The fact that the gradual souring of pulp in 5–gal. cans, even though very slight, is due to the action of living, acid forming bacteria can hardly be doubted. Bacteriological tests supplemented by chemical analyses would determine the matter definitely, but there is evidence in the experience of every canner to show that this souring is due to the fact that the pulp is not sterile.
Canned tomatoes are always sterilized and can be kept in storage for years without losing their sweetness. Pulp packed in 5–gallon cans is not sterilized and quite a after-sterilization percentage of it will lose much of its sweetness on being stored for six months, and after being stored a year still more is lost. It takes hundreds of pounds of expensive sugar to make up for this loss of sweetness when the pulp is made into soup.
Sampling
Of course, a cook is not infallible in his judgment as to just what is the right sweetness and he needs the co-operation of others at the factory. The factory manager should make it a point to sample the soup several times a day, and also to have others sample it. It is better to have it just a little less sweet than it should be than to have it too sweet. The consumer can always add a little sugar but cannot take any out.
The third point mentioned above—that of uniform consistency and smoothness, will be discussed fully under the cooking and shaking processes.
Proper Acidity
I stated as the fourth point that the soup should not curdle fresh milk, even though no soda is added by the housewife. One of the favorite ways of preparing canned tomato soup in the home is to mix it with an equal quantity of hot milk. The experienced housewife knows enough to add a pinch of soda to the milk before adding the soup, and she may also know that she will get a smoother mixture if she slowly pours the soup into the milk, stirring constantly, than if she pours the milk into the soup. The inexperienced housewife does not know these things, and often she opens the can and mixes it with milk without reading the admonition on the label as to how the cream of tomato soup should be prepared. Sometimes she forgets to add the soda, or does not thoroughly dissolve it, or does not add enough of it, and then there is disappointment. In order to guard against such disappointments the soup canner should so prepare his product that curdling of the milk cannot occur under any method of mixing. The way to do this is to add enough soda to the soup to reduce the acidity to a point where it will not curdle fresh milk, whether the soup is poured into the milk or the milk into the soup. The canner can then print a notice on the label of the can to the effect that it is not necessary to add soda to this soup when preparing it as “cream of tomato,” as the soda is already in it.
The acidity of the tomato is the active principle which causes the milk to curdle. If this acidity runs 0.60% it will curdle the milk badly. If it is reduced to 0.50% it will still curdle the milk, but if it is reduced to 0.40% it will not curdle the milk, if the milk is fresh. The aim of the soup maker should therefore be to keep the acidity of the soup between 0.30 and 0.40%, not allowing it to go above or below these limits. If it is reduced below 0.30% the flavor of the tomato begins to be effected—in fact, if possible, it is best to keep the acidity between 0.35 and 0.40% to avoid any possibility of loss of tomato flavor.
Now, the packer will ask—“How am I going to do this acidity testing? I’m not a chemist, and have no chemist at my factory, or any laboratory.” The acidity test on pulp and soup is a very simple one. The equipment required scarcely amounts to anything, as the sample can be taken by volume. Any chemist will be glad to show you how to make this test and to secure for you the few accessories needed.
Why is there less danger of curdling if the soup is poured into the milk than if the milk is poured into the soup? The soup contains all the acid, and the milk contains none. If the soup is poured into the milk slowly, with stirring, the acid is slowly added to the milk, and is diluted, there being at the beginning of the mixing a very small volume of acid and a large amount of milk. If, however, the milk is added to the soup, a small volume of milk is at first being thrown into a large volume of acid, and naturally the danger of curdling is considerably increased. But if the acidity of the soup is not above 0.40% and the milk is fresh, there will not be an appreciable amount of curdling no matter which way the two are mixed. A statement as to the proper way of mixing with milk should be made on the label.