Vinegar is added to nearly all ketchup. Formerly the acidity was obtained from the fermentation of the tomatoes and the resultant acid was probably mostly lactic. The flavor was different and not so agreeable. A good cider, grain, or malt vinegar may be used. Most manufacturers prefer to use grain vinegar of ten per cent acidity, as the volume required is less and interferes less with concentration. For real flavor, however, this may not be the best. Lately, glacial acetic acid has been substituted for vinegar, a practice which can not be approved and which ought to be abandoned. Citric acid is also added by some. Vinegar is usually added near the finish of the batch, as otherwise it attacks the kettle to some extent and a part is driven off in boiling. Experiments made by adding vinegar to pulp and evaporating to fifty per cent of its weight in twenty and forty minutes, respectively, show that in the former case the added acidity was decreased in almost the same proportion as the total evaporation, but in the latter case the acid was not driven off quite so rapidly as the moisture. This does not correspond with views held by chefs, as most of them seem to believe that practically all the vinegar is driven off. In order to obtain the sterilizing effect of boiling in an acid medium, it is advisable to make this addition at least five to ten minutes before the end of the cooking period. In home-made ketchup, vinegar is usually added near, or at, the start, and aids in sterilizing the product, as boiling alone may not, whereas, boiling in the presence of an acid will, sterilize. Oil is not an essential to ketchup, and while a small quantity is often used to prevent foaming, its use in large quantities is undesirable. Sugar is added to give the desired flavor. The higher the acidity, whether natural, or acquired by adding vinegar, the greater the quantity of sugar needed. In the high grades of ketchup, granulated sugar only is used, but in the cheaper grades, soft sugar or glucose, may be used, though the latter must be declared on the label. The sugar is usually added when the cooking is about one-half completed. There is an advantage in heating both the sugar and vinegar in a separate kettle and adding them while hot, as it will prevent a check to the cooking and lessen the sticking to the coils or kettle. Salt is used in small quantity and is added near the close of the cooking process. The use of flour or starch in any quantity for the purpose of making the body thick or heavy is properly regarded as an adulteration. This is also true of pulp from a foreign source, like pumpkin or apples.