Determination of soluble solids in fruit and vegetable products

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

1. Purpose requirements

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

(2) Have a preliminary understanding of the operation method for measuring refractive index.

2. Principles of Practical Training

When other conditions are constant, the refractive index of the same substance varies depending on its concentration.

The refractive index of the sample is measured using a refractometer, and the mass fraction of soluble solids in the sample is calculated based on the conversion table between refractive index and soluble solids. Then, the mass fraction of soluble solids in the sample is calculated based on the temperature correction table for soluble solids and converted to the standard soluble solids mass fraction at 20 ℃.

3. Training supplies

(1) Refractometer: The commonly used instrument is the hand

The handheld refractometer, also known as a sugar mirror or handheld sugar meter, is constructed as shown in Figure 2-11.

(2) Organizational masher.

(3) Pan balance.

(4) Mortar.

(5) Thermometer.

(6) Fruits and vegetables. Continuing to do

4. Safety Reminder

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

(1) The instrument should be placed in a dry, ventilated, and temperature appropriate place to prevent the optical components of the instrument from getting damp and moldy.

(2) Before and after using the instrument and when replacing the sample, the working surface of the refractive prism system must be cleaned first.

(3) The tested sample must not have any solid impurities. When testing solid samples, it is necessary to prevent the working surface of the refracting prism from roughening or generating indentations. This instrument strictly prohibits testing samples with strong corrosiveness.

(4) The instrument should avoid strong vibration or impact to prevent optical components from shattering or loosening and affecting accuracy.

5. Operation steps

(1) Preparation of sample solution

① Transparent liquid product: Mix the laboratory sample thoroughly and use this liquid for direct measurement.

② Non viscous products (fruit pulp, vegetable pulp products): Mix the laboratory samples thoroughly, squeeze out a portion of the filtrate with four layers of gauze, and take the remaining filtrate for measurement.

③ Viscous products (jam, jelly, etc.): Weigh 25-50g (accurate to 0.01g) of the sample and place it in a weighed beaker. Add 100-150mL of distilled water, stir with a glass rod, heat until boiling, and gently boil for 2-3 minutes. Cool and mix thoroughly. Weigh 20 minutes later, accurate to 0.01g, and then filter into a drying container using a grooved funnel or Brinell funnel, leaving the filtrate for measurement.

④ Solid phase and liquid phase separated products: Take a portion of the laboratory sample according to the ratio of solid and liquid phases, and then crush it with a tissue crusher. Mix the laboratory samples thoroughly, squeeze out a portion of the filtrate with four layers of gauze, and take the remaining filtrate for measurement.

(2) Measurement

① Correct the refractometer according to the instructions.

② Separate the two prisms of the refractometer and wipe them clean with a degreased cotton dipped in ether or alcohol.

③ Dip 2-3 drops of uniform sample juice into a glass rod with a rounded end, and carefully drop them in the center of the prism plane of the refractometer (be careful not to let the glass rod touch the prism).

④ Quickly close the upper and lower biprism, let it stand for 1 minute, and ensure that the liquid is uniform, free of bubbles, and full of field of view.