SECTION 2. POSTHARVEST TECHNOLOGY OF CHESTNUTS

2.5 Water Curing

In Italy, the most diffuse chestnut treatment is the water curing. Water curing was known in the past as "novena" which means that chestnut fruits were kept for nine days in the water. The objective of this method is to reduce the possibility of development of fungi during the storage process and at the same time to favour the killing of the worms from the chestnut. The efficiency of the method depends on partial lactic and alcoholic fermentation that takes place during the curing process, which reduces pH and allows diffusion of phenols from the episperm into the flesh.

Nowadays, the time of water dipping is reduced and can last even only three days.

The following steps are important for a good response of water curing treatment:

1. dip chestnuts into the water, better if at 15°C, and left for 3 to 7 days (start of fermentation is advised by bubbling; 7 days guarantee the killing of the worms); pH of the water goes down nearby one point from the initial pH;
2. remove chestnuts from the water, place plastic film over the floor and spread the chestnuts in a layer of 5-10 cm and let them to wipe very well in air stream;
3. if the external humidity is high and the temperature low, make a stream between two windows or placing a fan on one window which push the air inside;
4. rotate chestnuts preferably every day;
5. if external humidity is low and temperature not too low, use ventilated wet air system as described in the chapter 2.5.2.

Precautions:

• bleach can be added to the water (500 ppm of active chlorine); renew the water each time new chestnuts are dipped;
• clean very well the tank;
• wipe very well chestnuts before shipping and storage (take a sample of 10 kg of chestnut, place it in perforated net and then dip into the water curing tank together with the other chestnuts to be cured; after curing let the sample to wipe together with the other chestnuts being wiped; then weight the sample regularly; wiping process can be considered accomplished when the weight of sample is again 10 kg).

2.6 Storage

Storage is one of the major problem in chestnut postharvest process, since the product is very susceptible to fungi contamination. We must keep in mind that chestnuts are living tissue and, thus, perishable; the only way to slow down their living activity (respiration) is to reduce, first of all, the temperature.

2.6.1 Sand storage

Sand storage is a traditional chestnut storage method in China (and in some other countries) and consists in placing the chestnuts in the sand to allow the maintenance of constant thermohygrometric environment around chestnuts. In China, superficial sterilisation of chestnuts is done with dipping in 0.5% KMnO4 solution for 3 minutes. A layer of 10 cm high of wet sand - the optimum rate of water absorption is when the sand, squeezed in the hand, remains compact and when the hand is opened, the sand ball breaks up - is placed on the floor of the room; over the sand layer, a layer of chestnuts is placed, and successive layers of sand and chestnuts, up to a total height of 70 cm. The ratio of sand/chestnut is generally 7/3. Every week during the first two months, chestnuts and sand are turned upside down, discard the rotted nut and water added to the sand; this procedure is done every two weeks until January. Unfortunately, the total chestnuts loss is 30~40%, mainly occurring in the first two months of storage, due to decay. In the annex 3 some useful and sustainable modifications are suggested.

2.6.2 Ventilated wet air storage

Most of horticultural commodities require, together with low temperature, high relative humidity in the storage environment. Some of them, such as carrot and other radishes, need also the maintenance of high ventilation rate in order to permit uniform wet air distribution around the commodity. Different techniques are in use: ice bank cooling, Filacell system, jacketed system, wet-air cooling which differ among them for the architecture of the storage room. For chestnuts, wet-air cooling could represents an alternative to cold and controlled atmosphere storage in the case of reduced amount of product needs to be stored and less professional are the operators. In the annex 4 two systems are suggested depending on the economical capacity of the farmers.

2.6.3 Cold storage

The reduction of temperature in the range 10-0°C is the most effective technique to slow down the metabolic activity of fruits and vegetables. Even chestnuts benefits of the reduction of temperature since they are no susceptible to chilling injury. The following steps are essential for a good performance of cold storage of chestnuts.

1. water cure the chestnuts;
2. load them in perforated plastic or wooden boxes and place in the cold room, without wiping the chestnuts;
3. load the boxes over the pallet, and stack the pallets in lines from the evaporative cooler to the door; pallets must be close to each other while rows of pallets must be distant 20cm; cooling plant must be operating at room temperature at least, with door open and fan at the maximum speed to let the chestnut wipe; the morning after, cool down to 0°C;
4. check temperature and RH every day; they must be at 0° and 90% (place a glass of water in front of the door window, over a pallet, and mark with a pen the level of water, check daily if water is iced and the level of water; if the water is frozen the temperature is less than 0°C; if the water decrease too fast means that humidity in the room is low). Check the air speed with wind meter in different parts of the cold room especially among the chestnuts boxes;
5. check the chestnut condition by hand and even weighing at the beginning 10 kg of chestnuts taken in the 4 corners of the cold room and weigh again every two weeks;
6. check the evaporative cooler serpentine for frost formation; if you see ice, increase the number of defrosting.
   

