Tempering Wheat

by Teresa Acklin
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The process of adding water to wheat before milling aids in producing consistent, quality finished flours

   There has been much research and practice on the tempering, or conditioning, of wheat since well before World War II. There are as many opinions on the practice as there are systems.

   Essentially, tempering is carried out by adding water to wheat, entering most easily through the wheatgerm.

   There are several reasons why millers temper wheat. One is to toughen the bran skin so that it holds together rather than creating bran powder, which would otherwise lead to higher mineral content or darker flours. Another reason to temper wheat is to ease separation of the darker bran from the light colored endosperm. This also produces a higher extraction of flour of a given quality.

   Tempering is done to mellow the endosperm with the affect of reducing power consumption of rollermills, to enable clear and accurate sifting of mill stocks and flour, to ensure the desired moisture content of finished products and to aim for consistency.

   A typical wheat cleaning flow (see diagram on Page 34) involves conditioning from receipt of wheat from the elevator into the dirty or raw wheat bins through the I break rollermill. There are three points (A, B and C) at which water may be added. Virtually all mills use one point, many use two, and some use all three points. Point B is the most widely used. With up-to-date damping apparatus of varying degrees of sophistication (see World Grain, September 1997, Page 18), it is possible to add up to 8% moisture at one time.

   Point A is popular where particularly hard, dry wheats are initially wetted. A typical addition of moisture might be 2%. The idea is to give wheat kernels a longer lying time in which the endosperm can absorb moisture and become more mellow, or easier to mill. This technique is used in cold winter climates because water penetration is slower as the water or wheat becomes colder.

   The third point of water addition, C, has a different aim. Between 20 to 30 minutes before the I break, up to 0.75% of moisture is added to wheat using an atomizer water mist or spray. There are two benefits to this tempering method. The first is to hold bran together in the milling process to increase extraction. The other is to try to correct an earlier conditioning fault or mistake. However, water will not have time to penetrate the endosperm so it cannot affect final flour moisture content, although it clearly increases bran moisture.

      WHEAT CLASSES.

   Different wheat classes require varying moisture contents and lying times. North American and Australian hard spring wheats (such as Canadian Western Red Spring, Dark Northern Spring and Australian Prime Hard) benefit from a lying time of 24 hours and a final moisture content of over 16% at I break. Conversely, soft winter varieties ideally need less lying time — 12 hours and 15%, respectively. These should be varied in hotter or colder climates.

   Moisture loss across the mill on any particular wheat blend needs to be known in order to determine the ideal quantity of water to be added, where to do so, how long to rest the wheat and the ambient or climate-controlled conditions.

   Flours with high starch damage can lose up to 3% moisture from I break wheat to finished flour. This usually occurs in harder wheat varieties. Soft wheat flours made with a gentler grind may only lose 1% moisture in the final flour.

   Millers also must consider whether to condition or temper individual wheat origins separately, in blends of classes or all together. These choices all work, although the first is considered ideal, the second less so and the third a compromise. The difficulty in tempering wheat origins separately is trying to ensure a consistent lying time and good mill performance in the situation where customer demand dictates an immediate schedule alteration. No amount of computerized control will help.

   One solution in larger plants is to have two or more parallel, yet independent, cleaning and tempering lines so that particular schedules can be leapfrogged and re-sequenced.

   It has been said that conditioning grain before cleaning will cause unwanted dirt to remain and be “glued” onto the skin and increase capacity of wheat cleaning machinery by 10% or more due to damped wheat having lower bulk density. Experience shows the former to be debatable and the latter to be true.

   Those mills with laminar, first-in first-out bin discharge arrangements are in an excellent position to arrange job queues (via a programmable logic controller) and make schedules more easily. The benefit to the miller is knowing where a particular wheat change is expected in each bin.

   Product tracking — matching milled flours to parcels of wheat — is significantly aided. Changes on the mill can be pinpointed with precision, reducing the re-working of flour. Played carefully, lying times are more consistent.

   Then there is the possibility of running individual wheats or groups one after the other in series. The near opposite is to pre-blend, clean and condition the scheduled wheats and then mill according to prearranged plans. The latter tends to give a more consistent lying time.

