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SECTION 2. POSTHARVEST TECHNOLOGY OF CHESTNUTS



2.1 The three Cs

CLEANLINESS, CARE and COOLING are the 3 keywords for each postharvest process of fresh horticultural crops. For chestnuts, these 3 actions are very important from the harvest to the consumer table. Cleanliness means the adoption of all procedures to avoid, as much as possible, external contamination (microorganisms, toxic compounds, metals, pesticide residues, and more) of chestnuts; care means to deal with chestnuts as with a very delicate commodity, protecting them from each potential injury; cooling means to keep down the temperature as much as possible from the harvest to the consumer table.


2.2 Maturity index and Harvest

There are no commercial indices of maturity to apply for chestnuts harvest. Depending on the country, harvest usually is affected by the size of chestnuts tree and the labour cost:

1. When the trees are huge, even 20 m high, the only way to harvest is waiting for the drop of the spiny fruits which start to senesce and open. At this stage, chestnuts are perfectly ripe and the taste is very good. The problem with this kind of harvest is that frequently chestnuts remain on the ground, very humid, for more than one day, thus contamination of fungi is favoured. Low cost polypropylene net tarps are suggested, but often chestnut trees growth environment is rough because of mountain or hill areas. In this case, several pickings per day must be organised.
Clean baskets (plastic is better because is easy to clean and lasts longer) are required and care must be taken: in order to not break the chestnut tuft, to place chestnut leaves on the bottom for cushioning the basket.
If chestnuts must wait in the field, inexpensive canvas tents are very useful to shade chestnuts and to permit ventilation.

2. Another harvest technique is hitting branches when chestnut trees are not too tall. The advantage of this technique is that chestnuts drop in the burr and so fungi contamination is reduced; the disadvantages are:

  1. chestnuts are often unripe and so without flavour;
  2. spiny fruits must be opened and this is not so easy to handle;
  3. insect worms cannot escape in the ground, remaining inside and coming out later.

The spiny fruit is a protection for the chestnut but in the harvest basket the volume occupied is too high and less chestnuts are carried per time unit, wasting time to make several trips from the tree to the unloading point. This means that fruits can warm up waiting to be carried to the packinghouse. Inexpensive canvas tents are recommendable for shading jute sacks or baskets with spiny fruits, permitting ventilation. In any case, in this harvest system is always better to harvest early in the morning to reduce the field heat.

In conclusion the best procedure where useable is:

  1. waiting the chestnuts drop because of right maturity and the optimum flavour (sweet and less starchy)
  2. placing polypropylene net tarp
  3. picking up chestnuts quickly and placing them in the basket with care
  4. providing shading canvas tent for waiting chestnut or leaving under tree shadow but not waiting for more than 3 hours
  5. providing clean harvest baskets or sacks and clean shipping container (jute sacks, box)

In the case of hitting harvest technique:

  1. hitting only the yellow-brown, slight opened, fruits; this means making several hitting operations for each tree;
  2. picking up the fruits and placed them under shading canvas or under tree shadow

Mechanical harvest is becoming common by using vacuum system especially where chestnut orchards are located in flat area and ground under the tree is clean. The disadvantage of this technique is the tuft break (Fig. 7) which increases the potential for fungi contamination during water curing or heat treatment with hot water.

Fig. 7 On the left, chestnut without tuft after mechanical harvest

Fig. 7  On the left, chestnut without tuft after mechanical harvest

(Source: ARSIAL, 1999)


Transport from the field requires care and speed. Care because all kind of impacts due to uneven road or too fast driving can break the chestnut tuft; in the case of spiny fruits this precaution is less important. Speed is important to ensure the rapid processing of chestnut or rapid cooling. The shorter is the time from harvest to the farm house or packinghouse the less is the chestnut perishability with time.


2.3 Quality parameters and procedure

Quality standard for chestnut is usually the size, but quality must be something more. Quality is also appearance, taste, and free from internal defects. So, it is very important that growers or packinghouse managers guarantee high quality chestnuts keeping in mind these quality parameters and providing information on what has been done on chestnuts in the orchard. The packinghouse manager should follow these steps:

  1. select the grower which ensures yearly high quality chestnuts or, in the case of grower, selecting the orchard in which produces high quality chestnuts (size, taste, appearance, good response to storage);
  2. mark all containers coming from different growers sampling the chestnuts and scoring for the quality (Total Quality Score = TQS, see Annex 1); this is important for traceability during storage and distribution; give a code ;
  3. keep clean the equipment and all the facilities: yearly sterilise walls, floor, ceiling, equipment with compounds permitted for food; keep out mice and other animals;
  4. check accurately the equipment, few weeks before starting of chestnuts season;
  1. test job quality of sorters by sampling chestnuts and measuring how many are the rotted ones after the sorting table (pay the sorter on the basis of quality job); do not keep more than 2 h the sorter on the table sorting; guarantee good quality environment of work (safe, clean, less noisy it is possible); oblige them to use gloves and masks and, if the environment is too noisy, the headphones;
  2. pack chestnuts with a label where it is written beyond the weight, all the information on what you have done: code of the sample, water curing, sorting, brushing, storing, and suggestions for keeping in the shop and at home.


