In the popular sense the term “nut” is applied to a seed or fruit with an edible kernel inside a brittle or hard shell; the botanical definition is somewhat more complicated.
– remains have been found in archaeological sites dating back to before 10,000 BC. They were an important food item for the hunter-gatherers and were brought into cultivation at a very early date; it has been suggested that nut harvesting might have taken place before cultivation of cereals in agriculture. Most nut species are now cultivated, although Brazil nuts are harvested entirely in the wild. Collection of nuts from some other species in the weld (e.g. hazel) still takes place but this is usually as an addition to the normal diet.
Nut species are cultivated in temperate climates (e.g. hazel, filbert, sweet chestnut, almond, walnut species, pistachio, pecan) and in warmer climates (e.g. peanut, coconut).
A large number of the nut species are trees, but peanut is an annual growing to a height of 15-60cm (24 in).
Nut kernels are consumed raw, roasted, or salted, or in a great variety of products, e.g. nut butters, confectionery, curries, soups, stews, snack foods, sweetmeats, flour, bread, porridge, poultry stuffing, fritters, animal feed, cake, ice-cream, sauces, puddings, and meat and fish dishes. Immature green kernels of some species (walnut, almond) may be eaten as such or pickled.
Some nut products have achieved a considerable reputation, e.g. “groundnut (peanut) chop or stew” in West Africa, marrons glace (sweet chestnut) in France, and pesto sauce (pine).
As with all food analyses of plant products, the results will vary according to the environment, the variety (cultivar), the method of analysis, and some other factors.
By and large, the major nutrients in nut kernels are protein and fat (oil). The exception is chestnut, where starch in the dried kernel could be as much as 60%, and the amounts of protein and fat are low. Protein quantities vary according to species, with as much as 30% in peanuts, but the average seems to be about 15%. High-protein peanut flour has been used to supplement milk beverages in India and to raise protein levels in bread and biscuits.
With the exception of coconut, which contains saturated fat, nut species contain unsaturated fat (polyunsaturated and monounsaturated). The amounts are high, ranging from about 50% (e.g. peanut, pistachio) to about 70% (e.g. Brazil nut, pecan, pine, macadamia, walnut).
Nuts are good sources of minerals – calcium, iron, magnesium, phosphorus, potassium, sodium, zinc, copper, manganese, and selenium. Naturally, a food analysis of salted nuts will give a high reading for sodium. Vitamins present are B1, B2, B6, niacin, pantothenic acid, folate, and E. Vitamin C, not normally present in nuts, is found in green (immature) walnuts (1,300 – 3,000 mg per 100 g).
In reasonably recent times, interest has been shown in other substances present in nuts – flavonoids (quercetin and kaempferol) in almonds; resveratrol (a phytoalexin) in peanuts; sterols in pecans.
Nuts are a good source of dietary fiber (6-12 g per 100 g) and can be important commercial oilseeds (peanut, almond, coconut). The nutritional value of hazel and filbert is essentially the same, as is the nutritional value of the various walnut species.
Nuts have been used in medicine; e.g. tea made from peanuts has been used in Mexico and Peru to stimulate milk production in nursing mothers, and in Brazil as a nerve tonic; in Vietnam almonds are employed to cure dysentery.
However, as a health food, nuts provide a range of important nutrients such as protein, fat, minerals, vitamins, and dietary fiber. High intakes of saturated fat have been associated with heart disease and certain cancers; therefore, nut unsaturated fat is to be considered “healthy”. The role of antioxidants in dealing with free radicals and heart disease has already been described. Nuts include antioxidants such as vitamin E, flavonoids, and certain trace elements such as selenium; Brazil nut is a rich source of this element. In animal and human experiments, plant sterols (which reportedly occur in some nut species) have been shown to reduce total cholesterol and its low-density lipoprotein fraction. Resveratrol, found in peanut skins and kernels, is also found in grape skins and red wine. The “French paradox” postulates that those who consume red wine regularly; even with a high fat diet, have less incidence of coronary heart disease than inhabitants of England and Wales. Resveratrol, a phytoalexin, could contribute to this situation, but other substances, e.g. flavonoids, might be responsible.
It has previously been pointed out that unsaturated fat and antioxidants are good for general health. In addition, the presence of other substances, e.g. flavonoids, render nuts a good health food. Also, as in all other plant materials, nuts are cholesterol free. However, as has been stated, nuts are rich in fats, and about 30 g (1 oz) of kernels provide 200 kcal – a sizeable fraction of a normal day’s requirements. Therefore, there must be some control of the amount of nuts in a diet. There are those who claim that nuts constitute a “satiety” factor – they limit the amount of food consumed; however, there is sometimes a feeling that nuts are “moreish”.
A number of human epidemiological studies in the USA claim that frequent nut eaters have a lower risk of heart disease than non-consumers. Also, other studies claim that nuts in the diet reduce total blood cholesterol and the low-density lipoprotein fraction while not affecting the high-density lipoprotein fraction.
One of the difficulties associated with the consumption of nuts is that they may cause allergies – peanut is notorious in this respect. In extreme cases peanuts can bring about anaphylaxis, which can be fatal or near fatal. Those who are aware that they are allergic to peanuts obviously must avoid them, but it can be difficult because a wide range of products do contain peanuts or peanut oil, e.g. biscuits, cakes, ice-cream desserts, cereal bars, curries, and may others. Peanut oil is used in cosmetics. Discussion has taken place about the relative safety of unrefined and refined peanut oil. It is the peanut protein that is responsible for the allergy and therefore ea refined oil might well be safe, but those who are allergic should consider the wisdom of ingesting the oil. Considerable efforts are now made in food outlets and by manufacturers to warn of the danger of nuts.
Under certain conditions, peanuts may become infected with the moulds (fungi) Aspergillus flavus and A. parasticicus. These moulds can produce chemicals (mycotoxins) known as “aflatoxins”, which are carcinogenic. The situation is being carefully monitored in a number of countries (e.g. UK, USA, and others) because infected nuts must not enter the food chain.