Determination of amino acid nitrogen in soy sauce

1. Purpose requirements

(1) Understand the concepts of amino acids and amino acid nitrogen.

(2) Clarify the basic principle and calculation method of formaldehyde value method.

(3) Master the operational essentials of using formaldehyde value method to determine amino acid nitrogen in soy sauce.

(4) Proficient in using instruments and equipment such as magnetic stirrers, pH meters, and micro burettes.

2. Measurement principle

Amino acids have acidic - COOH groups and alkaline - NH2 groups, which interact with each other to make amino acids neutral inner salts. When formaldehyde solution is added, the - NH2 group combines with formaldehyde to eliminate its alkalinity. Then titrate the COOH group with a strong alkaline standard solution, determine the endpoint using an acidity meter, and determine the total amount of amino acids indirectly.

3. Training supplies

(1) Acidity meter.

(2) Magnetic stirrer.

(3) Microburette (10mL).

(4) Formaldehyde solution (36%): polymer free.

(5) Sodium hydroxide standard solution (0.05mmol/L).

4. Safety Tips

Formaldehyde is toxic and volatile. During the experiment, it is necessary to wear a mask and operate in a fume hood to maintain good ventilation in the laboratory.

5. Operation steps

(1) Sampling and sample processing

① Sampling:

Bottled soy sauce: Randomly select one bottle.

Bulk soy sauce: After mixing, extract approximately 500mL and place it in a clean and dry sealed container.

② Sample dilution: Take 5.0mL of the sample, place it in a 100mL volumetric flask, add water to the mark, mix well and make up to volume for later use.

(2) Prepare to install the titration device as required and calibrate the pH meter.

(3) Measurement

① Sample titration: Take 20.0mL of sample diluent, place it in a 200mL beaker, add 60mL of water, place it in a stirring rod, place it on a magnetic stirrer, insert the electrode, and start the magnetic stirrer. Titrate with 0.0500ol/L NaOH standard solution until pH 8.2 is indicated on the acidity meter, and record the volume of sodium hydroxide standard solution consumed (based on which the total acid content can be calculated). Add another 10.0mL of formaldehyde solution, mix well, and continue titrating with 0.0500ol/L NaOH standard solution until pH 9.2. Record the volume of sodium hydroxide standard solution consumed.

② Blank titration: Take 80mL of water and titrate it with 0.0500mol/L NaOH standard solution until the acidity meter indicates pH 8.2. Then add 10.0mL of formaldehyde solution, mix well, and continue titrating with 0.0500mol/L NaOH standard solution until pH 9.2. Record the volume of sodium hydroxide standard solution consumed.