Appl Microbiol Biotechnol DOI 10.1007/s00253-005-0274-5 APPLIED MICROBIAL AND CELL PHYSIOLOGY Am Jang . Jeffrey Szabo . Ahmed A. Hosni . Michael Coughlin . Paul L. Bishop Measurement of chlorine dioxide penetration in dairy process pipe biofilms during disinfection Received: 13 September 2005 / Revised: 22 November 2005 / Accepted: 27 November 2005 # Springer-Verlag 2005 Abstract Biofilms are considered a significant health risk in the food and dairy industries because they can harbor pathogens, and direct contact with them can lead to food contamination. Biofilm control is often performed using strong oxidizing agents like chlorine and peracetic acid. Although chlorine dioxide (ClO 2 ) is being used increas- ingly to control microbiological growth in a number of different industries, not much is known about disinfection in biofilms using chlorine dioxide. In this study, a microelectrode originally made for chlorine detection was modified to measure the profiles of chlorine dioxide in biofilm as a function of depth into the biofilm. In addition, discarded microelectrodes proved useful for in situ direct measurement of biofilm thicknesses. The chlorine dioxide microelectrode had a linear response when calibrated up to a ClO 2 concentration of 0.4 mM. ClO 2 profiles showed depletion of disinfectant at 100 μm in the biofilm depth, indicating that ClO 2 may not reach bacteria in a biofilm thicker than this using a 25 mg/l solution. Introduction Biofouling may be defined as the attachment of biological materials to a solid surface that quickly form thin layers of microbial colonies. The microbial colonies, otherwise known as biofilms, constantly and rapidly develop a complex aggregation of microorganisms including bacte- ria, protozoa, amoebae, fungi, and algae, which are encased in self-made extracellular polysaccharide. Biofouling commonly occurs in engineered systems, such as oil rigs, piping systems, drinking water distribution systems, heat exchangers, cooling towers, and ship hulls (De Beer et al. 1994; Meyer 2003). It causes equipment damage through corrosion, down time, and decreased energy efficiency due to the increased pressure drop in pipelines, which results in billions of dollars in losses each year (Meesters et al. 2003). Food and dairy industries frequently encounter persis- tent biofouling problems that lead to solids buildup, spoilage, and bacterial contamination in pipes and process equipment. Fatemi and Frank (1999) described that biofilms developed on wet food-processing surfaces were not completely cleaned or not cleaned often enough. Michaels et al. (2001) reported that in the food industry, even under refrigerated conditions, pipeline biofilms could include spoilage microorganisms as well as pathogenic species. Recently, it has been reported that several Bacillus cereus strains associated with dairy products potentially cause food-borne illness as a result of toxin production (Lindsay et al. 2002). Therefore, process efficiency and safety in the food industry requires novel approaches to the prevention, removal, and killing of biofilms. Biofilm control is often performed with strong oxidizing agents, of which the most commonly used is chlorine (Cl 2 ). Although gaseous chlorine has long been used as a simple and economic method for disinfection, its use as a microbiocide is declining because of safety, environment, and community impact considerations. Furthermore, it has been found that chlorination may form disinfection by- products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs), by oxidizing organic matter (fulvic and humic acids) in water. It is well documented that these products are harmful to consumers’ health (Junli et al. 1997; Chang et al. 2001). As a result of these concerns, various alternatives have been explored, includ- ing peracetic acid, chloramine, ozone, and chlorine dioxide (ClO 2 ) (Lopez et al. 1997; Fatemi and Frank 1999; Richardson et al. 2000; Chang et al. 2001). As one of the promising disinfectants, chlorine dioxide has become more widespread as it offers some unique advantages, including A. Jang . J. Szabo . A. A. Hosni . P. L. Bishop (*) Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221-0071, USA e-mail: Paul.Bishop@UC.edu Tel.: +1-513-5563675 Fax: +1-513-5563930 M. Coughlin JohnsonDiversey, Incorporated Innovation Center, 3630 East Kemper Road, Cincinnati, OH 45241, USA