Processing lower quality wheat
August 01, 1997
by Teresa Acklin
Industry consultant David Sugden offers solutions for handling and milling wheat with various defects.
Defects in wheat milled into flour occasionally have bedeviled and confounded the miller over the ages and no doubt will continue to do so. But customer satisfaction can be seriously upset if the miller does not recognize and deal effectively with lower quality grain.
Preventing the acceptance of undesirable wheat in the plant depends on good defenses and commercial decisions. Protection is provided by accurate sampling, by noting wheat's appearance and odor and by comparing wheat with specifications.
Accordingly, an experienced operative at the intake point is invaluable because his know-how can save untold problems, no matter what the defect may be. The intake laboratory also plays a large role. The two will not only save a great deal of money and trouble but also enable below-specification wheat to be directed to bins for potential blending.
Proper sampling is critical to the process because unrepresentative samples are misleading to the point of uselessness. Thus, operatives need not only experience but sampling techniques that assure each delivery is reasonably covered before discharge.
A dockage tester to determine different fractions by particle size and specific gravity will provide an early indication of abnormality. Hectoliter weight, moisture, protein and sedimentation tests will further characterize wheat.
Not to be forgotten in difficult years is some suitable measurement of sprouting such as the Falling Number test. Laboratory milling and gluten washing will quickly give a further indication of suitability. Additional tests at intake include dough rheology and even baking, but these are not as suitable as a screening method because of the time factor.
Depending on the terms of the wheat purchase, it may be possible to reject outright any unsuitable grain. If rejection is not possible, suspect wheat requires isolation pending a commercial negotiation of a discounted price. If wheat is unusable, resale at an agreed value is necessary.
The other classical defenses against defective wheat include a well equipped wheat cleaning plant and blending of wheat and flour. A complete answer to wheat defects seldom exists, but methods can be used to mitigate difficulties.Fungus Problems
A number of specific wheat defects are related to various types of fungus. Mycotoxins, molds that grow in high humidity and moisture conditions, include vomitoxin and aflatoxin. For vomitoxin, the U.S. Food and Drug Administration maintains an advisory limit of one part per million, while aflatoxins carry a more restrictive limit of 20 parts per billion.
Wheat with vomitoxin, also known as scab, may be rendered acceptable through blending, scouring with aspiration or debranning. The use of fluid bed and specific gravity separators also can be successful because contaminated wheat has a lower density than healthy wheat. Nonetheless, trial and error is the only truly reliable method of successful treatment.
Ergot, a fungus of wheat and rye, generally is larger than wheat, although similar in shape. Ergot is dark purple, and under ultra-violet light it shows a pale brown fluorescence. It is a poison that can cause abortion; contamination levels of 0.05% are considered by some to be safe.
Solutions to ergot problems in wheat include color sorting, an excellent if expensive method of separation. Specific gravity separation also may be used, although results may be less successful because ergoty kernels will be close in density to healthy kernels.
Karnal bunt, also known as stinking smut, is another fungus very much in the news in the past year. Infected kernels are slightly swollen, filled not with endosperm but black spores that have a pungent fish smell.
Karnal bunt poses particular difficulties because infected grain when broken spreads over sound wheat and can be a threat to crops. Apart from dilution by blending, the practical answer to infected wheat is to scour and aspirate after specific gravity separation.Insects and Other Defects
Contamination by weevils and mites is common, and the partial solution is fumigation by methyl bromide, phosphine or their substitutes. The use of an Entoleter will destroy all eggs as well as live insects.
Whether live or dead, insects should be removed by scouring and aspiration. The danger of insect fragment counts in products is heightened if long-dead insects remain in the grain because of shattering during processing.
Grain damaged by the wheat bug is relatively common in dry regions such as the Mediterranean Rim, the former Soviet Union and the Danube Basin. The wheat bug consists of two species, Aelia and Eurygaster.
Grain is attacked in the field before harvest. The insects inject a proteolytic enzyme that damages gluten to the point of total lack of cohesion, rendering the flour useless for baking. The pin-point puncture can be recognized by a black dot surrounded by yellowish color over 25% of the kernel.
