Energy Balances in Dough Mixers

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tralia, retired), W. Bushuk (University of Manitoba, Canada), 0. Maningat. (Midwest Grain Products, KS), and G. Stephenson (Reckitt Benckiser,. Hull, UK).
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COLUMN

ENGINEERING

Energy Balances in Dough Mixers ecause energy balances tell us about the mixed temperature of a dough, a knowledge of the energy balances involved in mixing doughs is important in understanding the operation and control of dough mixing systems. It is possible to estimate the temperature of a dough upon discharge from a dough mixer through the application of a simple energy balance equation. The governing energy balance is:

B

~ (enthalpy of ingredients)= Z: (energy inputs or losses from other sources)

t LEON LEVINE Leon Levine & Associates

For those readers who don't remember their thermodynamics (and how many of us do?), the enthalpy of a material is a fancy way of saying the heat content of a material relative to some reference temperature. The reference temperature chosen is often 0°C. The enthalpy of an ingredient, therefore, may be calculated by h =CPT

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where h = the enthalpy of the material; CP ::;; the heat capacity of the material; and T = the temperature of the material. The heat capacity of the material, Cp, may be measured with a calorimeter, but it is usually sufficient to estimate it through the use of published coffelations and proximate analysis. The coffelations for estimating heat capacity can be found in many text books, such as Physical and Chemical Properties of Foods (1). A typical value for the heat capacity of flour is ,;.2.02 kJ/kg-°C, which is about half that for pure water (~4.18 kJ/kg· C). The major inputs or losses of energy from a mixer come from three sources: 0

1) An energy input resulting from the viscous dissipation of

mixer energy, on the order of 10 W-hr/kg. 2) A loss of energy through cooling of the mixer surfaces. 3) A loss of energy through the addition of a material that undergoes a phase change, most commonly ice, but also through the incorporation of a liquefied gas such as carbon dioxide or through the addition of dry ice.

2) The energy associated with the evaporation of water from the mix. Because the dough temperature is low and mixing is normally done under atmospheric pressure, this is also normally a small amount of energy. 3) Heat losses to the surroundings, which are normally very small, because the dough and environment are about the same temperature. The large difference between the heat capacities of water and flour (about a factor of 2) immediately tells us something ~f value: because the ratio of flour to water in a dough is on the order of 60/40 and the ratio of the heat capacities of water to flour is about 2/1, changing the temperature of the water feed can have a much greater effect on the final dough temperature than changing the flour temperature if the temperatures of both ingredients are changed by an equal amount. This can have a significant impact on whether one chooses a flour- or water-cooling system or both to control dough temperature. In the absence of any cooling by the jackets, the final dough temperature will be independent of how fast one mixes the dough if the total energy input required to develop the dough is independent of the mixing speed. However, one seldom operates a large, industrial dough mixer without any cooling jacket. The presence of jacket cooling greatly complicates analysis of the problem. The quantity of energy removed is described by Heat removed by jacket= UAtmix

~Ta,ernge

where U = the heat transfer coefficient (about 200 W/m2- C); A = the cooled surface area; tmix = the mixing time; and ATavcragc = the average temperature difference of the dough coolant during the mixing process. Now analysis of the problem becomes considerably more complicated. I won't attempt to solve the problem in this column (I'll save that for my next column). One can simply look at the equations above and recognize a problem that automatically accompanies high-speed mixing. Suppose a bright young engineer, cereal chemist, or baker says, "I can increase the capacity of my mixing system by simply speeding up the rotational speed of the dough mixer." Three problems are immediately encountered: first, the motor driving the dough mixer must be capable of delivering energy at the higher rates required by the higher mixer speeds; second, the mechanical components of the mixer must be strong enough to tolerate the higher forces associated with the high energy input rates; and third, there is a temperature control problem revealed by the equation that describes heat removal by the jacket. 0

There are several, normally minor, energy inputs or losses that may be included: 1) An energy input resulting from heat generated by the heat of wetting the flour. The quantity of heat generated is small, on the order of 15 kJ/kg of flour.

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(continued on page 338)

In the 1920s, a limited outlet for the heat-dried gluten (termed "gum gluten") was found in the manufacture of low-carbohydrate bread for diabetics. Later outlets for gum gluten included the production of monosodium glutamate (in the 1940s) by acid hydrolysis and incorporation in Kellogg's Special K breakfast cereal (10). Until this stage of development, Jenks and Rossman had used a harsh drying process that devitalized the functional properties of the gluten, rendering it largely useless as an ingredient that could be used to enhance baking quality. However, in the late 1950s, the Huron Milling Company adopted the "new type of drying process used in Australia" (6) based on the ring- or flash-drying process. The resulting dry gluten retained its functional properties, and when added to a bread-flour dough, the gluten produced improved loaf volume and better crumb texture. Within a year, vital dry gluten was being used commercially in a variety of breads. However, at the time, attempts to blend the dry gluten with wheat flour at the mill to enhance protein quality were generally unsuccessful. Nevertheless, blending dry gluten with wheat flour at the mill has more recently become a common practice, especially in Europe where gluten fortification has become essential for many low-protein flours (9).

