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

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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:

oThe jelly set normally when tinned pineapple was added.

oWhen fresh, unripe pineapple was used the jelly set but was not as firm.

oWhen 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. The temperature and concentration will also affect the size of the wells as these are factors which affect an enzyme controlled reaction.

The investigation and prediction.

I am going to investigate the effect of protease concentration on the size of the wells, as it is related to the original problem. It seems apparent that the ripeness of the pineapple is related to the concentration of enzymes within it. Perhaps this increased protease concentration is needed for seed dispersal, as pineapple is a fruit and as it ripens the concentration of protease increases. I predict the greater the protease concentration which is used then the bigger the wells in the jelly will become, this is because the enzymes are breaking down the gelatin. I also predict that there should be some degree of proportionality related to the amount of enzyme used and the area of the hole over a range.

Why enzymes cause this to happen.

Enzymes are complex protein molecules which perform actions on other molecules whilst they themselves are not used up in the reaction. Enzymes are mainly affected by temperature and pH, and the rate of the reaction is affected by the concentrations of the enzymes. Enzymes can only break or make a molecule if they come into physical contact with the substrate, this is the reason why temperature and concentration affect rate. Temperature affects rate because the increased heat gives the enzymes and substrates energy and causes them to move around more rapidly, the faster they are moving means they have more chance of colliding with each other and so if the temperature is greater then the reaction will happen faster however if the temperature is too high then the enzymes will become denatured. An increase in concentration also causes the reaction to occur faster, this is for the same reason as temperature. Because there are more enzyme molecules then there is more chance of the substrate colliding with them, this will cause a very rapid reaction at the start and then it will gradually slow, as the substrate is being used up faster, so creating double the concentration every time the reaction occurs. Enzymes work on a lock and key mechanism where only a molecule of a certain shape will fit the enzyme, it is this specificity which makes the enzymes so effective. If the concentration or temperature are increased then there will be more substrate molecules fitting into the enzyme per second than there would otherwise be. The part that the substrate fits into is called the active site, this is the point where the enzyme has a specific shape which fits the substrate.

The reason the enzyme has this shape is because of the way it is folded, an enzyme is a long protein molecule, the way in which it is folded determines its shape and so the shape of the active site. The gelatin is fitting into the active...
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