2.4 Handling procedures


At farm

As soon the spiny fruit arrives to the farm house the following procedure must be applied:

  1. remove soon the chestnut from the burr paying attention not to injury or break the tuft; hand removal can be used but even a simple handy machine made by two counterrotating plastic rollers (even car tires) which squeeze out the fruit can fasten the operation;
  2. if chestnuts are sold immediately, do not dip in the water; dipping in the water is used frequently for removing the chestnuts which are floating (rotted) but the sorting efficiency is very low; if this system is required, chlorinate the water;
    if chestnuts need to be stored, dipping chestnuts in the water with some chlorine (approximately 500 ppm = 0.5 ml/l) for 3 to 7 days (7 days is more effective in killing the worms; longer time gives the risk of high amount of ethanol inside the flesh), in the shadow; removing the floating chestnuts;
  3. placing a plastic film over the floor and spreading chestnuts on the floor in a very thin layer and open the windows or turn on the fans to let them well drying superficially;
  4. fill the jute sacks (better if plastic or wooden box).


Precautions:


At packinghouse

The area for the handling equipment must be in a closed room with a door for the entrance of the chestnuts and a door for the exit of packaged chestnuts. During working time, the doors can be kept opened but thick, flexible, plastic strips curtains must be installed which provide some kind of protection.

Room temperature must be between 10 and 15°C and 3-4 meters around the equipment must be left free. This area must be nearby to the cold storage facility.

If chestnuts are in the burrs, they must be removed. This can be done by hand or by machine. The former takes longer time but it is more reliable and less injurious. The latter is faster and the capacity of the machine must fit with the capacity of the other machines.

Chestnuts can come directly from the farmer or from the storage facilities:


Case 1 - From the farmer and sold immediately

  1. sort by appearance (Fig. 8) rejecting chestnuts with superficial defects; if hot water treatment must be done (49-50°C/40') to kill the worms, be sure the tank is clean and the temperature is correct. After this treatment chestnuts must be cooled down;

 

Fig. 8 Sorting table for chestnuts

Fig. 8  Sorting table for chestnuts

 

  1. brush chestnuts by using gentle rotating brushes, blowing air to reduce the dust and removing residues;
  2. size chestnuts (Fig. 9): in China, in XinXian area, the chestnuts diameter ranges between 15 and 30 mm (approximately 42 chestnus /kg), so 3 sizes is enough; chestnuts are collected in 3 tanks from where they can be hand loaded on sorting table or by using a cup elevator (Fig. 10) which move from one tank to the other;

Fig. 9 Perforated sizer for chestnuts .

Fig. 9 Perforated sizer for chestnuts

 

Fig. 9 Perforated sizer for chestnuts (close view).

Fig. 9 Perforated sizer for chestnuts (close view)

 

Fig. 10 Chestnuts collector and cup elevator belt

Fig. 10 Chestnuts collector and cup elevator belt

  1. pack and label:


Case 2 - From the farmer and then to storage

  1. unload chestnuts in tank for water curing (Fig. 11) or in the case high CO2 shock treatment just brush chestnuts; drying well the water cured chestnuts before starting the storage (in the cold room or other ventilated place)


Fig. 11 Water tanks for curing chestnuts

Fig. 11  Water tanks for curing chestnuts


  1. load chestnuts in plastic (better) or in wooden box or special wooden container of bigger size (Fig.12) and place them in storage.

Fig. 12 Special wood container for handling chestnuts

Fig. 12 Special wood container for handling chestnuts

 


Case 3 - From the storage

  1. for the sorting, dip chestnuts in chlorinated water, wipe chestnuts by air stream then take to the sorting table; the reason of dipping in the water at this step is because the internal decay is very spread and so chestnuts can float and be discarded;
  2. sort very accurately by appearance rejecting the one which looks dull and black or with superficial defects;

The further steps are the same of the Case 1.

Testing equipment performance

Sizer

Brusher

Check the brightness of chestnuts, exposing them against the sun or white light: they should shine.

Hand sorting for appearance and external defects

Worker conditions




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