Saturday, December 10, 2011

Is there such a thing as totally safe food?

The short answer is "No!".  Even if we eat only sterilised food, such as canned meat, we are reliant on the proper delivery of the sterilisation process and prevention of post-process contamination.  Low acid canned foods are essentially sterile - the food is hermetically sealed in a can, which is then processed under pressure to destroy bacterial spores, specifically, the process is designed to reduce the chance of survival of a Clostridium botulinum spore to less than 1 in 10^12.  On that basis, you have a better chance of winning the PowerBall lottery than getting botulism.

But nobody wants to live exclusively on canned food.  Are there any other ways that we can make food safe, or at least reduce the risks of food poisoning to acceptable levels?

Yes!  Food microbiologists and food processors study the bacteria that can cause food poisoning - pH range for growth, water activity range for growth, requirement for oxygen, growth temperature range, response to preservatives etc. - and they also look at the intrinsic parameters of the food - pH, water activity - and then consider the storage conditions, such as relative humidity, temperature and atmosphere in the pack etc.

Taking all these factors into account, foods can be designed to be safe under proper conditions of handling and storage.  A significant part of the training of  food technologists centres around study of these conditions and manipulation of the conditions in the food to achieve the manufacture of safe foods.

As an example, look at a tank of raw milk on the farm.  Immediately after it is drawn from the cow, the milk is chilled.  This reduces the growth rate of bacteria, which come either from the cow or the equipment.  When the milk arrives at the processing factory, it is pasteurised - the temperature is raised to 72C and held for 15 seconds.  This kills all the vegetative pathogens, that is, the bacteria that are not in the spore form.  The milk is now safe, but will not keep indefinitely, because heat resistant spoilage bacteria are still present.  We can make the milk keep for a much longer time by ultra high temperature processing (UHT).  In this process, the milk is heated to around 140C and held for a few seconds.  This sterilises the milk, so there are no viable bacteria to cause food poisoning or spoilage.

The milk can be dried to powder.  This lowers its water activity below the threshold for bacterial or mould growth.  Alternatively, the milk can be fermented to yoghurt or cheese by addition of starter cultures.  The starters convert lactose to lactic acid and reduce the pH of the milk until it curdles and sets.  pH is a measure of the acidity of the food; the scale runs from 0 to 14.  Numbers below 7 are acid, those above are alkaline.  As the pH falls, fewer and fewer bacteria are able to grow.  Below pH 4.0 no pathogens are able to grow.  Typical pH of yoghurt is 3.65 to 4.40

Other foods may be made safer by the application of hurdle technology, where several preservation mechanisms are brought together to prevent growth of pathogens in the food.  There is a number of new technologies appearing, such as high pressure processing, electron beam irradiation and ohmic heating.  None of these processes can guarantee safe food, but each has its advantages.  I'll look at some of these techniques in future postings.

Safety management systems, like HACCP can be used to control food processing operations to ensure that safe food is produced.  However, some foods are inherently less safe, such as raw vegetable sprouts, raw shellfish, or very unusual foods that require highly skilled preparation, such as fugu.
Consumers must take some responsibility in their choice of food and eating habits.  If you are doing the traditional turkey for Christmas dinner, ensure that it is properly cooked - use a meat thermometer - and consider cooking the stuffing separately.

If you are going to barbecue this Christmas, don't cook chicken legs on the BBQ without first microwaving them - they are an irregular shape and ensuring that they are properly cooked is difficult.  Don't use the same plate or utensils for cooked meat as for raw.   Don't keep foods beyond their use-by date and ensure that all raw foods are refrigerated during storage. 

Have a wonderful holiday and keep safe this Christmas.

Friday, December 9, 2011

It's the pits - bacterial hideaways

Modern food processing is often carried out in stainless steel equipment - tanks, pipes, valves and conveyors are commonly made of various grades of stainless steel.  We tend to think of "stainless" as not suffering from corrosion.  To a large extent, this is true.  Stainless steel has a natural oxide coating that prevents water molecules from oxidising the iron.

However, stainless steel can still corrode where grain boundaries or embedded contaminants allow water to access the iron.  The contaminants might be grinding swarf from welding or repairs.  Stainless steels may therefore benefit from a process called passivation, in which the surface is cleaned with sodium hydroxide and then treated with nitric acid.  This restores the oxide film.

We use stainless steel in our laboratory experiments and routinely passivate with hot nitric acid.  One of my students used a bottle labelled "Concentrated nitric acid" from the chemistry laboratory to passivate some new samples.  Unfortunately, it appears that the contents were actually Aqua Regia, a mixture of nitric and hydrochloric acids.  (How often have I said that correct labelling is critical in food safety?).  

Chlorine ions are extremely electronegative and react strongly with certain compounds.  They can severely damage stainless steel.

The first photograph shows two coupons treated with the acid mixture.  It is obvious, even to the unaided eye, that the surface is pitted.  Chloride pitting tends to occur at right angles to the surface, so deep pits form rapidly.  Obviously, the use of aqua regia is a very extreme case of chloride attack, but even food materials containing sodium chloride will eventually attack stainless steel.  Even 316 stainless steel, which contains molybdenum that helps to stabilise the passive film, will corrode if exposed to high levels of chloride ion, or if the oxygen level is very low.  This is what may happen under a biofilm, where the bacteria use up the oxygen.  The area then becomes anodic and current flows, resulting in corrosion and the formation of a pit.

