SOP: Determination of Viscosity Using the Stormer Viscometer

GIFSQ — Global Institute for Food Safety & Quality

Principle

The Stormer viscometer measures consistency or viscosity by the time required for a rotor, driven by a falling weight, to make 100 revolutions in the sample. The number of seconds for 100 revolutions, measured against the time for the same number of revolutions in a standard liquid under identical conditions, gives the relative viscosity; by reference to calibration curves of liquids of known viscosity it may be expressed in centipoises. Driving weight versus flow (revolutions/sec) curves give the apparent viscosity of non-Newtonian products.

Apparatus required

• Stormer viscometer: rotor, platform with raising mechanism, brake, rewinding drum with driving weight box (driving weights as required for the product — cream style corn 750 g, tomato puree 250 g, tomato pulp 400 g), dial with pointer, revolution counter
• Stop-watch
• Distilled water (or castor oil or glycerol as standard liquid)

Procedure

Attach the desired weight for the particular food product (cream style corn 750 g, tomato puree 250 g, tomato pulp 400 g)

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Place the can of the food product (well stirred and at the desired temperature) on the platform

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Raise the platform until the rotor sits centrally in the sample with its top half an inch below the surface

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With the brake on, raise the driving weight by turning the handle of the rewinding drum counter-clockwise until the weight nearly touches the pulley

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Turn the brake control knob a quarter turn to free the dial; let the rotor turn until the dial pointer lies between 80 and 90, then re-set the brake — this gives the rotor 10 to 20 running-start revolutions before timing begins. If setting the dial has appreciably lowered the weight, rewind the drum to raise the weight box to the pulley before the test

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With a stop-watch in hand, release the brake and time, in seconds, 100 revolutions of the rotor on the revolution counter

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Set the brake and record the number of seconds. Make at least two readings of the same sample and average the results; the second reading may be made on the same sample by rewinding the cord on the drum. Before each successive reading, again permit the rotor to turn 10 to 20 revolutions before starting the stop-watch

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After each use, clean and dry all the parts of the instrument thoroughly

Calculation

The consistency or viscosity of the sample is reported as the relative viscosity:

Relative viscosity = (time for 100 revolutions in the sample) / (time for 100 revolutions in distilled water, under identical procedure, temperature and driving weight)

Where,

• Distilled water may be replaced by another standard liquid such as castor oil or glycerol if the liquid under test is beyond the viscosity range of water.
• If the viscosity is desired in centipoises (absolute viscosity), read it from calibration curves of liquids of known viscosity; the viscosity of water at 20 °C is very nearly one centipoise.
• If the revolutions per second are not directly proportional to the driving weight, the value is reported as the apparent viscosity; for viscosity determinations, draw a consistency or viscosity curve by plotting increased driving weight against flow (revolutions/sec) for each type of food product.

For cream style corn (using a 750 g driving weight), light consistency products require approximately 15 sec, while those of heavy consistency require over 175 sec. With tomato pulp of 12% solids (using a 400 g driving weight), readings from 10 sec to over 300 sec have been recorded. The instrument has also been used on the hot brine-corn mix (250 g driving weight) to predict the finished product consistency.

Precautions

• Keep all samples at the same temperature — viscosity decreases with any increase in temperature.
• Check the instrument periodically against known standards to make certain it is always in calibration.

References

Gould, W.A., Food Packer, 34 (4), 42 (1953).

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