Two methods of canning are commonly used both in the household and in the canning factory, the open-kettle method and the cold-pack method.

The open-kettle method is so called because the food to be canned is completely cooked in a kettle and then poured into the jar. Unless the jar, the cover, the rubber, and all utensils that come in contact with the food have been boiled for twenty minutes before the jars are filled, and unless the work is carefully done, there is always the risk that the food will be reinfected and that it may spoil after the jar has been sealed. For some products, such as preserves, conserves, jams, and marmalades, for which condensation and heat more intense than that of boiling water are needed, the open-kettle method must still be used. For beets, the open-kettle method is recommended, because the skins can be removed after the cooking, and thus less color is lost. Many persons prefer the open-kettle method for canning strawberries and tomatoes.

In the so called cold-pack method, the uncooked or partly cooked fruit, vegetable, or other food is packed in a can; the food is covered with some liquid, such as water, sirup, or juice; and both the jar and its contents are heated simultaneously in boiling water or steam. This method may be used for most fruits and all vegetables. It is recommended for meats, because it conserves the flavor and because meat may thus be canned under steam pressure. It is used by all canning factories for simple canned vegetables, fruits, and meats. It is also being adopted gradually by housekeepers, since in general it is a safer, easier way of canning most foods than the old open-kettle method, and since the product keeps much of its natural flavor.

Heat may be applied in the cold-pack method in one of two ways:

1. Continuously. (a) The cans may be covered with boiling water and may be cooked continuously for a given length of time and sealed. (b) The cans may be placed in a pressure canner and cooked under steam pressure for a given length of time and sealed.

2. Intermittently. The cans may be covered with boiling water and may be cooked for a stated length of time on each of three successive days, being sealed at the close of each period of heating.

When food is cooked in the can for a given period on each of three successive days, the process is called intermittent heating.

Acid or sweet foods may be canned safely by one period of heating. It is not always safe to can certain non-acid vegetables by a single period unless steam pressure is used. They may keep if cooked long enough but there is always a risk. This is because of the possible presence on them of resistant spores which are sometimes able to survive even twelve hours of continuous boiling. The intermittent method seems to be safer for these vegetables, but only the steam pressure method can be considered wholly safe. Many vegetables accepted by the housekeeper as having kept are poor in both flavor and odor, and this is due to spoilage. Food that has kept should not have either a flat or a sour taste.

Experiments in canning seem to indicate that the organism known as botulinus may not be destroyed either by the continuous or the intermittent methods of canning in hot water. When foods are canned at home, unless the pressure cooker has been used, it is urged that when the contents are removed from the can they be boiled for several minutes before being eaten. If, for example, beans are to be used as a salad, they should be boiled first and then cooled. Such a procedure makes for safety.

For peas, beans, corn, asparagus, greens, pumpkin, and squash, the intermittent method of canning is strongly recommended, unless steam pressure is used. Many canned foods, even non-acid ones, may keep without intermittent heating if the time of boiling is long enough, but there seems to be no certainty about it. Variations in soil, moisture, and climatic conditions from year to year may cause a failure one year when success has always before attended the one-period process of cooking. Even with the three periods of heating, there is risk of spoilage, if the first period has not been long enough or if too long a time elapses between the first and second periods.

The intermittent method is as follows: After a food is canned, boiled, and sealed, it is allowed to stand for twenty-four hours at room temperature. This gives time for spores to change to the vegetative form which they will do when warmth and food are supplied. The canned food is then cooked a second time in order to destroy the vegetative forms, and, still sealed, is allowed to stand for twenty-four hours at room temperature. Since all spores may not have changed during the first twenty-four hours, it is safer to allow a second twenty-four hours of standing and a third period of cooking.

The high temperatures or repeated heatings used in the canning of vegetables are not only troublesome and expensive, but often injurious to the flavor and texture. Experiments at the University of California * have developed a very simple and certain way of sterilizing vegetables as easily as fruit. This con-* Cruess, W. V. Circ. 158, Agr. Exp. Sta., Univ. of Calif.

sists in the addition of a little acid to the liquid in which they are canned. It was found that peas heated to 212° F. in a brine acidified by the addition of 5 ounces of lemon juice to every gallon, kept perfectly, while peas heated in the same brine without lemon juice spoiled. The same results were obtained with beans, pumpkins, beets, turnips, artichokes, and asparagus. Large quantities of these vegetables are lost by "spoilage" in the commercial canneries. The flavor of the vegetables sterilized at the low temperature was much superior to that of those sterilized under pressure.

The common household practice of canning corn and tomatoes together owes its efficacy to the same principle. Corn alone is very difficult to sterilize, owing to its lack of acidity. This lack is supplied by the tomatoes and the mixture is easily preserved by ordinary heating. Doubtless other wholesome acids, such as vinegar, citric, or tartaric acid, could be used for the same purpose. The amount of acid used is small and improves rather than injures the flavor.

The new method avoids both the expense of a pressure cooker and the trouble of repeated heatings, and can be highly recommended at least for home use. It consists essentially of making the vegetables slightly acid, thus rendering them as easy to sterilize as fruits. Lemon juice or vinegar may be used to acidify brines. If ordinary cider vinegar is used, twice the amounts given for lemon juice are needed (page 615).