Testing for food substances

 To test for the presence of a specified food substance in foods various agents are used.

We use Benedict solutions which is clear-blue in colour to test for reducing sugars such as glucose and fructose. We heat reducing sugars with Benedict solution in the presence of an alkaline solution. When the solution is orange-red there is a large amount of sugar present. A greenish-yellow colour indicates a small amount of glucose present. Once the solution remains blue there is no reducing sugar present. 

We use iodine solution brown in colour to test for the presence of starch. When iodine is added to a sample containing starch and turns blue or black starch is present. If the solution remains brown there is no starch.

 Biuret test blue in colour is used to test for the presence of proteins. When a solution containing proteins is treated with biuret reagent and dilute copper (ll) sulfate solution in an alkaline medium like potassium it becomes purple or violet in colour if protein is present. If there is no protein it remains blue. 

Ethanol, clear liquid is used to test for the presence of lipids. Once a sample containing lipids is mixed with ethanol and shaken vigorously, the lipids dissolves in the ethanol. The sample will then contain fat, tiny globules which will float. A milky or white suspension will indicate the presence of a lipid while it stays clear in the absence of a lipid. 

The DCPIP (dichlorophenolindophenol) blue in colour is used to test for the presence of vitamin C. Vitamin C sample drops are dropped into the DCPIP solution, the lesser the drops taken to change the blue DCPIP to clear, the more vitamin C is present in the sample. If the colour of the DCPIP does not change then there is no vitamin C present.