5g vanillin were heated in a 100ml beaker to 130°C, then 1g ascorbic acid dissolved. The mixture turned dark green after stirring it for several minutes. The reaction product is a soft, clear and elastic compound:
About an hour later the compound suddenly started to crystallize:
To avoid any contamination with metal ions, 5g vanillin and 1g ascorbic acid were placed into a fresh cleaned beaker without support of spoon or spatula. The mixture was then heated to 130°C. The temperatur sensor, which has a stainless steel jacket, was precautionary placed outside the beaker. The mixture was stirred with a fresh cleaned glass rod. The solution turned green after stirring it for 5 minutes and the ascorbic acid was completely dissolved in the vanillin.
To determinate if vanillin itself would turn green after heating it to 130°C for several minutes, the experiment was repeated without adding ascorbic acid. The vanillin turned slightly yellow after 10 minutes heating but not green. An idea is that the green color occurs not caused by a chemical reaction but rather by a special crystal configuration of vanillin and ascorbic acid. Example: anhydrous copper (II) sulfate crystals are grey -white, but if they containing water, the appearance is blue.
If pure vanillin get heated to 130°C for 20 minutes, the color change to yellow-orange. After cool down vanillin crystallize out immediately and scalelike. If ascorbic acid is added to pre-heated vanillin, the color changes not to green and the solubility of ascorbic acid is decreased. However, after cool down a soft, clear and elastic compound was found too.
Update November 12, 2011: Uploaded an extended article (in German) about this topic on scribd.com.
Update November 19, 2011: Uploaded an edited version of the article here.