Fluidized bed heating technique for controlling of post-harvest insect pests in paddy and milled rice

Abstract

The efficacy of heat fluidized bed (FDB) was studied on the laboratory heat fluidized bed (Christison Scientific, UK) on Khao Dawk Mali 105 paddy at 40, 45, 50, 55, 60, and 65°C for 60-240 seconds to control stored product insects. The result showed that an internal feeder; maize weevil (Sitophilus zeamais Motschulsky) and lesser grain borer (Rhyzopertha dominica (Fabricius)) at the pupal stage were the most tolerant to FBD compared to other stages, i.e. egg, larval and adult stages while an external feeder; the red flour beetle (Tribolium castaneum (Herbst)) and sawtoothed grain beetle (Oryzaephilus surinamensis Linnaeus) at the egg stage showed the most tolerated to FBD.

The optimal condition comprised of temperature and time of exposure of FBD to treat those tolerant stored product insects were carried out. The result showed that pupal maize weevil and lesser grain borer showed 100% mortality and no progeny (F1) at various conditions; 55°C for 150 seconds; 60°C for 90 seconds and 65°C for 60 seconds. The egg stage of the red flour beetle and saw-toothed grain beetle showed 100% mortality at conditions 40°C for 180 seconds; 45°C for 150 seconds, 45°C for 180 seconds, 50°C

for 60 seconds and 50°C for 120 seconds. At condition that causes the insects to die completely, the moisture content of paddy was reduced according to the increased temperature level and has a lower grain temperature of the paddy than the initial temperature setting. The percentage of whole kernels and head rice yield was not different from the control. However, milled rice was yellow color with the increased of b* value, and the decreased of L* value and whiteness index. The moisture content and amylose content were decreased. Although there was no statistically significant difference of the yellowness (b*) value among storage period. The yellowness (b*) rose significantly over time.

The Fluidized Bed (FDB) heat treatment at 60°C for 120 seconds can control all stages of maize weevil, lesser grain borer, red flour beetle, and sawtoothed grain beetle, so this condition had set on the fluidized bed system in rice milling at Charoen Phon Rice Mill Company Limited, Kong Sung, Mueang Nakhon Ratchasima District, Nakhon Ratchasima Province to control stored product insect in new harvested paddy in the production season of 2021. The industrial fluidized bed system, loading capacity of 20-25 ton/ hr. with a flowing rate of 8 m/seconds in the heating chamber were installed. Then the paddy samples were taken for 300 kg and the untreated control of paddy was prepared for 300 kg. In this condition, paddy at 28-30% mc. was reduced to 16-18%. Then paddy was air dried until 14% mc and kept in bag for 12 months. The 5 kg-bag samples of paddy were prepared individually in each month. The insect infestation in untreated control showed in the second months with maize weevil (24.50 insects/500 grams), Angoumois grain moth showed the highest number in the eight months (68.45/500 grams), and lesser grain borer showed the highest number in the eight months (5.75 insects/500 grams) while the number of insect pest in FDB was less than 1 insect per 500 grams. The number of insects was less than 1 insect/500 grams until month 8 for maize weevil and slightly increased up to 5.35 insects/500 grams. The result indicated that paddy was treated with FDB at 60°C for 120 seconds can significantly suppress number of maize weevils for 8 months. For Angoumois grain moth and lesser grain borer which is the primary insect pest found in paddy during harvesting, there was less than 1 insect/500 grams for all 12 months.

In terms of grain physical quality and color quality and chemical quality and the quality of cooking and eating of Khao Dawk Mali 105 paddy that has been heated by a fluidized bed and stored in plastic bags for 12 months. It was found that Moisture content in untreated control was significantly different from fluidized bed heat treatment. There is interaction between storage time and heat treatment (P<0.05). The moisture content of paddy was affected by temperature and relative humidity under both storage conditions. Storage time after paddy treated with fluidized bed heat treatment and untreated control had no effect on the chalky grain. The percentage of whole grain kernel and head rice treated with FDB (58.52±0.47%) was significantly different from untreated control (59.82±0.60 %) (P<0.05). The color values brightness (L*) and whiteness index decreased with longer storage periods significantly different from the untreated control. The gel consistency, alkali spreading value, and amylose content vary with storage time. With extended storage times, gel consistency, alkali spreading value, and amylose content tended to decrease. The concentration of 2-acetyl-1-pyrroline (2-AP), an aromatic compound, significantly decreased when paddy was stored. The average 2-AP contents of the fluidized bed (1.29 ppm) was significantly different (P<0.05) from the 2-AP contents of untreated control (1.50 ppm). However, all the characters were presented in the Thai Hom Mali Rice Standard.