Oxygen: friend or foe?

UNDERSTANDING YOUR FOOD<br /> the ultimate educational resource<br /> TEACHER NOTES<br /> <br /> teachers’ notes AO1.1<br /> AO1. Oxygen - friend or foe?<br /> Why has my apple gone brown?<br /> Pupils carry out investigations into conditions for apple browning.<br /> SAFETY NOTE:SOME ASTHMATICS ARE VERY SUSCEPTIBLE TO<br /> SULPHUR DIOXIDE VAPOURS ; THE SODIUM METABISULPHITE<br /> SOLUTION MUST BE PREPARED BY A TEACHER OR TECHNICIAN.<br /> The results given are for Bramley cooking apples. The cut surface of<br /> a Bramley showed noticeable browning after 10 minutes.<br /> The tasks on sheet AO1 show:<br /> 2. The cut surface browns more quickly than the broken surface. When<br /> an apple is broken it tends to break between the cells rather than<br /> through the cells as is caused by cutting. Breaking the apple causes<br /> less cell damage and less discolouration.<br /> 3. The pulped apple browns much quicker than the whole apple piece.<br /> Pulping produces extensive cell damage and allows for rapid diffusion<br /> of oxygen into the tissue. Intact cells have reducing characteristics<br /> which mean that oxygen is reduced in a controlled way in respiration<br /> rather than taking part in browning reactions.<br /> 4. The piece in the air should brown fastest; the one in water should be<br /> next; the piece in ascorbic acid takes longer to brown and the sulphur<br /> dioxide piece takes much longer to brown. The one in air has greater<br /> contact with oxygen than the one in water. Ascorbic acid is a reducing<br /> agent and hence will slow down the rate ofoxidation. Sulphur dioxide<br /> is a common preservative; in this case it is acting as anenzyme<br /> inhibitor.<br /> 5. The piece in boiling water should take a long time, if at all, to brown.<br /> If this was a purely chemical reaction, the rate of browning should<br /> increase with temperature. The result suggests an enzyme<br /> controlled reaction. This is enhanced by the fact that the piece at 30<br /> oC should brown more quickly than others at room temperature.<br /> 6. Bruising should have a similar effect to pulping since bruising also<br /> causes extensive cell damage. Pupils may wonder how the reaction<br /> occurs without the air/oxygen seemingly being in contact with the<br /> tissues. It is important for them to realise that very little oxygen is<br /> needed to initiate oxidation. Dissolved oxygen will be present in the<br /> tissues and this is taking part in the reaction.<br /> KS4<br /> science and food technology<br /> Timing - 40 - 60 minutes<br /> Pupil activity sheet AO1<br /> accompanies this activity.<br /> Requirements<br /> • apples (see below)<br /> • knives or scalpels<br /> • white tiles to cut on<br /> • a blender or pestle and mortars<br /> • 5%ascorbic acid (Vitamin C) sol.<br /> • sodium metabisulphite - to<br /> give a dilute solution of<br /> sulphur dioxide use 2 g in 100<br /> cm<br /> 3of water<br /> • beaker of boiling water<br /> • beaker of water at 30<br /> oC<br /> Apple selection- you need to<br /> choose a variety which browns in the<br /> shortest time possible; cooking<br /> apples and continental apples brown<br /> more quickly than English eating<br /> apples; it is recommended that you<br /> try a particular apple first since even<br /> different batches of the same apple<br /> type will show variations.<br /> Cooks and chefs add lemon<br /> juice to prevent browning.<br /> For teacher information:the<br /> browning reaction is really a series of<br /> reactions which result in the oxidation<br /> of compounds which contain the<br /> structure of<br /> 1,2-dihydroxybenzene. The first<br /> reaction in the sequence is catalysed<br /> by the enzyme polyphenoloxidase<br /> and this is the reaction that is<br /> important in the pupils’ investigations.<br /> It is an oxidation reaction which<br /> produces a quinone.<br /> OH<br /> OH<br /> Two important points come from these tasks. The browning reaction is:<br /> • enzyme controlled;<br /> • an oxidation reaction.<br /> Hence, the reaction may be prevented or slowed down by using methods<br /> which restrict enzyme action or methods which employ an antioxidant or<br /> reducing agent.<br /> O +<br /> 1<br /> /<br /> 2 O2 + H2O<br /> a quinone<br /> Further investigations<br /> The suggestions for further investigation may be suitable for assessment<br /> of Sc1. Since pulped apple browns more quickly and evenly than whole<br /> pieces, it is advisable to use pulp in further investigations.<br /> a. There are many other naturalantioxidantsthat could be<br /> investigated.<br /> There are many potential substrates<br /> for the reaction, but they are usually<br /> referred to as the ‘tannins’. The final<br /> reactions are polymerisation reactions<br /> which<br /> produce the highly coloured pigments<br /> that can be detected as browning.<br /> O<br /> O<br /> <br /> teachers’ notes AO1.2<br /> You could try a number of acids - malic acid (E296), this is the principalacid<br /> in apples; citric acid (E330), the principal acid in citrus fruits; acetic acid<br /> (E260) which is vinegar.<br /> b. A set ofbufferscould be used to produce a variety of pHs. Alternatively,<br /> mixtures of 0.4M disodium hydrogen orthophosphate and 0.2M citric acid<br /> solutions can be used.<br /> 0.4M disodium<br /> 3<br /> 3<br /> 0.2M citric acid/cm<br /> pH (approx.)<br /> hydrogenorthophosphate/cm<br /> 0.4<br /> 4.1<br /> 7.7<br /> 10.3<br /> 12.6<br /> 16.5<br /> 19.4<br /> 19.6<br /> 15.9<br /> 12.3<br /> 9.7<br /> 7.4<br /> 3.5<br /> 0.6<br /> 2.2<br /> 3.0<br /> 4.0<br /> 5.0<br /> 6.0<br /> 7.0<br /> 8.0<br /> c. Since this is an enzyme controlled reaction, a range of<br /> temperatures can be chosen to reflect knowledge of the effect of<br /> temperature on enzymes.