Cross contamination is the transferring of micro-organisms from contaminated food and contaminated areas to cooked or ready to eat food. This often involves transferring of micro-organisms from raw to cooked foods.
It is one of the main causes of food poisoning. Contaminated foods may arrive in a kitchen raw, but the bacteria causing food poisoning will be destroyed during the cooking process. However, the main danger in receiving contaminated food is: if food handling procedures are unhygienic or inefficient, then the "bad bacteria" will be spread around the kitchen. In turn, foods not considered to be at risk may then be contaminated. All foods, if handled poorly, can cause food poisoning.
Micro-organisms move readily in water. The dryer the surface, the lower the risk of cross contamination. Microbes will move with imperceptible levels of moisture, but they will move slower. Mostly they travel to find suitable environments to multiply, which include sufficient amounts of food, oxygen and water.
Temperature is also an important factor in cross contamination and food poisoning. Bacteria start to die above 60'C (140'F). As the temperature increases above 60'C, so does the rate of death for the bacteria. Below 5'C (41'F), the temperature of most cool rooms and fridges, the bacteria are classified as 'inactive'. Food spoilage and bacterial growth still occur but at a greatly reduced rate. Deep-freezing, -18'C (-0.4'F), stops growth completely. Note that below 5'C does not kill the bacteria. When the refrigerated food is raised above this temperature, the bacteria will continue to grow as normal.
The temperature between 5'C and 60'C is called the danger zone. The high danger zone exists between 20'C and 45'C, and bacteria are highly active when exposed to these temperatures.
It is common practice to assume that all raw flesh is contaminated, although it may not be. (Traditional, raw cuisine is a red herring to this writeup, and its extreme freshness and meticulous preparation makes it an exception.) Food poisoning, unlike food spoilage, is often only detectable with scientific equipment, or in the symptoms of the victim after the food has been eaten. Mostly, it cannot be seen, smelt or even tasted. Food spoilage, on the other hand, is fairly safe to consume, but the taste and smell are often offensive - so most people do not eat it (e.g. mouldy bread, off milk).
To avoid cross contamination, regularly clean and sanitise work areas and chopping boards. This is also important before any food preparation has begun.
Principles of avoiding cross contamination can also be seen in the Levitican kitchen. It is a good idea to keep high risk foods away from fruit, vegetables and dry store, as these might be served raw. Many larger restaurants have colour-coded chopping boards to reduce the risk of cross contamination between meats or dairy and fruit and vegetables.
Raw meat should be stored at the bottom of the fridge - covered, of course. Then any dairy products, then above that vegetable matter, and finally any cooked food should be stored at the top of the refrigerator. Should leakage occur in the fridge, meat and dairy will not drip on the foods that are already cooked or may be served raw.
Food should be covered at all times as bacteria can move around in the fridge, albeit a slow migration, as the fridge is a low-moisture, cool environment. Dishes left next to each other over a period of days will allow bacteria to commute between the food. Covering will reduce this risk.
The other major source of introducing micro-organisms to the kitchen is via the food handler.
Staphylococcus aureus naturally exists on human skin - usually originating in moist areas such as the nose, armpits, eyes, hair, etc. E. coli, Salmonella and Clostridium perfringens will be on human hands if they are not washed after going to the toilet.
Frequent hand-washing should be part of any food handling procedures. Particularly, after working with raw meat (including fish, shellfish, poultry and any other forms of flesh). Preferably using an anti-bacterial hand wash. Other times to wash hands include after sneezing, touching hair or piercings and wiping tears or sweat.
In a professional setting, food handlers should not wear their uniform to work. Bacteria will certainly be picked up from public transport and most likely if driving to work. Changing into a clean uniform at the work place will avoid bringing bacteria from areas a long way from the kitchen.
A Case Study
In a dodgy sandwich bar, the meat delivery is made early in the morning. The steak is checked and broken up into small pieces and stored in the deep-freeze.
The board and work bench is wiped over before preparing the bread for sandwiches, specifically buttering them, ready to be made to order at the lunch rush.
The board is wiped free from crumbs. The vegetables are then prepared: lettuce sliced, carrots peeled and grated, tomatoes sliced, etc.
The meat is likely to contain at least low-levels of micro-organisms. Whilst it is in the deep-freeze, these levels will stay low. When the meat is cooked, the micro-organisms will be killed. No danger in this handling method.
The board has not been sanitised, and will have some of the micro-organisms from the meat on it. When the bread is placed on the board, some of the micro-organisms will move onto the bread. However, because there is not a lot of moisture on the board or the bread, migration will be difficult. Cross contamination has most likely occurred, but at a rather low level.
The lettuce, on the other hand, is wet. Many of the micro-organisms that exist on the board can freely contaminate the lettuce. The lettuce will likely be kept in the coolroom, but this is more to prevent spoilage than a fear of contamination. During the lunch break the lettuce is left at room temperature. The longer the lettuce is left in the danger zone, the more poisonous it becomes.
There will be no sign that cross contamination has occurred.