STEC in Flour: What You Need To Know in 2025
Shiga-toxin producing E. coli, an underestimated hazard
Last week I wrote about Shiga toxin-producing Escherichia coli (STEC) in the context of a European Commission report about increased infections in Europe. I explained its prevalence, discussed case statistics and summarised the sources of STEC infections for the period of the report (2023). The outbreak foods in 2023 included raw (unpasteurised) milk cheese, ready-to-eat salad/ iceberg lettuce, buttermilk, and bovine meat.
What I didn’t talk about was STEC in flour.
Lucky for me, a fabulous reader wrote to tell me it should have been part of the story. The presence of STEC in flour has been top of mind in Europe since a deadly outbreak in France in 2022. More on that below.
The reader told me about the issues she and her colleagues face in the ingredient and spice industry in Germany when it comes to sourcing STEC-free flour.
Her company found it so difficult to find raw flour that was safe, no matter whether they looked at local, regional or Europe-wide sources, that they eventually switched to treated flour. This caused other problems because treated flour behaves differently to untreated flour from a water-binding perspective, and is presumably more expensive.
The German BfR has repeatedly detected STEC in baking mixes and dough samples, as well as in flour. The infective dose is very low, thought to be 100 colony forming units or fewer, and the bacteria can survive in wheat flour for two years.
Here are four outbreaks associated with flour and STEC
1: Frozen pizza outbreak, France (2022)
In 2022, France experienced its largest STEC outbreak, with 48 children sickened, and 2 killed after consuming frozen pizzas. The pizza dough had not been pre-baked, and final cooking had been insufficient to destroy the bacteria.
STEC O26 was isolated from both finished pizza and flour used to manufacture it. A nationwide recall was initiated, and manufacturing practices were overhauled to include adequate heat steps.
The case underscored that even cooked products may not reach temperatures high enough to reliably eliminate STEC.
2: Cookie dough outbreak, USA (2009)
In 2009, 77 illnesses across 30 U.S. states were traced to commercial prepackaged cookie dough. Most patients were adolescent girls who had consumed the product raw.
Extensive investigation pointed to contaminated flour as the probable source, though other ingredients were considered.
This led to a recall of 3.6 million packages, with the industry moving to manufacture “ready-to-bake” products as if they are ready to eat.
3: Pizza dough mix outbreak, USA (2016)
In 2016, 13 cases of STEC O157:H7 occurred across 9 states among consumers who had eaten dessert pizza made with a specific pizza dough mix, or breadsticks made with the mix. Eight were hospitalised; no one died.
Although the dry dough mix did not contain the outbreak strain of STEC, it did contain other enteric pathogens across multiple samples.
It’s suspected that the thicker dough used to make dessert pizzas might have predisposed it to undercooking, because the same mix was used for traditional pizzas at the restaurants where patients had dined.
4. Flour, USA (2016)
In 2016, flour produced at a flour mill belonging to a major brand was linked to an outbreak of STEC that sickened 56 people, causing 16 hospitalisations and one case of hemolytic–uremic syndrome (HUS). Many of the patients reported consuming homemade raw cookie dough in the days before they became ill.
The company recalled flour that had been produced during a four-month period. Approximately 250 other products containing the flour were also recalled.
Key takeaways for food safety professionals
STEC should be considered a hazard in dry goods such as flour. If it is present, it may have entered the flour during growing, transportation, or milling of the wheat. While STEC will not reproduce in flour, it will persist and can grow when the flour is added to products that contain more moisture.
If your company uses flour in products that may be eaten raw, such as cookie dough or cake batter, you may need to use treated flour, which has been treated to eliminate STEC, to keep consumers safe.
STEC can be inactivated by a temperature of 70oC for 2 minutes when in moist foods; however, dry heat is ineffective against STEC in flour.
Undercooked baked goods and subsequent dusting of baked goods with raw flour also pose STEC risks.
In short: 🍏 Shiga toxin-producing Escherichia coli (STEC) has caused outbreaks from raw and undercooked foods containing raw wheat flour, including cookie dough, and undercooked or improperly cooked pizza 🍏 It can persist for at least 2 years in flour 🍏 STEC can be inactivated by 2 minutes of moist heat at 70oC but is resistant to thermal inactivation when in dry flour 🍏 Hazard analyses for flour and foods containing flour should consider the risks posed by STEC, even if the food is expected to be cooked before consumption 🍏
Learn more about STEC and flour
Read an excellent overview of STEC in flour, including an explanation of how it gets into flour and how to treat and prevent it here:
BfR Germany (2024) Escherichia coli in flour and dough – What is important for enjoyment without regrets?
Did you know?
Nanoplastics with a positive surface charge attract E. coli O157:H7, and the charged surfaces caused physiological stress, which stimulates more shiga-toxin production.
