Macadamia is a genus of four species of trees in the flowering plant family Proteaceae.[1] [2] They are indigenous to Australia, native to northeastern New South Wales and central and southeastern Queensland specifically. Two species of the genus are commercially important for their fruit, the macadamia nut (or simply macadamia). Global production in 2015 was 160000t.[3] Other names include Queensland nut, bush nut, maroochi nut, bauple nut and, in the US, they are also known as Hawaii nut.[4] It was an important source of bushfood for the Aboriginal peoples.
The nut was first commercially produced on a wide scale in Hawaii, where Australian seeds were introduced in the 1880s, and for more than a century they were the world's largest producer.[5] [6] South Africa has been the world's largest producer of the macadamia since the 2010s.
The macadamia is the only widely-grown food plant that is native to Australia.[7]
Macadamia is an evergreen genus that grows 2– tall.
The leaves are arranged in whorls of three to six, lanceolate to obovate or elliptic in shape, 60– long and 30– broad, with an entire or spiny-serrated margin. The flowers are produced in a long, slender, and simple raceme 50– long, the individual flowers 10– long, white to pink or purple, with four tepals. The fruit is a hard, woody, globose follicle with a pointed apex containing one or two seeds. The nutshell ("coat") is particularly tough and requires around 2000 N to crack. The shell material is five times harder than hazelnut shells and has mechanical properties similar to aluminum. It has a Vickers hardness of 35.[8] [9]
| Image | Scientific Name | Distribution |
|---|---|---|
| Macadamia integrifolia | south east Queensland and northern New South Wales | |
| Macadamia jansenii | Queensland | |
| Macadamia ternifolia | Queensland | |
| Macadamia tetraphylla | Queensland | |
Nuts from M. jansenii and M. ternifolia contain cyanogenic glycosides.[10] [11] The other two species are cultivated for the commercial production of macadamia nuts for human consumption.
Previously, more species with disjunct distributions were named as members of this genus Macadamia.[2] Genetics and morphological studies published in 2008 show they have separated from the genus Macadamia, correlating less closely than thought from earlier morphological studies.[2] The species previously named in the genus Macadamia may still be referred to overall by the descriptive, non-scientific name of macadamia.
The German-Australian botanist Ferdinand von Mueller gave the genus the name Macadamia in 1857 in honour of the Scottish-Australian chemist, medical teacher, and politician John Macadam, who was the honorary Secretary of the Philosophical Institute of Victoria beginning in 1857.[15]
The macadamia tree is usually propagated by grafting and does not begin to produce commercial quantities of seeds until it is 7–10 years old, but once established, it may continue bearing for over 100 years. Macadamias prefer fertile, well-drained soils, a rainfall of 1000-, and temperatures not falling below 10C (although once established, they can withstand light frosts), with an optimum temperature of 25C. The roots are shallow, and trees can be blown down in storms; like most Proteaceae, they are also susceptible to Phytophthora root disease. As of 2019, the macadamia nut is the most expensive nut in the world, which is attributed to the slow harvesting process.[16]
A Macadamia integrifolia / M. tetraphylla hybrid commercial variety is widely planted in Australia and New Zealand; Dr. J. H. Beaumont discovered it. It is high in oil but is not sweet. New leaves are reddish, and flowers are bright pink, borne on long racemes. It is one of the quickest varieties to come into bearing once planted in the garden, usually carrying a useful crop by the fourth year and improving from then on. It crops prodigiously when well pollinated. The impressive, grape-like clusters are sometimes so heavy they break the branchlets to which they are attached. Commercial orchards have reached 18kg (40lb) per tree by eight years old. On the downside, the macadamias do not drop from the tree when ripe, and the leaves are a bit prickly when one reaches into the tree's interior during harvest. Its shell is easier to open than that of most commercial varieties.
A pure M. tetraphylla variety from Australia, this strain is cultivated for its productive crop yield, flavour, and suitability for pollinating 'Beaumont.'
A South African M. integrifolia / M. tetraphylla hybrid cultivar, it has a sweet seed, which means it has to be cooked carefully so that the sugars do not caramelise. The sweet seed is usually not fully processed, as it generally does not taste as good, but many people enjoy eating it uncooked. It has an open micropyle (hole in the shell), which may let in fungal spores. The crack-out percentage (ratio of nut meat to the whole nut by weight) is high. Ten-year-old trees average 22kg (49lb) per tree. It is a popular variety because of its pollination of 'Beaumont,' and the yields are almost comparable.
