Moisture by the infrared moisture balance (lab visit)
An infrared lamp dries the sample on a built-in balance; % moisture = loss in weight ÷ initial weight × 100, read within minutes.
Aim and principle
Lab visit: observed at a food analysis laboratory, not performed by you (but the principle, steps and calculation are asked). Aim: To find the moisture content of a food sample with an infrared (IR) moisture balance. Principle: The instrument is a balance with an infrared (often halogen) heater above the pan. The sample absorbs the infrared radiation and heats quickly, so its water evaporates. The balance weighs the sample all the time and stops when the weight no longer changes (constant weight). The loss in weight is taken as moisture, and the display shows the percentage directly: % moisture = loss in weight ÷ initial weight × 100. This is the same principle as oven drying (thermogravimetry), only much faster.
Requirements
| Item | What it does |
|---|---|
| Infrared moisture balance (IR or halogen lamp over a built-in balance) | Heats the sample and records the weight loss continuously |
| Aluminium sample pans | Hold a thin, even layer of sample |
| Food sample, ground or chopped (e.g. flour, rice, dal) | Material whose moisture is measured |
| Spatula, brush and tongs | Spreading the sample and handling hot pans |
| For comparison: hot air oven, metal dishes with lids, desiccator, analytical balance | The reference oven-drying method |
Procedure
Observed at the laboratory. Typical settings are given; follow the instrument instructions and your lab manual.
- 1Switch on, level the balance and let it warm up; set the drying temperature (often about 105 °C) and the end-point mode (automatic or timed).
- 2Place a clean pan and tare the balance to zero.
- 3Spread about 5 g of sample in a thin, even layer; note the initial weight.
- 4★Close the hood; the lamp heats the sample and the weight falls.
- 5The run stops when the weight is constant; read % moisture (or note the final weight).usually 5-20 minutes
- 6Lift out the hot pan with tongs, clean it, and repeat for a duplicate.
★ one-way, controls the pace
Worked example: IR balance and oven compared
IR balance: 5.000 g of wheat flour weighed 4.400 g after drying. Oven method on the same flour: empty dish 25.120 g, dish + flour 30.120 g, dish + dried flour 29.510 g. Find % moisture by both methods, % total solids, and moisture on dry basis (IR data).
- IR: loss in weight = 5.000 − 4.400 = 0.600 g.
- % moisture = 0.600 ÷ 5.000 × 100 = 12.0%.
- Total solids = 100 − 12.0 = 88.0%.
- Moisture on dry basis = 0.600 ÷ 4.400 × 100 = 13.6%.
- Oven: sample weight = 30.120 − 25.120 = 5.000 g; loss = 30.120 − 29.510 = 0.610 g.
- % moisture (oven) = 0.610 ÷ 5.000 × 100 = 12.2%.
Moisture = 12.0% by the IR balance and 12.2% by oven drying (total solids 88.0%). Values this close show the IR balance agrees well with the reference method for this flour.
IR moisture balance vs hot air oven
| Point | IR moisture balance | Hot air oven |
|---|---|---|
| Principle | Loss in weight on infrared heating | Loss in weight on heating at about 105 °C |
| Time | Minutes (about 5-20) | Several hours, until constant weight |
| Weighing | Automatic and continuous; reads % directly | By hand: weigh, dry, cool in a desiccator, weigh again |
| Samples per run | One | Many at once |
| Accuracy | Good for routine checks; calibrated against the oven | Reference (official) method |
| Limitations | Surface may scorch; other volatiles are counted as water | Slow; sugary foods may brown and lose volatiles |
Precautions and result
- Spread the sample thin and evenly, and use the same sample size each time.
- Choose a temperature that dries without scorching; sugary or heat-sensitive foods need a lower setting or a vacuum oven (alternative). Very dry foods are better tested by Karl Fischer titration.
- In the oven method, weigh dishes only after cooling in a desiccator; handle hot pans with tongs.
- Keep the balance level and out of drafts; run duplicates.
- Result: The moisture content of the given sample is ___ % (total solids ___ %).
Moisture decides shelf life and microbial spoilage: flour, rice, pulses and milk powder keep well only when dry, and FSSAI standards set maximum moisture for many such foods. Moisture is the first step of proximate analysis; it is needed to convert results between fresh and dry weight, and 'carbohydrate by difference' depends on it. Because food is sold by weight, moisture also affects price and label values.
Short answer: “State the principle of the infrared moisture balance.” An infrared lamp heats the sample on a built-in balance; water evaporates, and the loss in weight recorded until constant is taken as moisture: % moisture = loss ÷ initial weight × 100.
Structured question: “Compare the infrared moisture balance with the hot air oven method.” Same thermogravimetric principle. IR balance: minutes, automatic, reads % directly, one sample at a time, may scorch, calibrated against the oven. Oven: about 105 °C to constant weight, hours, desiccator cooling, many samples, reference method. Add the formula and a worked example.
Lost over Start × 100: % moisture is the weight you lost divided by the weight you started with.