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The Quest for the Perfect Leaf. Eucalyptus leaves and essential oil production.

May 26, 2020

I am not an expert at producing essential oil, maybe one day I will be as I’m spending a lot of time practicing the art of distilling,  experimenting with different leaf-picking, prepping and stuffing strategies and, when I get the time and manage to post them off, evaluating the chemical analysis (GC traces) from the few ml’s of oil that represent the fruits of our labour (the tree and I).

My husband and I purchased (white words) a 50 acre patch of beautiful box woodland out in the central west of NSW about six years ago and because of that I have access to a range of different gum (Eucalyptus) trees and hopefully once our bush regeneration efforts have really kicked in, we may also get some interesting tea trees and acacias (wattles) to experiment with.  As interesting as essential oil production is to me I am not looking to become a scaled-up oil producer. My aim in this venture is both personal and profession and centres around my core life purpose which is to elevate (maybe only inside of myself as I have limited influence over others) the value we as humans attribute to nature, the natural rhythm of life and the relationship between the resources it provides us and that which we give to it.  I just choose to apply that value to cometic science and as such, use that platform as a way to interpret what I see, feel, hear and learn.

Personally I’ve always found my mind becomes both quieter and sharper when immersed in the natural world.  I have both mind (very, don’t worry) vision and hearing impairments plus an ADHD wired brain which do contribute to the overwhelm and  palpable disadvantage I feel when placed in rushed, man-made spaces.  However, out amongst the woodlands of the central tablelands of New South Wales I tap into a whole other set of senses that see me pick up on the tiniest of fungi, the slightest whiff of an animal in the distance or the subtle changes in bird song as a predator (usually me unless I stay still long enough) comes closer.  This feeling of heightened awareness is addictive and is rewarded when nature opens up its arms and shows you more and more of its secrets.  What was once brown and green becomes different Eucalyptus species, what used to just be a stringy bark all at once becomes an ecosystem in its own right with soil fungi, birds, bees and echidna scratchings digging up the ants nests.  It is this level of attention and intent I bring to my essential oil practice and it is with the whole ecosystem in mind that I pursue this phase of my cosmetic chemistry journey.  As temporary custodians (much better word than owners) of this tiny patch of the planet I feel that interpreting this land through my cosmetic chemist eyes is the best way for me to protect it and share its value with the world.

The Quest for the Perfect Leaf. 

So recently I’ve become more interested in understanding how the age of a Eucalyptus leaf affects oil yield, understanding that the answer may be anywhere from ‘it doesn’t’ to ‘hugely’ of course.  What I’m finding out so far, and there is a lot of reading, comparing and looking for gaps and cross-overs (research, that’s the research part), is that it is likely not so much of a question of how old the leaves are but at what development stage they have reached.

I am no plant specialist either so bare with me while I excitedly share the rudimental understanding of where I’ve got to on this quest to identify the perfect leaf.

Eucalyptus leaves through their growing stages and ages. 

These leaves come from the same tree, one of the Eucalyptus trees that is growing on our driveway at Fox Hill Hollow.  The tree is currently about 2 meters tall and is in pretty good condition.

Eucalyptus trees grow all year round which is something that took a bit of a while adjusting to as I come from a place where trees have a natural break each autumn/ winter, dropping their leaves and going to sleep – maybe Australia is the land of 24/7 partying after all.  The top left are the tiny baby new growth and the bottom right is the fully-grown leaf shape.  According to this page in the book ‘Eucalyptus Ecology’ by Jann Williams and John Woinarski (and I don’t know if it applies to all Eucalyptus species) the average life span of a leaf is 18 months so maybe these leaves represent a 2-3 month development stage? If they do that would be great (but would also be a fluke as I’m only going by feel here).

As you may have noticed either just in these leaves or with Eucalyptus in general, their leaves can change remarkably between their baby and adult state.   While it is not uncommon for plants to have a juvenile and adult state, Eucalyptus really do take it to extremes at times.  When I first moved here I used to spend ages staring at trees trying to work out if some weird hybrid was developing, if maybe two trees were entwined to look like one or if I was just going crazy. Turns out that this leaf metamorphisis is just what Eucalyptus trees do and there are some important features attached to that ‘coming of age’ process when seeking out the ‘perfect’ leaf.

So, while you can see a change in size, shape and colour, you may not be able to tell that the leaves on the top row are all soft while the leaves on the bottom row become progressively stiffer due to a process called lignification.  Lignification is something I know very little about but it’s basically where a higher plant such as this Eucalyptus species produces an organic polymer (family of chemicals) called Lignin.  Lignin is a carbon-based chemical and is found across many plants, usually in their woody material where it helps to strengthen cell walls and keep the plants circulation functioning well.  The Eucalyptus tree starts off with very soft leaves but doesn’t take long to toughen them up in its bid to survive.  From what I’ve read so far, rather than the laying down of Lignin being just a biological-clock triggered process, it can also happen in response to trauma.  A good article on this can be found here.

