Raw cookie dough and Salmonella

A recent post on FaceBook caused me some concern, amusement and despair.  The argument centred on whether you can get salmonellosis from eating raw cookie dough.  Comments ranged from serious through flippant to borderline insane.

What concerned me was the level of ignorance on basic food safety and the authoritative, but incorrect, advice given.  For example, one writer said that there was no problem with cookie dough containing raw egg, because Salmonella is found only on the outside of the shell; washing the outside of the egg would remove faecal material, rendering the eggs safe.  Both of these comments are wrong.

The egg shell is not an hermetic seal; if eggs are washed and then they cool, moisture can be drawn into the egg and bacteria on the outside may also pass through.  There is a defence mechanism in the egg white.  Lysozyme is an enzyme that can attack the cell wall of Gram positive bacteria.  Unfortunately, Salmonellae are Gram negative, so may survive.  In addition, the hen may be infected with Salmonella, so the egg may be contaminated even before the shell is formed.

Laying flocks in New Zealand are unlikely to be infected with Salmonella because of the biosecurity measures employed on New Zealand poultry farms.  The situation may be different in other countries.

Commercial cookie dough may be made with pasteurised egg, in which case any Salmonellae will have been destroyed.  But unpasteurised eggs may contain Salmonella.

Statements that athletes drink raw egg drinks for breakfast and suffer no consequences are without scientific support.  Arguments that real mayonnaise is made with raw egg are also misleading, as mayonnaise also contains vinegar, which not only loosens the emulsion, but also reduces the pH.

Most of us at some time in our lives have scraped the mixing bowl with no ill effects.  The infecting dose for Salmonella varies with the strain, and we may have been exposed to only low numbers of bacteria.   However, to eat large quantities of raw cookie dough is playing Russian roulette.


My two favourite comments from the many posts:

"I know that if you put Nutella on salmon, you get Salmonella"

"I've eaten it twice, and died both times"

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550,000,000 eggs have now been withdrawn from the market in two recalls in USA.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

Hazardous Home-made Ice Cream

We’ve probably all eaten and enjoyed home-made ice cream at some time in our lives. It can be great fun for kids to help Mum make it during the summer holidays.

During the recent summer holiday (Southern Hemisphere), my neighbour gave me a recipe for making ice cream. It seemed simple enough – just combining eggs and cream, vanilla and sugar and then freezing it, stirring at intervals. “The children just love it”. There was no mention of pasteurization, which made me uncomfortable.

I looked for “Ice cream home made” on Google and found many recipes, some of which used eggs. Some included heating steps, some didn’t. This is worrying.

Eggs can potentially be infected with Salmonella during formation in the hen. External contamination can also occur, which may be transferred to the egg contents when the shell is cracked. The egg industry has procedures in place to minimize these risks and commercial egg pulp is pasteurized. This is a fairly delicate balance between providing sufficient heat to kill pathogenic bacteria and not causing the egg proteins to denature.

If you are going to make ice cream at home this summer, I recommend that you either use a recipe that does not call for eggs, or that you use a process that heats the eggs. A typical recipe can be found at:
http://www.cooks.com/rec/doc/0,1613,150189-248198,00.html
where the following instruction is given: “In heavy 3 quart saucepan with wire whisk, combine sugar, flour and salt. Beat in milk and eggs until well blended. Cook over medium low heat, stirring frequently, until mixture thickens and coats spoon, about 30 to 45 minutes. (Do not boil or mixture will curdle.) Cover surface with plastic wrap; cool completely, about 3 hours”.

The temperature of the egg mix needs to get high enough to kill Salmonella bacteria if they are present. This can be achieved by heating the mix to 80C and holding it for 15 seconds.

Should I eat the chicken?

A recent publication in New Zealand caused a lot of interest by claiming that sale of raw chicken should be banned, in favour of frozen chicken. The basis for the claim was that chicken meat is routinely contaminated with Campylobacter. The freezing process would kill the Campylobacter and render the meat safe.

Was this claim justified?

First we need to understand something about chicken farming, chicken processing and, not least, Campylobacter.

Let's start with Campylobacter. Campylobacter is a bacterium commonly found in animals and the environment. It can get into food via many different contamination vectors (carriers), including untreated water, birds, insects and animals. It is often found in the farm environment, which may be contaminated by animals.

The number of Campylobacter we need to eat to become infected is very low; various estimates put the "infecting dose" at between 6 and 800 cells. This means that even low levels of contamination in food can cause a disease if the food is consumed. Even contact with infected animals, for example in petting zoos, can put sufficient bacteria onto the fingers to give an infecting dose if they are put into the mouth (a problem with little children) or used to prepare food without first washing them carefully.

"But surely" you are asking "we cook chicken before we eat it". Well, yes we do. Assuming that we cook it properly, the Campylobacter cells will be killed and the chicken is safe to eat. But supposing that we handle the raw chicken and then prepare a salad; or we use the same chopping board to prepare the raw meat and then cut the salad on the same board, using the same knife? If we don't wash and sanitize the knife and board, we can "cross contaminate" the food from the chicken, via the knife and board.

The suggestion that raw chicken should be banned, and only frozen chicken sold now has some attraction. "Freezing kills Campylobacter, so that will make the chicken safe and we won't get cross contamination, right?" Well, it goes some way to reducing the contamination. But, as with most things to do with bacteria, it's not black and white. Again, estimates vary; freezing may destroy tens of millions of Campylobacter, but equally may have little effect. FAO/WHO data suggest a hundred-fold reduction if the product is frozen and held for a minimum of 3 weeks. A reduction of a hundred-fold sounds a lot, but in microbiological terms this might still leave millions of bacteria on the product. And as we have seen, this is plenty to give an infecting dose.

So, to go back to the title of this posting, should we eat the chicken? My personal opinion is a qualified "Yes". The caveat is that we must make sure to handle the raw chicken safely, preventing cross contamination of utensils, surfaces and other foods in the kitchen. Cook the meat properly and prevent recontamination.

The New Zealand Food Safety Authority (NZFSA) says that two of the most important ways of keeping food safe are to follow the 4Cs rule – Clean, Cook, Cover and Chill the food. Also follow the 20+20 hand wash rule before and after handling poultry and preparing food - wash your hands, using plenty of soap and warm water for at least 20 seconds, paying careful attention to finger nails and the webs between fingers. Rinse well and dry them for a further 20 seconds, using a clean dry hand towel or disposable paper towel. It has been shown that wet hands can transfer microbial contamination to utensils, refrigerator door handles, taps and other potential cross contamination points, so the drying is just as important as the washing.

Odd Spot: On 3 December 1988, Edwina Currie, the outspoken junior health minister in the UK Thatcher government was quoted as telling ITN, "Most of the egg production in this country sadly is now infected with salmonella." Sales of eggs plummeted 60 per cent overnight. The loss of revenue forced farmers to slaughter four million hens and 400 million unsaleable eggs had to be destroyed. The government had to provide a multi million pound rescue package for the poultry industry and the unfortunate Ms. Currie resigned on the 17th December. Undoubtedly, there was some substance to her claims. Vaccination of hens to protect against salmonella has now greatly reduced the levels of egg contamination to the present 0.3% reported by the latest FSA survey and since 1997 the incidence of salmonellosis in UK has fallen by a factor of about 4x. However, there is still a concern that eggs on sale in UK may contain Salmonella Enteritidis organisms, perhaps as a result of importation of eggs produced by unvaccinated hens.

Perhaps Edwina was right. Only time will tell if the advocates of frozen chicken are right too.