Technical Profile: Flour Heat Treatment

by Teresa Acklin
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   Why treat flour by heat?

   The chemical and baking characteristics of flour are primarily determined by the properties of the wheat that has been milled. But the reduction process also has a considerable impact on flour quality within the tolerance band allowed by the wheat.

   The purpose of a defined flour heat treatment process is to further influence the flour's quality features to permit applications in areas other than baking. Attention is focused in this process on the low- and high-temperature viscosity values of the flour, which are important parameters in such applications, in addition to the mineral content (ash), gluten and protein quantity.

   The maximum viscosity will vary as a function of the particular flour involved. An elevated low-temperature viscosity (a measure for starch gelatinization), for example, indicates a high water binding rate, which in turn provides a good basis for the production of soups, ready meals, baby food and various other flour-containing foods. If, for instance, the flour is used as a carrier material for the production of soup, its low-temperature viscosity must be different from that of a batter flour used for the coating of, say, fish sticks.

   For battered fish sticks, the goal is to achieve the most uniform layer thickness possible on the sticks, thanks to an optimal flour viscosity. An optimum viscosity will also prevent the dough from dripping after it has been applied to the sticks, and the batter will be more quickly bound in the frying process.

   Batter is also used for coating chicken nuggets, shrimp, vegetables and many other foods. In some cases, the flour used in these applications is also called bread-crumb.

   In the future, new flour heat treatment installations may allow expensive starch to be replaced by less costly heat-treated flour.

   Heat treatment not only allows the flour characteristics to be modified, but also produces a very welcome side effect — a reduction of the bacteria count. As a result of treatment at fairly high temperatures, the flour is partially sterilized, killing bacteria (e.g. coliform germs) and thereby allowing the total germ count of the flour to be considerably reduced.

   This is a desirable effect in feed meals, for example, which the latest laws require to be essentially germ-free. New applications are therefore created by this system, including some in the feed manufacturing industry.

   Heat treatment may allow current chemical flour processing methods such as the chlorination of cookie (biscuit) flour to be replaced. New, stricter food laws already in force or to be introduced in the very near future may accelerate the phasing-out of these chemical processes, or soon may even eliminate them altogether. The new flour heat treatment system may replace the chlorination process, although this development still requires verification by additional, more extensive laboratory testing.

The Heat Treatment Process

   The continuous process modifies the typical properties of the wheat gluten, produces a certain controlled gelatinization of the starch in the flour and inactivates enzymes. This selective starch gelatinization increases the flour's amylogram value, which provides a measure for the high-temperature viscosity and germ count of the flour, and its low-temperature viscosity, which provides a measure for starch gelatinization, adjusting it to a more favorable value for the subsequent application.

   A high amylogram value is achieved by longer exposure of the flour to the action of the heat. At the start of the process, the amylogram value of the flour is approximately 300 or lower, but it can be increased within a fairly short time to about 800 by heating.

   The time requirement can increase the possible value to 1,000 or higher. However, such high values are seldom necessary for further flour processing.

   It is possible to use inexpensive flour, so-called cookie (biscuit) flour, as a raw material for the heat treatment process. This type of flour is usually made from wheat that does not have the properties supporting the baking process, or only to a limited extent, and which otherwise would be used as feed meal.

   Mills equipped for the production of specialty flours already have some of the installations needed for flour heat treatment, but they are normally designed for the batch production of specialty flours. Very often, they allow only one specialty product to be made.

   Heat treatment in such conventional installations is normally performed in an enclosed vessel equipped with a mixing device. Very often, this equipment does not meet sanitation requirements.

   Batch processing also means that if the product range of a mill includes several specialty flours, the time requirement for cleaning and changing over the production equipment will be high when product changes are made, resulting in high, unproductive costs.

   Moreover, because the mixing efficiency of these batch processes is low and because the process is not continuous, uncontrollable agglomeration contents of up to 30% to 35% are sometimes created, making expensive regrinding necessary.

   According to information obtained from the Buhler Milling business unit, some systems already exist that may be said to allow a continuous process. Apart from the above-mentioned sanitation problems, however, the main problem associated with these installations is said to be process control, or reproducibility of the treatment results.