Only two solutions to wheat-bug damage exist, apart from rejection. These are blending and/or treatment of flour with ascorbic acid or bromate where allowed. Damaged wheat should be blended down to 1%.
Bin-heated wheat appears as a darkened interior of affected kernels with no gluten extractability possible. Rejection is the safest solution.
More subtle and less easy to recognize immediately is wheat that has been dried too quickly because appearance will not reveal the condition of non-elastic gluten. Gluten washing will highlight the problem, and blending is the solution.
The major problem with garlic is contamination of break roll corrugations. Garlic has a sharp taste and because it is soft, it all too easily finds its way into corrugated grooves. Contamination of corrugations makes extraction performance decline, and in extreme cases, loss of performance can amount to 3% or more.
Garlic can be separated from wheat by paddy separators or density/gravity tables. If garlic gets past well-set wheat cleaning machines to the break rolls, hand cleaning of the rolls is necessary. Various removal methods have been designed and tried, but none has been as successful as manual cleaning.
Rodent pellets are a constant threat to product purity. The supplier may provide guarantees against this type of contamination, but the miller must remain on guard.
Gravity tables and paddy and density separators are useful to clean wheat of rodent pellets, although they are not a perfect solution. Scourers also can be used as a complementary cleaning method.
Excess alpha amylase presents problems for bakers in particular. The level of alpha amylase activity typically is measured by the Falling Number test using a standard of 7 grams of ground wheat and 25 milliliters of water. Separate binning and judicious blending are solutions to excess alpha amylase.
Some mills divide light and heavy phase grain on density separators, which normally split wheat into 30% light and 70% heavy portions by hectoliter weight. The light or lower weight material carries an even higher alpha amylase content that the total stock.
These mills then have the option of selling the light portion of wheat for non-baking uses. Debranning also can be used to reduce alpha amylase because the excess is towards the outer side of the kernel.
The protein content of wheat also can create problems if too high or too low for the intended product. Blending is the most common way to attain desired protein levels, but other options exist.
One technique to reduce protein is the use of “flour protein shifting” by means of air classification. This process is expensive in terms of power consumption, although it is effective on soft wheat such as U.S. soft red winter, soft white wheat and many European varieties.
From a 10% flour protein on an “as is” moisture basis, the shifting process can yield up to 40% of 5% to 6% protein content on an “as is” moisture basis from the parent flour. This phenomenon works on the basis of particle size differentiation, which coincidentally shifts protein, because the particle size of low protein flour typically ranges between 15 and 40 microns.
This process also results in a 20% yield of high protein flour (15% to 18%) of fewer than 15 microns, where darker color and higher alpha amylase activity abound. The remainder of the flour is similar to the parent flour, although larger than 40 microns.
For wheat with protein content that is too low, higher protein wheat or flour blending will boost protein levels. The addition of gluten is another method of increasing protein.
The quality of wheat protein also can cause problems for the miller. If protein is too extensible, it can be adjusted by heat treatment, chlorine dioxide gas or bromate, if allowed. Protein with low extensibility can be treated with sulfur dioxide gas or a number of proteolytic enzyme preparations if legally permitted. But wheat selection and blending usually are the best solutions.
Wheat that is contaminated by agrochemicals can be improved by scouring and aspiration. For white flours, debranning is particularly effective because nearly all the problem resides on the outer bran layers. Debranning is less effective for whole wheat flours.
For bacteria on the wheat skin, the solution is treatment with chlorinated water. This normally is effective and is especially desirable in warm, humid climates.
In general, blending of both wheat and products is a must for ironing out troubles caused by defects in wheat. In really difficult circumstances, a run through the wheat cleaning department more than once can be beneficial, although a drawback for many mills usually is a lack of extra cleaning capacity.
Reselling below-grade wheat that has been separated from sound grain is another option, as resale sometimes will pay well when other possibilities have been costed.
David Sugden independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting Ingateston, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.