SOP: Determination of Jelly Grade with the Jelmeter

GIFSQ — Global Institute for Food Safety & Quality

Principle

Although sugars, proteins and starches influence the viscosity of fruit juices and extracts to a slight extent, the main influence on the viscosity is due to pectin. The viscosity of the extract is an indication of the quality and the quantity of pectin present and can be used as an index for the quantity of sugar to be added. The method is useful only for approximate and preliminary evaluation. The relative viscosity of a pectin extract or solution is the ratio of the time of flow of the pectin solution through a given orifice compared to the time of flow of an equal quantity of water at the same temperature.

Apparatus required

• Baker jelmeter: an Ostwald pipette cut off at the bottom of the capillary; capillary tube about 10 cm long sealed to a bulb of about 2 ml capacity; bulb blown on the end of a glass tube of about 4 mm inside diameter; tube slightly constricted as it enters the bulb chamber and extending about 5 cm from the bulb; calibration marks etched at A (the place of constriction) and at B (about 2 cm below the bulb on the capillary); the capillary should be so small that it takes water at room temperature (22–26°C) about 40 sec to run between marks A and B

Procedure — Measurement of relative viscosity

Bring the pectin extract or extracted juice to room temperature

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Draw up through the capillary into the bulb above the constriction marked A

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Note the time in sec for the liquid to flow back, of its own free will, from mark A to mark B, when the apparatus is held upright

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Wash the pipette thoroughly and note the time required for water of the same temperature to flow between the same marks

Calculation

Relative viscosity at temperature T = (Time in seconds for the extract to flow through) ÷ (Time in seconds for the water to flow through)

Where,

• T = temperature of measurement (same for the extract and the water)
• Time in seconds for the extract to flow through = flow time of the pectin extract from mark A to mark B (sec)
• Time in seconds for the water to flow through = flow time of water between the same marks at the same temperature (sec)

Using this device, Baker and Woodmansee made viscosity measurements of apple and citrus pectin solutions (0.5 and 1.0%) at pH 2.5 and 26°C. The approximate grade of pectin can be read from the curves given in the book’s Fig. 2.4 (approximate grades of apple and citrus pectin on the basis of a 0.5% and a 1.0% solution, determined by measuring relative viscosity). When the relative viscosity exceeds 20, the solution should be diluted with water for measurement.

Jelly grade can also be determined from the intrinsic viscosity by means of the curve shown in the book’s Fig. 2.5. Further work will be necessary to establish its reliability, because it represents data from only a few citrus pectins and molecular weight heterogeneity probably has a marked influence on the curve.

Note: grades are read off the printed curves — no grade numbers are computed here.

Precautions

• Bring the pectin extract and the water to the same room temperature before measuring.
• Hold the apparatus upright while the liquid flows back from mark A to mark B.
• Wash the pipette thoroughly between the extract run and the water run.

References

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

Baker, G.L., Food Ind., 6, 305 (1934) — cited in the book.

Baker, G.L. & C.W. Woodmansee, Delaware Agr. Expt. St. Bull., 272 (1948) — cited in the book.

Owens, H.S. et al., ibid. — cited in the book.