Determination of reducing sugar in candy (1)

There are two national standard methods for the determination of reducing sugars in food: direct titration and potassium permanganate titration.
This training uses direct titration (according to the first method of GB/T5009.
7-2008)
.

1.
Purpose requirements

(1) Further consolidate and standardize the redox titration operation
.

(2) Understand the principle and operation points of reducing sugar determination
.

(3) Master the operational skills for the determination of reducing sugars in candies
.

(4) Learn to control reaction conditions and master methods to improve the precision of reducing sugar determination
.

2.
Principles of training

After the sample has been removed of protein, under heating, use methylene blue (also known as methine blue) as an indicator to titrate the calibrated alkaline copper tartrate solution, and reduce the sugar to reduce the bivalent copper to cuprous oxide .
After all the valence copper is reduced, a slight excess of reducing sugar will reduce the methylene blue, and the solution will turn from blue to colorless, which is the end of the titration
.
According to the consumed sample liquid volume, the reducing sugar content can be calculated

3.
Training supplies

(1) Acid burette: 25mL
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(2) Adjustable electric stove with asbestos board
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(3) Hydrochloric acid
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(4) Alkaline copper tartrate (also called film reagent) solution A: Weigh 15g copper sulfate (CuSO ₄ ·5H ₂ O) and 0.
05g methylene blue, dissolve in water and dilute to 1000mL
.

(5) Alkaline copper tartrate (film reagent) B solution: Weigh 50g potassium sodium tartrate and 75g sodium hydroxide , dissolve in water, then add 4g potassium ferrocyanide , after completely dissolved, dilute with water to 1000mL, store in Rubber stopper inside the glass bottle
.

(6) Glucose standard solution (1g/L): Weigh 1g (accurate to 0.
0001g) of glucose dried at 98-100°C for 2h , add 5mL hydrochloric acid (to prevent microbial growth) after dissolving with water, and dilute to 1000mL with water
.
This solution is equivalent to 1.

4.
Safety reminder

Use an electric stove to heat, and be careful not to plug or unplug the power plug with hands wet in operation to prevent electric shock
.
When the solution is boiling, the titration should be slightly higher than the conical flask to prevent the steam from burning your hands

5.
Operation steps

(1) Sample processing Accurately weigh 1g of the sample and place it in a 250mL volumetric flask, add water to dissolve it to a constant volume, shake well, and it will be the sample solution
.

(2) Calibrate alkaline copper tartrate solution.
Accurately draw 5.
00mL each of alkaline copper tartrate A and B, place them in a 150mL Erlenmeyer flask, add 10mL water, add 2 glass beads, and add about 9mL glucose standard solution from the burette.
, Control to heat to boiling within 2 minutes, while it is hot, continue to drip the glucose standard solution at a rate of 1 drop/2s until the blue color of the solution just fades away as the end point
.
Record the total volume of the glucose standard solution consumed, and operate 3 copies in parallel at the same time, take the average value, and calculate the mass (mg) of glucose per 10 mL (5 mL each for A and B) of alkaline copper tartrate solution according to formula (1 )

m ₁ =p×V ₁ .
.
.
.
.
.
.
.
.
.
.
.
.
（1）

Where m ₁ ——10mL alkaline copper tartrate solution is equivalent to the mass of glucose, mg

p——The concentration of glucose standard solution, mg/mL

V ₁ ——The average consumption volume of reducing sugar standard solution, mL

(3) The sample solution is predicted to accurately draw 5.
00 mL each of alkaline copper tartrate A and B , place them in a 150 mL conical flask, add 10 mL of water, add 2 glass beads, and control to heat to boiling within 2 minutes and keep it boiling.
Add the sample solution from the burette at a fast and then slow speed, and keep the solution boiling.
When the color of the solution becomes lighter, titrate at a rate of 1 drop/2s until the blue color of the solution just fades as the end point, and record the consumption sample The volume of the liquid
.
Based on this, determine whether the concentration of the sample solution is appropriate for adjustment

When the concentration of reducing sugar in the sample solution is too high, it should be diluted appropriately and then formally measured, so that the volume of the sample solution consumed for each titration is controlled to be similar to the volume of the glucose standard solution consumed when calibrating the alkaline copper tartrate solution, about 10 mL About (that is, the concentration of reducing sugar in the sample solution is preferably 0.
1g/100g), and the result is calculated according to formula (2)
.

When the concentration is too low, directly add 10 mL of sample solution instead of adding 10 mL of water, and then titrate with glucose standard solution to the end point.
Record the difference between the consumed volume and the volume of glucose standard solution consumed during calibration, which is equivalent to the reduction contained in 10 mL of sample solution.
The amount of sugar
.
The result is calculated according to formula (3)

(4) Measurement of sample solution Accurately suck alkaline copper tartrate solution A and B were each 5.
00 mL, placed in a 150mL conical flask, add water, 10mL, glass beads were added 2, less added 1mL volume than predicted from the burette Put the sample solution into the Erlenmeyer flask, heat it to boil within 2 minutes, keep it boiling and continue titrating at a rate of 1 drop/2s until the blue color just fades away as the end point
.
Record the volume of the consumed sample solution, operate 3 copies in parallel with the same method, and get the average consumption volume

Related link: Determination of carbohydrates in food