Milling wheat

by Emily Wilson
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The overall suitability of a wheat for milling may be summed up in a single word — quality. While this is a simple statement, implying a simple situation, the reality is far from it. Anyone in the business of growing, marketing or processing wheat knows that it is not a uniform commodity.

In commercial trading, several species are involved, and within each species differences exist as a result of inherent characteristics and the manner in which crops are grown. This range of variation makes the wheat choice available to primary and secondary processors not only enormous but, to some at least, quite baffling. Add to this the changing needs of consumers and the picture of complexity expands even further.

Price should not be ignored because in a competitive industry with relatively small margins, premiums for the very best raw materials can only be justified by the primary processor if selling to end users also commands an adequate premium.

The successful end-user of the processed product from wheat — be it white flour, wholemeal flour or by-product such as bran or wheat germ — understands the requirements of his production system and sources appropriately to meet those needs as economically as possible. This means that the successful end-user takes an interest in the type of wheat from which his own raw material comes and the manner in which his supplier processes it. This article attempts to offer some broad definitions of quality in relation to individual wheat-based products.

QUALITY ASPECTS. Some quality criteria are fundamental to all wheat samples intended for food use, and are valuable for feed and industrial uses as well. Test weight (bushel weight, hectolitre weight, etc) measures the weight of grains that fill a container of a defined size and shape, under specified packing conditions.

As there is little correlation between test weight and any measures of processing performance, this is usually applied as a threshold requirement, to exclude samples with low test weight because they contain many shrivelled, insect-damaged or sprouted grains.

Subjective sensory tests include visual assessment, which can detect shrivelled grains; green, immature grains; and frost-damaged grains. Admixture of weed seeds and grains of other crop species might also be detected in this way.

A dull appearance of grains might suggest microbial contamination, either from the field or arising during storage or contamination with soil or other contaminant. Microbial contamination also usually leads to an easily detected characteristic aroma.

Diseases acquired in the field include smuts and bunts. Black-point is sometimes the result of fungal infection but it is non-specific in terms of the causative organism. Problems arising from fungal infestation include the presence of metabolites such as mycotoxins and alkaloids present in ergot. Foreign matter can be quantified by dockage testing.

Seriously sprouted grains are detectable visually, owing to the presence of elongated roots and coleoptiles. When abundant, they may also lead to detectable reduction in test weight; however, the germination process can begin without external evidence being present. Furthermore, some varieties growing under certain conditions have inherently high levels. Elevated levels of hydrolytic enzymes (most importantly alpha amylase) characteristic of such grains can be quantified by enzyme assays or, more commonly, by the Falling Number test. Low Falling Numbers indicate high enzyme levels.

Pesticides applied inappropriately to grain in storage can remain as a residue on grains. Detection of these is specialized and expensive and it is not carried out as a routine intake test. However, random testing should be done regularly by processors, industry representatives or government authorities.

In order to make products of consistently high quality, it is desirable to work with raw materials that are of consistently high quality. Increased use of mechanized secondary processing has exaggerated this requirement. Consequently, millers are expected to deliver, over long periods, flours with uniform characteristics, even when they have at their disposal wheats that vary in many respects. It is in this area that the expertise of buyers, as well as those responsible for gristing or blending, is vital.

PROCESSING INTERACTIONS. Most milling of white flour is performed by the gradual reduction roller milling system, whereby bran and endosperm are first separated on the "first break" passage, after which each component is further purified and reduced in size through separate passages. The number of passages adopted for the endosperm treatments influences the quality of the flour as short systems tend to involve greater roll pressures for each passage.

The main variable resulting from varying pressure and number of passages is the level of starch damage. As damaged granules hold more water than intact ones, this level determines how much water needs to be added to a flour to make a dough of the required consistency. The end use, processing method and local and national preferences determine acceptable levels of starch damage.

Alternative milling methods exist, the longest established being stone-grinding. This is used mainly for production of wholemeal flours and is preferred by advocates of traditional technologies. The perceived advantages are difficult to define, although particle shape and size distribution are undoubtedly different from roller-milled products.

A more recent commercial introduction for producing white flour is a debranning process in which bran layers are gradually abraded from the grain surface, leaving a core of endosperm that is subsequently ground into smaller particles. These flours differ from that of conventionally produced flours as the small amounts of bran included in the flour by each method originate in different bran components Thus, in conventional milling most bran specks are of seed coat origin while in flours from the debranning process more aleurone particles are present.

Another milling variable lies in the extraction rate, a measure of the proportion of the wheat milled that goes into a particular flour. The lower the extraction rate, the more likely flour is to consist entirely of ground endosperm while as extraction rate increases, more ground bran and embryo fragments are likely to be included.

Noodle flours may represent only 60% extraction; they are, therefore, very pure endosperm. Long extraction flours may represent over 80% of the starting wheat weight.

SPECIES VARIATION. The greatest differences among commercial wheat types occurs at a species level. By far, the predominant wheat species cultivated is the hexaploid Triticum aestivum L, known as bread wheat, common wheat, soft wheat (literal translation of the French ‘bl tendre'). It can be used for the greatest range of products because within the species there are many cultivars with very varied characters.

Some of the more fundamental variables are seed coat color (red or white), endosperm texture (hard or soft), and protein strength. Bread wheats may be spring sown or winter sown. Because of their shorter growing season, spring wheats accumulate less starch and thus give lower yields than winter sown types, but protein contents are generally higher.

