Ashing Techniques and Importance in Food Analysis

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What is dry ashing?

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Dry ashing refers to the use of a muffle furnace capable of maintaining temperatures of 500–600°C, where water and volatiles are vaporized, and organic substances are burned in the presence of oxygen to CO2 and oxides of N2.

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What happens to most minerals during dry ashing?

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Most minerals are converted to oxides, sulfates, phosphates, chlorides, and silicates.

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Which elements may partially volatilize during dry ashing?

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Elements such as Fe, Se, Pb, and Hg may partially volatilize during dry ashing.

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What is wet ashing?

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Wet ashing is a procedure for oxidizing organic substances using acids and oxidizing agents or their combinations.

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How does wet ashing differ from dry ashing?

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Wet ashing solubilizes minerals without volatilization and is often preferable for specific elemental analysis.

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What is the importance of ash content in food analysis?

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Ash content represents the total mineral content in foods and is important for nutritional evaluation and as a preparation step for specific elemental analysis.

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Why is ash content variable in plant sources?

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The elemental content from the ash of plant sources is variable, unlike that from animal products which is usually constant.

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What safety measure is required when using perchloric acid in wet ashing?

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A special perchloric acid hood is required when using perchloric acid in wet ashing.

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AOAC International has several dry ashing procedures (e.g., AOAC Methods 900.02 A or B, 920.117, 923.03) for certain individual foodstuffs. The general procedure includes the following steps: 1. Weigh a 5–10-g sample into a tared crucible. Predry if the sample is very moist. 2. Place crucibles in a cool muffle furnace. Use tongs, gloves, and protective eyewear if the muffle furnace is warm. 3. Ignite 12–18h (or overnight) at about 550◦C. 4. Turnoffmufflefurnaceandwaittoopenituntil the temperature has dropped to at least 250◦C, preferably lower. Open door carefully to avoid losing ash that may be fluffy. 5. Using safety tongs, quickly transfer crucibles to a desiccator with a porcelain plate and des iccant. Cover crucibles, close desiccator, and allow crucibles to cool prior to weighing.

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Wet ashing is sometimes called wet oxidation or wet digestion. Its primary use is preparation for specific mineral analysis and metallic poisons. Often, analyti cal testing laboratories use only wet ashing in prepar ing samples for certain mineral analyses (e.g., Fe, Cu, Zn, P), because losses would occur by volatilization during dry ashing. There are several advantages to using the wet ash ing procedure. Minerals will usually stay in solution, andthere is little or no loss from volatilization because of the lower temperature. The oxidation time is short and requires a hood, hot plate, and long tongs, plus safety equipment. Thedisadvantages of wetashingarethatittakesvir tually constant operator attention, corrosive reagents are necessary, and only small numbers of samples can

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A grain was found to contain 11.5% moisture. A 5.2146 g sample was placed into a crucible (28.5053g tare). The ashed crucible weighed 28.5939g. Calculate the percent age ash on (a) an as-received (wet weight) basis and (b) a dry matter basis.?

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Avegetable (23.5000g) was found to have 0.0940-g acid insoluble ash. What is the percentage of acid-insoluble ash?