2.3 Transport
The harvested raw plant material of the spice crop should be transported promptly to in clean, dry conditions. The crop They may be placed in clean baskets, dry sacks, trailers, hoppers or other well-aerated containers and carried to a central point for transport to the processing facility. All containers used at harvest should be kept clean and free from contamination by previously-harvested plant products and other foreign matter. If plastic containers are used, particular attention should be paid to any possible retention of moisture that could lead to the growth of mould. When containers are not in use, they should be kept in dry conditions, in an area that is protected from insects, rodents, birds and other pests, and inaccessible to livestock and domestic animals. Conveyances used for transporting bulk plant materials from the place of production to storage for processing should be cleaned between loads. Bulk transport, such as ship or rail cars, should be cleaned and, where appropriate, well ventilated to remove moisture from plant materials and to prevent condensation.
Threshing is the process of removing and separating the fruit or seed from the unwanted flower stems or plant stalks as well as removing damaged or immature material. This process can be undertaken by hand, assisted by sieves and screens, by use of winnowing or by mechanical shakers and sorters. Pepper is an example where spikes need to be washed prior to threshing to remove dust and dirt. Harvested spikes are spread onto a clean floor and threshed (decorned) manually by trampling or by using a mechanical thresher. The berries are also graded at this stage by removing undersized berries and put into a size category.
This the most critical process in the production of dried herbs and spices. The aim of drying is to reduce the moisture content of the product from actively growing in the field to a level that prevents deterioration of the product and allows storage in a stable condition. Drying is a two stage process: firstly the transfer of heat to the moist product to vaporize the water in the product and secondly mass transfer of moisture from the interior to the product surface where it evaporates. The most important and immediate management concern is to ensure the harvested crop will not rot or become grossly invaded with yeasts, bacteria and mould (producing aflatoxins) or become contaminated by pests. This is the start of the preservation process, which for most spice crops requires drying that will enable the long-term crop storage and the opportunity for further processing. The drying phase of post-harvest management can include four preliminary stages - the selection of high quality produce from the field; cleaning the crop by washing and disinfection; preparing the crop for drying by peeling or slicing; pre-treating with anti-oxidants, blanching or sulfurizing. In some cases, washing prior to processing is desirable to remove field contaminants (dust, soil) using anti-microbial solutions to reduce the microbial populations to a low level prior to the drying process.
There are four main types of drying. The most basic method of drying is to spread the crop on a surface exposed to the sun. In this case, the process is aided by a cover system that prevents wetting with rainfall. An improved method, speeding up the drying, is to use a fuel source (wood, oil/diesel, gas or electricity) to heat the drying room. Solar drying systems together with solar powered fans are also available [18] . The drying process should dry the crop as quickly as possible, at temperature levels which do not drive off the volatile flavour compounds. The drying temperature regime will be specific to each crop as will be the final moisture percentage for storage [19] .
The traditional open sun drying that is widely used in developing countries has major inherent limitations when trying to preserve product quality. High crop loss and low product quality result from inadequate drying, long drying times, fungal spoilage, insect infestations, bird and rodent damage and contamination plus the effects of sunlight and the weather. Even in the most favourable climate it is often not possible to get the moisture content of the product low enough for safe storage. In the tropics the high relative humidity of the air prevents drying of harvested crop products during the wet season.
The objective of a dryer is to supply the product with more heat than is available under ambient conditions. A relatively small amount of heating can greatly enhance the moisture carrying capability of the air. For example, heating air from a temperature of 20°C at 59% relative humidity (RH) to a temperature of 35°C at 25% RH increases the moisture holding capability three times. In a dryer the major requirement is the transfer of heat to the moist product by convection and conduction. The absorption of the heat by the product supplies the energy necessary for the vaporization of water from the product. The process that occurs at the surface of the product is simply the evaporation of the moisture. The moisture replenishment to the surface is by diffusion from the interior and this process depends on the nature of the product. Spices and essential oil crops derived from leaves or flowers are relatively thin and therefore relatively easy to dry due to their small diffusion thickness. Conversely in thick and fleshy materials such as roots, the drying process requires much more careful control of temperature, temperature ramp rate and airflow rate. If the temperature is too high, elevated too quickly or the airflow rate is too high when drying thick fleshy products, ‘case hardening’ may result, and only the outer surface will dry. This dry layer becomes impervious to subsequent moisture transfer.
In many rural locations, grid electricity and supply of other non-renewable sources of energy are too expensive, unavailable or unreliable and drying systems that use mechanical fans and electric heating are inappropriate. The high capital- and running-costs of fossil fuel-powered dryers presents an economic barrier to use by small-scale farmers. Solar-energy drying systems are put forward as technology for small rural farmers and enterprises in developing countries. Solar dryers can be classified into two generic groups, passive or natural air circulation solar dyers and active or forced convection solar-energy dryers (Fig. 6). Forced convective dryers employ motorised fans for circulation of the drying air. The electricity for the fan can come from a solar photoelectric panel and battery. Each group can also be sub-divided into three subgroups:
i) integral types (direct solar dryers where the crop is placed in a drying chamber with transparent walls, and the solar radiation falls directly on the crop, coupled with convection air flow from the heated surrounding air),
ii) distributed type (indirect heating, where solar radiation heats a solar collector external to the drying chamber),
iii) a mixed type where there is both direct and indirect heating.
The integral type dryers are simple in both construction and operation and require little maintenance. However, they are likely to operate at lower efficiency due to their simplicity and there is less control of the drying operation. The distributed type are much more elaborate structures so require greater investment in materials and running costs, but have higher efficiency and as a result product quality is generally higher. The solar dryer has two significant disadvantages: a limited ability to process crops when the weather is poor; and drying can only occur during the daytime. This not only limits production and extends the drying time but also may have an adverse effect on production and product quality particularly fleshy crop products such as roots and stems that typically have drying times of several days. This has led to the development of hybrid systems with auxiliary heating systems such as burners using biomass, biogas or fossil fuels. Alternatively, to achieve more efficient energy use, some active solar dryers are designed with thermal storage devices (mainly rocks or gravel) to extend the drying time during the night time and in periods of low sunshine. Desiccants (such as clay and rice husk) can be incorporated in the design to reduce the relative humidity of the drying air so as to improve the moisture carrying capacity. The use of desiccants is only possible in forced convection systems as they increase the resistance to airflow.
Figure 6. Typical Solar Dryer Designs reproduced from Ekechukwu & Norton [20]
Some information on inexpensive commercial driers suitable for spices is available on the INPhO website; see for example, a simple flat-plate drier from Pakistan :
A solar drier from Mali :
http://www.fao.org/inpho/isma?m=equipment&txt=spice&i=INPhO&p=SimpleSearchDetail&lang=en&op=or&n=1
and a drier for red peppers from Korea :
