FOOD ANALYSIS : MOISTURE TESTING
Heat near 100 to 105 °C until the weight stops changing, cool in a desiccator, weigh again
Heat a small sample on a built-in balance, the instrument reads the moisture directly
Figure 1: Both instruments measure moisture by loss on drying. The oven is the reference route and the analyzer is the fast route.
Introduction
A drying oven and a moisture analyzer both find moisture by drying a weighed sample and measuring the weight it loses. The drying oven is the reference method: the sample is dried at a set temperature until its weight stops changing, then cooled in a desiccator and weighed again. A moisture analyzer does the same drying on a small sample on a built-in balance and reads the moisture directly, in minutes, which is why it is used on the production line while the oven stays the reference in the laboratory.
Quick comparison
Table 1: Drying oven compared with moisture analyzer
| Aspect | Drying oven | Moisture analyzer |
|---|---|---|
| What it is | An oven held at a set temperature for drying samples | A combined heater and balance that dries and weighs in one unit |
| How moisture is found | Weigh, dry to constant weight, cool in a desiccator, weigh again | The instrument heats the sample and reads the moisture directly from the weight loss |
| Usual temperature | Near 100 to 105 °C for many foods | Set on the instrument, often with a halogen or infrared heater |
| Time per test | Hours, until the weight stops changing | Minutes |
| Sample size | Several grams, spread thin in a shallow dish | Small, a few grams on a pan |
| Samples at once | Many dishes in one oven | One sample at a time |
| Role | Reference method that settles disputes | Fast routine and production-line method |
| Skill needed | Weighing, desiccator use, and judging constant weight | Setting the temperature and the end point, and spreading the sample evenly |
| Example from the books | Flour dried to constant weight in the FTN practical work | Sartorius moisture analyzer for biscuit moisture on the line |
Scientific explanation
Definition
Oven drying: the reference method for moisture, in which the loss in weight on heating is called the moisture. Moisture analyzer: an instrument that dries a small sample and reads its moisture content directly, combining the heater and the balance in one unit.
Working principle
Both work by loss on drying: water leaves the sample as vapour, and the weight lost is counted as moisture. In the oven the analyst does the weighing before and after drying. In the analyzer the built-in balance weighs the sample while it dries, and the instrument works out the percentage itself and stops when the weight stops changing.
Apparatus
Drying oven: a convection or forced draft oven, shallow dishes, a balance, and a desiccator, a sealed container with a drying agent, for cooling before the final weighing. Moisture analyzer: a single unit with a heater, a sample pan, and a built-in balance, such as the Sartorius moisture analyzer shown in the biscuit manual, Chapter 22, Figure 22.3.
Measurement procedure
Oven: spread the sample thin in a shallow dish so water can escape, weigh it, dry it at the set temperature, then dry again for a set time and weigh again, and stop only when two weighings agree. Cool in a desiccator before the final weighing, so the sample does not pick up water from the air while it cools. Analyzer: place a small weighed sample on the pan, spread it evenly, start the run, and read the moisture directly when the instrument ends the test.
Factors affecting readings
Both methods read high when the sample loses more than water: foods rich in sugar break down on heating, and volatile flavour compounds escape with the water. A thick layer dries slowly and unevenly, so the sample must be spread thin, and particle size and surface area change the rate of loss. A high temperature dries faster but can break down the food, while a low temperature leaves water behind. For the oven, cooling outside a desiccator lets the dry sample pick up water again before weighing.
Formulas and calculations
Moisture percent = weight lost on drying divided by sample weight, times 100. Both instruments use this same calculation: the analyzer only does the arithmetic itself.
Worked example from the FTN Food Analysis notes: a flour sample weighs 5.000 g before drying and 4.400 g after drying to constant weight. The loss is 0.600 g. Moisture = 0.600 divided by 5.000, times 100 = 12.0 percent.
Industrial applications
- Drying ovens set the reference moisture for flour, milk powder, and biscuits, and settle disputes between laboratories.
- Moisture analyzers check biscuit moisture on the line against the target of 1.5 to 3.0 percent, tighter for each product type.
- Bakeries and mills use the analyzer for fast checks during production and the oven for the final laboratory record.
- Vacuum ovens dry heat-sensitive foods at a lower temperature, and microwave drying ovens shorten the time further.
Advantages and limitations
Drying oven: it is the reference method, it handles many samples at once, and it needs only simple equipment. Its limit is time: a test takes hours, and the analyst must judge constant weight by repeated weighing.
Moisture analyzer: it gives a result in minutes, reads the moisture directly, and fits production-line checking. Its limit is that it tests one small sample at a time, and like the oven it counts any weight lost on heating as water, so it reads high for sugary foods and foods with volatile compounds.
FAQ
Question: Why is the oven still the reference if the analyzer is faster?
Because the oven method is the long-settled standard that laboratories compare against. A fast method is checked against the oven before its results are trusted.
Question: Why must the sample be cooled in a desiccator before the final weighing?
So it does not pick up water from the air while it cools. A dry sample takes up water fast, and that would make the moisture read low.
Question: Why does oven drying read high for honey?
Because honey loses weight from sugar breakdown as well as from water. For such foods the Karl Fischer titration is used, because it reacts only with water.
Question: What moisture should a biscuit hold?
The FTN biscuit notes give a target of 1.5 to 3.0 percent, tighter for each product type, checked against the product specification.
References and further reading
- FTN Food Analysis, Unit 3: Moisture and ash (notes on disk): oven drying near 100 to 105 °C, constant weight, desiccator cooling, worked example 12.0 percent moisture.
- FTN Biscuits Technology, Unit 6: Quality control in biscuits (notes on disk): Sartorius moisture analyzer, moisture target 1.5 to 3.0 percent.
- Nielsen, S. S., Food Analysis Laboratory Manual, Springer: forced draft oven, vacuum oven, microwave drying oven, rapid moisture analyzer (consulted through Google Books).
- Nielsen, S. S., Food Analysis, Springer: moisture content and total solids methods (consulted through Google Books).
