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I've had this question in my head since forever, it's kind of a food sciencey question. I understand that cooking changes the properties of food including their nutritional value, sometimes positively, sometimes negatively. It appears per this link (https://healthyeating.sfgate.com/health-benefits-eating-cooked-tomato-products-4444.html) that cooking does not affect Vitamin C and lycopene (but it does affect Vitamin A).

But every time I go to buy a tomato sauce (see important note below***), the nutritional label shows literally no vitamins, why? Is it a labeling problem? Like lycopene doesn't show up on labels. Maybe Vitamin C becomes some new form that's not label-able?? Or is it that cooking doesn't kill vitamins but the high pressure canning process does? (implying that if I make homemade tomato sauce, I won't have this problem)

My motivation for this is that I like healthy eating and am just trying to understand how to healthily consume tomatoes.

***A note on the tomato sauce I'm talking about

I'm talking specifically about the kinds of tomato sauce that vendors at farmer's markets make using their own products. I can't speak for all farms, but having worked at one that did sell our products, I know exactly how this gets made. We harvest tomatoes, we cook & process everything in our own kitchen, we hire a contracted nutritionist to analyze the product & write the label, then we print the label and off it goes. Like, the process is basically exactly like a home cook's. It's not over-processed, over-salted like some industrial makers' products would be. I imagine this process is similar for some other local, premium organic brands that you find at small grocers (except they don't grow their own tomatoes). So given this is so similar to a home cook's process, why are there no nutritional points? I know what you're thinking... yes I am an idiot that I didn't think to ask our nutritionist this question. But I didn't and now I don't have a person to talk to.

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    I'm the UK, I'm not used to seeing nutrients like vitamins labelled on products generally, other than those specifically marketed for nutritional value. Most products I see have a list of ingredients (where naturally occurring nutrients wouldn't be listed) and a breakdown of quantities of carbohydrates, fat, protein etc. Orange juice is a natural source of Vitamin C but I wouldn't necessarily expect to see that on the label.
    – dbmag9
    Jan 9, 2021 at 23:10
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    Note that organic foods are chemically identical to non-organic, the only difference being in the pesticides that can be used with organic foods (some of which are far more toxic than those that can be used in regular agriculture, although it doesn’t matter in either case as the amount of pesticide in the end product is negligible in any case). Jan 10, 2021 at 3:24

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I would swear that there was already an answer about this on SA, but I can't find it.

Pressure canning destroys a lot of the vitamin C value of tomatoes:

The heating process during canning destroys from one-third to one-half of vitamins A and C, thiamin, and riboflavin. Once canned, additional losses of these sensitive vitamins are from 5 to 20 percent each year.

(there are other papers with more scientific information, but they're all behind paywalls)

This means that the finished jarred tomatoes in a year-old jar might have only 30% of the original vitamin C content -- and that will vary according to a lot of circumstances that are hard to predict. So, unless you test ascorbic acid content of the finished product over a lot of batches, you really don't know how much C you have, on average.

While the US FDA does want you to list C content on the label, you are unlikely to be punished for under-reporting the amount of C ... whereas overreporting is a lot more likely to get you an unwelcome phone call. So many canners simply leave it out.

For that matter, many commercial canneries report zero vitamin C as well. I have several cans of Muir Glen tomatoes in my pantry, and none of them report C content.

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  • ok great answer! does this generally ring true where because pressure canning kills (presumably) a lot of other vitamins as well and businesses don't want to be penalized for over-reporting, its just left out? so most canned products seem completely non-nutritious regardless of how "less processed" it is?
    – james
    Jan 10, 2021 at 7:29
  • Yes, that's correct. Particularly, I believe (no citation) that in the early 00's the FDA changed their standards to penalize for overreporting but not underreporting, and at that time all the commercial canned goods suddenly "lost" their vitamin C.
    – FuzzyChef
    Jan 10, 2021 at 20:32
  • Also: it's only C, E, A, thiamin, and riboflavin that are heat-sensitive and get destroyed during canning. Other vitamins (including beta carotene, the more common form of A) survive fine.
    – FuzzyChef
    Jan 10, 2021 at 20:33
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Most of the fruits and vegetables can lose up to 50% of their vitamin C within a week after their harvest, with a sad record for spinach which lose about 90% of its vitamin C within 24h. If you add to this the impact of the storage time and this type of cooking method, then it is not surprising that the amount of vitamin C in your sauce is so close to 0 that the producer would rather just leave it out. I think I found the related question with its interesting answer, but I don't know how to link it: What is the rate of loss of vitamin C in fresh vegetables?

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