Canning Safety

(Thinking of preserving some fruit for Holiday gifts? Whether you preserve it using sugar or honey and spices like cinnamon, or preserve it with liquor, there's a science behind it all.
Originally published at suite101, fall, 2011;

Safe canning – and safe cooking too, even when you're not canning – involve increasing a food's acidity (that is, lowering its PH) as well as the food's temperature. Some grain alcohols, including sherry, port, and whiskey, have also been used in food preservation

Besides acetic acid, you can add sugar or honey to food to preserve it. These like salt, dry out foods, soaking up the water in them. Whiskey likewise has a "drying" effect, where water is replaced with alcohol. The less water the less room for microbes. Note that salt itself, traditionally used to cure meat and fish which may not be cooked, is not that useful in conjunction with cooking, since salt can actually increase microbes' heat resistance.

Finally, herbs or spices, including cinnamon, cumin, garlic, mustard, oregano, sage, and thyme, all of which are popular in curries and barbecues, are also useful in preserving food.

Grain Alcohol in Canning

Like vinegar, grain alcohol is an acid. Grain alcohol makes proteins coagulate, and thus kills microbes. Finally, as noted, alcohol has a "drying" effect on foods. Grain alcohol can substitute for some vinegar in cooking, and works quite well if used along with vinegar.

In general at least 80 proof (40%) but not more than 140 proof (70%) alcohol is needed to kill microbes, although 15-to-20 percent alcohol (30-to-40 proof) can suffice for preserving fruits. Unlike acetic acid, grain alcohol can even kill salmonella effectively, provided that alcohol concentration is above thirty percent (60 proof). Alas, even half of that (15% alcohol or 30 proof), which is the minimum alcohol content used in preserving fruits, may be too much for the digestive system if much of the preserved food is consumed.

Alcohol and the Gut

Rubbed on the skin, grain alcohol does no damage as it kills microbes. However most grain alcohol damages the gut membrane, where it kills good bacteria along with microbes. Wine, which permits the growth of good bacteria, is the exception, but few wines make for good antimicrobials. Grain alcohol can also interfere with the production of gastric acid used in digestion.

It takes a while to cook that alcohol down. Some alcohol (which has a lower boiling point than water) will of course evaporate during cooking, even pressure cooking, but since pressure holds liquid in, not all alcohol will evaporate without overcooking the food, even dried beans (which tend to require more cooking than most fruits and vegetables; for more about cooking beans, see my "Beans About Beans"). According to Riaz's "Alcohol: the Myths and Realities," it takes an hour of normal cooking to evaporate alcohol to a quarter of its original concentration.

A Little Whiskey or Wine?

So do not overdo the spirits when you can food. Nevertheless, following Miss Vickie's suggestion (at "Miss Vickie's Pressure Cooker Recipes"), you still can "infuse" those beans with a bit of whiskey when cooking them. Besides adding flavor, this may help reduce some microbes as the beans cook, with the whiskey then cooking down to a reduced toxicity. Unlike whiskey, wines, other than port and sherry, are not effective at killing microbes.

Acidity and Acetic Acid

The PH of food you can should be 4.4 or below, with at least a 0.5%, or ideally, a 1% or more concentration of acetic acid (found in balsamic or other vinegars, or lemon juice; vinegar is about 5% acetic acid; since acetic acid boils at a bit higher temperature than water, the concentration of acetic acid may increase very slightly during cooking). You should measure the PH before canning food, making sure it's 4.4 or below. When you add vinegar, or lemon juice, the acetic acid in these penetrates microbes' cells. In proper concentrations, acetic acid will kill almost all yeast microbes according to research conducted by Sun-Young Lee for the Washington State University Department of Food Science (2004). Its effects on bacteria however vary.

Acetic acid is generally a more effective microbicide than hydrochloric acid. Benzoic and sorbic acids (note that "sorbic acid" is not the same as "absorbic acid" or vitamin C) may be more effective still (both sorbic and benzoic acid – the latter is discussed below – occur naturally in some foods; a synthetic preservative, sodium benzoate, is used instead of benzoic acid in commercial canning). The sorbic acids are more effective than acetic acid at higher PH levels (above 5), according to Lee. Acetic acid also has a tenderizing effect on meat.

Alas: acetic acid, like lactic acid, while it can reduce the amount of salmonella in food, may not be that effective at doing so, according to Consansu and Ayhan's (2010) research. Heat is also more important than acidity in preventing botulism, according to Chowhound. Thus cooking is needed, along with vinegar.