<br /> d. A range of suitable concentrations could be from 0 to 10% acid.<br /> e. Different types of apples, i.e. different substrates, do brown at different<br /> rates. Dessert apples tend to brown more slowly than cooking apples.<br /> This is due to lower levels of tannin in dessert apples which have been<br /> selected for this reason over many years.<br /> In all of these investigations, one of the most difficult aspects is the<br /> ability to measure the extent of the brown colour accurately.<br /> <br /> pupil activity A01 OXYGEN - FRIEND OR FOE?<br /> All life needs oxygen. It is used in all plant and animal cells in the process of<br /> respiration. This process releases theenergycontained in the food we eat so thatwe<br /> can do things. Reactions between oxygen and another substance are called<br /> oxidationreactions.<br /> But is oxygen always useful? Foods can also be attacked by oxygen. Food additives<br /> calledantioxidantsare used to try and slow down the reaction between food and<br /> oxygen. Antioxidants are numbered from E220 - E330 (you may find that some of the<br /> preservatives are found within these numbers too).<br /> Foods which containfatsand oils, in small or large quantities, are particularly<br /> susceptible to oxidation. The softer fats are described as beingunsaturated. These<br /> fats may be less likely to cause harm to our health than the saturated fats but they are<br /> more likely to be attacked by oxygen. When fats become oxidised they produce<br /> unpleasant and sometimes dangerous substances; the fats becomerancid.<br /> It is hard to believe, but pure oxygen can be dangerous to our health. It seems that<br /> oxidation reactions inside our bodies could lead to increased risk from heart disease.<br /> Our bodies have natural antioxidants (vitamins C and E are examples) and consuming<br /> these, and other antioxidants, through our diets, could prove to be very important in<br /> combating heart disease.<br /> Other effects of oxidation are less dangerous but also undesirable. You are going to<br /> investigate a very common oxidation reaction.<br /> Why has my apple gone brown?<br /> You may have noticed when peeling and slicing fruits such as apples, that the fruit<br /> starts to change colour and goesbrown with time. This is not a problem when eating<br /> a raw apple as a snack but it is of great importance in the large scale production of<br /> many fruit products.<br /> Cooks and chefs, perhaps by chance, found a simple way of slowing down this<br /> browning reaction. Do you know what they did?<br /> Carry out the following investigation which will allow you to find out more about this<br /> browning reaction. There are a number of small tasks to do. You may like to divide<br /> the work up between a few groups and then pool results.<br /> If you are the group using boiling water or water at 30<br /> o<br /> C, get this readybeforeyou cut<br /> your apple.<br /> SAFETY - USE SAFETY GOGGLES WHEN HEATING<br /> BE CAREFUL USING KNIFE or SCALPEL BLADES<br /> ASTHMATICS MAY BE SENSITIVE TO SULPHUR DIOXIDE<br /> <br /> OXYGEN - FRIEND OR FOE? pupil activity A01<br /> Method<br /> 1.You will need 11 pieces of apple for the entire investigation. Each piece<br /> needs to be about an eighth of a typical eating apple.<br /> 2.Take 2 pieces of apple. Leave them both to brown. When they are brown cut<br /> one of them in half using a knife/scalpel. Break the other into two pieces using<br /> your hands. What happens to the freshly cut surface and the freshly broken<br /> surface?<br /> 3.Take 2 pieces of apple. Turn one of them into a pulp (using a blender or a pestle<br /> and mortar. Compare the browning of the pulp with the unpulped piece.<br /> 4.Take 4 pieces of apple. Put one in water. Put one in ascorbic acid solution. Put<br /> one in dilute sulphur dioxide solution. Remember that apples will float so you will<br /> need to find a way to keep them below the liquid surface. Leave the fourth piece<br /> in the air. Leave them all for a few minutes and then compare.<br /> 5.Take 2 pieces of apple. Drop one piece into a beaker of already boiling water.<br /> Leave in the water for 2 minutes. Transfer the piece to cold water for 1 minute.<br /> Remove it and then leave it in the air.<br /> Drop the second piece into a beaker of water at 30<br /> o<br /> C for 2 minutes.<br /> Transfer the piece to cold water for 1 minute. Remove it and then leave it in<br /> the air.<br /> Compare the two pieces after a few minutes.<br /> 6.Take 1 piece of apple. It must still have the peel on. Bruise this piece, peel side<br /> down, by hitting it against a hard surface. Leave for a few minutes and then cut it<br /> open to see what has happened.<br /> If different groups have done these tasks, you will have to get together to give each<br /> other the results. Using the results, write a few sentences about what each of the tasks<br /> tells you about the browning reaction in apples.<br /> Further investigations<br /> a.Investigate if other substances such as vanilla, other acids (instead of ascorbic<br /> acid), mustard, sesame seeds and rosemary can also slow down the browning<br /> reaction.<br /> b.What effect does pH have on this reaction?<br /> c.Investigate in greater detail the effect of temperature on this reaction.<br /> d.What is the lowest concentration of ascorbic acid that can be used to significantly<br /> slow down the browning reaction?<br /> e.Do different types of apples, e.g. Bramley, Golden Delicious, Cox, Granny Smith’s,<br /> brown at different rates?