The post harvest processing of stevia comprises of mainly three operations :
Separation of leaves and stems
Baling and packaging
All the post-harvest processes are very important for the ultimate leaf quality, and preservation of steviol glycoside content in the leaves.
After the leaves are harvested, they are to be dried as quickly as possible to avoid the chance of browning and loss of steviol glycoside content. If the leaves are not dried properly and within a specific time window after harvesting, drastic loss of steviol glycoside may occur and loss up to half of the initial glycoside content in three days is reported. The initial moisture content of fresh leaves is approximately 80% and they are to be dried to a final moisture level of 10% for long term preservation. So the total loss of weight in drying process is 78%. Higher residual moisture percentage in the leaves during storage results not only in deterioration of the glycoside content but also mould growth and decomposition – making the leaves unusable for glycoside extraction. The most common methods of leaves drying are described below.
100 Kgs Fresh Stevia Leaves
22 Kgs Dry Stevia Leaves
For simple sun-drying, there are two processes. In one process, the leaves are spread in thin layer on plastic sheets – which is kept stretched on the ground. Black colored plastic sheets give better results due to better absorption of solar radiation and faster temperature raise. The leaves are turned occasionally with rakes for better exposure to solar radiation and uniform drying.
In another process, the leaves are dried on plastic mesh screens stretched over wooden/bamboo frames which are kept at a height (minimum 18” of 45 cm) from ground level with legs fitted at corners. In this system, the leaves are exposed to warm ambient air from both top and bottom which results in faster moisture removal and more uniform drying.
With low relative humidity, sun drying of a thin layer of leaves can be quite rapid (9 – 10hours). In tropical climates, it is not advisable to dry the leaves under direct sunlight as to harsh solar radiation may damage the leaves. In these areas, it is better to dry the leaves under partial shades created by shade-nets.
Solar Tunnel Drying
Solar tunnel dryer is a simple, walk-in type, convenient, and efficient dryer at low cost for drying large quantities of stevia leaves. It is essentially a poly house type structure in which drying takes place through natural flow of hot air.
The drying tunnel is built as a walk in type green house or poly-house with hemi-cylindrical roof. The system has a frame structure built with metallic or plastic tube sections. The entire structure is covered with ultraviolet stabilized transparent polyethylene sheet of at least 200-micron thickness. The polyethylene sheet allows smaller wavelength solar radiation to pass through it but is impervious to the longer wavelength infrared radiation radiated back by all the material surfaces within the drying tunnel. Thus, it effectively traps the solar heat to increase the temperature with the tunnel. The cement concrete floor is painted black for better absorption of solar radiation. Five-cm thick glass wool insulation is provided to reduce heat loss through the floor. Inlets for fresh air were provided all along the periphery of the tunnel near ground level. Chimneys for hot and humid air exhaust are provided at the roof of the tunnel.
A solar tunnel dryer can maintain a 10o – 15oC temperature differential between the inside and outside of the tunnel. In the tunnel, multi-shelved racks can be used as drying surfaces, thus, it can offer significant space saving over open air sun drying. The leaves dried in the solar heating tunnels are cleaner, as they are less contaminated with airborne dust and they are dried more uniformly.
Solar Tunnel Dryer Design
Drying of Stevia Leaves in Solar Tunnel Dryer
Modified Flatbed Grain Dryer
It is a mechanical dryer traditionally used for cereal grain and coffee drying, which is later used for stevia leaf drying with success. This dryer is essentially a box with top side open and having a false bottom made of perforated sheet metal or wire mesh. The box structure is generally constructed with wood, insulated metallic sheets or bricks and concrete. The dryer is also provided with a heat source – generally a gas/liquid fuel or biomass burner and an air-blower. The air blower draws hot air from the burner and forces it in the space between the false bottom and dryer floor – which is called plenum chamber. The material to be dried is spread over the false bottom uniformly and the hot air is forced upwards through the layer from the plenum chamber. To facilitate an even airflow through the bed the length of the drying chamber should be 2-3 times the width. The height of the plenum chamber is of the order of 0.3 m. Unloading ports can be fitted at intervals in the walls of the drying chamber.
The advantages of this dryer is that it can be constructed and operated indoors. With this equipment, the drying operation is faster and not dependant on the availability of solar radiation. It can be run in both day and night for better throughput. The initial capital expenditure for this equipment is high and it has considerable operation expenses for its fuel and energy requirement.
These are sophisticated equipment and highly capital intensive. Hence they can be considered more as part of the extraction operations and not as purely field level post harvest infrastructure. More skilled manpower is required for operating these machines and they are not amenable to field level maintenance.
Separation of leaves and stems
The stems of Stevia plants contain very little steviol glycosides, so their presence in the harvested part reduces the overall glycoside content. Moreover, stems elute a lot of tannin and waxy material during extraction, which interferes with the purification of glycosides. So, for making the leaves extraction worthy, the stems are to be separated from the leaves. For very small farms, it can be done manually, but mechanical processes are to be adopted for larger farms. The stems can be separated both before and after drying. The popular processes for leaf stem separation are described below.
A rudimentary defoliation system is used in Chinese farms – which consists of a small motor driven rapidly revolving rubber roller and chute, fitted on a metallic chassis. For leaf stem separation, a bundle of cut plants are placed and held against the rotating roller. It can remove leaves from freshly harvested or semi-dried stems. It’s a small machine with very low power requirement, but its operation is somewhat labour intensive.
Continuous Leaf – Stem Separator
The equipment shown at the left is claimed to be capable of separating leaves and stems for soft stemmed plants like Stevia and Mint. The plant has large throughput and it can work with fresh plants. Detailed technical description about the working principle of the equipment is not available. The most probable working principle of the equipment is a combination of separation with rotating perforated drum, vertical air flow and horizontal air flow classification.
Click on image to enlarge
These equipments are used for separation of leaves and stem from dried harvested part of Stevia. After drying, the leaf stocks become weakened and they can be separated by vibratory screening and air classification. These operations physically break the leaf stock, and then the lighter leaf portion can be separated by winnowing. These equipments are ineffective in case of freshly harvested plants. Some of these equipment have two stage separation system consisting of primary separation of loose leaf followed by stripping and then final separation and use stripping drum with rasp bars and concave.
Baling and Packaging
Dried and cleaned stevia leaves have very low bulk density and are thus very bulky in nature. For ease and efficiency in storing and transport they are compressed into bales and then packaged either in non-woven HDPE bags or in clear LDPE bags. For baling stevia leaves hydraulic horizontal or vertical bailing press is used.
Horizontal Baling and Bagging Machine
The horizontal baling machines have two hydraulic rams, one for compressing the loose leaves into compact and compressed bales, and the other is for ejecting the bales into packaging bags. The vertical baling machines are cheaper and have only one hydraulic ram for compressing the bales. The compressed bales are taken out manually for this type of machine.
The compressed bales can be stored in well ventilated warehouse away from dampness. Proper pest control in the warehouse premises is also a pre-requisite for proper storage of Stevia leaves.