Four kinds of things cause spoilage in preserved food:
(1) enzymes, which are naturally occuring substances in living tissues, and are necessary to complete growth and reproduction; and three types of micro-organisms.
(2) molds,
(3) yeasts,
(4) bacteria. All these micro-organisms are present in the soil, water, and air around us. They can be controlled adequately by care in choosing ingredients, scrupulous cleanliness in handling them, and faithful good sense in following the established safe procedures for putting food by at home.
How Enzymes Act
Nature has designed each plant or animal with the ability to program the production of its own enzymes, which are the biochemicals that help the organism to ripen and matu — in short, enzymes promote the organic changes necessary to the life cycle of all growing things. However, their action is reversible: they can turn around and cause decomposition, thereby causing changes in color, flavor, and texture, and making food unappetizing.
Their action slows down in cold conditions, increases most quickly between around 85 to 120 degrees Fahrenheit/29 to 49 degrees Celsius, and begins to be destroyed at about 140 F/60 C. However, the natural enzymes in some foods, notably cucumbers, are especially tough. Thus, in making pickles, the heat resistance of enzymes is as much of a concern as the heat resistance of bacteria, molds, and yeasts, all of which are controlled not only by the heat of the processing bath but also by the acid of the vinegar-loaded pickling solution.
How Molds Act
Molds are microscopic fungi whose dry spores (seeds) alight on food and start growing silken threads that can become slight streaks of discoloration in food or cover it with a mat of fuzz.
It used to be felt that “a little mold won’t hurt you,” but modern research has disclosed that only the mold introduced deliberately into the “blue” cheeses like Roquefort, Gorgonzola, or Stilton is trustworthy. The others, as they grow in food, are capable of producing substances called mycotoxins, and some of them can be hurtful indeed.
In addition, molds eat natural acid present in food, thereby lowering the acidity that is protection against more actively dangerous poisons—but we’ll have more to say about this in a minute.
Molds are alive but don’t grow below 32 F/Zero C; they start to grow above freezing, have their maximum acceleration at 50–100 F/10–38 C, and then taper off to inactivity beginning around 120 F/49 C; they die with increasing speed at temperatures from 140–190 F/60–88 C.
How Yeasts Act
The micro-organisms we call yeasts are also fungi grown from spores, and they cause fermentation—which is delightful in beer, necessary in sauerkraut, and horrid in applesauce. As with molds, severe cold holds them inactive, 50–100 F/10–38 C hurries their growth, and 140–190 F/60–88 C destroys them.
How Bacteria Act
Bacteria also are present in soil and water, and their spores, too, can be carried by the air. But bacteria are often far tougher than molds and yeasts are; certain ones actually thrive in heat that kills these fungi, and in some foods there can exist bacterial spores which can make hidden toxins. These spores will be destroyed in a reasonable time only if the food is heated at 240–250 F/116–121 C — at least 28 degrees higher than the boiling temperature for water at sea-level classification, and obtainable only under pressure.
Of the disease-causing bacteria we’re concerned with mainly, the most fragile are members of the genus Salmonella, which are transmitted by pets, rodents, insects, and human beings, in addition to existing in our soil and water. Salmonellae live in frozen food, are inactive up to 45 F/7 C, and are killed when held at 140 F/60 C for 3 minutes, with destruction almost instantaneous at 165 F/74 C.
The transmittable bacteria that cause “staph” poisoning, the Staphylococcus aureus, are responsible for the most common type of food- borne illness, the sort formerly attributed to “ptomaines.” The prevalence of the poisoning is traced to the fact that most meats and dairy foods carry the bacterium but its presence is not known unless the foods are allowed to sit unrefrigerated at room temperature; it is specially widespread during the summer, when picnic food is spread out, and it is found in meats kept warm for serving to a crowd. The staph bacteria are halted in their tracks fairly easily at temperatures that kill Salmonellae. However, their toxin is destroyed only by many hours of boiling or 30 minutes at 240 F/116 C; the growth of the bacteria themselves is checked if the food is kept above 140 F/ 60 C or below 45 F/7 C.
Most dangerous of the bacteria is the tough Clostridium botulinum, which deserves a section of its own.
