Food Analysis Lectures - Ash Analysis PDF

Ash Analysis 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. Determination of the ash content of foods is important for man...

Ash Analysis 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. Determination of the ash content of foods is important for many number of reasons:  Nutritional labeling. The concentration 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. Determination of Ash Content  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 (inorganic), and that they have a low volatility compared to other food components. 17  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 other various traditional methods.  Ash contents of fresh foods rarely exceed 5%, although some processed foods can have ash contents as high as 12%, e.g., dried beef. Sample Preparation  As with all food analysis procedures it is crucial to carefully select a sample whose composition represents that of the food being analyzed and to ensure that its composition does not change significantly prior to analysis.  Typically, samples of 1-10g are used in the analysis of ash content.  Solid foods are finely ground and then carefully mixed to facilitate the choice of a representative sample.  Before carrying out an ash analysis, samples that are high in moisture are often dried to prevent spattering during ashing. High fat samples are usually defatted by solvent extraction, as this facilitates the release of the moisture and prevents spattering.  Other possible problems include contamination of samples by minerals in grinders, glassware or crucibles which come into contact with the sample during the analysis.  For the same reason, it is recommended to use deionized water when preparing samples.  Many kinds of crucibles are available (Quarts, Pyrex, Porcelain, Steel, Platinum,…), all of them should be fired and cleaned prior to use. 18 Dry Ashing  Dry ashing procedures use a high temperature muffle-furnace capable of maintaining temperatures of between 500 and 600oC.  Water and other volatile materials are vaporized and organic substances are burned in the presence of the oxygen in air to CO2, H2O and N2.  Most minerals are converted to oxides, sulfates, phosphates, chlorides or silicates.  Although most minerals have fairly low volatility at these high temperatures, some are volatile and may be partially lost, e.g., iron, lead and mercury. If an analysis is being carried out to determine the concentration of one of these substances, then it is advisable to use an alternative ashing method that uses lower temperatures.  The food sample is weighed before and after ashing to determine the concentration of ash present. The ash content can be expressed on either a dry or wet basis: Advantages: Safe, few reagents are required, many samples can be analyzed simultaneously, not labor intensive, and ash can be analyzed for specific mineral content. Disadvantages: Long time required (12-24 hours), muffle furnaces are quite costly to run due to electrical costs, loss of volatile minerals at high temperatures, e.g., Cu, Fe, Pb, Hg, Ni, Zn.  Recently, analytical instruments have been developed to dry ash samples based on microwave heating. These devices can be programmed to initially remove most of the moisture (using a relatively low heat) and then convert the sample to ash (using a relatively high heat). Microwave instruments greatly reduce the time required to carry out an ash analysis, with the analysis time often being less than an hour. The major disadvantage is that it is not possible to simultaneously analyze as many samples as in a muffle furnace. 19 Wet Ashing  Wet ashing is primarily used in the preparation of samples for subsequent analysis of specific minerals (see later).  It breaks down and removes the organic matrix surrounding the minerals so that they are left in an aqueous solution.  A dried ground food sample is usually weighed into a flask containing strong acids and oxidizing agents (e.g., nitric, perchloric and/or sulfuric acids) and then heated.  Heating is continued until the organic matter is completely digested, leaving only the mineral oxides in solution.  The temperature and time used depends on the type of acids and oxidizing agents used.  Typically, a digestion takes from 10 minutes to a few hours at temperatures of about 350oC.  The resulting solution can then be analyzed for specific minerals.  Advantages: Little loss of volatile minerals occurs because of the lower temperatures used, more rapid than dry ashing.  Disadvantages: Labor intensive, requires a special fume-cupboard if perchloric acid is used because of its hazardous nature, low sample throughput.  Determination of Water Soluble and Insoluble Ash  As well as the total ash content, it is sometimes useful to determine the ratio of water soluble to water-insoluble ash as this gives a useful indication of the quality of certain foods, e.g., the fruit content of preserves and jellies.  Ash is diluted with distilled water then heated to nearly boiling, and the resulting solution is filtered. The amount of soluble ash is determined by drying the filtrate, and the insoluble ash is determined by rinsing, drying and ashing the filter paper.  Filter paper should be ash-free. 20