Using the proper amount of headspace pays off in quality products.
Adequate headspace is needed so food has enough room for expansion
during heat sterilization.
Generally, more headspace is needed for foods heat-sterilized in a
pressure canner than those in a boiling water canner, because of
increased expansion of foods at the higher temperature.
With too little headspace, liquid is more likely to cook out of the jars
during heat sterilization. This increases the danger of seal failure due
to food particles that may be trapped at the interface of the sealing
surface. With too much headspace, air remaining in the jar can cause
darkening of the food, oxidized flavor, and lower vitamin retention,
especially in food at the top of the jar.
Pressure canning low-acid foods is receiving increased emphasis today,
mainly based on the greater safety of this technique. The method also
offers an additional advantage that is little known to home
canners--improved quality and nutrient retention.
Generally an 18° F (10° C) rise in canner temperatures increases the
destruction of bacteria tenfold. At the same time, chemical changes that
affect color, flavor, texture and nutrients of the food are only
doubled.
This means that for every 18° F (10° C) increase in canning temperature,
bacteria are destroyed five times faster than are vitamins, flavor,
color and texture. Accordingly, in contrast to boiling water canning,
heat sterilizing in a pressure canner at 240° F (116° C) (10 p.s.i.)
destroys bacteria about 17 times faster than chemical changes are
effected, accounting for better nutrient and quality retention of
pressure canned low-acid foods.
Be cautious about interpreting this as an advantage in pressure
sterilizing acid foods. This technique can increase quality and nutrient
losses because of the total time the jars of food will be near or above
boiling water temperature.
For example, with tomatoes it takes about 5 minutes until steam begins
to escape, 10 minutes to exhaust the air, about 5 minutes to build up
pressure, 5 minutes for heat sterilizing at 240° F (116° C) and 30 to 45
minutes cooling before opening the canner. That adds up to about 55 to
80 minutes at temperatures near or above 212° F (100° C), as compared to
the recommended 35 minutes for heat sterilizing in a boiling water
canner. Assuming the average product temperature is elevated above 212°
F (100° C) about half of this total time, the loss in quality could be
at least doubled.
Properly managed heat sterilization has a great deal to do with the
quality of canned food. If the temperature is too high or the time of
sterilization too long, the result is a product with poor color, soft
texture, and less flavor. Tomatoes canned in the pressure canner are an
example.
After heat sterilization, a quality canned food will have liquid
covering the solid food in the jar. The lid must allow air, but not
liquid, to escape during heat sterilization. A low liquid level in the
jar may indicate the lid did not function properly, pressure was allowed
to fluctuate in the pressure canner, or the canner wasn't cooled
completely before opening, or raw pack was used. Foods not covered with
liquid will have poorer quality.
Jars must be air-cooled naturally after heat sterilization to maintain
quality as well as for safety. Delayed cooling in tightly enclosed
areas, such as cardboard cartons, will decrease quality of the product.
Storage conditions are vital to quality of canned foods. If stored in a
light, rather than a dark place, light-sensitive colors will darken
gradually. Some colors, such as carotenoids in tomatoes and carrots will
gradually fade with prolonged storage.
Storage temperatures between 40 to 50° F (4.5° to 10° C), if the spot is
dry, are best for quality retention. Quality losses are increased when
canned foods are stored at higher temperatures. For example, about a
third of the vitamin C is lost if foods are stored a year at
temperatures of 80° F (26° C) or higher.
Storage temperatures between 50° to 70° F (10° to 21° C) are acceptable.
Properly canned and stored foods should be safe to eat for more than two
years. However, for top quality, it is best to can only the amount to be
used in one year.
Top quality home canned foods have rich, jewel-like colors,
characteristic of the food canned. Colors are those of well-prepared
foods ready to be served, rather than fresh uncooked foods or
over-cooked foods. Light-colored foods should retain their color with no
signs of darkening at tops of jars. Fruits should look neither
under-ripe and hard, nor over-ripe and mushy. Vegetables ought to look
young and tender, rather than old and starchy. Vegetables and fruits
should be free of stems, cores, seeds, or pieces of skin, and be of
uniform size, shape and color.