After previously exploring colourful, enigmatic, poisonous and sadistic seeds, in his new blog, Wolfgang goes nuts about nuts.
I absolutely love nuts! They are healthy, taste great and being a Seed Morphologist, I obviously have a very special relationship with them. The latter is fuelled by the strange awareness that every time I eat a nut, I eat an embryo. Yuk? Well, read on!
To me, the most delicious nut of all is the cashew. No other nut can match its fine flavour and soft creamy texture. If you are also nuts about nuts, you will undoubtedly know cashews, even if you (incomprehensibly) prefer macadamias, Brazil nuts or pecan nuts. I wonder, though, how many people know the plant that cashew nuts come from and how unusual and amazing the actual fruit they are borne in looks like. Hence this blog...
The vibrantly colourful fruits of the cashew tree, Anacardium occidentale
Cashew apples and cashew nuts
Cashew nuts grow on trees with large, beautiful bright green leaves. Nowadays cultivated and naturalised almost all over the tropics, the cashew tree (Anacardium occidentale, Anacardiaceae) is originally native to the coastal plains of north-eastern Brazil, where it forms part of the so-called restinga* vegetation. Long before European colonisation in the sixteenth century, Brazilian Indians have relished the delicious fruits. Called ‘acajú’ by the members of the Tupi tribe, the name was converted by the Portuguese into ‘cajú’ and eventually became ‘cashew’ in English.
A cultivated cashew tree (Anacardium occidentale) in Brazil
Young cashew fruits with their stalks starting to swell, top right: flowers
From very humble, tiny, first white, then pink flowers arise rather large, brightly coloured (there are orange-red and yellow varieties) and somewhat weird-looking fruits. When ripe they resemble a very soft pear with a hard, kidney-shaped nut tucked in at one end. The pear-like part, also called ‘cashew apple’ (whoever came up with that name has never seen an apple next to a pear!) is extremely juicy and sensitive to pressure which is why you can’t find them in our supermarkets. In Brazil I have seen them offered in roadside stalls where they were carefully displayed in egg trays.
Ripe cashew fruits on the tree
'Cashew apples' are so soft, they need to be treated like raw eggs (here in a roadside stall in Lindóia, Brazil)
Hard sell methods aside, what’s most unusual about the cashew fruit is that the fleshy bit is not formed by the swollen ovary of the flower as in ‘normal’ fleshy fruits. Rather, the cashew apple is formed by the hugely swollen stalk of the flower which is why you wouldn’t find any seeds in it. If you want to find the seed you must open the fertile part that is formed by the ovary of the flower, namely the hard, kidney-shaped ‘appendix’, better known as the ‘cashew nut’.
A longitudinal section of a 'cashew apple' proves its origin from the flower stalk; there are no seeds, just flesh
Ever had to stuff your mouth with a cotton ball soaked in milk?
Sounds delicious but here’s a word of warning to those who love to travel in the tropics. If you ever encounter a fruit-laden cashew tree, restrain your enthusiasm, at least for the ‘nutty’ bit. Whilst the amazingly succulent cashew pear, err... apple, is harmless and best enjoyed by sucking out the sweet juice and discarding the stringy-fibrous residue, the shell of the cashew nut is poisonous owing to an acrid phenolic oil, called urushiol. Urushiol causes dermatitis which is why the cashew nut was once also called ‘blister nut’. The same nasty oily chemical is found in other members of the Anacardiaceae. Among these close relatives of the cashew tree are notorious plants such as the poison ivy (Toxicodendron radicans), poison sumac (Toxicodendron vernix) and poison oak (Toxicodendron pubescens & T. diversilobum) but also mango (Mangifera indica; urushiol is found in the sap and fruit peel!).
If you try to crack a fresh cashew nut with your teeth, you will soon be blessed with a painful blistering rash in and around your gob (if it’s any comfort, the pain can be soothed by stuffing your mouth with a cotton ball soaked in milk). That’s also the reason why you will never find unshelled cashew nuts on supermarket shelves whereas other nuts like walnuts, hazelnuts, almonds and pistachios are quite often sold ‘whole’.
