Food preservation

UNDERSTANDING YOUR FOOD<br /> the ultimate educational resource<br /> TEACHER NOTES<br /> <br /> teachers’ notes P2.1<br /> P2. Food preservation<br /> Pupils store frozen peas with a variety of preservatives in order to<br /> investigate their effects.<br /> SAFTEY NOTES:<br /> SOME ASTHMATICS ARE VERY SUSCEPTIBLE TO SULPHUR<br /> DIOXIDE VAPOURS; THE SODIUM META BISULPHITE SOLUTION<br /> MUST BE PREPARED BY A TEACHER OR TECHNICIAN.<br /> SODIUM NITRITE SOLUTION IS TOXIC; WEAR GLOVES WHEN<br /> HANDLING THE CONTAINER AND TRANSFERRING THE LIQUID.<br /> ALL THE SOLUTIONS MUST BE PREPARED BY A TEACHER OR<br /> TECHNICIAN.<br /> THE TUBES MUST NOT BE OPENED BY THE PUPILS.<br /> THE TUBES SHOULD BE DISPOSED OF BY AUTOCLAVING.<br /> A table for results is provided (third page of P2) for those pupils unable to<br /> design one easily.<br /> KS3<br /> science and food technology<br /> Timing - 15 minutesto set up<br /> apparatus;<br /> tubes to be left at least 24 hours.<br /> Two Pupil activity P2 sheets<br /> (plus optional table sheet)<br /> accompany this activity.<br /> Requirements<br /> • test tubes or boiling tubes<br /> • marker pens or labels<br /> • distilled water<br /> • salt solution - 20 g of sodium<br /> chloride in 100 cm<br /> 3of distilled<br /> water<br /> • sugar solution - 10 g sucrose in<br /> 100 cm<br /> 3of distilled water<br /> • vinegar<br /> • sodium nitrite solution - 0.1M<br /> maximum concentration<br /> • sodium metabisulphite - to<br /> give a dilute solution of<br /> sulphur dioxide use 2 g in 100<br /> cm<br /> 3of water<br /> • goggles<br /> • cotton wool<br /> • frozen peas<br /> • forceps<br /> • access to a refrigerator (not one<br /> where food for consumption is<br /> stored)<br /> Peas are used here merely as aneasily available medium. Alternatively,<br /> you could try cooked rice, cooked and pulped apple or cooked and pulped<br /> tomato. None of these foods are ever preserved by the methods<br /> investigated in this practical; in commercial food processing they are<br /> either dried, canned or frozen.<br /> After 24 hours the liquid in tubes C and E should already have gone<br /> cloudy (check this). It is possible to stop the experiment at this point.<br /> Leaving the tubes for 72 hours merely increases the cloudiness of these<br /> two tubes. Leaving the tubes for a week will show the growth of mould.<br /> You may like to prepare such a tube a week before it is needed.<br /> Tube G tends to look green as the colour is removed from the peas.<br /> Answers to questions on Pupil sheet P2:<br /> 1. Tube 2 should show more signs of spoilage than tube A. Tube A<br /> was at a lower temperature and thus microbial growth is slowed<br /> down.<br /> 2. Salt solution, vinegar, sodium nitrate solution and sulphur dioxide<br /> solution act as preservatives. The liquid in these tubes remain clear.<br /> After 24 hours the liquids in tubes C and E go cloudy; this shows the<br /> presence of fungal (mould) growth.<br /> Note: peas in the acidic vinegar and dilute sulphur dioxide solution<br /> have discoloured. In alkali the colours would have been much<br /> brighter (see C1It looks good enough to eat!, page 55). As<br /> preservation methods may well alter thenatural colour of foods, certain<br /> foods have colours added to them after processing.<br /> <br /> teachers’ notes P2.2<br /> 3. Strong salt solution removes, by osmosis, the water necessary for<br /> microbial growth.<br /> Vinegar is acidic; microbial growth is retarded at pH below 4.5; at<br /> certain concentrations acetic acid is bactericidal.<br /> Sodium nitrite and sulphur dioxide both kill microbes. Sodium nitrite is<br /> particularly important for inhibiting the growth ofClostridium botulinum<br /> (the bacterium responsible for botulism) in meat products. Its level is<br /> strictly controlled in food products.<br /> 4. Salt imparts a particularly strong flavour which would be undesirable<br /> for many products and tastes.<br /> 5. The most obvious are fruits and vegetables in jams, marmalades and<br /> pickles.<br /> 6. Sodium nitrite (E250) is used mainly for preserving meat products.<br /> 7. Sulphur dioxide (E220) is used to preserve soft fruits and alcoholic<br /> beverages.<br /> 8. Drying removes the water necessary for microbial growth.