The pistachio processing industry in Iran is gradually moving from traditional, low-capacity, home or farm based, sun-drying units towards specialized, industrial high-capacity plants utilizing hot forced air for drying. Traditionally, processing plants acted as service providers to the growers. Therefore, each grower retained possession of his/her produce after processing. However, with the advent of new industrial processing plants and the rising costs of processing in small scale units, new collective processing agreements are becoming more popular. In such schemes, the processing unit may even make the decisions about the selling of the processed pistachios on behalf of the growers.
Currently, the largest industrial pistachio processing plant in the country has a capacity of 350 tons of fresh pistachios per day. Although the number of high-capacity plants is small, the total number of mechanized processing units is quite large and growing every year.
A typical modern Iranian pistachio processing plant processes freshly harvested pistachios as follows:
Weighing, unloading and sampling: The trucks loaded with freshly harvested pistachios are weighed prior to and after unloading to get the net weight of the pistachios. The pistachios are unloaded into unloading bays (where samples are taken randomly), and are then moved onto conveyor belts to start the processing phase.
Hulling: Fully mechanized dry-operated rotating hulling machines remove the outer hull of the pistachios. The pistachios continue to the next step, while the removed hulls and other debris are rejected from the main product stream.
Water floatation tank: Blank nuts and nuts with immature kernels are removed from the main product stream when the pistachios are passed through a floatation tank. The underweight and blank pistachios (floaters) float to the top while the larger, heavier pistachios (sinkers) sink to the bottom.
Heated, forced air continuous pre-drying: The pistachios pass through a high-temperature pre-dryer, where the surface moisture added in the previous stage is removed and as a result, the next stage machine for removing nuts with adhering hull can operate more efficiently.
Adhering hull separation and removal: The pistachios drop over a large diameter roller, where pistachios with adhering hull are removed from the product steam through differential friction. These pistachios with adhering hull are returned to the beginning of the line for re-hulling.
Dryer and sun drying: Using batch or continuous forced-hot-air dryers, the pistachios are heated to reduce the moisture content to 7-11%. Then the pistachios are dried in the sun or by forced-ambient-air ventilation to reduce the moisture content to less than 5%.
Mechanical separation of open-shell nuts from closed-shell nuts: Open-shell nuts are separated and move to the next stage (closed-shell nuts are processed separately).
Mechanical sizing: The pistachios are separated into two or three grades depending on size.
Picking conveyor belt and/or picking table: The nuts with visible defects are removed.
Packing: The pistachios are packed in 50 kg sacks ready for transport or storage. Recently, there is a trend towards packing pistachios in big-bags.