FOOD ANALYSIS: SUGAR ANALYSIS
Diagram
Two optical sugar tests
Refractometry
Light bends when it enters a liquid. The more dissolved solid, the more it bends. A refractometer reads the bending as refractive index, or as degrees Brix.
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Polarimetry
Sugars rotate the plane of polarised light, each by its own amount and direction. A polarimeter reads the angle, and the angle tells how much sugar is there.
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The choice
Refractometry reads all soluble solids together, fast, from one drop. Polarimetry reads sugar itself, and can tell sugars apart.
Figure 1: Refractometry measures how much light bends; polarimetry measures how much polarised light is rotated. Drawn in our own style from the FTN Food Analysis notes, Unit 5, and the FTN Instrumental Methods of Food Analysis notes, Unit 4.
Introduction
Refractometry and polarimetry are the two optical methods of sugar analysis, and both read sugar with light instead of chemistry. The difference is what the light does: refractometry measures how much light bends when it enters a liquid, while polarimetry measures how much the plane of polarised light is rotated by the sugars in the solution. Refractometry is fast and reads all soluble solids together, while polarimetry reads the sugars themselves and can tell one sugar from another.
The FTN Instrumental Methods of Food Analysis notes, Unit 4, describe refractometry in full, and the FTN Food Analysis notes, Unit 5, describe polarimetry for sugars. This article follows those two units, with Joslyn, Methods in Food Analysis, chapters on refractometric methods and on polarimetry and saccharimetry.
Quick comparison
Table 1: Refractometry compared with polarimetry
| Aspect | Refractometry | Polarimetry |
|---|---|---|
| What is measured | The refractive index of a liquid, the amount by which light bends when it passes from air into the liquid | The rotation of the plane of polarised light by a sugar solution |
| Light used | Ordinary light entering a prism under the sample | Polarised light passed through a tube of the solution |
| Reading | Refractive index, or directly as degrees Brix, the percentage of soluble solids in the solution | The angle of rotation, in degrees |
| Sample | A drop placed on a prism, which must cover it fully and be free of bubbles | A clear solution in a tube of known length |
| What it tells | Total soluble solids together, not only sugar: for a sugar solution the Brix reading is close to the percentage of sugar | How much sugar is present and, in a mixture, which sugars are there, because each sugar rotates light by its own amount and in its own direction |
| Temperature | Temperature sensitive: the index falls as the temperature rises, so the reading is corrected to a standard temperature, often 20 C, or the instrument holds the prism at that temperature | The books used here give no temperature correction for polarimetry, so none is stated |
| Speed | Fast, needing only a drop, and destroying nothing, so it is used on the production line | A laboratory reading, used where the sugar kind and amount both matter |
| Best for | A jam maker reading the Brix of a batch in seconds, a juice maker reading incoming fruit, a honey packer reading whether honey is ripe | A sugar factory following its sugar content through the process, and measuring invert sugar from its change in rotation |
| Catching adulteration | A reading that does not match the expected Brix points to a change, and a chemical test then says what the change is | A honey that should rotate light one way but rotates it the other way, or by the wrong amount, has been mixed with a syrup |
| Limit | It measures all soluble solids together, so it is used where the soluble solids are mainly sugar, and checked against a chemical method where the composition is mixed | In a mixture the rotation is the sum of all the sugars present, so the reading needs care when several sugars are there |
Scientific explanation
Definition
Refractometry measures the refractive index of a liquid, the amount by which light bends when it passes from air into the liquid: the bending happens because light travels more slowly in the liquid than in air, and the denser the liquid, the more it bends. Polarimetry measures how much a sugar solution rotates polarised light: sugars rotate the plane of polarised light, and each sugar rotates it by its own amount and in its own direction, so the angle tells how much sugar is present and, in a mixture, which sugars are there.
Working principle
In refractometry, a solution of sugar in water bends light more than pure water does, and the more sugar, the greater the bending. A refractometer measures that bending and reports it as a refractive index, or directly as degrees Brix, the percentage of soluble solids in the solution, which for a sugar solution is close to the percentage of sugar. The instrument has a prism on which a drop of the sample is placed, a light source, and a scale seen through a telescope. Light enters the prism, meets the sample, and part of it is reflected and part refracted, forming a sharp boundary between light and dark on the scale, and the position of that boundary gives the refractive index. In polarimetry, sucrose rotates light to the right, while the glucose and fructose from its inversion rotate it to the left. That change of direction is the origin of the name invert sugar: the rotation inverts when sucrose is split.
