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When Garlic Gets the Blues

Aug 06, 2011

That food changes colour when it is cooked is not in itself surprising.   For those that cook, we witness it every day:  a blast of heat can intensify broccoli’s green and drive white carbohydrates brown.  But when your food turns blue, well, that one always gets a bit of a gasp.  

Partly it’s because blue is an under represented colour on our dinner plate.  Sure the Ameraucana chicken lays blue shelled eggs and the East Coast has the blue crab, but that’s just window dressing.   The stuff that actually makes it to your mouth is not blue, and so as a colour we are not used to eating it.   Go ahead and scrounge your kitchen for blue food.   After tossing out the packaged drink mixes and that bottle of blue ketchup from 2004, you’ll probably be left with the sole blue item that qualifies as a real food: the blueberry.   Though even here the claim to the colour blue is a bit dubious; add a little heat and our little round friends begin to look decidedly purple.    


So when your off-white coloured garlic turns blue, it is usually an experience you want to share.  The first time this happened to me, I was living in Savannah, Georgia, making a pizza in a picnic-table sized kitchen.  Being married to a garlic lover I thought tossing some minced raw garlic over the toppings and tomato sauce at the last moment before going into the oven would really boost the flavour.   What I was not expecting was for our pizza to emerge from the oven looking like a preschooler’s blue splatter paint project.   We ate the pizza of course, and after repeating the process a couple of more times through out the year, I moved away from the last minute garlic kick.  While I can attest to its edibility, it still remains, in my opinion, inherently unappetizing. 


What happened that night on top of my pizza, while new to me, is well known to those in the garlic industry.  Specifically if you are involved in garlic processing – that is, trying to sell bottles of pre-minced, or pureed garlic – then the problem at hand is usually referred to as ‘garlic greening’.  Why ‘greening’?  Well it turns out that the blue pigment formed in the minced garlic is not particularly stable (it has trouble keeping all of its atoms together) and so some of it falls a part a little bit.  In doing so, some of it changes from a blue colour to a yellow one – and voila, these two mix and green is born!

Given the economic interest in selling (uncoloured) processed garlic, a lot of research has been undertaken in attempts to understand the conditions required for pigment formation and what the chemical structures of the pigments are.   It’s been a tricky business – the pigment genesis is incredibly complicated and, it appears, that multiple blue pigments with different structures may be responsible for the end colour.  Still let’s delve a little deeper.   What do we know about garlic chemistry?

Well, garlic is a really dynamic food in the sense that so much of its flavour comes from change.   To unlock garlic’s full potential it needs to be crushed or minced, freeing its cells of their components and allowing them to mix and, importantly, to react.  You see, some of the more ‘garlic-y’ chemicals are not actually found in their delectable forms in the whole clove.   That these chemicals interact so pungently with out taste buds speaks partly to their reactivity, or instability, which is something that the garlic needs to keep in check if it wants to store these chemicals.  And it is this reactivity that underlies the colourful transformation.  If these reactive garlic-y chemicals (called organosulfur compounds) are not whisked away from the garlic, they can react with other neighbouring molecules in the garlic’s tissue.  And if the right precursor molecules are present, pigments can form.   Overall, most scientists consider pigment formation to involve two major processes, with the first step being the creation of the flavourful organosulfur compounds.  

This first step is an enzymatic one (it involves special proteins) and it is the friend of anyone who has every minced a clove of garlic freehand or wielded a garlic press.  The crushing of garlic cells frees a specific enzyme, allinase, from its vacuole prison and brings it into contact with various water soluble sulphur compounds.   Two of these, alliin and isoalliin, are both critical to eventual pigment formation.     Allinase converts these players into the volatile - and oil-soluble - organosulfur compounds that have put garlic on the flavour and natural medicine map.  From here, the second part of the process begins.  These organosulfur compounds then react with various free amino acids found in the garlic clove.  It is a multi-step and complicated process which has been revealed through making model systems of ‘mock garlic’.   Fun stuff!  Eventually there is a build up of the blue pigments which then breakdown into yellow to mix with the left over blue to give green.    These pigments also tend to be water soluble, and so if the coloured garlic is added to a water based sauce, they should also diffuse through out, and may even go unnoticed.    


The next question then becomes – why after cooking with garlic a million times, had I never noticed it turning blue before?  There are two key factors – one is a bit more relevant to our every day in the kitchen (the cooking process) than the other (the history of the garlic).  But let’s deal with the history issue as that probably explains why my first run in with blue garlic occurred while living in the very hot Southern US.    It turns out, that of the two flavour precursor compounds, alliin and isoalliin, the isoalliin has a more profound effect on pigment formation but it is usually a very minor component of most garlic varieties.   Thus most cooking experiences involve garlic that does not have enough of the ‘right stuff’ to turn blue or green.  But this can be changed – which is why a bulb of garlic’s past custody matters.  

Scientists are now able to explain what residents of Northern China have been taking advantage of for a long time: that cold storage, say refrigeration, of garlic bulbs over a few weeks significantly increases the amount of isoalliin in the garlic.  In Northern China garlic must be cold stored over winter to prepare it for making a green garlic condiment called ‘laba’.    So if you have ever experienced blue or green garlic then chances are you are working with a garlic bulb that has undergone cold storage at some point.   And if you have not had this colourful experience but really want to, you will need to start the process by storing your garlic in your fridge for a month or so.   

After making sure that your garlic has the ‘right stuff’ to turn blue or green, you’ll need to carefully consider the cooking process.   First, the reaction between the organosulfur compounds and the free amino acids proceeds fairly quickly at elevated temperatures and it benefits from acidic environments.     But the major point to ponder is easy to overlook – both types of molecules that we want to react must physically be present in the same environment for pigment formation to occur.   Normally when we cook with garlic it is added to oil which dissolves the oil-loving organosulfur compounds away from the water-loving free amino acids in the crushed tissue, and as the oil heats, it sends these molecules to into the air to give that garlic aroma.   So if you want to observe the colour change, you need to avoid oil.  Instead, opt for heating the crushed clove oil free – I recommend on top of a pizza like in my Savannah days.   A spattering of lemon or tomato juice may help the process, but you don’t want to add too much because as you may recall, these pigments can diffuse out of the garlic if surrounded by a watery sauce and you wouldn’t want to miss their appearance.   Bon appétit! 


Todd Barsby is a University-College Professor in the Department of Chemistry at Vancouver Island University in Nanaimo, BC.   He holds a Ph.D. in Natural Products Chemistry and is an avid cook.  Together with other VIU faculty he produces and hosts the science-themed news show Not Rocket Science on Nanaimo’s CHLY 101.7 FM and its contributing segment on food and chemistry, Atoms to Apples.  


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Posted by anita rafidi on
Thanks so much for writing this, I have always wanted to know the answer to that question...but no one I knew had ever experienced it.

But, I must admit the answer above was a bit much for me;

"because you're using cold garlic" would have sufficed...
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