Determination of Sugar
The sugar of tomatoes is probably always present as invert sugar. If cane sugar is ever present in the raw product it is doubtless inverted during the concentration of pulp. The per cent of sugar given in [Tables 2] and [3] was determined by the method of Munson and Walker.[14]
Determination of Acidity
Accurate results cannot be obtained by the titration of tomato products in the presence of the insoluble solids. If it be desired to determine the acidity in the entire sample of tomatoes or tomato pulp rather than in the expressed juice, the insoluble solids should first be removed by the method given in the determination of insoluble solids or by filtration through filter paper. The per cent of acid given in [Tables 2] and [3] was obtained by titrating the liquor obtained by filtering the pulp. In products of this nature, the addition of an alkali causes a brownish color which has a tendency to obscure the end point shown by the indicator. To obviate this, the sample should be diluted to at least 200 cc. and a larger amount of indicator employed than is necessary with a clear solution. The following details are suggested.
Dilute 20 grams of the filtrate under examination with over 200 cc. of water. Add at ½ cc. of phenolphthalein solution (prepared by dissolving 1 gram of phenolphthalein in 100 cc. of 95 per cent alcohol) and titrate with sodium hydroxide until the end point is obtained. Add 1 cc. of tenth-normal hydrochloric acid, heat the solution quickly to boiling and boil one minute to expel carbon dioxide. Cool the solution quickly to about room temperature, and then add tenth-normal sodium hydroxide until the end point is obtained. The volume of hydrochloric acid added must, of course, be taken into consideration in the final result. The filtrate may also be titrated direct with tenth-normal sodium hydroxide solution with satisfactory results.
Determination of Salt
This laboratory has been using the following rapid method which gives results agreeing closely with results obtained by the analysis of the ash:
Weigh out 20 grams of pulp, dilute in a volumetric flask to 200 cc., filter and titrate an aliquot portion with standard silver nitrate solution, using potassium chromate as indicator. The acidity of tomato pulp is not sufficient to interfere with this determination.
Determination of Specific Gravity[15]
The specific gravity of tomato pulp is used as one criterion for establishing the value of pulp that is offered for sale and is also used in connection with the manufacture of pulp to determine the point at which evaporation should be stopped.