Environmental sampling

Questions and answers relating to plant environment sample collections

Q1. How often should plant environmental samples be taken?

A1. The frequency of sample collections is decided by the food business operator (FBO). Many FBOs decide their sampling frequencies based on their historical results or as part of their procedures based on HACCP principles. Once a history of satisfactory results is established, it can be used as justification for reducing the numbers of samples collected and the frequency of testing.

For those businesses that are starting to test, they may want to consider collecting 10 samples every fortnight as a starting point. It is considered a good practice to vary the day of the week that the samples are collected.

Q2. What kinds of bacteria are counted from environmental swabs?

A2. The bacteria that are tested for depends on what the FBO wants to check. Plants that want to check cleaning effectiveness routinely count total aerobic mesophiles (TAM; all the bacteria that can grow in an air atmosphere at temperatures around 30oC) and Enterobacteriaceae. Cleaning effectiveness samples are normally collected after the completion of cleaning and just before the commencement of a day’s processing. L. monocytogenes testing can also be undertaken to assess cleaning effectiveness.

There is no legal requirement to undertake testing for TAM. However, EC 2073/2005 confers a legal obligation on FBOs to test processing environments used to manufacture ready-to-eat foods for L. monocytogenes. In practice, most FBOs test for general Listeria species, and if they isolate a Listeria will have it typed to determine if the species is monocytogenes.

Those plants that isolate L. monocytogenes from cleaning effectiveness samples, will want to investigate the sources of the contamination. As part of those investigations, environmental samples can be collected during processing to pinpoint any L. monocytogenes niches that are ineffectively cleaned. Isolates from samples collected during processing can by typed to determine if they are plant resident strains.

Further guidance on what to test for can be found HERE:

Q3. Where should I take samples from?

A3. The location of the sample collections is decided by the FBO. As a rule of thumb, many businesses decide that approximately:

Two thirds of the samples should be taken from food contact surfaces on processing equipment.
One third of samples should be taken from the plant environment but not from non-processing equipment.

The surfaces to be tested should be ones that are routinely cleaned and disinfected (see below). Surfaces should be ideally dry, flat, smooth and sufficiently large to take a sample (an area of 20-100cm2 is recommended).

Q4. When should I take my samples?

A4. The frequency of sample collection depends on what the FBO wants to determine. To check the effectiveness of cleaning, most FBOs clean down their plant (at least) at the end of a day’s processing. Surface sampling however is commonly delayed until just before the commencement of the next day’s processing. Commonly that can be 5-12 hours after cleaning has finished. The reason is that the FBO wants to determine if the surfaces are fit for processing. It is common to create aerosols when cleaning and if there are bacteria in the aerosols, they can settle out of the air onto surfaces over time, contaminating them. It is recommended that cleaning in any high-risk environment is done both pre- and post-processing.

Trends in test results should be analysed, as they are able to reveal unwanted developments in the manufacturing process, enabling the FBO to take corrective actions before the process is out of control.

If a FBO is routinely isolating L. monocytogenes from surfaces or products, they may wish to collect samples from food contact surfaces during processing. The strategy is designed to identify if there are places in equipment (inside conveyor belt rollers or in moving joints and hinges ) that do not get effectively sanitised and release L. monocytogenes from these sheltered areas onto products only during processing when the equipment is operating.

Q5. Can you give me some examples of where I should take samples from?

A5. A list of locations likely to harbour L. monocytogenes and how other FBOs have successfully decontaminated equipment and environments is provided on this page 

Q6. What equipment do I need to be able to take an environmental swab sample?

A6. There are a variety of ways to collect environmental samples. Guidance should be sought from the lab undertaking testing on their preferred method, and sampling kits may be provided as part of the testing service.

Two common methods are using drumstick swabs and sponge swabs. Drumstick swabs are commonly used to collect samples from impervious food contact surfaces, such as stainless steel. Sponge swabs tend to be used to collect samples from porous or cracked hard surfaces such as brick and concrete.

Typical equipment is shown in the picture below.

Q7. There are many different types of drumstick swabs, which ones should I use?

A7. There are lots of different styles of drumstick swabs. Varieties include shafts made from wood, plastic and paper shafts that require cutting to remove the head, plastic that shatters if you attempt to snap the heads from the stick and plastic shafts that break cleanly and easily. In addition, the heads can be different sizes and the absorbent material is variable but commonly cotton or cellulose.

