The seed coat of oilseeds, nuts and pulses, peels of fruits and vegetables, and bran of cereals contain considerably more fibre than the softer edible inner tissues.
So, the fibre content can be employed for assessing the proportion of outer coating of plant materials. The digestibility of food varies inversely with the crude fibre content.
- The crude fibre represents the cell wall material left after boiling with dilute acid and alkali.
- It contains a mixture of cellulose, lignin and pentosans, together with sand, silica and other mineral matter locked in the tissues and a little nitrogenous matter.
- Reagents Sulphuric Acid – 0.255 N [1.25 per cent ( m/v )], accurately prepared.
- Sodium Hydroxide Solution – 0.313 N [1.25 per cent ( m/v )], accurately prepared.
- Weigh accurately about 2 g of the dried material and extract the fat for about 8 hours with petroleum ether or hexane, food grade, using a Soxhlet or other suitable extractor or use the residue from the crude fat determination.
- Transfer the fat-free dry residue to a one-litre conical flask.
- Take 200 ml of dilute sulphuric acid in a beaker and bring to the boil.
- Transfer the whole of the boiling acid to the flask containing the fat-free material and immediately connect the flask with a reflux water condenser and heat, so that the contents of the flask begin to boil within one minute.
- Rotate the flask frequently, taking care to keep the material from remaining on the sides of the flask out of contact with the acid.
- Continue boiling for exactly 30 minutes. Remove the flask and filter through fine linen (about 18 threads to the centimeter) held in a funnel, and wash with boiling water until the washings are no longer acid to litmus.
- Bring to the boil some quantity of sodium hydroxide solution under a reflux condenser.
- Wash the residue on the linen into the flask with 200 ml of the boiling sodium hydroxide solution. Immediately connect the flask with the reflux condenser and boil for exactly 30 minutes.
- Remove the flask and immediately filter through the filtering cloth.
- Thoroughly wash the residue with boiling water and transfer to a Gooch crucible prepared with a thin but compact layer of ignited asbestos.
- Wash the residue thoroughly first with hot water and then with about 15 ml of 95 percent (by volume) ethyl alcohol.
- Dry the Gooch crucible and contents at 105 ± 1°C in the air-oven to constant mass.
- Cool and weigh. Incinerate the contents of the Gooch crucible at 600 ± 20ºC in a muffle furnace until all the carbonaceous matter is burnt.
- Cool the Gooch crucible containing the ash in a desiccator and weigh.
M1 = mass in g of Gooch crucible and contents before ashing,
M2 = mass in g of Gooch crucible containing asbestos and ash,
f = crude fat (on moisture-free basis), percent by mass, and
M = mass in g of the dried material taken for the test.
RESULTS AND INFERENCE
The difference between the results of two concurrent determinations carried out simultaneously or in rapid succession by the same analyst (repeatability) shall not exceed 0.1 % by mass.
• The fineness of the particles has an important bearing on the result accuracy. Hence, the sample should pass through a 1 mm sieve.
• The concentration of sulphuric acid and sodium hydroxide is very important for the separation of other food constituents from crude fibre.
• While dilution of sulphuric acid, always add acid to water but not water to acid.