Sesame is an erect, annual plant (Sesamum indicum L. Pedaliaceae), which is cultivated for its seeds, which are used for food flavouring and for the extraction of a prized oil. The seeds are yellowish-white, brown, and grey through to black.
Characteristics of the seeds are that they are flat, pyriform (pear-shaped), finely punctate (marked with minute spots or indentations), with four delicate, longitudinal ridges at the edges of the flat faces. The seed coat consists of an epidermis of radially elongated cells, with thin, cutinised outer walls; each of the cells contains a rosette calcium oxalate crystal.
The remainder of the testa (seed coats) are made up of collapsed cells containing numerous prismatic calcium oxalate crystals, while the endosperm and cotyledons consist of polygonal cells that contains fixed oil.
Harvesting is the manual or mechanical removal of a food product from its natural source or parent plant. Sesame seeds have a faint nutty odour and are ripe and ready for harvesting after about five months. During harvesting the whole plants are cut down and stacked to dry, after which the seeds are shaken from the capsules and stored, if immediate processing is not possible. In the harvesting process, considerable hand labour is required in order to prevent the loss of seeds.
The product can be kept under controlled conditions for a specific length of time until further use.
Appropriate conditions for storage:
The proper conditions under which to store grain prevent and discourage the growth of microorganisms and insects, and involve control of the following conditions:
- Moisture content of the grain
- Temperature of the grain
- Condition and soundness of the grain
- Oxygen supply of the storing environment
Conditioning processes of cereal grain:
Advantages of conditioning (drying)
- Allows for the harvesting of tough grain and thereby reduces losses due to weather and wildlife
- Extends harvest time
- Earlier harvesting is possible
- Drying tough or damp grain can reduce or eliminate spoilage in storage
- May improve market grade and acceptability of grain
- May afford alternative market outlets for grain
- Since artificially dried grain usually contains near maximum allowable water content, the extra weight generates more money when sold
Disadvantages of conditioning:
- Requires extra capital for equipment, energy, and operation
- Requires extra labour and inconvenience of handling unless centralised drying facilities are available
- Requires some experience to operate effectively
Types of grain dryers:
These dryers have a low setup cost, require minimal supervision and make it possible to obtain variable batch sizes, but their initial cost, including grain handling, is high, they require careful management to obtain acceptable uniform drying, and require matching loading and unloading equipment.
These dryers are capable of uniformly drying grain because of constant recirculation (in recirculation type dryers), but require supervision and there is a loss in drying time when the dryer is loaded and unloaded.
Continuous flow dryers:
These dryers can dry large quantities of grain without stopping, but their initial cost is high and they require careful management as well as matching loading and unloading equipment.
Storage facilities are intended to:
- Prevent losses due to leakage, rodents, and livestock
- Be centrally located
- Protect the harvested grain from damage due to moisture migration, snow, rainfall, heat, insects, and mould
- Provide easy access for handling or inspecting of the stored grain.
It is advisable to have several smaller storage facilities instead of only one or two large ones, as small volumes cool down faster and more evenly than large volumes. The best storage facilities are weatherproof, ventilated and of a single wall construction, and may be made of metal or wood.
Advantages of metal storage structures:
- Fire proof
- Rodent proof
- Require little or no maintenance
- Provide fewer places for insects to breed
- Infested grain can be fumigated more effectively
Disadvantages of metal storage facilities:
- If erected on reinforced concrete slabs, these slabs sometimes crack and allow ground moisture to seep into the grain
- Expansion of storage sides when filled with grain which requires waterproofing of the open seams
Advantages of wooden storage structures:
- Most of them are small and may therefore be moved when empty
Disadvantages of wooden structures:
- Require frequent repairs to keep them weather proof
- Not fire proof
- Not as rodent proof as metal structures
- Cracks in wood make it difficult to effectively control insects
Cleaning of cereals
Cleaning removes undesirable elements, impurities, damaged and shrunken, or broken grains. It involves the removal of material adhering to the surface of the grain by methods such as abrasion, attrition, or impact.
