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Macadamias – Australia’s contribution to the world’s fruits and nuts

Considering the thousands of edible tropical and sub-tropical fruits known worldwide, the USDA has suggested that only 300 of these are commercially important and less than 100 contribute to most production and consumption. Of fruits from Australia, the macadamia nut is the only one that makes these lists. Possible reasons for this laggard performance include firstly that our continent is the driest and has been relatively isolated botanically for a long time. As a consequence we don’t have anything like the diversity and profusion of edible and palatable species found in Asia and the Americas. Secondly, our indigenous people have had a nomadic lifestyle for thousands of years that did not gradually improve the stock by selection of occasional superior trees as happened more often elsewhere. As a result, our limited native bush foods have remained basically in their original unimproved forms, and consumption of the best of them remains largely at the fringes of society.

However despite these negatives, numerous surveys and personal evaluation have usually judged the macadamia to be the tastiest of all the commercial nuts. They taste good raw and even better roasted, but most of the world’s crop is eaten in more processed forms such as chocolate-coated kernels, nut-impregnated chocolates, muffins, pies etc. It was the Hawaiians who first realised their commercial potential, did the development work on them over several decades to produce superior cv’s and then built up a successful industry. Only then did we belatedly decide to get operations going here on our own native plant!

Macadamias have an extremely strong shell which is more difficult to crack than other tree nuts. For the home user this usually means that many kernels are fractured during preparation. Like all nuts, un-broken kernels command higher retail prices than fragments and commercial crackers can give a better proportion of unbroken nuts than that achieved by the home-user. As with cashews which have a toxic oil removal problem, much of the high cost of purchase is due to quality kernel preparation. However, if you only want to savour a few a day, the high retail price provides a good incentive to grow your own, given a few broken kernels is really neither here nor there – you still get to enjoy these delicious things and they store quite well in-shell if kept dry.

So macadamias are almost universally recognised as strong players on the pleasure front. But do they also have favourable nutritional qualities that can be of benefit in our current Western dietary habits which underlie so many chronic disease problems, given many of these have progressed to epidemic proportions? The following is a summary of a study (J. Nutrition (2003) 133: 1060–1063) by a research group at the University of Newcastle NSW illustrating that macs have substance and aren’t just a ‘pretty face’. It concerns unhealthy blood cholesterol levels which predispose to cardiovascular disease (CVD), the biggest cause of mortality in Australia.

This study was conducted to assess the cholesterol-lowering potential of macadamia nuts. Seventeen hypercholesterolemic men (mean age 54 y) were given macadamia nuts (40–90 g/d), equivalent to 15% energy intake, for 4 wks. Plasma total cholesterol, LDL-cholesterol, HDL-cholesterol, triglycerides and homocysteine (a CVD risk factor) concentrations and the fatty acid composition of plasma lipids were determined before and after treatment. Plasma monounsaturated fatty acids (MUFAs) were elevated after intervention with macadamia nuts. Essential polyunsaturated fatty acid concentrations(n-6 and n-3 PUFAs) concentrations in plasma were unaffected by macadamia nut consumption. Plasma total cholesterol and LDL-cholesterol concentrations decreased by 3.0 and 5.3% respectively, and HDL cholesterol levels increased by 7.9% in these hyper-cholesterolemic men after macadamia consumption. Plasma triglyceride and homocysteine concentrations were not affected by treatment. Macadamia consumption was associated with a significant increase in the relative intake of MUFA and a reduced relative intake of saturated fatty acids and PUFA. This study demonstrates that macadamia nut consumption as part of a healthy diet favourably modifies the plasma lipid profile in hypercholesterolemic men despite their diet being high in fat.

The quantity of nuts used in this study is higher than most people would consume per day but other studies since have considered more reasonable lower levels with comparable results. Although the ‘bad’ LDL-cholesterol only decreased by 5.3% on average, importantly the ‘good’ HDL-cholesterol increased by the larger margin of 7.9%, so that the more relevant ratio for predicting health benefits, namely LDL/HDL, changed to a far greater degree than either alone. Macadamias have the highest fat content of all the important tree nuts (76%) and 80% of this is MUFAs. Collectively, all these nuts have qualitatively similar effects on CVD and underlying risk markers, with the relative magnitude of effects depending on their particular makeup. No one of them is superior in all nutrient components so that the best strategy for health benefits is to eat mixed nuts rather than believing that any one of them is clearly superior. Three pillars of modern nutrition are balance, moderation and variety, and simplistic belief in the magic or sufficiency of ‘super foods’ that should be single-mindedly consumed is flawed.

