The relation between the hydrometer reading and the desired specific gravity is arrived at as follows:
When the boiling pulp approaches the finishing point, pour a sample into a tall gallon measure and take the hydrometer reading immediately. Save this sample, put it through the finishing machine along with several additional gallons dipped from the kettle so as to get a good sized sample through the finisher, taking care that the brushes are dry so as not to dilute the sample, and have the specific gravity determined on this finished sample when it gets cold. Take other samples from the kettle at five-minute intervals, and save them for the same purpose, after getting the hydrometer reading. We will say that one of these samples, when tested for specific gravity, ran 1.035. The packer looks up the hydrometer reading he got on this sample, when taken hot from the kettle, and finds the reading was 18 degrees. He then knows that for pulp of the degree of fineness he gets from his cyclone, 18 degrees on his hydrometer equals specific gravity 1.035 after finishing. He may also find that for the sample that was taken from the kettle five minutes later, specific gravity 1.04 is equivalent to 24 degrees under the same conditions. He now has established a relation between the degrees on his hydrometer and the actual specific gravity. (The reason for putting the pulp through a finisher before determining the specific gravity is that he wants to make this test under the same conditions that the buyer would make it.)
Now, if the packer decides that he wants to condense his season’s run of pulp to 1.035, he simply cooks it all to 18 degrees on his hydrometer, providing this is the comparison he got. Under his conditions he may find that 16 or 20 degrees equals specific gravity 1.035. As stated before, each packer must determine this for himself.
Method of Using Hydrometer
The cook should fill a tall gallon measure with the boiling pulp and allow the spindle to sink into it slowly. Do not drop it into the pulp so that it will fall suddenly, as the weight of the mercury or shot in the bottom will carry it farther than it should go. It will be noticed that the spindle will sink rapidly for a few seconds and then almost come to a standstill. At this point—when a standstill is almost reached—the reading should be taken. It requires quite a little time to reach an absolute standstill, and this is not necessary, as it is easy, with a little practice, to catch the other point and take the reading. After the reading is taken, lift out the spindle, stir the pulp with it, and allow it to sink again as a check determination, merely for the satisfaction of being doubly sure. You will find that you get the same reading unless you wait so long before making the check determination that the sample has had a chance to cool considerably. As the test only takes ten seconds, the two tests can be made in a half minute at the most, which gives the pulp no chance to cool.
The hydrometer method is not absolutely fool proof, but it is as near fool proof as anything I know of. Neither is it the acme of perfection in a pulp testing method, but it is, in my opinion, far better than anything that has been suggested to date. It is simple, and accurate enough for all practical purposes, and any ordinary laborer can be shown in a few minutes how to make the test, and the chances of error are very small indeed.
This method will not work where crushed tomatoes are being condensed. It is necessary that the tomato juice be previously cycloned. The pulp must be homogeneous and not lumpy and full of skins to be capable of being tested by the direct hydrometer method. When determining the finishing point on crushed tomatoes it is best to use either the eye or the specific gravity test by weighing.
Finishing the Cooking
The steam should be kept on the kettle continuously until the batch is done. While testing is being done it is not necessary to shut off the steam. If salt is used it should be scattered over the batch slowly a few minutes before it is done. Salt increases the specific gravity, and also the hydrometer reading. The relation between the hydrometer reading and the specific gravity can be worked out just as well where salt is used as when it is not used.
The addition of salt does add to the flavor, and some buyers prefer it, while others do not. I have seen buyers pick out a poor quality pulp with salt in it in preference to a much better quality with no salt.