As a food microbiologist I’m sometimes asked whether sterile foods exist, i.e. are there foods available which are guaranteed to be free of microbes. The answer is Yes, although we have to be careful about how we define sterility, so that we understand exactly what the concept of a sterile foodstuff means.
I’d like to begin, not by defining sterility, but by ruling out a whole raft of food that could possibly be sterile – if you processed them in an operating theatre! Many of the foods we eat, such as fruit, nuts, grains and animals are sterile on the inside, which happens to be the part we eat. Take the example of an orange: the skin is red-hot with bugs, with anywhere between 1,000 and 100,000 microbes per square centimetre lolling on the outside, but inside, unless the skin is broken or the orange has one of those systemic, penetrating fungal infections, it is sterile. There shouldn’t be a single bug hiding out among all that soft, juicy, sweet flesh. How would you manipulate the orange, however, in order to have sterile OJ sitting in front of you (in a sterile glass!) for breakfast? First of all, you’d have to surface sterilise the orange: a wash in detergent, 10 minutes in 5% bleach solution, a few washes in sterile water, and then two minutes in 70% ethanol should kill the vast majority of the bugs on it exterior. So, you have an orange, the surface of which is near-sterile – what next? You need sterile instruments, sterile surgical gloves, sterile containers, a sterile squeezer and a sterile environment in which to process the orange. That’s where your operating theatre (or a Class II biosafety hood) comes in. Not very practical, is it? Now, imagine trying to process cattle to achieve sterile meat. The hide is not sterile (and, in fact, is chock-a-block with faecal coliforms, Staphylococcus and the like from cattle not bathing regularly or using toilet paper), but the muscle inside, unless the animal is diseased, should be. But how would you go about extracting this sterile meat so that it remains sterile? Near impossible. So, let’s rule out these “naturally sterile” foods and accept the fact that if we want truly sterile food we have to apply a technological solution.
Now: back to sterility. The Oxford English Dictionary defines this as “Free from bacteria or other living microorganisms”, which is good enough for me. People often mix up sterility with asepsis: this is the condition of being free from disease-causing microorganisms, kinda like a working version of sterility for surgeons and the like. Us experimentalists also talk about asepsis – working in very clean environments where bugs from the environment don’t interfere with our experiments.
To achieve sterility one must kill all the microbes in or on a foodstuff. This is not such a difficult thing to achieve. In the lab, we use an autoclave every day to sterilise instruments, glassware and media, zonking stuff at a temperature of 121˚C for 15 min in the presence of steam and elevated pressure. But you can’t autoclave food – or, if you do, you convert everything into a tasteless, textureless, caramelised goo. And who wants to eat that?
Unfortunately, there are people who need to eat sterile food – severely immunocompromised patients and those unlucky enough to be born without functioning immunity, those famous boys and girls in the bubbles. And to heap misery upon misery, much of the time these patients have had no choice but to eat brown goop. Since the 1960s, though, another technology has been used to sterilise food – irradiation. This is the application of large doses of gamma irradiation to food. This radiation, which does not render the food it encounters radioactive, kills any microbes in the food, principally by rupturing their DNA and breaking their proteins. It has less of an effect on the taste, colour and texture of food than heat, but vitamins can be destroyed by it. Irradiation is also used to sterilise food for astronauts and troops, and, interestingly, is used to reduce the microbial load on one common item in our larders: spices. These are a high-value product for which it is economical to process using this expensive and tightly regulated technology.
Finally, there probably are sterile food products in all our larders – tinned goods – but the manufacturers cannot make claims about their sterility. Tinned goods receive a high heat treatment, not as extreme as autoclaving, but enough to kill the vast, vast majority of bugs the foods contain. Except with one exception: bacterial spores. This small number of spores, though, are highly unlikely to germinate and grow in the food. Therefore, such tinned goods are referred to as being “commercially sterile”. A good definition of commercial sterility is provided by Weddig et al (2007): application of heat, alone or in combination with other ingredients or treatments that render a product free of microorganisms capable of growing in the product at normal non-refrigerated conditions during distribution and storage”. The implication of this definition is that these commercially sterile foods do harbour the odd spore, which, if things go wrong in the supply chain (e.g. a shipping container might be stored on a sunny dock for a couple of days too many) could germinate, grow, reproduce and cause problems to the person unlucky enough to consume the foodstuff.
Weddig, L. M., Balestrini, C. G., & Shafer, B. D. (2007). Canned foods. Principles of thermal process control, acidification and container closure evaluation (7th ed.). Washington DC, USA: GMA, Science and Education Foundation.