Determining Protein in Soybean: a Kjeldahl Workflow

Soybean is one of China's major crops, combining an edible-oil resource and an edible-protein resource with high nutritional value. Soybean protein is one of the most abundant proteins in nature: its amino acid composition resembles the essential amino acid profile the human body requires, and it is also rich in calcium, phosphorus, iron, oligosaccharides and various vitamins.

Soybean protein is a plant protein whose amino acid composition is close to that of milk protein — except for slightly lower methionine, every essential amino acid is present in abundance — making it a complete plant protein, comparable to animal protein in nutritional value.

1. Preparing the digestion-tube reagents

1. Weigh 0.1–5 g of solid sample (soybean powder) on an electronic balance, record the mass as m, and transfer it into the digestion tube.
2. Add copper sulfate and potassium sulfate.
3. Add 20 ml of concentrated sulfuric acid.

2. Preparing the alkali reagent

Weigh 400 g of sodium hydroxide into a prepared beaker, add 1,000 ml of distilled water, stir until uniform, and pour into a plastic bottle ready for use.

3. Preparing the mixed indicator

1. Weigh 0.1 g of methyl red into beaker 1, and 0.5 g of bromocresol green into beaker 2.
2. Measure 100 ml of ethanol into beaker 1 and, likewise, 100 ml of ethanol into beaker 2; stir each until uniform.
3. Pour the solutions from beaker 1 and beaker 2 into a large beaker, stir until uniform, and transfer the reagent into a reagent bottle.

4. Sample digestion

Place the digestion tube in the tube rack and digest on the Sonnen Smart 20 aluminum-block digester. The instrument stores up to 510 user-defined programs; each program can set up to 21 heating stages, with a temperature setting range of 50–450 °C and a time setting range of 0–250 minutes per stage. When digestion finishes, the automatic lift raises the tube rack automatically, so the digestion result can be inspected while the tubes cool — no manual handling; the process is safe, convenient and saves bench space.

5. Distillation and titration

Place the digested sample, together with its digestion tube, at the tube position of the Sonnen K06 fully automated Kjeldahl nitrogen analyzer and start the instrument. The K06 is simple to operate with a friendly interface, and its dynamic titration technology titrates while distilling, saving considerable time — one sample takes only 3–5 minutes to complete. The 10-inch touch screen displays and monitors the experimental process in real time, and the nitrogen content and protein content are shown on screen as soon as the determination finishes.