Does Citric acid decompose in a lemon over time?

[iambadatscience]

Hi! So I am thinking of doing a concentration of citric acid in a lemon lab, by titrating it against sodium hydroxide.

I want my independent variable to be time, and so i plan to do it over the span of 25 days, in which I think it will be long enough to show some change. I'm just unsure about the science of how/why citric acid decomposes in a lemon, can someone help me? Thanks!

 

[Borek]

You may need to dig into the biochemistry of lemon ripping and degradation. These are not trivial processes in general (but they can be quite interesting).

Definitely read on the citric acid cycle. While technically it doesn't change the amount of the citric acid, it should be a good starting point to learn about processes in which the citric acid is involved.

 

[epenguin]

I really don't know what happens to citric acid in lemons or other fruits. I would not even be surprised if it increases in 20 days as the fruit loses water. Apart from that any change would be biochemical. But what I don't know. Of course if the citric acid is being turned into another acid, you might not see the change when you are measuring only acid.

I have to say I was at first bit sceptical about measuring it by such an unspecific method as alkali titration, when the juice will contain rather a mixture of things. What you would be measuring would be the total all substances in the juice that deprotonate between pH about 2, the pH of the juice, and pH 8.5, the pH of the phenolphthalein colour change. That would include any organic acids, and about half of the amino acids. But apparently citric acid is so predominant that this is sometimes done. I saw a protocol for it here. http://www.easychem.com.au/the-acidic-environment/acid-base-definitions/titration-experiment Of course even if most of the acid is citric acid if there is only a small change, it is hard to say what that represents. It could be significant, but you wouldn't be sure what of.

I'd think the first thing to do without waiting 20 days would be to do the experiment with half a dozen lemons to get an idea of the variability. You sure don't want to use just one lemon each day. You might want to standardise as much as possible by always using fruit only within a certain weight range or length range. I'd also play around with heating your extract maybe placing in boiling water for a minute or more, making up volume of any water loss, then centrifuging which should get rid of proteins and gunk not helpful to have in your titrations.

If it were me and I had some serious reason for wanting to know how the citric acid concentration varies I would be measuring by some more specific method. That means most obviously an enzymic method - that means the reaction involving an enzyme specific for citric acid or citrate that involves or is coupled to a reaction that gives a change measurable in a colorimeter or spectrophotometer. Chemical companies that you find on line advertise kits for this, but they are quite expensive, about £150 for 100 assays I think. Even with an enzymic method, I would test that a likely interference, particularly malic acid, isn't reacting too.

If you have a pH meter it might be amusing to test the effects of diluting your juice and seeing if it follows the square root law discussed just a few posts down. In any case let us know of your results and After you have finish the project.

 

[iambadatscience]

Alright, So now I'm doing the experiment. The values for the first 15 days stay around the same, and after 20, the titration results changed by around 0.5 ml (went up, more naoh used) for each trial. I find this interesting, but yet I still can't describe what's happening, or why it does.

 

[epenguin]

Alright, So now I'm doing the experiment. The values for the first 15 days stay around the same, and after 20, the titration results changed by around 0.5 ml (went up, more naoh used) for each trial. I find this interesting, but yet I still can't describe what's happening, or why it does.

Can you say the measured acid concentration went up from what to what?

 

[iambadatscience]

Alright, at 0 days the highest concentration was 0.35 mol/l , at 25 days it was 0.28 mol/l

 

[iambadatscience]

I also tried to look up things such as the citric acid cycle as mentioned earlier in the thread, but alas i can't seem to understand/find much, if you could elaborate a little bit more that would be amazing! Thanks :D

 

[iambadatscience]

Also, is there a generally accepted value for the concentration of citric acid in lemon juice? I've seen multiple sources say different things.

 

[epenguin]

Did you do a measurement of one lemon at one time nd another after 25 days, or did you have numbers of lemons done several times? Did you try to standardise as I suggested? Your phrase does not really amount to data yet.

 

[iambadatscience]

Did you do a measurement of one lemon at one time nd another after 25 days, or did you have numbers of lemons done several times? Did you try to standardise as I suggested? Your phrase does not really amount to data yet.

Apologies. Thanks for putting up with me, I am not very experienced in these sort of things.

