SOP: Determination of Specific Gravity by Specific Gravity Bottle

GIFSQ — Global Institute for Food Safety & Quality

Principle

Unfermented fruit and vegetable juices containing only traces of alcohol or essential oil always show a specific gravity above 1.0; the lower the soluble solids, the smaller the specific gravity, and sweet juices have higher specific gravity. The specific-gravity-bottle method is the reference method, giving an accuracy of 0.001. Hydrometers may be used for samples low in viscosity but not for pulpy juices (the spindles are affected by viscosity); carbonated juices must first be degassed.

Apparatus required

• Specific gravity bottle with thermometer stopper
• Water bath held at the specified test temperature
• Analytical balance
• Acetone and ether for cleaning

Procedure

Clean the specific gravity bottle by shaking with acetone and then with ether; dry the bottle and note the tare weight

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Cool the sample solution to a temperature lower than the test temperature

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Carefully fill the specific gravity bottle with the test liquid and insert the thermometer stopper

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Place the bottle in the water bath held at the specified temperature

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When the solution has reached the specified temperature, remove the surplus liquid on top of the side arm

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Remove the bottle, cool, dry and weigh

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Repeat the whole procedure using distilled water in place of the sample solution

Calculation

Specific gravity = (Weight of test sample held in the specific gravity bottle) / (Weight of water held in the specific gravity bottle)

Precautions

• Degass carbonated juices before determining the specific gravity — entrained gas falsifies the weight.
• Do not use a hydrometer for pulpy juices; the reference method is the specific gravity bottle.
• Bring the bottle to exactly the specified temperature before removing the surplus liquid.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.