2. Ashing Analysis
Introduction
Importance of ash in food analysis
Ash contents in foods
Methods
Sample preparation
Plant materials
Fat and sugar products
Dry ashing
Principles and instrumentation
Procedures.
3. Introduction
• Ash content is a measure of minerals in
the food.
• Ash refers to the inorganic components
remaining after either ignition or complete
oxidation of organic matter in a foodstuff.
• There are two types of ashing used:
- dry ashing
- wet ashing
4. Importance of Ash in Food
Analysis
• To proximate the nutritional
evaluation.
• Ashing is the first step in the
preparation of food sample for
specific elemental analysis.
5. Dry Ashing
• Refers to the use of a muffle furnace that
is capable to maintain temperatures of
500-600°C. Water and volatiles are
vaporized and organic substances are
burned in the presence of oxygen in air to
Co2, and oxides of N2.
• Most minerals are converted to oxides,
sulfates, phosphates, chlorides, and
silicates.
6. Wet Ashing
• Wet ashing is a procedure for
oxidizing organic substances by
using acids and oxidizing agents or
their combinations.
• Minerals are solubilized without
volatilization.
• Wet ashing is often preferable than
dry ashing as a preparation for
specific elemental analysis.
7. Alkalinity Of Ash
• Alkalinity of ash is a useful
measurement to determine the acid-
base balance of foods and to detect
adulteration of foods with minerals.
8. Ash Contents of Selected
Foods
Food Item
% of Ash
Rice, brown, long-grain, raw
1.5
Macaroni, dry, enriched
0.7
Milk, whole, fluid
0.7
Margarine, hard, regular, soybean
2.0
Apples, raw, with skin
0.3
Bananas, raw
0.8
Potatoes, raw, skin
1.6
Eggs, whole, raw, fresh
0.9
Chicken, breast meat only, raw
1.0
Beef, chuck, arm pot roast, raw
0.9
10. Sample Preparation
1) The sample needs to be very
carefully chosen so that it
represents the original materials.
2) A 2-10 g sample generally is used
for ash determination.
3) Milling, grinding, and the like
probably will not alter the ash
content much.
11. Sample Preparation
3) if ash is a preparatory step for specific mineral
analyses, contamination by microelements is of
potential concern.
4) Remember, most grinders and mincers are of
steel construction. Repeated use of glassware
can be a source of contaminants as well.
5) The water source used in dilutions also may
contain contaminants of some microelements.
Distilled water should be used.
12. Plant Materials
• Plant materials are generally dried by routine
methods prior to grinding.
• The sample may be used for multiple
determinations protein, fiber, and so on which
require consideration of temperature for drying.
• Fresh stem and leaf tissue probably should be
dried in two stages (i.e., first at a lower
temperature of 55°C, then a higher temperature)
especially to prevent artifact lignin.
• Plant material with 15% or less moisture may be
ashed without prior drying.
13. Fat and Sugar Products
• Animal products, syrups, and spices require
treatments prior to ashing because of high fat and
moisture (spattering, swelling) or high sugar
content (foaming) that may result in loss of
sample.
• Meats, sugars, and syrups need to be evaporated
to dryness on a steam bath or with an infrared (IR)
lamp.
• Smoking and burning may occur upon ashing for
some products (e.g., cheese, seafood, spices).
Allow this smoking and burning to finish slowly by
keeping the muffle door open prior to the normal
procedure.
• A sample may be ashed after drying and fat
extraction. In most cases, mineral loss is minimal
during drying and fat extraction.
15. Principles and Instrumentation
1- Muffle
• Dry ashing is incineration at high
temperature (525°C or higher).
Incineration is accomplished with a muffle
furnace. Several models of muffle
furnaces are available, ranging from large-
capacity units requiring either 208 or 240
voltage supplies to small bench-top units
utilizing 110-volt outlets.
16. Principles and Instrumentation
2-Microwave
• Both wet and dry ashing can be done
using microwave instrumentation.
• Time of ashing can be decrease up to 97%
• Example: dried egg yolks can be ashed in
the microwave in 20 minutes while it
needs up to 4 hours in the muffle furnace.
17. 3
-
Crucibles
• Quartz crucibles are resistant to
acids but not alkali, at high temperatures.
• Vycor® crucibles
are stable to 900°C
• Pyrex® crucibles
are limited to 500°C.
• Ashing at a lower temperature of 500-
525°C may result in slightly higher
ash values because of less
decomposition of carbonates and loss
of volatile salts.
18. 3
-
Crucibles
• Porcelain crucibles like quartz crucibles
in their properties but will crack with rapid
temperatures changes.
Porcelain crucibles are relatively
inexpensive and usually the
crucible of choice.
• Steel crucibles are resistant to both acids
and alkalis and are inexpensive, but they are
composed of chromium and nickel, which are
possible sources of contamination.
19. 3
-
Crucibles
• Platinum crucibles are very
inert and are probably the best
crucibles but they are currently
far too expensive for routine use
for large numbers of samples.
• All crucibles should be marked for identification. Marks
on crucibles with a felt-tip marking pen will disappear
during ashing in a muffle furnace. Laboratory inks
scribed with a steel pin are available commercially.
• The crucibles should be fired and cleaned prior to use.
