Eli5: Why is tomato-sauce so good at coloring plastic red in your dishwasher, unlike raspberries or strawberries for example?


We like tomato sauce, but one must be careful with what to put into the dishwasher, to not have plastic bowls, storage boxes or other things dyed red…Why is tomato sauce this potent in coloring plastic. It’s like it’s in the fabric of the plastic itself after it comes out of the dishwasher…why not the same effect with strawberries or raspberries? And is there a way to prevent this?

Edit: Wow, this got some momentum…I see a lot of people like tomato sauces. Thanks for the awards as well!

In: Chemistry

The colour in tomatoes that makes them red, and it is very concentrated in tomato sauce, is lycopene.

Lycopene loves plastic, it sticks to it and doesn’t want to let go. Similar to the yellow pigment in Tumeric (that spice in curry that turns everything a funny yellow colour). When a pigment sticks to a surface that well, it’s essentially dyed that colour. Because the pigment has bonded with the surface. You can’t just “wash it off”.

That red molecule isn’t in raspberries or strawberries. It’s different pigments that cause that and it’s not as adherent (sticky) to plastics.

Prevention – most simply, use glass or ceramics instead of plastic. It’s better anyway, will go in the oven, will go in the microwave, easy to clean, no roughness on the surface that germs can get stuck in, don’t get scratched by knives, generally amazing stuff.

If you absolutely cannot live without plastic in your kitchen, you can try putting a thin layer of oil on the plastic, to keep the pigment from contacting the container.

You can try to do some research on the molecule lycopene, and work out if there is an easy way to get it to release the plastic. See if washing it in vinegar gets it to let go, or whatever.

**The standard answer:**
The stain is from a hydrophobic (repels water) solid substance called lycopene, which is responsible for the red in tomatoes.

As it turns out. plastic is also a “hydrophobic solid”. When two hydroponic things get wet, they stick to each other, thus the stain.

**The standard fix:**
Lycopene is however soluble in oil. So spray on some oil and wipe of the red before getting it wet.

**But I am confused by your situation:**
I am not sure I understand what you mean by “tomato sauce”. In Australia tomato sauce is similar to “ketchup” but it tastes better. Both these sauces have significantly less lycopene than raw tomatoes and should not stain anything a tomato can’t. Typical tomato stains are from a puree (eg: “tomato paste”) which has up to 20 times as much lycopene than a raw tomato.

As other folks have already mentioned, the pigments are different. In fact, there are three types of red pigment you’ll commonly see in plant foods: anthocyanins (most common), carotenoids (tomatoes, peppers, turmeric), and betalains (mostly in beets).

If you cook a dish with red cabbage, say, you will see the color stays in the water based part of the food, and avoids any oil drops you see. Also, the color will shift to bluish (even greenish!) unless you add a lot of acid, which will shift it back to red. Both of these behaviours are characteristic of anthocyanins, though the color range depends on which variation you have.

If, on the other hand, you have a tomato sauce, then the oil droplets will turn red or orange, because the underlying pigment is more waxy/oily. Similarly, some plastics (especially polyethylene and polypropylene) are basically solid waxes/oils, so the pigment happily dissolves in them. Many carotenoids will shift between pale, yellow, and red depending on conditions but blue shades not so much.

Finally, beets! The red stuff in beets is different yet again. It also has a strong preference for water, so won’t stain plastic. They don’t color shift as dramatically as anthocyanins. Also your body doesn’t metabolize them as aggressively, so they turn your pee and poop red if you eat a lot. Betalains are only common in one order of plants, I think. So beets/poke/amaranth and maybe cactus.

Hello to anyone coming from /r/all or from the front page.

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