Vitamins and minerals in fruit and vegetable juice?

can anyone please tell me how quickly the vitamins and minerals disappear from the juice of fruit and veg? I've read it's best to drink immediately to obtain the maximum nutritional benefit.
Also, I've read that supermakets sell fruit and veg which may have been xrayed. if this is true how much nutritional value do these retain? Also, how much does insecticide sprayed on crops affect the nutritional content?
is there a way of measuring the nutritional content?

The process is chemical, not magical.

Different nutrients will react in different ways, so some deteriorate faster than others.

Heat, and exposure to the air, are the main accelerators for these processes. Fruit and veg that are in good condition will retain their goodness for longer, and they will contain more 'raw goodness' (unreacted nutrients) if they are eaten uncooked.

Unfortunately, the cellulose 'fibrous' parts of the plant are indigestible, so any nutrients enclosed by these fibres will not be available to absorb, unless the fibres are broken down somehow. The most common method of doing this is with heat. Cooking is designed to soften the fibres and allow access to the nutrients inside.

Very fresh veg can withstand more cooking than older stuff that's already begun to get soft. If you don't want to cook it, then you can break down the fibres by doing a lot of chewing.

(Most people don't chew their cooked food thoroughly enough to extract most of its nutrition, so chewing it raw might be exhausting. Good exercise for the jaw, though.)

Some of the 'Maillard reactions' that happen when food is heated, cause beneficial reactions between nutrients. Most of the negative kind of Maillard reactions (which produce toxic or carcinogenic compounds) happen when food is burned, and mainly affect overcooked meat.

To maximise its nutritional value, you should store fruit and veg somewhere cool and shaded, and not cut it up until you are about to cook or eat it. Cook it as lightly as possible, with as little water as possible (steaming is usually best), and eat it as soon as it is cooked.

Stews and soups can be kept and eaten some time after being cooked, because the heat drives dissolved air out of the water surrounding the vegetables, leaving them isolated from damaging reactions with oxygen.

Vacuum-sealing cooked veg will also isolate them from the air, but will bring them into contact with the plastic material of the bag. Plasticisers can 'leach' into the food and react with nutrients.

Fruit and vegetable juices will also deteriorate while exposed to air, heat or light, so the most nutritionus juice is that squeezed immediately into the glass to be drunk.

The skin of most fruit & veg will protect the interior from contamination by agricultural chemicals, and the outside should be washed (a short soak in vinegar is often recommended) to remove any traces that are present.

Some books give lists of the nutritional resources available from different fruit & veg, but these are based on average measurements.

Try:

http://healthyeatingclub.com

The 'terroir' (growing conditions) will affect the actual nutritional profile so much that it wouldn't be commercially possible to be accurate about any product. One grown in the corner of a field might contain much more of a nutrient than one grown in the opposite corner.

as soon as the fruit / veg is picked from the tree or plant it starts to loose mineral and nutrients.