How to prepare 'home-grown' cashew nuts: roasting the nuts in a fire destroys the poison and the charred shells can easily be removed
Why cashew nuts are expensive
Because of the problems caused by the toxic shell of the nut, Latin Americans, West Indians and West Africans have long used only the succulent ‘cashew apple’ making it into wine and refreshing beverages, similar to lemonade, such as the Brazilian ‘cajuado’. However, on a worldwide scale the seed of the cashew tree is still the main commercial product, despite the laborious cleaning process that makes cashew nuts one of the most expensive of all nuts (at least in the UK macadamias cost a lot more than cashews). The safest way to enjoy ‘home-grown’ cashew nuts is to roast them in a fire and then remove the charred shell as shown in the pictures above.
In the wild, the brightly coloured, 5-10 cm long cashew apple acts as a tasty reward for the animals it needs for seed dispersal. Fruit bats, monkeys and bats pick the fruits to feed on the yellow- to scarlet-coloured apple but discard the poisonous nut, leaving the seed inside unharmed.
Cashew nuts off the supermarket shelf as we all know and love them
About eating embryos
Every nut contains a seed and as such, logically, also an embryo (i.e. a little baby plant that ‘hatches’ from the seed upon germination). Therefore, every time you eat a nut, you eat an embryo. But this is not like eating a microscopically small embryo when you eat a chicken egg. The edible part of the nuts we eat as nibbles consists of nothing else but the embryo. Here’s an experiment for you that proves my point: next time you eat a cashew try pulling it apart and you will see that it splits into two halves, the cotyledons of the embryo. There is even a tiny shoot axis with miniscule leaves in between them!
A whole cashew nut, botanically an embryo, and one split in half, revealing its true nature: a baby plant with two leaves (cotyledons) and a shoot axis
Oh, one more thing....
We botanists use the term ‘nut’ in a very different and much more rigorous sense than ‘ordinary people’ do in their everyday language. For the food industry, chefs and 'regular' consumers who enjoy a tasty nibble, any large edible kernel that requires forceful liberation from a hard shell before consumption is unscrupulously addressed as a ‘nut’. In a botanical sense, a ‘nut’ is only a nut if it consists of nothing but the mature ovary of an indehiscent (= non-opening) fruit with a hard, dry shell, usually harbouring a single seed. This is true for hazelnuts (Corylus avellana, Betulaceae), walnuts (Juglans regia, Juglandaceae), pecan nuts (Carya illinoiensis, Juglandaceae), acorns (Quercus spp., Fagaceae), and unshelled peanuts (Arachis hypogaea, Leguminosae), though the latter often (and annoyingly to botanists!) contain more than one seed. Other culinary ‘nuts’ are, in fact, the stones of drupes (= stone fruits), such as unshelled almonds (Prunus dulcis var. dulcis, Rosaceae), pistachios (Pistacia vera, Anacardiaceae), and, in all honesty, also cashew nuts (Anacardium occidentale, Anacardiaceae). I didn’t want to make things too complicated but, working for Kew, I finally have to break the truth about the cashew nut. Although hardly recognizable as such, the shell of a cashew nut does indeed display the three defining layers of a drupe: an outer skin, a very thin, quick-drying but nevertheless soft middle layer, followed by the dominating hard, woody stone.
As a final blow to the culinary nut-concept, it has to be unveiled that unshelled Brazil nuts (Bertholletia excelsa, Lecythidaceae), macadamias (Macadamia integrifolia and M. tetraphylla, Proteaceae), ginkgo nuts (Ginkgo biloba, Ginkgoaceae) and pine nuts (Pinus pinea, Pinaceae) are purely seeds in a botanical sense because their shell consists of the seed coat.
- Wolfgang -
All photos by Wolfgang Stuppy
*Restinga: a distinct type of tropical and subtropical forest found on acidic, nutrient-poor soils at the the Atlantic coast of Brazil.
- "Difficult Seeds Project" - Anacardium occidentale
- Black Walnut on Kew's species pages
- More about the work of the Millennium Seed Bank Partnership
- Read Wolfgang's other blog post
The Millennium Seed Bank Partnership works with over 80 countries worldwide as we work towards our current goal of collecting and storing seeds from 25% of the world's wild plant species. To complete such a target requires a wide range of skills and expertise including training, research, seed processing, database management and fundraising.
- capacity building
- wet tropics
- focus families
- useful plants
- seed banking
- around the world
- South East Asia
- at risk
- new species
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