7 04.0Determination of total ash (official method 4 PDF

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CPUT Food Chemistry Laboratory Official Analytical Methods

Method Page 1 of 4 Number Last revised: 19 January 2015 4.0

DETERMINATION OF TOTAL ASH

1. INTRODUCTION Ash is defined as the inorganic residue from the incineration of organic matter. The content and composition depends on the nature of the food and the method of ashing. Determination of ash can be of three classes, i.e. total ash, soluble ash and insoluble ash. Total ash is often used as an indicator of the degree of refinement of a foodstuff. Ash is considered to be that grey, white and fluffy residue remaining after ignition. It is quite often difficult to get repeatable results using the same sample due to progressive decomposition of carbonates and chlorides. For this reason it is very important o use a uniform process for all determinations. 2. HEALTH AND SAFETY 2.1 HAZARDS a) The muffle furnace operates at high temperatures. b) Initial digestion is done using an open flame. 2.2 SAFETY REQUIREMENTS a) Initial digestion (charring) on open flame must be done in a fume cupboard. b) Goggles and safety gloves must be worn during charring and ashing step. a) Tongs must be used when handling and transferring containers. 2.3 PRECAUTIONS a) Avoid inhaling smoke and vapours produced during charring. b) Ensure that you treat hot surfaces and containers appropriately. c) Materials containing water should first be evaporated on a hot plate or steam bath. d) Ashing is usually considered to be complete when the sample ceases to give off smoke or is completely white. If any traces of black remain it means that carbonaceous material is still present. e) A temperature of 500oC is usually considered suitable for ashing. Some components of the ash may be volatilized at this or higher temperatures. This may be overcome by sulphating before ashing (see later). f) With some foods it is difficult to burn of all the carbon. Ashing can be assisted by breaking up the sample with platinum wire or treated with water and filtered through ashless filter paper. The residue on the paper can be returned to the dish and burned off at a higher temperature. The filtrate is then also evaporated in the dish and the ashing completed by a

CPUT Food Chemistry Laboratory Official Analytical Methods

Method Page 2 of 4 Number Last revised: 19 January 2015 4.0

further period in the muffle furnace. g) Take care when transferring the ashed sample to the dessicator because of the fluffiness seen when ashed. Cover with a lid or watch glass. h) Actual ignition may be done over a specified period of time or until a constant sample mass has been attained. i)

Determination of sulphated ash is done by gentle ignition in the container followed by cooling. Moisten with conc. H 2SO4 and gently ignite again. Determine ash as in 5.3 below (even higher temperatures may now be used – up to 800oC).

j)

The sulphation reduces losses due to volatilization and assists complete combustion. Fusible and more volatile substances are converted into more fixed substrates and give better consistency in ash composition and less dependence on ignition temperature.

3. APPARATUS a) Muffle oven capable of operating at 800oC. b) Analytical balance. c) Dessicator with active dessicant. d) Mortar and pestles, blender, spatulas, porcelain crucibles. 3.2 PREPARATION REQUIRED a) Ensure silica gel dessicant is blue (active). 3.3 MAINTENANCE a) Ensure fume cupboard, balances and muffle oven are clean before and after use. 3.4 CALIBRATION a) Calibrate balance using standard weights kit regularly. 3.5 STORAGE a) Standard procedures. 4. CHEMICALS 4.1 CHEMICALS REQUIRED a) None. 4.2 REAGENT PREPARATION a) None. 4.3 STANDARDIZATION a) None.

CPUT Food Chemistry Laboratory Official Analytical Methods

Method Page 3 of 4 Number Last revised: 19 January 2015 4.0

4.4 DISPOSAL a) Dispose of ashed material in a refuse bin. 5. METHOD 5.1 SAMPLING a) Take a representative sample. The AOAC recommends about 2g of dry matter for fish products, grain and feed, 3 – 5g for cereal food, milk or cheese, 5 – 10g of sugar, sugar products, meat or vegetables, 50ml of wine, 25g of juice or fresh fruit and 10g of jellies, jams and preserves. 5.2 SAMPLE PREPARATION a) Dry samples may be ground using a mortar and pestle until fine (specific types of samples may be passed through a sieve to ensure uniform grinding). If not to be analysed immediately, store in close container in a dessicator. b) Wet samples should preferably be dried first (determining moisture simultaneous – see Official Method 5.0 in this manual) and then ground as in (a) above. 5.3 TEST METHOD 5.3.1 Preparation of Crucibles

a) Clean an appropriate number of porcelain crucibles using a Bunsen burner and blow-torch or ash for an hour at 500oC. Crucibles only to be handled with clean tongs.

b) Allow crucibles to cool in a dessicator. c) Weigh each crucible (designated W1) on an analytical balance (note that one cannot use pens to label the crucibles since this will be burnt off during the ashing phase).

5.3.2 Sample Charring (Pre-ashing) a) Weigh between 1 – 5g sample into crucible (W2). b) Carefully char over an open flame in the fume cupboard until sample is black and no more smoke is generated (samples may sputter and cause loss of sample).

5.3.3 Ashing a) Pre-heat muffle furnace to 500oC (or other appropriate temperature depending on product and method specification). b) Use protective gloves and long tongs to carefully place crucibles in muffle furnace (keep a reference table handy indicating the “seating plan” of the different samples and crucibles). c) Ash until white (approximately 5 hours or overnight).

CPUT Food Chemistry Laboratory Official Analytical Methods

Method Page 4 of 4 Number Last revised: 19 January 2015 4.0

d) Switch off furnace and remove crucibles to dessicator to allow cooling. e) Weigh (W3). 5.4 CALCULATIONS Calculate % ash using the following formula: (W3 – W1 ) / (W2 – W1) Or % Ash = sample weight after / sample weight after X 100 6. VERIFICATION a) Using reference standards. b) Error survey against accredited laboratory....