Tough nuts

(Super)markets often sell the wrong varieties

By Bart Braun

Some people literarily have a bad taste in their mouth after eating pine nuts, a sensation that could last for days. Leiden geneticist Ben Zonneveld has designed a quick test to identify the wrong kernels.

Pine Nut Syndrome, or Pine Mouth: once experienced, never forgotten. It is a metallic taste that arises a day or two after eating pine nuts, giving everything you eat a bitter, “metallic” taste that can last up to two weeks. It is an unpleasant experience for anyone, and a disaster for restaurant critics.
This metallic taste – the medical term is “cacogeusia” or “metallogeusia” - is a relatively new phenomenon: the first description linking it to pine nuts only dates as far back as 2001. Not everybody suffers from it, and not all pine kernels produce this effect.
If you go to a supermarket to buy apples, you will find some five metres of shelves displaying Elstars, Jazzes, and Fijis – all variations of the same variety of Malus domestica. But a packet of pine kernels might contain anything. According to the United Nation’s Food and Agriculture Organisation FAO, there are 29 varieties of pine (Pinus) that have edible kernels, and any one bag could contain several kinds.
In his study, hidden away in the depths of the labyrinthine Van Steenis building, geneticist Ben Zonneveld tips four pine nuts from a tube. After careful examination, you might notice that one is longer than the rest, while the other three bear a superficial resemblance to each other, although they are actually three different varieties. “These are the four types available in the Netherlands.”
Zonneveld works at the National Herbarium and is specialised in determining the genome size - the amount of DNA per cell – of plants. Plant experts can identify tens of thousands of varieties on the grounds of certain features: the number of stamens, the shape of the leaf, the colour of the flower, etc. The genome size is also distinctive and has a number of useful properties: you can identify a plant using any part of that plant, so you do not need to wait until it flowers and a small piece of the plant will suffice, so a botanist at the other side of the globe can simply send a leaf by post. It is a rapid method of identification: genome size can be determined in just one day and in the case of some plants, it is a useful way of identifying two very similar varieties.
And this applies to pine nuts, as Zonneveld discovered. In the scientific journal Plant Systematics and Evolution, he describes his study into the genome size of the varieties available in shops and compares this to the amount of DNA in the twelve varieties of pine that produce the most common types of pine nut. He collected the nuts from Leiden supermarkets and asked friends from abroad to send him packets of pine kernels. Then he collected samples of the twelve types of Pinus from specialised arboretums so that he could match the right tree to the right type of kernel.
You cannot identify all varieties of pine by means of their genome size, but luckily, this method can be used for the four commercially available varieties of kernels. Zonneveld identified them as P. gerardiana (the long variety from Pakistan), P. koraiensis, P. pinea from Southern Europe and P. armandii from China. This last variety is not included on the FAO’s list of edible varieties.
It is easy to understand how a new variety could end up on Western plates: pine nuts did for food what U2 did for pop music – they became popular at some point in the eighties, you could get them everywhere in the nineties and now they refuse to disappear. Pine nuts are found in pesto sauces and in salads and sprinkled over meat. A whole generation of cooks cannot look at a plate of food without thinking: “Hmm, I know just what this meal needs: extra pine nuts.” The demand for them rocketed, but it takes years for pine trees to produce pine cones and consequently, people have been tempted to sell a new type of pine nut.
Zonneveld and his wife, who are both susceptible to Pine Mouth, tested the various sorts themselves. “We were very cautious to start with”, he recalls. “I didn’t really want to try them. First I would have one, then two, etc.” He would experience the metallic taste after six nuts, and found that it was caused by Pinus armandii. If you suffer from Pine Nut Syndrome, that is the type you need to avoid. They all taste the same, but you will notice the difference the next day.
By now, he can identify them without his lab equipment. “If you look closely, you will notice that the P. armandii are oval, smaller and greyish and the P. koraiensis are pear-shaped, yellower and larger. The armandii kernels always have spots at the tips but so do the koraiensis kernels some times, so that criterion is less reliable.” He holds up a tub from a local supermarket, which contains two types mixed together and points out the differences. “Nowadays, I can tell the difference really easily and when I’m at the market, I can see that a quarter of the nut sellers sell the wrong types of pine nuts, but at the same price, obviously.”
Import companies who want to make sure their customers won’t get a metallic taste but who do not spend time examining pine nuts can use the genome size identification method. “It is quite a straight forward test when done with the right equipment. If you give me ten samples today, I can give you the results tomorrow.”

Crappy taste

As Pine Mouth is a relatively new syndrome and not really a serious problem in medical terms, very little research has been done. It exists, some people are more sensitive to it than others, and it seems to depend on the precise variety of pine nut consumed. But we do not know exactly how pine nuts cause it, although there is an intriguing theory about it. 
People have five kinds of taste bud on their tongues: sweet, sour, salt, bitter and umami. The last one is sensitive to the amino acid glutamate, but does not have any relevance for this article – the ones we talking about here are the receptors for bitter foods. They are not only found on your tongue, but in your intestines as well. Bitter substances are often toxic, and these papillae help your body to decide whether or not the contents of your large intestine should be removed quickly - as diarrhoea.
American toxicologist Gregory Möller has suggested that these taste buds in your intestines have something to do with Pine Mouth. A certain acid in the pine nuts stimulates the extra production of gall, a bitter substance needed to digest fat. And you taste this gall with your special bitter receptors in your intestine. Your brain sweeps all this information into a big heap so the bitter taste in your food is actually the bitter taste of excrement in the making. To put it optimistically: the wrong type of pine nuts puts you in touch with your unchartered inner self.

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