The pistachio is highly appreciated for its flavour, its enormous nutritional value and its versatility in the culinary and cosmetic sector. However, for this fruit to reach the industry or the final consumer in optimum conditions, it is necessary to undergo a process that guarantees the quality of the product and its preservation.
Pistachio peeling and drying are key stages in this process, which affect the yield of the agricultural project and the marketing of the fruit. They are fundamental steps that prevent its deterioration and ensure an impeccable classification, for its subsequent sale on the market.
In this post we look in depth at the harvesting and subsequent processing of the pistachio. We explain each of its phases: peeling, cleaning, drying, classification and preparation for packaging. In addition, we highlight the factors that influence the success of each stage.
- Pistachio harvesting and processing
- Pistachio processing stages
- Looking to optimise pistachio processing?
Pistachio harvesting and processing
Pistachio harvesting is a delicate process that begins when the fruit reaches optimum ripeness on the tree, approximately between the last weeks of August and the first weeks of September. Although, depending on the variety cultivated and the climate of the area, harvesting may take place before these weeks or extend into October.
How can we recognise that this phase has begun? Basically, the skin (outer layer or peel) begins to turn pink or red. And the more uniform this new shade is, the riper the pistachio is. Also, inside the skin, the shell abruptly splits half open.
Another indication of ripening is that the fruits can easily detach from the cluster, with only a slight movement of the tree. However, the pistachios must be harvested before they start to fall to the ground. If it is done late, the risk of stained fruits, the attack of parasites or the loss of pistachios due to birds increases.
- At the harvesting stage, mechanical vibrators are used to shake the tree and loosen the nuts, trapping them in a tarpaulin as they fall. This method is safer because it avoids contamination. The traditional method consists of manual harvesting (widely used in Iran and Italy, due to the structure of the tree or the type of soil). In Spain and the United States, harvesting is done by machine. In young trees, knapsack shakers are often used to avoid injury.
- The next phase includes the immediate transport to the pistachio processing plant, for the shelling, drying, cleaning and classification of the fruit. This transfer must be carried out within 24 hours of harvesting, to avoid damage and contamination of the pistachios.
Pistachio processing stages
Pistachio processing begins with the reception of the fruit at the plant’s facilities. The lorries arrive with the load (the pistachios together with the leaves and branches that fall during harvesting) to empty them into the hoppers.
Afterwards, this harvest is moved to the peeling machines by means of conveyor belts. On the way, the fruits pass through a winnowing machine (which combines vibration and air currents) that separates the unshelled pistachios from the leaf litter and other impurities.
Peeling
This stage is carried out mechanically. The peeling machines remove the outer shell of the pistachio, the fleshy covering that gives it an ivory colour as it ripens. This prevents the fruit from deteriorating.
- Usually, shellers are installed in series for greater efficiency in the process. They are machines with soft rubber rollers and water injection systems, in order to peel the pistachios by friction and throw them out without external shells and completely washed.
The outer shell of the pistachio traps humidity, so if it remains for a long time it can stain it. Although these stains do not affect the taste of the fruit, they do change its appearance, which may cause the consumer’s rejection. For this reason, after harvesting the pistachios, no more than 24 hours should pass for peeling operations.
Drying
In order to avoid serious damage to the nut, this drying phase must not be accelerated or take more time than necessary (between 5 and 15 hours). To achieve the optimal drying of pistachios, the fruit must be dehydrated until it reaches a stable moisture content, without over-drying it. The difference between obtaining a good product or a high quality product depends on it.
For example, insufficient drying makes pistachios more prone to contamination with aflatoxins (toxin produced by fungi) during the storage stage. This leads to crop loss.
Different drying methods include:
- Hot air drying. This is the most widely used process. It consists in projecting hot air on the pistachios, with temperatures between 70 and 90 °C. It can last between 5 and 14 hours.
- Drying in two phases. In the first period (approx. 4 hours), hot air is used to dry the fruit to 12% humidity. In the second stage forced air circulation is used (between 24 and 48 hours).
- Sun drying. It consists of exposing the pistachios to the sun for 3 to 7 days. For this purpose, nets are put on the ground to spread the fruits. This method helps to obtain a high organoleptic quality of the pistachio, but it depends on the climatic conditions.
Modern techniques can also be combined with traditional ones, such as drying pistachios in the sun, assisted by ultrasound.
Classification
The last stage of pistachio processing consists of applying different classification criteria, with the help of various machines. For example:
- Machines with electronic colour detection systems to separate pistachios that look bad, have stains or are badly shelled.
- Winnowing machines. They are responsible for separating empty and full pistachios.
- Machines for separating open and closed nuts. They have a system of rods that rotate, hooking the open pistachios to pass them onto a conveyor belt.
- Machines for sizing, which classifies the pistachio nuts by size.
- Machines for the mechanical opening of closed pistachios.
The classification process is as follows:
- First the empty pistachios are separated from the full ones. The empty ones are discarded, while the full ones are separated again into open and closed.
- Open pistachios can be taken to the market as snacks, but they must be sorted according to size, because the price at which they will be sold depends on this.
- Closed pistachios and spotted pistachios can be shelled (double shelled) in order to sell the kernels.
Both open and shelled pistachios are stored, controlling the temperature and humidity to preserve the quality and flavour of the fruit.
Preparation for final consumption
Opened pistachios, properly sorted by size, may be subjected to the following processes:
- Salting. The pistachios are sprayed or immersed in brine solutions. They are then dried at high temperatures for approximately 30 minutes to re-optimise their moisture content.
- Roasting. Here, the pistachios are also brined, dried and roasted for 8 minutes in peanut oil. The excess oil is then removed by a centrifugation process.
Looking to optimise pistachio processing?
At Agróptimum we have a specialised team with vast experience in the development of pistachio cultivation. We apply modern production systems and control every parameter of the plantation, with the aim of guaranteeing the homogeneous growth of the trees and maximising the profitability of the project.
We analyse each variable to design personalised strategic plans that lead to the efficient management of the crop and ensure high production and respect for the environment.
We are your partner for innovation and optimisation of pistachio cultivation, harvesting and processing. Contact us. We accompany and advise you to build a profitable and sustainable project.
