Putting Food By - Janet Greene [58]
The average bushel of tomatoes weighs about 53 pounds/25 kilos, and will yield from 15 to 20 quarts when simply cut up and canned; it takes 2½ to 3 pounds of fresh tomatoes to do 1 quart. The yield in jars will be smaller, though, if the fruit is a modern hybrid that is heavy on flesh and light on juice. Since no water may be added to this pack—doing so will reduce the acidity of the pack to a dangerous degree—some of the tomatoes must be juiced beforehand to provide extra canning liquid for filling the jars. Some householders even have on hand an emergency outsize can or two of commercially canned tomato juice at the height of the tomato season.
And be picky if you’re buying from a farmers’ market or a produce stand. Sometimes poor cull fruit is advertised as “canning tomatoes.” But tomatoes that show any sign of rot or spoilage must never be canned.
Tomatoes’ third great virtue is still the relative ease with which they’re canned.
Adding Acid—Why and Which
Nowadays home-grown tomatoes often nudge the 4.6 pH cutoff, beyond which they are considered low-acid, and must be Pressure-processed accordingly. To ensure that they are satisfactorily within Boiling–Water Bath range at sea level, the USDA stipulates adding any one of three acid substances: powdered citric acid, OR bottled lemon juice (which always is a guaranteed 5 percent acidity), OR distilled white vinegar (which you use twice as much of as lemon juice, because vinegar is so much more volatile). Any of these acids may be added first at the bottom of the pack, or at the top; the swirl of the contents during processing will distribute it, along with the optional salt. Traditionally ½ teaspoon canning salt was added at the top of a packed pint, 1 teaspoon atop a quart; the salt is merely a flavor-enhancer, though, and often is omitted by today’s home-canners.
Word-Play and Some History
The introduction of new tomato varieties coincided with tragedy in 1974, and the result was alarm about canning tomatoes in the accepted way.
Seedsmen had established new strains of table tomatoes—fat, meaty, sweet varieties that were grown to be served sliced to garnish a juicy cut of steak. Less juice, fewer seeds, above all a new sweetness: these were the selling points. The public tended to equate sweetness with less acid, but this assumption was not wholly justified because hybridizers often were breeding for special sweetness, rather than for notably less acidity. The relationship will be underscored in Chapter 19 dealing with pickles and other tart things. In any case, too much tartness must never be offset by reducing the vinegar that makes the product safe, but rather by increasing the sweetness with additional sugar.
The new tomato varieties might have entered the scene without particular fanfare if the Centers for Disease Control of the U.S. Public Health Service had not dropped a bomb: it reported that early in 1974 there were two deaths from botulism poisoning traced directly to home-canned tomatoes and tomato juice. Consumer activists immediately queried if the “new, low-acid” tomatoes were responsible, and even suggested in print that, from then on, tomatoes would need to be Pressure-processed at 15 psig instead of in a Boiling–Water Bath.
Meanwhile the public health officers discovered what actually allowed the spores of C. botulinum to make the toxin that killed the victims. Common bacteria or molds grew in the food in the jars and thereby reduced the acidity because the natural acid in the tomatoes was metabolized by the micro-organisms as they grew and developed. It was established after compassionate, but thorough, investigation that these bacteria or molds survived either because the tomatoes were canned by the discredited open-kettle method, or entered under the lid of a jar that wasn’t adequately sealed. Inadequate processing is virtually always the cause of food poisoning that develops during shelf life.
. . . But the Care Never Varies
No matter which strain of tomato you can in which form—whole, stewed,