Canning Facts and Information
From FDA Consumer, September 1990, by Dale Blumenthal:
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.
The Canning Process
Food-spoiling bacteria, yeasts and molds are naturally present in foods. To grow, these microorganisms need moisture, a low-acid environment (acid prevents bacterial growth), nutrients, and an appropriate (usually room) temperature.
Foods are preserved from food spoilage by controlling one or more of the above factors. For instance, frozen foods are stored at temperatures too low for microorganisms (bacteria, yeasts and molds) to grow. When foods are dried, sufficient moisture is not available to promote growth.
During canning, the food is placed in an airtight (hermetically sealed) container and heated to destroy microorganisms. The hermetic seal is essential to ensure that microorganisms do not contaminate the product after it is sterilized through heating. Properly canned foods can be stored unrefrigerated indefinitely without fear of their spoiling or becoming toxic.
Foods with a naturally high acid content--such as tomatoes, citrus juices, pears, and other fruits--will not support the growth of food poisoning bacteria. In tests, when large numbers of food poisoning bacteria are added to these foods, the bacteria die within a day. (The exact amount of time depends upon the bacteria and amount of acidity.) Foods that have a high acid content, therefore, do not receive as extreme a heat treatment as low-acid foods. They are heated sufficiently to destroy bacteria, yeasts and molds that could cause food to spoil.
Canners and food safety regulators are most concerned about foods with low acid content, such as mushrooms, green beans, corn, and metas. The deadly Clostridium botulinum bacterium, which causes botulism poisoning, produces a toxin in these foods that is highly heat-resistant. The sterilization process that destroys this bacteria also kills other bacteria that may poison or spoil food. Low-acid canned foods receive a high dose of heat--usually 107 degrees Celsius (250 degrees Farenheit) for at least three minutes. (The amount of time the food is heated, though, depends upon the size of the container and the product.) The canned food is heated in a retort, a kind of pressure cooker.
Another critical element in the canned food process is sealing products in airtight containers. It is essential that air be removed from the container before sealing. Air could cause the can to expand during heating, perhaps damaging the seals or seams of the container.
A telltale sign of loss of this vacuum--and a possibly contaminated product--is a can with bulging ends. If a seal is not airtight, bacteria may enter the can, multiply, and contaminate the product.
Jams and Jellies Canning Information
All jams/jellies contain the four essential ingredients needed to make a jellied fruit product–fruit, pectin, acid and sugar.
Fruit furnishes the flavor and part of the needed pectin and acid. Some irregular and imperfect fruit can be used. Do not use spoiled, moldy or stale fruit.
Pectin is the actual gelling substance and is a naturally occurring carbohydrate in fruit that is concentrated in the fruit's skin and the core. The amount of pectin varies in fruits depending upon the kind of fruit and degree of ripeness. Under-ripe fruits have more pectin; as fruit ripens, the pectin changes to a non-gelling form. Usually using 1⁄4 under-ripe fruit to 3⁄4 fully-ripe fruit makes the best product. Cooking brings out the pectin, but cooking too long destroys it.
When dissolved and let cool, Pectin forms invisible strands that hold liquid in. Acid (such as lemon juice) helps draw even more Pectin out of fruit when it is heated. Water is attracted to sugar. Adding sugar causes some water to be drawn to the sugar molecules, leaving the Pectin molecules free to more easily get at and bind with each other, setting the preserve.
High pectin fruits are apples, crab apples, quinces, red currants, gooseberries, grapes, plums and cranberries. Any citrus fruit peel is also very high in Pectin.
Low pectin fruits are strawberries, blueberries, peaches, apricots, cherries, pears, raspberries, blackberries and pineapple.
Low-pectin fruits should be combined with one of the high-pectin fruits or with a commercial pectin. When a commercial pectin is added, fully ripe fruit can be used.
Acid is needed for gel formation and flavor. The amount of acid in fruits also varies with the fruit and degree of ripeness. When using low-acid fruits, add 1 tablespoon lemon juice or 1/8 tablespoon citric acid for each cup of fruit.
Sugar helps form the gel, serves as a preserving agent, firms the fruit and adds flavor. Beet or cane sugar can be used. Light corn syrup or mild honey can be substituted for part of the sugar. For best results, use recipes that specify honey or corn syrup.
