Potatoes: Measurement of specific gravity
Note Number: AG0324
Published: June 2000
Updated: August 2010
Specific gravity (SG), which is an expression of density, is the most widely accepted measurement of potato quality. There is a very high correlation between the specific gravity of the tuber and the starch content and also the percentage of dry matter or total solids. These are very important to the potato processor because they affect the quality and yield of the processed product. They also affect processing costs because the oil absorption rates during frying are related to dry matter levels. Higher specific gravity contributes to higher recovery rate and better quality of the processed product.
Specific gravity is the weight of the tuber compared to the weight of the same volume of water. It is a measurement of density and can be calculated as follows:
SG = Weight of tubers in air
(weight in air) - (weight in water)
Two methods are commonly used to measure SG. The most accepted method, and probably the more accurate, is to actually weigh the sample in air and under water. Five kg of tubers or more are taken at random from the sample to be measured. These should be sound tubers free of soil because any hollow heart or adhering soil can affect the result. The tubers are weighed in air, transferred to another tared weighing basket and weighed under water. The tubers sink in the water, so their weight is heavier than an equal volume of water. The weight measured is the difference between the weight of the sample, and the weight of an equal volume of water. The two weights are then applied to the equation; for example:
SG = wt in air (5067 gm)
wt in air (5067 gm) - wt in water (376gm)
= 5067 / 4691 = 1.080
The accuracy of the measurement can be increased by recording the tuber temperature and water temperature. Charts are available that allow for these temperature differences and a correction factor can be applied to the above equation.
However, under "normal conditions" (that is, if the water is at about room temperature and the tuber temperature is not too different) the correction factors are not generally applied.
Conversion tables based on the high correlation between specific gravity and dry matter are also available to give a corresponding dry matter percentage for a given specific gravity reading. In the above example, a specific gravity reading of 1.080 would be equivalent to a dry matter of 21.2%. For conversion tables see Agriculture Note AG0323: Potatoes - factors affecting dry matter. The other common method of measuring specific gravity is to use a potato hydrometer. The hydrometer consists of a float with the neck graduated to specific gravity readings. A basket containing the sample is hung beneath the float and the whole assembly placed in water.
After some time the float remains steady and the specific gravity is read from where the water level is on the neck of the hydrometer. The higher the specific gravity, the deeper the hydrometer will be in the water. The disadvantage of this method is that the hydrometer is calibrated to a fixed weight of potatoes in the basket and therefore the sample placed in the basket must be exactly this specified weight.
It can be time consuming finding tubers of the right size to make the exact weight, and also the hydrometer can bob up and down for some time before a reading can be made. If the hydrometer is knocked about, the chart of specific gravity readings inside the neck can be moved, thereby resulting in totally inaccurate readings. Hence, this method, although commonly used, is not thought to be as accurate as the weight in air/water method. A further disadvantage is the sample size: the hydrometer is limited to the amount specified in the basket, whereas the weight method can use a much larger and therefore more representative sample.
It should be noted that the reading obtained with either of these methods will be an average of the sample examined. The sample or samples taken should therefore be as representative of the bulk sample as possible. There are variations in specific gravity between tubers from the same plant, and even within a single tuber from end to end, but even allowing for this type of variation specific gravity estimates are still a practical way of assessing internal quality. The factors that can influence specific gravity or dry matter are discussed in the Agriculture Note AG0323 Potatoes-factors affecting dry matter.
Potatoes-factors affecting dry matter, Agriculture Note AG0323.
This Agriculture Note was developed by Andrew Henderson, Plant Standards Victoria in June 2000. It was reviewed by Neville Fernando, Farm Services Victoria and Andrew Henderson, Plant Standards in August 2010.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
This publication is copyright. No part may be reproduced by any process except in accordance with the provisions of the Copyright Act 1968.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication