Proximate Analysis/ Weende system
Proximate analysis is a system for estimating the nutritive value of
feed or material for feeding purposes. The principle of the analysis
is to separate the feed components into groups or fractions in
accordance with their feeding value. The various components are
moisture, ash, crude protein, crude fiber, and nitrogen free
extract.
Moisture and Dry Matter:
Moisture in a feed is estimated by drying a sample in a laboratory
oven at 100 oC until it reaches a constant
weight. First of all sample is weighed and this weight is called wet
weight. Then it is kept in oven and weighed after certain period of
time. It is again kept in oven and again weighed after certain
period of time. Sample is kept on keeping in oven and weighing until further change in
weight of sample stops. The weight at this point is called dry
weight. This is the amount of dry mater in sample. Subtract dry
weight from wet weight which will give the amount of moisture in
sample. The moisture free fraction is called dry matter.
% Moisture = |
Wet weight - Dry Weight |
× 100 |
Wet weight
|
% DM = |
Dry Weight |
× 100 |
Wet weight
|
Crude Protein:
Crude protein in feed is estimated by determining the nitrogen
content of feed with Kjeldahl Digestion and Distillation Procedure.
In this procedure feed is digested with sulphuric acid, which
convert all nitrogen present in feed into ammonia except nitrate and
nitrite. This ammonia is liberated by adding sodium hydroxide which
is distilled off and collected in standard acid. The quantity so
collected is determined by filtration or by an automated
colorimetric method. It is assumed that the nitrogen is derived from
protein containing 16 percent nitrogen and by multiplying nitrogen
with 6.25 an approximate value of protein is obtained. This is not
true protein as the method also determines nitrogen from sources
other than protein such as free amino acids, amines and nucleic
acids and the fraction is therefore digested crude protein.
% CP = |
Units of N × 6.25 |
× 100 |
Wet weight |
Ether Extract or Fat:
Ether extract refers to the fraction of the fat in the feed which is
soluble in ether and is estimated by continuous extraction of a feed
sample with diethyl ether for 6-9 hours in a Soxhlet Apparatus. The
residue after evaporating of solvent is ether extract. Pigments and
fat soluble vitamins, waxes, gums, resins are also included in this
fraction. The ether is then evaporated and the extracted material is
weighed.
% EE = |
Weight of Ether Extract |
× 100 |
Wet weight |
Carbohydrates:
Carbohydrates of the feed are obtained in two fractions, the crude
fibre and the nitrogen free extracts.
Crude Fibre:
In the laboratory crude fiber is measured by subjecting the residual
feed from ether extraction to boiling with diluted acid and then
with diluted alkali. The residue after filtration is considered the
crude fiber.
%CF = |
Weight of fiber residue |
× 100 |
Wet weight |
Nitrogen Free Extract:
This represents the more
soluble carbohydrates or non structural such as starches and sugars.
The nitrogen free extract is determined mathematically by
difference:
%
NFE = 100 – (% H2O + % ash +%EE + %CF)
Ash:
This represents the mineral components of a feed. A dry sample is
placed in a crucible and completely combusted in a furnace at 650
oC, the residue is the ash.
% ash =
|
Weight of ash |
× 100 |
Wet weight |
Van Soest Method of Feed Analysis
According to the concept of van soest there are two principal parts
of dry matter of plant origin which are cell wall and cell contents.
This method is highly efficient to take care of the defects in the
principle of estimating crude fibre and NFE by proximate analysis.
Plant cell contents consist of sugars, starch, soluble
carbohydrates, pectin, non protein nitrogen, protein, lipids and
water soluble materials including minerals and several vitamins.
The principal components of cell wall consist of cellulose,
hemicelluloses, silica, lignin etc. This part is considered fibre.
Fibre is classified as acid detergent fibre (ADF) and neutral
detergent fibre. Acid detergent fiber (ADF) consists of cellulose,
lignin, lignified nitrogen compounds (heat damaged protein), and
insoluble ash. Acid detergent fiber does not represent the total
fiber content in feed, as it does not account for hemicellulose.
Neutral detergent fiber (NDF) consists of ADF plus hemicellulose,
and is often called cell walls. Because NDF represents the total
fiber in a feed, it is highly correlated to intake, rumination, and
total chewing time.
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