From FDA Consumer, September 1990, by Dale Blumenthal:
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.
The Canning Process
Food-spoiling bacteria, yeasts and molds are naturally present in foods. To grow, these microorganisms need moisture, a low-acid environment (acid prevents bacterial growth), nutrients, and an appropriate (usually room) temperature.
Foods are preserved from food spoilage by controlling one or more of the above factors. For instance, frozen foods are stored at temperatures too low for microorganisms (bacteria, yeasts and molds) to grow. When foods are dried, sufficient moisture is not available to promote growth.
During canning, the food is placed in an airtight (hermetically sealed) container and heated to destroy microorganisms. The hermetic seal is essential to ensure that microorganisms do not contaminate the product after it is sterilized through heating. Properly canned foods can be stored unrefrigerated indefinitely without fear of their spoiling or becoming toxic.
Foods with a naturally high acid content--such as tomatoes, citrus juices, pears, and other fruits--will not support the growth of food poisoning bacteria. In tests, when large numbers of food poisoning bacteria are added to these foods, the bacteria die within a day. (The exact amount of time depends upon the bacteria and amount of acidity.) Foods that have a high acid content, therefore, do not receive as extreme a heat treatment as low-acid foods. They are heated sufficiently to destroy bacteria, yeasts and molds that could cause food to spoil.
Canners and food safety regulators are most concerned about foods with low acid content, such as mushrooms, green beans, corn, and metas. The deadly Clostridium botulinum bacterium, which causes botulism poisoning, produces a toxin in these foods that is highly heat-resistant. The sterilization process that destroys this bacteria also kills other bacteria that may poison or spoil food. Low-acid canned foods receive a high dose of heat--usually 107 degrees Celsius (250 degrees Farenheit) for at least three minutes. (The amount of time the food is heated, though, depends upon the size of the container and the product.) The canned food is heated in a retort, a kind of pressure cooker.
Another critical element in the canned food process is sealing products in airtight containers. It is essential that air be removed from the container before sealing. Air could cause the can to expand during heating, perhaps damaging the seals or seams of the container.
A telltale sign of loss of this vacuum--and a possibly contaminated product--is a can with bulging ends. If a seal is not airtight, bacteria may enter the can, multiply, and contaminate the product.
Jams and Jellies Canning Information
All jams/jellies contain the four essential ingredients needed to make a jellied fruit product–fruit, pectin, acid and sugar.
Fruit furnishes the flavor and part of the needed pectin and acid. Some irregular and imperfect fruit can be used. Do not use spoiled, moldy or stale fruit.
Pectin is the actual gelling substance and is a naturally occurring carbohydrate in fruit that is concentrated in the fruit's skin and the core. The amount of pectin varies in fruits depending upon the kind of fruit and degree of ripeness. Under-ripe fruits have more pectin; as fruit ripens, the pectin changes to a non-gelling form. Usually using 1⁄4 under-ripe fruit to 3⁄4 fully-ripe fruit makes the best product. Cooking brings out the pectin, but cooking too long destroys it.
When dissolved and let cool, Pectin forms invisible strands that hold liquid in. Acid (such as lemon juice) helps draw even more Pectin out of fruit when it is heated. Water is attracted to sugar. Adding sugar causes some water to be drawn to the sugar molecules, leaving the Pectin molecules free to more easily get at and bind with each other, setting the preserve.
High pectin fruits are apples, crab apples, quinces, red currants, gooseberries, grapes, plums and cranberries. Any citrus fruit peel is also very high in Pectin.
Low pectin fruits are strawberries, blueberries, peaches, apricots, cherries, pears, raspberries, blackberries and pineapple.
Low-pectin fruits should be combined with one of the high-pectin fruits or with a commercial pectin. When a commercial pectin is added, fully ripe fruit can be used.
Acid is needed for gel formation and flavor. The amount of acid in fruits also varies with the fruit and degree of ripeness. When using low-acid fruits, add 1 tablespoon lemon juice or 1/8 tablespoon citric acid for each cup of fruit.
Sugar helps form the gel, serves as a preserving agent, firms the fruit and adds flavor. Beet or cane sugar can be used. Light corn syrup or mild honey can be substituted for part of the sugar. For best results, use recipes that specify honey or corn syrup.