Determination of sucrose and total sugar in food

1.
Determination of sucrose

In food production, it is often necessary to determine the content of sucrose in order to determine the maturity of raw materials, to identify the quality of food raw materials such as sugar and honey, and to control the quality indicators of products such as candies, preserved fruits, and sweetened dairy products
.

There are high performance liquid chromatography and acid hydrolysis methods for the determination of sucrose

(1) Principle After the sample is defatted , it is extracted with water or ethanol , and the extract is clarified to remove impurities such as protein , and then hydrolyzed with hydrochloric acid to convert sucrose into reducing sugar

Determine the reducing sugar content in the sample solution before and after hydrolysis according to the reducing sugar determination method.

The difference between the two is the content of invert sugar (reducing sugar produced by the hydrolysis of sucrose), which is multiplied by a conversion factor of 0.

(2) Reagent

① Hydrochloric acid (1+1): Measure 50ml of hydrochloric acid , slowly add 50ml of water, and mix well after cooling
.

②Methyl red indicator solution (1g/L): Weigh 0.
1g of methyl red, dissolve it with a small amount of ethanol , and dilute to 100mL
.

③Sodium hydroxide solution (200g/L): Weigh 20g of sodium hydroxide and dissolve it in water, let it cool, and dilute to 100mL
.

④ Other reagents are the same as direct titration or potassium permanganate titration
.

(3) The instrument is the same as the direct titration method or the potassium permanganate method
.

(4) Operation steps Take a certain amount of sample and treat it according to the direct titration method or the potassium permanganate titration method
.

Take two 50ml each of the processed sample and place them in a 100ml volumetric flask respectively

Dilute the other portion directly with water to 100 mL, and determine the reducing sugar content by direct titration or potassium permanganate titration
.

(5) Result calculation: Determine the content of glucose in the sample by direct titration and calculate the following formula:

In the formula, w——the content of sucrose in the sample, g/100g

m ₁ ——the mass of reducing sugar in the unhydrolyzed sample solution, mg

m ₂ ——the mass of reducing sugar in the sample solution after hydrolysis, mg

V ₁ ——Total volume of sample treatment liquid, mL

V ₂ ——determine the volume of sample treatment solution taken for reducing sugar, ml.

m——sample mass, g

0.
95-the coefficient of reducing sugar (calculated as glucose) converted into sucrose

When the sucrose content is greater than or equal to 10g/100g, the calculation result retains three significant digits; when the sucrose content is less than 10g/100g, the calculation result retains two significant digits (that is, the calculation result is retained to one decimal place)
.

(6) Tips

① This method is the second method in GB/T5009.
8-2008.
Under the conditions specified in this method, sucrose can be completely hydrolyzed, other reducing disaccharides are not hydrolyzed, and the original monosaccharides are not destroyed
.

Therefore, the hydrolysis conditions must be strictly controlled to ensure the accuracy and reproducibility of the results

②Fructose is easy to decompose in acidic solution, so it should be taken out immediately after the hydrolysis is completed and cooled quickly to neutralize it
.

③When using reducing sugar method to measure sucrose, in order to reduce the error, the measured reducing sugar can be expressed as invert sugar, so when using direct titration method, the calibration of alkaline copper tartrate solution needs to be calibrated after sucrose standard solution is hydrolyzed according to the measurement conditions
.

④Calibration of alkaline copper tartrate solution:

a.
Weigh 1.
000g of pure sucrose dried at 105°C to constant weight, dissolve it with distilled water and dilute to 500mL, and mix well
.

1mL of this standard solution is equivalent to 2mg of pure sucrose

b.
Pipette 50mL of the above sucrose standard solution into a 100mL volumetric flask, add 5mL hydrochloric acid solution (1+1), heat in a water bath at 68～70℃ for 15min, take out and quickly cool to room temperature, add 2 drops of methyl red indicator solution, and use Sodium hydroxide solution (200g/L) neutralize to neutral, add water to the mark, and shake well
.

1mL of this solution is equivalent to 1mg of pure sucrose

c.
Take the hydrolyzed sucrose standard solution and calibrate the alkaline copper tartrate solution according to the direct titration method
.

Where m ₂ ——10mL alkaline copper tartrate solution is equivalent to the mass of inverted sugar, mg

m ₁ ——1mL sucrose standard hydrolysate is equivalent to the quality of sucrose, mg

V——Volume of sucrose standard hydrolysate consumed in calibration, mL

0.

95-the coefficient of converting sucrose to invert sugar

⑤ If potassium permanganate titration is selected, the inverted sugar item should be checked when checking Appendix 5
.

2.
Determination of total sugar

Many kinds of monosaccharides and oligosaccharides coexist in many foods
.

It is not easy and unnecessary to measure these sugars separately
.
Usually it is only necessary to know the total amount, the so-called "total sugar"
.
The total sugar in food usually refers to the total amount of reducing sugars (such as glucose, fructose, lactose, etc.
) present in the food or the total amount of carbohydrates that can be hydrolyzed into reducing monosaccharides under the measurement conditions
.

Total sugar is an important quality indicator of many foods such as malted milk extract, canned fruits and vegetables, chocolate, soft drinks, and its content directly affects the quality and cost of food
.
Therefore, total sugar is a routine inspection item in food production
.

The determination of total sugar is usually based on the method of reducing sugar.
The commonly used method is the direct titration method, and the onion ketone colorimetric method can also be used
.
The direct titration method is simple and fast.
It is a classic detection method.
Its principle and operation method are as follows (according to GB/T5009.
8-2008)
.

(1) Principle After the sample is processed to remove impurities such as protein, dilute hydrochloric acid is added to hydrolyze the sucrose into reducing sugar under heating, and then the total amount of reducing sugar in the sample after hydrolysis is determined by direct titration
.

(2) The instrument and reagent are the same as sucrose determination
.

(3) Operation steps

①Sample processing: same as reducing sugar determination (direct titration method)
.

②Determination: Hydrolyze the sample according to the method of measuring sucrose, and then measure the reducing sugar content according to the direct titration method
.

(4) Result calculation

Where x——the total sugar content in the sample (calculated as invert sugar), g/100g (or g/100mL)

m ₂ ——10mL alkaline copper tartrate solution is equivalent to the mass of invert sugar, mg

m——sample mass or volume, g or ml

V ₁ ——Total volume of sample treatment liquid, mL

V ₂ ——The volume of sample hydrolysate consumed during measurement, mL

(5) Tips

①The total sugar determination result is generally based on the product quality index requirements, and is calculated as invert sugar or glucose
.
Then, the calibration of alkaline copper tartrate should be carried out with the corresponding standard solution of sugar
.
When expressed by invert sugar, use standard invert sugar solution to calibrate alkaline copper tartrate solution; when expressed by glucose, use standard glucose solution for calibration
.

②Invert sugar is sucrose after hydrolysis.
Because the optical rotation of sucrose is right-handed, and the mixture of glucose and fructose obtained after hydrolysis is left-handed, this change in optical rotation is called conversion, so hydrolyzed sucrose is called Invert sugar
.