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If I eat 100 grams of raw fennel (Foeniculum vulgare) I eat also mg 414 potassium, K (according to ndb.nal.usda.gov, USA National Nutrient Database ).

But if I make a juice of 100 grams of raw fennel using cold pressing or centrifuging, does the juice contain 414 mg potassium?

If not, what percentage of potassium is lost during juice extraction?

Is there a nutritional value table for vegetable juices which accurately gives the quantity of potassium and other minerals like magnesium, iron, etc which are extracted?

Thank you

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Minerals are not destroyed by food processing, except when you discard cooking water, for example.

In this USDA.gov document, page 8, check for "veg, greens, cooked, water used" and you can see that all the minerals, including potassium, have 100% retention (Retention means how much of a nutrient stays in the food after processing).

As mentioned in the other answer, juicing can decrease the nutritional value of a fruit/vegetable when it destroys the fiber. Also, you can lose quite some minerals and vitamins when you remove the peel before juicing.

  • That's interesting data, but not relevant to the question, since the question is about how much is left after throwing away the pulp. – Nic Apr 15 at 14:10
  • The question is about fennel and potassium. You do not throw away any pulp from fennel. When it comes to other fruits/vegetables, you get the same amount of minerals if you eat all the edible parts of the raw food or if you juice all these edible parts. The point is that juicing does not destroy minerals. – Jan Apr 15 at 14:23
  • Juicing doesn't destroy minerals, but in this case the OP doesn't seem to plan to consume the pulp -- how much (if any) of the potassium comes through in the juice? – Erica Apr 18 at 10:20
  • One who does not consume the pulp from the raw fennel and also not from the juiced fennel, consumes the same amount of fennel, right? Which means the same amount of potassium, because potassium is not destroyed or lost during juicing. The document linked from my answer says that the retention of potassium after boiling (water used) of green vegetables is 100%, so no potassium is lost. After hard press juicing, which is much lighter processing, no potassium loss is expected, either. – Jan Apr 18 at 10:27
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I found many sources claiming that "most" of the vitamins and minerals are preserved through the juicing process. The only quantitative statement I found was an unsourced quote from this question on Quora.

Very little of micronutrients found in fruits and vegetables are thrown out with the insoluble fiber. In fact, the Department of Agriculture found that over 90% of the raw food nutrition is found in juice.

It seems that dietary fibre is the only nutrient that is significantly removed by juicing. But it is an important one, and that's why guidelines suggest that vegetable juices can only count for one serving of vegetables per day.

  • Thank you. My dietary is already rich of fibers but poor of Potassium or less than the recommended daily of 3 gr per day (OMS). If over 90% of Potassium is found in vegetable juice (such as fennel ) it could be a valid alternative to artificial dietary supplements (which are expensive and artificial) .However I have big doubts that only 10% of Potassium is lost. It's incredible that there is no scientific study (an official scientific Nutrient Database) for centrifugated juices. – gr68 Mar 6 at 16:09
  • BTW OMS recommends 3 grams potassium/daily , WHO recommends 3.5gr potassium daily for adults, not easy to reach also for Vegans/Vegetarians. – gr68 Mar 6 at 17:03

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