The effect of temperature on the setting time of Junket using rennet.

Paul Randlesome

Biology coursework

The effect of temperature on the setting time of Junket using rennet

Introduction

When milk is mixed with rennet a reaction occurs and turns the milk into small lumps which then merge to form a lumpy thick liquid that is almost solid. If made in a test tube it can be turned upside down and none will come out. The best way of describing it is that it looks like baby sick. This substance is called junket. Some people mix it with sweet things and serve it as a pudding.

The reason for this change is a reaction that goes on inside the test tube or whatever it is being made in. This reaction is because the rennet contains enzymes that convert caseinogen (a soluble protein) in to casein (an insoluble protein). It is found in young mammals including cows and humans. The stuff we use to make it obviously can’t come from a human so it is probably cows or a vegetarian substitute. However small children do have it, it is found in gastric glands in their stomach lining the same place as all young mammals do. This is because all mammals share one common thing (well one that’s relevant to this experiment) all the females have the ability to breast feed their young. But the milk would go through the digestive system way to fast for them to be able to get all they need from it. So this is where the rennet comes in, it curdles the milk (makes it more solid) this gives the digestive system longer to digest the milk. This is why junket looks like baby sick as that is what baby sick mainly consists of. It is found in the young because they need all the food they can get to grow and keep warm.

Rennet is also used in the making of cheese and other dairy products.

Aim

The aim of this experiment is to see what temperature the rennet works best at.

To do this I will conduct an experiment. In this experiment I will make junket at different temperatures and time how long it takes. By altering the temperature (the independent variable) this should alter the time taken (the dependant variable).

The control

To prove that it is the rennet setting the milk and note any thing else I will do a control. Where no rennet is put in the test tube and this will be left for 30 minuets. If nothing has happened to this after the time is up then this proves that, if left the milk will do nothing on its own. So the rennet is the thing that makes the milk curdle.

I will do this at room temperature.

After doing the control I found that nothing happened to the milk, so ...

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The control

I will do this at room temperature.

After doing the control I found that nothing happened to the milk, so any thing that happens will be to do with the rennet.

Preliminary test

I carried out a quick test where I tested to see how much rennet I would need to add to how much milk. From this I found that no matter how much rennet I used it did not set the milk hard enough to be able to turn the test tube upside down.

However it was possible to see when the substance was as thick as it was going to get so a new method is going to have to be planned. Up till now I had planned to just time it until it would tip completely upside down. But now I will have to find a way of measuring the solidness of the junket. I also found that to get it to its maximum thickness in the quickest time 1 ml of rennet is needed to turn 5ml of milk into junket.

Plan

To make this experiment fair I will have to change only one variable and make everything else stay the same to do this I will use the same amount of both milk (5ml) and rennet (1ml) and the same pH etc.

I will carry out the test at the following temperatures 0˚C, 10˚C, 20˚C, 30˚C, 40˚C, 50˚C, 60˚C, 70˚C and 80˚C. this gives me a wide range of temperatures to draw my conclusions from.

Planned method

Take 5ml of milk and place in a test tube, do this with a 5ml syringe.
Check the pH of this if it is the same in all of the experiments (which it should be if it’s from the same bottle of milk) then that’s ok, however if it is not then the experiment with a different pH should be repeated an the previous results discarded.
Get this to the temperature required by placing the test tube in a water bath with the correct temperature water in it. Also place another test tube with 1ml of rennet in to the same water bath.
When it is up or down to the right temperature pour the rennet into the milk test tube. And start the stop watch.
After 4 minutes take the test tube out of the water bath and filter what is left.
Scrape any solid off of the filter paper and weigh it. In a weighing boat on scales.
Repeat this 3 times for each temperature

From the amount of solid gathered you can work out how much set in a minuet and therefore how long it would take to do the whole 5ml of milk.

Prediction

I would expect that as the temperature increases, the amount of solid would also as the speed of the reaction would increase. When a particular temperature is reached I believe the rate of reaction will dramatically decrease. I believe this because most reactions happen faster when the temperature is higher. This is because at higher temperatures molecules mover around faster, they have more energy and they collide more which makes it easier for them to react together, and because the enzymes are made of protein they work faster the warmer it is. Until they reach their optimum temperature where they are most productive then the rate of reaction starts to decrease. This is because the proteins start to denature and this makes them useless, so in affect you are lowering the amount of working enzymes so of course the reaction rate will go down until suddenly it will stop as there will be no reaction, as all of the enzymes will have denatured and none will be able to take part in the reaction.

Actual method

Because of a mistake on the preliminary experiment the method has changed back to the original of turning the test tube upside down. The mistake was we were given the wrong type of milk it was a long life milk that doesn’t easily go off so consequently it didn’t go off and nothing worked as it didn’t set. When it came round to the proper experiment I used full fat pasteurised milk this worked fine but I had to change the method slightly. So it was changed to this;

Take 5ml of milk and place in a test tube, do this with a 5ml syringe.
Check the pH of this if it is the same in all of the experiments (which it should be if it’s from the same bottle of milk) then that’s ok, however if it is not then the experiment with a different pH should be repeated an the previous results discarded.
Get this to the temperature required by placing the test tube in a water bath with the correct temperature water in it. Also place another test tube with 1ml of rennet in to the same water bath.
When it is up or down to the right temperature pour the rennet into the milk test tube. And start the stop watch.
Time how long it take for the milk to set enough so it is possible to turn it upside down without it spilling (obviously put a cork on the end of the test tube when testing)
Repeat this 3 times for each temperature

I think this is a much more accurate way of doing it as if more than one temperature had set within 4 minuets then I would not have known what the optimum temperature was.

Conclusion

I conclude from my results that temperature affects the rate of reaction and so the time taken for the milk to be converted to junket. As I said in my prediction this is to do with the enzymes the higher the temperature the faster they work this is because there are more collisions and more energy involved.

When it got to a certain temperature (between 60˚C and 70˚C) the enzymes started to stop working as well. This is because enzymes do not actually take part in the reaction all they do is provide the site fore it to happen on and the high temperature damages this.

Enzymes are just natural catalysts. They have a very precise three-dimensional shape. This includes a ‘dent’, which is called the active site. It is exactly the right size and shape for the enzyme’s substrate (caseinogen in this case). When a substrate molecule slots into the active site, the enzyme ‘tweaks’ the substrate molecule, pulling it out of shape and making it split into product molecules (casein).
When high temperatures occur the enzyme is damaged because the molecules are moving faster, these bombard the active site of the enzyme changing its shape; when it’s shape has been changed then the substrate will no longer fit in the active site and therefore cannot be broken down in to the product or products. In this investigation it was found that rennet operated fastest at 50 to 60˚C and was damaged above this temperature. By 80OC the rennet was completely denatured. From this information I can conclude that the optimum operating temperature for the rennet is 50˚C.

Things to improve on

I think this experiment did not go to plan as many things went wrong so if I did it again I would sort these problems out for a start.

Was given the wrong milk for first experiment

Next time I would check before doing it that all the equipment is there and the right stuff.

Used to much rennet due to the mistake in the first experiment

This just goes to show how important doing a preliminary experiment is. So obviously next time I shall take a bit more time and care into doing it better.

The water baths did not keep their temp for as long as they needed to in some cases and had to be cooled with ice or warmed with a Bunsen.

Well in a perfect world with all the time and money then I would have electronic water baths with very sensitive thermostats.

Didn’t really have enough time to complete the experiments as some went on for over an hour.

Again in a perfect world then I could spend more time and then I could repeat the experiment more and get a more accurate average that would get rid of any weird results that may occur.