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Just about anything you would grow in your garden may be preserved by canning. Only a few foods cannot be safely canned at all, some may be canned in one form but not another, and still others don’t hold up well under the prolonged heat of processing. The biggest issue when it comes to home canning is to know which foods may be safely canned in boiling water or steam, and which items must be pressure canned.

The Acid Test

All foods may be categorized as being either low acid or high acid, as measured by pH. In case you missed, or forgot, this part of chemistry, here’s a quick review: The acidity of any substance is measured on a pH scale, where low numbers are high in acid and high numbers indicate low acidity. The letters pH stand for power of hydrogen, so-called because the pH scale is logarithmic (in powers of 10) and measures the concentration of hydrogen ions in a water-based solution.

A pH scale typically runs from 0 to 14, with each number 10 times less acidic than the previous number. Pure water is neutral and has a pH of 7. Working toward lower numbers, a pH of 6 is 10 times more acidic than pure water. Working toward higher numbers, a pH of 8 is 10 times less acidic than pure water. You can see, then, that a substance with a pH of 1 is strongly acidic, while a substance with a pH of 14 is weakly acidic.

In terms of canning, the line is drawn not at the neutral pH of 7, but at the acidic pH of 4.6. Any food that is below 4.6 is considered high acid, while any food that is above 4.6 is considered low acid. This distinction is important because high acid foods may be safely canned in boiling water or steam, while the only safe way to can low acid foods is under pressure. (These three canning methods—boiling water, steam, and pressure—will be discussed in detail in the next three installments of this series.)

As a general rule, most fruits are high acid foods, while most vegetables, as well as all meats, including poultry and seafood, are low acid foods. Any mixture that combines high acid and low acid foods is considered to be low acid unless it is acidified through the addition of a sufficient amount of vinegar, lemon juice, or citric acid. Low acid foods also may be acidified through pickling or fermenting, as in the case of pickled beets or sauerkraut.

The low pH of high acid foods is sufficient to prevent the growth of Clostridium botulinum, the bacteria responsible for botulism poisoning. C. botulinum spores grow in low acid foods in the absence of oxygen, and are not killed at 212ºF, the normal temperature of boiling water or steam. They are, however, killed at a prolonged temperature of 240ºF, which may be achieved only in a pressure canner.

Although low acid foods may not be safely canned in boiling water or steam, all foods may be safely canned under pressure. The reason high acid foods are not typically canned under pressure is that the total processing time is longer, which may cause deterioration in fruit texture. Many home canners choose to process only high acid foods, because water bath canning and steam canning are easier and faster than using a pressure canner.

Don’t Can

The list of things you shouldn’t can is pretty short and amounts to common sense. For instance, you don’t want to can overripe fruits. Acidity decreases as fruits ripen, and overripe fruit may not be acidic enough for safe water bath or steam canning. Further, overripe fruits tend to become bruised, moldy or damaged and therefore may contain microorganisms that make them unsafe for canning.

A notice in a local paper once told of two elderly sisters who died from botulism poisoning after eating their own home canned peaches. For many years I wondered how peaches, a high acid fruit, could develop botulism. I now realize those peaches must have been well overripe, and possibly bruised or otherwise damaged, to the point that their acidity fell below the level required for safe water bath canning.

Another thing not to can is any densely packed food. Even in a pressure canner, heat may not fully penetrate throughout the contents of the jar, making the food unsafe for pantry storage. Examples of densely packed foods include mashed items such as pumpkin, winter squash, potatoes, or parsnips; pumpkin butter; refried beans; and pâté. Among fruit purées, the following are not recommended for canning because safe processing procedures have not been developed: Asian pear, banana, cantaloupe and other melons, coconut, fig, ripe mango, papaya, tomato. While many of these foods may not be safely canned in mashed or puréed form, most of them are perfectly safe to can as chunks covered by liquid.

Some vegetables don’t maintain quality when pressure canned, so are best preserved by pickling. These include artichokes, broccoli, Brussels sprouts, cabbage, cauliflower, cucumbers, eggplant, summer squash,  and olives.

Dry pack canning shelled nuts is no longer recommended. Condensation that may develop inside the jars could prove potentially hazardous.

Years ago, an Extension home economist developed specific recipes for cake in a jar, and the idea took off. Today the internet is teeming with directions for baking cake or pie in canning jars, then sealing them as soon as they come out of the oven. USDA canning experts frown on this practice, because you can’t be sure the cakes or pies are entirely free of bacteria when the jars are sealed.

Oils and fats must be handled carefully. Poultry and other meat should have as much fat removed as possible before being canned. Melted fat tends to float in the jars and may interfere with sealing. Oils that are not heat-tolerant become unhealthful when heated, and all oils are low in acid, therefore canning flavored oils is not recommended. Approved recipes for such things as marinated peppers or mushrooms contain oil, but are also acidified with vinegar and lemon juice, making them safe for water bath canning.

