Refractometry: Checking for the Brix Level

We have been walking our vineyard to check on the Brix, or the sugar content of our Chenin blanc with our handheld refractometer. Although we have a few outlier Chenin blanc clusters whose Brix are already at 20^o, the majority of the Chenin blanc are between 10-14 ^oBrix. The cluster shown at the left is at 14 ^oBrix.
We had our friend Bob come and help us in the vineyard and I showed Bob how we took a reading with the refractometer. Since he is a retired ER physician, he wanted to know how the refractometer worked to measure the sugar. I was kind of at a loss to explain the physics of the refractometer, so I thought I would look into it and blog about it here.

The first thing to understand about how a handheld refractometer works is to define refractive index (RI):
Refractive Index: is the ratio of the speed of light traveling through a vacuum to the speed of light in the material being tested.
The physics of light is such that when it passes from a medium of one refractive index to one with a different refractive index, the light bends (refracts) at the interface, rather than passing straight through. The amount it bends or, in technical jargon, the angle of refraction, depends on the difference in the two media's refractive indices. This is easily demonstrated by putting a chopstick in a glass of water. I found a really nice schematic of a handheld refractometer in Reefkeeping.

How does it work?

• Light enters from the left and passes through the liquid sample
• The light hits the prism at the bottom of the liquid, it is slowed more than in the liquid because the prism has a higher refractive index
• As the light then travels down the refractometer, it passes through several lenses and lands on a scale
• The optical wedge that is mounted on the bi-metallic strip moves the lenses in response to temperature changes, ensuring that the readings are accurate regardless of temperature

• The bending of the light at the liquid/prism interface (this is where refractive index plays a role) sends the light higher or lower on the scale's grid
• The light covers a portion of the scale, and the remainder is dark
• Look through the viewfinder on the other end and read where the light is falling on the scale

In grapes, the refractometer scale (in degrees Brix) will measure the sugar content, in reality, it is measuring the amount of soluble solids but since sugar is the major component in the soluble solids, the scale is a good approximation of the sugar content in the grapes.
References:
1. Reefkeeping.
2. Refractometry; Savitribai phule pune UNIVERSITY SEMINAAR ON Miss. SNEHAL K. DHOBALE M-PHARMACY – SEMESTER-1 YEAR 2014-15 Padm. Dr. D.Y.PATIL COLLEGE OF PHARMACY,AKURDI, PUNE. GUIDED BY, PROF. M.T. MOHITE.
3. Hand Held Refractometer.