Triple - Decay
Investigating decay using decay of milk
Investigating decay using decay of milk
Decay
Measuring the pH change in Milk at different temperatures. This represents decay in an ecosystem. As bacteria grow they produce lactic acid which changes the pH (and makes milk turn to yoghurt)
Check your understanding with these quick common questions. Use the drop down boxes to see the correct answers.
Decay
This is an usual practical as it is labelled as decay and links to Ecology, but is really about an enzyme reaction. Lipase will break lipids down into glycerol and fatty acids. The fatty acids change the pH of the milk. A pink indicator is used to view this change.
Independent variable: Temperature (e.g. ice bath 10°C, room temp, 37 °C, 50 °C).
Dependent variable: Rate of pH change (time to reach a set endpoint pH/colour).
Controls:
Milk type/volume,
initial pH,
enzyme (lipase) concentration/volume if used,
indicator concentration (e.g. phenolphthalein),
sodium carbonate volume,
phenolphthalein (indicator) (Pink when alkaline, clear when acidic)
sodium carbonate solution (this ensures the pH is above 7, slighlty alkaline at the start. The reaction causes the indicator to change to clear, (but the sample becomes white as the milk is white)>
lipase
Improve accuracy by using a pH probe to measure the exact pH. The colour change is subjective, so an oppinion and will different between people.
Control validity by ensuring the temperature is maintained using a thermostatically controlled water bath. Use a measuring cylinder or syringe to ensure volume of lipase, milk and sodium carbonate are accurate.
If milk and lipase are mixed and THEN added to the water bath, they will not have had time to reach the temperature of the water bath.
Description: Higher temperature, faster rate of reaction
Explain: Because particles have more kinetic energy
Over 40C, the reaction will start to slow down
As enzymes denature
Set water baths at chosen temperatures
Add sodium carbonate + phenolphthalein to milk to give a pink starting solution.
Allow milk and lipase to reach the target temperature before mixing, then add lipase to start the reaction and start timing.
Stop timing when the solution loses pink (reaches acidic endpoint) or when pH probe hits target pH.
Repeat for each temperature; compare times/rates vs temperature.