I did 3 trials per day, which was 0 days (day of ), 5 days, 10 days ,15 days, 20 days. Here are my results:

(All in mols/l)

0 days: 0.35, 0.33, 0.33
5 days: 0.35, 0.33, 0.33
10 days: 0.33, 0.33, 0.33
15 days: 0.32, 0.33, 0.33
20 days: 0.28, 0.30, 0.30

Thank you so much!

 

[epenguin]

These are 15 different lemons? I'd say you could say you have shown that the acid level is practically constant for the first days, But it is possible that it is starting to decrease by the last day .ou measured.

 

[jim mcnamara]

FWIW From a botanical point of view, citrus fruits have evolved to do interesting things

1. under cool xeric conditions, they turn into 'hockey pucks' which have higher citric acid concentrations. The fruit cells become mostly dead. When the rains come the fruits absorb water, then decay attractively to attract animals. The attractants given off are from residual enzymes acting on remaining citric acid.

2. under moist and aerobic conditions the fruit ripens over time. Enzymes break down cell walls and the entire fruits becomes much softer, giving off attractants - polyacohols, for example. This involves a lowering of citric acid concentration. This is an example of seed dispersion - via the 'constipated small mammal theory of propagule dispersion.' My advisor in grad school used this very term.

In the US, grocery store lemons are generally underripe, considerably. I think because of consumers choices. And their storage methods.

Therefore, storage methods may affect the citric acid concentration results.

Also in FWIW category, cold underwater storage of most citrus fruits puts them on "hold" for several weeks. Meaning if you are like me and store lemons for future use on alternate Wednesdays in months that end in "q", this is an option. (This translates to: I forget. A lot. Often. Limes/Lemons are not on my radar. ) We use a plastic tupperware-like container filled with water in the "hydrator" section of the fridge. Does well. For month or two.

 

[iambadatscience]

Yeah, I'm thinking of saying that the concentration of lemons did not decrease until around the 15-20 day mark, as before then since citric acid is fairly stable.

The point made about the moist aerobic conditions is interesting - That's probably why the concentration of the acid decreased. I'll definitely look into that, if there's any sources you can suggest that I look at that would be great! thank you guys so much!

 

[jim mcnamara]

Try this older paper: http://fshs.org/proceedings-o/1945-vol-58/128-133(MILLER).pdf

 

[John Park]

I too want to know if your results were from different lemons.

Is aqueous citic acid stable against atmospheric oxidation? Would a control experiment using just citric acid solution exposed to the air be a good idea?

 

[jim mcnamara]

This thread really is about biological processes and experimental design. It is a really interesting topic.

@John Park - short answer - citric acid is used by many living things as a energy source. Humans included. Bacteria will colonize a solution of citric acid in a few hours. Lactobacillus spp. (like yogurt bacteria), Bacillus pyogenes and many other bacteria will use dilute citric acid (lemon juice) as a source of energy. The citric acid (Kreb's cycle) is part of anaerobic respiration in many living things, including humans. Low pH deters fungi from growing in citric acid, they colonize the skin of lemons instead.

So atmospheric oxidation is not a big deal compared with respiration.

Fruits are alive in general during ripening - which is why they can ripen pretty fast. Ethylene - the gas - in small concentrations speeds up lemons ripening. So the experiment should control for storage temperature especially. Plus humidity. Plus air circulation to keep ethylene from other fruits out of the picture. I don't think it does any of that. Lemons are non-climacteric - meaning they do not produce ethylene, but do respond to it, and they do not have explosively fast ripening in the absence of ethylene.

Fruits are there to entice animals into eating them, so many fruits self destruct by ripening, giving off plant hormones like ethlylene to get their neighbors to ripen at the same time, increasing attractants by mass action. Ripening releases all kinds of complex compounds to broadcast the fact that 'lunch is served'.

To reinforce that idea, observe that the seeds of many fruits actually require exposure to stomach acids before germination will occur. Mistletoe is an example.
Lemon seeds are unaffected by stomach acid. They just have to stay moist until they are in a spot to germinate.

 

[iambadatscience]

What kind of air condition would normal air be? Would it make sense for the concentration of the acid to decay in normal air?

 

[jim mcnamara]

In a word: no.

 

[jim mcnamara]

In Science experiments try to eliminate everything except one variable - in your case time. So temperature should not vary, kept in the dark at constant humidity without ethylene generating fruits nearby.

