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listeria monocytogenes

Current Recall a Reminder of Past Incident Involving Frozen Vegetables Contaminated with Listeria monocytogenes

Ryan Maus · September 6, 2023 ·

A new recall of frozen vegetables was published on August 23, 2023, due to the finding of Listeria monocytogenes in sweet cut corn. Recalled products were distributed to major retailers across the U.S. having best by dates as far away as December 2024. Listeria monocytogenes, like many pathogens, will not be eliminated by freezing and will persist in the frozen product. While little information is given in the recall, findings from a 2016 listeriosis outbreak involving frozen vegetables is highlighted in the following.


In 2016, the first multistate outbreak of L. monocytogenes involving frozen vegetables occurred in the U.S. Initially three persons reported illness in 2016, but six more illnesses occurring between 2013 – 2015 were identified using PulseNet traceback data. Two outbreak strains were identified in patients ranging in age from 56 to 91 years and three deaths were reported.


Routine sample of frozen vegetables, and the use of whole genome sequencing (WGS), revealed that eight illnesses were closely related to a strain found in frozen organic white sweet cut corn and one illness to a strain found in frozen organic petite green peas produced by CRF Frozen Foods of Pasco, WA. At the same time, an investigation of the Oregon Potato Company of Pasco, WA found that environmental samples matched the whole genome sequence associated with the eight clinical cases and the CRF frozen organic white sweet cut corn. However, a connection between the companies is unknown because the FDA is prohibited by law from releasing publicly certain information about supply chains, which may constitute confidential commercial information.


A recent publication describes the investigational findings by federal and state regulatory agencies at the two frozen vegetable manufacturers. Observations at the CRF Frozen Foods’ facility indicated that the materials and design of equipment and utensils did not allow proper cleaning and maintenance, and could be potential sources of contamination. An investigation of the facility did not find L. monocytogenes in the environment, but L. innocua was found in zones 1 – 3 which FDA considers evidence of conditions that would be suitable for L. monocytogenes.


Seven L. monocytogenes positive zone 1 swabs, two positive zone 2 swabs, ten positive zone 3 were found in the Oregon Potato Company’s facility. Two of these positives, one zone 1 swab and one zone 3 swab,- matched an outbreak strain also found in CRF’s frozen organic white sweet cut corn. Inspectional observations made include:

  • failure to clean food‐contact surfaces as frequently as necessary to protect against contamination
  • facility construction that did not prevent condensate from contaminating food‐contact surfaces
  • food‐contact surfaces not adequately cleaned and sanitized
  • failure to maintain physical facilities in a sanitary condition
  • facility construction not allowing adequate cleaning of floors and walls

A FDA warning letter was issued and noted the presence of L. monocytogenes in the facility as being indicative of inadequate sanitation efforts to effectively control pathogens in the facility. Although these products are considered not ready‐to‐eat, many consumers may use these products
without proper cooking. Cooking instructions should be clear and validated, and consumers should be informed that consuming undercooked or uncooked frozen vegetables could lead to foodborne illness.

A Recent Soft Serve Ice Cream Outbreak and a Look Back at Past Incidents Involving Pathogen Contamination

Ryan Maus · September 6, 2023 ·

A recent outbreak of listeriosis has been linked to Soft Serve On The Go ice cream cups, resulting in 2 illnesses. An investigation by the Pennsylvania Department of Agriculture detected the presence of Listeria monocytogenes in an unopened sample of ice cream taken from a patient’s home. The product isolate was matched to the clinical L. monocytogenes outbreak strain by whole genome sequencing. Five positive samples of finished product obtained from the production facility by the New York State Department of Agriculture and Markets also matched the outbreak strain.

A recall of products distributed nationally is currently ongoing. However, this is not the first outbreak or recall associated with ice cream and many others have occurred as shown in the tables below.

YearLocationPathogenIllnessesDeathsProducer
2022MultistateL. monocytogenes281Big Olaf Creamery
2014WashingtonL. monocytogenes22Snoqualmie Ice Cream
2010-2015MultistateL. monocytogenes103Blue Bell
2005MultistateSalmonella260Cold Stone Creamery
1994MultistateSalmonellaEstimated 224,0000Schwan’s
Past ice cream outbreaks are shown in the following table.
DatePathogenPathogen SourceProducer
07/13/2022L. monocytogenesEnvironmentBig Olaf Creamery, LLC
06/06/2022SalmonellaPeanut ButterTaharka Brothers Ice Cream
02/12/2022L. monocytogenesEnvironmentThe Royal Ice Cream Company, Inc.
04/27/2021L. monocytogenesNot MentionedVelvet Ice Cream
05/14/2020L. monocytogenesNot MentionedRamar Foods
03/17/2020L. monocytogenesEnvironmentWorking Cow Homemade Ice Cream
Recent ice cream recalls due to pathogen contamination are shown in the following table.

Contamination of ice cream products generally occurs after pasteurization, resulting from sources that include the addition of contaminated inclusions (e.g. fruits, nuts, candy, or bakery pieces) to the finished product, contamination from the production environment, and inadequate sanitation of equipment such as filling heads. Freezing ice cream can prevent pathogen outgrowth, but it will not eliminate pathogen presence. Low levels of pathogens causing illness were identified in the 1994 Schwan’s outbreak (Salmonella 0.004-0.46 MPN/g) and 2015 Blue Bell outbreak (L. monocytogenes 3.9-7.1 MPN/g).

