As bacteria are very similar to us they need similar conditions in order to grow. The
main conditions are food, moisture, warmth and time.
Food and Moisture
There are four main food types:
- High risk foods
- Raw foods
- Low risk foods
- Ready to eat raw foods
High risk foods are common in food poisoning outbreaks. They are usually
composed of protein, are ready to eat, stored under refrigeration and no further
processing such as cooking is required. Examples include pates, sandwiches, filled
rolls, fresh cream and artificial cream cakes, pies and cold cooked meats.
Raw foods are a major source of food poisoning organisms. High risk foods must be
kept away from raw foods at all times or cross contamination will result and food
poisoning will occur.
Low risk foods do not pose a food safety hazard mainly because they lack one or
more of the requirements for bacterial growth. They could be acid foods with a pH of
less than 4.5, they may have a high sugar, salt or fat content which are good natural
preservatives. They may be dried products or preserved foods not requiring
refrigeration. They may be foods that require ambient storage.
The last category of food is ready to eat raw foods.
These include fruit and salad vegetables and they must be thoroughly washed before consumption to minimise the
risk from low-dose pathogens.
High risk food contains sufficient moisture for bacterial growth.
Warmth
The next thing to consider is warmth. If we control warmth, then bacterial growth will
not occur.
The maximum recommended temperature for freezers is -18°C. At this temperature
there is no bacterial growth, bacteria remain dormant. Although some bacteria are
killed during the freezing process, many remain alive.
Fridge temperatures should be between 1°C and 4°C. (Although the maximum legal
fridge temperature in the UK is 8°C) At fridge temperatures some bacteria will grow
very slowly including Listeria and C botulinum.
The next range of temperatures is the temperature danger zone. This is between
5°C and 63°C. Once food is being prepared it must be kept out of the temperature
danger zone. If food is left in the temperature danger zone bacterial growth will occur
and food poisoning can result. As bacteria enter the temperature danger zone at 5°C
they start to grow slowly. As the temperature increases the bacteria will grow more
profusely. Bacteria grow best at 37°C, which is body temperature. As temperatures
increase from 37°C bacterial growth slows down. Most bacteria are killed by 52°C.
The UK recognised safety cooking temperature is 75°C, although foods can be
cooked at a low temperature over a longer period of time.
The hot holding temperature is 63°C, which is the top of the temperature danger
zone. If food is to be served hot, for example in a carvery situation, it must be
retained above 63°C. If the food is above this temperature, there is no time limit to
how long the food can be served for.
If cold food is refrigerated, it must be retained between1°C and 4°C. The time limit
would depend on the food product. The food could not be retained without time limit
as eventually there would be mould growth (fungi can grow at very low
temperatures).
The two hour hot rule allows foods that had been prepared and are meant to be
served hot, to be served at ambient temperature for no more than two hours. For
example if food is being retained at 63° C and suddenly the heating equipment
breaks down then that food which is losing temperature can be sold for a further two
hours. After two hours the food is allowed to be chilled and reused at a later
occasion after refrigeration.
The four hour cold rule allows foods that are normally sold at refrigeration
temperature to be served at ambient temperature for no more than four hours. For
example if sandwiches are prepared and refrigerated, and the refrigerator breaks
down, the temperature will rise. The sandwiches must be sold within four hours.
After four hours any remaining sandwiches must be disposed of.
Food Safety and the Four Hour Cold Rule
Temperature control must be strictly adhered to ensure the safety of food on the
receipt, storage, preparation, cooking and serving of ingredients. The temperature
danger zone is between 5°C and 63°C. Food, after preparation and cooking, must
be kept out of this zone. When hot food has been cooked, it must be retained above
63°C to remain free from bacterial growth. Food Safety pathogens will not grow
above this temperature. Once food is cooked to 75 degrees C it is safe to eat, all
bacteria have been killed. If food is retained at the correct hot-holding temperature,
it can be served indefinitely, as it is going to remain safe at that temperature.
(Although food quality would be greatly affected by retaining it for that length of
time!). No vegetative pathogens will survive temperatures above 60 degrees C, so
perhaps cooking to 75 is slightly overkill.
If high risk food is being served cold, it must be refrigerated between 1°C and 4°C
and held at that temperature until served. Sandwiches and baguettes with protein
fillings, pork pies, pates, cold meats, seafood, etc can all cause food poisoning if left
at warm temperatures, as any bacteria or spores present will start to grow to
dangerous levels. (Spores would only grow as pathogens after germinating.) It is the
vast numbers of bacteria that cause food poisoning. Foodborne diseases are
caused by fewer bacteria, which are termed low dose pathogens.
Although there is no time limit to how long hot food can held, from a food safety point
of view, cold food can prove to be a problem if held for too long at refrigeration
temperatures. Moulds, for example, can start to grow on food within several days,
even in very cold conditions.
In the UK, the Four-Hour Cold Rule can be used to good effect if there is a rapid
turnover of cold food or if refrigerated storage breaks down. If, for example, a batch
of sandwiches are prepared for service to the public, and is held in refrigerated
display, and that equipment was to subsequently break down, the temperature of the
food would start to increase to within the temperature danger zone. In those
circumstances, one would be allowed to sell the sandwiches for a maximum of 4
hours from the time of preparation. If any remain after the 4 hours, they must be
disposed of.
Some baguette/sandwich companies, which have a quick turnover of product, such
as in train stations and airports, are allowed to sell their wares without refrigerating
their products, due to this ruling. This ensures the sensory qualities of the food are
enhanced. (There is nothing worse than cold rolls or baguettes).
