Saturday, August 7, 2010

The joy of cleaning (yeah, right)

I was recently interviewed for a forthcoming television programme dealing with mould in the bathroom. I decided to brush up on my knowledge of cleaning chemicals to try to avoid getting caught flat-footed by the interviewer. When you really get into it, the science behind modern cleaning technologies is quite fascinating and more complex than you might expect.

My research team specialises in the study of biofilms. These accumulations of microorganisms and their sticky products on surfaces are extremely hard to clean. Since I have mentioned biofilms in earlier posts, I thought that it might be time to examine them in more detail here.

Pasteur and Koch laid the foundations of modern microbiology by culturing bacteria on solid media in pure culture. This development enabled microbiologists to study individual strains of bacteria without the interference of other types and we have continued to use their techniques. However, it is now generally accepted that bacteria grow preferentially as biofilms – complex communities growing on a surface and surrounded by polysaccharide slime known as glycocalyx. Among other things, this glycocalyx gives the bacteria protection from cleaning agents. Failure to take account of this when formulating cleaners and disinfectants can result in incomplete removal of the film. This is particularly important when the surface is a piece of food processing equipment.

Go and have a look at your beautiful stainless steel kitchen sink or the shower tray. They look perfectly smooth and should be easy to clean. However, when we use a scanning electron microscope to see the surface on the same scale as bacteria, it is clear that the surface is anything but smooth (see first figure). Bacteria can get down into the troughs between the grain boundaries and it’s obvious that getting them out of there is going to be difficult. The difficulty of cleaning is made worse if the bacteria are left to grow long enough to form a proper biofilm. The bacteria produce a sticky mixture of polysaccharides, which glues them to the surface and attracts other bacteria and traps food particles (see image at right).

When we buy a cleaning product from the supermarket, we are buying a carefully formulated mixture of chemicals that has a number of functions: it must bring the chemicals into close contact with the biofilm; proteins, carbohydrates and fats must be solubilised or suspended so that they can be rinsed away; for domestic cleaning it is also desirable that the cleaning product should kill bacteria. (In industrial cleaning, a separate sanitiser is usually applied after cleaning).

To satisfy these requirements, most cleaning products contain a surfactant to break down the surface tension of water (to make it “wetter”) and an alkali to solubilise proteins and fats. Sometimes an acid is used to remove scale deposits. Industrial cleaners for food processing equipment often also contain hypochlorite, which releases hypochlorous acid and ultimately an oxygen radical, both of which are strong oxidising agents that can break down dirt. Because of the potential danger to consumers, domestic cleaning products are usually much less alkaline and generally weaker than industrial cleaners.

I am often asked whether there is an alternative to the “harsh chemicals” used in cleaning products. Well, there are so-called “green cleaners” derived from plant materials, but the principles behind the formulations are the same – combination of surfactant such as an alkyl polyglucoside from palm and coconut, with citric acid and a solvent, D-limonene, from citrus skins. I have heard of white vinegar being used to remove bathroom mould instead of the chlorine-based cleaners. However, even the proponents of such substitutions admit that a lot more effort is required to remove the mould and that it soon comes back. This is partly because vinegar has no surfactant properties.

Successful cleaning requires four things: the right concentration of cleaning product, suitable temperature, mechanical energy (“elbow grease”) and sufficient time for the chemicals to penetrate the dirt and destroy bacteria. The best way to ensure that cleaning is successful is to follow the instructions on the label – the manufacturer has formulated and tested the product to be used in a certain way.

If done correctly, cleaning will remove biofilms from stainless steel. The two images at left show a piece of stainless steel before and after cleaning. The bacteria were stained with a fluorescent dye and observed under UV light in a fluorescence microscope.

However, a successful cleaning operation is only a temporary fix and regular cleaning is essential to prevent biofilms from forming. Like death and taxes, it’s not much fun and there’s really no getting away from having to clean.

