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.

Tasty but occasionally dangerous

If I asked you to name a food group that might be implicated in food poisoning, you would probably respond with "meat".  I did a survey (n=1) at home today and got that answer.  My wife thought a bit longer and said "Custard, but that's only because you have influenced me over the years".

Baked goods probably don't spring to mind as potential carriers of food poisoning bacteria.

In the last week, there have been reports from Rhode Island of zeppole, or St. Joseph's Day cakes, causing food poisoning by Salmonella.  I had never heard of zeppole before this report, but they are apparently popular in Italy, Sicily and Malta and in the Italian-American communities in the United States and date from the early 19th century.  They are deep-fried dough balls, or sometimes choux pastry, topped with powdered sugar or filled with custard, pastry cream or a mixture of butter and honey.  They sound delicious, though perhaps dangerous for the figure!

At the time of writing, there have been 76 cases and 29 hospitalisations.  Two people have died from Salmonella-associated illness.  It is too soon to say how this happened, but the possibilities include a carrier of Salmonella working as a food handler, or the use of ingredients contaminated with Salmonella.  The deep-fried pastry would probably be sterile immediately after frying, but could be contaminated by the food handler during the filling process.

Breads, fruit cakes and biscuits are usually pretty safe by virtue of low water activity, but filled pastries receive quite a lot of handling, while real cream may be contaminated with bacteria that rapidly grow under abuse temperatures to the point where they can cause food poisoning, either by infection or intoxication.

Sodium in food

Most people like to have salt on their food. In mediaeval England, salt was expensive and only the nobility could afford it, as it was made by evaporating salt water over a fire. The salt was placed in the middle of the high table; the commoners sat at lower trestle tables and did not have access to the salt. Thus they were "below the salt" and this came to be an indication of rank.

Around 1650, rock salt was mined in Cheshire and salt became more readily available. The connotation of the value of salt remains, however, in expressions like "He's worthy of his salt".

These days, we probably have too much salt in our diets. In New Zealand, for instance, we consume around 150% of the recommended upper intake level. Much of this intake is involuntary - manufacturers add it to foods including bread, sausages and pies. The recent television series "Master Chef" had the judges saying repeatedly "Don't forget the seasonings", meaning not just herbs and spices, but also salt.

So, should we just ban salt in food and let individuals add salt to taste?

The answer may surprise some readers. Salt (sodium chloride) contributes to the safety of food and is essential for developing texture and flavour in processed meats. It helps to bind proteins, improving texture; it increases water binding capacity of proteins, also contributing to texture and assists in stabilising meat batters by improving fat binding. It also decreases fluid loss in vacuum-packed, thermally processed products.

Not only that - salt improves safety and shelf life by inhibiting the growth of bacteria, though relatively high levels are required if salt is used alone. It helps to reduce the water activity^* of the food, making it more difficult for bacteria to grow. That's why salted beef and pork were carried on long sea voyages - the meat was preserved.

Stringer and Pin (Institute of Food Research, Norwich, UK) have noted that "There is scope to reduce salt in foods. However, as salt influences bacterial growth, survival and recovery after adverse treatments, reducing salt in foods will have consequences for food safety that must be considered". These researchers used predictive models to show that reducing sodium content from 1.5g/100g to 0.76g/100g food allowed a much greater growth rate of certain foodborne pathogens. This could be acceptable, but other preservative mechanisms would need to be put in place. For example, other preservatives might be added at low levels and refrigeration might be necessary. Above all, reducing salt content would require even stricter adherence to good manufacturing practices, particularly with respect to plant and operator hygiene.

I'll write more on sodium in food in a follow-up posting.

* See the end of "Free Choice or Safety of the Population" in this blog for an explanation of water activity.

It’s not quite about food safety…

Recently I was asked by a reporter to test magazines found in doctors’ waiting rooms for presence of pathogens. She wanted to do a story about the risks of going to the GP. I wasn’t sure what to expect, but asked her to collect a range of magazines from various waiting rooms. She brought me ten glossy or super glossy magazines covering popular women’s and men’s titles.

Each magazine was opened randomly and two pieces of the right hand page were cut aseptically, using sterile scissors. Each piece was 100mm x 100mm; one piece was cut from the top right, the other from the bottom right. This was to increase the chance of sampling the area most handled by readers.

The pieces of paper were transferred to a sterile bag with sterile diluent and massaged thoroughly by hand and then for two minutes in a machine called a Stomacher. I transferred samples onto agar plates that would enable me to assess general microbial contamination, faecal bacteria and Staphylococcus.