   The temperature and the moisture content of the flour cannot be duplicated, or only with modest success. In addition, the quality of the flour treated in these systems is very heterogeneous. Although the values achieved throughout a given batch may more or less meet the requirements when taken as an average, it is said that the flour quality varies considerably within the batch.

   The accompanying illustration shows a simplified version of the conventional continuous process: the system features a twin-shaft mixer into which water and steam are introduced. The flour is continuously conveyed forward by the rotation of the mixer shafts and is then discharged from the mixer.

   The adjustable discharge opening allows the heat treatment time to be varied and the low- and high-viscosity values to be controlled. Although this process is continuous, it will hardly provide regular, reproducible product characteristics.

   Because of the irregular treatment of the product, the percentage of agglomerates in the flour may rise to above 30%. In most mills, only one key system component such as a twin-shaft mixer is usually purchased for this process. It is then retrofitted with a heat and steam system.

   To date, a complete flour heat treatment process as a self-contained system including a moisture controller has not been available. The Milling business unit of Buhler Ltd. will be the first provider offering such an installation in the marketplace.

Operating Principle

   From the storage bins, flour is pneumatically conveyed to the surge bin and from there to the dosing scale, as seen at (2) in the diagram on page 47. The scale ensures that a defined flour throughput is maintained, thanks to its accurate proportioning action and screw conveying.

   After the scale, the flour is directed to the heated screw conveyor. Here the material is preheated before being fed to the twin-shaft mixer (3). In the mixer, the flour is conditioned and heated by the controlled addition of water and steam.

   At this point in the process, the decisive starch treatment is accomplished by the addition of water. Steam is introduced as a preparatory stage to protein treatment, with the main protein treatment stage taking place in the heated screw conveyors (4). The heated and moistened flour is then fed to the first large screw conveyor.

   Screws heated at their peripheries move the flour at a continuous and controlled rate through this processing section. The high temperature is maintained at a constant level while the flour passes through. It can be varied across a time range to suit the particular product formula.

   Maintaining the heat at a high level allows the desired quality modifications of the flour to be achieved. A steam barrier at the end of this section prevents the steam from escaping. After the product is discharged from these screw conveyors it is dried and cooled.

   To allow optimal storage of the product and to ensure that it has the best possible characteristics for further processing, it must ultimately have a defined moisture content of 4% to 14%, depending on its particular use. For this purpose, flour temperature readings are taken along a certain heat treatment section.

   Depending on the required water content of the finished flour, a pneumatic heating or cooling conveyor heats/cools the flour (e.g. for flours with a moisture content of 14%), or heats/heats/cools them (e.g. for flours with a moisture content of 4%). In the soup industry, the flours used frequently have a very low moisture content of 4%.

   At the end of this process stage, when the desired moisture content has been achieved, the product is sifted on a small sifter (5). This is necessary because agglomerates are created in the mixer by the addition of water and steam, which amount to about 8% to 15% of the flour, depending on the product formula. Because the agglomerates are up to several millimeters in size and have the consistency of dough, they would affect the low-temperature viscosity of the flour and must therefore be reprocessed by sifting out and micromilling (7).

   A second sifting stage after grinding ensures that all the flour to be further processed will have the allowable particle size of approximately 250 microns. After this process, the flour is pneumatically conveyed to a bin, where it remains in intermediate storage until packaging.

   Heat exchangers at various points in the process allow much of the process energy to be recovered and reused in an ecologically beneficial way.

   The process control system is provided with an integrated personal computer (1). The proportioning and conveying rates, temperature and other data for different product formulas are saved in a database. Each flour has its own product formula, which can be retrieved as required.

   This system allows 1 to 2 tonnes of flour to be treated per hour, depending on the product formula. This translates into about 3,500 to 7,000 tonnes per system, per year, based on 12-hour operations for 280 days.

   Contributed by suppliers, technical profiles feature new technology, products, specific applications or proprietary concepts. This material, provided by Buhler Ltd., Uzwil, Switzerland, is based on a paper by Georges Hanimann for the Economic Engineering post-graduate course at the St. Gallen School of Engineering, St. Gallen, Switzerland.