Credits for photographs provided by my research group:
First image by Steve Flint and Doug Hopcroft; Second image by Shanthi Parkar and Doug Hopcroft; Third and Fourth images by Shanthi Parkar.

(The description given above is still a simplification of cleaning technology. I have tried to capture just the essentials of the process and the cleaning products).


Helen Baron-St John said...

Proponents of the use of white vinegar to clean bathroom tiles etc often suggest using it with sodium bicarbonate. There is also the suggestion that oil of cloves in solution will help to prevent return of mould. Old wives' tales or a way to help protect the environment?

John said...

Vinegar with sodium bicarbonate probably does work, depending on the relative proportions. If the mix is essentially a paste, the sodium bicarbonate acts as a mild abrasive. If rather less bicarbonate is used, resulting in a solution, the mix will be alkaline and behave like the other cleaners I mentioned, but without the surfactant qualities.

Vinegar on its own probably enters the cell and there dissociates, damaging essential cell components.

Essential oils are often mildly antimicrobial and clove oil has been used for centuries as a preservative.

I'm not convinced that the amount of chemicals involved in cleaning shower cabinets will damage the environment, at least not in comparison with the discharge from a paper mill. However, every bit counts when we are protecting the environment.

Cleaning Services said...

World without bacteria is not possible. You just can't kill them all. Our organisms need bacteria to build a natural immunity. As we all know the bacteria saved us in "War of the Words" :) So please don't hate them :)

John said...

I agree with Cleaning Services - bacteria, moulds and viruses have provided my living for over 40 years and I don't hate them. The important thing to remember is that everything has its place in this world. Bacteria generally are undesirable in food processing equipment, as they cause spoilage or food poisoning. Of course, a few can be used to produce food, such as cheese, yoghurt and beer, so they're not all bad.

James said...

Biofilm can be successfully removed using combinations of enzymes. A Belgian company, Realco, has pioneered this technology. They are marketing it to the food processing industries, for use in their "Cleaning in Place" programmes.

John said...

You are correct James. However, the cost of enzyme cleaners may be much more than conventional alkali, so their use in equipment such as milk evaporators, which may have a product contact area of around 4,600 square metres, may be uneconomic. They are not 100% effective either. A PhD student in my research team recently published a paper on cleaning of dairy ultrafiltration membranes and found that enzyme cleaner was better than other cleaners, but still left about 1.2 log CFU per square cm. of culturable biofilm cells on the membranes. (See Tang, X., Flint, S. H., Bennett, R. J., & Brooks, J. D. The efficacy of different cleaners and sanitisers in cleaning biofilms on UF membranes used in the dairy industry. Journal of Membrane Science, 352(1-2), 71-75).

Anonymous said...

How far are alkyl polyglucosides helpful in cleaning of beer bottles? and what else could be their end use?

John said...

Alkyl polyglucosides are used in many cleaning formulations. According to the manufacturers' claims, they are mild, non-ionic products with good detergency and wetting properties. They are used where high, stable foam is required, or in highly alkaline formulations.

According to Dow, their TRITON™ BG-10 and TRITON™ CG-110 surfactants products are "ideal for glass cleaners, bottle washing formulations, metal cleaners, highly alkaline detergents, paint strippers, and aluminum brighteners".

Alkyl polyglucosides have also been used in personal care products because of their mildness.

I'm not sure what motivated your question, but please remember that these chemicals cannot generally be used on their own; they are combined with other essential components in cleaning formulations.

rudy said...

CoolPuck a product from" Special Water Holding bv" destroys the structure and viability of the biofilm. It is composed of salts of non biocidal chemicals. It is safe for humans. It is developed to detach easily the biofilm from all substrates in closed cooling water systems,especially for getting rid of slime and biofilm. It is also used to prevent the formation of biofilm

John said...

Thanks Rudy. I had a look at the manufacturer's website and also traced an independent test report on CoolPuck. There are no details given on the composition of CoolPuck, but it appears to be a detergent that has no biocidal power; it is a dispersant, which aids the access of biocides into the biofilm. For a test report, see:

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