FOOD ANALYSIS: ASHING
Diagram
Two ways to destroy organic matter
Dry ashing
The sample is burned in a muffle furnace until only the mineral ash is left. Simple, slow, and hot enough to lose some volatile minerals.
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Wet ashing
The sample is digested in strong acids, which oxidise the organic matter at much lower temperatures and leave the minerals in solution.
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The choice
Dry ashing suits total ash content. Wet ashing suits mineral analysis, especially for volatile trace elements.
Figure 1: Dry ashing burns the sample in a furnace; wet ashing digests it in acids. Drawn in our own style from the FTN Food Analysis notes, Unit 3, and from Joslyn, Methods in Food Analysis, chapter 5, Ash Content and Ashing Procedures.
Introduction
Dry ashing and wet ashing are the two ways of preparing a food sample for mineral analysis, and both work by destroying the organic matter so that only the minerals are left to measure. The difference is the destroyer: dry ashing burns the sample away with heat in a muffle furnace, while wet ashing digests it with strong oxidising acids at much lower temperatures. That difference decides which minerals survive: the furnace can lose volatile ones, while the acids keep them in solution.
The FTN Food Analysis notes, Unit 3, describe dry ashing for total ash, and Joslyn, Methods in Food Analysis, chapter 5, Ash Content and Ashing Procedures, covers both methods. This article follows those two sources, with Cantle, Atomic Absorption Spectrometry, which uses dry ashing in sample preparation.
Quick comparison
Table 1: Dry ashing compared with wet ashing
| Aspect | Dry ashing | Wet ashing |
|---|---|---|
| How organic matter is destroyed | Burning in air at high temperature: the organic matter is oxidised away | Chemical oxidation with strong acids in solution |
| Temperature | High, in a muffle furnace, commonly about 500 to 600 C | Much lower, heated with the acids, well below furnace temperatures |
| Equipment | A muffle furnace, and a dish or crucible for the sample | A flask or beaker, strong acids, and a fume hood for the vapours |
| Reagents | Almost none: only acid at the end, to dissolve the ash for testing | Large volumes of strong oxidising acids, such as nitric acid with perchloric or sulphuric acid, or hydrogen peroxide |
| Time | Slow, until the ash is white or pale grey and its weight stops changing | Faster, often completed in under an hour |
| Best for | Total ash content, for a label or a proximate analysis | Preparing a solution for mineral analysis, for AAS or ICP |
| Volatile minerals | Can be lost at high temperature: some sodium and potassium salts, and volatile elements such as lead, mercury and arsenic | Kept in solution, with little or no loss from volatilisation, because the temperatures are lower |
| After the step | The ash is dissolved in acid before single minerals are tested | The minerals are already in solution, ready for testing |
| Sample handling | The sample is first charred gently over a low flame so it does not flare and spit, which would throw material out of the dish | The acids are added to the sample in a flask, and the mixture is heated with care under a fume hood |
| Risks | Spitting and loss of material if the charring is too fast; losses of volatile minerals | Corrosive acids that can burn, and toxic fumes; perchloric acid needs special care, because it can react violently with organic matter when hot and concentrated |
Scientific explanation
Definition
Dry ashing is the burning of a food sample in a muffle furnace until the organic matter is oxidised away and only the inorganic residue, the ash, is left: the sample is first charred gently over a low flame so that it does not flare and spit, then placed in the furnace until the ash is white or pale grey and its weight stops changing. Wet ashing, also called wet digestion or wet oxidation, is the digestion of a food sample in strong oxidising acids, which break down the organic matter at much lower temperatures and leave the mineral constituents dissolved in solution.
Working principle
In dry ashing, the sample is ignited in the furnace, the organic matter burns away as carbon dioxide and water, and the inorganic residue stays behind as ash. The ash gives the total mineral content, which is part of the food’s composition and its label, and it is a check on the other results: in a proximate analysis, moisture plus ash plus protein plus fat plus carbohydrate plus fibre should add up to about 100 percent. Ash can also reveal adulteration: flour with added chalk or sand shows a high ash, and a spice cut with brick dust shows it too. In wet ashing, the organic matter of the sample is oxidised with concentrated nitric and perchloric acids, or with other acid mixtures. The acids are partly removed by volatilisation, and the soluble mineral constituents remain dissolved in nitric acid, ready for testing by AAS or ICP. Wet oxidation is preferred where volatile constituents would be lost in dry ashing, and where the ash residue dissolves slowly after burning.
