How to determine the presence of sugar in foods

What do grapes and milk have in com­mon? Find out in our cool video!

Safe­ty pre­cau­tions

Per­form this ex­per­i­ment only un­der adult su­per­vi­sion

Reagents and equip­ment

• grapes;
• milk;
• heat-re­sis­tant beaker;
• stove and can­dles;
• sodi­um hy­drox­ide so­lu­tion;
• cop­per sul­fate so­lu­tion.

Step-by-step in­struc­tions

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.

Pro­cess­ de­scrip­tion

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.

Are you keen on chem­istry? You’ll love the MEL Chem­istry sub­scrip­tion!