Botulism
The scientific books describe C. botulinum as a “soil-borne, mesophilic, spore-forming, anaerobic bacterium.” Which, translated into everyday language, means that it is present in soil that is carried into our kitchens on raw foods, on implements, on clothing, on our hands — you name it. Next, it thrives best in the middle range of heat — beginning at about room temperature, 70 F/21 C, on up to 110 F/43 C. Next, it produces spores that are extremely durable: whereas the bacterium is destroyed in a relatively short time at 212 F/100 C, the temperature of briskly boiling water at sea level, the spores are not destroyed unless they are subjected to at least 240 F/ 116 C for a sustained length of time. And finally, the bacterium lives and grows in the absence of air (and also in a very moist environment; these combined conditions exist in a container of canned food).
This description does not mention the poison thrown off by the spores as they grow: the toxin is so powerful that one teaspoon of the pure substance could kill hundreds of thousands of people.
The grave illness caused by eating toxin present in preserved food is comparatively rare — rare in relation to the cases of “staph” or salmonellosis — but, unlike them, it is often fatal unless life - support care is given right away. The symptoms are blurred vision, slurred speech, inability to hold up the head, and eventual respiratory arrest unless the victim is given help to breathe until the body can reverse the progress of the illness with the aid of medication. Between 1899 and 1949 the case-fatality rate of food- borne botulism was 60 percent. Since then it has declined markedly and steadily, thanks to quicker diagnosis and great improvements in intensive care at the outset. Th e case rate took a brief upward spurt in the mid-1970s however, because a whole generation of people went in for food preservation, especially home-canning, without having the information or equipment needed for a safe product.
The botulinum toxin can be destroyed by brisk boiling in an open vessel. This is why, throughout this book, we warn people to boil hard — for the time demanded by its density and acidity — any home-canned food about which there is the slightest safety problem.
C. botulinum is held inactive at freezing and comes into its own at room temperatures, as mentioned earlier. Type E, the comparatively rare food-borne strain that is found on sea- and freshwater seafood, begins to grow at refrigeration temperatures.
As with all micro-organisms, a moisture content of below 35 percent directly inhibits its growth.
DEALING WITH THE SPOILERS
Enter pH—the Acidity Factor
The strength of the acid in any food determines to a great extent which of the spoilage micro-organisms can grow in each food. Therefore acidity is a built-in directive that tells us what temperatures are necessary to destroy these spoilers within it, and make it safe to eat. (Heat alone, not natural acidity, controls the action of enzymes.)
Acid strength is measured on the pH scale, which starts with strongest acid at 1 and declines to strongest alkali at 14, with the Neutral point at 7, where the food is considered neither acid nor alkaline. The pH ratings appear to run backward, since the larger the number, the less the acid, but it may help to think of the ratings as like a “top-ten list”: number one is the highest.
The term “pH” is an abbreviation in chemistry for “potential of hydrogen,” traced by a sophisticated laboratory device. Crude indications can be made by litmus-like test papers available at scientific supply houses, which can be found on the internet or in the Yellow Pages.
pH RATINGS
This listing indicates in a general way the natural acid strength of common foods on the pH scale. Different varieties of the same food will have different ratings, of course, as will identical varieties grown under diff erent conditions. The Food and Drug Administration now uses a pH rating of 4.6 as the dividing point that dictates the preserving techniques to use: most notably, which canning method is right. Thus foods rated 2.2 (some lemons) on up to 4.6 (virtually every tomato) PFB¹ calls strong- acid—inserting “strong” as a sharper workable distinction than merely “acid,” the customary designation. Foods over 4.6 up to Neutral 7 are always specifi ed as low- acid. Foods over Neutral are alkaline. All low-acid and alkaline foods must be pressure-canned.
¹ PFB means "Putting Food By", the book.
