Enzymes: Vitamin C vs. Temperature and pH

[Fairedust1704]

Hi, I’m doing a piece of coursework on how cooking a lemon will affect it’s Vitamin C concentration. I have found out that when the lemon is squeezed and the juice is heated above 60 degrees the enzymes are denatured which causes the vitamin C to be oxidized. Therefore the concentration of Vitamin C decreases. I have also found out whilst researching that enzymes are affected by PH (acidity or alkalinity) and that extremes of PH can denature an enzyme and stop it working altogether. As a lemon is very alkaline, do you think this affects the Vitamin C concentration after it is cooked? I cannot find any information anywhere about the PH of a lemon. As you can see I am very confused and can’t seem to find any information anywhere. 🙁
Basically I just need to find outwhat the PH balance is in a lemon, and how the alkalinity of a lemon affects the enzymes in the lemon juice when it is heated? ❓

Help!

Many Thanks, Aimie [/code]

[mith]

You might get better luck looking for citric acid

[halitus87]

Hello u seam to think that lemons are basic when they are not
lemons and other citris fruit are acidic due to the vitman C becuase vitiman C is an acid it is called asorbic acid (pardon the spelling)

while im not sure how a low PH will affect it and im may do this in an upcoming practical at school

i do think that if there were a basic solution added the asrobic acid would react with it in an acid base reaction not a redox reaction

i hope this helps

i am doing an assesment to mesure the affects of temperature and or some other variables have on vitman C in fruite juice

[b_d_41501]

Lemons are acidic but they make the body more alkaline. If you are looking to see it’s effects on the body (which i’m not sure if you are, but if you are…..) you must measure the final results as they are in the body and not just the food since biological reactions are very complex. Lemon juice has a pH of 2, which means it’s very acidic. As to the temperature which would reduce the concentration of citric acid enought to balance the pH, this is where experimentation comes into play. You would set up a table of temperatures and test the pH of the end product after each temperature and record the results. Hope this helped some. 😀

[MrMistery]

As it was said before, lemons are not alkaline, but acidic. Highly acidic. The vacuolar juice of a lemon has a pH of 2-2.5, more acidic than many acids 😀 Now, i don’t think vitamin C can be oxidised, because it is an antioxidant. At high temperatures(i.e.- when you cook it) the cells burst and most of the vitamin C evaporates

[victor]

Who said that lemon is Alkaline? If lemon is alkaline, I wouldn’t drink the juice because it must be very bitter..he2…as you said that lemon has pH from 2 – 2.5 and the substances that is contained in the lemon are vitamin C (Ascorbic acid)…but, you said that even though it’s acid but inside human body it will become alkalic…um, how about the theory of the buffer liquid that our body have? Actually it’s not making it more alcalic but the buffer liquid of the body maintain the acid stage inside that.
And about Mr.Mistery..I don’t think that evaporation will affect the pH. Just test it on Sodium Acetic (a mixture of Acetic acid and Sodium Hydroxide). When you boil it and some of it are evaporating..just measure it again when it’s cooled down..you’ll get the same pH result..(I think the pH is 5 if you do the mixture of 80 mL of Acetic acid and 40 mL of Sodium Hydroxide)

[mith]

Ok, I think Fairedust1704 gets the point that lemon is acidic. There’s no need for everyone to point that out.

[MrMistery]

Of course boiling it won’t affect the pH. What i was saying is that when you boil it the vitamin C evaporates and degrades. It is still acidic, but not that benefical 😉
Also take note that is only if you boil it. if you put lemon into your tea it will still have vitamin C

[Jelanen]

Yikes! I’ve seen some craziness on this board before, but this getting bad… Ok, heres the real deal and some people may or may not follow me since this is college level chemistry. First, heres a lovely pic of L-ascorbic acid:
As with any biological organic molecule, and L-ascorbic acid in particular, they are subject to nucleophilic attack by oxidizing agents. Since Cl and F aren’t fairly common (thankfully) the nucleophile in this case is O2. This action is related to its role as an antioxidant. Remember that antioxidant doesn’t mean that its not susceptible to O2 oxidation, but that the antioxidant is good at arresting Oxygen radicals and keeping them from wreaking havoc in the cell. If the antioxidant takes a hit in the process and can no longer perform its antioxidant duties, oh well…taking getting rid of the radical was worth it.

-Jelanen

[mith]

Now, would boiling increase the oxidation rate?

[victor]

I don’t think that it will increase the oxydation rate.

[MrMistery]

Thanks for setting me straight. I was getting to the vitamins chapter anyway, so wish me happy reading… Hope after that i will be less stupid 😀
Regards
Andrew

[Jelanen]

Yes, boiling could increase the oxidation rate since the kinetics of a reaction can be influenced by heat. I understand there is some controversy on the matter of how much heat degrades L-ascorbate, but I’m not gonna get into it cause I really don’t care.

-Jelanen

[novacasanova]

Does the ordinarily acidic lemon juice become alkaline in humans after ingestion like the lemons themselves do? Why(not)?

[mith]

no, because if you read the previous posts you’ll see that it isn’t alkaline.

[novacasanova]

Sorry for going off topic here…

If a food’s potassium, sodium, or calcium content is higher than its phosphate, chloride or sulpher content then it’s considered alkaline producing when ingested. That’s why lemons are alkaline producing.

Why wouldn’t juice derived directly from lemons likewise be alkaline producing? Are the alkaline producing contents of lemons within the skin?

[novacasanova]

quote MrMistery:

i don’t think vitamin C can be oxidised, because it is an antioxidant. At high temperatures(i.e.- when you cook it) the cells burst and most of the vitamin C evaporates

Vitamin C has to be oxidized before it crosses the blood brain barrier via glucose transporters.

[mith]

I’m sorry I haven’t heard of that rule before, where did you read it from?
I think that rule doesn’t work very well since it would seem to ignore organic acids such as acetic acid which is composed of C,H, and O none of which are mentioned.

[sashell4817]

it has been suggeted that the sourness of citrus fruits is dependent on its citrus acid content,which experiment can i use to justify or refute this claim?

[barrybert]

Who said that lemon is Alkaline? If lemon is alkaline, I wouldn’t drink the juice because it must be very bitter..he2…as you said that lemon has pH from 2 – 2.5 and the substances that is contained in the lemon are vitamin C (Ascorbic acid)…but, you said that even though it’s acid but inside human body it will become alkalic…um, how about the theory of the buffer liquid that our body have? Actually it’s not making it more alcalic but the buffer liquid of the body maintain the acid stage inside that.

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