B3 RP Required Practicals: Food Tests and Enzymes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: required practical: food tests. Required practical: the effect of pH on the rate of reaction of amylase. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Carry out and describe the food tests for starch, reducing sugars, protein and lipids
• Plan an investigation into the effect of pH on amylase, naming the variables
• Calculate rates (1000 ÷ time) and means from results, and plot them
• Identify hazards, sources of error and improvements

Key words

• Qualitative test: A test that shows whether a substance is present (by a colour change), not how much.
• Reagent: A chemical used to test for something, e.g. Benedict’s solution.
• Independent variable: The variable you change on purpose (here, pH).
• Dependent variable: The variable you measure (here, time for starch to disappear).
• Control variable: A variable kept the same so the test is fair.
• Buffer solution: A solution that keeps the pH at a fixed value.

Required practical: food tests

Aim: to use chemical reagents to find which food molecules are in a sample. Preparing a solid food: (1) grind a small piece with distilled water using a pestle and mortar; (2) stir, then filter to get a clear solution; (3) put a sample into a separate test tube (or spotting-tile well) for each test.

• Starch; Method Add 2 drops of iodine solution to 2 drops of sample on a spotting tile.; Positive result blue-black; Negative result stays orange-brown
• Reducing sugars (e.g. glucose); Method Add about 1 cm³ Benedict’s solution to 1 cm³ sample. Heat in a water bath at about 75–80 °C for 5 minutes.; Positive result green → yellow → brick-red precipitate (more sugar → further along); Negative result stays blue
• Protein; Method Add an equal volume of Biuret solution. Do not heat.; Positive result lilac / purple; Negative result stays blue
• Lipids; Method Add a few drops of Sudan III stain and shake; or shake with ethanol, then pour into water (emulsion test).; Positive result red-stained oily layer on top; or cloudy white emulsion; Negative result no red layer; stays clear

Risk assessment: wear eye protection. Biuret solution contains copper sulfate and sodium hydroxide (irritant). Iodine solution is an irritant. Sudan III and ethanol are flammable – no flames. Heat Benedict’s tests in a water bath, not over a flame. Never taste food in the lab.

Exam tip: Benedict’s needs heat; Biuret does not. Sucrose (table sugar) is not a reducing sugar, so it stays blue. Colour changes are judged by eye, so food tests are subjective: use a colour chart or a control tube of water to compare.

Required practical: effect of pH on amylase

Aim: to find how pH affects the rate at which amylase breaks down starch. Apparatus: spotting tile, iodine solution, 1% amylase solution, 1% starch solution, buffer solutions (pH 4–9), water bath, boiling tubes, syringes or measuring cylinders, stop clock, dropping pipettes, thermometer, glass rod.

• 1. Put 2 drops of iodine solution into each well of a spotting tile.
• 2. Put 5 cm³ of starch solution in one boiling tube, and 5 cm³ of amylase plus 5 cm³ of pH 5 buffer in another. Stand both in the water bath at 30 °C for 5 minutes so they reach the bath temperature.
• 3. Pour the amylase and buffer into the starch, stir and start the stop clock.
• 4. Every 30 seconds, put one drop of the mixture into a fresh well of iodine.
• 5. Record the time when the iodine stays orange-brown (no starch left).
• 6. Repeat with each buffer, three times at each pH, and calculate a mean.

• Independent; In this practical pH (set with buffer solutions)
• Dependent; In this practical Time for the starch to be broken down (iodine stays orange-brown)
• Control; In this practical Temperature (water bath); volume and concentration of amylase and of starch; volume of buffer; time between samples

Risk assessment: wear eye protection – iodine is an irritant; amylase can cause an allergic reaction, so avoid skin contact and wipe up spills. Take care with hot water.

Processing results

A shorter time means a faster rate. Use rate = 1000 ÷ time (units s⁻¹). Leave any anomalous repeat out of the mean. pH values are separate categories, so a bar chart (or a line graph of rate against pH) can be drawn.

Worked example: mean and rate

At pH 6 the three times were 90 s, 60 s and 240 s. The 240 s result is anomalous (far from the others), so leave it out.
Mean = (90 + 60) ÷ 2 = 75 s. Rate = 1000 ÷ 75 = 13.3 s⁻¹ (3 s.f.).

Answer: mean 75 s; rate 13.3 s⁻¹

Errors and improvements

• Sampling only every 30 s; Effect End point is only known to within 30 s (low resolution); Improvement Sample every 10 s
• Judging the colour by eye; Effect Subjective; people disagree on the end point; Improvement Use a colorimeter, or compare with a colour standard
• Solutions not at the bath temperature; Effect Rate changes during the test; Improvement Pre-warm all solutions; check with a thermometer
• Big gaps between pH values; Effect Optimum not found precisely; Improvement Repeat with smaller pH steps near the optimum (e.g. 6.5, 7.0, 7.5)

Key fact: To show that enzymes are denatured by heat, boil some amylase, cool it, and repeat the test: the iodine stays blue-black because the starch is never broken down.

Common mistakes

• Forgetting to heat in the Benedict’s test, or heating the Biuret test.
• Writing “clear” when you mean “colourless”. Clear means see-through; it can still be coloured.
• Saying temperature is the independent variable in the pH practical. It is a control variable.
• Calculating rate as time ÷ 1000. The rate is 1000 ÷ time, so shorter times give bigger rates.
• Including an anomalous result in the mean.

Revision questions

Try these, then check your answers below each one.

Question 1. True or false? The Biuret test must be heated in a water bath.

Answer: False

Working: False. Only the Benedict’s test is heated. Biuret is added at room temperature.

Question 2. The diagram shows a spotting tile from the amylase practical. A drop of the mixture was added to the iodine every 20 s. At what time had all the starch been broken down?

Answer: 120 s

Working: The first orange-brown well (no starch left) is at 120 s.

Question 3. In the investigation of the effect of pH on amylase, what is the dependent variable? (Choose from: Time taken for the starch to be broken down, pH of the buffer, Temperature of the water bath, Volume of amylase.)

Answer: Time taken for the starch to be broken down

Working: You change the pH (independent) and measure the time until the iodine stays orange-brown (dependent).

Related lessons

• Previous lesson: B3.7 Making Digestion Efficient
• Next lesson: B4.1 The Blood
• In this chapter: B3.1 Tissues and Organs
• In this chapter: B3.2 The Human Digestive System
• In this chapter: B3.3 The Chemistry of Food

Continue in the full course

This lesson is section B3 RP (AGCSC0308) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.