Lecture 6- Ash Analysis.pdf

Food Analysis: Ash Analysis Saji George Associate Professor, Department of Food Science and Agricultural Chemistry, Macdonald-Stewart Building, Room-1039 (MS1-039) Tel: 514-398-7920, Fax: 514-398-7990 Email: [email protected] Web...

Food Analysis: Ash Analysis Saji George Associate Professor, Department of Food Science and Agricultural Chemistry, Macdonald-Stewart Building, Room-1039 (MS1-039) Tel: 514-398-7920, Fax: 514-398-7990 Email: [email protected] Web: http://safe-nano.lab.mcgill.ca/ SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY 1  Content Ash ▪ Relevance ▪ Methods of ash determination- dry, wet and microwave ▪ Characterization of Ash SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ash Analysis: Introduction Ash refers to the inorganic (mineral) residue remaining after the combustion or complete acid- facilitated oxidation of organic matter in food. Principle: Minerals are not destroyed by heating, and that they have a low volatility compared to other food components. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Relevance of Ash analysis 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 (e.g. In refining cane sugar, excessive amounts of minerals interfere with processing (decolorization and crystallization). Adulteration: types of ash reveals potential adulteration. E.g. Ash content is a useful parameter in distinguishing fruit vinegar from synthetic vinegar. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ash content of common food items Generally, ash content of oils/fat will be very low Cured meat may have ash up to 11% Generally, ash content of animal products does not vary too much whereas ash content in plant products tend to vary with type of soil and fertilization strategy SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures [O] Food sample CO2+ N Oxides (NO, N2O, NO2)+ Combustion 500-6000C H2O + Gray-white residue (Ash) contains a mixture of metals, metal oxides e.g. AlO2, SO4, PO3 and metal salts e.g. NaCl, KCl -Some metals (e.g. Pb, Hg) volatilize above 6000C, so temperature between 500-6000C is preferred for dry ashing SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY Types of crucible: Quartz crucibles are resistant to acids and halogens, but does not tolerate alkali at high temperature. They are stable until 9000C. Porcelain crucibles: inexpensive, the same properties as Quartz crucibles, they will crack with rapid temperature changes. Not good for alkaline samples SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY Steel crucibles: inexpensive, resistant to both acids and alkalies, they contain chromium and nickel. Platinum crucibles: Resistant to acid and alkali and high temperature. Very expensive for routine use for large numbers of samples. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 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. Four types of analytical procedure used to determine the ash content of foods are based on this principle: 1-Dry ashing 2-Wet ashing 3- Microwave ashing 4-Low temperature plasma dry ashing SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 1-Dry ashing: -Use a high temperature muffle furnace (500-600°C). -Water and other volatile materials are vaporized and organic compounds are burned in the presence of oxygen to CO2, H2O and N2. -Most minerals are converted to oxides, sulfates, phosphates, chlorides or silicates. -The sample is weighed before and after ashing SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 1-Dry ashing: General protocol 1. Weigh a 5–10-g sample into a tared crucible. Pre-ash if the sample is very moist and/or contain high levels of fat. 2. Place crucibles in a cool muffle furnace. Use tongs, gloves, and protective eyewear if the muffle furnace is warm. 3. Ignite 12–18 h (or overnight) at about 550OC. 4. Turn off muffle furnace and wait to cool down to at ~250◦C. 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 desiccant. 6. Cover crucibles, close desiccator, and allow crucibles to cool prior to weighing SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures Ash is the inorganic residue obtained after a complete combustion of the organic matter in a food. Wt of Crucible=18g Wt. of rice=2g Pre-ashing 1h In Fume hood Desiccator For food high in fats and organic matter Muffle furnace Wt. of ashed sample+ crucible= 18.04 g Incineration 500-600 0C for 2hr SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Wt. of crucible+ Ash= 18.04g Wt. of Ash= 18.04-18= 0.04g Ash content= Wt. of Ash / Wt. of sample *100 = 0.04/ 2 *100 =2% The ash content can be expressed on either a dry or wet basis. % ash (dry basis) = weight after ashing x 100 weight before ashing (dry) % ash (wet basis) = weight after ashing x 100 weight before ashing (wet) SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 1-Dry ashing: Advantages: ▪ Usually used for determining total ash content and characterization of ash ▪ Relatively safe and simple (not labor intensive) ▪ Many samples can be analyzed simultaneously, ▪ Ash can be used for determining specific mineral content Disadvantages: Long time required, Costly to run due to power consumption, Possibility of losing volatile minerals (at high temperature eg, Cu, Fe, Pb, Hg, Ni and Zn) SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures Black spots: - With high carbohydrate foods, sometime complete combustion is not easy to obtain. - At high temperature some CHO trap inorganic materials leading to tiny black spots. - This is due to fused salts e.g. NaCl, KCl. Phosphate salts and sulfate salts do not form fused salts. - Black spots are evidence of organic materials (incomplete combustion). - It can be solved by adding a small amount of water or HNO3 and re-ashing. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 2-Wet ashing: Primarily used in the preparation of samples for subsequent analysis of specific minerals. Wet ashing breaks down and removes the organic matrix surrounding the minerals so that they are left in an acidic solution. A dried ground food sample is usually weighed into a flask containing strong acids and oxidizing agents (e.g. nitric, sulfuric acid, perchloric acid, H2O2) and then heated. The process takes from 10 min to a few hours at 120-3000C. The resulting solution is assayed for metal ions using AAS or ICP-MS Metal content is expressed as ppm or ppb SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 2-Wet ashing: Oxidation of organic contents by acid and heat Advantages: Little loss of volatile minerals due to low temperature used Rapid than dry ashing Provides solution for determination of individual minerals Disadvantages: Labor intensive Requires a special fume-cupboard Low sample throughput May not be appropriate for total ash determination and characterization of ash SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 2-Wet ashing: 1. Weigh sample and add to Erlenmeyer flask (previously acid washed and dried) 2. Add 3 ml of concentrated H2SO4 followed by 5 ml of HNO3 (a combination of HNO3–HClO4 (acid and oxidizer) is used, especially when S is an element of interest) 4. Heat the sample on a hot plate at 200oC (or boiling in block heaters) 5. Once the brown-yellow fumes cease and white fumes from decomposing H2SO4 are observed, the sample will become darker. Remove the flask from the hot plate 6. Slowly add 3–5 ml of HNO3 7. Put the flask back on the hot plate and allow the HNO3 to boil off. Proceed to the next step when all the HNO3 is removed and the color is clear to straw yellow. If the solution is still dark in color, add another 3–5 ml of HNO3 and boil. Repeat the process until the solution is clear to straw yellow 8. Reduce the volume appropriately to allow for ease of final transfer and dilute using ultra pure water and use the resulting solution for determination of minerals using AAS or ICP-OES/MS SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Hot plate for sample digestion SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Ashing procedures 3-Microwave Wet ashing: Microwave as source of heat Faster and complete digestion at high temperature (2500C) and high pressure Nitric acid is used as the digestant Programmable instrument that handle multiple samples at a time Microwave dry ashing- Without acid SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  3-Low Temperature Plasma Ashing A sample is placed into a glass chamber which is evacuated using a vacuum pump. A small amount of oxygen is pumped into the chamber and broken down to nascent oxygen by application of an electromagnetic radio frequency field. The organic matter in the sample is rapidly oxidized by the nascent oxygen and the moisture is evaporated because of the elevated temperatures at low pressure. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY 3-Low Temperature Plasma Ashing -Advantages: Less chance of losing trace elements by volatilization. -Disadvantages: relatively expensive equipment and small sample throughput. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Characterization of ash Water-soluble/insoluble ash: Used as index of the fruit content of jelly and fruit preserves Alkalinity of ash: Ash of fruits and vegetable is alkaline. Ash Insoluble in Acid: An index of contamination. SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Characterization of ash Water solubility Adding Water Filtration Heat nearly to boiling Re-ashed Water-Insoluble ash Water-Soluble ash ▪ Water insoluble ash is an indication of the presence of siliceous (silica) matter Note: If titrated ▪ Lower water insoluble ash indicate higher fruit content of jam and with 0.1N HCL jellies (lower % of the skin, seed, stem etc.) (Alkalinity of ash) ▪ Ash of fruits and vegetable is alkaline, while ash of meats and some cereals is acidic SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY  Characterization of ash Acid solubility Adding 10% HCl Filtration Heat nearly to boiling Re-ashed Acid-Soluble Acid- Insoluble ▪ Acid insoluble ash indicate the surface contamination of fruits and vegetables and wheat and rice coatings. ▪ Generally silicates SAJI GEORGE @ S.A.F.E NANO, MCGILL UNIVERSITY