Plant and Equipment: Materials and Finishes for Plant and Equipment

Though hygienic design and operation of process equipment in dairy plants encompasses a very large number of issues, two critical aspects are correct selection of the material of construction for dairy equipment and specification of surface finish. If either is misjudged, then product integrity can be placed at risk. Stainless steel has been and continues to be the material of choice for dairy plants; particularly the grades American Iron and Steel Institute (AISI) 304 and AISI 316. Stainless steel has superb mechanical properties, and its corrosion resistance matches the demands of the dairy industry. However, it is not absolutely corrosion proof, particularly in environments where it is either exposed to free chlorides and/or where its oxygen-rich protective coat has been damaged. In the small number of instances where stainless steel is not appropriate, other metals can be selected. Polymers (plastic and rubber) can also be used to build process equipment but must be food-grade polymers. Irrespective of the material of construction that has been selected, a smooth surface finish on all product contact surfaces is necessary. The smoother the surface, the easier it is to clean and the less the possibility of product contamination. In particular, weld surfaces must always be ground down. There are a number of techniques to obtain the required surface finish for dairy equipment usually quantified as having an R_a less than 0.8 μm.