The Western Chestnut

by Stephen Morris
Sydney Postharvest Laboratory
Phone: 02 9490 8443 Mobile : 0410603170
Email: scmorris@postharvest.com.au

The CALM storage method was presented at the International Chestnut Congress, Oct 20-23, 2004, in Chaves, Portugal.

Summary
This storage system was developed over the last 3 years by Sydney Postharvest Laboratory with funding assistance from CGA (Chestnut Growers Australia) and HAL (Horticulture Australia Limited). The basic design has been finalized and is now protected by a patent.

• The basic principle is storage of chestnuts under high carbon dioxide levels (>15%) with adequate oxygen levels (>4%) maintained.
• This system enables excellent long term control of rots in chestnuts without use of chemicals for periods of up to 12 months, a very considerable increase over the up to four months currently obtained commercially.

Introduction
The basic concept of the system is that certain crops, like chestnuts, can tolerate high carbon dioxide in the storage atmosphere (15 - 23%) when stored in a cool room. High carbon dioxide in the storage atmosphere has the benefit that it prevents growth of storage molds and insects. Low oxygen gives some slight benefit to storage life of chestnuts (however, it has little effect on storage molds and insects).

Air contains 20.95% oxygen (most of the rest is nitrogen).

All fruits and vegetables use oxygen and convert it into carbon dioxide. By sealing up chestnuts within a plastic bag, oxygen is changed into carbon dioxide. However, a certain level of oxygen is required to keep the chestnuts healthy. Several year’s research has found that for chestnuts in cool storage, a good balance of the highest level of carbon dioxide and a safe level of oxygen is achieved by controlling the oxygen within the bag to 4%, under these conditions carbon dioxide levels are about 17%.

For the system to operate correctly, it is essential that the chestnuts are stored below 2°C (-2 to 2°C), they are completely sealed in the plastic bags and base unit and that the CALM unit is maintained on power and operating within the range of 3.5 to 7% oxygen at all times.

Development Leading to Current Model

Figure 1: The first model involved the controller being stuck onto the plastic bag enclosing the nuts, the body of the oxygen sensor being in the controller box and the large plastic bag sealed by twisting the bag and using a cable tie.

This technology was developed out of very promising controlled atmosphere work that was done supplementary to the chestnut research project examining sanitizers and packing funded by CGA and HAL. The research done to develop this technology has involved lots of work on the best oxygen/carbon dioxide levels for chestnut storage balancing performance and the safety margin under commercial storage conditions. It also involved testing different components for reliability and suitability under actual storage conditions and very importantly developing the best and most reliable sealing systems.

Figure 2: The second model involved moving the sensor well away from the heat of the electronics and placing it within the plastic bag.

Since the controller units were to be used in coolrooms at -2°C to 2°C with water often on the floor, they all are designed to work with safe low voltage DC power. The first model (Fig. 1) involved the controller being stuck onto the plastic bag enclosing the nuts, the body of the oxygen sensor being in the controller box and the large plastic bag sealed by twisting the bag and using a cable tie. This was along the lines of the original concept which was meant to be a stick on atmosphere patch (SOAP) to control carbon dioxide levels. However, sealing and oxygen sensor problems meant this concept needed to be further refined. The second model (Fig. 2) involved moving the sensor well away from the heat of the electronics and placing it within the plastic bag. The sealing was done by tying the plastic around a soft sealant strip surrounding the extension tube containing the sensor. The sensor operation in this model was greatly improved, but the sealing method while reasonable once set up, proved difficult to use.

Figure 3: Current CALM Model V6 sealed to chestnuts in plastic tube with interface plate.

