Principal contributor of the sections on basic principles of drying and on selecting a dryer, Britton D. F. Miller, and of the section on breakfast cereal drying fundamentals, Roy A. Gillespy.
Britton D. F. Miller and Roy A. Gillespy
Breakfast Cereals and How They are Made, Second Edition
Pages 133-159
DOI: https://doi.org/10.1094/1891127152.004
ISBN: 1-891127-15-2
Abstract
As pointed out in Chapter 2, most ready-to-eat breakfast cereals require drying as an intermediate or final processing step. This drying involves the controlled removal of water from the cooked grain and other ingredients to obtain the desired properties. Some of these properties are important for further processing such as flaking, puffing, forming, toasting, or packaging. The desired properties might include formability, internal viscosity, flavor, and shelf stability as well as reduced moisture content.
Breakfast cereals may be dried at several points. The cooked cereal mass may be “predried” to prevent agglomeration and product damage in further material handling and to create appropriate material properties before flaking or puffing. Because drying is used to change material properties for specific processing reasons, the existence of a moisture gradient in the mass as it leaves the dryer may need to be alleviated by tempering to create a more uniform moisture content within and among the cereal particles. Puffing and toasting operations involve further drying of products as well as changing their physical structure and chemical makeup. After a coating (e.g., a sugar syrup) has been applied, cereals are dried to set and harden the coating and to remove excess moisture.
Probably the most important principle to remember is that the purpose of drying is not simply to drive off moisture. Moisture removal can most efficiently be done at combustion temperatures. Rather, the purpose is to utilize drying to create the desired product, reliably and repeatedly. Accordingly, we are concerned not only with how to create the properties but also with when in the process those properties are needed.
Predrying and the dryers used for predrying and sugar coating are examined in this chapter. Puffing and toasting operations and the associated equipment are considered in Chapter 5, whereas this chapter focuses mainly on the cooked cereal mass and on cereal pellet drying and dryers. Important process and product parameters are defined and discussed in both cases. Finally, the application of the basic principles and parameters of drying to the design and selection of the dryer best suited to particular unit operations is addressed here.
Only a limited treatment of the principles and theories of drying can be included. The theory is well developed and the literature extensive. Most texts on drying have bibliographies for further reference (e.g., Sherwood, 1929; Slesser and Cleland, 1962; Williams-Gardner, 1971; Mujumdar, 1987). Accordingly, mathematical and physical derivations as well as property definitions and details available elsewhere will not be repeated.
The thermal drying process is the result of simultaneous heat and mass transfer whereby water is vaporized and removed from the product. In the case of particles or pellets of cooked cereal, the dominant mechanism of heat transfer is convection from air to the product and water, and a major mechanism of mass transfer would seem to be diffusion after the surface moisture has evaporated.
In the discussion that follows, constant drying temperature and relative humidity are assumed to prevail inside the dryer. This assumption is not trivial, since the rate of drying and the driving forces for drying are substantially affected by both temperature and relative humidity. If the discussion includes issues of product characteristics and product quantity and uniformity, then this assumption is critical.
