Putting Food By - Janet Greene [5]
Processing times vary according to acidity and density of the food concerned. Adequate temperature and length of processing time are given in the specific instructions for individual foods.
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Altitude and Metrics
This chapter is designed to provide the Whys and Hows for all altitude adjustments or metric conversions stipulated in the variety of preserving methods found in the chapters that follow. It is meant to quell any notions that home preservation is at all daunting as far as the numbers and “math” involved are concerned. For your altitude adjustments, especially, refer to the information here as often as is necessary—and make a note of any adjustments/conversions in the specific how-to’s as you go along especially in the Canning chapters (6–12).
CORRECTING FOR ALTITUDE
More North Americans who preserve food in some manner live below 1000 feet/305 meters—in the sea-level zone—than live higher. But there remain a great many householders who, to ensure that their home-preserved foods are safe and appetizing, must compensate for the perceptible decrease in atmospheric pressure as the altitude increases. It’s one thing to have a roast pork undercooked and yet dry; it’s another thing to court food poisoning in a canner-ful of green beans.
Before we go any further, we must consider altitude.
Boiling: What Is It?
Boiling is the process by which a liquid (for our purposes here the liquid is water) becomes a vapor, and this conversion is produced by heat. We put water into a pot, and put the pot over a source of heat, and soon the water boils. In boiling, bubbles of vapor are created at the bottom of the vessel near the heat; and they rise to the surface and break and waft into the air as steam, which is the gaseous form of water. As they rise, the bubbles agitate the water: gently as in a simmer, or violently as in a full boil.
When water is boiling—and by this we mean a thrashing, rolling boil—it looks the same at different altitudes but it is not boiling at the same temperature . The temperature is different because the blanket of air around us, called atmospheric pressure, is lighter when we live higher; and the lighter the air the lower the boiling point of water. At sea level our blanket of air weighs 14.7 pounds per square inch (psi), but at 10,000 feet/3048 meters the atmospheric pressure is only 10.1 psi; this difference of 4.6 psi will be the basis for the special thumbnail tables in the section Canning at High Altitude, coming soon.
However, back to boiling. Water at sea level, and for simplicity’s sake up to 1000 ft/305 m, boils at 212 F/100 C; at 5000 ft/1524 m, boiling point is rounded to 203 F/95 C; and at 10,000 ft/3048 m, water boils at a mere 194 F/90 C. Right away you see that this makes it harder for us to apply a heat treatment that will deal with bad micro-organisms in the food we intend to can. Say that Bacterium xyz is destroyed if the container of strong-acid food (pH of 4.6 or lower) is held at 212 F/100 C for 35 minutes—but we live above the sea-level zone. What do we do?
ALTITUDE AND WATER’S BOILING POINT (WITH METRICS)
We add processing time. Up to a feasible, sensible point, that is.
What if the food is low-acid (pH higher than 4.6) and of course we use a Pressure Canner to achieve the heat needed to destroy Clostridium botulinum spores, but we live well up in the mountains?
We add Pressure.
Time Out for “Simmer” and “Pasteurize”
There’s no easy way to put these below-boiling terms later, so this interruption does get them recorded for future reference, and indexing.
The descriptions of simmer range from the French cook’s “making the pot smile” on up to the one we mean when we say to simmer: which is 185