各類農(nóng)產(chǎn)品貯藏技術(shù)課程介紹-展示頁
2025-01-31 03:29本頁面
【正文】 ines the readiness of the crop for harvest. n This stage of crop development is reached prior to the crop being ready for bining as the kernel is relatively soft (easily dented with finger nail) and kernel moisture percentage is relatively high. n Once a plant has attained physiological maturity, it cannot accumulate any more dry matter.Harvestingn Harvesting methods differ according to the part of the plant to be used.n As regards forage crops, the whole plant is cut, but for underground crops (eg, groundnuts, roots and tubers), the crop is lifted while the soil sticking to it is removed. n With cereals, the crop is first cut either as a whole or partially (ears), and then threshed and cleaned to separate the grain from the ears and straw. n In the latter case two main alternatives exist: separate harvesting and threshing, or bined harvesting and threshing. HarvestingHarvesting methods Manual harvesting Mechanized harvesting n DIRECT METHODS n Defoliation and Desiccationn Leaving it in the field longer predisposes it to weathering. Sometimes, inclement weather threatens the quality of the harvest, thereby pelling the producer to make adjustments in the harvesting schedule. Such adjustments include “inducing” early harvest maturity by applying a defoliant or desiccant. Harvestingn A defoliant causes leafdrop, while a desiccant causes the plant to dry out in the field and die.n In cotton production, green leaves tend to stain the fibers. Defoliation of cotton prior to mechanized harvesting reduces not only the amount of undesirable plant debris in the harvested fiber but also the chance of tainting it with plant pigments. Harvestingn INDIRECT METHODSn Generally, crops are harvested when the conditions of the economic product are such that the product can be stored for a reasonable period of time without deterioration.n However, if the product has a facility for drying, the crop may be harvested sooner than normal harvest maturity then dried to storable moisture content at a later date.Harvesting(ii) Threshing methodsTraditional threshing Mechanized threshing Combined harvesting and threshing methods (iii)Cleaning Drying methods n There is an essential need to dry grain quickly and effectively after harvest and before storage to retain maximum quality, to attain a moisture content sufficiently low to minimise infestation by insects and microanisms (bacteria, fungi, etc.), and to prevent germination. n Natural methods of drying make use of exposure of the wet grain to the sun and wind. n Artificial dryers employ the application of heat from bustion of fossil fuels and biomass resources, directly or indirectly, and in both natural and forced convection systems. n Mechanical dryers, long used in developed countries, are finding increased application as farming and grain handling systems develop.DryingNatural and solar drying n The traditional practice of grain drying is to spread crop on the ground, thus exposing it to the effects of sun, wind and rain.n The logic of this is inescapable。 the sun supplies an appreciable and inexhaustible source of heat to evaporate moisture from the grain, and the velocity of the wind to remove the evaporated moisture is, in many locations, at least the equivalent of the airflow produced in a mechanical dryer. DryingDryingMechanical dryers n BatchinBin Dryersn The small capacity version of the batchinbin dryer, otherwise known as the flatbed dryer, has been developed for farm or villagelevel use. Its capacity is of the order of 13 tonnes/day with drying times of 612 hours. DryingRecirculating Batch Dryersn The dryer is a selfcontained unit with an annular drying chamber, 500 mm thick, around a central plenum chamber, a fan and heater, and
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