Transcript Food Preservation - preservationandgelatinisation

Food Preservation
What is Preservation
Because food is so important to survival, food preservation is one of the oldest technologies
used by human beings.

Refrigeration and freezing

Canning

Dehydration

Freeze-drying

Salting

Pickling

Pasteurizing

Fermentation

Chemical preservation
What Preservations do
The basic idea behind all forms of food preservation is either:




To slow down the activity of disease-causing bacteria
To kill the bacteria altogether
In certain cases, a preservation technique may also destroy enzymes naturally found in a food
that cause it to spoil or discolor quickly. An enzyme is a special protein that acts as a catalyst for a
chemical reaction, and enzymes are fairly fragile. By increasing the temperature of food to about
66 degrees Celsius, enzymes are destroyed.
A food that is sterile contains no bacteria. Unless sterilized and sealed, all food contains bacteria.
For example, bacteria naturally living in milk will spoil the milk in two or three hours if the milk is
left out on the kitchen counter at room temperature. By putting the milk in the refrigerator you don't
eliminate the bacteria already there, but you do slow down the bacteria enough that the milk will
stay fresh for a week or two.
Refridgeration and Freezing


Refrigeration and freezing are probably the most popular forms of food preservation in use today.
In the case of refrigeration, the idea is to slow bacterial action to a crawl so that it takes food
much longer (perhaps a week or two, rather than half a day) to spoil. In the case of freezing, the
idea is to stop bacterial action altogether. Frozen bacteria are completely inactive.
Refrigeration and freezing are used on almost all foods: meats, fruits, vegetables, beverages, etc.
In general, refrigeration has no effect on a food's taste or texture. Freezing has no effect on the
taste or texture of most meats, has minimal effects on vegetables, but often completely changes
fruits (which become mushy). Refrigeration's minimal effects account for its wide popularity.
Canning

In canning, you boil the food in the can to kill all the bacteria and seal the can (either before or
while the food is boiling) to prevent any new bacteria from getting in. Since the food in the can is
completely sterile, it does not spoil. Once you open the can, bacteria enter and begin attacking the
food, so you have to "refrigerate the contents after opening" (you see that label on all sorts of food
products -- it means that the contents are sterile until you open the container).

We generally think of "cans" as being metal, but any sealable container can serve as a can. Glass
jars, for example, can be boiled and sealed. So can foil or plastic pouches and boxes. Milk in a
box that you can store on the shelf is "canned" milk. The milk inside the box is made sterile (using
ultra high temperature (UHT) pasteurization) and sealed inside the box, so it does not spoil even
at room temperature.

One problem with canning, and the reason why refrigeration or freezing is preferred to canning, is
that the act of boiling food in the can generally changes its taste and texture
Dehyrdation









Many foods are dehydrated to preserve them. If you walk
through any grocery store you may notice the following
dehydrated products:
Powdered milk
Dehydrated potatoes in a box
Dried fruits and vegetables
Dried meats (like beef jerky)
Powdered soups and sauces
Pasta
Instant rice
Since most bacteria die or become completely inactive when dried,
dried foods kept in air-tight containers can last quite a long time.
Freeze Drying

Freeze-drying is a special form of drying that removes all moisture and tends to have less of an
effect on a food's taste than normal dehydration does.

In freeze-drying, food is frozen and placed in a strong vacuum. The water in the food then
sublimates -- that is, it turns straight from ice into vapor. Freeze-drying is most commonly used to
make instant coffee, but also works extremely well on fruits such as apples.
Salting and Pickling


Salting, especially of meat, is an ancient preservation technique. The salt
draws out moisture and creates an environment inhospitable to bacteria. If
salted in cold weather (so that the meat does not spoil while the salt has time
to take effect), salted meat can last for years.
The following