Sieving basic operation:
The sieve is also known as the flat bed screen. The multi-deck flat bed screen is made up of a number of inclined or horizontal mesh screens that have aperture sizes ranging from 20 to 125 mm. These screens are stacked inside a vibrating frame. As the frame vibrates, the food particles smaller than the screen apertures pass through them under gravity. This process continues until the particles reach a screen with an aperture size that retains them.
The rate of separation is controlled by a number of things:
- The shape and the size of the particles
- The nature of the sieve material
- The amplitude and frequency of the vibrations
- The effectiveness of the methods used to prevent blocking of the sieves
The main problems with sieves are:
- Blocking, especially if the particle size is close to that of the screen apertures
- High feed rates that cause the screens to become overloaded and cause small particles to be discarded with the oversize particles
- Large particles that may block the screens
- Excessive moisture or high humidity that will cause the small particles to stick to the screen or to agglomerate and so form larger particles that are discharged as oversize
Aspiration basic operation:
Aspirators use a moving air stream to separate food pieces from each other, and to remove contaminants from foods, based on differences in their densities. This principle is similar to that used by the cyclone separator that has found very wide application for removing solids from air or liquids from vapours. Aspirators are also widely used in harvesting machines to separate heavy and light contaminants from grain or vegetables.
Disc separator basic operation:
A disc separator consists of a series of vertical metal discs with precisely engineered indentations in the sides to separate grain from weed seeds. The indentations on the discs match the shape of the grain and as the discs rotate, the grain kernels are lifted out and removed. The discs may be changed for the separation of barley, oats, rice, wheat, and other grains.
Magnets basic operation:
The contamination of grain by metal fragments or bolts from machinery pose a potential hazard in all processing, and for this reason the grain passes through magnets. Ferrous materials are removed by either permanent magnets or electromagnets. The electromagnets are easier to clean, as their power source can be switched off, but permanent magnets are cheaper. It is important to regularly inspect permanent magnets to prevent a build-up of a mass of metal, which may be lost into the product all at once to cause gross re-contamination.
Dehulling involves removing the hulls of grain kernels.
Dehuller basic operation:
An impact dehuller consists of a rotating disc that feeds the grain and then flings it out to strike the wall of the cylinder. This causes the hull to come apart and be separated from the grain.
Extraction is the separation of various fractions or specific components that are required for further processing from raw food products.
Screw press basic operation:
The seeds are fed continuously into the press. While inside the press, a worm screw increases the pressure progressively as the material moves through a slotted barrel. The pressure applied varies from 700 to 2 100 kilograms per square centimetre and the extracted oil is squeezed out through slots, leaving behind the cake. This cake contains between 3 and 3,5% oil when pressed under optimum processing conditions, and between 4 and 5% oil under average conditions.
The screw press has numerous advantages, including:
- It is a continuous process
- This type of press has a greater capacity compared to other equipment
- The press requires less labour
- It removes more oil
The pressed sesame cake still contains a percentage of oil, and as sesame oil is highly priced, this remaining oil may be extracted by means of solvent extraction. Commercial methods of solvent extraction use volatile purified hydrocarbons. It must be noted that solvent extraction will get more oil out of seeds than is possible through pressing.
Solvent extraction basic operation:
In continuous solvent extractors, fresh flakes are added at a continuous rate and subjected to a counter flow of solvent. The solvent percolates through the seed flakes contained in perforated baskets, which move on an endless chain. When the extraction cycle is complete, the baskets containing the extracted flakes are automatically dumped and then refilled with fresh seeds to initiate another cycle.
Refining involves the separation of unwanted, coarse particles from a food product or the fragmentation of coarse particles to finer fragments. The refining of the crude oil may be done either by water refining or by alkali refining.
Remember that whereas water refining will settle much of the gummy material from the oil, alkali refining will settle additional minor impurities, which include free fatty acids that combine with the alkali to form soaps. These soaps can then be removed from the oil by filtration or centrifugation.
Alkali refining basic operation:
Many of the materials that contribute to undesirable properties in oils, may be removed through alkali refining. The oil is treated at between 40 and 85 °C with an aqueous solution of caustic soda or soda ash, and the refining process may be done in a tank or in a continuous system.