Supplier verification programs are critical to evaluate inclusions and reduce the risk associated with them. Inclusions and flavors must be ready-to-eat and any required Process Preventive Controls must have been applied at the supplier, unless the ice cream manufacturer assumes this responsibility.

Sanitation controls are critical to prevent post pasteurization pathogen contamination of finished ready to-eat ice cream products. This is especially true for L. monocytogenes. Cross contamination with L. monocytogenes from the environment is identified in Blue Bell’s 2015 outbreak and numerous recalls. Ice cream processes can be hard to clean, leaving behind nutrients for microbial growth. Listeria can grow,
form biofilms, and persist in harborage sites such as drains leading to cross contamination. Biofilm formation is a constant problem in ice cream producing facilities and requires proper cleaning and sanitizing procedures for their removal and prevention. The effectiveness of sanitation programs are verified by the results attained from a well-designed environmental monitoring program

FDA Releases Findings on RTE Dips and Spreads

Ryan Maus · July 14, 2023 ·

In 2021, the U.S. Food and Drug Administration (FDA) conducted a routine assignment to collect domestic multi-commodity ready-to-eat (RTE) refrigerated dips and spreads to test for Listeria monocytogenes and Salmonella spp. The agency’s goal in conducting this assignment was to determine the prevalence of these pathogens in RTE dips and spreads and remove adulterated products from the market, when possible.

The assignment, which ran from March 2021 to January 2022, found that four out of the 747 samples tested were contaminated with Salmonella or L. monocytogenes. Ready-to-eat foods can become contaminated with environmental pathogens from harborage sites in the food manufacturing environment or process. Cross-contamination from ingredients can also occur.   Additionally, dips and spreads often have a pH and water activity that favor bacterial survival and outgrowth.  Consumers typically eat these dips and spreads without a ‘kill step,’ such as cooking, to reduce or eliminate any pathogenic bacteria that may be present. As such, dips and spreads contaminated with L. monocytogenes or Salmonella can present a significant public health risk. Ready-to-eat dips such as hummus and cheese spreads have been associated with multiple recalls over the past few years. The FDA’s assignment was initiated due to five recalls of hummus products and six recalls of multi-commodity dips found to be contaminated with L. monocytogenes or Salmonella from FY2017 through FY2020.

During the sampling assignment, the FDA identified Salmonella Havana in a sample of hummus from a retail establishment in California.  The FDA notified the manufacturer of the hummus and a recall was initiated.  The FDA conducted a follow-up inspection at the manufacturer of the contaminated product to evaluate the firm’s manufacturing operations and collect environmental samples. The FDA found that the firm did not establish and implement adequate written procedures for monitoring sanitation control procedures (i.e., cleanliness of food-contact surfaces). The environmental samples collected at the follow-up inspection did not yield any positive pathogens, however, the agency still issued a warning letter after the follow-up inspection.  After taking corrective actions, the firm was unable to identify a root cause for the Salmonella Havana contamination in the hummus.

The FDA identified L. monocytogenes in three samples from a retail establishment in Colorado and the agency notified the firm where the positive samples were manufactured. All three of the samples contaminated with L. monocytogenes were produced by the same manufacturer. Upon notification of the contaminated samples, the firm recalled and destroyed the products associated with the positive findings. The agency issued a Consumer Advisory in conjunction with the firm’s recall.

The agency conducted a follow-up inspection at the manufacturer of the three contaminated products. The follow-up inspection sought to evaluate the firm’s manufacturing operations and collect environmental samples to determine potential sources and routes of contamination. The FDA found L. monocytogenes in 23 swabs collected within the production environment, including samples collected from food contact surfaces. WGS analysis was conducted on the L. monocytogenes isolates obtained from both the retail product samples and the inspection environmental samples. The isolates were a genomic match to each other.

The FDA’s follow-up inspection determined the firm’s employees were not trained in food safety or hygiene practices and found multiple instances of deficient sanitation practices related to equipment, utensils, and employees. The agency determined that the firm did not respond appropriately to the positive samples identified in the sampling assignment and had not addressed objectionable conditions observed by the FDA during a previous 2020 inspection.  The firm went out of business at the end of June 2021.

Underscored in the FDA’s report is the necessity for manufacturing firms to have good manufacturing practices in place, as well as a proper environmental monitoring program and adequate training for its staff as part of their standard business operating procedures.

Deibel Laboratories has years of experience in conducting shelf-life studies, formulation reviews, and validations for refrigerated dips and spreads.  Our staff of industry experts is ready to assist you in assuring that your manufacturing processes and facilities comply with the stringent food safety standards required by the FDA and USDA.  With a complete suite of accredited pathogen testing methods and the testing capacity of 15 laboratories strategically placed throughout North America, Deibel Laboratories is your one-stop for assuring food safety compliance.

Contact us at Sales@DeibelLabs.com for more information about a laboratory near you. Visit us at https://deibellabs.com/services/deibel-guardianemp/ to learn more about our environmental monitoring program, Deibel GuardianEMPsm. Link to FDA findings: FY21-22 Sample Collection and Analysis of Domestic Refrigerated RTE Dips and Spreads | FDA

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