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Please see the sentence beside the red heart. What's the chemistry behind this?

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From p. 68 in Williams-Sonoma Collection: Seafood 2005.

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    If you can’t be bothered to at least *type one sentence* instead of just posting a picture, I wonder why the community should bother to answer. You have been asked to do so previously, continuing the sloppy question is disrespectful, especially to users with physical limitations. – Stephie Jan 20 '21 at 11:08

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The "cook" in this case is not actually a cook. It is the acid in the citrus (all citrus contain... citric acid), which interacts with the proteins and precipitates/coagulates them, which is essentially the same process as happens when you heat protein.

In both cases the interaction causes loss of secondary structure in the protein, which results in essentially insoluble protein that then precipitates out of solution. Denatured proteins are generally whiteish ( e.g. egg white/albumen is ~10% protein, which is clear before heating, but white when denaturation then precipitation by cooking).

bob1
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  • Please don't add unsure/incorrect things to an otherwise correct answer. Stop at denaturation without referring to obscure or at least not general reference to protein solubility. This aspect is irrelevant when marinating fish. – Alchimista Jan 21 '21 at 17:12
  • @Alchimista but that is what happens to denatured proteins in the absence of detergents - you get precipitation as the proteins aggregate. This might be in the form of things like inclusion bodies and the like, but it essentially means that the protein comes out of solution and won't go back in. Denaturation alone won't result in a colour change that I am aware of. Source: 20+ years of molecular biology by me... – bob1 Jan 21 '21 at 19:07
  • I am not saying that what you wrote is wrong. It is unrelated to marinating the fish. One think is getting out of a solution / suspension. Another is to modify protein tertiary or secondary structure. The two can happen in egg, ok. Hair straightening does not involve precipitation, just to give another example. In other words you should have mentioned the eggs for further information only. – Alchimista Jan 22 '21 at 09:30
  • @Alchimista - it is directly related to the marinating. Hair contains 5% moisture as it is, so the keratin is essentially precipitated anyway. In the follicle it won't be and you can make keratin solutions, which are transparent and colorless, until you precipitate them (I've done this...). The color change is related to insolubility - all protein solutions that I know of are colorless until something precipitates, and generally proteins under normal conditions won't come out of solution unless denatured. Getting un-denatured proteins requires very specific conditions for preparation. – bob1 Jan 22 '21 at 21:11
  • Yes OK fine. The fish is liquid and after the marinade gets what we call fish shape. Fine for me. – Alchimista Jan 23 '21 at 09:45