A study by German researchers has raised concerns on the microbiological quality of ready-to-eat salad mixes and fresh sprouts.

The three-year research project by the Max Rubner Institute also found the microbiological quality of cucumbers, carrots and mushrooms was good. Mixed lettuce received a good to medium result and herbs were rated as medium.

The focus was on Listeria, Salmonella, E. coli, Staphylococcus aureus and Bacillus cereus.

Researchers said the microbiological quality of ready-to-eat (RTE) mixed salads was “inadequate” as 7 of 116 samples, or 6 percent, were contaminated with human pathogenic bacteria. This figure was 2.5 percent for lettuce, 1.3 percent for carrots, 1.2 percent for mushrooms, 1.1 percent for sprouts and zero for herbs and cucumbers.

The team sampled a total of 600 products sold in northern and southern Germany including 115 samples of herbs, 40 cucumbers, 79 carrots, 80 types of lettuce, 116 RTE salads, 81 edible mushrooms, and 89 samples of sprouts in the project from 2015 to 2018.

Results for cucumbers, carrots, herbs and mushrooms
In cucumbers no human pathogenic bacteria was detected. For carrots and mushrooms, bacteria could be identified, but only after culture enrichment in the lab.

Carrots came from Germany, Denmark, France, Israel, Italy, the Netherlands and Spain. E. coli O146:H28 and one isolate of Staphylococcus aureus was identified. However, the bacteria could not be detected without enrichment, so the assumption was they occurred at low levels and would not pose a risk to humans.

Mushrooms came from China, Germany, the Netherlands, Poland, Russia and Korea. Listeria monocytogenes was detected in one sample. Cucumbers came from Bulgaria, Germany, the Netherlands, Spain and Turkey.

Total bacterial load for herbs on average was relatively high. Countries of origin for organic and conventionally produced herbs were Germany, Ethiopia, India, Israel, Italy, Kenya, the Netherlands, Spain and Tanzania.

Salmonella, Listeria monocytogenes and STEC were not detected in the 115 fresh herb samples tested. However, two isolates were identified as Staphylococcus aureus. The bacterial count was insufficient, less than 104 CFU per gram, to assume a risk to people, according to the report.

Lettuce and RTE salad findings
Although human pathogenic bacteria were detected in lettuce, the germ burden is reduced by measures such as cleaning and washing before consumption by the consumer, the researchers reported. Salmonella Enteritidis and Listeria monocytogenes were isolated from one sample each. Countries of origin were Belgium, Denmark, Italy, Germany, Netherlands, Spain and Turkey.

Ready to eat mixed salads came from Germany, Denmark, Italy, Netherlands and Spain. Listeria monocytogenes was detected in three products and assigned to the serovar groups IIa, IIb and IVb after serotyping. Salmonella Enteritidis and Salmonella Szentes were detected in three products. In a mixed salad, Staphylococcus aureus was identified and E. coli O26:H11 isolated.

Sprouts came from Germany and Netherlands. Listeria monocytogenes was isolated from one of the 89 samples and Staphylococcus aureus detected in two samples.

The German Society for Hygiene and Microbiology (DGHM) has guidelines and warning values for RTE mixed salads and sprouts. For RTE mixed salads, some samples were above the guide value for total count, presumptive Bacillus cereus and yeasts. Warning values were also exceeded for presumptive Bacillus cereus and mold.

In the case of sprouts, presumptive Bacillus cereus was found to exceed the guideline and warning values in some samples. As the warning levels for presumptive Bacillus cereus were exceeded, a health risk for consumers cannot be ruled out, the researchers said.

The Max Rubner Institute recommends that sprouts should not be eaten raw and should always be blanched. Storage temperature of RTE salads should not exceed 4 degrees C (40 degrees F) and they should be eaten immediately after opening, according to the report.