ABSTRACT
The process of inactivating microbes by UV radiation involves two steps that occur in series with some overlap. In the first step, microbes must diffuse toward the radiation surface. When the microbes reach a distance l from the radiation surface, photons are absorbed by the microbes that become inactivated. The distance l represents the radiation penetration depth, where the photon flux is 10% of the initial flux I0 at the radiation surface. The penetration depth l within a fluid of absorbance A is defined by
I I
= −10 (6.1)
where lA = 1 and the penetration depth is thus defined by l = 1/A. Of the two steps representing diffusion followed by inactivation, the diffusion step
is much slower and is called the rate-limiting step in the microbe inactivation process (Forney et al. 2008). One must therefore focus on the diffusion process to predict microbe inactivation rates in a UV reactor. Here, the nature of the diffusion process will depend on flow conditions that can be either laminar, turbulent, or dominated by secondary flow such as Taylor or Dean vortices, as discussed in Chapter 8. In all cases, however, the microbes must diffuse across a laminar sublayer of thickness d next to the radiation surface, where d > l for most liquid foods.
