Science at home! Why do apples go brown Pt 3

Little demonstration of the experiment before you head off and run your own one. Okay so first things first I am going to cut up my apple. So first of all I’m going to slice it in half and then I’m gonna make some small pieces. I slice it in half again, and i’m going to cut it really small. And again don’t worry if you don’t have an apple, you can be doing this with a potato or lettuce or banana as I’ve said.We’re going to cut it again so we’ve got really thin pieces of apple.

We’re going to cut it again, nice small chunks of apple just like this so I’ve now got four pieces of apple. I’m going to cut another one as where I’ve got each cut another two, so let’s cut this one. Okay so now I’m going to put the pieces of apple into each of the bowls. This one is my control so this one is not going to have anything in it. We’re just going to leave that just as apple.

This one here is my bicarbonate of soda. Got my bicarbonate of soda here Next one is orange juice so I’m going to chop up my orange The next one is my lemon juice, so I’ve got my lemon juice here but you could be using lemon juice from an actual lemon or bottled lemon juice. And if we compare to the bicarb we’ve got a very similar amount of liquid in here.

Next one is our milk so we’re going to pour that in Lastly we have the vinegar so I’m going to pour that in as well. So that is our experiment nicely set up so it’s time for you to go ahead and be scientists and run your experiment. So you need to leave your experiment for one hour. Go and do something else in that time.

Little demonstration of the experiment before you head off and run your own one. Okay so first things first I am going to cut up my apple. So first of all I’m going to slice it in half and then I’m gonna make some small pieces. I slice it in half again, and i’m going to cut it really small. And again don’t worry if you don’t have an apple, you can be doing this with a potato or lettuce or banana as I’ve said.We’re going to cut it again so we’ve got really thin pieces of apple.

We’re going to cut it again, nice small chunks of apple just like this so I’ve now got four pieces of apple. I’m going to cut another one as where I’ve got each cut another two, so let’s cut this one. Okay so now I’m going to put the pieces of apple into each of the bowls. This one is my control so this one is not going to have anything in it. We’re just going to leave that just as apple.

This one here is my bicarbonate of soda. Got my bicarbonate of soda here Next one is orange juice so I’m going to chop up my orange The next one is my lemon juice, so I’ve got my lemon juice here but you could be using lemon juice from an actual lemon or bottled lemon juice. And if we compare to the bicarb we’ve got a very similar amount of liquid in here.

Next one is our milk so we’re going to pour that in Lastly we have the vinegar so I’m going to pour that in as well. So that is our experiment nicely set up so it’s time for you to go ahead and be scientists and run your experiment. So you need to leave your experiment for one hour. Go and do something else in that time.

There’s plenty of things you could be doing, if you haven’t done our sensory science video, you can go and find that on our website or on our YouTube sites you could read a book you could do some colouring go and tidy your bedroom lots of things you can be getting up to. Okay so your one hour is up everyone it’s now time to see what your experiment looks like and how much browning you have so I’m gonna get you to pause the video before we go ahead and explain the results and I want you to write down what you’ve observed go back to your hypothesis, see if you’re right and have a think about what you saw. You could maybe even write these down in a results table so that when we come back and do the explanations you can have a think about why your results are as they are. so I have my results in front of me and we’re now going to go through the science behind our browning experiment. So I’m just going to tilt my screen so that you can see my samples in front of me like this. These are my samples and I will lift them up to the camera when it’s time for us to go through them.

So starting with the control. So this is my control that had no treatments. We can see the browning spots here if I turn it over we can also see the browning spots there and I’ve got a picture coming up in the corner of our control so this have no treatments and this is what we are comparing all of our other samples to.

So starting off with the lemon juice sample. This for me was the least brown so if we look here no browning occurring so the lemon juice first of all is very acidic so lemon juice is pH 2 and that is very much below the optimum pH of our enzyme which is pH 5 to 7 so inactivate the enzyme were not able to have our reaction between polyphenol oxidase and oxygen occurring. But another really interesting thing about lemon juice. Lemon juice is really high in vitamin C which is also called ascorbic acid and ascorbic acid is an antioxidant and by antioxidant we mean something that will preferentially react with oxygen, it will use up oxygen.

So normally polyphenol oxidase can react with oxygen to produce those brown pigments but here we’re finding that the ascorbic acid will actually react with the oxygen before the polyphenol oxidase can do that, meaning there’s no oxygen left for the reaction in forming the brown pigments and so that’s why our lemon juice sample is really not brown and similarly the orange juice although it’s a slightly higher pH than the lemon juice we’ve still got no browning and that’s again because orange juice is really high in ascorbic acid and therefore the oxygen will preferentially react with the ascorbic acid and we’re not getting that browning reaction occurring. So I’m now going to move on to the bicarbonate of soda. This for me was the most brown.

