It’s the season of mists and mellow fruitfulness - not to mention chestnuts roasting on an open fire. But many of the “nuts” we eat are nothing of the sort

Any self-respecting pub quiz bore will tell you a peanut is a legume, a relative of the pea. If they are on top of their game, they will take pride in their certain knowledge that pineapples are, in fact, coalesced berries; there’ll be discussion of bananas no doubt. But, what of hips and haws? What about drupes? And, perhaps of more seasonal importance: what makes a “true nut”, in the strictest biological definition?

Facebook Twitter Pinterest Sweet chestnuts are tasty and also “true nuts”. Photograph: Richard Aspinall

I was pondering many of these questions as I roasted a dozen or so pleasingly-shaped chestnuts on our log burner, mindful of the prickly flesh that surrounds and protects them as they ripen on the tree. Is this the same as the husks of coconuts? What about the sweet, tasty flesh of the plum? Are these of the same biological origin? What about coffee beans? They’re not legumes, surely? What about the strawberry? It appears to wear its seeds on its exterior, whereas the blackberry cushions its seeds within multiple fleshy lobes, and rosehips … they just defy simple classification.

Understanding the biological origins of the seasonal harvest is, in part, an exercise in exploring an ever-expanding series of definitions, which demonstrate a divergence between simple, commonly-used culinary definitions, and the world of plant biology. Much of biology is about classification and categorisation of course, and arguably, in culinary terms understanding the origin of ingredients is irrelevant. Equally, it’s a self-evident truth that the birds and animals (which the plant kingdom is blatantly bribing to spread their genes with calorie-rich fruit) care not a jot, either.

Facebook Twitter Pinterest Acorns are biologically “true” nuts. Photograph: Richard Aspinall

To us humans though, our love of compartmentalisation has driven our desire to place species within groupings and in doing so we often find exceptions and specimens that defy simple pigeonholing. For example, not all plants rely on offering a meal to a passing vertebrate to persuade them to distribute the next generation. The coconut palm, for one, largely exploits ocean currents to spread its progeny, as do the various species of sea grasses. However, I digress.

Following pollination, either by wind, bird, bat or bee, plants have developed numerous ways to ensure the dispersal of their seed. Seeds can be defined as encapsulated plant embryos or as fertilised ovules; they are the next generation, in a nut-shell (or not, as the case may be - please pardon the pun). Seeds contain food reserves (endosperm) and rudimentary leaves (cotyledons). Cotyledons are either in a pair or singular, further classifying plants into dicotyledons and monocotyledons.

Facebook Twitter Pinterest Sloes, the fruit of the blackthorn, are drupes. Photograph: Richard Aspinall

Seeds are relatively simple structures, containing everything necessary for a plant to form, encased in a protective coat. Wondrous complications arise when the flower and its associated structures (depending on the plant family) undergo a range of modifications to encapsulate, augment and protect the seeds. Swellings and developments of differing parts of the flower that contain or support the seeds provide us with that catch-all culinary and biological term: “fruit”. From there, a great many shapes, sizes, colours and biological relationships follow.

Nuts

Nuts are dry things in shells, served at Christmas, found weeks later down the back of the sofa, right? Technically … no! Hazelnuts, acorns and chestnuts are true nuts. They are each formed when a fertilised flower’s ovary wall swells and hardens and, despite their diversity in form, shape, size and palatability, they share the same origins. They may be from different taxonomic groups, but their formation relies on the development of their parent flowers.

Facebook Twitter Pinterest Of significant value to many birds, haws are also drupes and to some, taste a little like avocado. Photograph: Richard Aspinall

A great many “culinary” nuts, are nothing of the sort. Almonds, for example, are in fact drupes, not nuts at all. Neither are cashews, pistachios and pine nuts. Many tree nuts are drupes, including walnuts and pecans (although confusingly these are known as drupaceous nuts as they difficult to categorise and are not true botanical nuts). Almonds, olives, peaches, coffee beans, cherries and plums are all classic drupes.

The seeds in drupes, are protected within fleshy three-layered husks or exocarps. In peaches and olives we value the exocarp, but in the case of almonds it is the seed contained within the endocarp, the layer which directly surrounds the seed, which is of culinary use. Coconuts are also drupes; the endocarp is the thick wood shell that surrounds the fleshy and liquid-filled seed.

Hawthorn fruits (haws) are also drupes, as are sloes. Of significant importance, ecologically these fruits provide food for countless migratory birds that visit the UK from Europe. Their stones are also of significant value to small mammals, which gnaw open the tough endocarp to reveal the nutritious seeds inside.

What about hips and berries?

Berries offer another source of confusion. Technically strawberries and blackberries are not berries, but aggregate fruits. A “proper” berry is formed from the outer layer of the ovary wall from a single flower, which forms the fleshy, and often nutritious pericarp which, in turn, encapsulates a seed.

Facebook Twitter Pinterest Frosty rosehips, which are rich in vitamin C. Photograph: Richard Aspinall

Blueberries are examples of simple berries. Blackberries are derived from multiple ovaries, whereas strawberries form, not from the ovule, but from the flower’s receptacle to which the ovaries are attached. The receptacle can be thought of as an extension of the flower stalk. The strawberry’s true fruits are the small pips, known as achenes, on its exterior, each containing a seed. The strawberry, then, is not a single fruit, but many fruits.

Facebook Twitter Pinterest Well-known as fruit, squashes and melons can be referred to as berries in the strictest sense of the word, but don’t try it at the grocers. Photograph: Richard Aspinall

Of a similar origin are apples and pears. These arise once more from a swelling of the receptacle, which in these flowers surrounds the ovary and ovules. As the apple ripens, the ovary wall forms the outer part of the core, surrounding the seeds. To add further terms to the growing list of definitions, fruits formed from enlarged receptacles, with tough central cores, are known as pomes, these include medlars, quince and rowan berries.

Rose hips, much revered for their vitamin C content, conversely contain their supply of achenes, within a structure called a hypanthium (this is the fleshy red structure we value so highly). The hypanthium is also sometimes known as the “flower tube”. Surprisingly perhaps, pomegranates are like rose hips biologically, and once more the hypanthium forms the outer wall of the aggregate fruit.

I should add that, whilst many wild fruits are edible, there are some that are deadly and others that are merely unpleasant. Confusingly, some parts of certain fruits are edible whilst other parts of the same fruit are deadly: yew fruits are a dangerous example. As a sensible precaution I would avoid eating any wild fruit unless you have correctly identified it to be edible.