Precautions for Determining Crude Protein in Flour Using the Kjeldahl Nitrogen Determination Apparatus
The crude protein content is one of the important indicators for evaluating flour quality. Based on protein content, flour can be classified into high-gluten flour, medium-gluten flour, low-gluten flo
The crude protein content is one of the important indicators for evaluating flour quality. Based on protein content, flour can be classified into high-gluten flour, medium-gluten flour, low-gluten flour, and non-gluten flour. Only flours with higher protein content and quality can be used to bake high-quality products. Therefore, using the Kjeldahl nitrogen determination apparatus to test crude protein in flour is crucial. However, there are several key points to be mindful of during the testing process.
The reagent solution should be prepared using ammonia-free distilled water.
During the digestion process using the Kjeldahl apparatus, do not use high heat. Maintain a gentle boil, gradually increasing the temperature to avoid nitrogen compounds adhering to the walls of the Kjeldahl flask, which may not be fully digested in the absence of sulfuric acid, leading to nitrogen loss.
If the sample digestion solution is not easily clarified, cool the Kjeldahl flask, add 2–3 ml of 30% hydrogen peroxide, and continue heating for digestion.
The distillation apparatus must be airtight and free from leaks.
If insufficient alkali is added before or after distillation, and the digestion solution appears blue without forming copper hydroxide precipitate, additional sodium hydroxide should be added.
When reusing the Kjeldahl nitrogen determination apparatus, the temperature of the boric acid absorbing solution should not exceed 40°C. Otherwise, the absorption of ammonia will weaken, causing losses. In this case, the solution can be placed in a cold water bath for use. Boric acid is used as the ammonia absorbent, which eliminates the need for standardizing alkali solutions, and the volume of boric acid is not strict, simplifying the operation.
After distillation, the lower end of the condenser should be raised above the liquid level to clean the pipe opening. Continue distilling for 1 minute before turning off the heat source to avoid the risk of backflow of the absorbent solution.
During distillation, ensure that the steam generation is uniform and sufficient. Do not stop the heating or gas supply during distillation, as this may lead to backflow.
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