The ash content is a measure of the total amount of minerals present within a food, whereas the mineral content is a measure of the amount of specific inorganic components present within a food, such as Ca, Na, K and Cl. Thus, ash content refers to the total mineral residue left after incineration of organic matter. It has no nutritional significance per se, but the value for ash is a useful check in summing up the proximate composition of food and a measure of its mineral content. It is expressed as g ash per 100 g sample. Determination of the ash and mineral content of foods is important for a number of reasons:
Nutritional labeling. The concentration and type of minerals present must often be stipulated on the label of a food. Quality. The quality of many foods depends on the concentration and type of minerals they contain, including their taste, appearance, texture and stability. Microbiological stability. High mineral contents are sometimes used to retard the growth of certain microorganisms. Nutrition. Some minerals are essential to a healthy diet (e.g., calcium, phosphorous, potassium and sodium) whereas others can be toxic (e.g., lead, mercury, cadmium and aluminum). Processing. It is often important to know the mineral content of foods during processing because this affects the physicochemical properties of foods.
As mentioned above, ash is the inorganic residue remaining after the water and organic matter have been removed by heating in the presence of oxidizing agents, which provides a measure of the total amount of minerals within a food. Analytical techniques for providing information about the total mineral content are based on the fact that the minerals (the analyte) can be distinguished from all the other components (the matrix) within a food in some measurable way. The most widely used methods are based on the fact that minerals are not destroyed by heating, and that they have a low volatility compared to other food components. The three main types of analytical procedure used to determine the ash content of foods are based on this principle: dry ashing, wet ashing and low temperature plasma dry ashing. The method chosen for a particular analysis depends on the reason for carrying out the analysis, the type of food analyzed and the equipment available. Ashing may also be used as the first step in preparing samples for analysis of specific minerals, by atomic spectroscopy or the various traditional methods described below. Ash contents of fresh foods rarely exceed 5%, although some processed foods can have ash contents as high as 12%, e.g., dried beef.