Edible mushrooms are a natural food with a unique flavor that is widely loved by people. Edible mushrooms are rich in protein, cellulose, polysaccharides and trace elements, and have low fat content, which can be used as a fat-reducing food.
Protein is an important material building block for the human body. At present, the commonly used protein detection methods are Kjeldahl nitrogen determination method and elemental analysis method, which determine the nitrogen content in the substance, and then convert the crude protein content of the sample.
Element analyzer detection method is often used to detect C, H, N, S and other non-metallic elements, in a high-temperature environment, the sample is fully burned and decomposed through redox reaction, and the inorganic metal elements in the sample to be measured are converted into gaseous substances, and the non-detectable gases (such as volatile halogen gases) are removed, and the gaseous substances to be detected enter the designated separation unit under the action of carrier gas argon. According to the principle of chromatography, the mixed components of the measured sample are loaded into the gas chromatography column, and separated in the order of N, C, H, S according to the time when the one-component gas flows out of the column (i.e., the retention time). The corresponding gas is detected by a thermal conductivity detector, the data is passed to a computer, and the content of different elements in the sample is automatically calculated. The nitrogen content of the sample is multiplied by a conversion factor of 6.25 to determine the protein content of the test sample. This method has the characteristics of no sample preparation, fast analysis speed and less contaminants, and can detect the total nitrogen content in the sample, including some non-protein nitrogen such as nitrate nitrogen, which is an emerging method to detect the crude protein content of the sample.
Kjeldahl nitrogen determination method is a traditional technology for detecting protein content, in line with the requirements of GB5009.5-2016 "Determination of Protein in Food of National Food Safety Standard", its principle is to decompose the organic matter in the sample with sulfuric acid, so that the nitrogen-containing substances are converted into ammonium sulfate, and then distilled with alkali, and then titrated with hydrochloric acid, the nitrogen content of the sample can be obtained.
The elemental analyzer was used to detect the protein content of three edible mushroom samples
Weigh (5.00±0.25) mg of edible mushroom (dry product) sample, wrap it in tin foil and compact it, and place it in a sample tray. Open the instrument, gas valve and the computer connected to the elemental analyzer, set the program to automatically detect, and the computer will automatically calculate the protein content of the sample to be measured according to the calibration curve of the reference material. Each sample was replicated 3 times, and L-cystine was used as the reference material for 3 control tests. The average of the test results is the protein content of the sample, and the coefficient of variation (RSD) is calculated.
Kjeldahl method was used to detect the protein content of three edible mushroom samples
Weigh 1.0~2.0g of edible fungus (dry product) sample, put it into the digestive tube, add an appropriate amount of concentrated sulfuric acid, copper sulfate and potassium sulfate, and then place the digestive tube in the digestion oven. After the edible fungus sample is digested, the volume is fixed to 100mL. 10mL of the diluted solution after constant volume was taken out, put into the analysis tube, and the protein content of the sample was detected by using the VELP Kjeldahl nitrogen determination instrument UDK159 in Italy. Each sample was tested in 3 replicates, and the average of the test results was the protein content of the sample, and the coefficient of variation (RSD) was calculated.
As can be seen from Table 1, the RSD of the elemental analysis method is 0.56%~1.75%, and the RSD of the Kjeldahl nitrogen determination method is 2.23%~2.96%, both of which are less than 5%, indicating that the two methods have high accuracy in detecting the protein content of the three edible fungi, but the elemental analysis method is more accurate. The nitrogen content of the three edible fungi detected by the Kjeldahl method is lower than that of the elemental analysis method, which may be that the Kjeldahl method can only detect the nitrogen content of organic matter that can be converted into ammonium nitrogen in edible fungi, and can not detect the protein converted into nitrate nitrogen, while the elemental analysis method is heated at high temperature, which can oxidize and decompose all nitrogen-containing substances, and edible fungi also contain trace amounts of nitrogen in the form of nitrate nitrogen. In addition, there is nitrogen loss during digestion and distillation by Kjeldahl method, so the nitrogen content detected by elemental analysis is slightly higher than that of Kjeldahl method, but the results of the two methods are relatively similar.
Table 1: Elemental analysis and Kjeldahl determination method were used to detect the nitrogen content of three edible fungi
The elemental analyzer was used to detect the protein content data of the three edible fungus samples for testing, which had the advantages of no pretreatment, low sample demand, fast determination speed and no pollution. Therefore, when detecting the protein content of Lion's Mane mushroom, Oyster mushroom and shiitake mushroom, the elemental analysis method can be used instead of the Kjeldahl method.