This lesson is called Food Tests: Practical and is from the unit, the Human Digestive System.

Hi there, my name's Mrs. McCready and I'm here to guide you through today's lesson.

In our lesson today, we're going to look to see how we carry out tests for nutrients and some of the substances that they are broken down into during digestion.

We're gonna come across a number of keywords in today's lesson, which are shown on the screen now.

You can pause the video to note them down or note down their definitions, but I will introduce them to you as we come across them.

In our lesson today, we're going to first of all look to see how we test for starch.

Then we're gonna see how we test for sugars before considering the differences between qualitative and quantitative measurements.

So if you're ready, I'm ready.

Let's get started.

So before we have a look at the actual test for starch, let's just remind ourselves what starch is.

So starch is a large, complex carbohydrate.

It's a large food molecule that must be broken down into much smaller pieces by the body before it can be absorbed.

Now it's possible to test for the presence of starch in food.

And this food test uses the reagent iodine.

So the reagent is the thing that is going to change colour to tell us whether the food stuff is there or not.

Now, iodine in its liquid form is usually a brown/orange colour, but in the presence of starch, it turns blue/black.

Now that's a very specific colour description.

Blue/black, it's neither black nor blue, but somewhere in between, blue/black.

So do make sure that you use that term, especially if you are answering exam questions.

So let's just quickly check our understanding.

What colour does iodine turn in the presence of starch? Is it brown/orange, blue/black, or is there no changing colour? I'll give you five seconds to decide.

Okay, got your decision? Did you choose blue/black? I hope so.

Well done if you did.

So we're gonna test a number of different foods for starch.

We're gonna see if starch is present in bread, in potatoes, and in meat, like sandwich meat, that sort of thing.

Now to test for starch, you're going to need some equipment.

You'll need a pestle and mortar.

You'll need some distilled water.

You'll need a spotting tile or white tile, and you'll need iodine solution.

Now, iodine is an irritant and it will also stay in your skin.

So you must wear goggles whilst you are handling iodine.

That's really important.

So let's just see how we test for the presence of starch.

Before you start, you will need a pestle and mortar, distilled water, a spatula, a white tile and iodine solution.

You will also need safety goggles because iodine is an irritant to test solid food, including bread, potato, and meat for the presence of starch, firstly, grind the food into small pieces using a pestle and mortar.

Add a few drops of distilled water to the sample and grind further to create a suspension.

A suspension is where the tiny food particles float in the water but do not dissolve.

Next, put a small amount of food sample onto the white tile.

Whilst wearing safety goggles, add one or two drops of iodine solution and note the colour.

So just to recap, the method that we're going to follow is to firstly grind our food into small pieces using the pestle and mortar.

Then we're going to add a few drops of distilled water to the sample to create a suspension where the food particles are suspended in the water but do not dissolve.

Then we're gonna transfer a small amount of that food sample onto the white tile or the spotting tile, and then add a few drops of iodine solution and note the colour.

And if there's a change, then starch is present if it turns blue/black.

Remember to wear goggles whilst you're handling iodine.

So let's just quickly check that you've got the method in the right order.

So starting with the first, put these statements in the correct order to describe the method of testing food for starch.

I'll give you five seconds to think about it.

Okay, let's check our answers.

So did you start with D? That we're going to grind food using a pestle and mortar first.

And then go to A, we're gonna add a few drops of water to create a suspension.

And then to C, where we're gonna transfer a small sample to a white tile.

And then finally B, where we're gonna add one or two drops of iodine solution and observe any colour change.

Did you get those all right? Well done if you did.

Okay, so your task now is to follow the method to test for bread, potato.

and meat for starch, and you can find the method in your additional materials.

Once you've done that, I'd like you to write a conclusion.

Do the bread, potato and meat contain starch? How do you know? So pause the video and come back to me when you've done all of those tests and you're ready with your conclusions.

Okay, so you should have completed the test by now, and let's have a look at what sort of conclusions you might have drawn.

