Of course, there is isn’t much real Ancient and Secret History – although it is a diverting acronym. Nonetheless, the fact that you can take pretty much anything you like and change its very nature by incineration at high temperature has been known and often deliberately obscured for many millennia. Indeed, although the apparent magic of metalworking has been around for at least 10,000 years, it arguably wasn’t until the 18th century that Lavoisier demonstrated the principles of constant mass and began to understand exactly what might be happening in a furnace. But what, one may ask, has any of that to do with food testing.
Analysis for ash is actually one of the oldest food authenticity tests. Determinations of ash were used by the Victorian food analysts, the original analytical chemists, to indicate adulterations in such essentials as flour and spices. It must have been tempting for the spice merchants of old to bulk out their products with a shovel or two of suitably coloured dry soil, and thereby help extend a healthy profit. It must have been pretty tempting then and it is probably pretty tempting now, but soil is pretty much full of ash and spices certainly aren’t, so this sort of gross adulteration is readily revealed. As well as adulteration, ash content can also be a very good indicator of specific features of a food product. For example, high or elevated levels of ash in minced meat can indicate high or elevated levels of bone being present, which could characterise a specific butchery procedure having been applied.
But what, after all, is “ash”? It will come as no surprise to learn that whilst it is almost impossible to specifically define the chemical nature of ash, it is very easy to define the physical process that it results from. Therefore, in food testing ash is simply “the residue remaining following incineration”. That incineration usually occurs at a temperature between 500°C and 550°C. It is possible to propose a lower temperature, and higher temperatures are occasionally used in specific circumstances, but for general purposes the 500/550 °C range seems to do the job pretty well. At that temperature virtually all the organic material is destroyed and, most of the time, a dark grey to white ash residue remains. It is obviously a collation of all the inorganic material that was either a thermally stable salt to begin with or has reacted to form a thermally stable salt within the furnace. This ash will then comprise such compounds as sulphates, chlorides, oxides, phosphates etc. This does mean that the ash of a food material can range from anything from zero (e.g. pure sugar) through to 100% (e.g. pure salt).
The main purpose of determining the ash content within food testing is usually as an estimation of the mass of inorganic material in the food. The result is then used to calculate a value for carbohydrate within the food and hence ultimately the energy content of a food. These calculations are based upon the very sound assumption that the major constituents of a food comprise only moisture, ash, fat protein and carbohydrate. It is relatively easy to determine levels of moisture, ash, fat and protein so whatever remains must be a carbohydrate of some form.
The actual process of reducing a food material to ash can also be used as a preparatory stage within food analysis. This is particularly true with reference to the determination of a specific mineral such as sodium. Complex materials such as food almost always need pre-treatment prior to elemental analysis, usually either a dry ash, or alternatively a wet oxidation using acid. Having determined the ash content of a food it is an easy subsequent step to then use that residue for a sodium determination.
However, it should not be assumed that the incineration of a sample of food is necessarily an easy or simple process. Although the physical conditions required are severe, the treatment of the test portion can be very important. Certainly, introducing a mass of wet material to a furnace already at 550°C is likely to be problematical, as the immediate and ferocious boiling of water will almost certainly cause the material to “spit” and send fragments flying in all directions. It is a lesson that many trainee analysts have to learn, and in my particular memory raw sausages and cooked rice were always samples to be wary of – simple drying or charring of the test portion is almost always a good idea before placing in a hot furnace. Even then, random samples sometimes appear astonishingly resilient to even a lengthy incineration process; although it is usually nothing that a drop of deionised water and another 30 minutes in the furnace cannot resolve.
There are also choices of instrumentation to be considered. Not only are traditional laboratory “muffles” used, but also microwave units that can complete an incineration in significantly shorter time periods. Both methods can be used most successfully. However, although there are a number of circumstances where a microwave would be my method of choice, there is no doubt that an overnight incineration in an old fashioned laboratory furnace is very hard to beat as a rugged and routine procedure.