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Whenever I speak with my husband’s 80-something Auntie Anne, I know that the subject will eventually turn to recipes and food safety. From grav lax to ganache, I’m her “go-to” gal. The last time we spoke, the topic was sprouts.

“I understand,” Anne said, “that sprouts in the supermarket are risky, and I don’t buy them anymore. But why can’t I buy the seeds at a health food store and sprout them myself?”

I explained that the seeds she finds in the health food store might be contaminated with Salmonella or E. coli.

“Then why,” she asked, ” can people eat the seeds without becoming sick?”

Putting aside the question as to how many people actually eat raw alfalfa seeds, arugula seeds, or mung beans, Anne’s question was a reasonable one. And the answer is in the arithmetic.

Electron microscope image of E. coli O104:H4 outbreak strain, courtesy of Robert Koch Institute

When a pathogenic microbe such as Salmonella, E. coli O157:H7 or E. coli O104:H4 is found in a batch of seeds, the level of contamination is typically very low. Researchers at the California Department of Health Services reported in 2001 (Journal of Food Protection, Vol. 64, No. 8, 2001, Pages 1240–1243) that they found Salmonella in contaminated batches of alfalfa seeds at levels as low as 0.07 per 100g – just a single live Salmonella in 3.15 pounds (1.4Kg) of seeds – and as high as 1.8 per 100g (or about one live Salmonella in 2 ounces of seeds).

If the level of pathogens in/on contaminated seeds is so low, why are sprouts such a problem? It’s all in the way the sprouts are produced.

  • Typical growing conditions for sprouts:- Moist, warm temperatures (typically, 70-80ºF)
  • Typical growing conditions for Salmonella and E. coli:- Moist, warm temperatures (optimally, 95-100ºF, but grow well in the 70-80ºF range)

And that’s the problem. The conditions required for sprouting seeds also encourage abundant growth of bacteria, including Salmonella and E. coli. A single Salmonella or E. coli cell can produce more than one million offspring during the first 10 hours of sprouting.

This risk is not just theoretical. Some of the German outbreak victims ate raw sprouts that they produced at home.

Another question I have been asked is how long pathogens such as Salmonella and E. coli can survive on or in seeds used for sprouting. The answer, in a word, is “years.”

The same study that documented the number of Salmonella present in the alfalfa seeds also mentioned that Salmonella had remained viable in the seeds after two years of storage at room temperature in the dark.

E. coli O157:H7 is just as rugged. In 1995, two Japanese sprouting facilities purchased radish seeds from the same US distributor. Between May and December 1996, more than 9,400 people in Japan became infected with a single outbreak strain of E. coli O157:H7, and 12 people died. Most of the outbreak victims were school children. The source of the infection was traced to radish sprouts produced in one of the two Japanese sprouting facilities.

Eight months later, in 1997, Japanese health authorities investigated another spurt of E. coli O157:H7 illnesses. The second series of illnesses was traced to radish sprouts produced in the second Japanese sprouting facility. The outbreak strain was identical to the 1996 outbreak strain. Clearly, this strain of E. coli O157:H7 survived on or in the US-produced radish seeds for more than a year.

It’s too soon for anyone to have determined the long-term survival of E. coli O104:H4; however, I would be astonished if it was very different from the others.

To summarize:

  1. The ideal conditions for sprouting seeds and for growing bacteria overlap.
  2. Bacteria will multiply into the millions per gram during the sprouting process.
  3. Salmonella and E. coli can survive for 1-2 years – or longer – on or inside the dry seeds.

Finally, my advice to Auntie Anne – and to anyone else who is interested: Do not eat raw sprouts.

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