So the images on the screen show some of the results that you should have got for bread, potato, and meat.

But what conclusions did you draw and how do you know that you're right? Well, you should have said that both bread and potato contain starch because iodine turned blue/black, whereas meat does not contain starch because it remained brown/orange.

So just check your work over.

Did you get the same results as me? Have you drawn the same conclusions? Have you explained why you are drawing those conclusions? Check your work over, and well done.

So our next part of the lesson is where we're gonna have a look at testing for sugar.

So starch is broken down into very small molecules called sugars, and we can test for the presence of these smaller molecules, these sugars using a different food test.

So a reducing sugar is a very specific type of sugar, which reacts with reagent called Benedict's solution.

So this is a different chemical that we're using to test for a different type of food molecule.

Now, reducing sugars are quite common, and whilst you might not necessarily have heard that term before, reducing sugars will be common to you.

Glucose, for instance, is found in energy drinks like lucozade and fructose is another reducing sugar, and that's found in fruit.

Now the presence of these reducing sugars can be identified using this reagent Benedict's solution.

Now, Benedict's solution is normally blue in colour and it changes colour depending on how much reducing sugar is present within your sample.

So if it stays blue, there are no reducing sugars within your sample.

However, if it turns green/yellow, then there are traces of reducing sugars.

Not very many, but a few.

If it turns orange, then there's a moderate quantity of reducing sugars.

But if it turns brick red, then there are a large quantity of reducing sugars.

So we can see that the colour change indicates an increasing quantity of reducing sugars from none at blue to lots at brick red.

So let's just quickly check our understanding.

Which colour does Benedict's solution turn if there is a large quantity of reducing sugar present? I'll give you five seconds to think about it.

Did you choose brick red? Well done if you did.

So we're gonna test three food substances for the presence of reducing sugars.

We're gonna test glucose, we're gonna test apple, and milk.

So let's have a quick look at what equipment we're going to need in order to test for reducing sugar.

So we'll need a pestle and mortar if the food is solid and we need to grind it up.

We'll need distilled water, a boiling tube, Benedict's solution.

Now Benedict's solution is also an irritant, so you must make sure that you wear goggles whilst you're handling Benedict's solution.

And you'll also need a water bath.

So let's watch and see how we complete the method for testing for reducing sugars.

Before you start, you'll need a pestle and mortar, distilled water, a spatula, a pipette, a boiling tube, Benedict's solution.

Wear safety goggles whilst using Benedict's solution as it is an irritant.

And a glass beaker for a water bath.

To test solid food such as apple for the presence of reducing sugars, grind the food into small pieces using a pestle and mortar.

Add a few drops of distilled water to the sample to create a suspension.

Next, transfer the food suspension from the pestle and mortar to the boiling tube.

You may need to add some distilled water to the boiling tube to recreate the suspension.

Finally, add an equal volume of Benedict's solution to the boiling tube.

You must wear safety goggles whilst using Benedict's Solution.

Place the boiling tube into the water bath and leave for about five minutes for the colour to develop.

So just to recap the method to test for the presence of reducing sugars, you firstly need to grind the food down into small pieces using a pestle and mortar if it's solid, and then add a few drops to create a suspension.

You then need to transfer that suspension into a boiling tube and then add an equal volume of Benedict's solution to the boiling tube, remembering to wear your goggles at this point.

You need to mix 'em thoroughly and then place them in a warm water bath of about 60 degrees centigrade, and leave for about five minutes until the colour has matured.

So just quickly recap our method.

Which of the following steps is specific for testing reducing sugars with Benedict's Solution? Remember that the methods are very similar for testing for starch and for Benedict's.

So is it specific to Benedict's solution that you grind food sample with pestle and mortar? Is it specific to Benedict's solution testing that you add water to create a suspension? Or is it specific that you place the boiling tube in a water bath and leave for five minutes? What do you think? I'll give you five seconds to think about it.

