ORIGINAL RESEARCH Induction of pitting corrosion on stainless steel (grades 304 and 316) used in dairy industry by biofilms of common sporeformers SOMIL GUPTA and SANJEEV ANAND* Midwest Dairy Foods Research Center, Dairy and Food Science Department, South Dakota State University, Brookings, SD 57007, USA This study hypothesised that there may be induction of pitting corrosion or microbially-induced cor- rosion on stainless steel (SS) dairy-processing surfaces by biofilms of common milk sporeformers such as Bacillus sporothermodurans and Geobacillus stearothermophilus. Scanning electron micro- scopy (SEM) and micrographs generated from energy-dispersive spectroscopy were used to observe pitting corrosion and to find the elemental composition and distribution on the control and pitted surfaces. From SEM images and energy-dispersive spectroscopy micrographs, it was observed that pitting corrosion on SS could be induced by biofilms of both B. sporothermodurans and G. stearothermophilus. Keywords Microbially-induced corrosion, Pitting, Stainless steel, Biofilm, Sporeformer. INTRODUCTION It is widely accepted in the food industry that the adhesion and colonisation of bacteria as biofilms cause problems by affecting product quality (Kumar and Anand 1998). In addition, biofouling of heat exchangers due to the build-up of biomass from biofilms can also lead to a decrease in the efficiency of heat transfer (Bower et al. 1996). Bacterial biofilms are more difficult to eliminate from within a system than free-living cells (Wir- tanen and Mattila-Sandholm 1992a; 1992b). In general, process biofilms are formed over a sur- face that comes in direct contact with a flowing product. They are characterised by rapid develop- ment, and counts as high as 6.0 log 10 cells/cm 2 (Bouman et al. 1982) have been reported that are often dominated by a single species (Hup and Stadhouders 1979). A common example is the development of biofilms in the regeneration sec- tion of a pasteuriser within 12 h of operation (Bouman et al. 1982). In dairy-processing plants, some other pieces of equipment operated at high process temperatures, such as evaporators, pre- heaters, plate heat exchangers and separators, are also prone to biofilm formation (Flint et al. 1997; Burgess et al. 2010). Milk powder manufacturing plants face the problem of spores of Geobacillus spp. (Palmer et al. 2010). Dead ends, valves and joints are susceptible to biofilm formation and, once established, they accelerate corrosion and material deterioration (Storgards et al. 1999a; 1999b). According to Marshall et al. (1971), the attachment of bacteria to a surface is the very first and essential stage in the formation of a biofilm, which is a two-step process. The first step is known as a reversible biofilm, in which the microorganism comes in to close proximity of the surface and gets weakly there held by electro- static forces. In the second irreversible step, the attached microorganism may produce exopoly- saccharides (EPS) that make the bacterial cell stick better to the surface. Russell (1993) pro- posed that bacterial biofilms might not just cross- contaminate dairy products, they may also cause corrosion of the metal surfaces. Some other work by Little et al. (1986) also suggested that bacte- rial attachment to a surface is essential for the ini- tiation of biocorrosion. As the cells in a biofilm propagate, they alter the electrochemical charac- teristics of the metal surface, leading to cathodic depolarisation (due to depletion of oxygen during microbial growth); the increased localised acidity around the microbial colonies may cause *Author for correspondence. E-mail: sanjeev.anand@ sdstate.edu © 2017 Society of Dairy Technology Vol 70 International Journal of Dairy Technology 1 doi: 10.1111/1471-0307.12444