Modification And Testing Of Biomass Dryer

CHAPTER TWO
LITERATURE REVIEW
2.1    Drying as an Element of Post Harvest
    Drying is an excellent way to preserve crop and solar dryers are appropriate crop preservation technology for sustainable development.  Crop drying is the most energy consuming process in all process on the farm.  The purpose of drying is to remove moisture from the agricultural produce so that it can be processed safely and stored for increase periods of time.  Crops are also dried before storage or during storage, to prevent spontaneous combustion by inhibiting fermentation.  It is estimated that 20% of the world crop production is lost after harvest because of inefficient handling and poor implementation of post – harvest technology, says (Hartman’s, 2001).  Grains and seeds are normally harvested at a moisture level between 18% and 40% depending on the nature of crop.  These must be dried to a level of 7% to 11% depending on application and market need.  Once a crop is harvested, it may have to be stored for a period of time before it can be marketed or used as feed.  The length of time a crop can be safely stored will depend on the condition it was harvested and type of storage facilities being utilized. This crop can be kept in storage for longer period of time before it quality will deteriorate.
    The application of dryer can reduce post harvest losses and is economical because of low cost and does not make use of electricity.  Thus improving the availability of crop in such country (Khalil et al, 2007)
Post harvest losses occur between harvest and the moment of human consumption. They may include on-farm losses, such as when crops are threshed, winnowed and dried as well as losses along the chain during transportation, storage and processing. (Harris et. al 2006).
The main cause of loss during drying is the cracking of grain kernel that are eaten whole, such as rice. Some crops may also be lost during the drying process. However, failure to dry crops adequately can lead to much higher levels of loss than poor. Quality drying and may result in the entire harvest becoming inedible.
Adequate drying by farmers is essential if crops are to be stored on farm and poorly dried crops for the market need to be sold quickly to enable the marketing processing chain to carryout adequate drying before the agricultural crop become spoilt. With a high moisture content, crop is susceptible to mould, heating, discoloration and a variety of chemical changes. Ideally, most crops should be dried to acceptable level within 2-3 days of harvest one of the problems in assessing levels of post harvest loss is in separating weight loss caused by the very necessary drying operation  from weight loss caused by other controllable factors ( Knoth, 2003 ).
2.1.1    Types of Losses
    FAO,( 2004) describes various types of losses.   The first distinction in Agricultural crop is that between quantity and quality.  Quantitative loss is a loss in terms of physical substances meaning a reduction in weight and volume and can be assessed and measured.  Qualitative loss, however, is a loss in terms of physical substance meaning a reduction in weight and volume and can be assessed and measured.  Qualitative loss, however, is concerned particularly with the crops and reproductive value of product and requires a different kind of evaluation.  It should be noted that we shall not consider losses occurring during the production period and caused by various crop pests (insects, weed, disease), even though they have a major influence on crop preservation conditions and account in part for the nature and size of post harvest losses.  
2.1.1.1    Moisture Content
    The moisture content is the amount of free water with a given product and is expressed either as a decimal proportion or a percentage.  In agriculture, moisture content or humidity rate is usually indicated as a proportion of the moist product, i.e. the moisture content is the proportion of the weight of moisture to the total weight of dry matter and moisture.
2.1.1.2    Damage
    Damage is a clear deterioration in the product e.g. broken of pitted grain, which affects most of its quality than its quantity and can in the long-term result in a definite loss.  Both damage and loss should be quantified in terms of weight and cost.
2.1.1.3    Direct and Indirect Losses
    Direct losses occur when the disappearance of agricultural crop is caused by leakage or consumption by pest (insects, rodents, birds), whereas indirect losses occurs when a reduction in quality leads to the consumer’s refusal to purchase.