How to determine the presence of sugar in foods

What do grapes and milk have in common? Find out in our cool video!
Reagents and equipment: grapes, milk, heat-resistant beaker, stove and candles, sodium hydroxide solution, copper sulfate solution.
Crush some grapes and pour the juice into a heat-resistant beaker. Add solutions of sodium hydroxide and copper sulfate. Heat the mixture on a stove with candles. Over time, the solution will change colors from blue to orange. If you prepare the same solution with milk instead of grape juice, the solution will also change colors.
Sodium hydroxide reacts with copper sulfate to form a loose blue precipitate – insoluble copper(II) hydroxide. Many foods, including grapes, contain glucose, which can act as a reducing agent thanks to its structure. Milk contains lactose, or milk sugar, the molecules of which consist of glucose and galactose. Thus, in both cases, copper(II) hydroxide reacts with glucose and is reduced first to orange copper(I) hydroxide when heated, and then to copper(I) oxide. The more glucose, the better the reaction and the brighter the color.
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Safety precautions: perform this experiment only under adult supervision.

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