Honey, Sugar, and Salt

Honey, sugar, and salt all dry out food. Only honey however provides nourishment for good bacteria and it may thus be one of the best preservatives. However it's not recommended for consumption by very young children (under a year of age), because honey itself can cause botulism in young children. (Sufficient pressure cooking however may make honey safe for infants, according to Momtastic's blog, which also suggests maple syrup as a possible alternative.)

Salt reduces oxygen activity in water as well as the water activity itself, thus reducing aerobic bacteria in addition to drying out food. Salt increases the effects of acetic acid, according to Sun-Young Lee, but, as noted above, not those of heat, so while salt helps with uncooked foods, it may not be a help in reducing microbe activity when you do cook.

In any case, it takes a lot of salt to reduce the water activity by much, so you don't really get any results from salt without a concentration of at least seven percent salt. It takes a concentration of 16-to-17 percent salt in water to reduce water activity to 92 percent of what it was. Of course, as water evaporates in cooking, the concentration of salt increases, but, again as noted, it may make bacteria less heat-sensitive. Finally at least one bacteria, salmonella, likes salt.

Herbs and Spices

According to Chemistry and Society, allicin, in garlic, has antimicrobial properties, as does allyl isothiocyanate, in mustard; also thymol, in thyme, oregano and sage, is an antimicrobial. Cumin and tumeric both contain curcumin, which has some anti-microbial properties too. Finally cinnamon contains benzoic acid, still another preservative, one perhaps more effective that acetic acid in some instances. No wonder these spices are popular in currying and barbecuing!

Benzoic Acid, Sodium Benzoate

Benzoic acid occurs naturally in fermented dairy products, cinnamon, and cranberries. Sodium benzoate, a synthetic product, because it's water soluble (benzoic acid on the other hand is only slightly water soluble) and also because it's effective in less acidic environments, is used instead of benzoic acid in commercial food preservation.

There are many unknowns about the long-term effects of both of these. Both of these need to be processed by the liver. There the toxins are converted into more harmless acids. Processing however can use up the amino acid glycine (an amino acid which the body normally synthesizes from other amino acids although some foods contain it).

Does this mean you should not consume foods such as cranberries, which naturally contain benzoic acid? Probably not; but it might just mean that you should limit your consumption of foods and beverages, such as soda pop, where these have been added. Studies reviewed by the World Health Organization ("Benzoic Acid and Sodium Benzoate," 2000) suggest that one of the top consumers of sodium benzoate, if not the top consumer, is the U.S., where it's consumed primarily in the form of soda pop.


Many but not all microbes are killed when temperatures reach 185 degrees f or above. Some microbes live at higher temperatures however, and for this reason pressure cooking is safest. When pressure is increased, water remains liquid at higher temperatures than 212 f (100 degrees C, its normal boiling point). Its boiling point goes to 240 degrees or above under roughly 11-to-15 lbs of pressure.

While cooking very acidic foods (apples and many other fruits, tomato sauces, eggplant, and, because of the added vinegar, pickles) at normal pressures and canning them at 185 degrees may be sufficient, pressure cooking is still recommended. Pressure cooking (cooking, not boiling) at around 240 degrees f is absolutely essential for less acidic foods, and for all meat. One popular way to do this is to cook foods first, place in clean jars, place the lids on the jars and loosely screw on the bands, and then immerse all for about ten minutes in a water bath in a pressure cooker.

Heat and Vinegar

Because acetic acid actually increases the heat sensitivity of many but not all bacteria, heat is an especially good partner for vinegar in canning (unfortunately acetic acid does not increase the susceptibility of yeasts or molds to heat; for more information on bacteria and heat, see "Resistance," below). Thus, although pressure cooking can reduce the need for an acid PH, when canning, you'll have a greater effect from heat if you add enough vinegar and lemon juice too.


Sun-Young Lee (citing Ryu and Beuchat, 1998; and Buchanan and Edelson, 1999), warns home canners (and consumers of canned goods) that, "[e]xposure to acidic environments . . . significantly increased the heat tolerance of various strains of E. coli O157:H7," particularly in milk and chicken broth, but not in dried beef powder, and not in apple juice. This is a reminder that canning meat or meat broths can be risky while canning things like apples is a bit less so. (Is it just the PH of the apples, or something else, that prevents E. coli from becoming heat resistant?)


Clostridium botulinum, the extremely toxic bacteria that causes botulism, grows in improperly processed home-canned food. Meat is especially difficult to can. Perhaps some mincemeat recipes, which combine meat with brandy or sherry, fruit, and sugar, if pressure-cooked (for example, Anju Mootilal's "Hearty Mincemeat") may can and store safely. (Prior to canning yourself, you might also check the information provided by the U.S. Department of Agriculture.)