Required Practical 4: food tests
Prepare the sample: grind the food with distilled water in a pestle and
mortar, and filter to get a solution
Test for
Reagent
Method
Positive result
Starch iodine solution add a few drops to the sample
orange-brown turns blue-black
Sugars (reducing sugars such as glucose)Benedict's solution add to the sample and heat in a water bath at about 80 °C for 5 minutes
blue turns green, yellow or brick red , more red for more sugar
Protein Biuret reagent add to the sample and mix
blue turns purple (lilac)
Lipids ethanol then watershake with ethanol, then pour into water
a cloudy white emulsion forms
Use a control : the same test on distilled water shows no colour
change, so any change is due to the food
Risk assessment, specific to the test: Benedict's needs a water bath, not
a Bunsen directly, because the tube could boil over and spit; iodine
and Biuret are irritants, so wear eye protection and rinse splashes; hot
water bath, so use tongs
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Required practical — food tests starch iodine solution blue-black a few drops of iodine — no heating sugars Benedict's solution brick red Benedict's, then a water bath at 75 °C for a few minutes protein Biuret solution purple Biuret solution — no heating lipids ethanol, then water cloudy white emulsion shake with ethanol, then pour into water Grind the food up and mix it with distilled water first, then filter — the test needs a solution. Each colour shown here is the POSITIVE result; if the food is absent, the reagent keeps its own colour (orange-brown iodine, blue Benedict's, blue Biuret). Four test tubes showing the positive result of each food test with its reagent labelled: blue-black for starch, brick red for sugar, purple for protein, cloudy white emulsion for lipids
Biuret is for protein , Benedict's for sugar. Mixing them up
loses the test and the resultA risk assessment names the hazard in this procedure and
what you do about it. "Wear goggles" and "be careful" with no hazard named
score nothing
Benedict's needs heat ; the others do not. Forgetting the water bath
is the most common omission in a method
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In the full topic
• Required Practical 4: food tests• Required Practical 5: the effect of pH on amylaseRead the free biology topics →