Processing of oil seeds – Part 17: General production of sunflower oil

By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

After discussing the production of sunflower oil for medium and small-scale farmers, we now take a look at the general and commercial production of sunflower oil.

Sunflower oil is an important oil seed product with good flavour stability, coupled with its high linoleic acid content. Sunflower seeds yield between 40 and 60% oil. The oil can be used as salad and cooking oils, shortening, and margarine production. Cold-pressed oil is suitable for table use and hot-pressed oil, which is of lesser value, is used for soap-making, candle-making, lubricants and mixing paints. The residue oil cake (press-cake) is used for animal feed.

 We thank the ARC Agricultural Engineering in South Africa who made the information available to the readers of ProAgri Zambia.

Harvesting and storage of sunflower seeds

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 as a result of 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 spoilt 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 metals, 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 and repair cost of the processing equipment.

Cleaning sunflower seeds

Unwanted materials adhering to the separated seeds are removed by cleaning through a combination of scouring and aspiration.

Scouring

Scouring is an abrasive treatment to remove dirt/soil and surface layers adhering to the surface of the seeds. The seed is moved inside a horizontal cylindrical wire mesh chamber by rotors, to which beaters or pins are fixed. Seeds enter at one end and are thrown against the mesh. Cleaning is achieved by friction of seeds against each other or against the mesh. Dust and fine particles pass through the mesh screen and are collected separately.

Aspiration

Aspiration removes particles loosened by scouring which are not removed from the seed during the scouring process. An air current is blown either through or over the seeds to lift of particles that are lighter than the seeds.

Hulling of sunflower seeds for oil extraction

The seed coat contains little or no oil; hence 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 press cake. Sunflower seed requires only 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 is insufficient, an oily paste will emerge from the expeller with little or no free-run oil. The hulls are utilised in animal feeds as roughage, or in the manufacture of chemicals, or used as a substrate for yeast cells.

Commercial hulling processes comprise of the following:

• Rubber roll hullers are a pair of rollers equal in diameter, that turn in opposite directions at different speeds. They must be replaced every 100 to 150 hours. The rough seeds are fed into the nip. Pressure exerted and the gap are adjustable to ensure best performance of the equipment. It is a highly effective method that can remove more than 90% of the hulls.
• A disk sheller uses abrasion by a horizontal abrasive wheel covered with emery or carborundum, which rotates on a vertical axis below a stationary abrasive surface wheel. The rough seeds are added through an opening in the centre of the stationary top wheel. Optimal hulling is achieved when the clearance between discs is slightly more than half the grain length.

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 to prevent contamination of the oil and complicated 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 rolls.

Roller mills are used with a pair of rollers, working against each other in opposite directions. One of the rolls rotates faster than the other (speed differential). One roller has 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 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 rolls. 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 to allow for chilled water to pass through, removing any excess heat to keep the temperature between 40 and 46° C. The surface temperature is between 40 and 48° C, and a pressure of 40 tonnes can be exerted by the rollers. Roller speed is 100 to 400 rpm and the 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 rollersurface. 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 done 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 30 to 60 minutes. Heating also combats enzymes in the plant tissue that would have a detrimental effect on the oil quality. If the oil cake is to be used for animal feed, controlled heating may be useful in increasing protein availability in the resultant oil-cake fraction.

Next month we shall look at the extraction, clarification, refinement, hydrogenation, winterisation and packaging of sunflower oil. Published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.