Detergent fiber analysis

The technique of using detergents to separate digestible and indigestible parts of plant tissues was initially proposed by Van Soest in 1963. The concept behind the detergent fiber analysis is that plant cell substances can be divided into less digestible cell walls (made of hemicellulose, cellulose and lignin) and highly digestible cell contents (containing starch and sugars). These two components are successfully separated by using different detergent systems.

>> NEUTRAL DETERGENT FIBER (NDF): the sample is digested in the Neutral Detergent Solution to separate the neutral detergent soluble fraction from the neutral detergent-insoluble fraction. The remaining dry matter is estimated and the proportion gives the neutral detergent fiber (NDF).

1. DIGESTION: the sample is boiled in the Neutral Detergent Solution NDS with a heat-stable a-amylase-treated enzyme to separate the neutral detergent soluble fraction (sugars, starches and pectins soluble, filtered) from the neutral detergent-insoluble fraction (cell walls substances, hemicellulose, cellulose and lignin, residues)
2. WASHING: the residues inside crucibles are washed with water to remove detergent residues
3. DEFATTING: it is the final step, where the last washes are performed with acetone

​​​​>> ACID DETERGENT FIBER (AFD): the Acid Detergent Solution (ADS) solubilizes the hemicellulose while lignin and cellulose remain insoluble. The residue is weighed for the determination of ADF. It includes cellulose and lignin.

1. DIGESTION: the Acid Detergent Solution (ADS) solubilizes the hemicellulose while lignin and cellulose remain insoluble in the crucible
2. WASHING: the residues inside the crucibles are washed with water to remove detergent residues
3. DEFATTING: it is the final step, where the last washes are performed with acetone

>> ACID DETERGENT LIGNIN (AFL): in sequential analysis, the remaining residues from ADF are solubilized by adding sulfuric acid 72 %. Only lignin and acid-insoluble ash remain after this step. The residue is then combusted in a furnace, and the difference in the weights before and after ashing yields the amount of lignin present in the sample.

The universally used method is Weende.  This method was developed by Henneberg and Strohmann during the 1860s at the Weende Experiment Station in Germany and is often referred to as the Weende System of proximate analysis.

The method is based on the differentiation of carbohydrates into digestible and indigestible fractions.

It is the reference for the legal measure of fiber grains, meals, flours, and feeds, even if it accounts for most of the cellulose but only a portion of the lignin and no ash, so it underestimates true fiber and is less than acid detergent fiber (ADF).

Even though Crude Fiber is not a very useful parameter for quantifying forage fiber where lignin content is substantial, it is a reasonable estimate of the fiber in grains because of their low lignin content.

Thus, it is still commonly used for the analysis of feeds for non-ruminants or monogastric animals.

The Crude Fiber determination can be summed up in 5 steps:

1. ACID DIGESTION (BOILING & FILTERING): the feed sample is boiled in sulfuric acid 1,25 % for the extraction of sugar and starch
2. WASHING: the residues in the crucibles are washed with water to remove acid residues and neutralize the pH
3. ALKALY DIGESTION: the second digestion with 1.25 % sodium or potassium hydroxide removes proteins, some hemicellulose and lignin.
4. WASHING: is performed again in order to remove the alkali residues into the crucibles with water and neutralize the pH
5. DEFATTING: the final step, where the last washes are performed with acetone