How do scientists determine the nutritional make-up?

[Kookaburra]

Lets say, an orange.

How do they know how much vitamin C it contains? How do they even know that it contains vitamin C in the first place. For all I know, they could be faking the nutritional values because they know the public will just take it at face value anyway.

I searched google to no avail. However I came across this page from wikianswers. It's about how the scientists find out how may calories a food contain.

http://wiki.answers.com/Q/How_do_scient ... a_food_has

They breakdown the food to its simplest form then place it in a metal cup which is in a water box. After they place the cup in the water they take the temperature of the water (Tinitial). When they have the temperature of the water they ignite the food and check the temperature after the fire(Tfinal). Then they use the equation

q=Masswater x specific heatwater x ΔTwater

(q=Heat in calories or joules) (T=temperature) (ΔTwater=Tfinal-Tinitial) (spefic heat of water= 4.18 J/g °C)

(Masswater = Mass of water in box - mass of box w/o water)
(joules to calore conversion 1 calorie=4.184 Joules)

They actually ignite the food as part of the process? Since we do not cook our food, won't the calories that the scientists have calculated not apply to us? Have we been using the wrong value of calories all along in the nutritional calculator?

[Oscar]

To calculate caloric value it's not about the actual food, or whether it's cooked or not. A calorie is the amount of energy needed to raise the temperature of water by 1 degree Celcius. Since this is very little we use kilocalorie or kcal, the amount of energy needed to heat 1kg of water by 1 degree Celcius.

Vitamin C is a biochemical substance with a specific chemical structure. You can analyze food to see how much of a given structure is present. I think there are several methods to determine vitamin C content.