FOOD ANALYSIS: MOISTURE
Diagram
Two ways to measure moisture
Karl Fischer titration
A chemical test: the water in the sample reacts with iodine and sulphur dioxide, and the reagent used tells how much water there was.
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Oven drying
A weighing test: the sample is weighed, dried in an oven until its weight stops changing, and weighed again. The loss in weight is the moisture.
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The choice
Karl Fischer suits low moisture and foods that heat would damage. Oven drying is the reference method for routine foods.
Figure 1: Karl Fischer titration measures water by a chemical reaction; oven drying measures it by loss in weight on heating. Drawn in our own style from the moisture units of the FTN Food Analysis notes, Unit 3, and the FTN Food Chemistry notes, Unit 2.
Introduction
Karl Fischer titration and oven drying are the two direct methods for measuring moisture in food, and both are direct because they measure the water itself, not a property that changes with water. The difference is how the water is counted: Karl Fischer titration counts it by a chemical reaction with iodine and sulphur dioxide, while oven drying counts it by loss in weight when the sample is heated. This article compares the titration method with the oven method: it is not about the instruments, which the earlier article on the drying oven and the moisture analyser covers.
The FTN Food Analysis notes, Unit 3, call oven drying the reference method for moisture, and the FTN Food Chemistry notes, Unit 2, set the two direct methods side by side. This article follows those two units, with deMan and colleagues, Principles of Food Chemistry, chapter on moisture analysis.
Quick comparison
Table 1: Karl Fischer titration compared with oven drying
| Aspect | Karl Fischer titration | Oven drying |
|---|---|---|
| What is measured | Water itself, by a precise chemical reaction | Loss in weight on heating, called the moisture |
| Principle | Water reacts with iodine and sulphur dioxide in a precise chemical reaction; the reagent used gives the water | The sample is dried until its weight stops changing, and the loss in weight is taken as the moisture |
| Heating | No heating: the test runs at room temperature | Heated in an oven, usually near 100 to 105 C for many foods |
| Time | Fast, a titration that runs in minutes | Slow, hours until the weight stops changing |
| Best for | Low water content, and foods that oven heat would damage, such as a dried powder | Routine foods, and as the reference method behind rapid methods |
| Sample handling | The sample is dissolved or suspended in the titration vessel, kept dry from the air | The sample is spread thin in a shallow dish so water can escape, and cooled in a desiccator before the final weighing |
| What can go wrong | Any water from the air or the reagents counts as sample water, so the apparatus and reagents must be kept dry | Some foods lose more than water: volatile flavour compounds escape, and sugars can break down at high temperature; some foods hold water so tightly that not all of it escapes at 100 C |
| Result | Water only, because only water reacts with the reagent | Water plus anything else volatile that leaves on heating |
| Reference status | A direct method used where the oven method fails | The reference method: rapid methods are checked against it for each food |
| Skill and cost | Needs a titration set-up, standardised reagent, and care with dry conditions | Simple and trusted, needing only an oven, dishes and a desiccator |
Scientific explanation
Definition
Karl Fischer titration is a chemical method that measures water by a precise reaction with iodine and sulphur dioxide, and it is used when the water content is low or when oven heat would damage the food. Oven drying is the reference method for moisture: the sample is weighed, dried in an oven at a set temperature until its weight stops changing, weighed again, and the loss in weight is called the moisture.
Working principle
In the Karl Fischer titration, the reagent is added to the sample until the water in it has reacted with the iodine and sulphur dioxide, and the volume of reagent used at the end of the reaction gives the water present. Because the reaction counts water only, the result is the true water content even in a food that would lose flavour compounds or break down in an oven. In oven drying, the sample is spread thin in a shallow dish so that water can escape, dried at a set temperature, usually near 100 to 105 C for many foods, until its weight stops changing, and cooled in a desiccator, a sealed container with a drying agent, before the final weighing, so that it does not pick up water from the air while it cools. Moisture content itself is simply the weight of water in the food, reported as a percentage of the food’s weight.
