Three instruments complete the measurement of protein content in food

There are many reports on the determination of protein content in foods, such as Kjeldahl method, phenol reagent method, double urine method, photometric method, formaldehyde titration method and current method. However, there are deficiencies such as cumbersome operation and time-consuming. The literature reports are basically imported instruments and are not suitable for general laboratory use. The classical Kjeldahl method is a widely used method in food analysis, nutrition and biochemical research, but it is also cumbersome and time-consuming. As a result of undergraduate students' experiments, the entire measurement process cannot be completed within the specified time. A large number of reagents consume resources.
In this paper, a temperature-controlled digestion furnace can be used to digest multiple samples at the same time, and the addition of alkali and quantitative distillation can be automatically performed using a nitrogen determination instrument. Automatic potentiometric titrators are highly accurate and highly automated. This paper measured the content of protein in food, explored the conditions for digestion, and compared it with the national standard method. Reagents: K2S04; CuS04•5H2O; H2S04; H3B03; NaOH (reagents are of analytical grade). Finely weigh 0.1～0.5g grain sample in the dried digestion tube, add 0.3g CuS04•5H20, 1.0gK2S04 and 5mLHzS04 to shake, heat and digest; wait until the reaction liquid is blue-green and clear and transparent, continue to heat for 10min, then remove and cool to Room temperature. The above digestion tube was directly clamped on the Kjeldahl apparatus, and the Erlenmeyer flask containing 20 mL of boric acid solution was placed on the Kjeldahl absorption rack, alkali was added until the liquid was brown, and distilled to the Kjeldahl meter signalling. After that, remove the Erlenmeyer flask and place it in a fully automatic potentiometric titrator. Titrate with a standard solution of sulfuric acid and examine the reagent blank as described above. Measurement III was performed according to the method of GB/T 5009.5-2003.
The influence of sample amount and amount of digestive agent on the determination result was investigated. When the sample amount was 0.1-0.5 g, the best digestion effect was obtained when the amounts of CuSO4, K2SO4, and H2SO4 reagents were 0.39, 1.0 g, and 5 mL, respectively. Fast speed, good absorption and potentiometric titration, its measured value meets the requirements. When the sample volume is too large, the digestion, absorption, and titration effects are not satisfactory. Set different digestion times and determine the protein content of the sample. The measured value of heating for 10 minutes is close to that measured by the national standard method. Taking 4 kinds of samples with different protein contents and adding the standard (ammonium sulfate) recovery test, the recovery rate of ammonium sulfate was above 99.0%. The protein content of the four samples was determined using this method and the national standard nitrogen determination method. The results showed that when the two methods had no significant difference (P=0.95), the value was <2.30, which met the requirements. Four samples with different protein contents were taken and measured in parallel for 5 times. The relative mean deviation (RSD) was less than 1.0%, indicating that the method had good precision.
The method utilizes three kinds of instruments to measure the protein content in food, has high accuracy and precision, is simple, rapid, time-saving, labor-saving, low in equipment cost, easy to popularize, and uses less reagents to reduce experimental costs. At the same time, it also reduces the pollution to the environment. This not only enables undergraduate students to complete the entire experimental process within the specified time, but also allows students to learn and use more advanced instruments in general basic laboratories.

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