By Joshua Kato
The Ministry of Agriculture Animal Industry and Fisheries (MAAIF) has selected macadamia as one of the money crops. According to Dr Samuel Mugasi, NAADS Executive Director, the nut has been selected because of its high value especially to the dollar markets. However, just like other crops, losses are made during harvesting and postharvest. In this article, we look at how to properly harvest macadamia.
In the month before the start of mature nut drop, ensure grass or weeds are under control and remove or use offset flail mulchers to chop up to ‘cornflake’ size pieces any significant quantities of unsound, old or immature nuts, leaf or foreign matter. Old, immature and pest or disease-damaged nuts left on the ground will reduce the quality of the harvest. To minimise the amount of material on the orchard floor requiring removal or chopping up, undertake pruning operations (side trimming and skirting) as soon as possible after the previous season’s harvest is complete. This ensures that prunings are well broken down by the time of the next harvest. When herbicides are used to control weeds, avoid spraying fallen mature nuts. Where it is necessary to use herbicides during the harvest season, spray where possible immediately after nut pick-up.
Nuts are harvested after they have fallen to the ground. Farmers can use mechanical harvesters. Hand harvesting off the ground is practised on some smaller orchards and where steep slopes preclude the safe use of mechanical
harvesters. It may also be necessary on occasions during extended wet weather. Most mechanical harvesters are of the finger-wheel type—a range of these is available to suit different orchard sizes and conditions. Clean mechanical harvesters before use to reduce contamination of harvested nuts. Also clean harvesters before shifting to a different orchard to avoid spread of diseases and weed seeds.
Harvest nuts at least every four weeks, particularly during extended wet weather or where nuts are exposed to direct sunlight. The less time the nuts are on the ground, the less the deterioration from mould, rancidity and early germination, and the better the kernel quality. Ensure pick-up is efficient to avoid nuts being left on the ground until the next harvest round. These may deteriorate and reduce the quality of the next pick-up. Keep nuts from different harvest rounds separate, particularly with early season harvests that often have high levels of immature nuts. It may be an advantage in the future to harvest, store and consign varieties separately. Where possible, harvest varieties with different processing characteristics separately.
Provided they are mature, nuts can be harvested directly from the tree. Tree harvesting is necessary with sticktight varieties such as Own Choice and Beaumont, unless ethephon is used to promote nut drop. For tree harvesting, the nuts are knocked, dropped or shaken from branches onto a mat spread below the tree. An indication of nut maturity is when the inside of the husk changes from white to brown. However, it is recommended that a sample of the nuts be first tested for maturity using the flotation test.
Unless you have access to processors who accept nut-in-husk (which is very limited), you will require on-farm facilities for postharvest handling including dehusking and drying. In planning and operating these facilities, there are three important considerations:
- The equipment must be designed and operated in accordance with legal standards under Work Place Health and Safety and other legislative provisions. For example, adequate lighting, machinery guards, ventilation and safety equipment needs to be provided to ensure worker safety.
- As dehusking nuts is a noisy operation, care needs to be taken to site postharvest handling facilities as far away from neighbours as possible, particularly where farms adjoin residential areas.
- Postharvest handling ystems must be designed and operated to prevent physical damage to the nuts, and reduce the risk of contamination and quality deterioration of the kernels. Important issues here include:
• Maintaining good hygiene and food safety practices. Keep the shed and equipment in a clean condition as dirty and poorly maintained equipment increases the risk of nut contamination from vermin and other pests. For the same reason, prevent birds, rats and other animals from entering the working areas. Provide containers for waste, including reject nuts, and frequently remove waste, disposing of it properly. Remove risks for nut contamination from either physical sources (for example bolts) or chemical sources (for example rat baits). Ensure all people handling nuts practice good hygiene.
• Careful design and maintenance of equipment. Nut sorting areas that are well lit and comfortable for workers improve the efficiency of sorting. Design the shed to avoid prolonged exposure of nuts to direct sunlight, as this increases the risk of rancidity and shell cracking. Ensure dehusking equipment is properly set up to avoid cracking of shells. Regularly clean silo fans and other areas where dust builds up to maintain equipment efficiency.
• Monitoring systems. Install a monitoring system to record daily movements of nut-in-husk (NIH) and nut-in-shell (NIS) through the shed, and into and out of storage. The MacMan farm recording system is ideal for this.
Some key points on drying, handling and storage are:
• Ensure the drying and storage facilities maintain an even and adequate air flow. If heating is used during drying, do not use temperatures greater than 30ºC (or more than 5ºC above ambient temperature). Excessive temperatures during drying, particularly of nuts with high moisture content, can result in internal browning and discolouration of the kernel during roasting, and reduced shelf life. With heating, care is also required to reduce the risk of fire. It is essential to have a secondary controlling system to act as a fail-safe should the primary temperature controller fail.
• Avoid prolonged exposure to direct sunlight as it can cause rancidity and shell cracking, predisposing the nuts to contamination.
• Minimise drop heights to minimise fracturing and bruising of kernels. As the moisture content of NIS decreases, the kernel is more prone to damage and the acceptable drop height decreases. The maximum acceptable drop height at 10% m.c. is 2 m. Effective letdown measures need to be in place for drop heights greater than 2 m.
• Keep the fan operating continually when moist nuts are being added to the silo. Switch the fan off at night when the nuts have been dried to about 10% m.c. and the ambient relative humidity is greater than 60%.
• Nuts will be re-wet if fans are run when the relative humidity (RH) exceeds the RH in the silo. Use simple hand-held RH meters to measure the RH of the inlet and outlet air. Alternatively, more sophicticated and expensive automatic switching systems are available. Seek professional advice from consultants or manufacturers of drying equipment.
• Ensure the ducting for the fan inlet is high enough above ground level to reduce the risk of blowing wet air onto dry nuts.
• Duct air from within the shed, preferably from higher up towards the roof. This air is generally drier and warmer.
• Adequate venting at the top of silos is required to allow sufficient air movement.
• The bed depth in silos should not exceed 3 m.
• Completely empty storage vessels when consigning nuts or transferring nuts to other storage vessels.