SOP: Determination of Moisture in Sugar
GIFSQ — Global Institute for Food Safety & Quality
Principle
The sample is dried to constant weight in a hot air oven or in a vacuum oven; the loss in weight is reported as moisture. The result obtained may not reflect the true moisture content, because carbohydrates and some other organic compounds are not stable at the high temperatures used.
Apparatus required
- Previously dried aluminium dish with a tight-fitting cover
- Hot air oven
- Vacuum oven (for the alternative procedure)
- Desiccator
Reagents
None.
Procedure — Hot air oven drying
Note the tare weight of a previously dried aluminium dish having a tight-fitting cover
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Remove the cover, place about 5–10 g of sample, and note the weight accurately
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Remove the cover and dry in a hot air oven — the recommended temperature and time are: AOAC, 3 hr at 100°C (raising the temperature to 105–110°C in the final heating periods to expel the last traces of occluded water); CAC/RS 4-1969, 3 hr at 105°C; ISI, 2 hr at 110 ± 1°C
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After drying, remove the dish, cool in a desiccator, and weigh
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Redry for 1 hr and repeat the drying, cooling and weighing until the change in weight between successive weighings at 1-hr intervals does not differ by 1 to 2 mg
Procedure — Vacuum oven drying (alternative)
Dry the sample in a vacuum oven for 5 hr at 60°C at a pressure not exceeding 50 mm of mercury (see Chapter 1 of the book)
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Allow a current of air, dried by passing through anhydrous CaSO₄, P₂O₅, etc., to pass through the oven during drying to facilitate removal of moisture
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After drying, cool and weigh; redry again for 1 hr as in hot air oven drying, until two successive weighings do not differ by more than 2 mg
Note: the cross-reference in the extraction reads “see Chapter !”; rendered here as Chapter 1, the book’s proximate-constituents chapter in which the vacuum-oven moisture procedure is given.
Calculation
% Moisture = (loss in weight on drying × 100) / (weight of sample taken)
Where,
- Loss in weight on drying = weight of dish and sample before drying − weight of dish and sample after drying to constant weight
- Weight of sample taken = about 5–10 g
Precautions
- Dry to constant weight — repeat the drying until successive weighings agree within 1 to 2 mg (hot air oven) or 2 mg (vacuum oven).
- In the final heating periods of the AOAC procedure, raise the temperature to 105–110°C to expel the last traces of occluded water.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Official Methods of Analysis, Association of the Official Analytical Chemists, 12th edn, 1975 — cited in the book.
Recommended International Standards for White Sugar CAC/RS 4-1969, Joint FAO/WHO Food Standards Programme, Rome — cited in the book.
Specification for Sugar used in Food Preservation Industry IS: 1679-1960, Indian Standards Institution, New Delhi — cited in the book.