A M. integrifolia / M. tetraphylla hybrid, this is a rather spreading tree. On the plus side, it is high yielding commercially; 17kg (37lb) from a 9-year-old tree has been recorded, and the nuts drop to the ground. However, they are thick-shelled, with not much flavour.
In 2018, South Africa was estimated as the leading producer of macadamia nuts, with 54,000 tonnes out of global production of 211,000 tonnes.[17] Macadamia is commercially produced in many countries of Southeast Asia, South America, Australia, and North America having Mediterranean, temperate or tropical climates.
The first commercial orchard of macadamia trees was planted in the early 1880s by Rous Mill, 12km (07miles) southeast of Lismore, New South Wales, consisting of M. tetraphylla.[18] Besides the development of a small boutique industry in Australia during the late 19th and early 20th centuries, macadamia was extensively planted as a commercial crop in Hawaii from the 1920s onward. Macadamia seeds were first imported into Hawaii in 1882 by William H. Purvis, who planted seeds that year at Kapulena.[19] The Hawaiian-produced macadamia established the well-known seed internationally, and in 2017, Hawaii produced over 22,000 tonnes.[20]
In 2019, researchers collected samples from hundreds of trees in Queensland and compared their genetic profiles to samples from Hawaiian orchards. They determined that essentially all the Hawaiian trees must have descended from a small population of Australian trees from Gympie, possibly just a single tree.[21] This lack of genetic diversity in the commercial crop puts it at risk of succumbing to pathogens (as has happened in the past to banana cultivars). Growers may seek to diversify the cultivated population by hybridizing with wild specimens.
Nuts from M. jansenii and M. ternifolia contain cyanogenic glycosides.
Macadamia allergy is a type of food allergy to macadamia nuts which is relatively rare, affecting less than 5% of people with tree nut allergy in the United States.[22] Macadamia allergy can cause mild to severe allergic reactions, such as oral allergy syndrome, urticaria, angioedema, vomiting, abdominal pain, asthma, and anaphylaxis.[23] Macadamia allergy can also cross-react with other tree nuts or foods that have similar allergenic proteins, such as coconut, walnut, hazelnut, and cashew.[24] The diagnosis and management of macadamia allergy involves avoiding macadamia nuts and their derivatives, reading food labels carefully, carrying an epinephrine auto-injector in case of severe reactions, and consulting a doctor for further testing and advice.
Macadamias are toxic to dogs. Ingestion may result in macadamia toxicity marked by weakness and hind limb paralysis with the inability to stand, occurring within 12 hours of ingestion.[25] It is not known what makes macadamia nuts toxic in dogs.[26] Depending on the quantity ingested and the size of the dog, symptoms may also include muscle tremors, joint pain, and severe abdominal pain. In high doses of toxin, opiate medication may be required for symptom relief until the toxic effects diminish, with full recovery usually within 24 to 48 hours.
Macadamias are also toxic to cats, causing tremor, paralysis, stiffness in joints and high fever.[27]
Raw macadamia nuts are 1% water, 14% carbohydrates, 76% fat, and 8% protein. A 100-gram reference amount of macadamia nuts provides 740 kilocalories and are a rich source (20% or more of the Daily Value (DV)) of numerous essential nutrients, including thiamine (104% DV), vitamin B6 (21% DV), other B vitamins, manganese (195% DV), iron (28% DV), magnesium (37% DV) and phosphorus (27% DV).
Compared with other common edible nuts, such as almonds and cashews, macadamias are high in total fat and relatively low in protein. They have a high amount of monounsaturated fats (59% of total content) and contain, as 17% of total fat, the monounsaturated fat, omega-7 palmitoleic acid.[28]
The trees are also grown as ornamental plants in subtropical regions for their glossy foliage and attractive flowers. The flowers produce a well-regarded honey. The wood is used decoratively for small items.[29] Macadamia species are used as food plants by the larvae of some Lepidoptera species, including Batrachedra arenosella.
Macadamia seeds are often fed to hyacinth macaws in captivity. These large parrots are one of the few animals, aside from humans, capable of cracking the shell and removing the seed.[30]