<br /> Freezing places microbes at a low enough temperature to slow down<br /> and sometimes completely stop microbial growth. It is important to<br /> remember that low temperatures do not kill microbes and that the<br /> normal processes of decay will begin as soon as the food is warmed<br /> up.<br /> In canning, food is heated to a temperature at which microbes are killed.<br /> The cans are then sealed to prevent the entry of new microbes.<br /> Further investigations<br /> Some of the following suggestions may be suitable for the assessment of<br /> Sc1 skills. The last two may be homeworks or projects.<br /> a.High salt concentrations are effective preservatives. However, pupils<br /> will realisethat this may produce unpalatable results and the lowest<br /> concentration which still gives some preserving power may be<br /> desirable. A range of concentrations between 0 and 20% could be<br /> investigated. If pupils have some scientific knowledge of osmosis, then<br /> this could be an investigation suitable for a complete Sc1 investigation.<br /> b. Sugar, when used in high enough concentrations, can act as a<br /> preservative because it binds (i.e. inactivates) the water necessary<br /> for microbial growth. This fact is used in jams, etc. Pupils could<br /> design a similar experiment to the original which investigates<br /> concentrations of sugar above 10%.<br /> c. Artificial sweeteners do not have osmotic activity and hence are not<br /> capable of copying the preserving properties of sugar. A simple<br /> investigation similar to the original one with peas may show this. You<br /> may like to use visking tubing to demonstrate that osmotic activity is<br /> indeed absent. The labels from diabetic<br /> (low sugar) jams may be used to help point out that these products do<br /> not have keeping qualities as good as traditional jams.<br /> <br /> teachers’ notes P2.3<br /> d. KS3 pupils should use sodium nitrite solution with a maximum<br /> concentration of 0.1M. Keeping this constant and adding salt to other<br /> tests will find out whether salt does enhance the performance. A range<br /> of salt concentrations could be investigated.<br /> e. Cook a few slices of raw apple for about 10 minutes in plenty of water.<br /> When cool rub through some muslin or a sieve or liquidise to produce a<br /> smooth pulp. This can then be used to see if the addition of cloves<br /> prevents the apple from going off.<br /> f. Most households will have food stocks which are preserved by a variety<br /> of methods. A lot of information can be gained as to the keeping<br /> qualities of these products.<br /> g. Louis Pasteur developed the process which now bears his name when<br /> he was approached by the wine producers of France. Many producers<br /> were finding that wine was turning into vinegar and hence the wine<br /> became an unacceptable product. The solution to the problem was to<br /> heat the wine to a temperature of around 60<br /> o<br /> C which killed the majority<br /> of the microbes responsible for the spoilage. Pasteur was responsible<br /> for saving the French wine industry.<br /> The pasteurisation process is applied to milk. The most common method<br /> is to heat milk to 72<br /> o<br /> C , for not less than 15 seconds. After treatment, the<br /> milk is rapidly cooled.<br /> The heat treatment kills most microbes but does not alter the flavour of<br /> the milk. Milk which is heated to a high enough temperature to sterilise it,<br /> results in a flavour that is not to the liking of the majority of consumers.<br /> <br /> pupil activity P2 FOOD PRESERVATION<br /> Wehave a constant demand for safe and nutritious food. However, most production<br /> of raw food, such as fruit and vegetables, is seasonal. There are times of the year<br /> when such food is plentiful and other times when certain foods are scarce.<br /> Food may have to be transported over large distances from the site of productionto<br /> the places where it is to be eaten. Imported food may travel thousands of milesover<br /> several days before it reaches our tables.<br /> Wehave been developing methods of food preservation for thousands of years.<br /> Preserving meat using thesmokefrom wood fires was probably one of the earliest<br /> methods as wasdryingfood in the sun.Irradiationis often thought of as the<br /> preservation method of the future.<br /> Preservativesare food additives which are numbered from E200 to E283 (you may<br /> find some antioxidants here as well). They are chemicals which destroy or slow down<br /> the growth of bacteria and fungi.