In the previous chapters we looked at how Perception vs. Reality can affect buying patterns, and Spiritual and Organoleptic methods of testing oils. In this, the third chapter, we will begin to look at more scientific methods of evaluation.
PART 3: General Physical Measures (Refractive Index, Specific Gravity and Optical Rotation).
While many scientific methods of evaluating products can be quite expensive, it is surprising how affordable some of the techniques can be. In this chapter we will focus on three that meet this criteria: Refractive Index, Specific Gravity and Optical Rotation.
Refractive Indexes are generated by measuring the angle of incidence when a ray of light passes from one medium into one that is more dense. Think of a simple glass of water. Now place a pencil in the glass and the angle of the pencil will appear to change in the water. Different substances will produce different angles that can be measured with affordable hand held devices.
Each oil has a specific refractive index, and by measuring this using a refractometer, the authenticity of oils can be established. An important point to remember is that this is easily influenced by temperature. For this reason refractive indexes are often displayed as a range. When testing against benchmarks it is important that the same temperatures be present.
Specific Gravity is simply the gravity of a substance in comparison to an equal volume of water (distilled, typically 20 degrees Celsius) expressed as a ratio. Results greater than 1 indicate a product that is more dense than an equal volume of water. Less than 1 indicates a product less dense than an equal volume of water. Oils tend to be under 1.
Results using this method are sometimes difficult to reproduce. For that reason specific gravities are often given with an acceptable range. An example of how this can assist with quality evaluation is comparing the accepted specific gravity range of a product against the common agents used for adulteration of that product.
If an essential oil normally had a specific gravity range of 0.85 to 0.90, and the most common adulterating agent for that oil had a specific gravity of 0.78 to 0.82, one would expect the specific gravity of an adulterated product to be below 0.85.
Lastly, we have Optical Rotation. Substances that have the ability to rotate a plane of polarized light are termed Optically Active. Counterclockwise rotation is referred to as laevorotatory, and is symbolized by ‘l–‘. Clockwise rotation is dextrorotatory, and this is symbolized by a ‘d-‘.
A single substance can have different molecular orientations, but still have the same molecular weight and formula. By using Optical Rotation we can determine how the differences will affect quality, as well as identifying in some instances natural versus synthetic compounds. An example is as follows:
Citronellol exists in many oils. However, not all Citronellols have the same molecular orientation. If Optical Rotation is applied, the l-citronellol is thought to have a superior rose odour than the d-citronellol variety despite both being exactly the same in formula. The orientation of the various elements (carbon, hydrogen, etc…) can play a significant role in your final product.
Each of these methods has a large volume of technical information available on-line so even a novice could get started. They are common in our industry, and most importantly, they are unlikely to cost you an exorbitant amount of money.
In the next chapter we will look at the Gas Chromatographs.
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