Pulp should not come in contact with iron at any stage, as the union of the acid of the fruit with the metal will cause discoloration. When such discoloration does occur, it becomes uniform throughout the mass, and not in the neck of the bottle as has sometimes been described.
Darkening in the neck of the bottle is frequently due to the spices used, as has already been pointed out. It can be redistributed throughout the whole by placing the bottle in a shaker for a short time.
Darkening at the top may sometimes be due to extraction of color from the corks. Soaking corks in two per cent acetic acid, then in hot water before drying, and paraffining, will assist in preventing discoloration on cheap grades.
Discoloration in the neck also results from the small amount of air incorporated and from any subsequent addition which may come in through the cork or seal. Bottles which are full to the cork may show no darkening, those having a space of an inch or more between the contents and cork may show little discoloration, while those having more space will show much more marked discoloration. This holds for both pulp and ketchup and in this case the discoloration begins on the surface and works downward. The product made from some fruit will discolor more than that made from fruit grown in another section of the country.
A bright red color is secured in some brands of ketchup by means of paprika, as indicated under spicing.
A light colored ring in the bottom of a bottle is generally due to organisms and debris, indicative of the use of barrel or trimming-stock pulp, or it may result from changes after the process of manufacture. It has been mistaken for sand.
Ketchup must not only keep while in the unopened bottle, but for a reasonable time after opening, if it is to be a commercial success. Every canner understands that if he puts food in a hermetically sealed package and sterilizes by heat, that it will keep until opened. The same principle applies to ketchup in the bottle, but there are some packers who wish to be spared this expense and trouble and prefer to use a substitute for heating.
The keeping quality after opening depends upon the utilization of the same principles followed in the household operation of making fruit butters, ketchup, preserves, and pickles, that is, sufficient concentration and the use of sugar and vinegar. A ketchup can be made essentially a pickle with an excessive quantity of vinegar and it will keep; it can be made a preserve with excess of sugar and it will keep; or, it can be made a distinctive sauce well concentrated in which the vinegar and sugar are used only in sufficient quantity to give proper flavor, and it will keep. Apple juice or cider will spoil quickly if allowed to stand in a warm place; apple sauce will behave in like manner only a little more slowly; but if the juice and sauces be boiled together until they have acquired the consistency or state known as apple butter, they will keep very well. The acidity, sugars, and solids have been increased by the concentration. In the making of tomato ketchup, the fruit does not have sufficient acidity and sugar of itself to give preservative property at the concentration desired for a sauce, so these are augmented by the addition of vinegar and sugar.