report
2.3.3.2.1 Equipment coatings

2.3.2.1 Equipment coatings

Coatings for food processing equipment must be, first and foremost, non-hazardous to human health.  Secondly they should minimise (or ideally prevent) biofilm formation which can lead to food spoilage and contamination, and finally they should be durable.  Traditionally such equipment was manufactured from stainless steel as this is both durable and non-hazardous to human health.  However, stainless steel is susceptible to pitting and scoring, which serve as focal points for microbial growth.  As a result they require regular cleaning and disinfection, which at the very least means some production downtime, and can often require partial dismantling to allow access to internal spaces.  The areas that are most prone to biofouling are heat exchangers.  Bacteria that commonly contaminate food processing equipment include Bacillus subtilis, Listeria monocytogenes, Staphylococcus aureus, Salmonella typhimurium and Escherichia coli.

It has been known for a number of years that biofilms will grow in any nutrient rich medium and will strongly adhere to many different surfaces[i].  More recently it has been determined that the nanoscale structure of a surface can control the adhesion of biomolecules and by extension microbes[ii]^,[iii],[iv].  Biofilms are a major concern to the food processing industry (as well as many other sectors, including medicine and marine industries) as bacteria within the biofilm are resistant to antibiotics and normal cleaning practices, but have the potential to ‘break off’ and contaminate foodstuffs.

Nanotechnology enabled processes can help resolve the issues of durability and biofilm prevention.  This can be achieved through application of a coating or through the direct nanostructuring of the surface layers of the material.  Both act to decrease the material’s surface free energy thus decreasing the strength of microbial adherence.  This can either help prevent adherence in the first instance or increase cleaning efficiency[v].  An established material that is widely used is Teflon (polytetrafluoroethylene), which has a low surface free energy, but poor abrasion resistance.

There are several methods for applying coatings to material surfaces:

• gas phase synthesis (e.g. chemical and physical vapour deposition, CVD and PVD, plasma and laser ablation).  The material is vaporised by intense heat (e.g. laser) and then deposited on a substrate (usually under vacuum).  This is generally expensive, difficult to scale-up and not suitable for temperature-sensitive materials (e.g. polymers, biomolecules).
• sol-gel processes.  Reactants are mixed under defined temperatures and pressures to produce colloids of nanoparticles.  Major issues include strictly defining particle size distribution (or porosity), preventing particle agglomeration, and the amount of waste material produced.
• electrospray and electrospinning.  Reactants are passed through a fine nozzle, which is subject to a high voltage, causing the reactants to form charged droplets or fibres that are collected on a grounded collector.  Such processes can be used to coat large surfaces.
• self-assembly.  Reactants combine in a predefined manner to form a layer on the desired substrate.   

Of these the most practical (in terms of cost, versatility and scale-up) is the sol-gel approach.  Sol-gel techniques allow control of the porosity and nanoscale structure of the final coating (thus limiting microbial adherence) and can also incorporate anti-microbials (such as silver) and photo-catalytic materials (such as titanium dioxide).  Both of these types of material help reduce biofilm growth. 

For certain equipment parts, high durability is required.  Diamond-like carbon (DLC) coatings (which are deposited by gas phase processes) show high durability and minimal biofouling.  They are used in many different industries: for example, personal care (e.g. razor blades), car engine parts, medical device industry (e.g., implants such as stents and catheters).  In food processing the applications are more likely in non-food contact areas, as there is experimental evidence that DLC coatings do not withstand the repeated cleaning cycles necessary in the food processing industry[vi]. 

Other promising research in this area includes electroless plating with nickel and PTFE to produce a nanostructured surface on stainless steel[vii], and the use of polymer coatings with and without antimicrobial nanoparticulates on a variety of surfaces, but which do not require high wear resistance[viii]^,[ix].