Lesson 6 - Hygienic Design

Materials play a vital role in hygienic design. If the material does not meet hygienic specification, there is no way the equipment made with it or the installation thereof will itself be hygienic.

One of the important things to bear in mind with any material is whether it will it be susceptible to either corrosion or degradation. You may remember these terms from the Classification and Properties lesson. If the shiny hygienic material is attacked, the resulting weak spot will render the material unhygienic. Let us quickly review some materials and their hygienic properties.

Metals

We learnt in the lesson on material properties that there is a wide range of metals available, and we base our choices on a wide range of properties. When it comes to wet-cleaned process plants, there is normally only one choice.

Stainless steels are chosen because of their resistance to corrosion and the durability they bring. Designers should have a careful look at the class of stainless steel used, as not all of them are fit for every purpose. Certain combinations should only be used for more industrial purposes, like pipe construction, not for building food manufacturing equipment.

Other metals such as titanium and platinum will also meet the criteria, but they are cost-prohibitive. Copper has been traditionally used in the brewing industry; however, it is not advisable for high-acid applications because acid tends to leach the metal, with the potential for forming toxic compounds. Aluminium is often used for food processing machinery designed with a lighter weight in mind. Aluminium is, however, extremely susceptible to corrosion and becomes warped and cracked after a long period of use. Oxidising cleaning products also are not compatible with aluminium.

Non-metals

Plastics in contact with food are governed by legislation in the EU and the FDA. For them to be suitable for hygienic applications, they should be:

• Suitable for the proposed temperature range

• Abrasion resistant

• Resistant to stress-cracking and brittleness

• Hydrophobic and non-reactive

• Cleanable in terms of surface roughness and topography (the deviations in the surface just like those found in a landscape).

Elastomers

Elastomers need to be able to ‘bounce back’ without losing their form, be resistant to ageing, and have low thermal expansion coefficients to be suitable for hygienic applications.

Composites

Composites can be a produced from a wide range of component materials, and each of these has to be checked for their compatibility. Additions of fibres can cause porosity and therefore, can increase the risk of microbiological contamination.

Glass

Glass is not always all it’s cracked up to be. Various glasses have different characteristics, and care needs to be taken when selecting the type for a given application. The significant risk with glass is always breakage. Whilst we are specifically not covering packaging materials here, it is worth noting that a large percentage of product recalls in the beverage industry are as a result of glass splinters or shards in bottles.

Coatings

The biggest issue with coatings is that they may break off and end up in the product. One practical recommendation has been, where possible, for coating to be a distinctive colour so that if it does break or flake off, it will be easy to detect.

Coatings are constantly evolving and may soon be playing a greater role in hygienic applications.

Other

Ceramics are widely used in membrane filtration systems. They are easy to clean as filtration systems and remove unwanted components such as yeast and bacteria from beer.

Wood is porous and has poor durability. Wood is a high risk substance for foreign bodies and the use of wood should be avoided for the construction of food production equipment/facilities (unless as a process input as in wood aging beers). Wood is regarded as a difficult to clean material and is not generally used in modern brewing. However, some companies have adopted wood for specific reasons.

You can read more about the challenges of cleaning wooden tanks and the novel way of doing it using dry ice below. Note, this is for illustrative purposes only and is not a recommendation or endorsement of the supplier’s product.

https://www.dryicecleaning.com/en/media/pdf/case-studies/dogfish-brewery-case-study.pdf

Finishes of stainless steel

For a hygienic surface, finishes should not exceed the maximum roughness value (Ra < 0.8 µm) advised by EHEDG for large surface areas.

The finishes of stainless steel play a huge role in the overall cleanability of the surface. A recent study was done to compare different finishing methods, and the results are shown in the table below:

Table: Percentage of tested areas left unclean

You can read about a novel process to treat stainless steel surface in the EHEDG Yearbook 2017/2018 pages 50-53. Note, this is for illustrative purposes only and is not a recommendation or endorsement of the supplier’s product.

https://www.ehedg.org/fileadmin/user_upload/PDF-public/EHEDG_17_LR_Ac9.pdf

Now that we have learnt about the materials commonly used for hygienic applications let us look at equipment that is typically used within a brewery such as pipes and vessels, along with in-line equipment such as valves and pumps.