I took a couple of coupons to Dr. Jen Wilkinson who runs our Scanning Electron Microscope.  She took the following images, which show clearly the damage to the surface and the deep pits caused by the corrosion.  The second image below shows the interior of the pit.  Bacteria could easily enter the pit and would be very difficult to remove during cleaning.  If the bacteria form a biofilm, they will be protected by the extracellular polymeric substances (EPS) which glue them to the surface and may inactivate disinfectants.  The bacteria will be impossible to remove.

Wednesday, December 7, 2011

More on antibiotic use in animal rearing

I wrote last month, suggesting that it is high time we banned the sub-therapeutic use of antibiotics currently used in human therapy for animal rearing. 

When I wrote the article, I was not aware that the Food and Drug Administration had, around 9th November 2011, rejected two petitions to ban antibiotics from being used in food animal production. The petitions were filed by a coalition that included  the American Public Health Association and the Union of Concerned Scientists.

It is troubling that in its denial letter, the FDA acknowledged that its “experience with contested, formal withdrawal proceedings is that the process can consume extensive periods of time and agency resources.”  I interpret that to mean that FDA may well think that antibiotic use should be banned, but it can't afford to force the issue.

Meanwhile, researchers at McGill University have shown that bacteria resistant to tetracycline and tylosin can be isolated from pigs raised in a swine complex 2.5 years after administration of these antibiotics ceased.  See Microbial Ecology  DOI 10.1007/s00248-011-9954-0  published on-line 14th October 2011.  Antibiotic resistance genes were found in the bacteria, though the workers were not able to explain their persistence long after antibiotic use ceased.  The results are of significance for both animal and public health because these antibiotic resistant bacteria can be transferred between animals, humans and the environment.

Friday, December 2, 2011

Don't always blame the chicken - it spoils my sleep

"The human understanding when it has once adopted an opinion ... draws all things else to support and agree with it. And though there be a greater number and weight of instances to be found on the other side, yet these it either neglects or despises, or else by some distinction sets aside or rejects".   Francis Bacon, 1620.

A few years ago, I was flying from Hong Kong to Auckland.  I sat with two attractive young women and enjoyed a glass of champagne with them before dinner.  After the meal and film, the girls went off to spend time with other members of their party and I settled down for a sleep.

Some time during the flight, the cabin attendant shook me awake: "Dr. Brooks.  Are you a real doctor?".  Suppressing the urge to take him by the throat and educate him about academic qualifications, I asked what was the problem.  It seemed that one of the girls and two others of their party had been taken seriously ill, and the attendant wanted me to open the airliner's medical pack to use the rehydration fluids for the victims.  I explained that I was not qualified to do this and headed off to the galley to chat to the stewardess and persuade her I was really in need of a beer and some sandwiches.

Some time later, the girl who was not ill told me that her friends were vomitting and had diarrhoea.  "It was the chicken pizza we had in the lounge before we boarded".  The timing and symptoms didn't seem right to me and I asked a few questions, including what they had been doing in China.  "Oh, we've been at a furniture expo for the last week".  It didn't seem to occur to her that she had been snacking on canapes and eating buffet-style meals for the previous seven days; she immediately blamed the chicken.  When we landed, her friends were taken by ambulance to the hospital, suffering from severe dehydration. I never heard from them again, but my money is on the meals during the week, not the chicken.

Now whenever I fly international, I ensure that my ticket is booked in the name "Professor Brooks".  I'm more likely to get a decent sleep during the flight.

Your all-time favourites

I have just reviewed the most common search terms resulting in visits to this blog.

By far the most common has been "Coliforms in food".  This has been a regular search since I started writing Safe Food in 2006, but has perhaps appeared more frequently since the Escherichia coli O104:H4 outbreak in Europe.

Coliforms are used as "indicator organisms" to show whether the food has been processed under hygienic conditions.  Presence of these bacteria mean that the food has potentially been contaminated with faecal material and hence faecal organisms, but to confirm this, we look for Escherichia coli presence in the food.

Surprisingly, the next most common search has been "Safety of Probiotics".  Probiotic bacteria are those that confer some benefit on the host.  Thus many foods, such as yoghurt, contain live bacteria that may colonise the gut of the consumer and confer some benefit.  These bacteria are often lactic acid bacteria.  Many claims have been made for the benefits of consumption, though there are some indications that administration to critically ill patients with acute pancreatitis may have deleterious effects.  There is also some evidence that administration to infants under 6 months of age may render them more prone to development of sensitivity to allergens.  For the majority of consumers, however, probiotics will not be hazardous and may be beneficial.

Do not confuse probiotics with "prebiotics".   These were so named by Marcel Roberfroid in 1995, who later wrote the definition: "A prebiotic is a selectively fermented ingredient that allows specific changes, both in the composition and/or activity in the gastrointestinal microflora that confers benefits upon host well-being and health."  A major class of prebiotic is soluble fibre.

Much lower down the list are "Listeria in vegetables" and "Safety of chicken".  There has been a small spike in Listeria searches, probably as a result of recent recalls and the cantaloupe outbreak in USA that has killed approximately 29 people.  The incidence of listeriosis is very low and victims are usually neonates, the elderly and those people with compromised immune systems, but this outbreak seems to have been particularly dangerous.  Chicken has made regular appearances in the news, either being a vehicle for Salmonella or Campylobacter.  The incidence of Campylobacter infections in New Zealand has dropped significantly since the introduction of more stringent controls and biosecurity systems.

One new search now appearing is a bit disturbing: "Will eating spoiled food make you sick?".  I have previously written on this after receiving a direct question from a reader.  Is the increased interest a result of hardship in the community, or merely a thirst for knowledge?  I hope it is the latter.