The vigor with which the washing is done is always apparent in the finished product. The poor washing usually given to tomatoes for canning, accounts in a measure for the relatively large numbers of organisms found in ketchup made from trimmings.
After washing, the tomatoes may be reduced to a pulp in one of three ways: by running the raw tomatoes directly through a grinder and into the cyclone; by passing the tomatoes through a scalder and into the cyclone; and by turning the tomatoes into jacketed-kettles or tanks and cooking them until soft before running through the cyclone. There is a difference in the product obtained by these methods. The first one gives a somewhat larger yield, as the hard parts are cut and torn so that more will be squeezed through the sieve. The color is generally stronger and inclined to the purple side rather than the yellow. The color, however, does not hold so well when exposed to light. The pulp inclines to froth and there is a marked separation of red pigment on the top. A raw pulp will begin to separate into a clear layer below and solids at the top in about fifteen to twenty minutes after standing in a tank. This is due to the air incorporated in the solids and possibly to difference in specific gravity, and not to fermentation, as frequently alleged. Changes will take place more rapidly in such pulp than in that made from scalded fruit.
There is not a great deal of difference between the second and third methods, the object in both cases being the same. If a long scalder be used, the skins will be loosened and the tissue softened so that it will be easily separated from the green parts, hard cores, or black rot. There will be no acquisition of color from the stems to discolor the ketchup. The loss is a little heavier in scalder heating than where the fruit is cooked in the tanks, but there is the compensation that there is less carrying of hard or objectionable material. A scalder to be effective should be much longer than that used in canning, or a greater volume of steam should be used. The tomatoes should be heated to about 180 deg. F. There is little choice in the two methods, but the preference is with the scalder, both being preferred to the raw ground fruit. A pulp made in this way separates slowly and there will be no material increase in organisms for a rather long time (three or four hours). There is less separation of pigment on cooking and there is a clean look to the tissue under the microscope.