Grain temperatures
The average temperatures of grain at three different depths beneath the apex of the platform over the storage period are given in Fig. 3. This shows the very marked influence of seasonal temperature fluctuations in the ambient on temperatures at 0.3 m within the grain bulk, whereas at 1.0 and 3.0 m fluctuations were progressively attenuated. This resulted in the establishment of strong temperature gradients within the bulk particularly between 64 and 200 days of storage (November 1989-March 1990), around 500 days (January-February 1991) and around 850 days (January-February 1992). These temperature gradients were considered responsible for convection currents that caused moisture transfer to the peak area of the bulk.
Moisture content
Fig. 4 shows the maximum and minimum MCs recorded at a depth of 0-0.3 m, and the average MCs from all sampling stations at depths of 1 and 3 m during storage. The average "initial MC was 8.8% (range: 8.0-10.8%). In the 0-0.3 m layer, the MC rose more rapidly at some sampling stations than at others. The critical MC of barley above which storage fungi are capable of rapidly developing is that above the ERH of 75%, namely 14.5% (Pixton and Warburton, 1971). After the MC had risen above this critical MC subsequent samples were found to be mouldy resulting in caking at the surface layer. Fig. 4 shows that there were cyclic moistening and drying effects caused by seasonal temperature fluctuations that resulted in a wide range of MCs recorded at the upper layer. By the end of storage, MCs above critical had been recorded at least once at all sampling stations, and mouldy grain was recorded from surface layer at seven of the eight stations. Samples from depths of 1 and 3 m were relatively uniform in MC throughout storage with no samples above critical MC. This and previous storage trials (Navarro et al., 1993) showed that the presence of mouldy grain was limited to the area at the peak of the grain bulk. This phenomenon was attributed to convection currents arising from temperature gradients, particularly occurring at pointed apices where condensation was marked. The daily cycles of warming and cooling of the surface layers determine whether there will be a net increment or decrement of moisture.
Gas composition
The CO2 and O2 concentrations of the intergranular air within the platform are given in Fig. 5. It can be seen that there was a rapid decrease in O2 and increase in CO2 concentration over the first 50 days followed by fairly stable levels for the rest of the storage period except for a sudden increase in O2 and decrease in CO2 after 540 and 900 days of storage. On these two occasions it was found that one or more of the plastic screw-caps were absent and the changes in gas composition were attributed to a temporary break in the hermetic seal of the platform. From experience on hermetic storage in Israel it has been found that in contrast to insect infestations, active microbial spoilage can raise CO2 readings well above 12%. Since very few CO2 recordings were made above this level during the storage period, this indicates that no extensive microbial respiration took place in the grain bulk.
Insect infestation
Of 450 samples examined for infestation (from three depths at the eight sampling stations) live insects were recorded from only 34 samples during the storage period. The distribution of infestation was: 26 infested samples from the surface layer; seven samples from a depth of 1 m; and one sample from 3 m. The species were: Tribolium sp. in 27 samples; Oryzaephilus sp. in five samples; and Rhyzopertha dominica in three samples. All were recorded during the first year of storage, mainly from the upper layer. Of these, only three infested samples were recorded during the second half of the first year.
No samples containing live insects were recorded at the end of storage except from samples taken from the grain bulk surface. To quantitate the intensity of this infestation, eight composite surface samples were taken after removal of the overliner before unloading the grain. Analysis of these eight samples taken from the surface revealed that three were infested with a total of 16 Tribolium sp. and three R. dominica live adults. This residual infestation may have survived due to the high MC prevailing at the surface of the bulk. No live insects were recorded from the 1-m depth samples.
Results from previous trials in Cyprus and those in Israel, all revealed initial infestations by stored product insects. The very rapid reduction in O2 and rise in CO2 concentrations after the platform was sealed, was attributed mainly to insect development especially at the surface layer of the bulk.
Germination
Samples taken after 19 months storage from 1 and 3 m depth served for analysis of viability. Sampling ports 1-4 formed one composite sample and samples from ports 5-8 formed another. Viability of the in-bulk composite grain samples after 19 months storage was 94-98%.
The germination tests of nine composite samples obtained during unloading after 34 months of storage, gave a germination level of 88.3 ± 1.6% (SD) with an additional 4.6 ± 1.0% abnormal seeds. The significance of these high levels of germination after prolonged storage lies in the possibility of utilizing the hermetically stored barley for seed grain, should this be needed.
