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Why doesn't jelly set when fresh pineapple is used?

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Why doesn't jelly set when fresh pineapple is used? Basic Problem: Jelly is made from protein, gelatin, this is dissolved in hot water and as it cools, it sets. For a childrens party it was decided to make some fruit jellies, these were made by putting sliced pineapple into the jelly before it set, the following observations were made: o The jelly set normally when tinned pineapple was added. o When fresh, unripe pineapple was used the jelly set but was not as firm. o When fresh, ripe pineapple was used, the jelly did not set at all But why? The reason the jelly is not setting is due to protease, which is breaking down the protein in the jelly, protease is an enzyme, and its rate of activity is affected by many things, such as temperature and concentration. Theoretical investigation. If the pineapple juice contains an enzyme which is stopping the jelly setting then this can be investigated by using jelly in Petri dishes with holes or wells cut into it. If some pineapple juice is added to these wells then they should get bigger as the enzyme in the pineapple juice will be breaking down the gelatin around the outside of the well, causing it to expand. ...read more.


Because it is the enzymes which are causing the gelatin to stop from setting, it is reasonable to believe that because the tinned pineapple does not stop the gelatin from setting then the enzymes must not be working properly. This is because they have become denatured, this is when the tertiary structure that gives the enzyme its specific shape is broken up, causing the enzyme to become the wrong shape, and therefore useless. The tertiary structure is broken up usually under one of two circumstances. Firstly the enzymes could be in a solution which is too acidic, ( the pH is wrong ) this acidity interferes with the hydrogen bonds in the enzymes tertiary structure, and blocks them, causing them to break up. Another way of the enzymes becoming denatured is if the temperature is too high, when this happens the enzymes are given too much energy, and with all molecules they vibrate and move faster. If the temperature is too hot, and the enzymes are given too much energy then the bonds holding the enzyme together are overpowered by the amount of energy. During the canning process one of these two things must happen, and that is the reason why the canned pineapple has no effect. ...read more.


Rejected Ideas: Among these were immersing the finished dish in water and then comparing the difference with that of a dish with just the hole cut out. However to do this the vessel in which the dish would be placed would have to be so big that the difference in the level of the water would be almost immeasurable. Next was the idea of measuring the mass of the dish after it had been washed out, and comparing it with that of the disk with just the hole in. this is a fairly good idea, however it is very difficult to determine whether all of the solution has been removed, whether any of the water used to wash it out has been left in, or whether there may be slight differences in the weight of the Petri dishes. Control: A control was set up to make sure that no other factors were present in causing the reaction other than the enzymes, the control just consists of a disk with the hole cut out, and water in place of a solution. Tinned pineapple: This was set up the same as the other experiments. It is the only real juice used, as it can not be reproduced as a solution. ...read more.

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