Food and Ingredient Categories, Carrier Systems and Food Technology HFV1004 (HFV1004)
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Lecture glucose determination
ice cream
The assay for glucose determination is normally used in medical facilities to detect glucose in urine
and to identify if someone is diabetic. The name of the assay is the Fehling’s test, it is an chemical
test and you can differentiate between reducing and non-reducing sugars. Reducing sugars can be
present in monosaccharides or some carbohydrates with a free carbonyl group (= aldehyde or ketone
group). Normally, the Fehling solution has a deep blue colour due to the copper.
- The Fehling solution consists of a copper sulphate with has a pentahydrate which has some
water in its salt mixed with potassium sodium tartrate in a strong alkali environment (due to
sodium hydroxide).
- Upon heating, especially the tartrate salt will create a distartrate 2 complex, this complex can
oxidize aldoses to aldonic acids. In this process the copper ions of the complex will react with
this reducing sugars (a.k.a. aldehydes) to form a Cu2O ion and this will create a red
precipitate. The ketones or aldehydes, on the other hand, are oxidised to shorter chains
acids.
- So, why do we need to add the potassium sodium tartrate ions? This is to prevent the
formation of insoluble copperoxide that will be formed when coppersulfate reacts with
sodiumhydroxide when there is no complex present and this will actually give a black
copperoxide and this is why we have the addition of the tartrate salt.
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