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The aim of preserving is to slow down the activity of microorganisms and enzymes or destroy them altogether.
They cannot survive in acidic or dry conditions, in high concentrations of salt and sugar, in alcohol, or in high temperatures. A preserve will often employ different techniques, for example jams combine heat with a high concentration of sugar.
The colder a food is, the slower its rate of deterioration. Bacterial action reduces with refrigeration, while freezing stops it altogether; enzyme activity, however, is only slowed down. Vegetables must be blanched in boiling water first to destroy enzymes and microorganisms, while herbs can be mixed with oil and fruit should be sprinkled with sugar to limit enzyme activity while frozen.
> Freezing herbs
• Herbs in water can be frozen in ice cube trays
• Once frozen food is thawed, the enzyme and microorganism activity accelerates again
• If food is frozen to 0°F (-18°C), microorganisms can’t function
Boiling or blanching food at high temperatures destroys all enzyme activity and almost all microorganisms. The more acidic the food, such as fruit, the more easily microorganisms are destroyed by heat. Boiled preserves must be sealed in airless conditions (e.g. airtight jars) to prolong their shelf life.
> Boiling to make fruit preserves
• Most bacteria will be killed at 212°F (100°C), the boiling point of water
• Can preserves while still hot to maintain the benefits of boiling
• Enzymes start to be destroyed above 140°F (60°C)
3. Use strong concentrations
Alcohol, acid, and salt and sugar in high concentrations all create environments that prevent the growth of microorganisms or, in the case of alcohol, destroy them completely. Naturally acidic fruit is usually preserved in a concentrated sugar solution or alcohol. Vegetables, which are more alkaline, are preserved in acidic vinegar or a salt solution, or a combination of both.
> Pickling in vinegar
The acid in vinegar stops food from spoiling
> Fermenting alcohol
Yeasts can ferment and spoil foods but they can also be harnessed to preserve fruit juices by converting them into an alcoholic concentration.
4. Exclude air
A seal of fat or oil can prevent any airborne microorganisms from coming into contact with food and spoiling it. It also starves aerobic bacteria present in food of oxygen, which it requires to survive and increase. Heat processing jars and bottles of preserves prolongs shelf life by forcing air to escape as steam to leave a sterile vacuum.
> Heat processing bottled produce
• Air escapes from partially sealed containers as they are boiled in water
• Heating produce also helps destroy harmful organisms
5. Remove moisture
Microorganisms need moisture to grow, and die off in dry conditions. Food can be dried using warm air or an oven, or sealed in a concentrated solution of salt or sugar that draws out moisture by osmosis.