Food Tests - GCSE Science Required Practical

hi this is dr. Sheffield from Mars beauty school science and today we're going to be having a look at the required practical that is the food tests it's important to say at this point but these food tests while they are require practical and could be examined on that basis often crop up in other exam questions for example the iodine test for starch often comes up in questions about photosynthesis so please bear that in mind these questions crop up at different points in the exam the first thing we have to do is prepare some samples of

food for testing and then we're going to use the food tests to identify which different chemicals are found in these different food types we're going to test some apple some biscuit and some cheese the way I would do this if you're going to do these food samples I would do them in this order oh dude biscuit and then apple and then cheese essentially they are an order of messiness and you have to clean up between each one so I think it's the it's the best way to go and you leave the cheese for last because

it causes a say the most mess so start with a biscuit the first thing we do is take ourselves a small sample of biscuit we then grind that biscuit sample up in a pestle and mortar just going to grind up two nice small pieces I'm going to show you the biscuit but you need to prepare all three food samples in the same way so grind up nice and small we're then going to transfer the biscuit into a small 100 ml beaker [Music] and then add a small amount to Stillwater doesn't really matter how much we

add just a reasonable volume I'm not too big not too small sort of that amount it then needs a really good stir and what we're trying to do here is dissolve the chemicals in the food into the booster so particularly what we're testing for today are sugars proteins and fats which I don't get those chemicals out of the food and into the water very good stir well that's dissolving we have to prepare a conical flask a filter funnel and some tools paper we're going to filter the sample the reason we do this is that food

tests are subjective you are looking for a color change that you have to decide if that kind of change has occurred or not the food itself can be colored or can produce cloudy suspensions that could make it much harder to see the color change you are looking for in the food test so the idea of filtering here it removes much of the solid food as we can to help us more clearly distinguish the kind of changes that we are going to see filter the food see the conical flask eye fields funnel does paper one last

step and in it goes so in goes this is our biscuit and water and distilled water and it's going to filter away that takes some time so I'm gonna put that to one side so I do have some samples that I've made earlier we're now going to take you through how to do the for food tests using a pre-prepared sample but that is how you prepare all the samples just the same way filtering the food out and collecting the fluid afterwards I should also mention that once you've ground your biscuit up the pestle and mortar

needs to be cleaned and you then use fresh glassware and a clean pestle and mortar to grind up and test your next foods on so you must make sure this is clean between its purpose so take a food sample here so I've got some filtered apple solution here so this is an apple suspension so I've cut up some apple I remove the skin because the skin is it gonna be particularly useful in this test it's just going to block up the filter funnel so I've cut off sex and Apple up remove the skin I've ground

up in the pestle and mortar add some distilled water and now filter this we've got a suspension here of apple solution we're going to do the for food tests so we need four test tubes and I'm gonna add a small amount of apple solution to each tube don't particulate we add roughly somewhere about two mil be good so I now have four tubes each with roughly two mid to milliliters or two centers cubed of apples solution in it so it doesn't particular that's exactly how much you use just need to make sure you've got an

apple solution to go into each of the four tubes the first two tests we're gonna do are both for carbohydrates these are sugars the first test is for very short carbohydrates such as glucose and this is called the Benedict's test so we take our first two Blackwell solution and we're add this blue solution of Benedict's reagent if you asked about the methods for this technique in an exam you add Benedict's solution and it starts as a blue color you then have to place the tube of food and Benedict's solution in a hot water bath this

is a recently bored kettle again if you asked about the method in an exam you must say the sample and these heating so you add Bennett reagent to the food sample and then heat can leave that now until I what kind of change we'll get later the second test for a carbohydrate is a test for starch this is a large polymer of glucose that's found in plants the test this is an iodine test this is one that can come up particularly in question about photosynthesis the products photosynthesis being glucose plants then store excess glucose as

starch and so a presence of starch is a sign that photosynthesis has occurred but in this case we're looking for starch in this food sample nice easy test all we have to do is add some iodine a squirt of iodine now the iodine is initially an orange color and we'll see what color change we get at the end the third test we need to do is a test for lipids or fats so the test for that involves using ethanol I should say at this point the a canary method asks you to grind up some food

for this test add some distilled water and not to filter it and just test the food sample with water together that works that's fine you can do that it might be easier you've already made the filtered sample suggest use a filtered sample if your food here that's what I'm going to do and the test works perfectly fine so it just saves a little bit of work if you want to do it that way so we're going to add some still water a sample the salt already has to still water on it so that's not a

