ASH DETERMINATION: METHODS AND EXAMPLES - CHEMISTRY - 2025

The determination of ashes is a technique or process to estimate the total amount of minerals normally present in a sample of food. It corresponds to one of the essential analyzes in quality studies and characterization of the food industry.

Ash is understood to be the non-volatile residues that are obtained when incinerating food. These consist essentially of metal oxides and are rich in metal ions that represent the mineral content of food. Depending on the product, the amount of ash affects its quality, being a factor to take into account in quality analysis.

The ashes represent the non-volatile inorganic residues that remain after the incineration of a material or food.

The determination of the ash content is carried out inside a muffle (high temperature furnace), placing the sample in refractory containers known as crucibles. There are many materials, the most used being porcelain. Said content is expressed as a percentage on a dry or wet basis; that is, taking into account or not the humidity of the food.

On the other hand, some analyzes support that the sample is transformed into ash by a wet method. In this way, the “fly ash” is analyzed which, due to the high temperatures of the muffle, ends up escaping from the crucible.

Ash determination methods

Ash determination is carried out using three methods: dry, wet and plasma at low temperatures. Each of them has its advantages and disadvantages compared to the others; however, the dry method is by far the most familiar and intuitive: incinerate the sample until it is scorched.

Dry

The sample is processed according to the standard method (national or international). It is weighed into a crucible that has been previously heated and weighed together with its lid, until its mass does not vary. This reduces the error of weighing due to moisture or negligible residues.

The crucible, with the sample inside, is then placed in the flask and left to heat at a temperature of 500 to 600 ºC for 12-24 hours. Here the organic matter in the sample reacts with oxygen to become water vapor, carbon dioxide and nitrogen oxides, as well as other gaseous compounds.

After the stipulated time has elapsed, the crucible is left to cool and is transferred to a desiccator to prevent it from absorbing humidity from the environment. Once it has completely cooled, it is weighed on the balance and the difference in masses between the crucible and the sample at the end is equal to the mass of the ashes, M _ashes.

Thus, the percentage of ashes is:

% _ash = (M _ash / M _{dry sample}) 100 (dry basis)

% _ash = (M _ash / M _sample) 100 (wet basis)

This percentage on a dry basis means that the sample was dehydrated before even being weighed for incineration.

Muffle. Werneuchen

Damp

The problem with the dry method is that it consumes a lot of electricity, since the muffle must be running for a whole day. Also, the high temperatures volatilize some minerals that are not found in the ash; like those of the elements iron, selenium, mercury, lead, nickel and copper.

For this reason, when you want to analyze the minerals of the metals mentioned above, you use the wet method of ash determination.

This time, the sample is dissolved in acids or strong oxidizing agents, and heated until its organic components are digested.

In the process, the organic matter ends up volatilized, even when the oven works at temperatures no higher than 350 ºC. The water-soluble minerals remain in solution for subsequent spectroscopic (absorption and atomic emission) or volumetric analysis (precipitation titrations or EDTA complexing).

The problem with this method is that, although it is much faster, it is more dangerous because of the handling of corrosive substances. Also more challenging in terms of technical expertise.

Plasma at low temperatures

In the third most used method. The sample is placed in a glass chamber, where it is partially dehydrated by vacuum. Then, a volume of oxygen is injected, which is decomposed by the action of an electromagnetic field, to generate radicals that violently oxidize the sample, while at the same time it is dehydrated at a temperature below 150 ºC.

Examples

Flours

The ash content in flours is of special interest because it is believed to affect the quality of your baked goods. A wheat flour with a lot of ash reveals that it has been ground with too much bran rich in minerals, and that therefore it is necessary to refine its purity, as well as to improve its grinding.

This percentage of ashes should be between 1.5 and 2%. Each flour will have its own ash content depending on the land where it is harvested, its climate, fertilizers, and other factors.

Cookies

The ash content in cookies is subject to the flour with which they are made. For example, those made from banana flour will have the highest amount of ash or minerals. Therefore, fruit cookies can be expected to be richer in minerals than chocolate cookies; or at least in the beginning.

Croquettes for dogs and cats

Dogs and cats need the ash content of their kibbles to be at least 2%; otherwise they will be very low in minerals. For dog food, this percentage should not exceed 6.5%; while for cats, the percentage of ashes in their kibbles should not be higher than 7.5%.

When their kibbles have very high percentages of ash, dogs and cats run the risk of developing kidney stones, just as the excess of minerals impairs the assimilation of other essential for their physiological functions.

meats

For the determination of ashes in meats, they are first defatted, since the fat interferes during incineration. To do this, they are macerated in apolar and volatile solvents, so that they evaporate completely when the sample is placed inside the flask.

Following the same reasoning, a meat with more ash means that its mineral content is higher. In general, meats are rich in protein, but poor in minerals, at least when compared to other products in the food basket. Of the meats, chicken and sausages contain the most ash.

Fruits

Nectarines are fruits rich in ash or minerals. Source: Pixabay.com

Fruits with a relatively high ash content are said to be rich in minerals. However, this does not imply that they are not deficient in other minerals, since each metal is analyzed separately from their ashes. In this way, a nutritional table is built which highlights which minerals make up the fruit in greater or lesser abundance.

For example, nectarines contain a lot of ash (around 0.54%), while pears are low in ash (0.34%). Pears are also low in calcium, but rich in potassium. That is why the ash percentage alone is not a good indicator to determine how nutritious a fruit is.

Someone with a potassium deficiency would be better off eating pears or bananas, while if their body demands calcium, then peaches would be better for them.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Dr. D. Julian McClements. (2003). Analysis of Ash and Minerals. Recovered from: people.umass.edu
3. Ismail BP (2017) Ash Content Determination. In: Food Analysis Laboratory Manual. Food Science Text Series. Springer, Cham
4. Courtney Simons. (October 29, 2017). Determination of Ash Content. Food Science Toolbox. Recovered from: cwsimons.com
5. Wikipedia. (2020). Ash (analytical chemistry). Recovered from: en.wikipedia.org
6. Guest Post. (August 8, 2017). Estimation of Ash Content in Food. Recovered from: discoverfoodtech.com
7. Wheat Quality & Carbohydrate Research. (March 27, 2018). Flour Analysis. Recovered from: ndsu.edu
8. Loza, Angélica, Quispe, Merly, Villanueva, Juan, & P. ​​Peláez, Pedro. (2017). Development of functional cookies with wheat flour, banana flour (Musa paradisiaca), sesame seeds (Sesamum indicum) and storage stability. Scientia Agropecuaria, 8 (4), 315-325. dx.doi.org/10.17268/sci.agropecu.2017.04.03
9. Pet Central. (June 16, 2017). The Importance of Ash Level in Pet Food. Recovered from: petcentral.chewy.com
10. Farid and Neda. (2014). Evaluation and Determination of Minerals Content in Fruits. International Journal of Plant, Animal and Environmental Sciences.