From Plant Resources of South-East Asia No 2: Edible fruits and nuts, PROSEA Foundation
by R. A. Stephenson, and C. W. Winks
Taxon
Macadamia integrifolia Maiden & Betche
Protologue
Proc. Linn. Soc. N.S. Wales 21: 64 (1897).
Family
PROTEACEAE
Chromosome Numbers
2n = 28
Synonyms
Macadamia ternifolia F. v. Muell. var. integrifolia (Maiden & Betche) Maiden & Betche (1899).
Vernacular Names
Macadamia, Queensland nut, Australian bush nut (En).
Origin and Geographic Distribution
The
macadamia is the only commercial food crop indigenous to Australia,
originating in the rain forests of south-eastern Queensland and
north-eastern New South Wales. The crop was first developed in Hawaii,
with trees imported in the 1880s, but not until the 1950s did the
Hawaiian success encourage other countries to grow macadamia on a
commercial scale.
Close to the equator macadamia is grown
commercially in East Africa. In South-East Asia some scattered trees
occur, but in northern Thailand trial plantings have been made on a
series of promising sites.
Uses
Hawaii dominates the world
market, with over 70% of production (20 400 t nuts-in-shell (NIS) from
8100 ha in 1987). Australia supplies 10—15% of world markets (5000 t
NIS from 6000 ha) and Costa Rica is similarly poised to commence
production on a large scale (500 t from 2000 ha). Production in Africa
appears to have levelled off (1700 t NIS from 4250 ha). The United
States is the largest market for macadamias and demand is increasing.
Properties
The quality
of the kernel is related to its oil content and composition. Nuts are
mature when the kernels accumulate 72% or more oil, as determined by
specific gravity (SG). Kernels also contain 10% carbohydrates, 9.2%
protein which is low in methionine, 0.7% minerals, particularly
potassium, and niacin (16.0 mg/kg), thiamine (2.2 mg/kg) and riboflavin
(1.2 mg/kg). The characteristic, subtle macadamia flavour is probably
due to volatile compounds, the major ones being similar to those in
other roasted nuts.
Description
Large
spreading tree, attaining a height of 18 m and a spread of 15 m. Leaves
in whorls of 3, oblong to oblanceolate, 10—30 cm x 2—4 cm, glabrous,
coriaceous, irregularly spiny-dentate when young, in later stages
entire; petiole 5—15 mm long; in the axil of each leaf 3 buds are
arranged longitudinally, of which usually only the top bud shoots out,
making a sharply acute angle with the trunk.
Racemes axillary on
mature new growth or on leafless older shoots, pendulous, 10—30 cm long
with 100—500 flowers; flowers in groups of 2—4, about 12 mm long,
creamy-white; pedicels 3—4 mm long; perianth tubular with 4 petaloid
sepals; stamens 4, inserted 2/3 above the base of the tube; pistil
about 7 mm long, ovary superior, 1-carpellate with 2 ovules. Fruit a
globose follicle, 2.5—4 cm in diameter; pericarp fibrous, ca. 3 mm
thick. Seed (nut) mostly 1, globular, with a smooth, hard, thick (2—5
mm) testa enclosing the edible kernel.
Growth and Development
Seeds
germinate readily and seedling growth, initially slow, gathers momentum
as saplings produce a series of extension growth flushes in a year. The
juvenile phase lasts for 7 years or more, but grafted trees come into
bearing after 3 years.
Floral initiation takes place when
temperatures drop and trees become quiescent in autumn, the optimum
temperature being 18°C. The initials remain dormant for 50—96 days and
the racemes extend after a rise in temperature and some rain. In
Australia high yields are associated with a strong and early spring
flush prior to anthesis, followed by minimal shoot growth throughout
the 6-month nut development period. At the end of that period there is
a late summer flush; meanwhile nuts may be retained on the tree for a
further 3 months, but gradually they fall.
The flowers are
protandrous, the anthers dehiscing 1—2 days before anthesis, whereas
the stigma does not support pollen tube growth until 1—2 days after
anthesis.
