[Hadrian (OP)]

How do they measure the carbohydrate content of a given liquid like say milk?      

[Hadrian (OP)]

i don't know Hadrian you better ask someone  [^]

[paul.fr]

found this:

"when manufacturers measure carbs, they do so by the method of "carbs by difference". They measure everything else(protein, fats, water..), and what's left is ASSUMED to be carbs"

[Bored chemist]

They might measure the cabohydrates by chromatography. It wouldn't be very difficult.

[Hadrian (OP)]

They might measure the cabohydrates by chromatography. It wouldn't be very difficult.

thanks for that it most helpfull

i fond this on the subject among a lot other intresting stuff

A mixture of various components enters a chromatography process, and the different components are flushed through the system at different rates. These differential rates of migration as the mixture moves over adsorptive materials provide separation. Repeated sorption/desorption acts that take place during the movement of the sample over the stationary bed determine the rates. The smaller the affinity a molecule has for the stationary phase, the shorter the time spent in a column

[another_someone]

They might measure the cabohydrates by chromatography. It wouldn't be very difficult.

But I would have thought that carbohydrates represents a range of molecules covering quite a range of molecular weights?  If that is so, how could you uniquely identify carbohydrates through chromatography?

[Soul Surfer]

Looking up "carbohydrate assay" in google shows that carbohydrates in relatively dilute aqueous solutions (like milk) are assayed by adding phenol and then concentrated sulphuric acid and measuring the absoption of the solution at a particular wavelength.

The precise process is not explained in the references I skimmed through

From my chemistry I remember being shown that concentrated sulphuric acid will  "dehydrate"  carboydrates leaving them as  carbon

[lightarrow]

How do they measure the carbohydrate content of a given liquid like say milk?      

Once, they used Fehling and Tollens reactions (especially the first one) after have decomposed, with concentrated HCl, the complex sugars in simple ones. So they became "reducing sugars" that reduce Fehling reactive (Cu⁺⁺ complexed with tartrate in basic solution) to Cu₂O, which forms a "brick red" precipitate (and you can weigh it):
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA0/MOVIES/FEHLTEST.html
http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Fehling-e.htm

With the Tollens reactive (containing Ag⁺ ions) a "silver mirror" is formed in the reduction of Ag⁺ into metallic Ag:
http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Tollens-e.htm

[another_someone]

The problem is, how much of these carbohydrates are actually digestible (after all, is not cellulose also a carbohydrate, yet it is not digestible by humans)?

Do all humans gain the same energy value from the same carbohydrates, or do different people's digestive systems obtain differences in efficiencies in digesting carbohydrates?

[Bored chemist]

"But I would have thought that carbohydrates represents a range of molecules covering quite a range of molecular weights?  If that is so, how could you uniquely identify carbohydrates through chromatography?"

In milk there are not many different carbohydrates so it's not difficult. The question isn't any different from the general question of how do analytical chemists measure anything in anything and the (OK I realise this is rather unhelpful) answer is by doing analytical chemistry. In this case one possible answer would be to couple the outlet of the chromatographic column to a mass spectrometer. The other thing that is commonly done is to run chromatograms of known compounds for comparison.

"Do all humans gain the same energy value from the same carbohydrates, or do different people's digestive systems obtain differences in efficiencies in digesting carbohydrates?"
In the particular cases of milk carbohydrates the answer is no. Anyone with lactose intolerance will not only fail to gain any calories from the lactose in milk but will suffer ill effects from it. On the other hand broadly speaking, all people gain about 4 Calories a gram from any carbohydrate that is digestible. Cellulose is listed in nutritional information as fibre rather than carbohydrate.

[eric l]

The problem is, how much of these carbohydrates are actually digestible (after all, is not cellulose also a carbohydrate, yet it is not digestible by humans)?

Cellulose may be a carbohydrate, but is NOT decomposed by reaction with HCl (which is not "concentrated" after it has been added to milk).

There are chromatographic processes that allow to make distinction between monosacharides, bisacharides, trisacharides and so on.  I am not sure if they work very well if the sacharides have not been isolated before.  Of course this "sugar spectrum" does not only give an indication on the ratio between "slow" and "fast" sugars.

[Bored chemist]

There are, and I have used, specialised chromatographic columns that do a very nice job of separating all the (common)monosacharides and all the (common) disacharides. If you put a selective detector on the end you could do a very good job of analysing carbohydrates.
There are only a finite number of sacharides worth worrying about; the important ones are well characterised.

[eric l]

There are, and I have used, specialised chromatographic columns that do a very nice job of separating all the (common)monosacharides and all the (common) disacharides. If you put a selective detector on the end you could do a very good job of analysing carbohydrates.

My question :  do you use the technique on a mix of sacharides, or on a complete product containing various sacharides (like the milk in this case) ? 
I never did this kind of chromotography myself, or watched id done, I am more familiar with long chain polysacharides like starch or cellulose.

[Bored chemist]

I was actually looking for lactose but I ran a whole bunch of different monosacharides and also sucrose to prove that they were not present in the sample.

[eric l]

I was actually looking for lactose but I ran a whole bunch of different monosacharides and also sucrose to prove that they were not present in the sample.

So you could inject milk in the chromatography system.  Was that gel permeation chromatography or was it an other system ?

[Bored chemist]

You wouls probably have to remove the fats and proteins first or they would mess up the column.