Saturday, March 31, 2012

Bacteria in canned food

Looking through the search terms that produced hits on Safe Food this month, I came across "Listeria in canned food".

Is canned food hazardous from the point of view of Listeria?  In my opinion, the answer is "No" with just one small caveat:  "unless the food has been grossly underprocessed, or post- processing contamination has occurred".

Canned foods are divided into two main categories: Low Acid (LACF) and Acid Canned foods (ACF).  The dividing line is pH 4.6, low acid foods having a pH greater than this, and acid food having a pH lower than 4.6.

The significance of this pH value is that Clostridium botulinum can grow at pH above 4.6. 
C. botulinum is able to form very heat resistant spores and can grow in the anaerobic conditions in canned food.  Since C. botulinum also produces a lethal toxin, it is essential to destroy the spores.  So we heat the food in the cans to temperatures above boiling point (in fact we heat to the equivalent of 121.1C for 2.52 minutes and this is called a 12D process).  Properly processed LACF is safe.

When we look at acid foods, we find that C. botulinum is incapable of growth under these acid conditions and so no toxin can be produced.  Since we don't need to deliver a 12D process, we can heat for a shorter time or at lower temperature.  This has the advantage that the food is changed less and it costs less to deliver the process.  Depending on the food and the degree of acidity, we really need only destroy vegetative (i.e. non-sporulating) pathogens and spoilage microorganisms.  This will then destroy Listeria.

What about my conditional clause above?  If the food canner doesn't properly control the process, it might be possible for cans to be underprocessed.  This might occur if the steam supply to the retort (pressure cooker) is lacking or turned off too soon.  Another possibility is that cans might by-pass the retort altogether.

Post-processing contamination could occur if the cooling water is contaminated.  The can seals are not 100% watertight while they are still hot, so using contaminated water may allow a few bacteria to enter the can during cooling.  This resulted in a typhoid outbreak in Aberdeen in 1964, when people consumed imported Argentinian canned  corned beef that was cooled in untreated, sewage-contaminated river water.

Damage to the can seams caused by rough handling can also allow contamination, as there is a partial vacuum in the can.  Momentary opening of the seam caused by a blow can permit bacteria to be drawn into the can.

In summary, providing cans are properly processed and handled with reasonable care, LACF are essentially sterile, so all bacteria, including Listeria will have been killed; ACF are pasteurised and this process will also kill Listeria.

Tuesday, March 20, 2012

Giving bacteria a cold

I hope that title got your attention. I could equally have used "Fight fire with fire" or "Biological warfare".

Food Standards Australia New Zealand is calling for submissions on an application to use viruses to aid in the control of Listeria monocytogenes.  L. monocytogenes is a bacterium found in the environment that can infect food and may cause listeriosis.  The majority of the population is unaffected by Listeria, but it is a hazard for pregnant women, young children and the elderly, in addition to those who are immunocompromised.  Note that you don't have to have had an organ transplant to be immunocompromised - you might just have another infection stressing your immune system.

You might think that deliberately putting viruses into foods is crazy.  However, these viruses are special.  They exclusively target bacteria and are called bacteriophages.  Bacteriophages (or "phages") are extremely common; they occur at levels up to 9×108 per mL in bacterial mats at the surface of the sea.  They were first identified in 1915 by Frederick Twort in England and, independently, in 1917 by Felix d'Herelle in France.

Bacteriophages are deceptively simple - essentially a piece of nucleic acid contained within a protein coat called a capsid.  They can't reproduce independently and have to have a means of infecting a living cell.  Perhaps the most famous phage is the T4 phage that infects Escherichia coli and always reminds me of the lunar lander(I sometimes wonder how something like this could have evolved by chance).  See image from Wikipedia below.  

 When a phage infects a bacterial cell, it injects its DNA into the cell.  The viral nucleic acid then takes over the bacterial synthetic machinery and makes copies of itself, and synthesises new phage coat and other components.  The components are then assembled into new phage particles, whereupon the bacterial cell is lysed and releases the phage.  Burst sizes may be around 100 viruses per bacterial cell.  Since these are all infective, the infection of the population proceeds rapidly, resulting in the death of the majority of the bacterial cells.  Phages cause the cheese-making industry a lot of trouble, because they kill the starter bacteria.

The application currently under consideration is for the use of a mixed bacteriophage preparation, sold commercially as LISTEX™ P100, as a processing aid.  This preparation was the first phage product to be classified as Generally Recognized as Safe (GRAS) by the FDA  and USDA.

A number of scientific papers have been published on the efficacy of the P100 preparation, showing that the phage significantly reduces the population of L. monocytogenes on foods, such as salmon fillets or surface-ripened cheeses*.

The use of bacteriophage to control pathogens, such as Listeria and Salmonella, can reduce the risk of food poisoning, though it is unlikly that it can be a total solution, as the pathogen population may not be totally destroyed.  However, when used in an integrated food safety programme, the processing aid can be a valuable tool, reducing the reliance on chemicals to inhibit the bacteria.

