Chilly Food: Fat Tom for Food Safety

FAT TOM is an acronym, or a mnemonic device which is useful for remembering six factors important for food safety. Whether you are in the food service industry or a person with a cooler and a couple of days' worth of food, knowing these six things can help you prevent spoilage and benefit from more time to eat your vittles. The key is to know what conditions bacteria need to grow and multiply, then minimize those conditions in an effort to suppress the bacteria. The sections below will explore each of the six aspects of FAT TOM and food safety.

F is for "Food"
A is for "Acidity"
T is for "Time"
T is also for "Temperature"
O is for "Oxygen"
M is for "Moisture"
The Food Safety Danger Zone"

F is for "Food"

For starters, the foods you must work hardest to protect include meats, seafood, poultry, milk and eggs. These are high in protein and naturally moist. Considered as "perishable" foods, they must be kept in a manner which slows down the rate of bacterial growth. The most serious problem with bacteria in these foods is that they are the most harmful to us. We can suffer illness or death because of an infection by the bacteria or poisons produced by them.

Fruits, vegetables, and starches can also spoil because of bacteria and molds. There is a bit less urgency with these foods, as they may be kept at room temperature or refrigerated and will stay fresh for a much longer time than the high protein perishables mentioned above.

Foods which may spoil must be preserved with some kind of treatment or action, such as refrigeration, freezing, pickling, smoking, canning, or maybe irradiation.

A is for "Acidity"

Acidity is a characteristic of all substances, which is measured in units of pH, on a numeric scale from 0 to 14. A pH of 7 is considered to be neutral, with acidity increasing with lower pH values. Alkalinity is the opposite, and increased with pH values above 7. Bacteria grow best in neutral environments, but can cope with slight acidity, down to a pH of about 4.6. They do not tolerate alkaline environments, with a pH over 7.5 being mostly unlivable.

Mixtures of foods containing some acidic types are more resistant to pathogenic bacteria. In fact, food may be preserved for very long periods by pickling it, which means immersing it in an acidic liquid, such as a brine or vinegar.

As an example, here are some typical edibles and their typical pH values:

Let's Look at pH Values

    Substance               pH
----------------------------------------
    pure water              7.0
    sea water               8.1
    lutefisk                12.0
    vinegar                 2 to 3
    lemon juice             2 to 2.5
    orange juice            3.3 to 4.1
    brine for pickling      4.5
    fruit jam               3.1
    ketchup                 3.6

FDA publications indicate pickled and canned foods with pH values below 4.6, I repeat: hermetically sealed, can be considered "shelf stable" and be stored at room temperature.

T is for "Time"

Over time, all foods will spoil. Some foods, such as freeze dried, irradiated, and salt cured, may be edible for many years. Fresh ground beef, at room temperature, will become unsafe to eat after a couple of hours. The primary issue, above all others, is the effect of bacterial growth and the resulting build up their toxins. Experts have given time limits for safety, depending on how a food is handled.

Bacteria do not grow linearly, which would mean their numbers are proportional to the time. No, you do not double the bacteria in double the time. Truly, bacteria grow exponentially. Given an increment of more time, they grow their population (and their toxins) by a multiple of that increment.

Pathogens build in a cumulative manner. Thus, the time taken toward a safety limit is cumulative. If a piece of meat at room temperature, which should be safe for two hours, is refrigerated after one hour, it does not reset the clock. When that meat is removed from the refrigerator, it only has an hour of safety left.

Food will remain safe for days if kept at a temperature just above freezing. You should be very aware of these food safety time limits, when food is not being kept as cold:

Safe Food Times versus Temperature

Temperature                           Time Limit
--------------------------------------------------
  1°C / 34°F up to  4°C / 39°F        several days
  4°C / 39°F up to 30°C / 86°F        2 hours
  30°C / 86°F up to 60°C / 140°F      1 hour

T is also for "Temperature"

Cold temperatures, from below the freezing point to slightly above freezing, are ideal for preserving food through refrigeration. Bacterial growth is limited at cold temperatures.

Bacteria grow best at a temperature range of 4°C / 39°F to 60C / 140°F, which is referred to as the “danger zone.” Food can spoil quickly in this range, as shown in the temperature vs time table shown above. This is so important to remember; make every effort to limit the amount of exposure of food to these temperatures.

The top of the danger zone for food is bounded by temperatures which kill bacteria and actually cook the food. Cooking temperature should exceed the danger zone and sustain above it for enough time to kill all of the bacteria. The duration of the temperature should therefore be longer in time if peaking lower. For example, see these examples for cooking chicken:

Cooking Chicken - Safe Temperatures vs Time

Internal Temperature                  Time
-----------------------------------------------
  145°F / 63°C                        9 minutes
  165°F / 74°C                        1 second

O is for "Oxygen"

Harmful bacteria in food typically need oxygen in order to live. In just a few instances, certain bacteria can live without oxygen. As examples, Salmonella needs oxygen; botulism does not.

Food will remain edible for very long times if it is first heated to kill bacteria, then sealed in a manner which keeps out oxygen. Here are some examples:

Methods of Sealing Food Without Oxygen

  Method            Description
-------------------------------------------------------------
  Confit            Cook then immerse in hot fat.
  Cannig            Heat and seal in an airtight can or jar.
  Vacuum Packing    Heat and seal in an airtight bag or box.

M is for "Moisture"

Bacteria, yeasts, and molds must have water in order to live. If you want to avoid spoilage, it is possible to deny moisture to the undesirable organisms.

Moisture in food available to bacteria may be expressed as water activity (aw), with very dry / minimal activity 0 and wet / maximal activity 1.0. It is not a measure of total water content, but whether the water is available for bacteria (not chemically bound to proteins, salts, or sugars). For long edibility, foods preserved by water reduction should have an aw no higher than 0.85.

Dry crackers, flours, noodles, candies, or hard pretzels are examples of foods we can easily recognize as low moisture foods which are shelf-stable and do not require refrigeration.

Jerkies are an example of meats often preserved with salt. Jams and jellies use sugar for preserving some meats and more fruits or vegetables. In these, the water molecules are taken by the salt or sugar by the process of osmosis.

Here are some examples of water activity levels in foods. Note the high levels in most foods, in their normal conditions. Then note the low wa numbers for preserved foods such as beef jerky:

Water Activity of Common Foods

Food                            Water Activity (aw)
---------------------------------------------------
fresh meat and fish 	        0.99
raw vegetables                  0.99
raw fruits                      0.98
cooked meat, bread              0.91-0.98
liverwurst                      0.96
caviar                          0.92
moist cakes (ex : carrot cake) 	0.90-0.95
sausages, syrups                0.87-0.91
flours, rice, beans, peas       0.80-0.87
salami                          0.82
soy sauce                       0.80
beef jerky                      0.60-0.80
jams, marmalades, jellies       0.75-0.80
peanut butter                   0.70
dried fruits                    0.60-0.65
dried spices, milk powder       0.20-0.60
biscuits, chocolate             0.40-0.60

The Food Safety Danger Zone

Times are short for food temperatures between 4° C / 39° and 60° C / 140° F because that range is considered to be very favorable for bacterial growth. Temperatures in the upper part of the range, above 30° C / 86° F, are especially favorable, and food is considered unsafe for times over one hour.

You now have an idea, and some details, of what "FAT TOM" means in terms of food safety. Knowing what it means and following the guidelines will help you avoid sickness due to micro organisms within your food.