Apparatus
Refractometry needs a prism, a light source, and a scale seen through a telescope, with temperature control or correction to a standard temperature, often 20 C. Polarimetry needs a polarised light source, a sample tube of known length, and a scale to read the angle of rotation.
Measurement procedure
For refractometry, a drop of the sample is placed on the prism, the prism must be clean, the sample must cover it fully, and the drop must be free of bubbles, because a bubble bends light in its own way and moves the boundary. The reading is taken from the scale, as refractive index or as degrees Brix. For polarimetry, the clear solution is placed in the tube, polarised light is passed through it, and the angle of rotation is read. The books used here give no step by step procedure with volumes for polarimetry, so none is invented here.
Factors affecting readings
For refractometry, temperature is the largest factor: the index falls as the temperature rises, so the measurement must be made at a known temperature or corrected to one. The wavelength of the light matters, because the index changes with wavelength, and the instrument uses a standard wavelength. The composition matters: a solution of sugar and a solution of salt with the same refractive index hold different amounts of dissolved solid, so the Brix scale is true only for sugar solutions and is an approximation for mixed solutions. For polarimetry, the kind and amount of each sugar decide the reading, because each sugar rotates light by its own amount and in its own direction, and the rotation of a mixture is the sum of its parts.
Formulas and calculations
The books used here give no formula with numbers for either method, so none is given here. Refractometry is quantitative through a calibration curve of refractive index against concentration, just as absorbance gives concentration in colorimetry: the index of a solution rises with the concentration of dissolved solids, and the relation is steady enough to give a rapid measure of total soluble solids. Polarimetry is quantitative through the angle of rotation, which rises with the amount of sugar present.
Industrial applications
- Refractometry: a jam maker reads the Brix of a batch in seconds and knows whether to cook longer.
- Refractometry: a juice maker reads the Brix of incoming fruit and knows its sweetness.
- Refractometry: a honey packer reads the Brix and knows whether the honey is ripe.
- Refractometry: measuring the sugar in a fermenting must, the fat in milk by the refractive index of its serum, and the purity of an oil by the index of the oil itself.
- Polarimetry: a sugar factory follows its sugar content through the process with a polarimeter.
- Polarimetry: measuring invert sugar in a syrup from the change in rotation when sucrose is split.
Advantages and limitations
- Refractometry, advantage: fast, needing only a drop, and destroying nothing, which is why it is used on the production line for minute to minute control.
- Refractometry, limitation: it measures all soluble solids together, not only sugar, so it is used where the soluble solids are mainly sugar, and checked against a chemical method where the composition is mixed.
- Polarimetry, advantage: it reads the sugars themselves, tells sugars apart by their direction and amount of rotation, and catches adulteration such as syrup mixed into honey.
- Polarimetry, limitation: it needs a clear solution and a laboratory reading, and in a mixture the rotation is the sum of all the sugars present, so the reading needs care.
FAQ
What is the main difference between refractometry and polarimetry?
Refractometry measures how much light bends when it enters a liquid, while polarimetry measures how much polarised light is rotated by the sugars in the solution.
What is degrees Brix?
The percentage of soluble solids in a solution. A refractometer measures the refractive index, the bending of light, and reports it as Brix for sugar solutions.
Why is sucrose called invert sugar after it is split?
Sucrose rotates light to the right, while the glucose and fructose from its inversion rotate it to the left, so the rotation inverts when sucrose is split.
Why must a refractometer reading be corrected for temperature?
The refractive index falls as the temperature rises, so the reading is made at a known temperature, often 20 C, or corrected to one.
Can a refractometer tell sugar from salt in a solution?
No: it measures all soluble solids together, so a sugar solution and a salt solution with the same refractive index hold different amounts of dissolved solid.
References and further reading
- FTN subject: Food Analysis, Unit 5: Carbohydrate, Fibre and Sugar Analysis; FTN subject: Instrumental Methods of Food Analysis, Unit 4: Flame Photometry and Refractometry.
- Maynard Joslyn, Methods in Food Analysis: Applied to Plant Products, Elsevier, 2012 (Google Books page read)