Pollination is by insects; most cultivars are at least
partly self-incompatible. Planting pollinator trees and introducing
bees are important for good fruit set. Fruitlets continue to be shed up
to 2 months after bloom.
Other Botanical Information
Besides
Macadamia integrifolia, Macadamia tetraphylla L.A.S. Johnson also
produces edible kernels and is also cultivated. It differs from
Macadamia integrifolia by its always spiny leaves, usually occurring in
whorls of 4, up to 50 cm long, often sessile (petiole up to 3 mm), pink
flowers and a rough-shelled nut. It occupies a more southern area than
Macadamia integrifolia. Sterile natural hybrids exist in an overlapping
area of distribution in Australia.
Although Macadamia tetraphylla
has excellent culinary qualities, its kernel is not favoured by
processors because it has a higher residual sugar content which
caramelizes on roasting.
Macadamia hildebrandii Steenis is a native
species from Sulawesi (Indonesia). [Celebes nut (En), pérandè (Tado),
tinapo (Toradja), balo molaba (Tobela)]. Presumably the nuts are edible
and the species may be better adapted to a wet tropical climate. Its
fruit is indehiscent, but the seed has a thin soft testa.
Ecology
The
xerophytic characteristics of the macadamia tree, including the
sclerophyllous leaves and proteoid roots - dense clusters of rootlets
formed to explore poor soils, especially soils low in phosphorus -
suggest adaptation to relatively harsh environments. However, the
conditions required for optimum production may be quite different from
those for survival.
The macadamia occurs naturally in the fringes of
subtropical rain forests. It appears to tolerate only a narrow range of
temperatures (optimum during the growing season is 25°C) and
temperature is the major climatic variable determining growth and
productivity. Trees in South-East Asia grow fairly well but flower and
fruit sporadically throughout the year. In East Africa, also close to
the equator, orchards are planted at elevations of 1000—1600 m, in
areas with a prominently seasonal climate, leading to a synchronous
resumption of growth and flowering after a cool overcast season.
Abnormal tree growth, low yield and poor nut quality have been noted in
Africa at higher altitudes with little sunshine during the main season.
The
mature macadamia is capable of withstanding mild frosts, but only for
short periods. The minimum annual rainfall for macadamias is about 1000
mm, well distributed through the year. The brittle wood makes trees
susceptible to wind damage.
Macadamia can be grown in a wide range
of soils but not on heavy, impermeable clays and saline or calcareous
soils. The trees are most suited to deep, well-drained soils with good
organic matter content (ca. 3—4% organic carbon), medium cation
exchange capacity and pH of 5.0—6.0.
Propagation and planting
Early
plantings of macadamias were established from seedlings, but these were
slow to commence bearing and variable in yield and nut quality.
Successful grafting was achieved by using vigorously growing, healthy,
recently mature stock and scion material, 1—1.3 cm in diameter and by
cincturing scion wood 6—8 weeks before grafting to promote accumulation
of carbohydrates. The rootstocks are seedlings 9—12 months old.
The
current trend is for high density, hedgerow plantings which maximize
early yields. Inter-row spacings of 10 m are most common (7 m if
mechanical pruning is carried out). The distance between trees within
rows should be 4—5 m, depending on cultivar and growing conditions.
Windbreaks should be well established prior to planting.
Husbandry
It
is preferable for the tree to develop a strong central leader.
Corrective pruning will often be required to prevent later limb
breakages. Upright cultivars such as 'Kau' (formerly known as selection
no 344) and 'Mauka' (741) require less pruning as they are naturally
more wind resistant. Mulching is recommended for young trees (when the
trees come into bearing it interferes with nut collection). Some weed
control measures are generally necessary. Fertilizer management should
be guided by leaf and soil analysis, the phenological cycle and yield.
Macadamia trees appear to be sensitive to nutrient deficiencies and/or
imbalances, and positive responses to N, P, K, Zn, B, S, Mg, Fe, and Cu
have been observed.
