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Gelatine, more commonly known as Jelly, is a substance that consists mainly of collagen, a protein found in animal tendons and skin. The gelatine used for cooking purposes is usually in the form of granules. These granules swell when they are re-hydrated in water, but only fully dissolve in hot water. As this solution cools it sets to a moisture holding gel. This gel forms due to the proteins in gelatine joining to form a web like structure. In Module 1 A-level Biology, we learn about the structure of a protein molecule.
A protein molecule is formed when amino acids join together by condensation, forming a peptide bond and water as a bi-product. A chain of many amino acids is known as a polypeptide and a protein can consist of one or more of these. The opposite of condensation is hydrolysis. When hydrolysis occurs a peptide bond is broken and water is used up in the reaction. There are specific enzymes called proteases (Module 2), which can be found in fruits such as pineapple, that speed up the hydrolysis reaction that breaks down protein molecules.
From research I have found that it is a protease called bromelain found in pineapple, which in the scenario is preventing the gelatine from setting by breaking up the proteins forming the web like structure. It is also in Module 1 that we learn how enzymes perform such tasks, and the conditions that best suit them. Enzymes are proteins which act as catalysts. They have a tertiary structure that provides them with an active site; a groove in the enzyme surface that combines precisely with a substrate of a specific shape and charge.
When Is A Pineapple Ripe
The lock and key hypothesis states that the substrate binds to the active site to form an enzyme substrate complex. The substrate is then altered to form the product of the reaction and is released from the active site. The induced fit hypothesis is a more recent theory which suggests that the active site actually changes shape to mould itself to the substrate. The tertiary structure of enzymes also causes them to be sensitive to temperature and pH, and an enzyme will denature in extremes of these conditions.
When an enzyme denatures it is no longer functional because the active site has changed shape and consequently the substrate molecule will not be able to combine with the enzyme. Increasing the temperature gives molecules more kinetic energy, so they collide more frequently and the rate increases. This is also true for enzymes up to a certain point: the optimum temperature. Above this temperature, enzymes vibrate so much that their structure is damaged and the active site altered. A change in pH disrupts the charges; consequently the active site cannot bind to the substrate. Plants produce fruit to acts as a delivery system for seeds.
Fruit consist of carbohydrates that make them taste sweet (Module 1), providing attractive food for animals, which will help aid the dispersion of the seeds. Ageing of fruit is known as ripening, and this process is designed to stop animals from eating the fruit before the seeds are fully developed. When under-ripe, pineapples would not be appealing to animals because they are green in colour, tough to eat and acidic. There are enzymes responsible for the ripening of fruit which break down the starch content to produce more sweet sugars and make the fruit softer, making it more edible.
Therefore, altering the conditions which effect enzyme rate of reaction, will effect how quickly a pineapple will ripen. Other enzyme activity increases in the fruit during ripening, due to certain hormones (such as ethylene). Applying this rule to pineapple: the bromelain enzyme activity will increase as the pineapple ripens. If I were to put a pineapple in cold conditions, this would slow down the ripening process because the enzymes responsible would have less kinetic energy, and I am therefore indirectly reducing the activity of bromelain.
In this investigation scenario, when under-ripe pineapple was used in jelly, it set better than when ripe pineapple was used. Taking in to consideration the information I have found out above, I propose that this could have occurred due to a protease enzyme that breaks down the protein in the jelly, which is more active in the ripe pineapple than the under ripe pineapple. I will now plan a full investigation to prove my proposal by testing pineapples at different stages of ripeness.
I will place one pineapple (A) in a freezer for two weeks to stop the ripening process. Another pineapple (B) will be placed in a freezer for one week and kept at room temperature for the second week. The third pineapple (C) will be kept at room temperature for two weeks. I will make sure that any pineapples kept at room temperature will not be placed near a window or radiator where the temperature may fluctuate. The pineapples in the freezer will be kept on the same shelf as each other. I will take the pineapples out of the freezer and place the in the fridge 24 hours before the experiment, to make sure they are all the same temperature at the start.
- I will prepare the jelly, according to instructions on the packet. I will then pour it in to four petri dishes and put it in the fridge to set. Before pouring the jelly in to the dish, I will measure 3/4 of the way up of the dish and make a mark. This mark is where I will pour the jelly up to to make sure that there is the same amount of jelly, which reaches the same height in each dish.
- To prepare the pineapple, I will remove the top and bottom, stand vertically and remove the skin, cut in to quarters and remove the core. I will not use the core because this is not usually eaten so does not apply to the scenario where the pineapple was being used in food. I will be as consistent as possible with each pineapple to make sure that I am using the same type of tissue. I will then place the quarters in the blender for ten seconds and place in a labelled beaker (labelled A, B or C). Blending the pineapple will break walls of the pineapple tissue, meaning that the enzymes will be more exposed and take effect more quickly than if the pineapple had not been blended.
- (To take place as soon as possible after step 2) I will take the petri dishes out of the fridge and with a borer make three holes, as far away from each other as possible in the jelly of each of the dishes. I will label the dishes A, B, and C, and measure the diameter of each of the holes made by the borer with a ruler. I will number each of the holes by writing the number on the lid and placing it underneath the dish with numbers in the same place as the corresponding hole.
- I will use a pipette to place the pineapple pulp in to the holes. Each type of pineapple will be in its own, labelled petri dish. One of the petri dishes will not have any pineapple put in it, and will be used as a control.
- I will then put the petri dishes back in the fridge and leave them there for seven days. This is enough time for the enzyme, of even the ripest pineapple to take effect. Any longer than this, and the liquefied gel from each hole may join up, making it difficult to take measurements. Putting the dishes back in the fridge will prevent any bacteria or foreign bodies attaching to the jelly which could effect the results. I will start the timer the minute that I have placed the pineapple into the holes, and have a different timer for each petri dish so I know they have had exactly the same amount of time in the fridge.
- After every 24 hours, I will remove them from the fridge again. From research I have done, I have found that enzymes in the pineapple will turn the gelatine from a gel to a liquid. I will measure the diameter of the area that is liquid and record my results in a table as below.