Lemons 2.2–2.8
Rhubarb 3.2–3.4
Gooseberries 2.8–3.0
Raspberries 3.2–3.9
Grapes 2.8–3.8
Pineapple 3.2–4.0
Plums 2.8–4.4
Sauerkraut 3.3–3.6
Grapefruit 3.0–3.7
Apples 3.3–4.0
Strawberries 3.0–3.9
Peaches 3.3–4.0
Blueberries 3.1–3.3
Apricots 3.3–4.8
Mangoes, ripe 3.4–4.8
Spinach 5.5–6.8
Pears 3.5–4.6
Beans, string,
Oranges 3.7–4.3
Cherries 3.8–4.5
Oysters 5.7–6.2
Blackberries 3.9–4.5
Melons 5.7–6.7
Tomatoes 4.0–4.6
Salmon 5.8–6.4
Persimmons 4.4–4.7
Tuna fish 5.9–6.1
Pimientos 4.4–4.9
Mangoes, green 5.8–6.0
Peppers, green, bell 4.6–5.4
Carrots 5.9–6.4
Pumpkin 4.9–5.0
Corn 5.9–7.3
Figs 4.9–6.0
Asparagus 6.0–6.7
Papaya 5.2–6.0
Mushrooms 6.0–6.7
Squash 5.2–6.2
Meats 6.0–6.9
Cabbage 5.2–6.8
Hominy 6.0–7.5
Sweet potatoes 5.3–5.6
Peas 6.2–6.9
Turnips 5.3–5.9
Milk, cow’s 6.4–6.8
Beets 5.3–6.0
Beans, Lima 6.5
Potatoes, white 5.4–6.0
Shrimp 6.5–7.0
The Importance of Sanitation
Even though the spoilage micro-organisms in a food are rendered inactive by cold, they kick in with a vengeance as soon as they warm up.
Or, adequately preheated food can be contaminated by air-borne spores while it, or its unfilled container, stands around unprotected; and, unless it is cooled quickly or refrigerated, the new batch of spoilers can start growing— and growing fast. In this connection, it’s interesting to trace back the notion that one must not put hot food in a refrigerator in order to cool it quickly. Th is idea is a holdover from the days of the wooden ice chest, which was kept cool by a big block of ice: of course the hot food warmed the inside of the ice chest, and there was a long lag before the ice (or what was left of it) could reduce the internal temperature of the cabinet to a safe-holding coolness. With modern refrigerators, though, a container of warm food merely causes the thermostat to kick on, and the cooling machinery goes to work immediately.
Another good way to deal with the spoilers is simply to wash them off the food as carefully as possible, and to keep work surfaces and equipment sanitary at every stage in the procedure. Food scientists refer to the “bacterial load” in describing the almost staggering rate of increase in bacteria if the food is handled in an unsanitary manner or allowed to remain at the optimum growing temperature for the spoilers. The procedures described in this book have been established after years of research by food scientists, and naturally there is an allowance made for extra micro-organisms that must be destroyed. But in many cases the food would have to be processed almost beyond palatability if the bacterial load had been allowed to increase geometrically to an enormous extent; the alternative of course would be that the treatment was not intensified, so the food spoiled after all.
Household Disinfectants
Boiling water can destroy many organisms if it is indeed boiling and if it has long enough contact with a contaminated surface after it has dropped a hair below the boiling point. Cleaning work surfaces with boiling water is a cumbersome exercise, though; better to use a good household disinfectant, which, for our money, is the liquid chlorine bleach on hand in most kitchens.
Chlorine dissipates aft er a while: when you can smell it in the air, it is dissipating. (The best way to rid big-city tap water of too much chlorine is merely to draw a pitcher/bottle of water and let it sit uncovered in the refrigerator for several hours.)
The carbolic-acid-based disinfectants, generally pine- scented, and always milky in solution, are likely to leave their flavor on surfaces.
Heat + Acidity = Canning Safety
Combining (1) the temperatures that control life and growth of spoilage micro-organisms with (2) the pH acidity factor of the foods that are particularly hospitable to certain spoilers, gives the conscientious home-canner this rule to go by: it is safe to can strong-acid foods at 212 F/100 C (at sea level to 1000 feet) in a Boiling–Water Bath, but low-acid/non acid foods must be canned at 240 F/116 C (again, at sea-level zone) — a temperature possible only in a Pressure Canner at 10 pounds pressure — if they are to be safe.
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.
By Janet Greene, Ruth Hertzberg and Beatrice Vaughan in "Putting Food By", Plume Book (Penguin Group), USA,2010, excerpts pp. 3-10. Adapted and illustrated to be posted by Leopoldo Costa.