Dr's Casebook: The pumpkin seed oil phenomenon

Pumpkin seed oil has a strange colour changing phenomenon, essentially, when you see it in the bottle it looks red, but when you use it in a dressing it appears green. Photo: AdobeStockPumpkin seed oil has a strange colour changing phenomenon, essentially, when you see it in the bottle it looks red, but when you use it in a dressing it appears green. Photo: AdobeStock
Pumpkin seed oil has a strange colour changing phenomenon, essentially, when you see it in the bottle it looks red, but when you use it in a dressing it appears green. Photo: AdobeStock
​​Halloween is just round the corner and homes are bedecked with ghosts, ghouls and pumpkin lanterns. I have a personal tradition of doing pumpkin research and experiments at this season, which you may have noticed over the years.

Dr Keith Souter writes: This year I came across some interesting research on pumpkin seed oil, which resulted in me searching delicatessens for some pumpkin seed oil. It is used in Central European countries as a salad dressing. It has a strong and characteristic nutty flavour. Yet it has a strange colour changing phenomenon. Essentially, when you see it in the bottle it looks red, but when you use it in a dressing it appears green.

Hide Ad
Hide Ad

The change in colour is due to a change in oil layer thickness. As the oil layer thickens, the oil changes its appearance from bright green to bright red. The observed colour is neither dependent on the angle of observation nor on the direction or type of light.

If you get some pumpkin seed oil and pour a thin film in the bottom of a glass it will look green, but if you tilt the glass so that the oil bunches up and gets thicker it appears red, and as the film thins it will go green again.

The reason for this is that pumpkin seed oil is dichromatic. This means that its colour depends on the thickness of a layer, because it will split white light into its component coloured lights. It then depends upon how we perceive colour, and this relates to particular cells called cones in the retina of the human eye. There are three types, which are designated by the letters LMS. These stand for long, medium and short wavelengths. The L cones respond mostly to red light, the M cones to yellow to green light and the S short wavelength cones to blue light. So, the oil colour change is a result of the thickness of the oil and to the cones in your eyes that are being stimulated.

Hide Ad
Hide Ad

Understanding how the cones work helps explain colour blindness. Dichromacy is a type of colour blindness when only two types of cones respond. There are three types. Protanopia or red colour blindness, occurs when red cones are absent. Deuteranopia is green colour blindness because green cones are absent. Tritanopia is blue-yellow colour blindness when blue cones are absent.