FOOD ANALYSIS : MOISTURE AND SOLIDS
The whole sample, water included
Protein 11.5 percent as received
Dry matter only, water taken out
Protein 13.1 percent on a dry basis
Same flour, same protein, different denominator.
Figure 1: The same flour reported on a wet basis and on a dry basis. The protein weight does not change: only the weight it is divided by changes.
Introduction
Wet basis and dry basis are two ways of reporting the same laboratory result. A wet basis result is worked out as a percentage of the whole sample, water included, and it is the same as reporting a result as received. A dry basis result is worked out as a percentage of the dry matter only, with the water taken out of the calculation. The difference matters because flour takes up water from damp air and loses water to dry air, so the same sack can read differently on a humid day and a dry day while its dry matter stays the same.
Quick comparison
Table 1: Wet basis compared with dry basis
| Aspect | Wet basis | Dry basis |
|---|---|---|
| What it means | Result as a percentage of the whole sample weight, water included | Result as a percentage of the dry matter, water removed |
| Other name | As received basis | Dry matter basis |
| Denominator | Total sample weight | Sample weight minus water weight |
| Value | Lower, because water adds weight to the denominator | Higher, because the denominator is smaller |
| Changes when the food picks up water | Yes: the result falls even though the dry matter is unchanged | No: the result stays the same when only water changes |
| Used for | Labels, trade, and payment, where the food is bought as it is | Comparing samples and setting specifications, where water must not hide the difference |
| Needs | The moisture content of the sample, reported with the result | The moisture content, so the dry matter can be worked out |
| Example | Flour protein 11.5 percent as received | The same flour protein 13.1 percent on a dry basis |
Scientific explanation
Definition
Wet basis: a result worked out as a percentage of the food exactly as it arrived, including its water. Dry basis: a result worked out as a percentage of the dry matter, with water removed, so wetter and drier samples can be compared. Moisture content itself is the weight of water in the food, reported as a percentage of the food weight.
Working principle
The two bases differ only in the denominator. The weight of the part being measured, such as protein, stays the same. On a wet basis it is divided by the whole sample weight. On a dry basis it is divided by the dry matter weight, which is the whole weight minus the water weight. Because the dry matter weight is smaller, the dry basis number is always higher than the wet basis number for the same sample.
Apparatus
Both bases use the same laboratory work: a balance, a drying oven, dishes, and a desiccator for the moisture test. No separate instrument is needed for the basis itself: the basis is a calculation made from the moisture result.
Measurement procedure
First, the moisture of the sample is found by oven drying: weigh the sample, dry it until its weight stops changing, cool it in a desiccator, and weigh it again. Second, the part of interest, such as protein, is measured on the sample as received. Third, the dry basis value is worked out by dividing the as received value by the dry matter fraction.
Factors affecting readings
The reading changes with the water in the sample, which follows the air around it: flour in damp air reads wetter than the same flour in dry air. The sample must be sealed at once after sampling, and the moisture result and the basis must be reported together, because a protein result without its basis cannot be checked. Two laboratories that test the same flour will disagree if one reports as received and the other reports on a dry basis.
Formulas and calculations
Moisture percent (wet basis) = weight lost on drying divided by sample weight, times 100. Dry matter percent = 100 minus moisture percent. Dry basis value = as received value divided by the dry matter fraction, where the dry matter fraction is dry matter percent divided by 100.
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. Dry matter = 100 minus 12.0 = 88.0 percent. If the same flour tests at 11.5 percent protein as received, its protein on a dry basis is 11.5 divided by 0.880 = 13.1 percent.
Industrial applications
- Flour mills sell and label by protein as received, and compare lots on a dry basis so that a damp lot is not judged against a dry lot.
- Milk powder buyers pay for total solids, which is the dry matter, not for water.
- Biscuit makers compare batches on a dry basis, because batches can leave the oven with different moisture.
- Proximate analysis reports protein, fat, and ash on a dry basis to remove the effect of changing water.
Advantages and limitations
Wet basis: it describes the food as it is bought, eaten, and labelled, and it needs no extra calculation. Its limit is that it moves with the water content, so it cannot fairly compare a wet sample with a dry one.
Dry basis: it removes the effect of water, so samples can be compared fairly and specifications stay steady. Its limit is that it is not the food as eaten, so it must always be labelled as dry basis or it will be read as the as received value.
FAQ
Question: If a flour reads 12.0 percent protein as received and 13.6 percent on a dry basis, has the protein changed?
No. The protein weight is the same. Only the denominator changed: the dry basis divides by the dry matter instead of the whole weight.
Question: Why does the same sack of flour read differently on a humid Friday and a dry Monday?
Because flour takes up water from damp air and loses water to dry air. The flour did not change its dry matter: its water content followed the air around it.
Question: Which basis should a buyer use to compare two flour lots?
The dry basis. It removes the water, so the comparison shows the real difference in the flour and not the difference in how wet the two lots are.
Question: What must always be reported with a moisture or protein result?
The basis: as received or on a dry basis. A result without its basis cannot be checked by another laboratory.
References and further reading
- FTN Food Analysis, Unit 3: Moisture and ash (notes on disk): wet and dry basis, worked examples for moisture 12.0 percent and protein 11.5 percent as received, 13.1 percent on a dry basis.
- FTN Food Analysis, Unit 1: Why analyse food, sampling and sample preparation (notes on disk): as received and dry basis results can differ for the same flour.
- Nielsen, S. S., Food Analysis, Springer: moisture and total solids, wet and dry basis reporting (consulted through Google Books).
- Nielsen, S. S., Food Analysis Laboratory Manual, Springer: determination of moisture content (consulted through Google Books).