Apparatus
Dry ashing needs a muffle furnace, a dish or crucible, a balance, and a desiccator for cooling the dish before weighing. Wet ashing needs a flask or beaker, strong acids, a heater, and a fume hood, because the acid vapours are toxic.
Measurement procedure
For dry ashing, the steps are: weigh the sample, char it gently over a low flame, place it in the muffle furnace until the ash is white or pale grey and its weight stops changing, cool the dish in a desiccator, and weigh it again. The worked example is given under Formulas and calculations below. For wet ashing, the steps are: add the acids to the sample in a flask, heat the mixture under a fume hood until the organic matter is destroyed and the solution is clear, and keep the minerals in solution for testing. The books used here give no step by step procedure with volumes for wet ashing, so none is invented here.
Factors affecting readings
For dry ashing, the temperature is the main factor: volatile minerals, such as some sodium and potassium salts, can be lost at high temperature, so the stated temperature is followed exactly, and the dish is cooled in a desiccator before weighing, just as in the moisture test. For wet ashing, the choice of acids and their purity decide the result: large volumes of acids are used, and if they are not pure enough they can add the very elements being measured, giving results that are too high. The sample matrix also matters: wet or dry ashing is chosen by the elements being investigated and the analysis that follows.
Formulas and calculations
Worked example for dry ashing, from the FTN Food Analysis notes, Unit 3: a biscuit sample weighs 3.000 g, and the ash left after burning weighs 0.045 g. Ash = 0.045 divided by 3.000, times 100 = 1.5 percent. If the same biscuit tested at 4.0 percent moisture, 8.0 percent protein, and 20.0 percent fat, the remaining carbohydrate by difference is about 66.5 percent, which is the kind of cross-check the ash value makes possible.
For wet ashing the books give no worked numerical example, so none is given: the minerals are read from the solution by AAS or ICP, against standards.
Industrial applications
- Dry ashing: measuring total ash content for a food label and for the proximate sum.
- Dry ashing: checking for adulteration, such as flour with added chalk or sand, which shows a high ash.
- Dry ashing: preparing a sample for mineral testing, where the ash is dissolved in acid first.
- Wet ashing: preparing samples for mineral analysis by AAS or ICP, where the minerals must stay in solution.
- Wet ashing: measuring volatile trace elements such as lead, mercury, arsenic and cadmium, which would be lost in a furnace.
- Wet ashing: digesting foods whose ash residue dissolves slowly after dry ashing.
Advantages and limitations
- Dry ashing, advantage: simple, needing almost no reagents, so there is little risk of adding trace elements from outside, and many samples can be run at once.
- Dry ashing, limitation: slow, and the high temperature can lose volatile minerals, so it does not suit trace analysis of volatile elements.
- Wet ashing, advantage: faster, at much lower temperatures, and it keeps volatile elements in solution, so it suits trace mineral analysis.
- Wet ashing, limitation: it needs large volumes of strong acids, which must be very pure, and it must be done under a fume hood with constant attention, because the acids are corrosive and their vapours are toxic.
FAQ
What is the main difference between dry ashing and wet ashing?
Dry ashing burns the sample away with heat in a muffle furnace, while wet ashing digests it with strong oxidising acids at much lower temperatures.
Why is wet ashing chosen for lead or mercury?
Because the furnace temperatures of dry ashing can turn volatile elements into vapour and lose them, while wet ashing keeps them in solution.
What is ash in food analysis?
The inorganic residue left after the organic matter is burned away: it gives the total mineral content of the food.
Why is the sample charred gently before dry ashing?
So that it does not flare and spit, which would throw material out of the dish and lose sample.
Why must wet ashing be done under a fume hood?
Because the strong acids give off toxic vapours, and perchloric acid can react violently with organic matter when hot and concentrated.
References and further reading
- FTN subject: Food Analysis, Unit 3: Moisture and Ash; Unit 6: Mineral and Vitamin Analysis.
- Maynard Joslyn, Methods in Food Analysis: Applied to Plant Products, Elsevier, 2012 (Google Books page read)
- J. E. Cantle (ed.), Atomic Absorption Spectrometry, Elsevier, 1986 (Google Books page read)
