Food Tests - GCSE Science Required Practical

Summary

In this video, Dr. Sheffield from Mars Beauty School Science demonstrates food tests for detecting sugars, proteins, fats, and starch in different foods such as apple, biscuit, and cheese. The process includes preparing food samples, filtering them, and conducting Benedict’s test for sugars, iodine test for starch, ethanol test for lipids, and Biuret’s test for proteins. Each test shows specific color changes to indicate the presence of these chemicals. The video also highlights the importance of knowing test methods, expected results, and offers additional resources for teachers looking to improve practical science skills.

Takeaways

• 🔬 The video covers required food tests, including testing for sugars, proteins, fats, and starches.
• 🥐 The food samples used in the test are biscuit, apple, and cheese, with biscuit being tested first to minimize mess.
• ⚗️ Preparing the samples involves grinding the food, adding distilled water, stirring, and then filtering the mixture to remove solids for clearer results.
• 🧪 The Benedict's test is used to detect sugars (glucose). The reagent starts blue and, when heated, changes color depending on sugar levels, with the final color being brick red.
• 🌾 The iodine test is for starch, commonly used in questions about photosynthesis. If starch is present, iodine turns from orange to blue-black.
• 💧 The test for lipids (fats) involves adding ethanol and distilled water, then shaking. A cloudy white color indicates the presence of fats.
• 🥚 The Biuret test detects proteins, where the solution starts blue and turns pink or purple if proteins are present.
• 🍎 Results for the apple sample showed glucose was present (orange color in Benedict's test), but no starch, fats, or proteins were detected.
• 🍪 The biscuit sample had positive results for sugars, starch, and fats, but no proteins.
• 🧀 The cheese sample gave unexpected positive results for starch due to a food additive in grated cheese. It also tested positive for fats and proteins, but negative for sugars.

Q & A

What is the purpose of the food tests in this practical?

The purpose of the food tests is to identify different chemicals, such as sugars, proteins, and fats, found in various food types like apple, biscuit, and cheese.

Why is it important to filter the food samples?

Filtering the food samples removes solid food particles, helping to make the results of the color-change tests clearer and easier to interpret.

Why does Dr. Sheffield suggest testing biscuit, apple, and cheese in that specific order?

The food samples are tested in order of increasing messiness, starting with the biscuit, followed by the apple, and leaving cheese for last as it is the messiest to handle.

What is the Benedict’s test used for, and what color changes indicate a positive result?

The Benedict’s test is used to test for glucose and other short carbohydrates. A positive result shows a color change from blue to green, yellow, orange, or brick red, depending on the amount of sugar present.

How is the iodine test for starch performed, and what result indicates the presence of starch?

The iodine test for starch involves adding iodine to the food sample. A positive result is indicated by a color change from orange to a blue-black color.

What is the procedure for testing lipids (fats) using ethanol?

To test for lipids, ethanol is added to the food sample, followed by distilled water. The tube is shaken vigorously, and a positive result is indicated by a cloudy white suspension.

What does the Biuret test detect, and what result shows the presence of proteins?

The Biuret test detects proteins. A positive result is shown by a color change from light blue to pink or purple.

Why might grated cheese give a positive result for starch in the iodine test?

Grated cheese may give a positive result for starch because manufacturers often add starch powder to keep the grated pieces separate.

Why is it necessary to clean the pestle and mortar between preparing different food samples?

The pestle and mortar need to be cleaned between food samples to avoid cross-contamination, ensuring that the results of the tests are accurate.

What should you include in an exam answer when describing the Benedict’s test for sugars?

In an exam, you should describe adding Benedict’s solution to the food sample, heating the mixture in a hot water bath, and observing the color change, which can range from blue to brick red depending on the amount of sugar present.

Outlines

00:00

🔬 Introduction to Food Tests in Practical Science

Dr. Sheffield from Mars Beauty School Science introduces the topic of food tests, which are important not only as required practical exams but also in other scientific topics like photosynthesis. The lesson will focus on testing foods such as apple, biscuit, and cheese to identify the presence of sugars, proteins, and fats. The food samples are prepared in order of cleanliness (biscuit, apple, and cheese) by grinding them, adding distilled water, and stirring to dissolve chemicals for testing. The samples are then filtered to remove solids and make it easier to observe color changes during the tests.

05:01

🧪 Benedict’s and Iodine Tests for Carbohydrates

Dr. Sheffield explains the process for testing carbohydrates, starting with Benedict’s test for short carbohydrates like glucose. Benedict's reagent is added to the food sample, which is then heated in a hot water bath. The second test, the iodine test for starch, involves adding iodine to detect large carbohydrates. The iodine initially appears orange and changes color based on the presence of starch. These tests help detect carbohydrates in food samples.

10:02

🥛 Ethanol and Biuret Tests for Fats and Proteins

The ethanol test is used to detect fats (lipids), with the addition of distilled water and ethanol followed by vigorous shaking. The resulting cloudy white solution indicates the presence of lipids. For the protein test, Biuret reagent is added to the food sample, which starts as light blue and changes color to pink or purple if proteins are present. Dr. Sheffield emphasizes the importance of noting both the methods and color changes in the food tests.

🍎 Results of Food Tests Using Apple

The results of the food tests using apple are discussed. The Benedict’s test turned orange, indicating a high presence of glucose. The iodine test showed no color change, meaning no starch was present. The ethanol test for lipids also showed no positive result, as the solution remained clear. Lastly, the Biuret test for protein was negative, showing no color change. These results confirm that apple contains glucose but lacks starch, fats, and proteins.

