When I make my vegetables, I boil tomatoes, onions, broccoli, cauliflower, mushrooms, collard greens and eggplants in plain water. To my surprise I always find oil on top of the water, even though I added none. What could be the cause?

My thought is the seeds from the eggplants and tomatoes emit oil when boiled. Is this possible?

  • 2
    Does oil float on top of water/gravy ? Or are you describing the texture of the vegetable pieces ? Is this texture present when egg plants are not added ? Does your experience match this ?
    – AJN
    Commented Feb 26, 2023 at 4:39
  • 2
    How are you cooking them (boiling, baking etc.)? It sounds odd, none of the vegetables you list have significant oil content.
    – User65535
    Commented Feb 26, 2023 at 18:23
  • I boil them and the water that I used to boil them becomes oily. What could be the cause for this?
    – Eliana
    Commented Feb 26, 2023 at 20:01

1 Answer 1


I think you are likely mistaking wax for oil.

Plants generally have a protective wax layer on their surface, just like humans have sebum on their skin. There are also other nonpolar substances within plant cells that would have a waxy or oily texture when clumped together, but they are less likely to be the cause, because they are not, in fact, clumped together, but exist as a single molecule here, another one there.

the seeds from the eggplants and tomatoes will emit oil when boiled

This is highly unlikely. First, tomatoes and eggplants are not an oil crop, and their seeds contain very little oil, when compared to other seeds such as walnut or sunflower seeds. Second, boiling doesn't really get oil out of an oil-containing seed in any appreciable amount.

And third, if there is anything in your soup emitting actual oil, you wouldn't end up with the water feeling oily. Oils are way too light, and boiling disperses them into an emulsion. You can easily observe this when you start a regular soup. Sweat some onions in oil, then add water and let it boil up. The water will be cloudy, but it won't feel oily to the touch, even though there obviously is oil in the mixture. The oil droplets at these small amounts of fat are just too small to coalesce into anything that makes your soup feel oily. In contrast, plant waxes are much heavier and like clumping into a skin on the surface. So when you touch the water there, you can easily perceive it as oily.

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