Low-temperature drying, also known as near-ambient drying, is one of in-storage drying methods. There are four major factors which influence low temperature drying the variability of weather, the harvest moisture content, the air flow in the storage bin and the amount of heated air. Scanair low-temperature heat pump dryers are built to dry grain as slowly as possible, while in the same timeless spoilage on the grain. It is suggested that low-temperature drying system is better operated when the average daily temperature is between 30°C and 50°C. Rather than control the drying air temperature, the low-temperature drying focuses on the relative humidity in order to achieve equilibrium moisture content (EMC) in all grain layers. Low-temperature drying process usually takes 5 days to one month depends on several important variables weather, airflow, initial moisture content and amount of heat used. Among which, airflow is the key factor. Without appropriate airflow rate, spoilage will occur before drying is completed. By using heated dried air with Scanair Solar Assisted Heat pump Dryers the relative humidity of the drying air is better controlled to achieve the desired moisture content in the air of drying process.
CARDAMOM DRYING PROCESS
Cardamom curing is a process in which the moisture of freshly harvested capsule is reduced from 80% to 10to 12% at an optimum temperature of 50oC so as to retain green colour to the maximum extent. This is the most important part of the process as it affects the quality of the final product. It is important to dry the cardamom capsules as soon after harvest as possible to prevent the loss of flavour. It is also important that the drying process is as short as possible so that mould does not grow on the capsules and the bright green colour is retained. The drying temperature should not be above 50°C as this affects the colour and delicate flavour of the final product. In most places, cardamom capsules with a good green colour can be sold for a premium price.
The moisture content of a fresh cardamom capsule is about 85%. This needs to be reduced to 10% in the dried product so the cardamom capsules can be stored. If the drying period is too long mould can start to grow on the cardamom. There are several options available to the small-scale processor, depending upon the size of the business and the local weather conditions at the time of processing. Each method has different advantages and disadvantages.
Humidity-controlled drying Scanair drying chamber has been developed that helps to reduce colour loss and to produce high-quality pods. The cardamom capsules are placed in the drying chamber, which is at a temperature of 50°C. During the first two hours of drying, the humidity builds up within the chamber. This allows the cardamoms to ‘cook’ and at the same time destroys the enzymes that break down the chlorophyll (chlorophyll gives the pods their green colour). No light is allowed into the drying chamber. After two hours the humid air is blown out of the chamber and the humidity reduced. The capsules are left in the chamber to dry until they have a final moisture content of 10% in the drying chamber
PEANUT DRYING PROCESS
The combination of average drying conditions of 34.6ºC temperature and 27% relative humidity were very effective in drying peanut. There was no loss of seed coat colour and darkening of the cotyledons The physiological quality of the peanut seed was maintained by the new technology dryer. Drying with low temperature and relative humidity at recommended air flow rate did not reduce the observed that slow drying or low temperature/low humidity drying reduced physical damage and maintained high vigour and viability during storage. The maximum recommended plenum temperature is 35°C and the minimum plenum humidity is approximately is 50% in peanut seed vigour index based on germination speed when the drying temperature was increased from 43.33 to 104.44°C in a hot air drying system. A slight increase in germination percentage from 82 to 89% was observed which could be attributed to seed pathogen control due to the HPT drying process. The seeds after drying produced no abnormal seedlings due to fungi infection as observed in the test with wet seeds dried peanut seed at 34.1°C and 40% RH and obtained germination value of 76% which was 13% less than observed using the HPT dryer system. Peanut seed drying provided by the heat pipe technology is an efficient process and caused no detrimental effects on seed physiological quality.
Drying is an energy-intensive process which involves the removal of moisture from a crop until the moisture content of the crop is in equilibrium with the surrounding air. Crops are usually dried to a moisture content that will allow safe storage and/or further processing eg. Milling .The drying behaviour of different crops depends to a large extent on the physical characteristics of the crop for example particle size, internal structure and composition. These characteristics are crop specific, and so the drying behaviour of crops is usually different. In order to select an appropriate dryer and the drying parameters such as drying time and temperature, it is important to understand the drying behaviour of the material to be dried. This principle forms the basis of many drying studies, which are conducted to determine moisture loss characteristics, physical and biochemical changes of specific crops.
The traditional method for crop drying in the tropics is sun drying. Sun drying usually involves the spreading in thin layers of crops such as paddy, coconuts, coffee, shrimp and fish on concrete floors, large trays, galvanized sheets or simply on pitched roadsides until the crop is sufficiently dried. This method has its inherent advantages of no operational energy cost for moisture evaporation, together with a low or negligible initial investment, except for the larger sun drying installations such as those used in Guyana for paddy or in Grenada for cocoa beans. These may require as much as 180 m2 of drying floor space per ton of paddy.
There are several limitations associated with traditional sun drying. During periods of intermittent and particularly during continuous rainfall, crop drying is not possible and the risks or crop losses are high. Sun drying is slow and weather dependent compared to other alternative drying systems. Crop quality may be low due to contamination by dust, dirt, stones and insects, while direct crop losses from theft and livestock consumption can be high. Sun drying also suffers from a high labour requirement and excessive crop handling particularly in periods of inclement weather which can result in high costs, crop damage and a loss in quality.
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