Inactivation of Aerobic Bacteria, Yeast, and Mold Contained in Solution Discharged from Soaking Germination Process by a Laminated Photocatalyst

Abstract

In order to establish an efficient inactivation process of aerobic bacteria, yeast, and mold contained in a solution discharged from a soaking germination process in producing germination brown rice, the germs were inactivated in a photocatalytic reactor where a laminated catalyst was prepared by coating and drying emulsion of commercial photocatalyst on a stainless steel plate. A combination process of filtering with the diatomaceous earth produced in Akita Prefecture and photocatalytic reaction was also evaluated for the effective inactivation of the germs.
A prototype of laminated photocatalyst with blacklight lamps was effective for inactivation of aerobic bacteria, yeast, and mold contained in a solution discharged from a soaking germination process. Inactivation rates of the germs were affected by distance between blacklight lamp and photocatalyst surface (r₂) and thickness of solution layer (l); their rates tended to be lower as l became larger than 3×10⁻³ m even under the thin layer zone. By applying suction filter with the diatomaceous aid to sample solution, ca. 90% of aerobic bacteria and 100% of yeast and mold were removed. A combination of diatomaceous filter and photocatalysis proved to be efficient for inactivation of aerobic bacteria.
To kinetically formulate the rate of inactivation for germs by a laminated photocatalyst, a single-hit with multitarget model with the light intensity on the photocatalyst surface I, estimated by the radial light model, was applicable only under thin liquid-layer conditions, while introduction of the effective light intensity I_eff proved to represent observed results well by adopting a single set of target number m and inactivation efficiency η for the inactivation of aerobic bacteria under various values of r₂ and l irrespective of filtering or unfiltering of sample solution. The value of I_eff/I became smaller as l got bigger, and it was nearly identical irrespective of filtering or unfiltering of sample solution. The values of I_eff obtained for the aerobic bacteria also gave good relationship to the observed results for yeast and mold; the values of target number and inactivation efficiency were almost the same as the ones for aerobic bacteria.