SOP: Determination of Moisture by Oven Drying

GIFSQ — Global Institute for Food Safety & Quality

Principle

Moisture is determined gravimetrically by measuring the weight lost by the food on evaporation of water in a hot-air oven at atmospheric pressure. The loss in weight may not be a true measure of the water content of the sample, because some volatile constituents may be lost at the drying temperature, and in foods high in protein only a proportion of the free water may be driven off while the bound water remains associated with the proteins. The proportion of free water lost increases with the drying temperature, so results must be compared only when obtained under the same drying conditions. Where decomposition is likely, as in fruit products containing an appreciable proportion of sugars, drying is done at lower temperatures, preferably in vacuum.

Apparatus required

• Drying oven whose temperature can be controlled accurately, preferably fitted with an internal fan for circulation of air (temperature differs on different shelves)
• Flat-bottom metallic dish
• Finely divided asbestos (Gooch grade), as a thin layer spread in the dish
• Lid for the dish
• Desiccator
• Silica dish (without any asbestos aid) when the dried sample is to be used for ashing and estimation of minerals

Procedure

Spread a thin layer of finely divided asbestos (Gooch grade) in a flat-bottom metallic dish

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Dry the dish at 110°C for 1 hr; cover the dish, cool and weigh

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Spread 20 g of sample uniformly over the asbestos layer; weigh as quickly as possible to avoid loss of moisture

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Remove the cover and dry in a hot-air oven at atmospheric pressure, maintaining 70°C for fruits or their products, or 100°C for vegetables or their products

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Heat for 16–18 hr (the duration varies with the type of tissue; this is sufficient for most tissues)

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After drying, replace the lid, cool the sample in a desiccator and reweigh

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Reheat the sample if necessary until consecutive weighings do not vary by more than 3–5 mg

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For tissues or products containing volatile organic constituents or high percentages of sugars, which cannot be brought to constant weight, use the compromise procedure: dry at 55°C for 4 days, employing one standard technique throughout

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The sample after moisture determination may be used for the estimation of ether extractives; if it is to be used for ashing and estimation of minerals, dry it in a silica dish without any asbestos aid

Calculation

Moisture % = (Loss in weight on drying × 100) ÷ Weight of sample taken

Where,

• Loss in weight on drying = decrease in weight of the dish with its contents on drying (g)
• Weight of sample taken = weight of the sample spread over the asbestos layer (g)

Precautions

• Weigh the sample as quickly as possible to avoid loss of moisture.
• Use an oven with accurate temperature control and circulating air; check that the required temperature is maintained on the shelf in use.
• Compare results only when the same drying conditions have been used.
• For sugary or volatile-containing products, do not attempt to force a constant weight — use the 55°C, 4-day compromise procedure.
• If the dried sample is intended for ashing and mineral estimation, dry it in a silica dish without asbestos.

References

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