High on the don’t-do list are using canning instructions from outdated sources and making up your own recipes for canning. Do not can any food item for which you cannot find a recently tested recipe published by a reliable source. Such sources include your local county Extension office, the National Center for Home Food Preservation (nchfp.uga.edu), the 2015 edition of USDA Complete Guide to Home Canning (nchfp.uga.edu/publications/publications_usda.html), and the 2015 edition of Ball Blue Book Guide to Preserving (available at www.freshpreservingstore.com). These sources publish scientifically tested directions for canning specific foods using precise processing methods. For safety’s sake, follow instructions exactly as they are published.

A Special Case for Tomatoes

Both heirloom and hybrid tomato varieties have a wide range of pH values, and within each variety acidity varies with growing conditions and stage of maturity.

Tomatoes, tomato juice, and other tomato products are the most popular foods for home canning. And they are borderline when it comes to pH. Traditionally, tomatoes have been classified as a high acid food, making them safe for processing in a water bath or steam canner. When grown under normal conditions and harvested at optimal ripeness, most tomatoes have a pH below 4.6.

However, acidity can vary quite a bit from one variety to another. In a list I compiled of pH values derived from four reliable sources, among 118 varieties the pH ranged from a low of 3.70 for Celebrity to a high of 5.20 for Super Marzano. Among all the hybrid varieties tested, 66 percent had a pH above 4.6, compared to the heirlooms with only 8 percent having a pH above 4.6. Factoring in heirlooms with borderline values, like 4.56, ups the percentage to 15 percent. According to these statistics, hybrid varieties tend to be less acidic than heirlooms.

A few of the same varieties were tested by more than one source, and it’s interesting to compare the results. The hybrid Celebrity had a pH of 3.70 in one study at the University of Utah, and 3.92 in a follow-up study the next year, while in a study at North Dakota State University the same variety had a pH of 4.93. The heirloom Opalka had a value of 4.51 in a study at the University of Illinois, but 5.08 at North Dakota State. The heirloom Super Italian Paste had a pH of 4.33 at the University of Illinois, but 5.06 at North Dakota State. Why the discrepancy?

Many conditions in the garden affect a tomato’s acidity. The acidity varies as a tomato matures, being highest in unripe tomatoes and decreasing as the tomato ripens. Acidity may also be influenced by growing conditions such as extreme heat or excessive moisture, and by bruising, cracking, or other damage to the fruit including from frost, insects, or blossom end rot. Tomatoes that grow in shade or are ripened off the vine are less acidic than the same tomatoes when vine ripened in full sun. Tomatoes picked from dead vines are lower in acid than tomatoes from healthy vines. Combining different tomato varieties having different pH values affects the total pH of tomatoes in the canning jar.

Since you can’t determine a tomato’s acidity by looking at it or tasting it, the USDA recommends adding ¼ cup vinegar, 2 tablespoons lemon juice, or ½ teaspoon citric acid to each quart jar before processing. Of these options, citric acid (usually available where canning jars are sold) is least likely to adversely affect flavor, although in all cases the acidified tomatoes, and any dish made with them, will be quite puckery. The recommended remedy is to add sugar to each jar or to the recipe in which acidified canned tomatoes are used. Since no method has been tested and approved for pressure canning tomatoes without acidification, the USDA recommends acidifying even tomatoes processed in a pressure canner.

Any recipe that combines tomatoes with other foods—for example, salsa or spaghetti sauce with meat—increases the total pH. Such recipes must always be processed in a pressure canner.

Examples of High Acid and Low Acid Foods

High Acid (PH < 4.6)

• Apples
• Apricots
• Berries
• Cherries
• Cranberries
• Fruit Juices
• James & Jellies
• Oranges
• Peaches
• Pears
• Pickles
• Pineapple
• Plums
• Rhubarb
• Sauerkraut

Low Acid (PH > 4.6)

• Beans
• Beets
• Carrots
• Corn
• Greens
• Meat & Poultry
• Mushrooms
• Okra
• Onions
• Peas
• Peppers
• Potatoes
• Pumpkin
• Soups & Stews
• Spinach

CANNING CODE:

ACIDIFIER—An acidic ngredient such as citric acid, lemon  juice, or vinegar added to decrease a food’s pH to below 4.6, making it safe for water bath canning.

BOILING WATER CANNING—Processing jars of food surrounded by boiling water; also called water bath canning.

DRY PACK—Food processed in jars without added liquid.

HIGH ACID FOOD—Any food having a pH less than 4.6.

LOW ACID FOOD—Any food having a pH of 4.6 or more.

PH—A measure of acidity in which low numbers indicate higher acidity and high numbers indicate lower acidity.

PRESSURE CANNING—Processing jars of food surrounded by pressurized steam.

STEAM CANNING—Processing  jars of food surrounded by steam at ambient pressure.

WATER BATH CANNING—Processing jars of food surrounded  by boiling water; also called boiling water canning.