🍪 Results of Food Tests Using Biscuit

The biscuit sample yielded different results from the apple. The Benedict’s test resulted in a strong orange color, confirming the presence of glucose. The iodine test turned blue-black, indicating a significant amount of starch. The ethanol test produced a cloudy white suspension, showing the presence of fats. However, the Biuret test remained negative for proteins. These findings suggest that biscuit contains glucose, starch, and fats but little to no protein.

🧀 Results of Food Tests Using Cheese

The food tests with cheese showed mixed results. The Benedict’s test stayed blue, indicating no glucose in the sample. However, the iodine test unexpectedly produced a blue-black color, likely due to starch additives used in grated cheese. The ethanol test for lipids resulted in a cloudy white suspension, confirming fats in the cheese. Finally, the Biuret test showed a positive result for protein, turning purple. These results reflect cheese’s high protein and fat content, with some unexpected starch from food additives.

🎓 Final Remarks and Educational Resources

Dr. Sheffield concludes by encouraging teachers who may not be confident with practical science experiments to explore professional development courses offered by the Avon Teaching School Alliance. These courses can help educators improve their skills outside their usual specializations. Teachers interested in learning more can click the link in the description for additional resources.

Keywords

💡Food Tests

Food tests are practical experiments used to identify the presence of different chemicals in food, such as sugars, proteins, fats, and starch. In the video, the tests are a core focus as Dr. Sheffield guides the viewer through how to conduct them using apple, biscuit, and cheese as samples.

💡Benedict's Test

Benedict's Test is used to detect the presence of reducing sugars like glucose. The test involves adding Benedict's reagent, which starts blue, and heating the mixture. A positive result is indicated by a color change, ranging from green to red, with more intense colors indicating higher sugar levels. This test is demonstrated using apple and biscuit samples.

💡Iodine Test

The iodine test is used to detect starch in food samples. Iodine starts as an orange color and turns blue-black if starch is present. This test is highlighted as a key method for identifying starch, particularly in plant-based food and is demonstrated using biscuit and cheese samples.

💡Ethanol Test

The ethanol test is a method used to detect lipids (fats) in food samples. When ethanol is mixed with a sample, a milky white emulsion indicates the presence of fats. This test is demonstrated on the biscuit and cheese samples, both of which test positive for fats.

💡Biuret Test

The Biuret Test is used to detect proteins in food samples. Biuret reagent starts as a light blue color and turns pink or purple if proteins are present. In the video, this test is performed on the cheese sample, which tests positive for protein, while the biscuit sample shows no protein presence.

💡Carbohydrates

Carbohydrates are organic compounds that include sugars and starches. The video focuses on two types of carbohydrate tests: the Benedict’s Test for simple sugars and the Iodine Test for starch. Carbohydrates are highlighted in the context of apple and biscuit samples.

💡Lipids

Lipids, or fats, are one of the key components tested in the video. The ethanol test detects lipids by forming a cloudy white emulsion. Lipids are found in the biscuit and cheese samples, which both show a positive result during the demonstration.

💡Proteins

Proteins are large molecules essential for life, made of amino acids. The video explains how to test for proteins using the Biuret Test, where a positive result turns the solution from blue to purple. Cheese is shown to contain proteins, while the other food samples do not.

💡Distilled Water

Distilled water is pure water free of impurities and is used in the preparation of food samples for testing. It is added to the ground food samples to dissolve the chemicals before testing for sugars, proteins, and fats. The video emphasizes its role in creating a solution for clearer test results.

💡Filter Funnel

A filter funnel is used to remove solid particles from the food solution during preparation. This step helps prevent cloudiness in the test results, making color changes more visible. Dr. Sheffield highlights the importance of filtering in ensuring accurate and clear results in the food tests.

Highlights

Food tests often appear in exams, not only as required practicals but also in other areas like photosynthesis.

Food samples are tested for sugars, proteins, and fats by using specific tests on different types of foods such as apple, biscuit, and cheese.

Preparing food samples involves grinding them in a pestle and mortar, mixing them with distilled water, and filtering the mixture to remove solids for better test visibility.

The Benedict’s test detects small carbohydrates like glucose, starting with a blue solution and changing color based on the sugar content.

The iodine test checks for starch, with a color change from orange to blue-black indicating the presence of starch.

For lipids or fats, the ethanol test is used, resulting in a cloudy white suspension as a positive indicator of fats.

The Biuret test detects proteins, where a positive result changes from blue to pink or purple.

Results from apple samples show a positive Benedict's test (orange color) indicating glucose, but no starch, fats, or protein detected.

Biscuit samples showed positive results for sugars (Benedict’s test), starch (iodine test), and fats (ethanol test), but no proteins.

Cheese samples showed no sugar (Benedict’s test), a surprising positive starch result due to an additive in grated cheese, fats (ethanol test), and proteins (Biuret test).

The Benedict’s test for glucose is semi-quantitative, with color changes indicating the sugar concentration, ranging from blue to brick red.

Each food test requires careful preparation, and cleaning between samples is crucial to avoid contamination.

Cheese contains starch only if additives are present, as animals store glucose as glycogen rather than starch.

The video demonstrates step-by-step methods for all four food tests, showing clear examples of positive and negative results.

For teachers outside their specialism, courses are offered to build confidence in conducting required practicals in science.

Transcripts

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 in these areas

please click on the link in the

description below

to find out more about the courses that

we offer