Most mills process either hard wheat or soft wheat; those that do both must make compromises
Most flour mills are designed to specifically process either hard wheat or soft wheat. It is the rare mill that handles both hard and soft wheats interchangeably, according to Henry H. Stevens, a former miller and milling instructor, now director of technical services with U.S. Wheat Associates in Portland, Oregon, U.S.
"Mills designed for grinding both very soft and very hard wheats always involve compromises in design that will affect either flour quality or mill capacity or both," Stevens told a recent gathering of the Association of Operative Millers.
In many regions of the world, it is considered uneconomical to mill a wide range of wheat hardness on the same milling unit. "Normally, durum wheat is milled on a unit dedicated exclusively for this very hard wheat," Stevens said. "There are exceptions, but they are rare."
He described a typical hard (non-durum) mill unit as one that might grind medium-hard to hard wheats, such as U.S. hard red winter and hard red spring wheats. A typical soft mill unit might grind soft to medium-hard wheats, such as U.S. soft red winter, lower protein hard red winter wheats, soft white and western white wheat.
The endosperm of hard wheat requires a lot of compressive force before it will fracture, Stevens said. In contrast, a soft wheat’s endosperm fractures very easily, with a minimum of compressive force. But that simple definition leaves many unanswered questions, Stevens said.
"Who decides whether the amount of compressive force required to fracture the endosperm is ‘a lot’ or ‘a minimum’?" he asked. "What is the dividing line between ‘a lot’ and ‘a minimum’ amount of compressive force? What do we do when we encounter a sample of wheat of a variety known to exhibit ‘hard’ end-use quality characteristics but whose endosperm is relatively ‘soft’?"
Hard wheats tend to have higher protein content than soft wheats, Stevens said. But he noted that protein content is not a reliable indicator of hardness. For example, samples of high protein (11% and higher) still have soft endosperm and weak gluten strength.
Hard wheats tend to have vitreous, or "glassy," kernels, while soft wheats tend to have non-vitreous, or "chalky," kernels. But vitreousness also is a very poor indicator of hardness, Stevens said.
He noted that although supporting data are not available, it is well-known in the industry that hard red winter wheat may have a wide range of vitreous kernel contents while still retaining the milling, and end-use, properties expected of a hard wheat.
"It can be a big mistake to assume that a non-vitreous wheat kernel is a soft wheat kernel, or that a vitreous kernel is a hard wheat kernel, although this is a less common problem," he said.
So what makes a hard wheat hard? Stevens concluded that the difference between a hard wheat and a soft wheat depends mostly on whether the interstitial proteins inside the endosperm cell can effectively "glue" the starch granules together. This, in turn, depends on the quantity of the interstitial protein and whether the interstitial protein "wets," or adheres to, the starch granules.
"Research indicates that the wetting ability depends, to a large degree, on the characteristics of the protein ‘skin’ of the starch granules," Stevens said. "If the interstitial protein ‘wets’ the starch granules, there will be a firm bond between starch granules, thus the kernel will be relatively hard."
This is all academic to millers, he said. "We know that we have real milling problems when we attempt to grind a very soft wheat on a mill designed for harder wheats. We know that we can have quality problems when we attempt to make a hard wheat flour on a mill designed for softer wheats."
The conditioning, sifting and grinding processes are very different for hard wheat and soft wheat. Mills designed for softer wheats often have a relatively short conditioning time, Stevens said. Some soft wheat is conditioned for only five hours prior to milling while hard wheat is typically conditioned for 12 to 24 hours.
"The interstitial spaces between the starch granules of soft wheat actively suck in any available water by means of capillary action," Stevens said. "This water penetration is relatively rapid.
"There are no interstitial spaces between the starch granules of hard wheat. Thus, water penetration must proceed by the relatively slow mechanism of diffusion."
Hard wheat that is milled after only a short conditioning time will still have a very hard endosperm, Stevens said. The higher roll pressures required to break the endosperm are enough to fracture the bran and germ.