Apparatus
Karl Fischer titration needs a titration vessel kept dry, a burette or automatic titrator, the Karl Fischer reagent, and a means of finding the end of the reaction. Oven drying needs an oven held at a set temperature, shallow dishes, a balance, and a desiccator for cooling the sample before the final weighing.
Measurement procedure
For oven drying, the steps are: weigh the sample, dry it in the oven until its weight stops changing, cool it in a desiccator, weigh it again, and call the loss in weight the moisture. The worked example is given under Formulas and calculations below. For Karl Fischer titration, the steps are: keep the vessel and reagents dry, add the sample, titrate with the reagent until the reaction is complete, and read the water from the volume of reagent used. The books used here give no worked numerical example for the titration, so none is given.
Factors affecting readings
For oven drying, the temperature, the time, and the food itself decide the result: some foods lose volatile compounds other than water, sugars can break down and gain weight at high temperature, and some foods hold water so tightly that it does not all escape at 100 C, so the method states a different temperature or a different approach for those foods, such as drying under vacuum at a lower temperature. Moisture results also change with the air, because flour takes up water from damp air and loses water to dry air, so results are always reported with the method and the basis, as received or on a dry basis, and samples are sealed at once. For Karl Fischer titration, water from the air or from damp reagents is the main error, so dry conditions decide the result.
Formulas and calculations
Worked example for oven drying, from the FTN Food Analysis notes, Unit 3: 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. Dry matter = 100 minus 12.0 = 88.0 percent. If the same flour later tests at 11.5 percent protein as received, its protein on a dry basis is 11.5 divided by 0.880 = 13.1 percent.
For Karl Fischer titration the books used here give no worked numerical example, so none is given: the water is read from the volume of reagent used at the end of the reaction, against a standardised reagent.
Industrial applications
- Karl Fischer titration: measuring the low moisture of a dried powder that oven heat would damage.
- Karl Fischer titration: measuring moisture in foods where the water content is low and oven drying would be too rough.
- Karl Fischer titration: checking moisture in foods that lose volatile flavour compounds in an oven.
- Oven drying: routine moisture testing of flour, biscuits and other foods, as the reference method.
- Oven drying: checking rapid moisture methods, such as a moisture balance or an electrical meter, for each new food.
- Oven drying: giving the dry basis for protein, fat and ash results in a proximate analysis.
Advantages and limitations
- Karl Fischer titration, advantage: it measures water only, by a chemical reaction, so it works for low moisture and for foods that heat would damage.
- Karl Fischer titration, limitation: the vessel and reagents must be kept dry, because any water from the air counts as sample water, and it needs a titration set-up and standardised reagent.
- Oven drying, advantage: simple, trusted and cheap, and it is the reference method against which rapid methods are checked.
- Oven drying, limitation: slow, taking hours, and it can mislead when a food loses volatile compounds other than water, or when sugars brown and gain weight.
FAQ
What is the main difference between Karl Fischer titration and oven drying?
Karl Fischer titration measures water by a chemical reaction with iodine and sulphur dioxide, while oven drying measures it by loss in weight when the sample is heated.
When is Karl Fischer titration chosen over oven drying?
When the water content is low, or when oven heat would damage the food, such as a dried powder.
Why is oven drying called the reference method?
Because it is simple and trusted, and rapid methods such as a moisture balance or an electrical meter are checked against it for each food.
Why is the sample cooled in a desiccator after oven drying?
So that it does not pick up water from the air while it cools, before the final weighing.
Can oven drying overstate the moisture?
Yes: some foods lose volatile flavour compounds as well as water, and sugars can break down at high temperature, so the loss in weight is more than the water alone.
References and further reading
- FTN subject: Food Analysis, Unit 3: Moisture and Ash; FTN subject: Food Chemistry, Unit 2: Water.
- John M. deMan, John W. Finley, W. Jeffrey Hurst and Chang Yong Lee, Principles of Food Chemistry, 4th ed., Springer, 2018 (Google Books page read)
