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There is an old Indian parable of a king who wanted to reward the wise man who had invented chess.

The wise old man asked for rice but had an unusual stipulation. He requested the amount of rice equal to a single grain of rice placed on the first square of a chessboard, two on the second, four on the third and so on, the amount of rice doubling until all 64 squares were filled with rice.

The King, who had clearly never dealt with a bank, readily agreed. Though it didn’t take long for him to realise he’d been duped.

By square 16 the total amount of rice on the board was over 65,000 grains of rice. By square 32 over 4 billion grains of rice were needed and by square 64 the King would have had to supply a whopping 18 quintillion grains, more rice than existed in all the world.

What happened next? Was it wise for the wise man to fool a king this way? Different versions of the story have the wise man executed or made a trusted adviser, so I guess we’ll never know.

What we do have though is a good parable of exponential growth. We usually use it to illustrate the power of compound interest, but, for a food scientist, it is rather more reminiscent of bacterial growth.

Especially bacterial growth in food.

Bacteria like our food as much as we do and when they get onto some food their numbers can increase like the rice on the chessboard. For cooks this means that feeding anyone always carries the risk of causing harm, or even death, because of bacterial infection.

Having some knowledge of the basics of bacterial reproduction in food is a good thing. It can help you keep your family and friends safe. It could also bring harm to your enemies I suppose, but I’ve been watching too many murder mysteries lately.

Bacteria Reproduce by Binary Fission

The secret to bacteria’s exponential growth is in the way that they reproduce. Although they are capable of a limited type of sexual reproduction by far the major mode of bacterial reproduction is an asexual process called binary fission.

In binary fission a bacterial cell replicates its DNA, kept in a circular ‘chromosome’ called a nucleoid, and then splits into two distinct daughter cells.

This process can be completed pretty quickly. Some bacteria, like Escherichia coli, can complete a round of binary fission in about 10-20 minutes.

This means a population can potentially double in size every ten minutes.

Bacterial Populations have Defined Stages of Growth and Decline

If a bacterium, or some bacteria, have gotten themselves onto a suitable substrate, like your lunch, they will enter an initial stage called the lag phase.

You can think of this as a nesting stage where the bacteria are getting used to their new home. They produce macro-molecules needed for growth, repair any damage and get used to the new conditions (temperature, pH, oxygen availability etc).

The length of the lag phase can vary depending on the conditions but eventually the bacteria will move into the log (or exponential) phase. This is where all the action happens.

It’s in the log phase that the bacteria are like the grains of rice on the chessboard. The total number of bacterium doubles with every generation produced by binary fission.

To visualise this, I’ve plotted a growth curve (below) starting with a single bacterium that is undergoing binary fission every 10 minutes. If you wait seven hours (420 minutes) our single bacterium will have around 4 trillion descendants.

Note that this is the exact same geometrical progression as the parable of the chessboard. If you want to express things in chess squares rather than time we are on square 43.

Exponential Growth Can’t Last Forever

Of course, exponential growth cannot last for ever. Eventually a point is reached where the available resources cannot support further population growth.

Things like space, nutrients, water or oxygen can put a limit on further population growth and a buildup of toxic by-products of bacterial metabolism can do the same thing.

When this point is reached the population will enter a stationary phase where the ‘birth’ rate is greatly curtailed and the production of new bacteria becomes roughly equal to the death rate.

The bacterial population will continue like this until resources are depleted and the population enters the death phase. Resources decline, waste products increase and the bacteria begin to die off.

Bacterial Growth and Our Food

Because of exponential bacterial growth, as soon as animal products are produced the clock is ticking.

Animals, including the humans that are slaughtering the other animals, are covered with bacteria. Though modern facilities should be pretty hygienic, it is a pretty safe assumption that there will be some bacterial contamination on the meat after an animal is slaughtered.

The objective is to stop any bacteria that are present from entering log phase growth. Traditionally this meant that meat needed to be immediately eaten, or further bacterial growth prevented by smoking, fermentation, salting or drying.

In the modern world, although we still preserve meats for the flavour, to preserve fresh meat before consumption we use refrigeration.

Bacterial Growth is Temperature Dependent

Refrigeration works because different species of bacteria are comfortable, and grow, at different temperatures.

The ones we are mostly concerned with in food production are called mesophiles and these bacteria reproduce best between 20-45$\degree$C, generally dying at temperatures above 65$\degree$C.

All the bacteria of our microbiome are mesophiles, as are most of the bacteria responsible for food spoilage, food poisoning and disease.

If food is kept at a temperature below the lower limit of the mesophile temperature range any bacteria present will not go into log phase growth or they will do so very, very slowly.

The modern approach is to ensure our logistics networks never allow our animal products to stray into the optimal temperature range for mesophile bacteria before they are cooked and eaten.

That is, we keep them below 4$\degree$C.

Cooking, Our Major Defense Against Food Poisoning

While keeping a mesophile below 4$\degree$C stops them from reproducing, subjecting them to temperatures above 65$\degree$C actually kills them and this is where cooking comes in.

Cooking can kill any bacteria that may have contaminated your meat as it made its way to your kitchen.

To help quantify the temperature sensitivity of bacteria, scientists use a decimal reduction time, usually denoted as ‘D’. The D value is the amount of time is needed to reduce bacterial numbers by 90% at a specific temperature.

If we have 1000 individual bacterium and the D₆₀ is 2 minutes, this means that after 2 minutes at 60$\degree$C only 100 bacteria will survive. If it’s kept at 60$\degree$C for four minutes then only ten bacteria will survive and so on.

