INTRODUCTION
Dietary fiber can be defined as sum of polysaccharides and lignin that are not digested by human digestive enzymes. The major components of dietary fiber are cellulose, non-cellulose such as hemicelluloses and pectin, lignin,
and hydrocolloids (gums, mucilages,
and algal
polysaccharides). Human foodstuffs contain mainly non-cellulose polysaccharides, some cellulose and little lignin. The average proportions of non-cellulose polysaccharides. Cellulose and lignin for common foodstuff are about 70%, 20% and 10% respectively. The crude fiber method was developed in the 1850s to estimate indigestible carbohydrate in animal feeds. Crude fiber method is one of the gravimetric method that measures the organic food residue remaining after sequential digestion with 0.255N sulphuric acid and 0.313N sodium hydroxide solutions, followed by oven-drying at 104ºC overnight and ignition in muffle furnace at 600ºC for 3 hours. The compounds removed are predominantly protein, sugar, starch, lipids and portions of both the structural carbohydrates and lignin. Crude fiber method measures variable amounts of the cellulose and lignin in the sample, but hemicelluloses, pectins, and added gums or hydrocolloids are solubilised and removed. Therefore, crude fibre measurement drastically underestimates dietary fibre in foods since it measures only cellulose and lignin. As a result, crude fiber method is only adequate for determination of fibre in animal feed product, but not suitable for human food analysis as lignin is significant to human health.
ACID AND ALKALI DIGESTION METHOD
OBJECTIVE To determine the crude fibre content in food sample using acid and alkali digestion method.
PROCEDURE 1. 1 g ±0.001 (W) of dried ground sample was weighed into a crucible. 2. 200 mL 1.25% N H2SO4 was added. 3. Exactly boiled for 30 minutes. 4. Contents were filtered and washed the residue with hot water until free from acid. 5. 200 mL 1.25% N warm NaOH was added into the beaker containing the insoluble material and boiled for 30 minutes. 6. 2-3 drops of octyl alcohol was added to prevent foaming during boiling. 7. Contents were filtered and washed the residue with hot water (four times), then with 15 mL of 1% hydrochloric acid (twice). 8. The sample was washed with hot water again until it is neutral or free from acid. 9. The crucible was transferred with insoluble material into the oven at 105˚C to a constant weight. The weight of residue and crucible (W2) were recorded. 10. The residue was char gently over a Bunsen burner until it has ceased smoking. 11. The residue was ignited in a muffle furnace at 550˚C for 3 hours. 12. The crucible was cooled in dessicator and weighed. The weight of ash and crucible (W3) were recorded. 13. The loss in weight on ignition represents the weight of crude fibre.
CALCULATION
g crude fibre per 100 g sample = (W2 – W3) x 100 W1
Where, W1 = weigh in gram of sample taken W2 = weigh in gram of crucible + dried residue W3 = weigh in gram of weigh of crucible + ash
QUESTIONS
1. Calculate the volume of: a. Concentrated sulphuric acid required to make up a litre of 1.25% M H2SO4 Volume = = 13.03 m / 0.01302 0.1302
of H2SO4 required to make up a litre of 1.25% M NaOH
b. Weight of NaOH used to prepare a litre of 1.25% N NaOH Mole =
1.25 = Mass = 50 g
2. Why is crude fibre determination important in food analysis? It is used as an index of the feeding value of poultry and stack feeds, seeds high in crude fibre content are low in nutritional value. It is also used in the chemical determination succulence of fresh vegetables and fruits.
3. What are the factors that are greatly affected the accuracy of crude fibre determination? Factor that are greatly affected the accuracy of crude fibre determination is the manipulation and the procedures of the experiment itself.
DISCUSSION
In this experiment acid and alkali digestion method is used to determine the crude fibre content in food sample. The food sample that we used in this experiment is carrot. Crude fibre refers to the residue of a feed that is insoluble after successive, boiling with dilute acid and alkali. The alkaline hydrolysis will removes protein and some carbohydrates. This process also removes some hemi-cellulose and lignin. Besides, only partial recovery of fibre components is achieved. Crude fibre is the portion of the total carbohydrate of a food that is resistant to the acid and alkali treatment. In this experiment, we have used carrot as a sample. The sample we need to make this experiment is 1g. Some soluble hemicelluloses are solubilised in the acid digestion and are also extensively and variably solubilised in the alkaline digestion. In addition, this indigestible components will be considered as part of the nitrogen-free extract, causing a serious error in the calculated nutritive value of the food. The chemical reagent we was used in this experiment is sulphuric acid (H2SO4) 1.25%, sodium hydroxide (NaOH) 1.25% and hydrochloric acid (HCl) 1%. The results of experiment acid and alkali digestion method for three trials were taken and percentage were calculated by formula given. The mean percentage of total ash that we get is 91.08%.
CONCLUSION
In conclusion, we can achieve the objective of the experiment after conducted the experiment successfully. In addition, any errors during conducting experiment should be avoided or less to make sure we can get the experiment results is almost equal with theoretical results.
REFERENCES
Aisyah Bujang, (2011). Food Analysis laboratory manual for Diploma Food Technology. Published by UiTM Press.
Retrieved on Feb 18 2013, from, http://www.scribd.com/doc/24474809/Determination-of-Fiber
Retrieved on Feb 18 2013, from, http://wiki.answers.com/Q/Acid_alkali_digestion_of_crude_fibre
DATA SHEET
Sample
Trial
Wt of sample (g)
Carrot
1 2 3 Mean
1.0209 1.0501 1.0125
CALCULATIONS
Trial 1 = (88.7728-87.7625) x 100 1.0209 = 98.96 %
Trial 2 = (88.3129-87.4634) x 100 1.0501 = 80.90 %
Trial 3 = (88.5565-87.6111) x 100 1.0125 = 93.37 %
Mean = 98.96 + 80.90 + 93.37 3 = 91.08 %
Wt of crucible + dried residue (g) 88.7728 88.3129 88.5565
Wt of crucible + ash (g)
% Crude Fibre
87.7625 87.0634 87.6111
98.96 80.90 93.37 91.08
