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Figure 4. Nonlinearity of Arrhenius plot for penicillin production rate with Penicillium chrysogenum. Source: Ref. 26.

been shown that the activation energy for cell growth is sufficiently different from that for penicillin production. This difference suggests that the enzymes involved in each sequence are different. (Penicillin, being a secondary metabolite, is produced by mature culture of ¡he Penicillium.) Studies such as these are helpful in elucidating the kinetic mechanisms of reactions. To determine whether a biological or a biochemical reaction is kinetically controlled or mass-transfer controlled, the Arrhenius plot and the calculation of activation energy can be used. The activation energy of a catalyzed biological or biochemical conversion is in the range of 8 to 10 kcal/mol or higher, whereas that of diffusion is smaller than that (27).

The activation energy calculation according to Arrhenius only deals with one type of energy source: heat or thermal. It is known that there are reactions that can be activated by other types of energy source, such as photoenergy, electrical energy, shear energy, and pressure-related energy. The process of photosynthesis is initiated by photo-energy. The mechanism of the transduction of light energy into chemical energy has been a challenge to the scientific community. Electrochemical reactions are initiated by electrical energy. Manufacturing of chlorine and sodium hydroxide are examples. Shear energy has been shown to initiate free-radical formation. What is the efficiency of these different energy source in causing the reactions to happen? Activation energy can be looked on as the energy barrier to the initiation of a reaction. What are the activation energy requirements for photochemical, electrical, or shear energy induced reactions? It is possible to look at the activation energy as an index for the efficiency of a particular energy source. The smaller the value, the higher the efficiency. Using starch conversion as an example, starch can be converted (based on DSC thermogram) by supplying thermal energy or shear energy in an extruder. It is known that starch can also be converted (based on DSC thermogram) by shear energy alone (30-32) that is without external heating or dissipated heat. The efficiency of shear energy in causing this conversion is two to three orders of magnitude higher than that by thermal energy. The thermal activation energies shown in Table 3 are in the range of kilocalories per mole, whereas those of shear, as shown in Table 4, are in the range of calories per mole. This is an important reason why extruders, due to their capability of providing high shear, are efficient reactors in which to carry out starch conversion for food processors.

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