SOP: Determination of Moisture by Drying in a Vacuum Oven

GIFSQ — Global Institute for Food Safety & Quality

Principle

Moisture is determined gravimetrically by drying the sample in a vacuum oven at 70°C under reduced pressure. The reduced pressure lowers the effective drying temperature and is preferred where decomposition is likely, as in fruit products containing an appreciable proportion of sugars. Dry air is admitted to the oven during drying so that moisture is carried away.

Apparatus required

• Vacuum oven
• Flat-bottom dish with a tight-fitting cover
• Boiling water bath (for predrying liquid products)
• Drying oven at 55–60°C (alternative for predrying)
• Desiccator

Reagents

a. Sulphuric acid (H₂SO₄), concentrated: dry air is admitted to the vacuum oven by bubbling it through concentrated sulphuric acid.

Procedure

Use a dish, preferably flat-bottomed and with a tight-fitting cover

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Prepare the dish and spread the sample as in the oven-drying method

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Predry liquid products such as juices, beverages, syrups and purees by evaporation on a boiling water bath, or in an oven at 55–60°C, until the samples reach apparent dryness

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Tomato products, vegetable purees and comminuted products may be dried to apparent dryness in a vacuum oven at 70°C at a pressure not exceeding 450 mm Hg

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Place the partially dried samples in a vacuum oven maintained at 70°C, with the bottom of the dish in direct contact with the shelf; the temperature variation from one part of the shelf to another must not exceed ±2°C

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Admit dry air (bubbled through concentrated sulphuric acid) into the oven at the rate of 2–4 bubbles per second

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Dry the samples at 70°C (the oven temperature may fall as low as 65°C at the start of drying, but must reach 70°C within 1 hr) at a pressure not exceeding 100 mm Hg

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Dry for 6–7 hr, which is generally sufficient; dry fine fruit juice powders for 16 hr

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Redry the sample if necessary until consecutive weighings made at intervals do not vary by more than 3 mg

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As the dried sample absorbs moisture on standing, cover it quickly, cool in a desiccator, and weigh as soon as possible after the sample reaches room temperature

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 in the dish (g)

Precautions

• Keep the bottom of the dish in direct contact with the oven shelf, and check that shelf temperature variation does not exceed ±2°C.
• Do not exceed 100 mm Hg pressure during drying (450 mm Hg during the predrying of tomato products, vegetable purees and comminuted products).
• Make sure the oven reaches 70°C within 1 hr of starting, even if it dips to 65°C at the start.
• Cover the dish quickly after drying and weigh as soon as the sample reaches room temperature — the dried sample reabsorbs moisture on standing.

References

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