The composition of dietary fiber is complex, and domestic methods for measuring in this area have not yet formed a unified standard. They have always followed the old methods or used foreign methods of measurement, and have not formed their own methods for measuring crude fiber. Conventional crude fiber determination methods include neutral detergent fiber measurement methods and acidic detergent fiber measurement methods. Although traditional methods are still in use today, there are many shortcomings. Therefore, some domestic scholars have introduced the latest foreign crude fiber determination methods: the enzyme gravimetric method and the Englyst method. The enzyme gravimetric method is widely used in some countries because it can measure soluble fiber. However, enzymatic analysis requirements are quite strict and the cost is relatively high.

In the following we mainly analyze the enzyme analysis method that is the weight method.

Crude fiber measurement method:

Principle: Under the action of hot dilute sulfuric acid, the sugar, starch, pectin and other substances in the sample are removed by hydrolysis, and then treated with hot potassium hydroxide to dissolve and saponify the protein, which is then removed by treatment with ethanol and ether. Remove tannins, pigments and residual fats, the resulting residue is crude fiber, such as containing inorganic substances, can be deducted after ashing.

Reagents and raw materials: bean dregs were air-dried, vacuum-dried at 57°C, ground, and dried bean dregs after passing through a 60-mesh sieve. 125 % sulfuric acid (AR), 1.25% potassium hydroxide (AR), anhydrous ethanol (AR), ethyl ether (AR), methyl red (AR), phenolphthalein (AR).

Instrument and equipment: blast drying box, high temperature furnace, griddle, reflux device, 200 mesh nylon filter cloth, suction filter device.

Determination procedure: Accurately weigh about 5 grams of bean dregs sample, place it in a 250-mL Erlenmeyer flask, add 200 mL of boiled 1.25% sulfuric acid in a conical flask, and install a reflux device, heat to make it slightly boiled, reflux for 30 minutes, every other Shake the Erlenmeyer flask for 5min, fully mix the contents of the flask, remove the Erlenmeyer flask, filter immediately with a 200 mesh filter cloth, and wash with hot water until the wash solution is not acidic (using methyl red as indicator).

Use 200ml of boiled 1.25% potassium hydroxide solution to wash the remaining contents of the filter cloth into the original conical flask, heat to boiling, reflux for 30min, remove the Erlenmeyer flask, immediately filter with 200 mesh filter cloth, wash with boiling water Until the washing liquid is not alkaline (indicated by phenolphthalein).

The residue on the filter cloth is washed with water into a 100 mL beaker, transferred to a dry GZ pot that has dried to constant weight, suction filtered, washed thoroughly with hot water, and drained, followed by ethanol, ethyl ether in that order. Wash once and dry the contents of the crucible in a 105°C oven until constant weight.

If the sample contains more inorganic substances, the asbestos tower pot can be used instead of the G2 vertical expansion pot filter, after drying, weighing, moving into a high-temperature furnace at 550 °C to burn to constant weight, after the dryer is cooled to room temperature and weighed, The difference in weight before and after burning is the amount of crude fiber.

Crude fiber calculation: After the above processing, we obtained some data, and then calculated the crude fiber content according to the following formula. Crude fiber = G/m*100%, where G is the mass lost after ignition at high temperature, equal to the mass of the residue before ignition minus the mass of ash obtained after burning; m is the mass of the sample.

The results of the determination of crude fiber in bean dregs are attached below.

Determination of crude fiber in table bean dregs

The result of this measurement method is 23.96%, which is very different from the actual content. Because dietary fiber components such as cellulose, hemicellulose, and lignin will degrade to varying degrees, the results of the tests will be less. However, the gravimetric method using the gravimetric method is simple, rapid, and applicable to all kinds of foods, and is the most classic and most extensive analytical method.

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