Minimizing the chance of bacterial contamination by designing a piece of equipment for ease of cleaning should be the goal of all processing equipment design engineers. This goal is readily achieved by using good manufacturing practices and appropriate material selection.
In a sense, the cleaning and sanitizing of food processing equipment should begin before the equipment is built, as many cleaning and sanitizing problems are the result of improperly designed or manufactured equipment.
As you walk through a typical food processing plant you can see equipment of varying ages, constructed of a myriad of materials, including carbon steel, aluminum, stainless steel and plastics.
Corrosion can be an enemy of many of these materials. Corrosion is an attack on a material due to a chemical or electrochemical reaction with a surrounding medium. Metals, such as aluminum, iron, steel, chromium and titanium form a thin oxide film under oxidizing conditions. This oxidizing film increases the resistance of the metal to corrosion.
If this oxide film is damaged by chemical or mechanical action, local corrosion can occur in the damaged areas while the rest of the surface remains protected. If the surface is contaminated with remnants of scale or welding slug, the formation of this protective oxide film is impeded and the risk of corrosion is increased.
Aluminum is a material used extensively for processing equipment. It has a very good strength to weight ratio and is highly resistant to corrosion. Aluminum is also resistant to many acids, but contact with alkalis attacks the oxide skin and causes corrosion. Although the metal can safely be used in the presence of certain mild alkalis, with the aid of inhibitors, direct contact with alkaline substances should be avoided.
Aluminum is readily formed and welded, and due to its relative light weight it is considered more economical than stainless steel.
Sanitary problems can arise with the use of aluminum due to improper handling during fabrication. Aluminum has a relatively low surface hardness, and therefore is susceptible to scratching. The use of phosphoric acids to clean welds and sometimes the entire piece of equipment removes the aluminum oxide layer and usually results in corrosion. Another problem can occur when conveyor belting runs directly on aluminum beds. The oxide film transfers to the belting and
causes it to darken.
Steel is the most common material used in manufacturing. However, plain steel has a relatively weak passivity and, unprotected, it will continue to rust and corrode. Plain carbon steel can be augmented by alloying with chromium which has a very high tendency to passivate. When the chromium content of steel reaches 12 to 13 percent the passivity is so good the material will not corrode in ordinary atmospheres or in fresh water. Such alloys are generally called stainless steel. However, stainless steel is not stain or rust proof. Some stainless steels being produced have chromium as their sole alloying element but most stainless steels also contain significant amounts of other alloying elements. The purpose of these additives is to improve corrosion resistance of the steel or to increase its strength.
The two most common grades of stainless steel used in processing equipment are:
1. Type 304 - most common and versatile stainless steel. It has excellent forming and welding characteristics. It is readily brake or roll formed into a variety of parts for equipment. Type 304 has outstanding welding characteristics. Post weld annealing is not required to restore the excellent performance of this grade.
2. Type 316 - contains slightly more nickel and has a better resistance to corrosion than type 304, especially in chloride environments that tend to cause pitting. Type 316 is generally more expensive than type 304.
It is these properties that make stainless steel the preferred metal for fabricating processing equipment.
Stainless steels are also identified by their surface finishes which are produced by three basic methods:
1. Rolling between polished or textured rolls
2. Polishing or buffing with abrasive wheels or belts
3. Blasting with glass beads, which produces a uniform surface
Common stainless steel finishes found in food processing equipment are #2B, which is a smooth, dull finish and #4, which is a general purpose polished finish. Both these finishes are considered smooth. Smoothness is important - crevices provide places for bacteria to growth. FABRICATION PRACTICES
Cleaning problems with stainless steel which occur because of bad fabrication practices include:
1. Mild Steel Contamination:
a) Metal particles from steel dies can become imbedded in the surface at pressure points. This can be solved by covering either the stainless steel with a plastic coating or the press dies.
b) Grinding tools previously used on carbon steel can leave particles imbedded in stainless steel which will later rust and stain. This is a big problem in plants that fabricate in both stainless and mild steel.
Mild steel contamination can be removed by chemical passivation. Cleaning the material with solutions such as nitric acid and water dissolves carbon steel particles thus providing the proper surface conditions necessary for the reformation of chromium oxide passive film.
a) Improper welding and weld finish techniques can leave crevices and cracks. These areas leave ideal locations for bacterial growth. Use of proper filler materials, grinding and polishing welds, and glass bead blasting can eliminate this problem.
a) Short radius bends can cause stress cracking in the stainless steel which also leave areas for bacterial growth. Proper use of dies can eliminate this problem.
4. Improper Handling During Fabrication (Scratches - Dents):
a) Care should be taken to reduce scratching and denting during fabrication.
5. Design Considerations:
a) The use of dissimilar metals, such as aluminum and stainless steel, in direct contact with each other should be avoided. A galvanic reaction can occur, leading to corrosion. When this cannot be avoided, the use of an insulator is a must.
b) For structural members, round or square tubing is preferred over angle or channel sections. Using continuously welded, totally sealed round tubing, or square tubing "on it's edge", reduces the areas where product or dirt can accumulate on a piece of equipment. However, an incomplete weld or a carelessly placed fastener can open the tubing to water infiltration.
c) Potentially hard to clean areas of equipment to consider are tight double bends in sheets, overlapping of sheet and structural members, plastic wear strips, intermittent welded joints and bolted sections. The equipment should be designed to eliminate these problem areas, or make them accessible for cleaning.
Although it is the designers choice, the authors opinions are that all processing equipment should be built with as large a percentage of stainless steel as practicable, using the methods mentioned herein. There will be a greater initial capital expenditure, but that will be offset by long term savings due to the properties of stainless steel we have mentioned.
Food processing equipment which can easily be made sanitary can be designed and fabricated by using good manufacturing practices and selecting appropriate materials.
Jim Coady, P.Eng., Mike McKenna, P.Eng.