Are fresh sprouts among the foods most at risk of bacterial contamination?

We can find them in salads, garnishing soups and other ethnic cuisine recipes: we are talking about fresh sprouts, foods often prized for their nutritional value, but which have also been at the centre of food scares linked to bacterial contamination. In Europe, in 2011, a serious outbreak of Escherichia coli was traced back to sprouts, with dramatic consequences for public health. That episode prompted European health authorities to review safety measures for these foods. So let's see what the scientific evidence says about this risk.

Sprouts are obtained from the germination of seeds immersed in water, in a warm and humid environment. These conditions are ideal for plant growth, but equally so for bacterial growth. Those of greatest concern to experts are Salmonella, some dangerous varieties of Escherichia coli and Listeria (which we have already discussed in 'Are many foods contaminated with listeria?')

The main problem is that contamination is often already present in the seed before germination even begins. A study conducted by the US Food and Drug Administration (FDA) on more than 800 samples found Salmonella in 2.35% of the seeds analysed. What makes this risk insidious is the insignificant amount of bacteria sufficient to trigger an outbreak: the European Food Safety Authority (EFSA) has estimated that as little as 4 bacteria per kilogram of seeds are enough to cause an epidemic outbreak. Once germination starts, these bacteria multiply explosively over the following days, reaching potentially very dangerous levels without the product showing any visible signs of alteration. In other words, a contaminated sprout may appear perfectly healthy to the touch and to the eye, a problem that affects many other contaminated foods, as we explain in 'Can I trust my nose to tell if a food is spoiled?

For this very reason, the FAO and the WHO have classified sprouts as a high-priority plant food from the point of view of microbiological risk. According to a scientific review published in the International Journal of Food Microbiology in 2021, there have been at least 64 outbreaks of food-borne illnesses worldwide that can be traced back to sprouts since 1988. The bacteria most frequently involved were Salmonella and Escherichia coli.

Many people grow sprouts at home thinking they will get a fresher, more controlled product. Unfortunately, the risk does not depend on where one sprouts seeds, but on the conditions under which germination takes place. The same temperatures and humidity needed to sprout seeds are favourable for bacterial growth in any environment. EFSA emphasised in a 2011 scientific opinion that even those who grow sprouts for personal use need to be aware of the microbiological risk. It also recognised that, despite advances in research, no chemical, physical or biological decontamination method is currently able to completely eliminate pathogenic bacteria from all types of seeds without compromising their ability to germinate.