 

[Cahill11]

Hey I'm doing a high school experiment on the effects of Temperature on the degradation of citric acid in lemon juice. We are juicing the lemons and storing the juice inside a refrigerator. We then take 20mL out and expose it to different temperatures (50, 70, 90 degrees C) and titrate the juice with NaOH to find where the end point point occurs. I know that there are other juices present in lemon juice that will potentially impact our results but lemons consistent mainly of citric acid. We tested with 90 degrees C, exposing the lemon juice for 15 minutes and the endpoint occurred at the same volume of NaOH as room temperature. Although we only did one trial due to time constraints, why would this be the case? And is the degradation of citric acid in lemon juice from heat impacted on by the krebs cycle, enzymes or bacteria?

 

[Borek]

90°C is way too high for a standard biochemistry to work.

 

[Cahill11]

We also tried with 50 and 70 and yielded the same results. Also would the degradation of the citric acid be due to the equilibrium based on Le Chateliers principle since adding heat should move it to the towards reaction? And does the Krebs cycle impact its degradation?

 

[Borek]

IMHO 15 minutes is way too short for results that can be measured with just titration.

If you want to see effects of the biochemical degradation (that is, one involving cellular engine) you need to keep your samples in the temperatures typical for the plant - that would probably mean something between 20-40°C. If it gets too hot you are killing the cell with the temperature, enzymes get denatured and stop to wok (compare the image in the top right: https://en.wikipedia.org/wiki/Denaturation_(biochemistry) )

 

[Cahill11]

IMHO 15 minutes is way too short for results that can be measured with just titration.

If you want to see effects of the biochemical degradation (that is, one involving cellular engine) you need to keep your samples in the temperatures typical for the plant - that would probably mean something between 20-40°C. If it gets too hot you are killing the cell with the temperature, enzymes get denatured and stop to wok (compare the image in the top right: https://en.wikipedia.org/wiki/Denaturation_(biochemistry) )

Ok thanks. Was going to try leaving it in for 3 hours in those temperatures and see what happens. However, being a school experiment it is limited with time. So what your saying is in the Citric Acid cycle the temperature that is being applied is denaturing the enzymes present which caused the cycle to occur? And do you know if citric acids degradation has a lot to do with Le Chateliers principle?

 

[Borek]

No idea about exact numbers, so take what I wrote with a grain of salt - but I would be quite surprised if any of the cell cycles would still work OK above 40-45°C. Some of the enzymes would definitely survive but the cycle itself contains many steps and chances of all enzymes being resistant are slim (we are talking about a normal plant, not an extremophile).

Le Chatelier's principle is more of a rule of thumb than a real principle, it works reasonably well for simple reactions but not necessarily for a multi step process. So it may work, or it may not work, hard to tell.

 

[Cahill11]

No idea about exact numbers, so take what I wrote with a grain of salt - but I would be quite surprised if any of the cell cycles would still work OK above 40-45°C. Some of the enzymes would definitely survive but the cycle itself contains many steps and chances of all enzymes being resistant are slim (we are talking about a normal plant, not an extremophile).

Le Chatelier's principle is more of a rule of thumb than a real principle, it works reasonably well for simple reactions but not necessarily for a multi step process. So it may work, or it may not work, hard to tell.

Ok

 

[Borek]

This is just dissociation, not decomposition.

 

[Cahill11]

This is just dissociation, not decomposition.

Also is there a difference between decomposition and degradation because I've seen these terms been used interchangeably in articles focusing on the citric acid cycle? Thanks for all your help btw

 

[Borek]

Also is there a difference between decomposition and degradation

Perhaps in some contexts decomposition and degradation are intended to mean something different, but in this case they seem to mean the same.

 

[bill1243325]

Y'all are smart AF. Came here to see if my lemon juice is still at a 2 pH for tummy issues(Don't ask). Now I'm leaving blown away at the science you guys/gals did on this. Someone has to do the hard work. I'm toasting a drink to all you scientist. Remember to relax every once in a while though. Don't be too smart for your own good heheh. You can learn that from an operator.

 

[Yiwei Zhou]

So, does the concentration of citric acid in a lemon change over time under standard temperature and pressure? If so, why?