Measurement of food optical rotation

1、 Basic concepts

1. Polarized light

Ordinary light waves are rays that vibrate in various directions, with their vibration direction on an infinite number of planes perpendicular to the direction of light propagation. Figure 2-4 shows a beam of light shining towards our eyes, and the arrows in the figure represent the vibration plane of the light. When this ordinary light wave is directed towards a Nicol prism, only light vibrating in a plane parallel to the axis of the prism can pass through. Therefore, the light passing through the Nicol prism has only one plane of vibration, which is the plane parallel to the axis of the prism. Light that vibrates only on a certain plane is called polarized light. Its vibration plane is called the polarization plane.

2. Optical rotation and optical rotation

The property of matter that causes the direction of polarized light to rotate is called the optical rotation of matter.

Some organic substances (such as sucrose, glucose, amino acids, and other compounds containing chiral isomers) can rotate polarized light by a certain angle. This property that causes the direction of polarized light vibration to rotate is called optical rotation, and substances with this property are called optically active substances. The angle at which polarized light is rotated in the direction of vibration by a optically active substance is called rotational rotation, denoted by a. Observing from the direction of incident light, what can rotate the vibration plane of polarized light to the right (clockwise) is called right-handed rotation, represented by (+); On the contrary, it is called left-handed and is represented by (-). Sucrose and glucose have right-handed properties, while fructose has left-handed properties.

3. Specific rotation

The magnitude of optical rotation mainly depends on the molecular structure of the optically active substance, as well as the wavelength of the light source（ λ)、 The temperature (t), concentration of the solution, thickness of the liquid layer (d), and type of solvent are related. The optical rotation measured when the concentration of a optically active substance is 100g/mL and the liquid layer thickness is 1dm is called the specific rotation, denoted by the symbol [a] λ t. Represent.

In order to facilitate the comparison of the optical rotation of different substances, it is usually specified to use sodium light D-line (wavelength 589.3nm) to measure at 20 ℃. Under these conditions, the specific rotation is represented by the symbol [a] D20.

Specific rotation of pure liquid:

The specific rotation of the solution:

In the formula, a represents the measured optical rotation,.

P - Density of liquid at 20 ℃, g/mL

C - Mass of optically active substance per milliliter of solution, g/mL

D - length of the optical rotation tube (liquid layer thickness), dm

20- measured temperature, ℃

S - The solvent used (if the specific rotation of the solution is not indicated, it indicates that the solvent is water).

Due to specific rotation under certain conditions [a] λ t. It is known that d (the thickness of the liquid layer or the length of the optical rotation tube) is constant, so the concentration c in the optical rotation solution can be calculated by measuring the optical rotation.

4. Variable rotation effect

After the dissolution of optically active reducing sugars (such as glucose, fructose, lactose, honey, etc.), their optical rotation initially changes rapidly, then gradually becomes slower, and finally reaches a constant value. This phenomenon is called variable optical rotation. This is because these reducing sugars have two isomers, namely type A and β Type, their specific rotation