Re: How calorie content of foods is determined (minus nondigestables)

Hello,
the exact amount of (useful) energy available in food is almost impossible 
to say exactly. This is so due to the differences in the metabolism of each 
person. Some people have more muscles than others, are more "active", etc. 
and all these parameters change the "pattern" of use of energy. What I can 
say as a general rule is that the worse way of calculating it is using a 
calorimeter. You probably know why: the non-digestible carbohydrates (the 
fibres). In the industry, the major "calorific compounds" (digestible) in 
the food are separated and quantified. The total amount of calories in the 
food is the sum of all the compounds.
If you heat (75-80 degrees Celsius) a sample in ethanol, you extract all 
the low molecular weight carbohydrates (sucrose, glucose and so on). Now 
you can quantify carbohydrate in this ethanol-soluble fraction. In another 
sample you can quantify lipids simply extracting it with organic solutions 
(usually chloroform or hexane, but BE VERY CAREFFUL because these compounds 
are flammable and toxic. The amount of starch can be calculated using 
enzymes directly in the non-soluble fraction extracted with ethanol (to 
avoid contamination with free glucose) or you can extract with DMSO and 
then quantify (this will make starch soluble).
Once you have all these parameters, you simply calculate the "digestible" 
calories by considering that each gram of carbohydrate (in any form, free 
or in starch) gives 4 (four) calories, and lipids gives 6 calories per 
gram. You can compare with the data in some industrialised foods. Of 
course, pentoses (from DNA, for example) will give another value, but no 
one lives on a diet rich in xylose or ribose, anyway. That is a good 
approximation. It may sound too complicated and, indeed, it is not feasible 
in a classroom, but if you take a look at some labels in the stuff you buy 
in the market, you will see each one of these compounds quantified 
separately and that these are only a few of the measurements they make 
before putting a new product in the market.
One final remark: I did not tell you many things you MUST know before 
trying to do it in the classroom because I would not suggest you to do such 
measurements with teenaged students. Carbohydrate measurements, for 
example, use phenol (a carcinogenic compound) and sulphuric acid (which is 
very corrosive). Extraction with chloroform MUST be done in a hood to avoid 
inhalation (one may die during the experiment if one inhales too much of it). 
And these are only some of the problems you will find. As you can see, very 
serious safety procedures have to be taken. That is why I will not give 
the details of how to do these experiments over the internet, since anyone 
can read this answer. If you really want to go ahead, get help from someone 
in an university or other kind of lab (try the food or plant physiology 
department).
Hope you are not (very) frustrated. Hugs,
Marco.