Food tests 🍔🧪

Describe the test for reducing sugars (2)

• Heat with Benedict’s solution

• An orange / brick red precipitate shows the presence of a reducing sugar

Describe the test for non-reducing sugars (i.e sucrose)

• Heat with Benedict’s solution and stays blue/negative

• Add HCl and heat

• Add sodium hydrogen carbonate (alkali) to neutralise the acid

• Add Benedict’s solution and heat

• An orange/ brick red precipitate shows the presence of a reducing sugar

Colour only orange/brick red precipitate because often disaccharides so double the monosaccharide

Describe the test for starch

• Add drops of iodine to sample

• It turns blue/black if starch is present

Describe how you would use calorimetry to identify the concentration of glucose in an unknown solution

• Carry out the Benedict’s test on sample of glucose with known concentration of glucose

• Use a calorimeter to find the % light transmission of known samples of glucose

• Draw a graph with glucose concentration on the x-axis and % light transmission on the y-axis

• Plot the pints on the graph and draw a calibration curve

• Cary out Benedict’s test on the solution of glucose with an unknown concentration

• Find the % light transmission for this sample using the colorimeter

• Find this % light transmission on the y-axis and draw a horizontal line until you reach the calibration curve. Then draw a vertical line down and read the conc. of glucose from the x-axis

Describe the test for lipids

• crush/rind the food

• Add ethanol

• Then add water and shake/mix

• If positive/ ethanol is present it turns white/ milky emulsion

Describe the test for proteins

• Biuret test

• Add Biuret reagent (NaOH and copper sulfate)

• If it turns purple protein is present

Describe the biochemical tests you would use to confirm the presence of lipid, non-reducing sugar and amylase is a sample (5)

Lipid

• Add ethanol then add water and shake

• White/ milky emulsion if present

Non-reducing sugar Colour only

• Do Benedict’s test and stays blue/negative

• Boil with acid then neutralise with alkali

• Hat with Benedict’s test and becomes red/orange precipitate

Amylase

• Add Biuret reagent and becomes purple

• Add starch, (leave for a time), test for reducing sugar/ absence of starch