*  See:  doi:10.1016/j.yrtph.2005.08.005

Electron micrograph of T4 phage adsorbed to a bacterial cell.
Image by Elizabeth Kutter, Bacteriophage Ecology Group

Odd Spot:  When I was researching this post, I searched on "Listeria virus".  I got many hits, predominantly in popular press and websites, where the Listeria bacteria were described as "viruses".  In modern microbiology terms, this is totally wrong.  However, "virus" is derived from the Latin word for poison and this apparently appeared in the English lexicon in 1392. "Virus" was also used in 1728 to describe an "agent that causes infectious diseases".  I'm sure that most of those press writers didn't know this, but, strictly, they were correct.

Friday, March 2, 2012

Toxic salt in Poland?

Anonymous sent me a comment after I posted "Is Vinegar just for Fish and Chips?".

In fact, the comment was not relevant to the original post, but I followed up on the quote Anonymous sent me.  It appears that an investigation has been broadcast by an independent Polish television station, TVN.  In the report, the reporter presented evidence of industrial salt - obtained as a waste by-product of calcium chloride production and claimed to contain dangerous carcinogens - being sold wholesale to the food industry as edible salt.

I have been unable to verify this report, because the commentary is, presumably, in Polish, but it has also been reported on CNN iReport, labelled as "Not vetted by CNN".

The programme claimed that up to 1000 tons per month of the waste product, labelled as being intended only for de-icing of roads or as a chemical industry raw material, has been purchased over the last 10 years.  The salt was made as a by-product by a fertiliser company in Poland.  Three Polish businesses have repackaged and on-sold the salt to numerous food processing plants as edible. 

The Polish government has made some arrests of those thought to be responsible, but has not published the identities of any food processors who received the salt.

The concern is that toxins and potential carcinogens have been incorporated into the Polish food supply and, by implication, to some exported foodstuffs.

Food fraud is by no means a new thing and knows no national bounds.  Where there's a fast buck to be made, you can bet that someone will try it.  Unfortunately, this is not a victimless crime - those who suffer are often the unwitting consumers or their children, as we saw in the melamine milk scandal.  Only the vigilance of regulatory authorities and investigative reporters can hope to reduce our exposure to food fraud.

Can I build a business around Mum's secret recipe?

I am often asked this question.

On the face of it, this is a simple question.  Mum has for years made a special dish or sauce and the whole family enjoys it.  Perhaps it is a traditional dish made back in "the old country" and an enterprising emigrant wants to make it commercially in the adopted country.  All that is necessary is to scale up production, right?

In some cases, this might be so.  However, there may be hidden pitfalls.

Perhaps the most important difference between Mum's cooking and a commercial operation is the timing - Mum cooked the dish or sauce and served it straight from the kitchen, whereas commercial manufacture involves packaging, storage, transport, retail display and purchase.  The shelf life must also leave time for the consumer to store it at home before consumption.

How about putting the sauce into a glass jar or a plastic pouch?  This introduces a new variable not present in the original.  This is exactly the scenario presented to me recently and I want answers to some additional questions before I'll agree that the product is safe.

For example: what is the pH of the product - acid or low acid?  This is not just about flavour.  If Mum poured a low acid sauce over your food and you ate it straight away, there was no problem.  But if we now wish to sell it in an hermetically sealed container, it may support the growth of Clostridium botulinumEven if the raw materials are heated during preparation, spores will have survived and can germinate and grow, producing botulin toxin during storage.  There may be no apparent change in the product, but it could be lethal.  In the case of a low acid product, a full 12D process must be applied and this process must be filed with the regulatory authority, must be followed for every batch and under the control of a registered person and full records kept.  Special equipment, capable of heating the product to well over 100C is needed to deliver a 12D process.

What about stability?  Will the sauce separate during storage and transport?  It may need a stabiliser to be added to prevent separation and thus ensure that it looks good to the consumer.  It's not a safety issue, but dissatisfied customers are unlikely to be repeat purchasers.

What shelf life should we put on the label?  Properly conducted storage trials are essential. For that matter, what are the requirements for labelling?  You can't just put a picture of the product onto the label and call it "Mum's special pasta sauce"; in most jurisdictions, there are very specific requirements, including name and address of the manufacturer and a list of ingredients, possibly with warnings about potential allergens like peanuts.  These requirements extend to specifying the size of type required for the nutritional information label.

Not least is the requirement for the product to be manufactured in a suitable premises.  These premises must be inspected and it is unlikely that the home kitchen can be approved for even minor commercial production.

In New Zealand, food manufacturers must have a suitable food safety programme in place.  New legislation will require this programme to be based on an assessment of risk posed by the product and process, and the programme must be documented and detailed records kept, so that premises and process can be audited.

There are many wonderful products on the market today that had their origins in Mum's kitchen in some part of the world.  To avoid tears, the budding entrepreneur should seek the advice of a professional food technologist before putting the product on the market.