Tempering is the lay term given to the sophisticated physical process that occurs when moisture is applied to the outside of the wheat berry. If dampened grain is left to sit for an extended period of time, the moisture added to the external surfaces migrates inwards and disperses itself through all the components of the wheat berry, including the germ, endosperm and bran layers. Thus, the overall moisture content of the berry is increased to a consistent level.
Wheat grains are harvested in many countries at very low moistures. These moisture contents are unsuitable for milling and moisture correction of the wheat must be carried out. The reverse happens in temperate climates where wheat must be dried in order to insure correct preservation of the wheat in storage and optimum milling characteristics.
Typical intake moistures at mills range from 11% to 16% depending on harvesting and drying conditions, while the optimum moisture for milling is generally in the range of 15% to 16%. Therefore, accurate moisture measurement devices are an extremely important part of any milling plant.
EFFECTS OF TEMPERING
The objective of tempering immediately prior to milling is to specifically treat the outer bran layers. The added moisture causes the bran layers to swell and thus lose some adhesion to the endosperm. Secondly, the added moisture makes the bran layers less friable so they are more resistant to the physical actions of the milling process. This results in larger bran particles within the process. These particles are easier to separate and, where bran is a mill product, the product is more attractive and thus more marketable.
In the case of de-branning processes, the final moisture addition is critical to the success of the process, so this addition must be carefully monitored and controlled. Final tempering prior to de-branning serves to enhance the distinction between bran layers and endosperm. Thus the process of removing bran layers from the wheat berry using friction and abrasion, while not producing a bran powder, becomes easier.
Tempering is sometimes used post milling to modify the moisture content of the final product. This is used as a tool to make product moisture more closely match customer specifications, but also it allows the mill manager to operate the process with the raw material at its optimum moisture content rather than operating the process with the end product moisture in mind. This approach can yield significant process and operational benefits, not to mention economic benefits.
Tempering of grain has a number of profound visible effects on the milling process itself and on the characteristics of mill products. They all relate to the elasticity of the component parts of the wheat berry post tempering and to the coefficient of friction of the mill products. Tempered grain entering a mill produces larger bran and endosperm particles than non-tempered grain. This affects all subsequent sieving processes. Tempered grain will thus load the later break passages more heavily, produce less flour on the break system and load the early reduction passages. The bran particles don’t fragment as easily, and so endosperm particles tend to be whiter with less contaminating bran particles.
The effects of these tempering induced characteristics compared to non-tempered grain are profound on the mill process. A change in the tempering regime can lead to a change in the ash content of the mill products, the average particle size of bran produced, the starch damage of the flour produced and also the yield of product. These particles do not flow as well in spouting and in silos as the products of non-tempered grain. Chokes can occur in spouts and silos can fail to empty if the equipment is not correctly designed for the materials and quantities being handled. Thus, tempering is not an operation to consider lightly, and certainly it is an operation that one cannot afford to get wrong!
Moisture addition is performed in two different ways in many plants. Moisture can be added at a fixed rate per tonne of grain passing through the plant, or moisture can be added in accordance with the measured moisture content of the grain. The former method is often used where low moisture grain is taken in and the average moisture needs to be raised close to optimum milling moistures. This is usually carried out in the wheat silo and can be performed in one, two or even three stages, depending on the level of sophistication of the moisture addition equipment. For example, the newer generations of intensive dampeners can add up to 7% moisture in one pass, whereas a simple mixing conveyor may only add about 3% moisture in one pass.
Fixed rate moisture addition is also performed just prior to milling. This is sometimes referred to as first break damping. Moisture addition at this point is usually of the order 0.3% to 0.5%. The function of this moisture addition is to modify the properties and moisture content of the outer bran layers only.
The de-branning process employed in some mills prior to the conventional milling process employs this type of tempering to great effect. The only drawback is that in these processes, moisture addition is very critical to the success of the process and so must be monitored and regulated very closely. This may not suit every mill operation.
Moisture measurement is a critical purchasing parameter for wheat intake, but is just as critical in deciding how wheat is dealt with in the silo and subsequently in the mill. Initial moisture measurement is carried out using ovens and rapid testers. The results from initial moisture testing determine how much moisture addition is necessary in the silos and how long grain is held before transfer to the mill. Moisture measurement after first damping is usually done manually using ovens. Where the addition of moisture in the wheat silo is normal practice, it is advisable to have an automated moisture measurement and addition process, because this type of system delivers continuous moisture measurement and adds moisture accordingly. The result is a homogenous moisture content in wheat entering the mill silos.
The main moisture addition process in flour mills is usually a well controlled and automated process. Feed forward or feed back moisture measurement is used, and the throughput of grain is also measured. Tempering time is also well regulated so that the grain destined for the mill is homogenous and has been left sitting after moisture addition for a constant period of time prior to milling. The result is (most often) a predictable raw material for milling.
Tempering equipment varies from the very simple to the very complex. The simplest equipment are worm conveyors with a slight backward incline that promotes mixing as moisture is added, using simple dribble bar mechanisms. More complex mechanisms have specially designed screw conveyors that ensure complete mixing in a matter of seconds. Water addition can be via dribble bar or atomizing nozzles.
The most complex devices incorporate a scouring and mixing action and are known as intensive dampeners. These devices are capable of adding large amounts of water to wheat as it is processed.
As mentioned already, many moisture addition devices employ sophisticated technologies to control water addition. Microwaves, gamma rays and capacitance are the most common technologies used to perform moisture measurement.
Quite often PLC control is used in conjunction with the above equipment to control the timing of tempering processes, so that moisture addition occurs a fixed time before actual milling occurs. This enables the exact milling properties of the wheat grist to be manipulated.
However, the most common method of controlling tempering time is manual. This works very well in most circumstances, but most millers will admit to variations in the milling performance of the grist as discrete batches that have been tempered pass through the mill. This can be a minor inconvenience or the reason for process problems, such as unexplained changes in mill balance and even chokes.
Tempering alters final product moisture content and properties, thus ongoing measurement of these properties gives an indication of the effectiveness of tempering, potential problems and opportunities.