Addendum to IV.8. Determination of crude fiber

A measure of the proportion of crude fiber in the particulate carbohydrates may also be obtained by use of the anthrone reaction. Crude fiber as determined in agricultmral products is not a chemical entity but consists of the acid- and alkali-insoluble carbohydrates and lignin obtained under standardized reaction conditions. In determining microquantities of crude fiber by use of the anthrone reaction the value obtained is predominately a measure of the amount of cellulose and similar polysaccharides which may have little direct food value. 

Dilute 9.0 ml of concentrated analytical reagent quality concentrated sulphuric acid (sp gr 1.82) to 1000 ml with distilled water. 

Dissolve 12.5 g of analytical reagent quality sodium hydroxide in 1000 ml of distilled water. Store the solution in a well-stoppered polyethylene bottle. 

Place the tubes in a sand-bath or block heater at 100 C for exactly 30 min (Note a). Remove the tubes and centrifuge the contents for 10 min at 5000 rpm. After centrifuging carefully withdraw the supernatant liquid with a capillary pipette attached to a vacuum source, being careful not to disturb the sedimented material (Note b). 

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