The tetraploid T. durum, known as durum wheat, macaroni wheat, pasta wheat, hard wheat (French ‘bl dur'), etc., is second in abundance, accounting for about 5% of total world wheat production. Fewer varieties of this species have been developed and less variation in characteristics exists.

As its name implies, all durum wheats have hard endosperm. Although some red varieties are grown, mainly for feed, milling varieties have "white" seed coats, and all cultivars used successfully in food have strong protein. Amber pigmentation in the endosperm is desirable and the description "amber durum" is applied to those varieties that meet this requirement.

The range of durum wheat products is more limited than that of common wheat, mostly falling into the pasta category. Some bread is made with this variety.

All varieties of the tetraploid T. compactum have soft endosperm. Although red types exist, much of their commercial advantage lies in their white color, so only these are grown for processing. Club wheat flours do not make good commercial bread but their typically low content of weak protein makes them suitable for cookie and pastry production or noodle manufacturing.

A species that is generating some interest is the hexaploid T. spelta. L, or spelt wheat. This ancient type has been cultivated since the Bronze Age and is one of the hulled wheats; in other words, it does not thresh freely so that grains remain attached to the leaf-like structures, which in free-threshing varieties are separated as chaff. The hulls provided protection against predators during storage but needed to be removed before grinding.

Spelt and some other hulled wheats have been cultivated continuously in some mountainous regions of Europe and Asia since ancient times, but recent interest has arisen mainly among those who reject modern agricultural practices in favor of the organic approach.

Bread and other products made from spelt have a distinct character and stronger flavor, providing variety and novelty for the wider consuming public. Some success also has been achieved with pasta manufacture from spelt wheat.

Other ancient species being considered as potential alternatives to T. aestivum in familiar products are Polish wheat, T. polonicum, Einkorn T. monococcum and Emmer T.dicoccum. One of the attractions of the ancient species is that fertilizer and disease protection requirements are minimal. These species also will grow in soils that would not support bread wheat crops, and are highly compatible with the principles of organic production because application of fungicides and inorganic fertilizers can be avoided.

This does not mean that only ancient species can be grown organically, as much bread wheat is grown in this way.

INHERENT CHARACTERISTICS. In some of the major exporting nations, marketing of wheats has undergone radical change in recent years as a result of widespread decontrol. Under government-controlled schemes, it was usual for classes or grades to be arbitrarily defined in terms of compositional and morphological criteria.

All grains for trading were transported to receivals, where they were blended with other grains deemed to be of the same grade. Customers bought the grade that most closely approached their requirements. This system had the advantage of making available large quantities of uniform blends and it worked when, within a market catchment, the same few varieties predominated.

New, improved cultivars created characteristics not defined by the grading criteria. Thus, in blending, particular virtues associated with a single variety became diluted. Grade criteria were inadequate to exclude varieties with undesirable characteristics and the overall quality of blends became vulnerable.

The increased range of flour-based products within the same regional market also has influenced market philosophy. End users and technologists, presented with a wider range of varieties, were able to recognize varieties with desirable attributes in the contexts of their products, even though cereal chemists were unable to define the properties in chemical terms. This has promoted an increased interest in identity preserved wheats, whereby purchasers can buy samples of individual varieties known to best fit their needs.

Sometimes the requirement may be less specific and blending may be acceptable, but it is not unusual for a customer to insist that certain varieties be excluded. Customers now know better what they want and are better able to persuade exporters to meet their needs.

While it is possible to define some criteria of flours, and hence wheats, for certain purposes, requirements change in response to technological advances and political needs. Examples of engines of change are tariff barriers, the desire of nations and trading blocs for self-sufficiency in wheats, and advances in breadmaking techniques that allow bread to be made from flour of lower protein content.

In spite of technological advances and reduced protein content requirements, protein type remains the main criterion for breadmaking. All varieties have complements of many different protein types that contribute to the gluten that is produced on wetting and mixing. Scientists now have a wide but not complete knowledge of the types of glutenin sub-units that contribute to good breadmaking qualities.

Genetic engineering offers quicker, cheaper and more efficient means of producing tailor-made varieties. We are now reaching the stage where political barriers impede progress more than technological ones.

Protein content and grain hardness are criteria that define processors' requirements more than any other. Protein is not the most important component for all end uses. The economics of starch production from wheat demand that a market exists for both products. As prices of these commodities fluctuate, industry requirements also vary. When the market for gluten is high, a high protein wheat is favored. But when the market is stronger for starch, it is better to start with a high starch, lower protein wheat.

Because damage to starch is always undesirable in starch production, soft wheats are preferred. Starch granules occur in two distinct size ranges in wheat, and the best quality commercial products are made from the larger granules. The average contribution of large granules is 70% w/w but there are a few varieties with greater proportions of large granules under development, and these clearly carry advantages.

Similarly, advantages exist for varieties in which the ratio of starch polymers differ from that normally expected for the species. Thus, high amylopectin (waxy) and high amylose varieties command a premium when available as these have higher value applications.

Amylose/amylopectin ratio also is important in some flour applications. Some Australian wheats particularly suitable for Japanese noodles had a reduced proportion of amylose.

Clearly, quality, like beauty, is in the eye of the beholder. In 1997, an international wheat quality conference was held at Kansas State University, Manhattan, Kansas, U.S., to provide a forum for the discussion of national, international and individual concepts and strategies for defining and meeting processors' requirements. A second international conference is planned for May 20-24, 2001, also at Manhattan. For information, go to

Tony Evers, a former head of the Cereals and Milling Department at Campden and Chorleywood Food Research Association (U.K.) Ltd., is now a consultant for the cereals industry.