Australian plants have to deal with lots of things nibbling and attacking them from fluffy animals right through to creepy crawly things and microbes.  They also have to deal with a rather harsh sun for long periods of time.   One of the tools the tree has to defend its self is its ability to lay down this hard polymer and stiffen its leaves up.  Stiffer leaves are harder to eat and are more resilient so it makes sense to invest in this protection.  However, for the essential oil hunter, this defence mechanism does seem to make it slightly harder to access the essential oil meaning that the perfect leaf for essential oil production may well be one that is fully grown but not fully lignified (as stated in the article I’ve linked to just below).

Here is a really good overview of the way different ages and stages of leaves have influenced essential oil yields. I do wonder though if the study that found younger leaves to contain more essential oil (as a percentage yield) were actually finding that or was it just a case of the oil being more accessible? I say that because other papers I’ve read have found the oil yield to be similar throughout the tree’s age.  As essential oil is produced by the plant for its benefit – it’s often designed to be bitter to the taste so animals give up eating it and contains anti-microbial actives so again, pests either die or are put off attacking it.  It makes little sense to leave your soft fresh leaves vulnerable on two counts – less pesticide (essential oil) and less lignification/ cuticle wax.  Anyway, that’s what I’m trying to dig into more deeply now.

Cuticle or epicuticular wax is another thing that may reduce the ability of an essential oil distiller to access the oil within the cell.   This paper is about floriculture but it does look into the variability of waxes between Eucalyptus species and how the wax levels vary with leaf age.   This paper is also very interesting as it looks at the chemistry and differences in thickness of wax between two common Eucalyptus types.   

Maybe I’m looking to find the wrong word but I can’t quite work out yet if the amount of wax (percentage wise) changes form young to older leaves, I have read that the structure of the wax develops over time but that’s not quite the same thing.  To me, so far, it looks like the epicuticular wax is performing a role similar to either a raincoat or sunscreen or both for the leaf.  The chemistry of this wax is definitely determined by the species of Eucalypt but I feel it likely that the amount of wax expressed is there in response to the environment rather than age although I may be wrong. Further, over time, these waxes have been shown to change.  Maybe the waxes get a chance to polymerise…  This happens sometimes with essential oil chemistry, some compounds react with each other over time, turning what was a liquid essential oil into a thick resin-like substance.  Anyway, the long and short of this part is that I don’t feel at this point, that the wax protection layer is as much of a barrier to finding the perfect leaf as the lignification is – as odd as that may seem given that wax is waterproof and we distill essential oils using steam to which the waxes may be somewhat resistant… I feel the waxy layer could be a barrier to distillation of some species, but within a species less of factor in choosing to distill young, middle-aged or old leaves.  What looking at this has given me an appreciation for is the potential role that a solvent other than steam/ water could play in improving oil yield in some species.  Maybe leaves could have a cold wash with a surfactant to remove some of the wax before distillation?  I’m not sure how that would go,  solvents like hexane are often used in the papers I’ve linked to but most people don’t want mention of that in their oil distillation and I completely understand why.

So where are the perfect leaves then?

When I consider waxiness, leaf size and leaf stiffness I’m edging towards the perfect leaves being those that have just taken on their adult form and are well developed although not fully grown but still retain the softness and flexibility of their youth – pre lignified leaves.  I don’t know what age that is for the trees I have but I know how they look and feel – probably top right and/or bottom left (just getting harder). Another thing to consider is leaf physical condition – as stress toughens the leaves up, maybe prematurely, I feel it is important to stress that undamaged leaves that haven’t been too stressed by sun, wind, frost or nibbling things are going to be easier to work with than less pristine leaves.

Testing my theories. 

The next step now is for me to try and collect enough of the leaves at those intermediate stages to distill in a meaningful and scientific way (or as much as possible) and then compare that with a fully grown leaf distillation round.  A rough go at that may give me a bit more insight into the perfect leaf or it might just send me batty. Either way, I’ll end up with a bit more essential oil and a good amount of hydrosol to play with which doesn’t sound too bad at all.  This may take some time to do but I’ll see what I can come up with.

In the meantime the perfect leaf for oil distillation is not too soft or too hard, not too big or too small and hopefully in great condition.  My guess is that it is also probably around 9-11 months old (well, for my type that is, yours may be different).



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