The type of swab used is a personal preference of the sampler or may be dictated by the lab used for testing. Many samplers prefer a larger head on their swabs so that a larger volume of diluent or neutraliser is applied to the sampled surface. Swabs with plastic shafts that shatter when the head is removed are not recommended for use in a food processing plants due to potential contamination of products with plastic splinters. Many samplers avoid using swabs that have perforated plastic or wooden shafts because these tend to break if too a firm pressure is applied during sampling. 

Q8. How do I take a sample using drumstick swabs?

1. Put on the gloves.

2. Take an alcohol-soaked wipe and wash your hands with the wipe. Make sure you get in-between your fingers and also the tips of your fingers as shown in the picture below.

3. Open two swabs. Keep hold of one (and keep it dry). Dip the other in the neutraliser for at least 10 seconds. (NB: a neutraliser is required to neutralise any sanitiser residues and ensure that any collected bacteria are not killed by these residues on the way to the lab)

4. Squeeze the end of the swab against the inside of the tube of neutraliser to remove excess fluid.

5. Discard the opened neutraliser.

6. Begin with the wet swab. Use a firm pressure and roll the shaft of the swab between the fingers and thumb to rotate the swab whilst rubbing.

7. Rub the wet swab the surface. Rub horizontally, then vertically then diagonally as shown below. It is important that you press firmly on the swab whilst sampling to collect a true representation of the number of cells on the surface.


8. Keep the wet swab in your hand. Rub the dry swab across the same area of surface that was used with the wet swab. Rub horizontally, then vertically then diagonally as shown above. The basic idea is that the wet swab coats the surface with a film of neutraliser. Bacteria are released into the neutraliser, and then the neutraliser and bacteria are absorbed into the dry swab.

9. It is common for people to use a template when they first being collecting environmental samples. However, within a short period of time, most people acquire an ability to predict where the swab would hit the edge of the template, without the template being present. Since imperfectly sanitised templates can increase the test count, many experienced samplers dispense with using any template.

10. A short video clip of how to take a swab can be viewed by clicking here.

11. Cut or break the swabs off into a tube of peptone salt solution (peptone salt solution is also called MRD [maximum recovery diluent])

12. Screw the top onto the sample tube, and then label the sample with

the date
the area sampled (e.g. 100cm2)
a description of surface sampled (e.g. fridge door).

13. Immediately put the sample into an insulated cool box containing frozen freezer blocks wrapped in bubble wrap, or crushed ice. Keep the samples chilled (but do not allow them to freeze). Keeping samples cold will prevent bacteria from multiplying and make sure your counts accurately reflect the bacteria present on the surface.

14. When all samples have been collected, package up the samples for sending to the lab. Make sure samples will stay at 0oC to 4oC. Samples should arrive at the lab a maximum of 24 hours after being taken. Transport in a cool box, but do not let the samples freeze or come into direct contact with any frozen blocks. 

Q9. How do I take a sample using sponge swabs?

1. If the use of gloves is required, put on the gloves and take an alcohol-soaked wipe and wash your hands with the wipe. Make sure you get in-between your fingers and also the tips of your fingers as shown in the picture below.

2. Pour two vials of neutraliser (roughly 40mls) into the bag containing the sponge swab and let the sponge absorb the liquid for at least one minute. (NB: a neutraliser is required to neutralise any sanitiser residues and ensure that any collected bacteria are not killed by these residues on the way to the lab). The sponge should be saturated and drip neutraliser when it is ready for use.

Grasp the bottom of the bag containing the sponge and fold the bag back over your hand (Figure A below) so that the sponge is exposed to the air and the bag protects the sponge from contamination from the sampler’s hand (Figure B below).

Place the sponge on the porous area to be sampled and press onto the sponge so that the neutraliser is expelled. 

Relax the pressure on the sponge so that the neutraliser is re-absorbed. Continue expelling and reabsorbing the neutraliser into the surface/into the cracks at least 10 times. If the surface is not too abrasive (e.g. vinyl or wood), the sponge can be squeezed out into the bag and then rubbed across the sampled area in two perpendicular directions to mop up any excess diluent that was not re-absorbed.

Rubbing sponges across abrasive surfaces such as concrete however, can cause physical damage and reduce the number of bacteria captured (see below)

Many processors will re-clean and sanitise sampled surfaces to remove chemical residues and potential sources of nutrients for bacteria.