Both types of retorts are provided with ther mometers and pressure gages. In the use of retorts of either kind it is essential that a current of steam under pressure be passed continuously, this "circulation" being effected by leaving the ex haust valve slightly open. The temperature may be kept constant by regulating the amount of steam entering the retort and the amount of the exhaust.
As already mentioned, this method is most effi cient in its action on the resistant spores of bac teria, consequently is the safest method to employ in the preparation of canned goods. It is neces sary, however, to avoid excessive heating, as dam age to the foods may be done in this way. One result of over-cooking is to produce discoloration of the food substance, a defect which sometimes interferes with the commercial value of the article. Temperatures above 120° C. (218° F.) are rarely used, the best temperature for any material being determined directly by experiment.
In sterilization of canned foods, it is necessary that the whole contents of the can be subjected to the required temperature for a period of time long enough to destroy all germs whether spore-produc ing or not. This period of time can be determined accurately only by experimental tests. It is of equal importance to know the length of time necessary for the required heat to penetrate to the center of the cans, this time varying very much with different materials, owing to their different conductivity for heat. Liquids are, in general, good conductors, while solid or semi-solid substances conduct but poorly. Knowledge on this point is absolutely essential in order to prescribe a satis factory process.
The vacuum.
It is customary in the preparation of canned foods to have a partial vacuum in each can, and for many years it was thought that this vacuum was the principal factor in keeping the goods. While this is untrue, it is desirable to have the vacuum as it allows a means of inspection of the cans. The vacuum is indicated by the concavity of the ends of the cans and should always be present in sound cans. If, however, putrefaction or fer mentative changes take place, in which gases are produced, the ends bulge out, owing to the pres sure of the gas within, and so may be easily de tected. Even in case no swelling of the cans takes
place, skilful inspectors can distinguish between good and bad cans by the sound when the cans are struck on the ends. The vacuum is generally pro duced by filling the cans with the material in a hot condition and sealing them immediately. When water-bath sterilization is employed, the cans are sometimes unsealed or punched while hot and the steam allowed to escape, the aperture being closed again at once.
Principles involred in canning specific crops.
In the canning of fresh vegetables, the raw materials are substances high in their percentage of water and relatively high in carbohydrates, but relatively low in proteid matter. Because of dif ferences in texture and composition, no hard and fast rules of procedure can be laid down. The details of the processes for various kinds of canned goods cannot be given here, but the general prin ciples involved in the different classes may be mentioned.
In the preparation and preservation of all kinds of canned goods the necessity for cleanliness is evident, since the entire operation is one in which the aim is to prevent bacterial action. Although in the final process absolute sterilization is to be brought about, the length of time necessary to produce this end may be much shortened if care is taken to exclude the organisms from external sources. Owing to the preponderance of carbohy drates, fermentations taking place are most likely to give rise to acids, lactic acid, probably, being the one most frequently found. Putrefactive fer mentations sometimes occur, especially in those vegetables having considerable nitrogenous sub stances, as beans, peas and asparagus.
Asparagus is packed in large quantities in Cali fornia and the middle Atlantic states. After plac. ing the stems in the cans, a dilute salt solution is added in sufficient amount to fill the cans. Unless the freshly cut plant is used, a poor product is obtained, as, on standing, it rapidly becomes with ered and tough. If not sufficiently " processed" it undergoes fermentation, losing color and assuming a rather bitter, acid taste. If too highly heated, it is darkened and has an overcooked taste.