Onion cooking: why does adding garlic change the color and add bitterness?

[Philip-Thai]

I have a problem.

I need to cook a lot, (20 or so kilos,) of finely diced onions at a time in making a commercial sauce. They are gently fried in olive oil.

Long slow cooking of onions alone produces the desired sweet caramellisation. However if I add garlic at or near the start of cooking, the onions turn a greenish colour and take on an unpleasant, slightly bitter flavour.

Does anybody have any idea why this occurs and is there anything I can do, short of leaving the garlic out of this process, to overcome this problem?

Edited April 22, 2024 by Smithy
Adjusted title for clarity (log)

[jende]

I have a problem.

I need to cook a lot, (20 or so kilos,) of finely diced onions at a time in making a commercial sauce. They are gently fried in olive oil.

Long slow cooking of onions alone produces the desired sweet caramellisation. However if I add garlic at or near the start of cooking, the onions turn a greenish colour and take on an unpleasant, slightly bitter flavour.

Does anybody have any idea why this occurs and is there anything I can do, short of leaving the garlic out of this process, to overcome this problem?

←

Garlic burns easily and produces the bitter flavor you described. I would just add the garlic once the onions are fully cooked, and let it go five minutes or so. Stop cooking just when the garlic is turning a bit translucent and smells good, but before it browns.

[Qwerty]

What kind of pan are you using?

[Philip-Thai]

I have a problem.

I need to cook a lot, (20 or so kilos,) of finely diced onions at a time in making a commercial sauce. They are gently fried in olive oil.

Long slow cooking of onions alone produces the desired sweet caramellisation. However if I add garlic at or near the start of cooking, the onions turn a greenish colour and take on an unpleasant, slightly bitter flavour.

Does anybody have any idea why this occurs and is there anything I can do, short of leaving the garlic out of this process, to overcome this problem?

←

Garlic burns easily and produces the bitter flavor you described. I would just add the garlic once the onions are fully cooked, and let it go five minutes or so. Stop cooking just when the garlic is turning a bit translucent and smells good, but before it browns.

←

Neither the garlic, nor the onions, ever brown. They are cooked over a very gentle heat.

When cooked to the stage I desire, the onions turn translucent and then a slight yellow colour. This takes place over about two hours cooking.

If I add garlic, the reaction occurs within minutes.

I should point out that I am a food professional with over thirty years experience in the business.

[Philip-Thai]

What kind of pan are you using?

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I have tried many different pans, different shapes and metals. from stainless steel, heavy iron, copper and bronze. Paella pans, woks and different types of large saucepan. I have a well equipped restaurant kitchen and over 30 years experience in the business.

[C. sapidus]

I have a problem.

I need to cook a lot, (20 or so kilos,) of finely diced onions at a time in making a commercial sauce. They are gently fried in olive oil.

Long slow cooking of onions alone produces the desired sweet caramellisation. However if I add garlic at or near the start of cooking, the onions turn a greenish colour . . .

The greenish color could be a manifestation of the blue garlic phenomenon (click). Blue garlic happens most commonly in the presence of acid and when using young garlic.

[Philip-Thai]

I have a problem.

I need to cook a lot, (20 or so kilos,) of finely diced onions at a time in making a commercial sauce. They are gently fried in olive oil.

Long slow cooking of onions alone produces the desired sweet caramellisation. However if I add garlic at or near the start of cooking, the onions turn a greenish colour . . .

The greenish color could be a manifestation of the blue garlic phenomenon (click). Blue garlic happens most commonly in the presence of acid and when using young garlic.

←

Thank you for this. I followed your link and on down a path of extra research.

It seems both onion and garlic contain anthocyanins, which can turn blue under acidic conditions.

I now need to find out what chemical reaction occurs when onion, garlic and olive oil are gently heated together. How does it differ from cooking the onion and garlic separately.