Each bacterial species, and even strain, has its own D value. The table below sets out some experimentally derived D values for E. coli, Salmonella, and Listeria monocytogenes, bacteria commonly involved in food poisoning (I got the values from this paper).

Using these values, we can see that if we want to avoid poisoning our dinner guests, we will need to hold our food at 55$\degree$C for 27 minutes to kill 90% of any E. coli present, 67 minutes for any Salmonella and 81 minutes for L. monocytogenes.

On the other hand, the D₇₀ values for the same bacteria are 2.4, 13 and 18 seconds, meaning, for example, any E. coli will be reduced by 90% after holding the food for 2.4 seconds at 70$\degree$C.

A 90% reduction in bacterial load is pretty good, but it depends how many bacteria were originally present. Ten percent could still be enough bacteria to cause sickness if you had a lot of them to start with.

This is why most health authorities suggest cooking to relatively high temperatures, mostly in the low 70$\degree$C range for most types of meat.

At these temperatures you can burn through several log-fold reductions of bacteria in a few seconds.

You’ll Take My Rare Steak from My Cold Food-Poisoned Hands

Anyone who likes rare steak, myself included, is now saying that there is no way I’m cooking my steak to an internal temperature of more than 70$\degree$C. I’d rather die of food poisoning.

But we don’t have to worry. The inside of a beef steak is generally considered sterile. Bacterial contamination of beef steak occurs on the surface, and the bacteria is unable to penetrate into the muscle.

This means that if you give your steak a good seer at very high temperatures, which you want to do anyway so yet get a nice char from Maillard reactions, then you are usually good.

The official recommendation is still 63$\degree$C but I’m happy to take the risk that a rare steak presents. Pregnant woman, those with chronic illness and children should probably stick with that guideline though.

Ground Beef is a Different Risk Altogether

When it comes to ground beef though the scenario changes. Mincing meat drives the surface of the meat into contact with the internal parts of the meat and mixes them all up together.

When this happens all of the meat is now potentially contaminated, inside and out.

This has some ramifications for hamburgers.

A lot of people like a medium rare burger but I’m very wary. Medium-rare is about 54-57$\degree$C and we can see that even for the most thermally sensitive bacteria, E. coli, you’d need to hold your burger at 55$\degree$C for 27 minutes to get a 90% reduction in bacterial numbers.

I’m sure this rarely happens when burgers are being cooked to medium rare.

If you aren’t sure of the provenance of your minced beef, then this starts getting a little risky for my liking. If it’s supermarket mince, forget about it; I’m going to 70$\degree$C.

Chicken is Very Risky Unless Well Cooked

Pork, since we got Trichinella spiralis under control, is pretty much the same story as beef.

Chicken is another matter all together. Salmonella and L. monocytogenes are much greater risks in poultry then they are in beef. And there is some evidence that Salmonella can penetrate chicken flesh and contaminate poultry meat internally as well as on the surface.

So not only do we have more Salmonella and Listeria in poultry meat but searing the surface of the meat, like we do with beef steaks, will not save you if there is internal contamination.

This is why recommendations for poultry are generally around the 74$\degree$C internal temperature range.

This temperature is not so bad for thigh and wings, that have much more connective tissue, but it is also the reason that we often experience dry and tough chicken breast. It is difficult to cook lean chicken breast to the recommended temperature without drying it out.

Leftovers

Once food is cooked we can’t relax because bacteria are fond of cooked food as well.

Bacteria are everywhere so cooked food can quickly become contaminated. Once the temperature of the cooked food enters the comfort zone for mesophile bacteria, we are back to the dynamics of bacterial growth.

What this means is you don’t want to leave food out for more than two hours. Any longer than this and any bacteria present will have passed through the lag stage and be well into exponential growth.

Also, the growth rate for mesophiles is highest around 32$\degree$C, so if it is a hot day reduce that time to an hour.

Bacteria Don’t Have to Still Be There to Cause Harm

If you have some leftovers that are potentially contaminated, you can always reheat it to bacteria killing levels. This is good practice, but in some food stuffs bacterial growth causes a build-up of toxins that can still cause illness, even after re-heating.

The classic example of this is leftover rice. The bacteria Bacillus cereus grows on rice left out for more than a couple of hours and produces the toxins that cause food poisoning.

These toxins include cereulide, that causes vomiting, as well as hemolysin BL, non-hemolytic enterotoxin and cytotoxin K all of which cause diarrhea.

In this case reheating the rice wont help because it may kill bacteria but it wont destroy the toxins.

Conclusion

One of the main drawbacks of having worked with bacteria and other nasties in a scientific setting is that you tend to spend the rest of your life thinking about how many bacteria there may be on your food or the surfaces you touch.

That is, it gives you, at best, a mild case of germaphobia, at worst, a full-on latex glove wearing pathology.

Nonetheless, it is important stuff to know if you are in the habit of feeding anyone, including yourself. About 48 million people get food poisoning every year in the USA, and 3,000 of these people die.

Here, I’ve only scratched the surface. Apart from a little bit about rice, I’ve only talked about meat but meat products only account for 29% of the deaths attributed to food poisoning. It is plant products that are responsible for 46% of all food related illnesses.

Apart from meat and vegetables, things like dairy, eggs, egg products and, of course, seafood all bring their own risks with them when we start eating them. Not to mention food-borne parasites.

All this means that I will be doing more posts on food safety, but in the meantime a good understanding of bacterial growth can go a long way towards keeping your dinner guests safe and your reputation untarnished.