Practical skill 17: Biological washing powders and temperature

Aim

To investigate the effect of temperature on how quickly a biological washing powder removes an egg yolk stain from cloth.

Background

Biological washing powders contain enzymes that digest stains. Proteases break down protein stains (such as blood or egg) into amino acids. Lipases break down fat and grease stains into fatty acids and glycerol. Egg yolk contains both protein and fat. The small, soluble products wash out of the clothes.

Biological powders clean well at low temperatures, such as 30 to 40 °C, so less energy is needed to heat the water. At very high temperatures the enzymes are denatured: the shape of the active site changes, the substrate no longer fits, and the powder removes stains no better than a non-biological powder.

In this investigation, the same stain is put on identical pieces of cloth, which are placed in the same biological washing powder solution at different temperatures. The time taken for the stain to disappear measures how fast the enzymes work. Two controls are used: the stain in water only, and the stain in powder solution that has been boiled and cooled so that its enzymes are denatured. The boiled powder still contains detergent, so the stain may fade a little, but it is not removed.

Hypothesis

The stain will disappear fastest at about 40 °C, because enzymes work best at a suitable temperature; it will be removed more slowly at lower temperatures and at 60 °C because the enzymes work slowly in the cold and are denatured when it is too hot.

Variables

Equipment

• Biological washing powder (containing protease and lipase)
• Balance, to measure to 0.1 g
• White cotton cloth cut into 5 cm × 5 cm squares, stained with 5 drops of egg yolk from a dropping pipette, then dried for 24 hours
• 5 beakers (100 cm³) for the powder solutions, and 5 thermostatically controlled water baths (or 250 cm³ beakers of water kept at the right temperature)
• Measuring cylinder (50 cm³)
• Thermometers, −10 °C to +110 °C
• Kettle, ice and tap water to set up and adjust the water baths at 20, 30, 40, 50 and 60 °C
• 5 glass rods for stirring
• Forceps
• Stop-clock reading to 1 s
• Unstained white cloth, to compare the colour
• Marker pen or glass-marking pencil for labelling beakers
• Eye protection and gloves
• Powder solution that has been boiled for 5 minutes and cooled (prepared by the teacher)

Risk assessment

Method

1. Label five 100 cm³ beakers 20, 30, 40, 50 and 60 °C. Weigh 1.0 g of biological washing powder into each and add 50 cm³ of water to each. Stir until the powder has dissolved.
2. Set up water baths at 20, 30, 40, 50 and 60 °C. Check each temperature with a thermometer.
3. Stand each labelled beaker of powder solution in the water bath at the same temperature. Wait until a thermometer in the solution shows that temperature.
4. Put a stained cloth square on a white tile and compare the colour with an unstained piece, so you know what ‘no stain’ looks like.
5. Working with a partner, use forceps to put one stained cloth into each of the five beakers at the same time. Start the stop-clock.
6. Stir each solution five times every minute, using a separate glass rod for each beaker.
7. Look at each cloth every 30 seconds and compare it with the unstained cloth. Record the time when the yellow stain can no longer be seen. If the stain is still visible after 1800 s (30 minutes), record ‘> 1800’.
8. During the test, check the water bath temperatures and add hot or cold water if needed.
9. Repeat the whole investigation a second time at each temperature, using fresh stained cloths and fresh solution.
10. Control 1: at the same time as a 40 °C test, put a stained cloth into 50 cm³ of water only at 40 °C and test it in the same way.
11. Control 2: warm 50 cm³ of the boiled and cooled powder solution to 40 °C, put a stained cloth into it and test it in the same way.
12. Calculate the mean time at each temperature. Wash your hands and clean the bench.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Line graph of mean time for stain to disappear / s on the y-axis (for example 400 to 1400, 2 cm for 200 s; the axis does not have to start at 0) against temperature of solution / °C on the x-axis (20 to 60, 2 cm for 10 °C). Plot each point as a cross (×) and draw a smooth curve of best fit. The curve will have a minimum (the fastest removal) near 40 °C. Do not plot the two control results.