Vanillin (4-hydroxy-3-methoxybenzaldehyde), is one of the most easily identifiable flavourings throughout the world.  Its pleasant taste and smell make it a useful ingredient in the formulation of food flavourings, fragrances, pharmaceuticals and perfumes.  In fact my most recent toothpaste purchase is advertised as having a “refreshing vanilla mint” flavour!

The compound was first obtained from the seed pods of the orchid “Vanilla planifolia”.  The extract from the pods is 98% vanillin and 2% other complex molecules.  The 2% actually causes the extract to have a noticeably different taste than pure vanillin.  This is the reason that gourmet chefs extol the virtues of “good” vanilla over the “inferior” synthesized variety.  Today only 1% of vanilla is obtained from orchid pods.  Much of the rest is obtained as a byproduct of the pulp and paper industry.  Chemical synthesis accounts for the remainder.

Since the melting temperature of pure vanillin is low, about 82 ^oC, a melt specimen for observation with the polarizing microscope, is easy to prepare.  A small quantity of the compound, a white to pale yellow crystalline solid, is placed on a microscope slide, covered by a cover-glass, and heated gently with an alcohol lamp.  When the crystals melt completely and form a thin layer between slide and cover-glass, the slide is removed from the vicinity of the flame and allowed to cool.

I must say that the preparation of vanillin melt specimens is a particularly pleasant experience.  Many of the chemicals that I work with while preparing articles for Micscape have extremely unpleasant odors.  One of the most malodorous is elemental sulphur.

It should be noted however that caution should be exercised while using pure vanillin.  The MSDS safety document for the compound states the following.

WARNING! MAY BE HARMFUL IF SWALLOWED, INHALED OR ABSORBED THROUGH SKIN. MAY CAUSE IRRITATION TO SKIN, EYES, AND RESPIRATORY TRACT.

CARBON DIOXIDE AND CARBON MONOXIDE MAY FORM WHEN HEATED TO DECOMPOSITION.  (Carbon monoxide is an extremely poisonous gas.)

The structural formula and molecular shape of vanillin are shown below.  (Both illustrations were prepared using HyperChem Pro software.

As the molten compound re-crystallizes, some sections may have greater thickness than others.  This difference is one of the factors that cause different colour pallets to be displayed between crossed polars.  The light gray sections in the image below are particularly thin crystal formations.

The three images that follow were obtained while the melt was re-crystallizing.  Great speed is required since the crystal growth front seen in the image is advancing at an alarming speed.  Seconds later, the entire field would be covered with crystals.  (Crossed polars)

Occasionally circular growth fronts, called spheroliths are observed. (Crossed polars)

The higher magnification image below shows a rectangular growth that has started to encroach into the edge of a large spherolith. (Crossed polars)

Interesting patterns are often seen at the center of spheroliths. (Crossed polars)

Compensators called “wave-plates” can be inserted into the light path in a polarizing microscope in order to change the colouration of the image.  The two images that follow are of exactly the same field. (Crossed polars + two lambda/4 plates   –  One lambda/4 plate was rotated to produce the colour difference.)

Here is a second example of the use of compensators.  Again the field shown is the same for both images.  (Left image: Crossed polars + two lambda/4 plates    –    Right image: Crossed polars + lambda/4 plate + lambda plate)

The three images that follow show typical vanillin fields.  (Note: The first image in the article is identical to the first image below, however, it was post-processed using Adobe Photoshop’s “Invert (colour)” command.  A “Levels” adjustment was also performed in order to increase the contrast.)

Finally, here are two fields showing serpentine gaps that often occur in melt specimens. (Left image: Crossed polars + lambda/4 plate + lambda plate  –   Right image: Crossed polars + two lambda/4 plates)

The Mexican Aztecs first used the vanilla plant as a flavouring in the 14^th century.  It’s not surprising that its popularity remains undiminished in the 21^st century!


Photomicrographic Equipment

The images in the article were photographed using a Nikon Coolpix 4500 camera attached to a Leitz SM-Pol polarizing microscope.  Images were produced using a polarizing condenser.  Crossed polars were used in all images.  Compensators, ( lambda and lambda/4 plates ), were utilized to alter the appearance in some cases.  A 2.5x, 6.3x, 16x or 25x flat-field objective formed the original image and a 10x Periplan eyepiece projected the image to the camera lens.