Damage by moulds
Mouldy grain was only located along the apex of the grain bulk, and to a distance of 24 m from the peak along either slope. However, it did not form a continuous strip along the apex and the depth of affected grain ranged from 5 to 50 cm.
The following microflora were identified from three visible mould-damaged grain samples taken from the surface after the overliner had been removed: Aspergillus ochraceus, A. flavus, A. fumigatus, Fusarium sp., and Penicillium sp. All species were identified from both surface sterilized and non-sterilized seeds.
Although mould presence alone is not evidence of mycotoxin production, some of the above species are capable of producing mycotoxins under favorable conditions (Mirocha and Christensen, 1974; Bullerman, 1979). Therefore, it was most important to remove all mouldy grain from the sound grain of the bulk before unloading.
Aflatoxins were not found in the pooled sample formed from the composite samples of sound grain taken from the bulk after the platform had been uncovered.
Other grain quality parameters
Chemical analysis: No initial analysis of the barley was made. However, for comparative purposes chemical analyses of the barley after 34 months storage were made together with those of two local barley varieties from the 1993 harvest analyzed before storage. Results given on a dry weight basis showed that crude protein in the stored barley was 11.8%, while in fresh barley var. Cantara it was 11.9% and in fresh barley var. Athinais it was 13.0%. Fat in the hermetically stored barley was 2.0%, in fresh barley var. Cantara it was 1.5% and in fresh barley var. Athinais it was 1.9%. Crude fiber in the hermetically stored barley was 7.5%, in fresh barley var. Cantara it was 6.7% and in fresh barley var. Athinais it was 6.3%.
During uncovering of the platform, the sound grain had the characteristic odour of fresh, recently harvested barley.
Physical analysis: The average results of the nine composite samples after 34 months storage showed 96.7% pure seeds, 0.2% weed seeds, 2.5% broken seeds, 0.6% inert material, and test weight was 603.2 kg/m3.
Durability of the PVC overliner
The durability and resistance of the overliner was shown to be suitable to protect the grain over the storage period from adverse climatic conditions of solar UV irradiation, high temperatures and high velocity winds. The same liner was used for a previous one-year storage period. Laboratory tests, carried out on samples of material cut from the liner at the end of storage, showed that its resistance to tear remained almost unchanged, elasticity changed by 10-20%, and gas permeability improved by 60% over that of the original liner. Improvement in gas permeability is due to the loss of plasticiser components.
High velocity winds in the Nicosia area occur particularly in November from the north. Since the liner forms an hermetic seal, air is unable to penetrate it, and the covering provides excellent resistance to stormy weather. Periodic examination of the liner over the 34 months of storage showed no weather damage to the liner, and the welded sections remained undamaged. Consequently, water penetration from the surface to the grain bulk was not possible.
The grain loading method and the overliner held under tension provides a slippery surface that makes it extremely difficult for rodents to make an incision in the material with their teeth. This has been confirmed by laboratory trials carried out with liners kept under tension and exposed to rodents (unpublished results). Although rodents may be able to climb over the walls, damage to the liner was not perceived.
Polyethylene underliner
The concrete platform was constructed at an elevated site to prevent water infiltration from underneath and also well protected from rainwater from above by the. PVC liner. An adhesive tape was used for holding the polyethylene underliners together until the grain was loaded and held the liners against the floor. The underliner served as a gas barrier rather than protection from water.
Losses
The weight of mould-damaged grain removed from the peak of the platform was 26.72 tonnes, representing a 0.665% weight loss for 34 months or an annual loss of 0.222%. Additional losses (insect damage and spillage) based on inlet and outlet weigh-bridge data, amounted to 12.08 tonnes. Total storage losses for the 34 months were 38.8 tonnes or an annual loss of 0.34%. These losses were relatively high in comparison with trials conducted in Israel (Navarro et al., 1984, 1993). The major loss in the present trial was from mould-damaged grain due to moisture migration. This problem has been significantly alleviated in recent bunkers in Israel which have a flattened area at the peak. We believe this serves to broaden the area over which moisture is deposited during phases of convection currents and condensation beneath the liner, and in consequence mouldy grain is only rarely encountered.
© 1995 Elsevier Science B.V. All rights reserved.