Precautions:

• place the thermostat close to the door but check temperature in the 4 corners and in the middle of chestnuts boxes;
• calibrate thermometer by wrapping with a plastic bag filled with ice: temperature must be 0°C; calibrate the hygrometer placing it over the cooking steamer (it must be off) covered with a plastic bag: it must give 100% ;
• clean and sterilise accurately the room every year before starting the storage season;
• place chestnuts inside the cold room only when the temperature is already down;
• keep the corridor in front of the cold room as cooler as possible;
• before carrying the chestnuts outside for truck loading (if it is not refrigerated) keep them in the corridor, to reduce water condensation.

Troubleshootings for the Cold Storage Plant

RH too low	too small cooler	larger cooler
	too low T	check expansion valve
	use of dry containers	plastic container or waxy wood or cardboard
	too long defrosting	reduce time or cycle number
	excess of air circulation	decrease fans speed
	defective insulation	check the insulation

2.6.4 Controlled Atmosphere (CA) storage

A complement to cold storage is CA storage, which means the use of low temperature together with the modification of atmospheric oxygen and carbon dioxide concentrations. Commercially applied for apples, pears, and kiwifruits it has been successfully used even on chestnuts. The advantage of CA storage over the regular cold storage for chestnuts is the extension of storage period because it delays sprouting and reduces decay.

All the indications given for cold storage are even essential for CA storage. Moreover, CA storage room needs air tightness and the equipment for producing the required atmosphere.

In the case of chestnuts, an alternative treatment to water curing is the High CO₂ Shock Treatment (HCST). The treatments consists of increasing the CO₂ up to 40-50% and keeping it for one week. The advantage of this technique over the water curing is the control of decay without requiring too much work as for water curing (Fig. 13)

Fig. 13 High CO₂ shock treatment (right)control the decay development
better than water curing (left) after long term refrigerated
storage at 0°C and 95% R.H.

This technique is especially worthy if it is followed by Controlled Atmosphere storage (Fig. 14)

Fig. 14 High CO₂ + Controlled Atmosphere (3% O₂ + 15% CO₂ ) (right)
control the development of decay and superficial mould better than
water curing + CA (left) after 5 months of storage at 0°C and 95% R.H.

The following are the steps for CA storage with CO₂ treatment:

1. stack the chestnuts boxes in the cold room soon after the sanitising treatments (chlorine washing for few minutes);
2. leave to wipe out at 15°C with the door open and then cool down to 0°C with cooler fans at the maximum speed;
3. turn on the O₂ burner (Fig. 15) to reduce the oxygen to 2-3%; the burner will increase CO₂ and humidity; then open the CO₂ flushing from the tank up to the required concentration;
   

Fig. 15 Oxygen burner to use in order to rise CO₂ in chestnuts storage

4. when the right gas mixture is reached, this atmosphere shock will be maintained for one week (the system will work automatically); if the O₂
5. if the O₂ concentration decreases (less than 2%) turn on the air compressor to push air inside the room (or open the door);
6. after that week, turn on the air compressor or open the door to remove the excess of CO₂ until the 15%; then turn on the burner to reduce again the oxygen (if CO₂ remains or rises more than 15% is not a problem; the higher CO₂ the better is);
7. keep the atmosphere 2-3% O₂ and >14% CO₂ for the months you need; keep in mind that every time you open the door to remove chestnuts pallets, atmosphere will be changed and you need to reduce again oxygen and increase carbon dioxide.

Precautions:

• keep in mind the free volume of the room and the flow speed to calculate how long it takes to reach approximately the CO₂ concentration;
• wear always the oxygen mask to get into the CA room;
• check accurately the security pressure valve; they must be calibrated at the right pressure when you inject CO₂ ;
• check constantly the gas analyser to see the condition of the atmosphere inside the room (calibrate the analyser by using a specific gas atmosphere);
• check the connection of breathers on the roof.

Troubleshootings for the CA Storage Plant

*To check the air tightness, with CA room loaded and perfectly closed, creates an overpressure of 25 mm H₂O (use a glass water pressure meter) and wait for 1 h; the water pressure must not be reduced more than 15 mm.