SOP: Polarimetric Determination of Sucrose and Commercial Glucose in Jam, Jelly and Marmalade
GIFSQ — Global Institute for Food Safety & Quality
Principle
The sample is clarified with neutral lead acetate, the excess lead is removed, and the rotation of the solution is measured in a polarimeter before and after inversion of the sucrose. From the direct and invert readings the sucrose content is calculated. Because invert sugar shows zero rotation at 87°C, the rotation observed at that temperature after inversion is due to added commercial glucose syrup alone.
Apparatus required
- Polarimeter with 200 mm tube (or 400 mm tube — see notes under Procedure A)
- 200-mm jacketed metal tube (for the reading at 87°C)
- 200-ml and 100-ml volumetric flasks
- Water bath (maintained at 60°C and at 20°C)
Reagents
a. Alumina cream: prepare a saturated solution of alum in water. Add ammonia by stirring until the solution is alkaline to litmus. Allow the precipitate to settle. Decant and wash the precipitate with water until the washings give only a slight test for sulphate with barium chloride. Decant the excess of water and store in a glass-stoppered bottle.
b. Saturated solution of neutral lead acetate.
c. Anhydrous sodium carbonate (Na₂CO₃) — for removing excess lead after clarification.
d. Hydrochloric acid (10 ml used for the invert reading).
e. Sodium hydroxide solution — for neutralizing the inverted solution before the glucose reading.
f. Dilute hydrochloric acid — for slightly re-acidifying after neutralization.
g. Phenolphthalein indicator — for the neutralization.
Sample preparation
Use the prepared sample as received. Work with a constant temperature for both the direct and the invert readings, since the readings are temperature dependent.
Procedure — A. Direct reading
Mix 52 g (double normal) of the sample with water, transfer to the 200-ml volumetric flask, and add sufficient neutral lead acetate solution for clarification (avoid adding excess lead acetate solution)
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Shake and dilute to volume with water; filter, rejecting the first 25 ml of the filtrate
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Remove the excess lead by adding anhydrous Na₂CO₃ a little at a time (avoid adding excess); mix and refilter, rejecting the first 25 ml of the filtrate
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Dilute 50 ml of the clarified solution to 100 ml in a volumetric flask with water (this gives the normal solution, 26 g/100 ml) and polarise in the 200 mm tube
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Find P, where P = Direct polarimeter reading × 2 (P = polarization before inversion)
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If a 400 mm tube is used, do not multiply by 2
Note on mutarotation: jams, jellies, marmalades and preserves may exhibit mutarotation, i.e. a change of polarization after standing for some hours, caused by the slow build-up of balance between the sugar forms, each of which rotates light differently. In polarimetric methods only constant rotations should be used. To obtain them, allow the solution to stand overnight before taking the reading. Alternatively, heat the neutral solution (pH 7.0) to boiling and add a few drops of ammonia before diluting to volume; determine completion of mutarotation by measuring at intervals of 15–30 min.
Procedure — B. Invert reading
Pipette 50 ml of the clarified solution into a 100-ml volumetric flask; add 20 ml water and 10 ml HCl with mixing
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Keep in a water bath maintained at 60°C; shake the flask continuously for 3 min in the bath, and leave the flask for another 7 min (total heating time 10 min)
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Transfer immediately to a water bath maintained at 20°C; when the liquid in the flask has cooled to 35°C, dilute with water just near to the mark
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Allow the flask to remain in the bath at 20°C for 30 min longer; thereafter dilute to the mark, mix well and note the reading I in the polarimeter, where I = polarimeter reading × 2
Note: if readings are taken at temperatures slightly varying from 20°C, both the direct and the invert readings must be taken at the same temperature.
Procedure — C. Inversion at room temperature (alternative)
Instead of the water-bath inversion, the sample to which acid has been added may be kept at 22°C and above for 24 hr, or at 28°C and above for 10 hr
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Make up the volume and take the invert reading as before; use the same formula for the calculation of sucrose, except for replacing 132.56 by 132.66
Procedure — Determination of commercial glucose
Carry out the inversion of 50 ml of the clarified solution at 60°C for 10 min as required for the invert reading in the determination of sucrose
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After inversion, cool the solution; neutralize with NaOH solution using phenolphthalein as indicator
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Acidify slightly with dilute HCl; add 5–10 ml of alumina cream and make up the volume
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Filter the solution and note the polarimeter reading at 87°C in the 200 mm jacketed metal tube
Calculation
Sucrose:
S = [(P − I) × 100] / [132.56 + 0.0794(m − 13) − 0.53(t − 20)]
Where,
- S = % sucrose
- P = direct reading of the normal solution
- I = invert reading of the normal solution
- t = temperature at which the readings are made
- m = g of total solids from the original sample in 100 ml of the inverted solution
To determine m for liquids: determine the refractometer solids and apply the correction for temperature and invert sugar as described in the soluble-solids determination; multiply the value so found by the weight of the original sample in 100 ml of the invert solution.
For inversion at room temperature, replace 132.56 by 132.66 in the formula.
Commercial glucose:
G = (a × 200) / 196
Where,
- G = % commercial glucose solids polarizing at +211°S
- a = % soluble solids (carried exactly as printed in the source)
Note: at 87°C the positive rotation of dextrose equals the negative rotation of fructose, so invert sugar shows a zero rotation at 87°C and the rotation after inversion is entirely due to glucose syrup. Owing to the expansion of the solution at the higher temperature, the readings taken at 87°C should be multiplied by 1.0315.
Precautions
- Avoid adding excess neutral lead acetate during clarification and excess sodium carbonate during lead removal.
- Take the direct and the invert readings at the same temperature.
- Allow mutarotation to complete before reading, or heat the neutral solution to boiling with a few drops of ammonia first.
- Keep the total heating time of the acid inversion at exactly 10 min (3 min shaking + 7 min standing) at 60°C.
- Record the temperature of the polarimeter readings for the sucrose calculation.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.