To illustrate the varying strengths of nuts and the lack of any that are tops all round, consider how macadamias stack up against some of the others regarding familiar macro- and micro-nutrients (expressed per 100g edible fresh food): they have 7.9g protein cf 25.8g in peanuts, 6g fibre cf 10.4g in hazelnuts, 12.1g saturated fat cf 3.9g in almonds, 58.9g MUFA cf to 8.9g in walnuts, 1.5g PUFA cf 47.2g in walnuts, 1.3g n-6 essential fatty acids (eg linoleic acid) cf 20.6g in pecans, 0.2g n-3 (alpha-linolenic acid) cf 9.1g in walnuts, 11mcg folate cf 145mcg in peanuts, 0.54mg vitamin E cf 27.1mg in almonds, 85mg calcium cf 248mg in almonds, and 130mg magnesium cf 376mg in brazil nuts.

With other less familiar plant phytochemicals, macadamias have roughly half the level of phytosterols cf pistachios and peanuts, and total antioxidant phenols in pecans is 28 times that in macadamias. All nuts including macadamias are very good nutrient-rich foods, and positive health effects that have been reported in addition to the study above on CVD risk factors include type 2 diabetes, reduction in overweight and obesity, gall stones, hypertension, cancer and inflammation.

Fruits and nuts may be processed into various forms for a number of reasons such as cultural practices, improved storage properties, improved palatability and taste, aesthetics and nutrition. If nutrition is important to you with the nuts you eat, then it should be recognised that preparation and processing can have a considerable influence. Most fruits have an unequal distribution of nutrients throughout the edible parts, usually with higher levels found towards the exterior. Simply removing apple skins before eating (eg for cooking in apple pie where heat/time causes its own additional losses) can result in loss of over 90% of phytochemical antioxidants.

With nuts this surface concentration can be similarly extreme eg in walnuts 95% of antioxidants are in the brown seed coat, almonds are often blanched to remove their brown coat for aesthetic reasons and so lose 80% antioxidant capacity, and peanuts minus their red-brown seedcoat lose 70%. Then once the kernel is isolated, simple dry roasting can further degrade nutrition (although usually enhancing flavour) eg with pistachios the antioxidant loss can be 44%. More extensive processing into popular products such as peanut butter loses another 50%. As with many foods, raw is frequently best nutritionally unless there’s a specific reason to modify, eg inhibit, microbial spoilage, reduce anti-nutrient factors or soften tough fibrous tissue.

Roasting macadamia nuts

The macadamia had its origins in Australia and is now enjoyed as one of the premier nuts worldwide.  Like all nuts and many seeds, the flavour is enhanced with roasting, but it has to be done carefully to avoid burning.

The following process should give you a delicious product to enjoy:

1. Nuts should be collected as soon as possible after falling to avoid any mould growth.
2. Remove the husk as soon as possible.  Some trees will do this for you before falling but for others you will have  to do it.
3. You can eliminate nuts with malformed, unsatisfactory or absent kernels by immersing them in a bucket of water.   Discard the ones that float.
4. Place in a shaded location on flat trays, spreading them out to maximise air exposure, and allow to air dry for at  least a few weeks depending on the prevailing temperature.  The ideal time to crack the hard shell is when you can  shake the nut and hear the kernel rattling inside.
5. Crack the very hard shells.  Short of having  a properly designed macadamia nut cracker with a hopper for cracking  large quantities, this will inevitably be one by one.  There are many crackers advertised on the Internet, but one  of the best for the home grower is TJs, designed and made in Queensland.  They sell for  around $100  according  to whether you can get one second hand or new.  Many others are cheaper but may not be quite as convenient or  effective if you’ve got a couple of kilos to do.  The final  ‘no-cracker’ approach of course is to use a hammer or vice,  but this can be very slow and result in most kernels being broken, sometimes into many pieces. It is also hazardous,  as the nuts break explosively and throw sharp shrapnel in all directions.
6. Spread the kernels out in a single layer on a flat baking tray and place in the oven or food dryer, probably on your  lowest setting, hopefully to achieve about 37°C.  Let them dry at this temperature for at least two days, then increase  the temperature to 60°C and continue heating till kernels are crisp.  The purpose of these two steps is to ensure the  kernels are sufficiently dry.  If not followed, the retained moisture will caramelise or brown when completing the  next higher temperature roasting stage.  The goal is to achieve the roasting with kernels remaining a creamy  colour.
7. Finally, roast kernels at 135°C for 20-30 minutes with occasional stirring until a creamy light golden colour is obtained.
8. They are now ready to be enjoyed as is, or by adding whatever fancies you choose. Store smaller quantities in an  air-tight jar.
9. For longer term storage, you could vacuum seal or freeze.