So if you look it’s really really brown here and it’s really brown here so it is pH 9:00 to 10:00 and it’s really alkaline and that means it’s got an abundance of hydroxide ions so o h minus ion as opposed to our acid samples which have abundances of hydrogen ions that means there’s actually more oxygen for our polyphenyl oxidase to react to, meaning that we get a quicker reaction. then going to move onto my milk sample. So I had a little bit of browning in our milk so we can see that there’s a little bit of browning here and if we compare it to our lemon juice you can see that there is a little bit more browning in our milk sample. Milk is around pH 6 to 7 depending on your milk so it is a within the optimum range for the polyoxidase however it is not really alkaline so we’re not getting an abundance of hydroxide ions but we’re still able to have the browning reaction occurring although it is slower than the one with no treatment and that’s probably because the milk is covering the apple slightly which means that there’s a little barrier for the oxygen to have to get to the apple. So lastly moving on to our vinegar reaction, so the reason why we left this till last is because we actually had a really unexpected result. It went really brown and even more brown than the control which is something we did not expect.

So if I show you our vinegar sample it looks something like this. Look all this browning all over here and if we compare that to our control you can see just how brown it is. Now I don’t know about you but I expected that because of the low pH of the vinegar approximately pH 3 it would have inhibited or denatured the enzyme with the enzyme being a protein and slowed down or completely stopped the browning reaction and as you can see that didn’t happen at all. So being scientists we repeated this experiment twice more so we had three samples to go from and each of these still remained brown.

More brown than the control and more brown than the bicarbonate of soda reaction. So then I did some research looking at the ingredients in our vinegar which was distilled malt vinegar and malt vinegar comes from barley and barley grains actually have a really high level of phenolic compounds like we talked about earlier, so the compounds that can make blueberries blue and can provide carrots with their orange color. Now because they had such a high concentration of phenolic compounds it meant that we had a really increased concentration of substrate because if you remember in the reaction it’s the phenolic compounds that are reacting with the oxygen and that reaction is catalyzed by the polyphenol oxidase and therefore were able to have a much larger extent of browning because we have such an increase in substrate concentration as we put quite a lot of vinegar in the glass.

So that probably will explain why we had such browning when we used the malt vinegar. So I tried it with another vinegar as well and that was white wine vinegar. So if I show you that one look like this so we’ve got around about the same level of browning in the white wine vinegar as we do with the control. White wine vinegar contains sulfates which inhibit the reaction as they are inhibited of the polyphenol oxidase. So what we could see from this is that it’s not just the acidity that affects this reaction, we’re getting effects from antioxidant so they are really slowing down the reaction or stopping it.

In the lemon juice and the apple juice sample were having inhibitors like the sulfates in the white wine vinegar and we’re also seeing that substrate concentration to the concentration of phenolic compounds really plays an important role in this reaction. Okay so if finally we now know that poly phenyl oxalates can cause browning’s in fruits and vegetables so why do you think that PPO is important in the food industry why does it matter? Pause the video, have a think about what you know about polyphenol oxidase and write down a few reasons why you think it’s important to both the food industry and to plants have a think pause it and we’ll come back and discuss in a minute. So I don’t know what you’ve written down but some of you may have said that polyphenol oxidase might be a cause of food waste in the food industry and that’s because if you were to buy a packet of cut-up apples and you saw that they were brown I don’t think you’d want to buy them because you think they didn’t look very nice.

And so polyphenol oxidase if this browning reaction happens during the processing of fruits and vegetables can lead to a lot of wastage because then they can’t be sold in the shops so we have to work out ways to inactivate the polyphenol oxidase and that’s either by using a really high temperature as I explained that polyphenol oxidase is inactivated by high temperatures for example if we’re cooking like apple sauce we’ll make sure it’s heated really high to inactivate the enzyme. And secondly by adding in preservatives or ingredients so the next time you buy guacamole you might see that actually on the back of the packaging it will contain lemon juice or ascorbic acid and that’s vitamin C and lemon juice added in to inactivate polyphenol oxidase to prevent your avocados going brown. The same is when you buy cut fruit in supermarkets it’s often dipped in a preservative solution which prevents it from going brown otherwise we’d end up with really spoilt and low quality and not very nice looking fruits and vegetables which wouldn’t be very appealing for you and I to buy in the supermarkets.

Now you may have noticed that if you have a plant that’s split off or at least that’s broken that you get some brown spots or some brown patches on the leaf. Now that’s because it’s been broken off and the polyphenol oxidase just like in our food has been exposed to the air so you’re allowed to have that browning reaction occurring now we don’t know this for definite because well I’ve never spoken to an insect, I don’t know about you, but it’s been thought that insects are actually deterred from eating the leaf anymore because of the brown color so it prevents any more destruction to the leaf. If you’ve really enjoyed today’s video I hope you’ve learned something and the next time we see a brown apple in your lunch box or the next time you see some potato going brown on the side you can tell your friends and family why that is! Stay tuned for our next video which is going to be looking at used by and best before dates and how to understand them better to reduce food waste Bye everyone you

Leave a comment

Your email address will not be published. Required fields are marked *