Sciencemadness Discussion Board

Effect of microwave heating on Ascorbic acid molecule

Xrpdguy - 22-1-2020 at 03:46

Hello there, we all know that Ascorbic acid is thermally labile and after cooking the amount of this vitamin becomes lower. Frying is the worst method, after its boiling.
Now I am curious about microwave heating (cooking). Internet says its the best method because of saving the most of nutrients (the research was based on calculating the amount of vitamin c in broccoli and pepper before and after cooking).

Here is my question: Which program can calculate the total energy of all bonds in the molecule of vitamin c (or any other)? I have tried with ChemDraw 3D but I am not sure about the result (8.4831 kcal/mol) is pretty low.

After that how I can calculate the maximum energy and time of microwave radiation which is not strong enough to break the molecule bonds?

My idea was to convert power to kcal (400W = 0.096 kcal/s) so according to ChemDraw, heating with 400W can be applied a max 88.4 sec.


Metacelsus - 22-1-2020 at 04:35

Quote: Originally posted by Xrpdguy  
Hello there, we all know that Ascorbic acid is thermally labile and after cooking the amount of this vitamin becomes lower. Frying is the worst method, after its boiling.
Now I am curious about microwave heating (cooking). Internet says its the best method because of saving the most of nutrients (the research was based on calculating the amount of vitamin c in broccoli and pepper before and after cooking).

Here is my question: Which program can calculate the total energy of all bonds in the molecule of vitamin c (or any other)? I have tried with ChemDraw 3D but I am not sure about the result (8.4831 kcal/mol) is pretty low.

After that how I can calculate the maximum energy and time of microwave radiation which is not strong enough to break the molecule bonds?

My idea was to convert power to kcal (400W = 0.096 kcal/s) so according to ChemDraw, heating with 400W can be applied a max 88.4 sec.


To calculate bond energies you need to use a specialized computational chemistry program (such as Gaussian09 or something similar).

But microwaves don't directly break bonds. They just heat up stuff (by exciting rotational energy modes). Therefore you can simply compare cooking methods by temperature and time.

Swinfi2 - 22-1-2020 at 05:59

You can approximate the energy of bonds in a molecule just by adding up the bonds by type eg: 1 (C=C), 6 (C-O), 1 (C=O), 4 (C-C), 4 (C-H), 4 (O-H). All the average bond strengths are in tables online. But be cautious the method is kinda fast and loose, and can be pretty bad when resonance is involved. But useful for rough calculations.

Xrpdguy - 22-1-2020 at 10:26


[/rquote]

But microwaves don't directly break bonds. They just heat up stuff (by exciting rotational energy modes). Therefore you can simply compare cooking methods by temperature and time.

[/rquote]

I know, but if put only ascorbic acid in the microwave will it get melted?
And how high energy can I apply before it becomes carbon?

The mechanism of degradation is by free radicals.

unionised - 22-1-2020 at 11:22

YOu don't need to know the total bond energy in ascorbic acid.
You just need to know the energy needed to break the weakest bond.
That's probably (roughly) equivalent to somewhere in the near UV.
(ascorbic acid is white- it does not absorb visible photons and IR or MW photons are not going to have enough energy to break bonds).
So microwaves can't get anywhere close to breaking the bonds.
Any degradation during microwaving must be thermal.

If the broccoli had higher levels of vit C after microwaving rather than other cooking the reason must be a combination of things like not washing it out with boiling water and not scorching it by baking.