After incubation, I examined the plates, looking for typical bacterial colonies. The levels of contamination on all media were low. The limit of detection in this analysis was 3.7 colony forming units/10 cm². In the food industry, a count of less than about 100 cfu/cm² is considered acceptable, though the count on food contact surfaces at the start of food processing should be close to zero. Thus, the analysis I used was sufficiently sensitive to indicate whether the magazines were microbiologically hazardous with respect to Staphylococcus and faecal bacteria.

Staphylococcus is typically found on the skin and can be transferred to food and there produce enterotoxin, which causes food poisoning when we eat the contaminated food. It is unlikely that the low levels of S. aureus detected in this work would lead to significant transfer between readers. No typical colonies of faecal coliforms were detected, suggesting that there were no, or very low numbers of faecal bacteria on the magazines and hence that the likelihood of faecal contamination of the pages was also low.

All of the samples were printed on glossy paper, which doesn’t absorb water, a fact I confirmed by measuring the water activity* of the samples. There was thus no opportunity for bacteria to grow on the pages and in fact, they may have died off reasonably quickly.

One thing I should point out is that it is not possible by any simple analysis to detect viruses, so the work done here doesn’t completely rule out contamination by viruses of upper respiratory tract or gastrointestinal tract origin. Some, such as the Norwalk virus that causes gastroenteritis, can survive on surfaces for long periods.

I conclude from this cursory examination that, with the possible exception of viruses, the pages of the magazines were relatively free of pathogens. Handling the magazines is therefore relatively hazard free. However, as always, it would be advisable to wash or sanitise hands before consuming foods, snacks or sweets after visiting the surgery.

Have a happy and safe Christmas, everyone. Make sure your Christmas fare and BBQs are safe.

* For a description of water activity see:

http://foodsafetywithjaybee.blogspot.com/2007/06/free-choice-or-safety-of-population.html

Free Choice or Safety of the Population?

Governments and regulators are always in a difficult position. Do they allow total freedom of choice, or does their responsibility to the population require them to make restrictive decisions based on the greater good of the people? The government then performs a balancing act, running the risk of being labelled as Big Brother (or at least Nanny Government) or as being irresponsible. For example, should we all be allowed to use any chemical we can obtain to relieve the monotony of our daily lives, or should there be regulations to control the use of mind-altering substances and thus protect us from ourselves? If we allow unfettered use of any substance and thus maintain personal freedom, how do we protect the innocent - those who are too young or too poorly educated to know that these materials may be harmful?

Most people would probably come down on the side of some restrictions for the use of drugs, though some would argue for freedom of choice. The same applies to the sale and consumption of raw milk and raw milk products.

I have discussed raw milk before in this blog. I might have left it at that, but the New Zealand Government, in the form of the New Zealand Food Safety Authority has now called for submissions on a proposal to permit direct imports of Roquefort, a soft raw milk cheese made in France, and extra hard Parmesan-style raw milk cheeses Grana Padano, Pamigiano Reggiano, Romano, Asiago and Montasio. An extensive programme of risk assessment has already been undertaken and the Authority is now consulting with industry groups.

This development may lead some members of the community to believe that Big Brother has been wrong all along and is now backing down. Extra hard cheeses have a low water activity* of about 0.693 that prevents the growth of most bacteria. Soft cheeses, such as Brie, Camembert and Roquefort have much higher water activities and thus may permit the growth of pathogens such as Listeria monocytogenes (see post To Pasteurize or not? - 10th December 2006). The truth is that cheeses made from raw milk are not as safe as those made with pasteurized milk.

European Community (EC) legislation sets microbiological, food safety and process hygiene criteria that reduce the risks in consumption of raw milk cheeses. NZFSA has recommended that if these products are to be directly imported and ultimately manufactured in New Zealand, additional risk mitigation measures should be introduced:

• 100% verification of certificates attesting that the relevant EC standards have been met

• continuing monitoring of products to check for E. coli levels, which can indicate unsafe manufacturing conditions leading to faecal contamination of the cheese.

It is expected that leaflets, posters and point of sale brochures will be required to educate the consumers on the risks of eating cheeses made from unpasteurized milk and that labelling of the products will be mandatory. Yet another acronym has been added to our lexicon: YOPI - Young, Old, Pregnant or Immunocompromised. This group of the population should not consume raw milk products.

*Water activity is a measure of the ability of water to take part in biological and chemical reactions. It is measured as a ratio of the partial vapour pressure of the food to that of pure water. That sounds complicated, (it's not) but it results in a scale of water activity running from 0 to 1. Generally speaking, the lower the water activity, the harder it is for micro-organisms to grow. Water activity in foods can be controlled by salts and sugars. So we can formulate food to have a particular water activity and thus preserve the food.