Speaker one All right. So have you ever left your milk outside the fridge at room temperature, and then wondered why it’s gone sour and then smelt sort of weird?
Speaker two No, (laughs) but. it’s because bacteria in the milk has started the decay process, converting the sugar in the milk to lactic acid, a by-product of anaerobic respiration.
Speaker one In today’s investigation, we’re going to model this reaction by using the enzyme lipase.
Speaker one My favorite
Speaker two It’s your favorite enzyme? Now lipase breaks down the lipids into fatty acids. And glycerol. And the more fatty acids are produced and the milk becomes more acidic and the pH decreases.
Speaker one Okay. So we can observe this change in pH with an indicator called ‘cresol red.’ If it turns red, the solution is alkaline. And if it turns yellow it’s acidic.
Speaker two Okay, that makes sense. Now for this experiment you need the following equipment. So do pause the video and take a look.
Speaker one Before we start, you’re going to need to put on the safety specs. So, I want you to add five centimetres cubed of lipase into this boiling tube. Yeah?
Speaker two All right got that.
Speaker one And then you’re going to add another five of milk into this boiling tube.
Speaker two In it goes.
Speaker one Now, fantastic. Now you’ve done that I’m adding seven centimetres cubed, of sodium bicarbonate.
Speaker two Ooh yeah, yeah.
Speaker one That’s going into the full fat one.
Speaker two Okay. So you’re adding that to the milk?
Speaker one Yeah.
Speaker two To make it alkaline.
Speaker one Exactly.
Speaker two Okay. That sounds good.
Speaker one And then now I’m going to add five drops of cresol red.
Speaker two All right. Let’s give it a shake. So that’s alkaline?
Speaker one Hmmm mmmm. Let me stick both of these into a water bath.
Speaker two at 30 degrees?
Speaker one 30 degrees. Let’s double check that.
Speaker two I guess we need these to get up to temperature as well.
Speaker one You need to get these up to temperature. Exactly.
Speaker two So give it a few minutes.
Speaker one Give it a few minutes.
Speaker two And we’ve had the solutions in there for a little while. So let’s check the temperature.
Speaker one Yeah we’re at 30.
Speaker two Perfect. So we can start the reaction okay. So what we need is one cubic centimetre of the lipase. And we need to put that in the milk solution. But at the same time I need to start the stop-watch as well
Speaker one okay.
Speaker two So, three, two, one, inch perfect. Now yes. If you take out the thermometer we can stir it because we need to get the reaction going.
Speaker one Oh I’m stirring.
How long do I have to stir that for?
Speaker two Well, after five minutes, if we don’t see a reaction, then we’ll call it quits. We’re at five minutes now.
Speaker one Okay.
Speaker two Can you see any change in reaction?
Speaker one Nope. No change in colour.
Speaker two So it might be because of the temperature. So let me write that down and name change.
Speaker one So that’s a 30 degrees, no change.
Speaker two No change.
Speaker one So repeat the experiment for five different temperatures. We’re going to be using 20,30,40,50 and 60. Now 20. That’s just room temperature. So don’t use that.
Speaker two No water bath.
Speaker one. No water bath, I’m sorry.
Speaker two Now the independent variable is the temperature. The dependent variable is the time taken for the milk solution to turn yellow. Now there are other variables too… the control variables are: the volume milk, the sodium carbonate, lipase and cresol red.
Speaker one So you’ve got to make sure the same for each temperature. To get reliable results. Repeat the test for each temperature three times. Calculate the mean time taken for the solution to turn yellow. And then you can you can start looking it right.
Speaker two
I like a good graph. Now, enzymes work slowly at low temperatures and this can be seen from the graph as the rate of reaction takes longer.
Speaker one
As the temperature is increased, the time taken for the solution to turn yellow decreases. Now this is because the rate of lipase action increases. At 50 degrees, the reaction is at its fastest as it takes the least amount of time for the indicator to turn yellow.
Now this is the optimum temperature for the activity of the lipase. The temperature at which it works the most effectively and therefore decay occurs the fastest. At 60 degrees, there will be no reaction. Now this is because the enzymes will have all denatured, therefore there’ll be no data point to plot in your nice little graph.
Speaker two I know, I’m disappointed actually.
At low temperatures, bacteria and fungi are less active, so the decay is slow.
Speaker one And this is why we keep food like milk in a fridge. You get it from the experiment?
Speaker two I do, yes.
Speaker one Now, would you like to have a sniff?
Speaker two No no please no no no no no no
Speaker one No, I’m not going to. `
Description
A presenter-led live action video showing a science experiment that demonstrates how temperature affects the rate of decay, with Dr Maggie Aderin-Pocock and Dr Alex Lathbridge.
