The weight of potatoes in water

Summary

Approximately 2000 samples of potatoes have been examined in order to evaluate the different methods suggested for routine determination of dry matter and starch. Comparisons between the weight of 5-kilogram samples of potatoes in water and the percentage of dry matter found by oven-drying of pulp from the same samples showed that the average weight of 5 kilograms of potatoes in water multiplied by a regression coefficient (b) of 0.05 equals the percentage of dry matter minus 2.00. It was later found possible to calculate b from the equation

$$b = \frac{v}{{v - 1}}.$$

where v denotes the specific gravity of the dry matter; this way of calculation is probably the most exact as well as the most convenient.

Potatoes have a strongly varying content of air that causes a certain buoyancy during weighing in water. Largely owing to this buoyancy the percentage of dry matter is found approximately two units too low if calculated by multiplying the weight in water by b. The air may be removed by keeping the potato sample immersed in water for one-half hour under vacuum.

Dry matter in potatoes may then be determined by three methods: (a) by oven-drying the pulp, (b) weighing a 5-kilogram sample of untreated tubers in water, and (c) by similar weighing after evacuation of air from the potatoes. Oven-drying may yield reliable results, but is rather laborious. Weighing in water is therefore usually preferred as a routine method, and the data given in table 2 permit a more accurate determination of dry matter than does Maercker’s table. To obtain exact results it is, however, necessary to remove the air from the potato tubers before weighing in water; this may be done without too great inconvenience if a suitable number of samples are vacuum-treated simulaneously.