Putting Food By - Janet Greene [22]
From Ruth Klippstein—“Klippstein of Cornell”—concerning material sent out to the county agents in home economics across New York State: “[apropos very old-time instructions from a number of sources] they are either very imprecise, or use what is now an expensive ingredient, or result in a product which by modern standards is not acceptable.”
And from Isabel D. Wolf, of the University of Minnesota, about home-canning recipes: “avoid following the home-canning advice of celebrities, old cookbooks, ‘back to nature’ publications, and out-of-date home-canning leaflets. Some potentially dangerous instructions can be found in old official publications, even those of this state!”
CONVERSIONS FOR CANNING
Do look at the conversions for metrics (with workable roundings-off) and for altitude—both in Chapter 3—and apply them.
The two methods for canning everything else at home are the Boiling–Water Bath and Pressure-processing: the first is for strong-acid foods; the latter, for low-acid ones.
For its effectiveness, canning relies on applied heat and the exclusion of air. Between them, these functions destroy the dangerous, targeted things in the food that cause spoilage or poisoning, and drive out air from the contents; thus is created a condition that will form the vacuum that seals the containers against outside contamination during storage.
How much heat depends on the acidity of each particular food we intend to can, as was discussed at some length in Chapter 2. In our kitchens this means that every carefully prepared food with a pH rating of 4.6 or below may be canned safely in a Boiling–Water Bath at 212 F/100 C at the sea-level zone (which includes up to 1000 ft./305 m; we’ll deal with higher altitudes in a minute). Every carefully prepared food with a rating of higher than 4.6 pH must be processed in a Pressure Canner—which at sea level produces temperatures ranging from 1 degree hotter than the boiling point of water on up to 250 F/121 C, and beyond.
The type of heat is vitally important, because the ability to transfer heat varies between wet heat and dry heat. Illustration: hold your hand in a steady, strong flow of steam from the spout of a teakettle for one minute, and it will be burned enough to blister badly; hold it in the dry air of an oven at the same temperature reading for a minute and it will be pleasantly warmed. To carry the idea further, the 240 F/116 C steam in a Pressure Canner at 10 pounds psig (pounds per square inch by gauge) at sea level has a much greater effect on the heat transfer process than does the atmosphere of an oven operating at 240 F/116 C.
So Never Forget This: only under GREATER THAN ATMOSPHERIC PRESSURE can you produce wet heat that is hotter than the boiling point of water at your altitude.
And you will need wet heat under pressure to reach deep enough inside a container of low-acid food to destroy the spoilers that can make it nasty, even dangerous, to eat.
If you remember this Why, you’ll always be able to keep track of the How in canning food at home.
The rate of heat penetration and the acidity of the food (discussed in Chapter 2) are the criteria for determining the length of time needed to process food safely when it’s canned at home.
How Heat Penetrates a Container of Food
Heat from outside the jar or can comes through the wall of the container and moves deep into the contents either by conduction, i.e., by being passed from one particle of food to another particle next to it; or by convection—being carried, almost swirled, by currents of liquid within the container. Usually both types of heat are involved, of course in varying proportions, when adequate processing occurs.
Conduction
In dense, closely packed food, and with relatively little fluid, heat is passed from the surface of the container inward from top, bottom, and sides, leaving a spot in the center as the last place that will heat. There is extra worry when the food is low in acid. Strained pumpkin and mashed winter squash are extreme examples