Some researchers have worked on adapting the Single Kernel Characterization System for sorghum grains. This system determines some important attributes of the kernel such as hardness, weight and size. Because antioxidants are contained in a particular sorghum kernel structure between pericarp and endosperm, it is imperative to develop new industrial milling technologies which can aid in processing these kernel structural components, and which incorporates them into the sorghum flour for usage in food products.
In a research paper entitled, “New Processing Alternatives for Production of Low Fat and Ash Sorghum Flour,” Florin Iva, a graduate student in the Department of Grain Science and Industry at Kansas State University, noted that most sorghum flour available in the market place is whole grain flour with inferior stability and baking characteristics.
“While the demand exists for high quality stable sorghum flour with low fiber and fat content, the current decortication step used for separating the bran from endosperm in sorghum milling is not economically viable and the alternative techniques, which are based on abrasion and fractions, do a poor job and to increase endosperm loss,” he said in the paper, which was published in 2012.
The inspiration for the research paper was “the lack of information regarding sorghum dry milling to obtain low fat and low ash white sorghum flour.”
The main method developed in this study for processing sorghum without decorication consisted of the following systems: prebreak, gradual reduction system with purification, and an impact technology. It was
named F20105. Also, two short laboratory methods were designed for obtaining white sorghum flour for comparison purposes. These were named F20106 and F20107.
The method F20106 was based on the use of a Buhler Experimental Mill, a Great Western Gyratory Sieve and Quadrumat Brabender Sr. Experimental Mill. The method F20107 was based on processing decorticated sorghum in a process which uses a hammermill, a Great Western Gyratory Sieve and an Alpine Pin Mill.
A commercial white sorghum flour was evaluated along with the flours from the different methods in order to make comparisons among them.
The flour obtained from F20105 (long reduction system), F2105 (Buhler-Quadrumat short laboratory flow) and F20107 (Hammer/Pin milling) were identified as 148, 210 and 200, respectively. The commercial flour was identified as 144. The milling procedure in F20107 produced the largest flour yield of the three procedures used. However, fat, ash and fiber contents were highest in that flour. This is undesirable in white sorghum flour.
The flour yield from 20105 and 20106 were similar, and they also had the best color. The fat, ash and fiber contents differed very little among flour samples 148, 144, 210, and in some cases, these differences were not statistically significant.
The percentage of starch damage was similar for the samples 148 and 144, and also these flours had the same particle size distribution as well. The milling procedure that used both the Buhler and Quadrumat Mill (sample 210) produced the highest amount of damaged starch in the flour, while the lowest was attained by the hammermill (210) and pin mill (200). The lowest protein content was found in sample 210 and the highest level was found in commercial sample 144.
The cumulative curves of fat and ash content for the samples 148 and 210 were very similar. However, the slope was very small throughout the range of sorghum flour yield tested (5% to 75%). This indicated that the marginal gain of fat and ash content with every unit of sorghum flour produced was very small for both F20105 and F20106. It can be shown from these figures and from the proximate analysis of the resulting flour that these milling procedures were effective in reducing the particle size of endosperm and separating it from the bran and germ.
This finding made the use of a short laboratory flow, specifically F20106, as a check for the long reduction system more relevant. Nevertheless, the high degree of damaged starch associated with flour from short diagram F20106 should be considered as a drawback of its utilization.
The long reduction system (FS20105) which included impact detaching techniques, produced white sorghum flour with a high extraction rate and good baking properties. An impact dehulling machine and a prebreak roller mill were effective in collecting glumes and cracking the sorghum kernels before first break.
The three sets of sorghum flour produced by the milling procedures from F20105, F20106 and F20107, plus commercial flour, were tested using the Mixolab, manufactured by Chopin Technologies.
There were visible differences, from a qualitative point of view, in the mixing and pasting behavior of these samples.
The batter of samples 144 and 148 were more stable after gelatinization.
The flour sample 200 had the best stability during the mixing time. The flour also had the best proofing and baking behavior. The highest amount of water was added to sample 210, while the lowest was added to sample 200.
Concomitantly with water addition, peak dough viscosity is higher for sample 210 and lowest for sample 200 due to high and low, respectively, starch damage.
The study found that a long reduction system which included impact detaching techniques produced white sorghum flour with a high extraction rate and good quality flour (compared with existing flour on the market), baking and bread properties.
An impact dehulling machine and a prebreak roller mill were effective in preparing the sorghum kernels before first break. The shattering effect of the fragile sorghum bran was avoided by implementing air separation of bran from endosperm before each break. A purification system effectively cleaned and sorted the sorghum grits by size.
Sorghum flours with different protein contents were evaluated for their baking quality properties.
The protein content of sorghum flour was found to have a strong positive correlation with the amount of water added to the batter, cell wall thickness, cell diameter and cell volume, and was strongly negatively correlated with the number of cells per square centimeter and L-value of the bread crust. It was also correlated with the a-value and b-value of the bread crust. The flow diagrams F20105, F20106, and F20107 can be used successfully in their current form or with small adjustments to obtain flour from different sorghum hybrids at the laboratory scale. These diagrams also fill a gap in the currently available milling literature. Additionally, they can be scaled up in the sorghum processing industry.
The study said a growing gluten-free food product market would potentially provide a rapid return on the necessary investment.
Sorghum flour increasingly popular as gluten-free solution
The U.S. Department of Agriculture’s Agricultural Research Service (ARS) reports that growers in the U.S. are increasing their interest in producing human food products from sorghum due to the development of white sorghum.
White sorghum flour has a light color and a neutral flavor. Processed similar to wheat flour, white sorghum flour has a bland flavor that can be beneficial because it does not add an unfamiliar or distinctive taste. Whole grain white sorghum flour can be used to provide nutritional benefits associated with whole grains.
Sorghum flour is becoming increasingly popular as it offers a gluten-free solution that is more economical than specialty starches and competitively priced with other flours.
Traditionally a niche market, gluten-free food products have become a dynamic sector, with an annual growth rate of nearly 30%.
According to a survey conducted by the market research company, Mintel, in 2008, 8% of the U.S. population was in search of gluten-free when they shopped for groceries. By 2014, the U.S. market is expected to grow by more than $500 million, making the U.S. population 53% of the world gluten-free market.
According to a 2010 Datamonitor analysis, the gluten-free market is expected to reach $4.3 billion by 2015.
White sorghum flour is designed for people with intolerance to wheat, and specifically for people with celiac disease, who suffer from a lifelong inflammatory condition of the intestinal tract caused by gluten, a protein found in wheat, rye and barley. It is estimated that 1 in 33 people have the disease in the U.S. and must avoid gluten.
Wheat gluten can be difficult to avoid because it is found in many packaged foods such as soups and sauces, under the following names: modified starch, hydrolysed/textured vegetable/plant protein, binders, fillers, extenders and malt.