Analysis c5

• Ash

  Inorganic residue remaining after either ignition or complete oxidation of organic matter in foodstuff
• Ash content

  Estimate the total mineral content in foods
• Minerals are not destroyed by heating, and they have a low volatility
• Ash content of most fresh foods rarely > 5%, but could be higher in processed food (e.g.: dried beef = 12%)
• Pure oils & fats contain no ash / little
• Importance of ash analysis

  • Nutritional labeling
  • Quality (appearance, texture, taste or stability)
  • Microbiological stability
  • Nutrition (essential and toxic minerals)
  • Processing (physicochemical properties)
• 3 major types of ashing

  • Dry ashing
  • Wet ashing
  • Low-temperature plasma dry ashing
• Dry ashing

  Incineration of sample at high temp. (500 – 600oC)/ 24 h, in a muffle furnace
• Dry ashing

  • Water & volatiles are vaporized
  • Organic substances are burned in the presence of O2 in air to CO2, H2O of N2 (gas ↑)
  • Most mineral are converted to oxides, sulfates, phosphates, chlorides, and silicates
• Cu, Fe, Se, Pb, Hg, Ni, Zn may partially volatilize with dry ashing procedure - need to use lower temperature ashing method
• Crucible selection for dry ashing

  • Quartz
  • Porcelain
  • Steel
  • Platinum
• Dry ashing

  • Safe method, no added reagents or blank subtraction
  • Little attention is needed once ignition begins
  • Can handle a large number of crucibles at one time
  • Resultant ash can be used for analysis of individual elements, acid insoluble ash, water-soluble & insoluble ash
• Dry ashing

  • Time consuming (12 – 18 hr, overnight)
  • Expensive equipment – electrical cost
  • Lost of volatile elements: As, B, Cd, Cr, Cu, Fe, Pb, Hg, Ni, P, V and Zn
  • May have interaction between crucibles and elements
• Dry ashing procedure

  1. 5 – 10 g sample (pre-dry if very moist)

  2. Place in muffle furnace

  3. Ignite 12 – 18 hr (or overnight), 550oC

  4. Turn off, wait until temp drop to <250oC

  5. Open door carefully to avoid ash that may be fluffy

  6. Quickly transfer crucibles to desiccator
• % ash (dry basis)

  wt after ashing (g) - wt of empty crucible (g) / dry matter (g) x 100
• % ash (wet basis)

  wt after ashing (g) - wt of empty crucible (g) / original sample (fresh) weight (g) x 100
• Completely oxidized sample: white in color
• Procedure for samples with persistent carbon

  1. suspend the ash in water

  2. filter through ashless filter paper

  3. dry the filtrate

  4. place paper & dried filtrate in muffle
• High fat sample: extract or burn off the fat
• Prevent spattering (jem/jellies): mixed sample with cotton wool
• Glycerin, alcohol & hydrogen accelerate ashing
• Cereals: a solution of magnesium acetate as can accelerate ashing, use a blank
• Wet ashing

  Chemically oxidizing organic substances by using strong and oxidizing agents acids (hydrochloric, nitric, perchloric, sulfuric acids) or their combinations, then heating
• Wet ashing

  • Mineral stay in aqueous solution, little or no loss from volatilization because of the lower temp. (350°C)
  • Time used is shorter than dry ashing (10 min to few hours)
• Wet ashing

  • Need constant operator attention
  • Use corrosive reagent (HNO3, HClO4 & H2SO4)
  • A special perchloric acid hood is required
  • Only small numbers of samples at a time
  • Single acid does not give complete & rapid oxidation, need to combine with H2SO4 or HClO4 – diff. combination for diff .samples
  • Very hazardous
  • Do not determine the total ash content
• Wet ashing procedure

  1. 1 g sample

  2. Prepare blank of 3 ml of H2SO4 followed by 5 ml of HNO3

  3. Heat the sample on a hot plate (200oC). Brown yellow fumes - white fumes and finally the sample turn dark.

  4. Continue heating to allow HNO3 to boil off. Once the colour is clear to straw yellow, proceed with boiling to reduce the volume. Cool down

  5. Transfer to an appropriate volumetric flask and dilute with ultrapure water
• Low-temperature plasma ashing


  • Dry ashing in a partial vacuum with nascent oxygen
  • Ashing temperature is much lower than furnace (150oC or less)
• Low-temperature plasma ashing

  • Prevent loss of volatile or trace elements
  • Low temp → allows the microscopic & crystalline structure to remain unaltered
• Low-temperature plasma ashing


  • Small sample capacity
  • Expensive equipment
• Microwave ashing can be used for both dry & wet ashing
• Microwave wet ashing


  • Working with open or closed vessels for ashing a variety of samples
  • Closed vessels can work at high pressure and temperature
  • Advantages : reduce time of ashing greatly to 40 min = 4 h time ashing with a conventional muffle furnace
  • Wet ashing in a closed system is rapid & safe
• Microwave dry ashing

  • Can reach a temperature of 1200oC
• Soluble & Insoluble Ash in Water

  • As an index of the fruit content of preserves & jellies
  • A lower ash in the water-soluble fraction is an indicator of higher fruit content in the product
• Procedure for soluble & insoluble ash in water

  1. Dry ashing → Weigh the total ash → add 10 ml deionized water → cover & heat to boiling → filter & rinse with hot deionized water → dry & re-ash filter paper at least 30 min → weigh & calculate as % water insoluble ash

  2. amount of water-soluble ash is determined by drying the filtrate
• Ash Insoluble in Acid

  • Refers to ash that is insoluble in acids
  • Determination of the surface contamination (silicates) of fruits & vegetables, wheat & rice coatings, primarily from soil, and generally not tested in food
  • Silicates remain insoluble in acid
• Procedure for ash insoluble in acid

  Dry ashing →Weigh total ash → add 25 ml 10% HCl → cover & boil → filter with hot distilled water → dry & re-ash → weigh & calculate as %
• Alkalinity of ash

  • Alkaline ash: Ca, Mg, K, Na (fruits & veg.)
  • Acid ash: P, S, Cl (meat, some cereals)
  • The alkalinity of ash has been used as a quality index of fruit & fruit juices
• Procedure for alkalinity of ash

  Total ash from dry ashing → add 0.1 N HCl → add boiling water if necessary & warm on a water bath → cool → titrate the excess HCl with 0.1N NaOH (methyl orange as indicator)→ express as ml of 1N acid / 100 g sample
• Factors to consider when analysing ash

  • High fat sample can be solvent extracted first to remove fat
  • High moisture sample can be pre-dried first
  • Alcohol, magnesium acetate, and glycerol may speed ashing
  • Spattering can be controlled with lids on the crucibles
  • Wet ashing is often used as prep for mineral analysis