In the batch operation, the emulsion of soaps formed by the free fatty acids, as well as the other impurities, known as soap stock, settles to the bottom of the tank and is then drawn off, while in the continuous operation, the soaps and impurities are separated with centrifuges.
After the refining process, the oil is washed with water to remove any traces of alkali or soap stock. Oils that have been refined through the use of soda ash or ammonia usually require a light re-refining with caustic soda in order to improve its colour. After the washing process, the oil is dried by heating it in a vacuum or by passing it through dry filter-aid material.
Water refining basic operation:
Water refining, also known as degumming, means treating the oil with a small quantity of water, followed with centrifugal separation. Water refining is applied to many oils that contain phospholipids and since the separated phospholipids are waxy or gummy solids; the term degumming was applied to this kind of separation. The degummed oil can be used directly for industrial applications, or it may be refined through alkali refining for ultimate edible consumption.
Even after the refining processes, the oil contains various plant pigments. These pigments are removed by passing the oil, after a short heating process, over fuller’s earth, which is a natural earthy material that decolourises oils, activated carbon, or activated clays.
The plant impurities are absorbed onto the agents and so removed by filtration. Bleaching also reduces the resistance of oils to rancidity as some natural antioxidants are removed together with the impurities.
Winterising (the removal of fats, lipids, and other unwanted materials from crude oil extract)
It is also desirable to remove the traces of waxes, such as cuticle wax from seed coats, as well as higher melting glycerides from fats and oils. The waxes are removed by rapid chilling and filtration, while the separation of high-melting glycerides require very slow cooling for the formation of crystals that are large enough to be removed through filtration or centrifugation. Also, crystallisation and settling in a refrigerated product such as salad oil is prevented by cooling and then removing the crystals before the final product is bottled.
The natural oil and fat obtained from seeds, meat and fish can contain lowmolecular-weight odorous compounds, which are desirable in some products and so are not removed, but in other oils, such as fish oils and seed oils, they give rise to disagreeable odours.
These odours may be removed by heat and vacuum or by absorption into activated charcoal. The required heat is supplied by injecting steam into the oil in low-pressure evaporators.
Bottling involves preserving and sealing the product in glass bottles.
Glass bottles are made by heating a mixture of sand, sodium oxide and calcium oxide with a proportion of broken glass, after which it is shaped in a mould through the use of either the blow-and-blow process or the pressand-blow process. The glass is then annealed (heated and cooled to make it less brittle) at a temperature of between 540 and 570 °C in order to remove stresses, after which it is cooled under carefully controlled conditions to prevent distortion or fracturing.
Glass containers have numerous advantages. They:
- are impervious to moisture, gases, odours, and microorganisms,
- are inert and so do not react with or migrate into foods,
- have filling speeds comparable to that of cans,
- are suitable for heat processing when hermetically sealed,
- are transparent to microwaves,
- are re-useable and recyclable,
- are resealable,
- are transparent to display contents,
- can be moulded into a wide variety of shapes and colours,
- are perceived by consumers to add value to the product,
- are rigid, which allows stacking without container damage.
The disadvantages are:
- The higher weight of glass incurs higher transport costs than other types of packaging,
- lower resistance to fractures, scratches, and thermal shock,
- more variable dimensions than other containers,
- potential hazards from glass splinters or fragments in food products
The selection of the right filling machine depends on a number of factors, including the nature of the product and the production rate required. Volumetric fillers are used for the filling of liquids, pastes, powders and particulate foods, and the filling heads are either in line or in a carousel arrangement. All fillers should accurately fill the container without spillage or contamination of the seal, they should have a ‘no container-no fill’ device and be easily changed to accommodate different container sizes. Glass containers should have a headspace of 6 to 10% of the container volume for normal sealing temperatures.
Glass containers are sealed with one of the following seals:
Pressure seals: This type of seal is used mostly for carbonated beverages and include cork or injection moulded polyethylene stoppers, crown caps and aluminium roll-on screw caps. Normal seals are used for pasteurised milk or wine bottles, and include cork stoppers fitted with tinned lead or aluminium capsules and aluminium foil caps.
Vacuum seals: These seals include omnia and twist-off lids, as well as lever-off or pry-off types.
Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.