Prevention of condensation in shipping containers containing bagged stored products

13 May 20208 min reading

Condensation in shipping containers is greater especially when transporting from warm climates to cooler ones. The reason for such condensation was investigated on bagged groundnuts and a solution to reduce the risk was detailed. Placing desiccants in the container to absorb the condensed water is common. However, water extracted from the cargo continues by convection currents, causing the absorbing agents to be saturated easily. To reduce such risk, use of a container size large bag named TransSafeliner (TSL) was proposed. The purpose of using a TSL is triple: it reduces the chances of condensation, controls infestation and free fatty acid (FFA) levels.

Shipping containers play a significant role in the handling and transportation of many agricultural commodities. Many agricultural dry products are being been shipped in bags of 25 to 60 kg capacity. Among them most common are: coffee, cocoa beans, all kind of edible nuts, pulses, beans, rice, flour and spices. In principle all these products may suffer from condensation damage during ship transportation (Fig. 1).

Condensation and moisture migration cause an increase in the moisture content of stored agricultural commodities, which can lead to serious damage as a consequence of fungal activity (Fig. 2). This phenomenon is typical in particular of products transported in containers shipped overseas for relatively long journeys of several weeks, and thus exposed to temperature fluctuations, especially when the cargo is exposed to alternating warm and cool weather. The primary aim of our investigation was to study the occurrence of condensation. In addition, we investigated the efficacy of calcium chloride in preventing moisture damage and a novel method based on a large bag named TranSafeliner (TSL) to reduce the intensity of condensation.

MATERIALS AND METHODS Analysıs of condensatıon Four 20 ft metal containers (internal dimensions 2.43 m wide, 2.37 m high and 5.3 m long) were placed on the forecourt of a groundnut-processing plant at Kefar Saba, Israel. The containers were loaded with bags containing in-shell groundnuts with an average initial moisture content (mc) of 7.7% and 8.5 % for shelled and in-shell groundnuts, respectively. The bags were positioned in layers of 30 bags each to form a stack to a height of six layers and raised on pallets. Thus each container contained 180 bags (about 5400 kg groundnuts).

Calcıum chlorıde absorbıng capacıty To test the moisture­ absorbing capacity, calcium chloride (technical grade) was placed (in lots of 4-5 kg) in plastic-coated cardboard boxes. The amounts of calcium chloride used were 30, 60 and 120 kg per container, while a fourth container without calcium chloride served as a control. The trial was carried out during October-November and lasted 3 weeks.

Test of TranSafelıners (TSL) TranSafelinerTM is made of transparent multi-layer PE film of 100 microns thickness which has superior gas and moisture barrier properties. These liners fit in 20 ft and 40 ft container for bulk or bagged commodities. The TSLs are manufactured by GrainPro Inc.

Table 1. Average maximum and minimum temperatures recorded at different locations inside containers, and ambient temperature ( 0C) Table 2. Average maximum and minimum relative humidities (%) recorded during the trial in the headspace and on the floor of the containers.

Of 7 containers, 3 contained cocoa beans inside TSLs, 3 contained a desiccant and in one container the cocoa bean stack was covered by ordinary PE liner providing non-hermetic conditions. The 7 containers were loaded in Ivory Coast and emptied after 55 days in Malaysia. The ship voyage took place during August-September 2019.

RESULTS AND DISCUSSION In our trial, temperature gradients inside the con­ tainers were apparently initiated by daily ambient temperature fluctuations and the exposure of the containers to direct solar heating (Fig. 3). The r.h. of the air inside the containers and the mc of the groundnuts constituted sources of moisture which enhanced the condensation phenomenon.

