Neutral lead acetate: suitable for plant extracts, it can remove protein, tannin, organic acids, and pectin. Disadvantages: Poor decolorization power, it cannot be used to clarify dark sugar liquid, otherwise it will be treated with activated carbon.

Alkaline lead acetate: suitable for dark sucrose solution, which can remove pigments, organic acids and proteins. Disadvantages: The sediment particles are large and can take away fructose.

Potassium oxalate, sodium oxalate, disodium hydrogen phosphate, sodium sulfate

The protein is removed from the sample, and the calibrated Fehling's reagent is directly titrated under heating conditions. The reducing sugar reacts with the potassium sodium copper tartrate complex in Fehling's reagent to generate red cuprous oxide, which precipitates cuprous oxide and then reacts with the reagent. Potassium ferrocyanide reacts to generate a soluble compound, using methylene blue as an indicator. At the end point, a slight excess of reducing sugar reduces the blue oxidized methylene blue to the colorless reduced tetramethyl blue. Finally, the amount of reducing sugar is calculated based on the volume consumed by the sample.

Sample processing: sample → extraction → protein removal → filtrate

Calibrated Fehling's reagent solution

Predetermination of sample solutions

Determination of sample solution

After removing the egg matter from the sample, the reducing sugar reduces the copper salt to cuprous oxide. After adding iron sulfate, the cuprous oxide is oxidized to copper salt. The ferrous salt generated after the oxidation is titrated with potassium permanganate solution. According to the consumption of potassium permanganate, calculate the content of cuprous oxide, and then look up the table to get the amount of reducing sugar.

After the fat and soluble sugars are removed from the sample, the starch is hydrolyzed and converted into reducing monosaccharides under a certain acidity. The weight of the starch is determined by measuring reducing sugars and multiplying by the conversion parameter 0.9.

It is suitable for samples with high starch content and low other polysaccharide content such as hemicellulose and polysaccharide. This method is simple to operate and widely used, but its selectivity and accuracy are not as good as the enzymatic method.

Starch%=Glucose%x0.9

After the fat and soluble sugars are removed from the sample, the starch is hydrolyzed into disaccharides using amylase, and then the disaccharides are hydrolyzed into monosaccharides using hydrochloric acid. The reducing sugar content is then measured using the reducing sugar determination method and converted into starch content.

This method is not interfered by polysaccharides such as hemicellulose, polypentose, pectin, etc., and is suitable for samples with high polysaccharide content. The analysis results are accurate and reliable, but the operation is complicated and time-consuming.

Starch%=Glucose%x0.9

First treat the sample with 70% ethanol to precipitate pectin, and then wash the precipitate with ethanol and ether in order to remove soluble sugars, fats, pigments and other substances. The residue is extracted with acid or water to extract total pectin or water-soluble pectin. Pectin is saponified to produce sodium pectate, which is then acidified with acetic acid to produce pectic acid. Calcium salt is added to produce calcium pectate. The precipitate is dried and weighed.

It is suitable for all kinds of food, and the method is stable and reliable, but the operation is cumbersome and time-consuming. Calcium pectate precipitates are easily mixed with other colloidal substances, making this method less selective.

Under the action of hot dilute sulfuric acid, sugar, starch, pectin and other substances in the sample are hydrolyzed and removed, and then treated with hot potassium hydroxide to dissolve the protein and saponify the fat and remove it. It is then treated with ethanol and ether to remove tannins, pigments and residual fat. The resulting residue is crude fiber. If it contains inorganic substances, it can be deducted after ashing.

This method is simple and rapid to operate, suitable for all types of food, and is the most widely used classic analysis method. However, the measurement results of this method are rough and have poor reproducibility.

Crude fiber (%) = (G/m) x100%

After the sample is soaked in hot neutral detergent, the residue is fully washed with hot distilled water to remove free starch, protein, and minerals in the sample. Then alpha-amylase solution is added to decompose the bound starch, and then washed with distilled water and acetone. , to remove residual fat, pigment, etc., and the residue is dried to become neutral detergent fiber

This method applies to samples of grains and their products, feed, fruits and vegetables, etc. This method has simple equipment, easy operation, high accuracy and good reproducibility. The measured results include all cellulose, hemicellulose, and lignin in the food, which are closest to the true content of dietary fiber in the food, but do not include water-soluble non-digestible polysaccharides, which is the biggest shortcoming of this method.