   Although not strictly a tempering question, the addition of chlorinated water to wheat at about 200 to 250 ppm reduces bacteria counts in finished products. In that situation ancillary equipment and machines need regular internal cleaning in order to avoid neutralizing the chlorinated water affect. Stainless steel parts placed close to chlorinating equipment is desirable for anti-corrosion reasons.

   Durum wheats are conditioned to relatively high moisture contents of around 17% for relatively short times of eight hours, although this practice varies up to 24 hours. The purpose is to hold water in the bran while retaining hardness in the endosperm for milling long extraction, low mineral content semolina destined for pasta or couscous.

   The aim in durum milling is to keep the particle size of endosperm as large as possible for as long as possible. This is the virtual opposite of flour milling.

   Debranned wheat is able to absorb water at up to six times the rate of wheat that has not been debranned. This technique removes about 70% of the bran skin before being milled. The remaining 30% lies in the crease.

   At this step, the space required for tempering bins is greatly reduced. It is necessary to keep such freshly damped debranned kernels “alive” by movement for at least 15 minutes in order to prevent them from sticking together because of their starchy surface nature.

   Hot air or radiator conditioners were once used to speed up the process, to save bin space and to modify gluten. Radiator conditioners, popular in the 1950s, were capable of converting an extensible wheat with a low resistance to a less extensible one with a higher resistance as measured by laboratory dough rheology apparatus. This made a bolder loaf of bread with better volume. This practice was difficult to control consistently, however, and the machines, which were often 10 meters or more in height, were expensive and consumed high amounts of energy.

      FINDING THE ‘IDEAL.'

   Notwithstanding the mountain of excellent scientific research on both sides of the Atlantic and beyond, the ideal consistent conditioning of wheat remains elusive. It is worth considering some of the consequences of faulty tempering in virtually any mill.

   One of the most common difficulties is found in the situation where too much grain has been prepared before the weekend and not milled until the following week. Wheat will then have to lie for 48 to 72 hours before milling. Milling wheat where moisture has fully migrated to equilibrium produces flour of higher than desired moisture content.

   On one hand, this can be beneficial financially; on the other, the flour will probably be out of specification. One solution is to blend the higher moisture content flour with a drier flour. The endosperm when milling will be relatively soft, or mellow, and the damaged starch will be lower than expected.

   This can be corrected if recognized in time. If not, the customer will notice a change in less water uptake, which should lead to a complaint. However, most mills probably will be able to grind such over-tempered wheat without choking.

   Should the damping system fail to add enough water, the characteristic result is a high ash content, long extraction, dirty-looking flour with low moisture. The effect on the mill is that it runs easily, almost like “sand,” because the flour races through. If imperfect, the instant correction is to use the I break damper. This does not bring up flour moisture, but it does tend to reduce dark color and high mineral content.

   If the opposite happens, the mill may “choke,” caused by a wheat blend too high in moisture. Rollermill and sifter machinery will run full, possibly too full. Finished product moistures will be too high, yet mineral or ash content will be lower and flour brighter. Extraction will drop.

   Then there is the scenario experienced when a sudden climate change takes place — from relatively warm to cold or from relatively high humidity to low humidity. The effect is similar to milling dry wheat because products are drier, extraction longer, ash higher and the appearance is of an “empty” mill due to speed and ease of milling.

   The practiced miller is all too aware of these distinctions and can act as soon as particular milling characteristics have been observed. He will know and sense difficulties before static laboratory instrumentation picks them up, often before on-line equipment registers a change.

   Even with the best intentions in the world, customer re-scheduling, climate changes and unwanted stoppages conspire to confound consistent conditioning, which is the basis for the all-important regular, non-variable set of finished product.

   In the final event, taking the customer's requirements into account as well as the availability of different wheat, economics, local climate changes of temperature and relative humidity, consistent conditioning of wheat is the aim with whatever equipment is available. With complicated customer demands to satisfy, it is best to try to keep schedules simple in order to avoid pockets of under-conditioned or over-conditioned wheat.

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