This 4 hour ruling can be considered to be a food safety hazard, however, especially
if buffet items are festering for 4 hours, after being prepared, in a warm room. If
there are already bacteria present, which there will be, (from handling, environmental
particles, cross contamination), they can multiply quite significantly in 4 hours.
Bacteria can double in size every 10 minutes. In four hours bacteria will double in
size 24 times. If only 10 bacteria are present at the start of the 4 hours, 1024 bacteria
will grow in that time. (That is quite a few! In the order of trillions. It only takes 106
Salmonella to cause food poisoning).
In order to check temperatures of freezers, fridges, cooking temperatures and
delivery of foods, a temperature probe must be used. It must be a thermocouple
temperature probe which is able to be calibrated. You can calibrate with boiling
water, when the probe should indicate a temperature of 100°C. The bottom range of
temperatures can be checked with a mixture of ice and water. The probe should
indicate a temperature of 0°C. Probes must be cleaned and disinfected between
uses. This can be achieved by using alcohol wet wipes or simply boiling water. The
temperature probe must be inserted into the thickest part of the food which is known
as the core temperature.
Is Undercooking Beef Steaks A Food Safety Hazard?
As all raw meat and soil-contaminated vegetables contain pathogens, we have to
look at ways of controlling them, to prevent food poisoning.
Pathogens, along with all other good bacteria require food, moisture, warmth and
time. It’s a bit like a jigsaw; provide all four pieces to the jigsaw and bacteria will
grow. Take away one or more pieces of the jigsaw and bacteria will not grow.
We can control warmth by adequate cooking of food ingredients, which kills
pathogens. In the UK it is recommended that the internal core temperature of food
reaches 75 degrees Celsius, in the USA they recommend 70 degrees Celsius. The
UK has always been accused of overcooking their food, perhaps this is where it
originated!
We can cook our food to lower temperatures, however, as pathogens stop growing
at approximately 50°C. Pates, for example, can be cooked at 55°C degrees Celsius,
but it must be held at that temperature for 2.5 hours. Food can be cooked at 63°C,
but it must be held at that temperature for 1.5 hours. It is based on a
time/temperature graph. The higher the temperature, the less time the food has to
be held at that temperature to guarantee food safety. 75°C is an “instant
temperature”, in other words, as soon as the temperature of the food reaches a core
temperature of 75°C, it is safely cooked. There is no need to retain this temperature
for any period of time. If the food is cooked to 70°C, it is recommended that the food
is held at that temperature for 2 minutes.
Many delegates on my training courses ask about undercooking steaks, joints, etc.
How can you have a filet au bleu at 75°C core temperature? The answer is you
can’t. The core temperature of the steak has to be a lot lower than 75°C. Bacteria
are only found on the outside surfaces of single muscle, raw meat. They do not
migrate to the inside of the meat, as do some parasitic pathogens, due to their
inability to cross membranes. This being the case, the steak can be seared either
side and around the edges at high heat in a griddle or pan. This will kill all surface
pathogens. As there are no bacteria in the centre of the meat, it is safe to eat, even
if the inside is raw.
Cooking Pork Incorrectly Can Be A Serious Food Safety Hazard
Although beef, lamb and game can be undercooked, (single muscle joints only, not
diced, rolled or minced) pork should not. Pork contains a parasite called Trichinella
spiralis. This is a pathogenic parasite ingested by pigs (also bear and horse). This
parasite will penetrate into muscle joints in the living animal. If the pork is
subsequently undercooked, the parasite will survive and enter the small intestine of
the new host. The T spiralis larvae are protected by nurse cells, which, when in
contact with the stomach acids, release the larvae into the small intestine, where
they will grow to maturity and release up to 1500 larvae, per female adult, into the
blood stream and lymph system. Initial symptoms of nausea, vomiting, stomach
cramps and diarrhoea, can start in as little as 3 days after eating undercooked meat.
Other symptoms include fever, general malaise, respiratory disorders, disorders of
the central nervous system and eventual death.
Some religious faiths do not eat pork as it is classed as unclean. The uncleanliness
is due to this parasite.
So always cook pork and pork products (or bear/horse products if available) to a
core temperature of 75°C to guarantee food safety.
Freezing will not kill the parasite, as it is protected by an encrusted cyst. Therefore
even freezing the meat before cooking will not allow undercooking.
Fresh fish, however, can be safely undercooked if frozen first. Do not under any
circumstances eat undercooked fresh fish as they also contain parasitic infection.
The main problem parasites are Anisakis simplex, and Pseudoterranova decipiens.
The main symptoms are the immune reaction by the body’s defences, such as
nausea, vomiting, stomach pains. Surgical procedures have sometimes to be
adopted to remove the parasites.
Undercooking fresh fish or eating raw fresh fish can lead to this infection. Under UK
legislation, all fish used for Sushi as a raw ingredient, must be frozen at -20 degrees
Celsius for at least 24 hours. It can then be defrosted and used as fresh.
“The freshest fish to eat is frozen fish”. This is because fish caught at sea or fresh
water, meant for freezing, is frozen within 1 hour of capture. Fresh fish, however,
can take as long as 7-10 days to reach the supermarket or fishmonger. If the fish
has not been refrigerated during the 7-10 days, further complications can arise, such
as the build up of toxins or bacteria. One such toxin is scombrotoxin. This is a
poison which develops in fish that has been temperature abused between capture
and sale and is a serious food safety hazard. Histidine in the fish develops into
Histamine, causing an allergic reaction to the consumer. This could result in
anaphylaxis, which is life threatening.