Diseases and Pests
In
their place of origin macadamias are attacked by more than 150 pest
species, although parasites and predators usually provide considerable
control. Insects which commonly reduce yields include macadamia flower
caterpillar (Homoeosoms vagella), fruit spotting bug (Amblypelta nitida), banana spotting bug (Amblypelta lutescens), macadamia nutborer (Cryptophlebia ombrodelta), and macadamia felted coccid (Eriococcus ironsidei). Any of these has the capacity to destroy much of the crop during severe infestations. The macadamia twig-girdler (Neodrepta luteotacetella) and macadamia leafminer (Acrocercops chionosema)
destroy foliage and may therefore reduce yield indirectly. Many of the
minor macadamia insect pests cause damage sporadically. An integrated
pest management system for control, involving monitoring of pest
population levels and delaying spraying until action levels are reached
is recommended. Rats are particularly fond of macadamia nuts and can be
a problem in some areas. In comparison with other fruit trees,
relatively few diseases are serious in macadamia.
Harvesting
Nuts
fall from the tree naturally in autumn and early winter when they are
mature. They are gathered by hand or swept into windrows and picked up
by machines; machine harvesting requires a smooth and clean orchard
floor.
Yield
Yields of 45 kg nuts-in-shell from better trees or an average of 3.2—3.5 t/ha per year are obtained in Hawaii.
Handling After Harvest
The
husk must be removed from the follicle as soon as possible to prevent
over-heating, mould development and deterioration in quality. The
husked nuts are then dried either artificially in silos or air-dried
(10% moisture) on wire racks on the farm. At the factory, nuts are
dried to 1—1.5% moisture for longer term storage, most efficient
cracking of the shell and more complete recovery of the whole kernels.
Drying is done in stages (at 52°C down to 4.5% moisture and then at
77°C down to 1.5% moisture) to avoid adverse effects on kernel quality.
The kernels can then be lightly roasted in coconut oil and salted or
packaged raw in vacuum-filled foil-laminate bags which prevent
development of rancidity. The product is then kept in cold stores.
Under these conditions, the kernels can be safely stored for a year.
Genetic Resources
Considerable
genetic diversity exists in the range of cultivars and in germplasm
collections in Hawaii and Australia. No thorough examination of
diversity has been completed in natural populations, which are under
threat owing to clearing and poor regeneration through rodent damage.
Breeding
Most
commercial cultivars were developed from seedling selection in Hawaii.
Of the 120 000 seedlings examined, less than 900 were given Hawaii
Agricultural Experiment Station (HAES) accession numbers, 13 were named
and 9 of these are planted in substantial numbers. Selection is based
on tree characteristics, high yield and superior nut quality.
Prospects
The
demand for most tree nuts is good and there is scope for per capita
consumption to be increased substantially. The world production of
macadamias is increasing steadily but is still less than 1% of world
nut production. Production is expected to treble within the next
decade. Within South-East Asia only the climates of northern Thailand
and eastern Indonesia appear to be sufficiently seasonal to provoke
good flowering and fruiting in macadamia, but this needs experimental
confirmation.
Literature
Allan, P., 1972. Use of climatic data in predicting macadamia areas. California Macadamia Society Yearbook 18: 77—87.
Cavaletto,
C.G., 1983. Macadamia nuts. In: Chan, H.T. Jr (Editor): Handbook of
tropical foods. Marcel Dekker Inc., New York. pp. 361—397.
Hamilton, R.A., Ito, P.J. & Chia, C.L., 1983. Macadamia: Hawaii's dessert nut. University of Hawaii Circular No 485. 13 pp.
Ironside, D.A., 1981. Insect pests of macadamia in Queensland. Queensland Department of Primary Industries, Brisbane. 28 pp.
Liang,
T., Wong, W.P.H. & Uchara, G., 1983. Simulating and mapping
agricultural land productivity: An application to macadamia nut.
Agricultural Systems 11: 225—253.
Shigeura, G.T. & Ooka, H.,
1984. Macadamia nuts in Hawaii: History and production. University of
Hawaii Research and Extension Series 039. 91 pp.
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