Changes of red colour in jam

[IvanV]

I just opened a jar of my red gooseberry jam, from last summer's red gooseberries. It was minimally cooked, no added water, as makes nicer jam, and it was red when I made it. Not very dark red, but perceptibly red. After 6 months in the cupboard, it's barely different in colour from the yellow gooseberry jam. Where did the red go?

I made some blood orange marmalade recently. It was a wonderful deep red colour before I cooked it, from the juice inside the oranges, the peel itself was mostly orange. But it came out only a little darker than normal marmalade, with a gold-russet tinge. Unfortunately with marmalade you have to boil it up for a couple of hours to soften the peel, before you add sugar. It lost the red colour in the initial boil, before adding sugar. Where did the red go? Would it be better if I kept some red juice back to add after the boiling period?

Curiously the opposite happens with quince jam. That's another one that needs long boiling up to soften it. It helps to include quite a lot of lemon juice. Eventually it turns red. Takes about 3 hours. That's always been another mystery. Why does quince eventually go red as you boil it up?

Less colour excitement with my redcurrant, blackcurrant, mulberry, damson and dark plum jams. Those are all minimally cooked, no added water.

[tom p]

Where you moving the jar towards you? Is it possible that it was blue-shifted?

[monkey]

I just opened a jar of my red gooseberry jam, from last summer's red gooseberries. It was minimally cooked, no added water, as makes nicer jam, and it was red when I made it. Not very dark red, but perceptibly red. After 6 months in the cupboard, it's barely different in colour from the yellow gooseberry jam. Where did the red go?

I made some blood orange marmalade recently. It was a wonderful deep red colour before I cooked it, from the juice inside the oranges, the peel itself was mostly orange. But it came out only a little darker than normal marmalade, with a gold-russet tinge. Unfortunately with marmalade you have to boil it up for a couple of hours to soften the peel, before you add sugar. It lost the red colour in the initial boil, before adding sugar. Where did the red go? Would it be better if I kept some red juice back to add after the boiling period?

Curiously the opposite happens with quince jam. That's another one that needs long boiling up to soften it. It helps to include quite a lot of lemon juice. Eventually it turns red. Takes about 3 hours. That's always been another mystery. Why does quince eventually go red as you boil it up?

Less colour excitement with my redcurrant, blackcurrant, mulberry, damson and dark plum jams. Those are all minimally cooked, no added water.

Chromophores in the jam/pre jam mass absorb light in a wavelength dependent way, so I'd start out by finding out what's in them that's giving them their colour. Then you can work out what might cause those to change. Might be heat, a reaction, or both - some reactions might be slow, as in the case of gooseberry, it seems.

I am not good at chemistry, so that's about as far as I can go with this.

Where you moving the jar towards you? Is it possible that it was blue-shifted?

[IvanV]

Where you moving the jar towards you? Is it possible that it was blue-shifted?

The speed jam gets eaten in this household... Mainly in yogurt.

[IvanV]

The red colour in blood orange, google tells me, is anthocyanins. That's the same class of chemicals that makes everything else I have mentioned red. Also red wine, blueberries, autumn leaves, etc. Google also tells me that quince goes red because anthocyanins form when you cook it up. Anthocyanins can change colour with changes in pH. Probably why the lemon juice matters in the quince jam.

Red wine tends to lose colour over time. So probably some of these other things tend to lose colour over time also. But it is less noticeable when there is a lot of it. So that might explain why the only slightly red gooseberry jam lost its colour more easily than blackcurrant jam. Still seems a bit weird that the marmalade lost the anthocyanins by 3 hours boiling up, while the quince gained the anthocyanins by 3 hours boiling up.

[science_fox]

It's been a very long time since I studied chemistry! Somewhat generalised. Take with a pinch of salt, which might work in quince but probably not jam.

It might be helpful to think of why anthocyanins are red.

They're red because they have a delocalised electron ring with energy gap sufficient to absorb most visible frequencies except red, (https://en.wikipedia.org/wiki/Anthocyan ... absorbance) and so they appear red to the eye, each compound has a very specific range. Chemical changes to the electronic structure (such as forming or breaking bonds) will change this absorption gap and so change their colour. Some chemical reactions are much faster than others, and some perhaps far away from the main absorption sites will have less impact, than those directly affecting it.

pH changes directly add charge and can be very fast. Hydrolysis, oxidation and reduction can all be very slow as the electron ring adds chemical stability to the molecule.

The Quince may be forming the right compounds either by bringing two halves together, or by losing a part that was shifting the colour out of the visible range. It is the same process as the marmalde losing colour by either breaking the molecule or adding extra parts to it.

[Grumble]

Time to have a chat with my dad I reckon. His phd was on organic dyes.