食品分析. 4. Ash and minerals

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1 The ash content of a foodstuff is the residue remaining all the moisture has been removed and organic matter has been burn away at ca. 550 C. => Expressed on wet weight or on dry weight basis. Organic matter: C => CO 2 ; N => nitrogen oxides; S => SO 2. Total ash: Determined by weighing the dry mineral residue of organic materials heated at elevated temp (550 C). Total ash content is a useful parameter of the nutritional value of some foods and feeds. Water-soluble ash: An index of the fruit content of jelly and fruit preserve. Acid-insoluble ash: An index of mineral matter (dirt or sand) and determined after digesting the total ash in 10% HCl. Salt-free ash: Important in seafoods and determined as the difference between total ash and NaCl in the ash. *The amounts of mineral elements present in food are generally small, often less than 1% of the food in total. 1) Major elements (>0.01% or 100 ppm): 7 elements. Calcium (Ca), chlorine (Cl), magnesium (Mg), phosphorus (P), potassium (K), sodium (Na) and sulfur (S). 2) Trace elements (<0.01% or 100 ppm): 15 elements. Arsenic (As), chromium (Cr), cobalt (Co), copper (Cu), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), selenium (Se), silicon (Si), tin (Sn), vanadium (V) and zinc (Zn). 3) Trace but toxic elements: 6 elements. Beryllium (Be), cadmium (Cd), mercury (Hg), lead (Pb), palladium (Pd) and thallium (Tl). 18

2 *Sources of minerals: Ca: Most dairy products, cereals, nuts, some fish, eggs and certain vegetables. P: Most dairy products, grains and grain products, nuts, meat, fish, poultry, eggs and legumes. Fe: Most grains and grain products, flours, meals, baked and cooked cereals, most nuts, meat, poultry, seafoods, fish and shellfish, eggs and legumes. Na: All salted foods. K: Most dairy products, fruits, cereals, nuts, meat, fish, poultry, eggs and vegetables. Mg: Nuts, cereals and legumes. Mn: Cereals, vegetables and some fruits and meat. Cu: Seafoods, liver, cereals and vegetables. S: Protein-rich foods and some vegetables. Co: Vegetables and fruits. Zn: Some seafoods. 19

3 *Ashing procedure: Wet or liquid samples are usually dried in ovens. Plant materials with 10% moisture may be ashed without prior drying. Ashing of the same spl may follow drying and fat extraction. 1) Drying ashing: For most samples. 2) Modified dry ashing: 3) Wet ashing: For samples high in fat content. *Drying ashing: For most samples. In dry ashing, the sample is weighed into a dish and the organic matter is burned off without flaming and heated for a fixed time or to a constant weight. The residue must be free from carbon (black color), is cooled in a desiccator and the amount of total ash is weighed. Crucible materials: quartz, Vycor, porcelain, steel, nickel, platinum and gold-platinum alloy. Dry ashing for the detection of organic matter prior to determination of trace elements is not sued extensively due to losses occurred from volatilization. 20

4 *Modified dry ashing: Certain modifications and additives are proposed to accelerate the process, to prevent overall losses of minerals, or to improve the retention of critical components. Liquid and moist materials should be dried prior to ashing, and it always done for moisture determination. High fat samples should be extracted before ashing or burned off before closing the furnace. Pure cotton wool of known ash content is added to aid the ashing of various moist materials, such as jams. Pure olive oil is added after drying and before ashing to carbohydrate-rich materials, which swell and foam excessively. To accelerate ashing, the following can be added: Pure glycerin or alcohol; chemical oxidants, e.g. hydrogen peroxide; ammonium carbonate; aluminum chloride and magnesium acetate (blank). *Wet ashing, wet digestion or wet oxidation: Sample preparation procedure for further specific mineral analysis. 3 acid mixtures: 1) Nitric and sulfuric acids; 2) Sulfuric acid and hydrogen peroxide; and 3) Mixtures containing perchloric acid. Mixture of sulfuric and nitric acids is the usual reagent for the decomposition of organic materials in wet-ashing procedure. 21

5 *Dry ashing vs. wet digestion: Dry ashing: Used to determine water-soluble, water-insoluble and acid-insoluble ash. Applicable to determine most common metals. Little attention required, simple and well suited for routinely large numbers of samples. No reagents added or no blank subtraction required. Takes a long time, can be accelerated by modified methods, or can be done overnight. Losses of volatile elements. Wet oxidation: Relatively low temp, liquid conditions, simple apparatus, and rapid oxidation. Required large amounts of corrosive reagents and the correction for the reagents. Difficult to handle large numbers of samples and consume the operator's time. 22