Monday, January 21, 2013

It's not CSI

Most of us have seen the odd episode, or at least been aware of the popular CSI-Miami or other incarnations of the crime drama TV series.  Those of us who are scientists are somewhat cynical about the ease with which samples from crime scenes can be analysed, seemingly within hours, by whizzy machines in gleaming laboratories.

In most cases, this is far from the truth; analysis of DNA samples, for example, requires painstaking care during the collection and processing of the material.  You only have to see the real world courtroom challenges to forensic laboratory evidence to realise that the whole process is much more complicated than the TV programmes would have us believe.

However, the detection of pathogenic bacteria in foods, or even the deliberate adulteration of beef burgers with other meats, is now benefiting from these molecular techniques.

The ability to sequence whole genomes of bacteria, coupled with the cheap synthesis of primers (probes) that will bind to specific parts of the bacterial DNA has enabled us to test for the presence of pathogenic bacteria in food samples. 

In principle, we extract the bacterial DNA and add our probes to bind to unique sequences in the DNA.  If the probe binds to the DNA, then we can use the Polymerase Chain Reaction** to amplify that piece of DNA and then detect it, separating it on a gel to produce a pattern of bands similar to what we see being examined on CSI.  If there is no binding, no amplification occurs and no detection, so the bacteria are absent from the food.

In practice, it's a bit more complicated and time-consuming.  We normally have to 'selectively enrich' our sample to increase the number of bacteria to a level at which we can detect them.  We do this by adding the food to a culture medium that inhibits most other bacteria and encourages our target bacteria to grow.  The actual preparation of the media, weighing the sample and putting them together takes only a couple of hours.  However, we need to incubate the mixture for up to 48 hours under controlled conditions before we can run the PCR.

Over the past few weeks, I have been working with my research assistant, testing a new PCR-based method of detecting Listeria.  It looks as though the method will be quicker and easier than existing methods and we'll publish our results in the near future.

Manufacturers continue to develop new rapid methods, many based on DNA and using automated equipment, making the detection of pathogens in food easier and quicker, allowing products to be checked and released to the market earlier.  These methods can also be used to track down sources of contamination, such as in the European E. coli O104:H4 outbreak of 2011.

**  For those readers keen to know more about PCR, I'll post a more complete description of the technique, trying to keep it relatively simple.

Wednesday, January 16, 2013

"Faster" food creates a furore

The finding of horse meat at up to 30% of a "beef" burger in Ireland has created a furore this week, both in the mainstream and social media.

The Irish Agriculture Minister, Simon Coveney, has blamed a single meat processor and has further stated that the horse meat entered the product via an imported additive, though the nature or identity of the additive was not mentioned.

You can't tell by looking at a burger just what sort of meat is present, so some fairly sophisticated laboratory methods must be used.  Though not stated in the press articles I have read, the chances are that the burger was tested for specific DNA markers for horse meat.  Indeed, both horse and pig DNA were found in some burgers.

It appears that "traces" of pig DNA were also found in 85% of the burger products tested.  The fact that only traces were detected suggests to me that either a pig-derived additive was used in the manufacture of the burgers, or the beef was ground in equipment improperly cleaned after processing pork.  This latter suggestion is perhaps unlikely, as the traces were found in burgers supplied by a number of manufacturers.

In itself, the finding of traces of DNA from meat animals other than beef cattle should not be cause for alarm - no harm can come from consuming the burgers.  However, some religious and cultural sensibilities may be offended - if you are not allowed to eat pork for whatever reason, you should be able to purchase "beef" products with the expectation that they don't contain pork.  Of course, 30% horse meat in a beef burger is not a trace and the implication is that in this case there was a deliberate attempt to deceive the customer.

While I don't condone the addition of meats not appearing on the label, I wonder if this is another of those situations where the use of highly sensitive analytical techniques has raised a storm.  Something similar happened many years ago when milk was tested by a new, sensitive method for detecting dioxin.  In that case, the dioxin came from the chlorine-bleached paperboard used in manufacture of the milk cartons and was at extremely low levels in the milk. 

How many other foods that we take for granted contain minuscule traces of DNA, and, by implication, foreign plant or animal materials, which get there as a result of the use of natural additives?

Monday, January 7, 2013

Fancy a snot pie?

The thought of this is totally gross.  Why would I contemplate writing under such a heading?

Unfortunately, it's quite possible that some customers have in fact been sold pies contaminated by the profuse nasal secretions of a baby held in the arms of the cook filling the pies.  Talia Shadwell wrote a piece in the Manawatu Standard at the end of December 2012 about a family-owned Rangitikei food premises forced to close because of a very unfavourable food safety inspection report.  In addition to the highly probable contamination of the pies by the infant, the milkshake blender was encrusted with rotten milk.  It is likely that there were other violations of good catering practice and the Food Hygiene Regulations.

Legislation is changing in New Zealand, but it is safe to say that the intent of the regulations will not change.  District councils are responsible for registering food premises and it is illegal to sell food not produced in registered premises.

"No persons shall be issued with a Certificate of Registration for food premises (except a vehicle used solely for the carriage or delivery of food for sale) unless -

• That person has been issued with a Council recognised Certificate in Food Hygiene or another qualification approved by Council; or
• There is, working on the food premises, a Manager or a staff member with specific responsibility for staff training in food hygiene, who has been issued with a Certificate in Food Hygiene, a Certificate in Basic Food Hygiene or other qualification approved by Council."
 It is perhaps surprising that the inspection report noted that the family had 'very limited' knowledge of food hygiene and food safety practices.

Unfortunately, this state of affairs is probably more common than we would like to think - many food premises are run by immigrants who provide the rich diversity of foods available in New Zealand, but who may have very limited knowledge of the language and regulations. This is no excuse for poor hygiene and food production practices.

Consumers have a right to expect their food to be safe to consume and to be produced under aesthetically acceptable conditions.