Would you like your Salmonella over easy or sunny side up, sir?

550,000,000 eggs have now been withdrawn from the market in two recalls in USA.

380m eggs were recalled early last week by Wright County Egg and now 170m eggs have been recalled by Hillandale Farms of Iowa.  Eggs from both companies have been linked with salmonellosis affecting at least 1700 people.

Salmonella is a bacterium found in the gut of man, animals and birds.  It causes nausea, vomiting, abdominal pain and diarrhoea that may last for several days and can be fatal in the very young or elderly.  It is usually contracted by ingestion of faecally contaminated food, though this is not the only route of infection.  The disease is described as a “food borne infection”, because after ingestion of infected food, the bacteria grow in the gut, causing the symptoms, which appear 12 to 14 hours after ingestion. 

Why have so many eggs been withdrawn?  I think it’s probably because of the way the regulations governing poultry farming in the US are written.  As I understand it, testing is conducted under the FDA “Egg Rule” -Environmental Testing for Salmonella Enteritidis (21 CFR 118.5).  Samples are taken from the environment to determine if S. Enteritidis is present, because this is an indicator of the effectiveness of the SE control plan.  The pullet environment is tested when the pullets are 14 – 16 weeks old and in addition, the environment in each poultry house is tested when any group of laying hens reaches 40 – 45 weeks of age.  

If rodents, birds or insects introduce Salmonella to the poultry house, between testing times, the infection could spread rapidly throughout the flock and eggs could be produced from infected hens. Hens can become infected internally and eggs can be infected before the shell is formed.  There is no way to tell if an egg is infected with Salmonella, other than cracking it and culturing the contents. Eggs are required to be tested only when an environmental sample is shown to be positive.  So the huge recall is to ensure that all potentially contaminated eggs are removed from the market place and homes.

How can consumers protect themselves from contracting salmonellosis?  The most obvious precautions are the proper refrigerated storage and cooking of eggs, washing hands after handling eggs and avoidance of products containing raw eggs, such as home made ice cream and mayonnaise, or coddled eggs.  If you have eggs subject to the recall, dispose of them or return them to the retailer for a refund.

Eggs in New Zealand are unlikely to contain Salmonella.  In the next posting, I’ll discuss this and the latest on Campylobacter in New Zealand.

Just a thought:  How are they going to dispose of 550m eggs?  At 60g (about 2 ounces) each, that is about 33,000 tonnes (36,376 US tons) of eggs.

More on Salt in Foods

It’s generally accepted that we eat too much salt in New Zealand – up to 150% of the maximum recommended intake (see Sodium in Food, 13 July 2010). Excessive consumption of sodium raises blood pressure and may increase the risk of cardiovascular disease. Bread is the greatest contributor to our sodium intake, followed by sausages and processed meats.

One of the concerns of manufacturers of foods that contain added salt is that the consumer will detect the change if salt content is reduced and refuse to buy that brand.

In 2003, I took part in a trial in which we tested three commercially baked breads with varying levels of salt, from the standard content at that time of 550 mg/100g, 5% reduction (530 mg/100g) and 10% reduction (490mg/100g). We performed controlled trials in which 60 consumers were given three samples - two identical and one different - and asked to pick the odd one out. This is called a triangle test. Twenty eight percent of the panellists correctly identified the 5% reduced sample and 37% identified the 10% reduced salt bread. This relatively small trial showed that these differences in perception of salt content were not statistically different i.e. that the consumers could not detect the lowered salt breads. Recent figures show that some breads now have 20% less salt than equivalent products in 2003.

As I wrote in “Sodium in Food”, July 2010, sodium chloride has many functions in foods besides flavouring. What are the alternatives to salt? We can replace some sodium with other ions, such as potassium, magnesium and calcium. We can purchase reduced sodium table salt, though the UK Food Standards Agency does not recommend the use of salt substitutes, as they don’t reduce consumers’ taste for salt. Replacement of 40% of sodium by potassium in manufactured foods may result in detectable flavour changes and there may be problems for people with kidney conditions. We could use other preservatives, but consumers have been fed the line that preservatives are bad for them, so there is likely to be resistance to this approach. We could target other sources of sodium in the diet, such as monosodium glutamate (MSG, a flavour enhancer), or water binding agents, such as sodium tripolyphosphate.

How will we know if reducing salt in our food will result in safe food? We can conduct computer-based modelling experiments, using the vast resources of microbial growth models stored in databases. Some of these databases are freely available and allow us to predict such things as “time to spoil” or “time to toxicity” or simply “how long will it take this initial level of contamination to grow to an unacceptable population?” We can vary the formulation of the food and run the model again to see how it performs. In a matter of minutes, we can do extensive trials of alternative formulations.

Unfortunately, these models are not real foods. Once we have modelled the likely shelf life etc. we have to make samples and test them under normal storage and abuse conditions. This is not straightforward and can be very costly. The likelihood therefore is that we will not see rapid reductions in salt content of our manufactured foods, but rather a progressive reduction, as was the case with bread. We can, however, make a start on personal salt intake reduction by using other seasonings and spices in our homes.

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).