The ash usually varies from 2 per cent to 4 per cent, owing to the different amounts of salt, which varies in general from 1½ to 3 per cent.
The acidity ranges from .4 to 2.3 per cent. The acidity is one of the most important factors in preventing the growth of bacteria and yeasts in the ketchup after being opened. In order to secure the best results the ketchup should have an acidity of over 1 per cent (expressed as acetic acid) and an acidity of 1.25 per cent or higher adds to the keeping quality of the ketchup after the bottle is opened. An increase in acidity will necessarily require an increase in the amount of sugar in order to secure the proper flavor; or, vice versa, an increase in the sugar will necessitate an increase in acidity. In some ketchups about one-half of the acidity is due to the citric acid of the tomatoes and the remainder to the vinegar added in manufacture. With ketchups of exceptionally high acidity, the proportion of citric acid to total acid may be much less than this. There may be considerable difference in the acidity in ketchup of the same brand due to variations in manufacture.
The sugar present in the ketchup is derived both from the pulp and from the added sugar. In ketchup ranging from 12 to 30 per cent total solids, from 9 to 22 per cent of the solids may consist of sugars.
METHODS OF MANUFACTURE
Ketchup may be prepared either from the fresh tomatoes, or from pulp. The most common practice is to prepare it from fresh tomatoes, although some manufacturers prefer to make ketchup during the winter, when they are not so busy with other products, and therefore use pulp. Presuming that the same quality of stock is used and the same care used in manufacture, there are some advantages in making ketchup from fresh tomatoes. The pulp loses some of its color by bleaching, and a ketchup made from pulp is naturally subjected to more heating than that made from fresh tomatoes.
In the manufacture of ketchup the fresh tomatoes may be broken by steam or by the use of a mechanical breaker. Both methods have their advantages, some preferring the one method, some the other.
In securing good quality in ketchup the same factors must be considered as in the making of pulp. These factors are care in manufacture and the use of a raw product of good color and quality. For discussion of these points in regard to pulp, see page 7.
The constituents used in the manufacture of ketchup in addition to the tomatoes are sugar, vinegar, salt, onions and spices. The sugar generally used is granulated cane or beet sugar. Some of the lower grades of cane sugar may be used satisfactorily. The terms used to designate grades of sugar below granulated do not always give a correct idea of the purity of the sugar and in buying such grades it is best to have samples submitted and have analyses made for sugar content.
The vinegar generally used is 100-grain distilled vinegar.
The salt used is of the grade known as dairy salt.