A review of theory and practices by Dave Malcolm.
Just before harvest this year Southern Seeds Technology (SST) gave notice to walnut growers, that they would no long be able to dry walnuts. This left quite a few growers looking for alternatives for drying their walnuts. Fortunately, some larger growers offered their drying facilities.
As a consequence, as the field events coordinator at the time, I organised a field day on walnut drying, to let growers learn about setting up their own walnut drying facility. In this article, I will report on the drying facilities of the three properties visited and also background some theory behind walnut drying.
Firstly, the theory. I had a look back at an article by David McNeil called Improved Management Practices of Walnut Quality Factors under Grower Control, available from the Internet and our WIG website and also a presentation from Lincoln University scientist, Leo Vanhanen to NZWIG members back in 2016.
Walnut drying is a crucial step in producing saleable walnuts. If nuts are not dry enough, moulds and kernel darkening occurs. If walnuts are too dry growers receive less per kilogram for their crop. Also, the kernels become brittle and during cracking more pieces and fewer half kernels are obtained. There are two ways of measuring moisture content of walnuts, shell and kernel and kernel only. 
The shell and kernel method is preferred as the shell and kernel need to be at equilibrium. (the shell contains a higher moisture content because of the kernels higher oil content). The nuts are initially cracked, then passed through a mincer or grinder so that both shell and kernel are broken down to fine particles (3mm or less). This broken down mixture is put into a moisture meter for testing. 
For the kernel only method, the kernels are broken down to a fine state (3 mm) and measured with a moisture meter. The maximum moisture level for shell and kernel is 9.5% and kernel only is 5.5% (requirement for Walnuts NZ Cooperative). It is important to note that walnuts are dried and tested after harvest in April and stored until processing. The moisture content may increase during the damper winter months in storage so it is recommended that nuts are dried below the maximum level to allow for slight increases in winter.
Many growers use a Wile55 moisture meter from FF Instrumentation 48 Hayton Rd Sockburn Christchurch. Cost is roughly $850. It is essential to calibrate your meter each year so that it is reading accurately. Alternatively, you can get another grower with a meter or send in a sample to Walnuts NZ Cooperative to measure the moisture levels of your walnuts.

The following factors affect walnut drying:

1. The outside climatic conditions. Walnut drying will be quicker on warmer drier days and slower on cooler wetter days. This will vary depending on the time of day. You may decide that it is not economic to run driers on cool and damp nights as the drying rate will be lower.  If you generate your own electricity from solar PV panels then you lose the benefit of this free electricity by running your dryers or dehumidifiers at night. 

2. Airflow volume and speed. 750W carpet dryers deliver sufficient amounts of air (3,000 cubic metres per hour) at sufficient speeds (40 cubic metres per minute)

3. Relative humidity. When drying walnuts the moisture removed from the nuts needs to be vented out of the drying room. This can be done with a dehumidifier or extractor fans. This is crucial in smaller drying rooms and not so important in larger sheds. The lower the humidity in the room the faster the drying time.

4. Temperature. Increasing the temperature will decrease the drying time. At temperatures above 43 degrees walnuts go rancid. Dehumidifiers can create their own heat and this will speed up the drying. If you are using carpet dryers, heat can be added using gas burners or electric fan heaters.

5. The depth of walnuts in the drying bin. The greater the depth of walnuts in the bin, the longer the drying time, due to the amounts of nuts needing drying.

6. How dry the walnuts are before going in the drying bin. After washing, the nuts will be dripping wet and it is a good idea to allow excess moisture to dry off before placing in the drying bins. 

7. It is important to know that walnuts start drying on the trees once hull cracking occurs. Nuts harvested earlier in the harvest season will need more drying than nuts harvested later in the season, because these nuts have already had some time drying while on the trees. 

 
As a general guide, it takes 24 hours to dry walnuts using warmed air and 48 hours using cold air. Obviously, this will depend on what drying system you are using and climatic conditions. 

Drying Systems
There are two main types of drying systems. 

1. Continuous flow systems are usually used in large commercial orchards where large amounts of walnuts need to be dried. Walnuts are fed into a tower dryer at the top and work their way down through a series of sloping shelves to the bottom where a hatch is open to release the dried walnuts. Warm moving air is blown up through the tower to dry the walnuts. Equipment like conveyer lifts are needed to feed walnuts into the top of the drying tower.
2. Batch drying systems. There systems tend to be found on smaller orchards although can also be used to handle large amounts of walnuts like at Frank and Margaret Brenmuhl’s property. Walnuts are dried in bins with vented grating in the base using forced air or dehumidifiers. Bins can be placed in a line so that air can pass easily between each bin or stacked on top of each other to increase capacity. More fans can be added and the air can be warmed using electric fans or gas blower heaters. The bins are either emptied by hand or lifted and tipped mechanically using a forklift or other means.