Selection of sampling sites

Selection of environmental sampling sites likely to harbour L. monocytogenes

The table below is a list of niches from a number of food processing plants that have been reported to contain L. monocytogenes. Also included in the table is a list of corrective actions that successfully removed the contamination. The table contents have been modified from a list originally assembled by Tompkin (2002). The information is provided so that plant operators can devise strategies to assess the contamination status of likely niches in their processing plants and previously effective decontamination strategies for equipment and environments.

Contaminated equipment Contamination source Successful corrective actions taken
Continuous brine chill chamber for product on racks with wheels

Doors made of rubber-coated fabric, large metal hinges extending the width of the door, and hollow bump guards at bottom of door

The doors were replaced with rigid cleanable plastic material; the large hinges and the bump guards were removed
Ammonia unit used to chill brine solution Fibreglass insulation on ammonia line to brine chilling unit became saturated with brine that was splashing from chilling unit The contaminated insulation was removed; the pipe and the area were cleaned and sanitised; the insulation was not replaced on the pipe close to the brine chiller
Brine chill exit Hoses and spray nozzles at exit end of brine chill tunnel used to spray product The hoses and the nozzles were replaced; daily cleaning and sanitation was initiated
Brine chill Construction of brine chill tunnel had stainless steel framing with metal touching metal, causing an uncleanable hollow space at the point of contact The framing was modified to facilitate cleaning and to prevent the material from getting into the space between the tunnel wall and the framing
Brine chill tunnel Damaged rubber seals on stainless steel door at end of tunnel The damaged seals were replaced; the cleaning procedures were tightened
Conveyor belt Contaminated liquid was discovered within a hollow sprocket that had split open The hollow sprocket was replaced with a solid sprocket
Conveyors (multiple events) Hollow rollers The rollers were replaced on detection; where possible, conveyors were replaced with sloping steel chutes to remove rollers and the danger of re-contamination
Conveyor between shrink tunnel and boxing Worn conveyor made of rubber-coated fabric A new belt was fitted
Collator and conveyor Undetermined The equipment was covered with a large tarpaulin and steam was injected at the bottom of the tarpaulin

1. Exit from spiral freezer

2. Spiral freezer

1. Exit from spiral freezer

2. Spiral freezer

1. The bearings were removed and replaced

2. The cleaning frequency was increased and the equipment was allowed to defrost properly before commencement of cleaning

1. Between freezer and packing machine

2. Wire mesh conveyor between oven and freezer

1. Overhead conveyor

2. Hollow support rods for conveyor

The hollow rods were replaced with solid rods
Slicer Worn hydraulic seals at base of slicer, oil with water and product residue The slicer was stripped, cleaned and sanitised; the parts were placed in the oven and moist heat was applied. Oil with sodium benzoate (an anti-listerial chemical) was used for lubrication
Slicer  Product detritus build-up inside safety cover on gear and drive belt; the material further contaminated conveyor belt below The cover was changed so it could be removed for cleaning each night

1. Conveyor leading to packaging machine

2. Cooked product work table

1. Fabric conveyor belt material

2. Hand held knives 

1.The conveyor was replaced with a stainless steel chute

2. The knives were cleaned and sanitised daily in automatic washer and were no longer stored in the employees lockers

Work table Air duct used for blowing product storage bags open The table was modified to make the duct accessible for nightly cleaning
Packaging machine Crack in stainless steel covering on top edge of machine near product loading area The area was cleaned, sanitised and the crack was welded shut
Packaging machine Steel rods for pushing product into cartons The rods were removed from the packer and cleaned/sanitised on a daily basis
Insulated concrete block wall The insulation between fibreglass and concrete wall was contaminated by condensate from overhead pipes All fibreglass/insulation was removed from the wall; the concrete wall was cleaned with an acid base cleaner, sanitised, and sealed. The overhead pipes were rerouted to be closer to the floor
Ingredients hopper Cinder blocks around an opening in the wall The cinder blocks were sealed to prevent moisture from accumulating in the blocks
Scoop Join between handle and scoop  Preferably use single piece scoops avoiding niches for harbourage
Refrigeration unit near ceiling of holding cooler Condensate from refrigeration unit The refrigeration unit was cleaned and sanitised