"Because of this, soft wheat mills grinding hard wheats often have problems with flour color and flour ash," he said
Mills that try to grind soft wheat after conditioning the wheat for a relatively long period may find that the endosperm has literally sucked the water out of the bran, resulting in brittle bran and "gummy" endosperm. "The brittle bran may cause flour color and flour ash problems, but primarily the ‘gummy’ endosperm will result in sifting and flow problems in the mill," Stevens said.
The relative absence of interstitial proteins in soft wheat makes the endosperm very weak, he added. A minimum of force is required to break the bonds holding the endosperm structure together.
"When compressive forces are exerted on the structure, it tends to break apart into many small particles, most of which are individual starch granules," he said.
Small particles are highly susceptible to surface attractions. Soft wheat flour particles tend to stick to other surfaces and other flour particles, causing sifting problems, materials transport problems (chokes) and packing problems.
Stevens said soft wheat mills tend to use more aggressive sifting technology than hard wheat mills, such as "reverse" sifting, more extensive use of "turbo" or vibratory sifters, more extensive use of bran finishers to knock adhering flour particles loose from the bran and higher overall sifter surface. Chokes are usually solved by a higher angle of slope on pipes. Packing problems are addressed by using larger bags to hold equivalent amounts of flour.
When soft wheat must be milled on a hard wheat mill, Stevens said the load should be decreased by 10% to 20% in order to allow the sifters to perform adequately.
"Hard wheat flour is of a much higher average particle size than soft wheat flour," he said. "This makes sifting, materials transport and packing relatively easy. But milling hard wheat on a soft wheat mill can cause quality problems due to too much sifting."
Because oversifting can cause flour color and flour ash problems, hard wheat mills usually have less overall sifting surface than soft wheat mills, Stevens said, and they avoid the use of aggressive sifting technologies except where absolutely necessary.
When hard wheat must be milled on a soft wheat mill, "overload" the mill in order to avoid flour quality problems. Stevens noted this also will result in a lower extraction rate.
Because it is relatively easy to break down the endosperm for soft wheat, the grinding surface of a soft wheat mill is usually somewhat less than for a hard wheat mill, Stevens said.
"It is difficult to achieve high degrees of starch damage on a soft wheat mill," he said. "If the flour is intended for applications in which higher water absorption is important, it is necessary to under-load the rolls in order to achieve the desired result. In addition, the reduction rolls may be running with a differential of only 1.2:1, with detachers under the rolls in order to achieve more water absorption.
"For hard wheat, the roll surface is usually longer than for soft wheat because of the harder endosperm. If a mill is designed only for hard wheat, it may use reduction roll differentials of 1.5:1 or even higher, without detachers."
Fluted rolls in the reduction system are usually avoided in order to achieve higher flour quality, Stevens said. Some common exceptions are during the first reduction, if there is good purification; during the last reduction roll, if there is a market for industrial-grade flour; and in regions where mill capacity is more important than final product quality and flour extraction rate, or where there is a good market for relatively low-quality flour.
If a mill must handle both hard and soft wheats interchangeably, what compromises must be made to the mill’s design and operation?
Stevens said such a mill will normally have the sifting capacity of a typical soft wheat mill and the grinding capacity of a hard wheat mill, and will condition the wheat for a length of time consistent with good hard wheat milling. The mill may be equipped for a final short conditioning — about a half an hour — to be applied to soft wheat.
He noted that the same (or nearly the same) roll releases must be maintained on both hard and soft wheats. "This means there will be large readjustments of the rolls when changing between wheat types," he said.
Stevens suggested decreasing the load to first break on soft wheat by 10% to 20% of the normal load for hard wheat. "Expect chokes in the pipes and sifters when grinding soft wheat, and watch critical points carefully," he said. "Drop the moisture by 0.5% to 1% to maintain good flow through the pipes, sifters and purifiers."