Vol. XII · Food Safety Plate 8 of 12cross-contamination, cutting board, hands, handwashing, raw chicken, salad, cloths, separate boards, raw below cookedST

Cross-Contamination.

The chicken will be cooked; the salad will not. The danger is the board between them.

Raw meat, poultry, fish and eggs often carry pathogens on their surfaces; cooking will kill them. The risk is that some reach food that will not be cooked — salad, bread, cooked meat, fruit — by the routes of the kitchen: hands, chopping boards, knives, cloths, sponges, tap handles, and drips in the fridge. Each contact transfers a share of the bacteria on one surface to the next, and the chain can be several links long.

Each link can be cut. Wash hands with soap for twenty seconds after handling raw food. Use separate boards for raw meat and for ready-to-eat food, or wash the board in hot soapy water between them. Keep raw meat on the lowest shelf of the fridge, below cooked food. Do not wash raw chicken under the tap, which splashes bacteria across the sink. Replace or hot-wash cloths and sponges often. The workshop follows bacteria from a chicken to a salad.

Below: the bacteria at each step of the chain from raw chicken to board to salad, in logs, for the board treatment you choose (teaching transfer and removal values within published ranges).

What you will learn
Workshop
Teaching model: raw chicken surface 4 log of Salmonella or Campylobacter on the piece; each contact passes about a tenth of the bacteria on one surface to the next (published transfer rates range from under 1 % to over 50 %); a dry cloth wipe removes about half (0.3 log), a cold-water rinse 1 log, a hot soapy wash and rinse 3 log, a separate board the whole route.

Cut the chain

With 10 000 bacteria on a chicken breast and the board used straight away for salad, about 100 cells reach the salad in this model — for some pathogens, such as E. coli O157 or Campylobacter, a few hundred cells can be enough to cause illness. A hot soapy wash of the board between the two leaves about 0.1 cells on average: a thousandfold safer. A separate board removes the route entirely, which is why professional kitchens colour-code boards (red for raw meat, green for vegetables).

Hands are the commonest link of all. Wash them with soap for twenty seconds, scrubbing between the fingers and under the nails, after touching raw meat, eggs, rubbish, pets or the toilet, and before handling ready-to-eat food. Sanitise boards and surfaces after raw meat; dry them, because bacteria thrive on damp surfaces; wash sponges and cloths hot or replace them weekly. Store raw meat sealed on the bottom shelf. These habits matter more than any single temperature.

At the stove

Dust your hands with a pinch of cinnamon and then make a sandwich. Afterwards, look for brown specks on the bread, the knife, the tap and the fridge handle: that is the route bacteria would have taken.

Ignaz Semmelweis showed in 1847 that handwashing with chlorinated lime cut deaths from childbed fever in a Vienna hospital from about one in ten to about one in a hundred; the habit took decades to spread, in hospitals and kitchens alike.

Terms

Cross-contamination
The transfer of microbes from one food or surface to another.
Ready-to-eat food
Food that will not be cooked before eating.
Sanitising
Reducing microbes on a cleaned surface to safe levels, with heat or a sanitiser.
Colour-coded boards
Separate boards for raw meat, fish, vegetables and cooked food.
Bacteria travel by hands, boards, knives and cloths. Wash hands 20 seconds, separate or hot-wash boards, raw on the bottom shelf.

Sources

  1. World Health Organization, Five Keys to Safer Food Manual, 2006.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.