By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek
The medium-scale oil processor can process 300 kg to 1 tonne of sunflower seeds per hour. Sunflower seeds yield between 40 and 60% oil of which 65 to 80% may be extracted using medium-scale oil presses or expellers.
The oil is usually cold-pressed and yields high quality oil for frying, salad or shortening purposes. The oil generally retains anti-oxidants better than large-scale produced oil. This is important from a health and shelf-life perspective.
We thank the ARC Agricultural Engineering in South Africa who made the information available to the readers of ProAgri Zambia.
Medium-scale processors are generally close to the sunflower production area, resulting in the minimum need for transportation. Marketing possibilities of the oil range from direct selling and marketing to the local rural people in retail packaging, or as bulk packaged products for large plants. Sunflower oil is a very popular product in fast food franchises as well as health shops.
A great advantage of the medium-scale processor is the fact that the equipment needed is locally available.
A popular by-product of sunflower oil is oilcake. It can be sold to nearby stock farmers as animal feed. This also saves on transportation costs. It is important to exercise caution to maintain the quality of the protein in the oilcake. During processing, a significant amount of protein denaturation can take place. For feeding purposes, the denaturation is generally desirable, because of the resultant improvement in digestibility. However, excessive heat generated during processing, may result in intensive loss of amino acids. The extent of such a loss will depend on the processing time and temperature, the moisture and the reducing sugar content of the oilcake, as well as other variable constituents and foreign materials in the original material.
Harvesting and storage of sunflower seed
The seed is harvested when the moisture content is approximately 10%. To have seed available throughout the year, large quantities need to be stored. Proper storage is very important. Seeds may be spoiled by mould, insect damage or overheating during storage. This may lead to damage ranging from a moderate increase in free fatty acid content in the oil, to seeds that are thoroughly spoiled and unfit for processing. The moisture content is the single most important factor for storing seeds.
The moisture content must be below 10% throughout the entire storage period. If the seeds are harvested at a moisture content above 10%, the seeds should first be dried, and this adds to the processing cost. The capital cost of proper storage may be substantial, but must be weighed against the overall investment costs. To make maximum use and profit from the capital invested in the processing machinery, processing needs to take place continuously, and this is only possible with a steady supply of properly stored seeds.
Separation of sunflower seeds
The harvested seeds are contaminated with foreign matter that needs to be removed. The foreign matter includes metal, sticks, straw, animal residues, dust and stones. Separation is performed using various apparatus and methods including:
-Magnets to remove ferrous metals
-Metal detectors to detect non-ferrous metals
-Destoners or a series of sieves (rotary or table sieves): coarse screens remove larger objects while fine screens remove particles smaller than the seed.
-Disc separators remove unwanted foreign seeds as well as damaged, shrunken and broken sunflower seeds on the basis of their length
-Aspirators remove light, air-borne particles.
Proper cleaning will not only increase the processing capacity and improve the oil cake quality, but will also reduce the maintenance of the processing equipment.
Hulling of sunflower seeds for oil extraction
As the seed coat contains little or no oil, its inclusion would make oil extraction less efficient. If the hulls are not removed prior to extraction, they will reduce the total oil yield by absorbing and retaining oil in the pressed cake. Sunflower seeds only require partial hulling, since the seed coats play an important role in the separation of the oil and the cake. If the percentage of husks present were insufficient, an oily paste would emerge from the expeller with little or no free-run oil. The hulls are utilised in animal feeds as roughage, in the manufacture of chemicals, or as a substrate for yeast cells.
Two methods/processes can be used for medium operations:
-The Engelberg huller which combines milling and hulling in one machine. The ribbed cylinder rotates on a horizontal axis within cylindrical chamber. The first/early stages of the machine are dedicated to hulling. The ribs for this are diagonal curves, and the other ribs are parallel to cylindrical axis. The bottom half consists of screens through which fine material may pass. The gap between the cylinders and the flow rate controls the severity of treatment. Hulls removed help to abrade the grain surface further. Hulls, bran and broken endosperm are discharged through screens.
-Two-stage process – hulling is done by running the seeds through two rubber-coated rollers turning in opposite directions at different speeds. The rollers must be replaced every 100 to 150 hours. Pressure is exerted and the gap is adjustable to ensure best performance of the equipment. It is a highly effective method that can remove more than 90% of the hulls.
Size reduction of sunflower seeds (optional)
The extraction efficiency is improved by reducing the particle size and/or increasing the surface to volume ratio. The pieces should, however, not be too small as they may then contaminate the oil and complicate filtration/clarification. The seeds are reduced in size through either crushing, breaking, grinding or flaking. The choice of method depends on the oil content of the seeds, as well as the availability of equipment and the subsequent extraction method to be used. Typical equipment includes roller mills and flaking rollers. Roller mills use a pair of parallel rollers that rotate in opposite directions.
One of the rollers rotates faster than the other (speed differential). Fluted rollers used for the breaking system consist of flutes, which resembles an italic V (one side shorter than the other). This process relies on the separation of the botanical tissue of the seed (endosperm from endocarp, testa [seed coats] and embryo) and reduction of the endosperm into grits. It involves both separation and size reduction. A thin curtain of seeds is fed into the nip (opening) between the rollers. Flutes shear open the seed and the seeds are reduced to a coarse meal.
The broken seeds may be rolled or flattened with large heavy rollers (flaking rollers) if required. Rollers are hollow inside to allow for chilled water to pass through, removing any excess heat to keep the temperature between 40 to 46 °C. The rollers are counter rotating with a surface temperature of 40 to 48 °C. 40 Tonnes of pressure can be exerted by the rolls. Roller speed is 100 to 400 rpm and capacity can vary from 100 to 400 kg/h.
Sharpened steel knives mounted on adjustable holders are used to shave off the flakes that adhere to the roller surface. The thickness of the flakes can be between 0,25 and 0,45 mm.
Conditioning of sunflower seeds (optional)
The seed is conditioned to weaken the cell walls, to lower the viscosity of the oil and to coagulate the proteins in the cell walls. Conditioning is performed in a cooker that heats the seeds directly with water and steam. The seed is kept at a temperature of 90 to 95 °C for a period of 30 to 60 minutes. Heating also combats enzymes in the plant tissue, which would have a detrimental effect on the oil quality. If the oil cake is to be used for feed, controlled heating may be useful in increasing protein availability in the resultant oil-cake fraction.
Oil extraction from sunflower on medium-scale
Oil extraction is accomplished using continuous screw presses or expellers. The expellers apply pressure to the seeds through a screw that rotates in an enclosed casing or housing. The tapered auger feeds the seed into the housing to fill the gap between the auger and the sleeve. The pressure increases from the intake of the barrel to the end.
An adjustable cone closes the housing at the end. The pressure in the sleeve may be adjusted by screwing or unscrewing the cone. Cake emerges from the press through the gap between the housing and the cone.
Very high temperatures may easily occur in these expellers due to the enormous pressure that is generated. The heat aids the extraction of the oil. Care must be taken to ensure that the oil does not overheat as this could render the oil inedible.
Clarification of sunflower oil
In the case of pressed oil, an oil settling and filtering process is required to remove suspended debris. The crude oil is pumped to settling tanks and allowed to stand for at least 48 hours for most of the impurities to settle. The clear oil is then drawn off and the remaining suspended solids in the oil may be removed by either filtration or centrifugation.
Next month we shall look at the refining process of sunflower oil and the recovering of oil from oil cake.
Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.