Determination of soluble solids in fruit and vegetable products

This training uses the refractometer method (according to GB/T12295-1990)
.

1.
Purpose requirements

(1) Understand the principle and significance of refractive index measurement
.

(2) Preliminarily grasp the operation method of refractive index measurement
.

2.
Principles of training

Under other conditions, the refractive index is also different for the same substance with different concentrations
.

Use a refractometer to measure the refractive index of the sample, check the mass fraction of the soluble solids in the sample according to the conversion table of refractive index and soluble solids, and then convert it into 20℃ standard soluble solids according to the soluble solids versus temperature correction table Quality score
.

3.
Training supplies

(1) Refractometer: the commonly used instrument is hand

Portable refractometer, also called sugar mirror, hand-held brix meter, the structure of the instrument is shown in Figure 2-11
.

(2) Tissue masher
.

(3) Frame pan balance
.

(4) Mortar
.

(5) Thermometer
.

(6) Fruits and vegetables
.

4.
Safety reminder

Figure 2-11 Schematic diagram of the structure of the handheld refractometer

(1) The instrument should be placed in a dry place with good air circulation and temperature to prevent the optical parts of the instrument from being damp and moldy
.

(2) Before and after the instrument is used and when changing samples, the working surface of the finger net refracting prism system must be cleaned
.

(3) The sample to be tested is not allowed to contain solid impurities.

(4) The instrument should avoid strong vibration or impact to prevent the optical parts from being shattered or loosened and affecting accuracy
.

5.
Operation steps

(1) Preparation of sample solution

①Transparent liquid products: Mix the samples in the laboratory thoroughly, and use this liquid for direct measurement
.

②Non-viscous products (fruit pulp, vegetable pulp products): fully mix the laboratory samples, squeeze out part of the filtrate with four layers of gauze, and take the remaining filtrate for determination
.

③Viscous products (jam, jelly, etc.
): Weigh 25-50g (accurate to 0.
01g) of sample into a weighed beaker, add 100-150mL distilled water, stir with a glass rod, heat to boiling, and gently boil 2～3min, cool and mix thoroughly
.

④Products with separated solid and liquid phases: Take a part of the laboratory samples according to the ratio of solid to liquid phase, and then mash them with a tissue masher
.

(2) Determination

① Calibrate the refractometer according to the instructions
.

② Separate the prisms on both sides of the refractometer and wipe clean with absorbent cotton dipped in ether or alcohol
.

③Dip 2 to 3 drops of even sample juice with a glass rod with a rounded end, and carefully drop them on the center of the prism plane of the refractometer (be careful not to make the glass rod touch the prism)
.

④Close the upper and lower prisms quickly and let them stand for 1 min.
The liquid is required to be uniform and bubble-free and full of vision
.

Table 2-2 Temperature correction table of soluble solids

⑤Aim at the light source, observe through the eyepieces, and adjust the indicator so that the field of view is divided into two parts: light and dark
.

⑥ If the scale of the refractometer reading scale is percentage, the mass fraction of soluble solids can be converted into the standard soluble solids mass fraction at 20℃ according to the soluble solids versus temperature correction table (Table 2-2)
.

⑦If the scale of the reading scale of the refractometer is the refractive index, the refractive index can be read out, and then check the mass fraction of the soluble solids in the sample according to the conversion table of refractive index and soluble solids (Table 2-3), and then press the soluble solids The object-to-temperature correction table (Table 2-2) is converted into the mass fraction of soluble solids at 20°C
.

Table 2-3 Conversion table of refractive index and soluble solids

6.

(1) If it is an undiluted liquid or semi-viscous product, the soluble solid content is equal to the number read on the refractometer
.

(2) If it is a diluted viscous product, the soluble solid content (X) is calculated as follows:

Where X——The content of soluble solids in the sample, g

W _D ——The mass fraction of soluble solids in the diluted solution,%

m ₀ ——The mass of the sample before dilution, g

m ₁ ——The mass of the diluted sample, g

7.

(1) The temperature should be controlled at about 20℃ during measurement, and the calibration range should be narrowed as much as possible
.

(2) Perform two measurements on the same test sample
.

(3) The difference between the results of two subsequent measurements by the same analyst should not exceed 0.

(4) If the reproducibility of the measurement can meet the above requirements, take the arithmetic average of the two measurements as the result

Related Links: Determination of Food Refractive Index (1)