<br /> There are many traditional ways of preserving food. You will probably be familiar with<br /> most of these.<br /> In this investigation you will be looking at some ways of preserving food.<br /> SAFETY<br /> NOTE<br /> SODIUM NITRITE SOLUTION IS POISONOUS; USE WITH EXTREME CARE<br /> AND WEAR GLOVES.<br /> SULPHUR DIOXIDE SOLUTION IS HARMFUL; USE WITH EXTREME CARE.<br /> ASTHMATICS MAY BE SENSITIVE TO SULPHUR DIOXIDE.<br /> WEAR GOGGLES WHEN USING THE SOLUTIONS.<br /> DO NOT REMOVE THE COTTON WOOL BUNGS FROM THE TUBES.<br /> Method<br /> 1.Label eight test tubes A - H. Put your initials and the date on each tube.<br /> 2. Use forceps to put three peas (these were frozen peas) in each test tube.<br /> 3.Treat the tubes and the peas in the following ways:<br /> Tube A - add nothing to this tube<br /> Tube B - add nothing to this tube<br /> Tube C - half fill with distilledwater<br /> Tube D - half fill with a strongsalt solution<br /> Tube E - half fill with a strongsugar solution<br /> Tube F - half fill withvinegar(this is acetic acid (E260))<br /> Tube G - half fill with sodium nitrite solution(E250)<br /> Tube H - half fill with a dilutesulphur dioxide solution(E220)<br /> 4.Put a cotton wool bung in each tube.<br /> 5.Put tube A in the refrigerator.<br /> 6. Leave tubes B - H in a warm place until next lesson.<br /> <br /> FOOD PRESERVATION pupil activity P2<br /> Results<br /> 1. Construct a results table into which you will be able to put your observations fromeach<br /> tube. You will be looking at what has happened to both the peas and the liquid. You<br /> will need to decide whether the peas look different to normal. A cloudy liquid shows<br /> that mould is beginning to grow.<br /> 2.Look at the peas and the liquid in each tube after 24 hours and 72 hours. You<br /> may also like to leave the tubes for a week and then examine them again.<br /> Questions<br /> 1.Compare tubes A and B. What differences are there? How can you explainthese<br /> differences?<br /> 2.Which of the solutions used in tubes C - H act as preservatives? How were you able<br /> to tell this?<br /> 3.Why are these solutions able to act as preservatives? You will have to do some<br /> research to find this out. Your teacher may discuss this with you.<br /> 4.A very strong salt solution may act as a very good preservative. Why might it be an<br /> unsuitable preservative for some foods?<br /> 5.What sort of foods are preserved using a strong sugar solution?<br /> 6.What sort of foods are preserved using sodium nitrite?<br /> 7.What sort of foods are preserved using sulphur dioxide?<br /> 8. Other methods of preservation include drying, freezing and canning. Explain how<br /> these processes prevent food from going bad.<br /> Suggestions for further investigations<br /> a.Design an experiment to find out the lowest concentration of salt solutionwhich will<br /> stop a particular food from going bad in a particular length of time.<br /> b.Design an experiment to find out at what concentration sugar will act as a<br /> preservative.<br /> c. Sugar is being replaced in some food situations by artificial sweeteners. Do these<br /> sweeteners have similar preserving properties to sugar? Design an experiment to<br /> find out.<br /> d.It has been suggested that the antimicrobial action of sodium nitrite can be<br /> enhanced (made even better) by the presence of salt. Design an experiment to<br /> investigate this.<br /> e.Design an experiment to find out whether cloves added to cooked apple pulp<br /> have a preservative effect or not.<br /> f.Make a survey of the sorts of foods you and your friends have at home. What<br /> methods of food preservation are used? For how long will the different foods using<br /> the different methods of preservation stay fresh?<br /> g.One of the most important methods of food preservation is pasteurisation. Find out<br /> the history behind this method and the details involved in its modern day usage.<br /> <br /> pupil activity P2 FOOD PRESERVATION<br /> Results - after 24 hours<br /> Tube What do the peas What does the liquid<br /> look like? look like?<br /> A<br /> in refrigerator<br /> B<br /> room temperature<br /> C<br /> distilledwater<br /> D<br /> salt solution<br /> E<br /> sugar solution<br /> F<br /> vinegar<br /> G<br /> sodiumnitrite<br /> solution<br /> H<br /> sulphur dioxide<br /> solution