Principal contributors of the section on tempering, James Ievolella, Harry Levine, and Louise Slade; on flaking and flaking mills, Leon Levine and Charles Lauhoff; and on toasting and toasting ovens, James Breslin, Fred Shouldice, and Kevin Knott.
Elwood F. Caldwell, Robert B. Fast, James Breslin, James Ievolella, Kevin Knott, Charles Lauhoff, Harry Levine, Leon Levine, Fred J. Shouldice, Jr., and Louise Slade
Breakfast Cereals and How They are Made, Second Edition
Pages 161-213
DOI: https://doi.org/10.1094/1891127152.005
ISBN: 1-891127-15-2
Abstract
The term tempering in a breakfast cereal process denotes a unit operation different from the operation with the same name in flour milling. In flour milling, tempering refers to the addition of a small amount of moisture to the cleaned grain before milling. The added water toughens the bran coat so it will remain in larger pieces during the grinding and sifting of the flour fractions. In a breakfast cereal process, tempering usually follows a drying or cooling step and is the period during which the cooked grain mass or cereal pellets are held in collection bins to allow the equilibration of moisture within and among the particles and the development of desired flakability or shredability as a result of starch retrogradation.
The effect of tempering on final product quality depends to some degree on drying conditions. Proper drying conditions greatly cut down on temper time. Manufacturers should resist the temptation to raise dryer temperatures to push up production rates. Prolonged tempering cannot make up for poor dryer operation; problems caused by improper drying are only compounded during tempering.
The drying step in a cereal process usually leaves the cooked grain or pellets with a nonuniform distribution of moisture. The centers of the particles may contain more moisture than the surfaces, as in grits dried from about 30% moisture down to 14–20% for flaking. In the case of shredded whole wheat, on the other hand, where the grain has been cooked in excess water rather than a limited amount that is all absorbed during cooking, the step following cooking is primarily a cooling one, with some evaporation of surface moisture. This results in a moisture disparity opposite to that in grits, with the interior being drier than the areas closer to the surface. In both cases, tempering time provides for equilibration of moisture within the particles. A third situation concerns grain that is cooked to about 30% moisture in limited water (which is all absorbed) and then cooled rather than dried as such. It remains in the 28–30% moisture range, but other changes take place during tempering time (such as the starch retrogradation explained later) that result in improved shredding characteristics.
In all three of these situations, cooling the grain before tempering greatly improves the quality of the tempered material. If grain dried to 17% moisture for flaking is binned at temperatures much over 100°F, its color darkens during tempering. The additional color stays with the product through flaking and toasting and shows up in the finished flakes. The same is true for grain cooked and cooled to 30% moisture for shredding. Wheat cooked for shredded wheat and binned at elevated temperatures remains overly sticky and will not shred properly or produce the desired long, straight shreds that result in attractive biscuits. Thus, the importance of cooling before tempering cannot be overemphasized.
Tempering bins come in a variety of sizes and shapes, depending on the product. Some bins for flaking grits are merely large, open-top, rectangular containers (Figure 1). The bottom of such a bin is a slow-moving conveyor belt. The vertical sides and one end are mounted at the outside edges of the conveyor belt, and the other end of the bin typically consists of a set of rakes inclined back toward the center of the bin about 15°. The rakes are driven at moderate speed so that those closest to the pile of grits in the bin are moving upward. Such bins are typically about 10 ft wide, 20 ft long, and 5 ft high (3 m × 6 m × 1.5 m). The top may be closed but need not be.
These bins are loaded from a central point from an overhead conveyor running the length of the bin. The first product loaded into the bin is also the first to be discharged at the end of tempering. The forward movement of the conveyor bottom toward the rakes at the end of the bin discharges the product. Grits have a tendency to block in a solid mass when allowed to stand in a bin like this. The rakes moving across the face of the pile break up the blocks and loosen the grits from the bin surfaces. The grits drop onto a conveyor to move to the next step, which is usually sifting to ensure that only grits of the correct size go on for further processing. Sifting removes any fines generated and separates the “overs,” or large pieces, for more breaking and resifting.
In another, much simpler type of tempering setup, the tempering facility is merely an additional section of the dryer. Because humidity is controlled as well as temperature, no case-hardening occurs during drying, which greatly reduces temper time to equilibrate the moisture within particles. In this type of system, a cooling section is incorporated into the dryer between the drying and tempering sections.
In a third type of tempering setup, used in whole-grain shredding, the cooked and cooled grain is held in large, cylindrical bins before shredding. The bins are usually fed from above by a conveyor, and the tempered grain is discharged from the cone-shaped bottom through a central opening.