A typical programme for the collection of environmental swabbing in a fish processing plant

Below is a table of sample collection locations collated from a number of different sizes of fish processing and smoking plants in the UK. The table is provided because some of the larger processors have determined where the problem areas in their plants are and deliberately target these locations to ensure their plants don’t become colonised with persistent L. monocytogenes. Typically, the areas that are sampled repeat every 4-9 weeks in the majority of UK plants. In addition, to the sample collection sites listed below, fish processors may want to consider sample collections from areas in their plants that are frequently wet. L. monocytogenes has a tendency to colonise places in food processing plants that are dry only occasionally. Advice on sample collections is available here.

Week Sampling locations

1

Packing feed conveyor x6,

Raw fish skinning machines x3,

Cooker/hot smoker door handles and seals x5 on the refrigeration side of the oven,

Cold smoker door handles and seals x5 on the high care side of the unit,

Blast chiller entry door handles and seals x10 on the side of the ovens (which may exhaust hot, moist air)

Blast chill floor x10,2

Drains in high care area, especially if located near slicing equipment

2

Pack line weighing balances x8,

Cooker/hot smoker door handles and seals x5 on the refrigeration side of the oven,

Cold smoker door handles and seals x5 on the high care side of the unit,

Final product chiller door seals x10,

Drains in post cook/smoke work room x4, Blast chiller exit door handles and seals x10,

Blast chiller drains x5

3

Main packing exit conveyor, concentrating on rollers and belts x6,

Rejected product packing exit conveyor, concentrating on rollers and belts x6,

Kiln/oven door handles x10,

Final product chill door handles,

Ceiling condensation squeegee for area between chillers and smokers/cookers,

Drain in area between chillers and smoker/cookers,

Drains in post cook/smoke work room x4

4

Slicer on/off switches and controls x6,

Packing line control panel x6,

Kiln/oven door handles x10,

Randomly selected fish transport racks, concentrate on the wheels, x10,

Ceiling squeegee for area between chillers and cookers,

Staff changing area floor,

Random areas of high risk area floor x10, , Drains in post cook/smoke work room x4,

Blast chiller floors x10

5

Knife blades (x6) as a pooled sample,

Ingredient hoppers and shelving in ingredients store x10,

Kiln/oven door handles x10,

Final product chill door handles,

Ceiling squeegee for area between chillers and cookers,

High risk area floor dry detritus squeegee,

Blast chiller floors x10,

Drains in raw product chiller, especially if melting ice leaks onto floors x5,

Random areas of the dry ingredient store floor x5

6

Packing feed conveyor x6,

Product racks in packing area x6,

Cleaner’s anti cut gloves as a pooled sample,

Blast chiller exit handles and door seals x10,

High risk area brushes and squeegees x5,

A new pair of randomly-selected disposable gloves from a glove’s dispenser x5

7

Drain gully in front of cooker high risk exit at smokers/cookers x5,

Random areas of the dry ingredient store floor x5,

Blast chiller entrance handles and seals x5,

Random boot soles in high risk area entrance x10

Hosepipe nozzles x5

Automatic boot washing equipment x5

8

Blast chiller exit handles x10,

Blast chiller exit seals x10,

High risk area brushes and squeegees x5,

Blast chiller floors x10,

Drain gully in front of cooker high risk exit at cookers x5

9

Blast chiller evaporator coils pooled x5,

Product temperature probes x5,

On/off switches on temperature probes x5,

Kiln/oven door handles x10,

Kiln/oven door seals x10, High risk area drains at random points x10,

A used pair of randomly-selected disposable gloves from a glove’s dispenser x5

Fish processing plants are under a near continuous assault from L. monocytogenes and it is likely once you begin testing that sooner or later you will have a positive isolation. You can find advice on how to sample your plant environment for L. monocytogenes on these pages. How to effect decontamination is described on these pages. In addition, a positive isolation of L. monocytogenes from a plant environment may be grounds for a review of processing practices by a FBO. 

Tompkin, R. B. (2002). Control of L. monocytogenes in the food-processing environment. Journal of Food Protection 65, 709-725