Diverse Uses for Gluten Vital dry gluten is still primarily used to fortify flour for bread manufacture. In bakery products, added gluten enhances volume, texture, and shelf life. In addition, it is used in a wide range of food products, including breakfast cereals, meats, cheeses, and snack foods, and for seafood analogs and texturized foods (3,8). Nonfood applications include pet foods, aquaculture feed, biodegradable plastics, films, coatings, adhesives, inks, cosmetics, and pharmaceuticals. Gluten also has been modified in various ways to produce a wide range of food ingredients, providing water-binding and emulsifying properties for meat products, nutritional advantages for sports drinks and medical supplements, dairy analogs such as coffee whitener and calf milk, emulsifying properties for powdered shortenings, and milk replacement in bakery applications.

(Levine-continued from page 334)

Let's assume that the motor of the mixer is large enough to deliver the required energy input rate (power) and that the mechanical components are strong enough to tolerate the higher energy input rates. As the ambitious engineer, cereal chemist, or baker has assumed, the dough will develop faster, and the turnover time per batch of dough will be shorter. However the mixing time, tmix, in the heat removal equation has been overlooked. The equation reveals that the total quantity of heat that will be removed by the jacket will be reduced. In other words, the final dough temperature will increase. It doesn't take much of an increase in dough temperature to render the dough unacceptable. This oversight is one source of problems associated with high-speed mixing systems. I have a suspicion that this physical fact might have been overlooked by some of the original developers of high-speed systems. There is also a scale-up problem that can be identified by examining the equation governing heat transfer through the jacket. As a mixer is made larger, the volume (capacity) of the mixer increases with the cube of the vessel diameter, while the surface area, A, only increases with the square of the vessel diameter. Consequently, the ability to remove heat, in terms of quantity of heat per unit mass of dough, decreases with scale. This means that at the same coolant temperature the dough produced will be hotter as the mixer gets bigger. The only solution to this problem is..t9 decrease the coolant temperature. At some point, this creates a problem: the mixer walls become so cold that significant water condensation can occur, or, in the extreme, ice can form on the mixer walls. This puts an ultimate limit on the size of the dough mixer one can build and successfully operate. My next column(s) will illustrate the calculations associated with the equation given above. Reference l. Okos, M. R., ed. Physical and Chemical Properties of Foods. Ameri-

can Society of Agricultural Engineers, St. Joseph, MI, 1986.

Acknowledgments

This account of the origins of gluten production is based on information provided by several scientists, particularly D. Tucker (Procera, Australia, retired), W. Bushuk (University of Manitoba, Canada), 0. Maningat (Midwest Grain Products, KS), and G. Stephenson (Reckitt Benckiser, Hull, UK). Some of the text fonns part of a much larger story (11). (This section is reproduced with permission from the Royal Australian Chemical Institute.) References 1. Anonymous. Mr. Frederic Reckitt. Reckitts' Mag. 6(7):1, 1912. 2. Bailey, H. C. A translation of Beccari's lecture "Concerning Grain" (1729). Cereal Chem. 18:555, 1941. 3. Bietz, J. A., and Lookhart. G. L. Properties and non-food potential of gluten. Cereal Foods World 41:376, 1996. 4. Buller, A.H. R. Essays on WheaL Macmillan Company, New York, 1919. 5. Bushuk, W., Briggs, K. G., and Shebeski, L. H. Protein quantity and quality as factors in the evaluation of bread wheats. Can. J. Plant Sci. 49:113, 1969. 6. Dubois, D. K. History of vital wheat gluten. Am. Soc. Bakery Eng. Bull. 233:991, 1996. 7. Finney, K. F., and Barmore, M.A. Loaf volume and protein content of hard winter and spring wheats. Cereal Chem. 25:291, 1948. 8. Maningat, C. C., Bassi, S., and Hesser, J.M. Wheat gluten in food and non-food systems. Am. Inst. Baking Res. Dep. Technic. Bull. 16(6):l, 1994. 9. Spooner, T. F. The unsung hero of any successful baking procedure. Milling & Baking News, December, 1995. 10. Thompson, J. J., and Raymer, M. M. Production ofready-to-eat composite flaked cereal products. U.S. Patent 2,836.495, 1958. 11. Wrigley, C. W. Contributions by Australians to grain quality research. Pages 268-329 in: An Introduction to the Australian Grains lndusfl)'. L. O'Brien and A. B. Blakeney, eds. Royal Australian Chemical Institute, Melbourne, 2000.

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