Cleanability of Stainless Steel Used as a Food Contact Surface

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THE CLEANABILITY OF STAINLESS STEEL USED AS A FOOD CONTACT SURFACE:

AN UPDATED SHORT REVIEW

John J. Milledge

 

Introduction

A previous review, of the cleanability of stainless steel concluded; that if surface finish had an effect, on the cleaning process, it is probably small in relation to other factors during cleaning ¹.   The object of this paper is to briefly review subsequent information so that an update assessment can be made.

Measurement of Surface Roughness

The costs of different surface finishes can vary considerable and the cost of imparting a highly polished finish can be expensive ^{2, 3}.

The roughness of a surface has been most commonly measured by an instrument in which a stylus travels across the surface, the movement of the stylus being amplified and the signal recorded. The result is most commonly expressed as Ra or Average Roughness in “microns Ra” (µm Ra) which is the arithmetic average value of the deviation of the trace above and below the centre-line⁴.  Although, Ra is a useful average it does not differentiate between peaks or valleys and very different profiles can have the same Ra value for example rolling and jagged profiles of the same amplitude^{2, 5}. Other expressions of surface roughness are available and have been suggested as better descriptions of the profile, such as Rz (the sum of the maximum highest peak and lowest valley within the sampling length) and may give additional insight regarding surface character ⁶.

Stainless steel surface finishes are very often described by a numbering system representing a processing method rather than its roughness in terms of Ra values ^{3, 7 8}. Estimates of surface topography as indicated by Ra value for standard finishing process description have been given, but Ra values of a given material can vary ^{2, 5, 9}.

Current Regulations & Recommendations

Regulations normally only give general guideline on food contact materials and surface finishes ^10-13.

European legislation requires that handling, preparation, processing, packaging, etc. of food is done hygienically, with hygienic machinery in hygienic premises. How to comply with these requirements, however, is left to the industry ¹⁴.

The EHEDG (European Hygienic Engineering and Design Group) is a consortium of equipment manufacturers, food industries, research institutes, universities and public health authorities, founded in 1989 with the aim to promote hygiene during the processing and packing of food products ¹⁵. It provides practical direction on hygienic design. The EHEDG may also authorise the use of the certification logo on equipment that complies with the relevant hygiene criteria^{16, 17}.  

In the USA the 3-A Sanitary Standards, Inc. publish uniform standards for the hygienic design of equipment, with the first 3-A Sanitary Standards being developed in the late 1920s. The 3-A Sanitary Standards serve as important references for many state and federal regulatory authorities ¹⁸.

In 1993 the EHEDG and the 3-A organisation concluded that it would be beneficial if US and European standards were similar. Cooperation between the two organisations has been established with the aim of investigating how to produce standards for food processing equipment that would be acceptable in both Europe and the USA ¹³.

The EHEDG recommends surface roughness of less 0.8µm and all 3-A Sanitary criteria now also always include surface finish requirements which are equivalent to smoother than a 0.8 µm (32 µin.) Ra^{9, 19, 20}. The American Meat Institute Equipment Design Task Force also recommends that surfaces should also not exceed 0.8 µm ²¹.

Adhesion

The amount of residual soil after cleaning has been shown to be influenced by the initial contamination on a surface^{22, 23}.  In the assessment of surface finish on cleanability it may be useful to consider the effect of roughness on bacterial and soil adhesion.

In the dental industry a surface with Ra value of less than 0.2µm is deemed to accumulate plaque less well²⁴.  A study of four bacterial species appeared to support this view with bacterial adhesion being lowest on a stainless steel surface with an Ra of 0.16 µm with rougher and smoother surfaces having greater adhesion²⁵.  A later study of Staphylococcus aureus would also appear to give additional support to this view ²⁶.  Small changes in surface topography of 0.1µm have also been found to influence bacterial adhesion with attachment to features only 0.2 µm ²⁷.

In a study of rougher surfaces (Ra 0.029 – 3.2 µm) the minimum adhesion was found at Ra 0.8 µm with both smoother and rougher surfaces having greater adhesion²⁸, but this is contradicted by a study of the adhesion of Streptococci which found that surface roughness did not greatly impact on adhesion with “entrapment” being greatest at Ra 0.9µm²⁹.

Many other studies have found no correlation between surface roughness and bacterial adhesion for stainless steel with a variety of finishes with Ra of up to 1.37µm ^30-35.

One study found no correlation between surface finish and adhesion, but a re-examination of the result suggested a significant correlation ^{6, 36}.

Studies of welds (up to Ra 1.19µm) found that there was no difference in the adhesion to the weld and the smoother base stainless steel ^{37, 38}.

A study of the transfer of bacteria from a contaminated meat product to stainless also found no significant difference between various finishes (Ra 0.25-0.75µm)³⁹.

The adhesion distribution and the effect of surface roughness have been shown to be different for bacteria of different surface energies and for rod and cocci bacteria ^{40, 41}.  These difference between different bacteria together with the type of soil, enumeration method and the range of roughness may help to explain the apparent contradictions in the results⁴².

Drainage

The first step in cleaning is the drainage of residual product from the system. In the drainage of sucrose solution, although surface roughness was found to be significant, there was no correlation between Ra 0.025-0.45µm and it was concluded that heavy investment in highly polished surface is probably unwarranted ⁵. In study of the drainage of edible oils and food emulsions it was concluded stainless steel surface roughness encountered in the food industry is not a major factor in Newtonian food liquid drainage⁴³.

Cleaning

In a modelling study it has been suggested that surface topography only has negative effect on cleanability if crevices are “large and deep”⁴⁴.  Roughened stainless steel (Ra 5.38µm) have been shown to be unacceptable⁴⁵ and abraded stainless steel has also been found to be more difficult to clean than un-abraded, but resistance of stainless steel to abrasion makes it more likely to retain its hygienic properties than many other materials ^46-48.  Rough surfaces caused by welding (Ra 1.97 to 4.56µm) have also been shown to be less cleanable than bright annealed stainless steel⁴⁹, but others have found no difference in cleanability of well executed welds and the surrounding material⁵⁰.

“Highly adhesive milk soil” was found to be more difficult to remove with increasing surface roughness (Ra 0.11-0.3µm)⁵¹ and E. coli was also found to be more difficult to rinse from rougher surfaces (Ra 0.04- 1.37µm) although no difference in initial bacterial adhesion was found³⁵.

Some studies have found an influence of surface finish on cleanability, but without a correlation with surface roughness as indicated by Ra ^52-55, indicating that it may be nature of the surface rather than the magnitude of roughness of the surface that is important.

The majority of studies have found little or no significant difference in the effect of surface roughness on cleanability or rinse-ability^56-62.  In a study mimicking intensive cleaning found in brewery industry there was little difference in the cleanability of 4 common surface finishes² and no difference in cleanability was found between 3 different commercial finishes in a study of milk plate heat-exchangers⁶³.

A study of spray cleaning of 9 different finishes found no evidence that a surface finish less 1µm Ra has any effect on cleanability⁶⁴ and another study of 4 commercially available finishes that for Ra values below 0.8µm surface topography did not affect cleanablity⁶⁵.

Conclusion

This updated brief review would appear to confirm the conclusion of my previous review¹ and a recent review for the pharmaceutical industry ⁶⁶ that  if surface finish does have an effect it probably small in relation to other factors in cleaning and that the weight of evidence would not normally justify the food industry investing in mechanically highly polished surfaces.  The suggested maximum surface roughness of less Ra 0.8µm for stainless steel for the food industry would appear to be reasonable and appropriate.

 

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