Okay so the specific part of this method for using Benedict's solution is to place the boiling tube into a water bath and leave for five minutes.

Did you spot that difference? Well done if you did.

So your task now is to follow the method to test for glucose, apple, and milk for the presence of reducing sugars.

And then once you've done that write your conclusion approximately how much reducing sugar does each food contain? And again, how can you tell? So pause the video and come back to me when you're ready.

Okay, so let's see what results you might have got and which conclusions you should have drawn.

So the images on the screen show the results you should have got for glucose, apple, and milk, but what conclusions did you draw? Well, you should have said that glucose contains the most reducing sugar and turned brick red.

Apple contains traces or moderate amounts of reducing sugar, but because it turned green/orange, somewhere in between.

That would depend on how sweet the apple was.

Some of you might have got more of a green colour, some of you might have got more of an orange colour.

So go with your own results.

And you should have said that milk contains no reducing sugar because the Benedict's solution remained blue.

So just check your work over, make sure you've drawn the correct conclusions based on the colours that you achieved for your samples that you were testing, and that you've written it up in enough detail to state not only what colour it changed, but also what conclusion you can draw from that.

And well done.

That was a bit trickier, that was.

So our final part of today's lesson is about comparing qualitative and quantitative measurements.

And what are these? So we've seen two food tests for starch and for reducing sugars using the reagent's iodine, and Benedict's solution.

Now, both of these tests are qualitative.

They provide us with qualitative results.

That means that they can say whether they can show whether the food stuff is present or not, or perhaps in how much rough quantity, but they cannot give us a precise measure or a quantity, a number based value against that.

So with the iodine solution, there's either starch present or there isn't.

But it doesn't tell us how much starch is present.

With Benedict's solution, there's no sugar present, a very little bit, some, or quite a lot.

But again, it doesn't tell us exactly how much sugar is present in terms of grammes or percentage.

So all of these results are qualitative.

They are just indications as to whether something is there or not and not the actual value.

How much of it is there.

If you compare that to a quantitative test, quantitative tests provide us with exact numbers of how much of something there is.

For instance, how far something has travelled, or how much mass has been gained, or how much by how much their temperature has changed.

These are all giving us exact numbers that we can calculate, and therefore these values are quantitative.

So let's just check our understanding.

True or false, Benedict's solution is a qualitative test.

What do you think? So you should have said that that is true, but why is that statement true? Justify your answer by choosing the correct option.

So did you justify your answer by saying that the results indicate an approximate range? If so, well done.

So Benedict's Solution is qualitative because the results are an approximate range rather than a precise numerical value.

So my final task for you for today is to complete the table to show the expected colour of Benedict's Solution for the various food tests that's listed in the table.

So take your time, think about it really carefully.

Use the information that you've been given, pause the video, and come back to me when you're ready.

Okay, you ready to mark your work then? So let's have a look.

If lucozade drink has a high quantity of reducing sugar, then we would expect Benedict's solution to turn brick red.

Orange juice turned orange and therefore has a moderate quantity of reducing sugar.

Tea without milk has no reducing sugar and therefore Benedict's solution should have stayed blue.

But tea with milk will have had a trace of reducing sugar, and so should have turned green/yellow.

Pineapple juice turned Benedict's Solution brick red and therefore has a high quantity of reducing sugar.

And cranberry juice has a moderate quantity of reducing sugar and therefore should've turned Benedict's Solution orange.

So just check your work over, make sure you've got the right answers and well done again.

So just to summarise our lesson today, we've seen that reagents can be used to test food and that the reagent iodine can be used to test for the presence of starch.

If starch is present, it turns blue/black.

Whereas Benedict's Solution can be used to test for the presence of reducing sugars, and it turns from blue to green/yellow, orange, and then brick red as a concentration of reducing sugars increases.

And we've also seen how both of these tests for both iodine solution and Benedict's Solution are examples of qualitative tests.

Thank you very much for joining me today.

I hope you've enjoyed today's lesson, and I hope to see you again soon.

Bye.