problem so we add distilled water to our sample and we're now going to add some ethanol just a little bit of meth not a few drops is plenty so if the ethanol test coming out of the distilled water and the ethanol you know into shake vigorously so I'm gonna place a bun in the top of the tube to vet any accidents give it a really good shake and again we'll come back and see what results we got for the food samples in a moment the forth food test is a test for proteins to do this

test again we have our filtered food sample this time we're going to add some clued buyer X reagent this is the test for protein it's a light blue color so it's a blue color to start with so we add some to our futon pool and we can just leave it again to look at results later so nice again nice and easy if you add by rekhs reagent it starts as light blue and we'll see the color change that's how we do our four tests you need to know the methods that the tendons we've done there

the chemicals you've added the start color and anything else like adding heat that you've done in the method equally important note is to know the color changes you should see for a positive test for those different types of food so positive test for glucose for a small carbohydrate for starch a large carbohydrate for lipids or fats and for protein I've got my few times I made of it earlier and I did the all these tests earlier I want to show you the results now you can see some of the color changes you should see these

are all for Apple samples so we can see here this is the Benedict's test and it's gone from a blue color to orange Benedict's test used is quite an interesting array of color changes bending it starts blue if glucose is present you see a sequence of color changes the further along the color change you get the more sugar you actually have in your sample so you can describe this as a semi-quantitative test because it gives you an indication through color changes of actually roughly how much sugar you have a lot or a little the other

tests simply tell you if the chemicals are present or not it goes from blue to green to yellow to orange and eventually to brick red all of those colors are a positive indication of a sugar like glucose any of those color changes are an acceptable answer for an exam but brick red would show you the most sugar present is kind of the end result we've got a nice dark orange color here suggesting there was quite a lot of sugar a lot of glucose in our apple but then got the iodine test so that is Meyer

Dean to see some starch and we actually got no real changes there so we haven't got the positive result there so no indication yet of the positive result for starch got the fats or lipids test we've got the solution here is clear and colorless still so again there's no indication of fats s so we don't we don't know what positive result looks like lipids yet so that's a negative result and finally the protein result again there's no obvious color change here so it's a negative result so here's the sample of biscuit I prepared a little

while ago and a lovely Benedict's result here it's got a nice orange color Virgen towards dark brick red and that's a positive also there is sugar glucose in biscuit not too much surprise there the I didn't test me now let's see the positive is off here iodine initially started as orange and we now have a positive result of a black color or a blue-black color that is an indication we have lots of star chips so there's a positives of starch the ethanol tests for lipids or fats we have a cloudy suspension now it's gone milky

colored cloudy white is the way to describe this color it's cloudy white not just cloudy that is a positive result for fats or lipids so again unsurprising biscuits there is looser fat content there and got the by rats test here not much of a change so we haven't got a positive result for proteins their final sample was my cheese sample for food testing Benedict's test has remained blue so nothing really had no real blue case in this sample and sugar that's not a surprise she's not particularly sugary the iodine test has actually got a positive

result this is very strange in an animal product which is cheap animals do not store glucose as starch they store glucose as another chemical another polymer called glycogen however we use grated cheese if you use grated cheese you may will see a positive result for iodine because they keep the bits of grated cheese separate the manufacturers add a food additive which is starch powder so we're actually seeing an additive here that's given a positive zone so again we've got the blue-black color a positive test for starch and that is unexpected if you don't use grated

cheese you shouldn't see that we now have the ethanol test test for lipids again gone slightly cloudy cloudy white so positive test for fats or lipids again in cheese that's nuts not a huge surprise she's quite a fatty food so this cloudy layer here is a positive result for fats and finally we do actually have a nice positive result for protein so cheese again very high in protein content and we've got here the bio 3 agent started as a blue color a positive result for protein with bioactive agent it goes from blue to pink or

purple so it's a lovely positive result there so we've shown here how to do all the for food test they're showing you all four potential positive results and identified different chemicals in our different types of food finally if you are a teacher and you've enjoyed this video but you are teaching outside your specialism and you feel some of these require practicals you're lacking a limited confidence in them then we are part of the Avon teaching school alliance we've run a variety of courses that help you brush up on skills outside of specialism building your confidence

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