**1. Introduction**

In aquatic environments, microorganisms have a tendency to attach to surfaces along with organic and inorganic soil. For example in breweries, microorganisms have been shown to accumulate on sterile stainless steel surfaces within hours after the start of production [1].

Consumer demand is driving the development of a new group of more sensitive beverages with less alcohol, hop substances, and preservatives; however, these products are more prone to spoilage than are traditional drinks [2]. There are numerous operations involved in making beer. Each stage has a level of cleanliness that needs to be achieved and fouling is encountered at each stage [3]. Attachment of primary colonizers to stainless steel has been shown to be increased by sugars and sweeteners [1]. Thus removal of these deposits is essential since conditioning of a surface may be followed by biofilm formation. Biofilms on bottling plant surfaces are considered as serious sources for potential product spoiling microorganisms in the brewing industry [4]. Further, Fornalik [5] noted that minor fouling organisms resistant to cleaning in place (CIP) may become more resistant with time. Rheological studies indicated that increasing the temperature of the deposit generated a more elastic deposit which may decrease cleanability [3]. Thus, regular daily cleaning is needed. The following media are usually used in the cleaning process in brewing industry: water and steam, peroxide and alcohol based disinfectants, alkaline and acidic detergents and organic solvents [6,7]. There are however numerous drivers for a revision of CIP operations including the need to minimise utility usage (energy and water) and production downtime, minimisation of waste and greenhouse gas (GHG) emissions, and the need for product safety and quality [3].

One way to reduce cleaning costs and to improve process hygiene could be to use self-cleaning and antimicrobial coatings which can prevent the attachment of microorganisms and soil, or facilitate their efficient removal in the cleaning process.

TiO2 is a widely used semiconductor. It has many different applications in optics [8], the environment [9], photovoltaics and solar cells [10,11], self-cleaning [12,13] and antimicrobial coatings [14]. In the self-cleaning and antimicrobial applications, the intended mechanism of action is often photocatalytic; in which the action of light on the TiO2 coating generates active species that may be detrimental to microbes. For these applications, thin TiO2 films with submicron thicknesses are usually employed. Several studies have been carried out to investigate the effect of crystal structure on the photocatalytic performance of TiO2. Whilst some studies have found a higher activity of the anatase form [15,16], others have reported the mixed phase anatase/rutile to show a better photocatalytic performance [17]. Comparative studies of single phase anatase and rutile TiO2 have concluded that the photocatalytic activity is dependent on the reaction being studied and different kinetics and intermediaries may be produced in each case [18,19]. As the surfaces used in the food and beverage industries are exposed to adverse environments (contact with water and beverages, cleaning solutions, abrasive wear during cleaning), scratch and corrosion resistance play important roles in their mechanical durability and chemical stability. Hence, it is important to satisfy several requirements, including good adhesion to the substrate, the retention of high activity and resistance to chemicals.

The adhesion of any film to its substrate is one of the most important properties of a thin film. The level of adhesion depends on the force required to separate atoms or molecules at the interface between film and substrate. The adhesion of a film to the substrate is strongly dependent on the chemical nature, cleanliness, and microscopic topography of the substrate surface [20]. The presence of contaminants on the substrate surface may increase or decrease the adhesion depending on whether the adsorption energy is increased or decreased, respectively. Also the adhesion of a film can be improved by providing more nucleation sites on the substrate, for instance, by using a fine-grained substrate or a substrate pre-coated with suitable materials. Of the deposition processes available, magnetron sputtering has been shown to produce well adhered and uniform coatings over wide areas [11]. In this process, the adhesion of the film to the substrate can be improved by ion-cleaning of the substrate prior to the coating deposition as well as additional ion bombardment during coating deposition which improves adhesion by providing intermixing on an atomic scale [21].

It has been shown throughout the literature that the chemical and structural properties of the active film have a profound impact on the overall photocatalytic performance. Photocatalytic performance is influenced by film characteristics including; composition, bulk and surface structure and nanostructure, atomic to nanoscale roughness, hydroxyl concentration, and impurity concentration (e.g., Fe and Cr) [22–25].

The work described in this paper investigates the chemical and mechanical durability, wettability and the retention of photocatalytic activity of selected coatings after being placed in different brewery process environments, in this case bottle/can filling lines in three Finnish breweries.
