Analysis of Ash and Minerals

1. Analysis of
Ash & Minerals

2. Dry Matter
1. Organic compounds:
– Carbohydrates
– Lipids
– Proteins
– Nucleic acids
– Vitamins
– Organic acids
2. Inorganic Compounds:
- Minerals

3. • “Ash content” - total amount of minerals
present within a food.
• “Mineral content” - amount of specific
inorganic components present within a food,
such as Ca, Na, K and Cl.

4. Determination of Ash Content
• Ash - inorganic residue remaining after water and
organic matter removed by heating in the presence
of oxidizing agents.
• Principle: Minerals are not destroyed by heating,
and they have low volatility compared to other
food components.

5. • Main types Ashing: dry ashing, wet ashing,
plasma ashing
• Method chosen depends on the reason for
carrying out analysis, type of food analyzed
and equipment available

6. Sample Preparation
• Representative sample.
Typically, samples of 1-10g are used.
Solid foods are finely ground and carefully mixed
(representative sample).

7. • Before ash analysis, samples high in moisture are
dried to prevent spattering.
• High fat samples are defatted by solvent extraction
(facilitates release of moisture and prevents
spattering).
• Avoid contamination of samples by minerals in
grinders, glassware or crucibles etc.

8. Dry Ashing
• Use a high temperature muffle furnace (To between 500
and 600oC).
• Water and other volatile materials are vaporized and
organic substances are burned in the presence of O2 in
air to CO2, H2O and N2.
• Most minerals are converted to oxides, sulfates,
phosphates, chlorides or silicates.

9. • Some minerals are volatile and may be
partially lost, e.g., iron, lead and mercury.
• For such minerals - use an alternative ashing
method that uses lower temperatures.

10. • Food sample is weighed before and after ashing to
determine the concentration of ash present.
• The ash content can be expressed on either a dry or
wet basis:

11. • The following data was obtained on a sample of hamburger:
sample wt, 2.034 g ; wt after drying, 1.0781 g; and wt of ash,
0.0233 g.
What is the percentage ash on a) a wet weight basis and b)
dry-weight basis?
Wet Basis Ash % = (0.0233 / 2.034) *100
= 1.15 %
Dry Basis Ash % = (0.0233 / 1.0781) * 100
= 2.16 %

12. • Different types of crucible - quartz, Pyrex,
porcelain, steel and platinum.
• Selection depends on sample being analyzed and
furnace temperature used.
• Most widely used - porcelain (relatively
inexpensive, resistant to high temperatures
(< 1200oC) and easy to clean).

13. Platinum
Pyrex
Porcelain
Quartz

14. • Advantages: Safe, few reagents are required, many
samples can be analyzed simultaneously, not labor
intensive, and ash can be analyzed for specific
mineral content.
• Disadvantages: Long time required (12-24 hours),
muffle furnaces are quite costly to run due to
electrical costs, loss of volatile minerals at high
temperatures, e.g., Cu, Fe, Pb, Hg, Ni, Zn.

15. Wet Ashing
• Primarily used in preparation of samples for specific
minerals analysis.
• It breaks down and removes organic matrix
surrounding minerals and left them in an aqueous
solution.
• A dried ground food sample weighed into a flask
containing strong acids and oxidizing agents (e.g., nitric,
perchloric and/or sulfuric acids) heated.

17. • Heating is continued until organic matter is
completely digested, leaving only mineral oxides in
solution.
• Temperature and time - depends on type of acids
and oxidizing agents used.
• Typically, a digestion takes from 10 minutes to a
few hours at about 350oC.
• Resulting solution can be analyzed for specific
minerals.

18. • Advantages: Little loss of volatile minerals
occurs because of the lower temperatures used,
more rapid than dry ashing.
• Disadvantages: Labor intensive, requires a
special fume-cupboard if perchloric acid is used
because of its hazardous nature, low sample
throughput.