The third model (not shown) involved using a sealing plate attached to the plastic bag which was interfaced to the oxygen/ carbon dioxide controller by an airtight pressure fit connection. The current model CALM V6 (CALM stands for Controlled Atmosphere LongLife Module) (Fig. 3) is similar to the third model. However, it has improved electronics and module design and works with lower power consumption. Further it has the sealing and plastic bag/tubing configured so that instead of storing 250 kg with one unit in a plastic bag inside a wooden storage bin, one can store 4 bins (up to 1.6 tons) of chestnuts with the bins of

Figure 4: Several CALM Units running showing the bins of chestnuts in plastic tube and sealed to pallet base.

chestnuts enclosed within a plastic tube. This system is illustrated in Figure 4 which shows several CALM units set up with four bins of chestnuts and running in a cool room.

Parts of the CALM Storage System
Figure 5 illustrates the basic features of the CALM system. The various elements are (1) the plastic bag (tube) to cover the chestnuts, (2) Pallet Base under storage bin (to which tube is sealed), (3) cable tie to seal top of plastic bag, (4) CALM unit, (5) Interface Plate (to connect unit to plastic tube), (6) Oxygen Sensor, (7) flexible tubing to add and extract air to plastic tubing (8) rigid nylon tubing to deliver air to the chestnuts farthest from the CALM unit. Also included is a 12V DC plug pack (so that only safe 12V DC power cables will be on the cool room floor).

Figure 5: Parts of the CALM System.

Each unit can be used to store 1 to 4 bins (400 to 1600 kg) of chestnuts. Basically the bins of chestnuts are placed on a pallet base which seals the bottoms of the bins, a plastic sleeve is placed over the bins and sealed at the pallet base with duct tape. A pallet bag interface is then attached to the plastic bag at the top of the first bin the CALM unit seals to the pallet bag interface with air oxygen sensor and tubing going through the interface to provide just enough fresh air to the chestnuts to maintain the ideal storage atmosphere. The CALM unit is powered by l2V DC power so that only safe low power cables are near the chestnut bins. Finally the top of the plastic bag is sealed with a cable tie. During storage operation the CALM system should be checked daily to ensure correct oxygen levels, power to units etc.

Figure 6: Control of external rots by the CALM system.

Figure 7: Control of internal rots by CALM system Model 2.

The Benefits of the CALM System
The CALM system, largely through maintaining a constant high carbon dioxide atmosphere around the chestnuts, greatly reduces rots especially external rots. This is seen in Figure 6, where after six months storage the CALM system has essentially maintained mold or rot levels at those initially found. There is a slight, but not significant improvement by also using sanitizers or fungicides. This excellent control of external rots was consistently observed. Internal rot levels were very low in these experiments and consequently there was usually no effect of the CALM system. However, in one experiment there was significant levels of internal rot and it was possible to demonstrate significant control of internal rots by the CALM units. This control of internal rots is shown in Figure 7.

The oxygen levels within the CALM system fluctuate within a range of values determined by setup of the CALM system and the volume of air within the system. The variation in oxygen levels for a CALM Model three unit is shown in Figure 7. The current unit runs within a much smaller range, typically from 4 to 4.5% oxygen.

Conclusion
Several years of research has shown that the CALM system combined with careful preparation of the chestnuts prior to storage gives excellent extension of storage life. The main benefit is on reduction of external mold on the soft hilum. The long term costs of operating the CALM unit will only be precisely known after several years experience. However, based on current experience, one could estimate on the CALM unit lasting about 6-9 years (including the pallet base and interface plate), the oxygen sensor, air pump and DC power supply needing replacing every two to three years and the plastic bag replacing every one to two years. Based on these estimates and current prices, the cost of storing with this system would be about $150 to $250 per year to greatly reduce external and internal rots and greatly extend the storage life up to 8 to 10 months if required. For a stack of one ton, CALM storage costs are estimated at 15 to 25 cents per kilogram. Besides this amount, the only costs to the farmer are a few dollars of electricity, a short time to set up the bins and regular (preferably daily) checks during storage to ensure the units are connected to power and working normally.

(N.B. Royalties from the sale of each of these units goes to CGA and HAL as a continual return on their research investment)