Table 3. Average moisture content (± SE) of in-shell and shelled groundnuts taken from different locations in the containers at the end of the trial

Table 1 shows that the extreme temperature fluctuation (from an average minimum of 16.4°C to a maximum of 39.4 °C), which can cause water condensation, took place near the roof. Indeed, when the temperature in the headspace of the containers was 39.4°C, the r.h. at the same location was 43% (Table 2). Under these conditions, analysis using a psychrometric chart indicated that cooling of the roof to 23.5°C can bring the air to its dew point, and any further cooling can cause water condensation. That was the situation when the roof of each container was cooled to 16.4°C (during the nights) (Fig. 4). Subsequently, the average maximum r.h. of the headspace was 93% (in the control and the 30 kg calcium chloride-treated container, Table 2). However, the average maximum r.h. recorded within the containers treated with calcium chloride dosages higher than 30 kg were in the range 82-89%, indicating the influence of calcium chloride on moisture absorption under the trial conditions. A similar tendency was observed when the r.h. values recorded in the lower layers of the containers were analysed. The average maximum r.h. recorded at the floor of the 120 kg calcium chloride-treated container was the lowest (75%) (Table 2).

Comparison of the mc of groundnuts (both in-shell and shelled) of the control container and with that of the 30 kg calcium chloride-treated container (Table 3), showed that for the maximum r.h. (93%) recorded in both containers (Table 2), the mc was expected to be similar. However, results shown in Table 3 indicate that for the 30 kg calcium chloride-treated container, a mc lower than the control was recorded (Fig. 5). In our opinion this difference may have derived from the absorption capacity of calcium chloride, which shortened the period of the maximum humidities which prevailed in the headspace (r.h. >93% lasted for 101 h in the control, versus 66 h in the container with 30 kg calcium chloride).

Fig. 2- Condensation damaged of boxes during transportation in a container.

Results achıeved usıng TSL The operation principle of TSL is based on the gas-tightness achieved which isolates the commodity from the headspace above the stack (Fig. 6). Such isolation prevents moisture migration from the commodity to the headspace. This prevents the daily moisture equilibrium of the cocoa bean moisture with the increased temperature of the headspace air volume. The low gas permeability property of TSL traps the CO2 gas inside the TSL, thus controlling and killing the insects. This gives the TSL the advantage of ‘green’ and safe fumigation.The modified atmosphere created, prevents infestation, mould growth and oxidation.

In the trials described in this work, shipment of containers that were loaded with cocoa beans stacks in TSLs (3 containers) and without TSLs (4 containers) were examined upon their arrival after 55 days of voyage.

Fig. 3 - During the day external heat and solar irradiation increase the headspace temperature of the container. Fig. 4 - At night temperatures are lower and water holding capacity of air is reduced at the headspace of the container.

It should be noted that the voyage took place during August-September 2019. The loading port was in Ivory Coast in Africa, then the ship passed South African coast when the temperature there was winter with average night temperatures around 7o to 8oC, and during the day 17o to 18o C. Then the ambient temperature rose to an average of 30oC at arrival close to Malaysia. Such temperature fluctuations may cause significant moisture migration.

In containers without TSL (Table 4) the average FFA measured on samples taken from the bags was 2.30 and the mc was 8.73%. The critical mc for safe storage of cocoa beans (70% equilibrium r.h.) is at 28°C, 7.4% (Gough, 1975), whereas the average mc recorded in the container without TSL was 8.73%. Such mc is equivalent to a r.h. of 78% that enables microflora development and the consequent excess humidity damage of the beans. That conveys to the result of average 2.30% FFA that is above the acceptable limit of 1.75% FFA. It should be noted that those containers without TSL were equipped with the desiccant "Clariant blanket". Similar to the tests carried out on groundnuts the use of desiccant could not satisfactorily prevent moisture migration.

Fig. 6 - Use of TranSafeLiners of GrainPro prevent condensation. Fig. 5 – Moisture is translocated from bottom layers to deposit on the cool surface of container roof and the commodity.

In containers with TSL (Table 5) (Fig. 7) average mc was 7.41% close enough to the critical mc. The average FFA was 1.56% below the acceptable upper limit (1.75%) for processing cocoa beans. These results clearly indicate the successful storage during the shipment of the cocoa beans through a cold region (winter in South Africa) and back to a tropical climate in Malaysia. The TSL was demonstrated to have triple advantages; it effectively reduces the chances of condensation, controls infestation and FFA levels.

Fig. 7 – View of sealed hermetic liner termed TranSafeLiner by GrainPro.
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