Wednesday, December 31, 2014

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"


Anonymous said...

Ugh, why do people post "facts" that are really just opinions? Why don't U.S. poultry farms have the same standards?

John Brooks said...

It has been pointed out to me that flour may also contain pathogenic bacteria, though it appears that the most common is Escherichia coli. This has caused illnesses when people have eaten raw batters, but also in children who play with home made playdough.

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