By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

Sunflower seeds are a rich source of oil, which has many applications in the food industry. The oil cake is a by-product that finds application in animal fodder. This month we introduce sunflower and the diverse processed products.

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

Defatted sunflower meal

Sunflower meal is good source of protein but has limited application as human food because of the chlorogenic acid and caffeic acid content which cause an unappetising dark-green discoloration of the products when the pH increases above a certain point.

Milled sunflower seeds have potential use in bread, pastries, cake and as a thickener in various products, but finds its main application in animal and bird feed as a protein supplement.

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 as a result of the effects 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 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 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 on a continuous basis 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;
• de-stoners 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.

Cleaning of sunflower seeds

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

Scouring

Abrasive treatment to remove dirt/soil and surface layers adhering to the surface of the seeds. The seed is moved within a chamber by rotors, to which beaters or pins are fixed, which rotate within a horizontal, cylindrical wire mesh. Seeds enter one end and are thrown against mesh. Cleaning is achieved by friction of seeds against each other or against 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 off particles lighter than the seeds.

De-hulling of sunflower seeds for processing

De-hulling may be achieved in different ways. One method involves passing the seeds through a number of drum-shaped chambers with revolving plates that impel the seed against the walls by centrifugal force. The hulls are loosened by the friction created, and separated by a combination of forced air and gravity.

Rubber roll de-hullers are a pair of rolls equal in diameter, which turn in opposite directions at different speeds. They must be replaced every 100 to 150 hours. The rough seeds are fed into the nip. The pressure exerted and the gap are adjustable to ensure best performance of the equipment. It is a highly effective method and more than 90% of the hull may be removed.

The disk sheller uses abrasion by a horizontal abrasive wheel covered with emery or carborundum, which rotates on a vertical axis below a stationary wheel, also with an abrasive surface. 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. The hulls are utilised in animal feeds as roughage, or for the manufacture of chemicals, or as a substrate for yeast cells.

Steeping of sunflower seeds

The seeds are steeped in a solution of 0,3 to 4% citric acid for 4 to 12 hours to soften them. Steeping is performed in a series of tanks through which the steep water is pumped. The moisture content of the kernels increases rapidly to between 35 and 40% and then more slowly to between 43 and 45%.

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 crushing, breaking, grinding or flaking. The choice of the 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.

A roller mill consists of a pair of aligned rollers rotating in opposite directions at different speeds. A fluted roller used to break seeds consists of flutes, which resemble italic V’s (one side shorter than the other). This process relies on the separation of the botanical tissue of the seed (endosperm from endocarp, test 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 rollers are counter rotating with a surface temperature of 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 roller surface. The thickness of the flakes can be between 0,25 and 0,45 mm.

Extraction of oil from sunflower seeds

Oil is extracted from the seed by means of a pressing action or by the use of solvents or a combination of both. As a rule, solvent extraction only becomes cost-effective when the processing capacity exceeds 200 tonnes of fodder per day.

Pressing:

Cold pressed oil is of superior quality. Mills or screw presses are used to break down the cell walls and set the oil free. The pressure is applied for 5 to 9 minutes. The oils are squeezed from the seeds by physical pressure using (1) a hydraulic press or (2) a screw press.

• The hydraulic press consists of a vertical perforated metal cylinder. A pressure plate is lowered onto the material and the pressure is gradually increased. The liquid expelled flows through the perforations and is collected. The residual oil content in the pressed cake is approximately 3%.
• The screw press consists of a horizontal metal cylinder in which a stainless steel helical screw is present. The pitch of the screw flights decreases towards the discharge end of the cylinder, increasing the pressure as the product moves through the cylinder. The final section of the cylinder is perforated to allow the expelled liquid to be collected. The product is fed into the barrel at the wide end and is expelled without the oil at the other end. The pressure is approximately 300 atm. The cylinder may be water cooled to ensure that no damage occurs to the pressed oil as a result of heat developed during the movement of the product through the cylinder. The residual oil content in the press cake is approximately 3%.

Solvent extraction:

The oil can also be extracted by dissolving the oil in a solvent. The oil is present in vacuoles in the cell walls of the seeds, and the cell walls must be softened for optimum recovery of the oil. Solvent extraction requires the seeds to be rolled between rollers to break cells and set the oil free. The solvent, usually hexane, is added at a ratio of 1:1 to 1:3. Extraction conditions vary, but typically times of 15 to 60 minutes and temperatures of 40 to 65°C are used. Two types of extractors are used, either in combination with each other or in combination with one of the above pressing processes.

• The percolation extractor: The solvent is pumped over a bed of the seeds and the solvent percolates down, leaving the cylinder through perforated bottom of the cylinder.
• Immersion extractor: The crushed seeds move through a pool of solvent. The seeds are propelled by chain conveyer or a screw conveyor. The recovered solvent-oil mixture `Miscella’ is heated to recover the solvent for re-use during future extraction. The extracted seeds/flakes are called “spent flake” or “cake”. The residual oil content in the cake is approximately 1%.

Drying of the defatted sunflower seed

The defatted seeds are then dried until the moisture content is 4%. Steam tubes or louvre dryers are used. Single or multiple pass conveyor dryers can also be used. Particles are separated according to size by scalping, dusting and grading. Sieves are used, the most common being the plansifters which are flat, rectangular, framed sieves held within chests suspended on flexible canes in such a way that they can gyrate.

Milling of sunflower seeds

Sunflower seeds are converted into meal with roller mills. These mills consist of numerous pairs of reduction rollers aligned in parallel and rotating in opposite directions at different speeds. 10 to 15 sets of reduction rollers are usually used. The crushing action of the rollers reduce the size of the endosperm, reducing some of them to the fineness of flour and at the same time flattening out the bran and germ-containing particles. The stock leaving each pair of rolls is sieved. The sieved product is separated into two fractions – the finer and purer fraction that moves on to the next pair of rollers while the coarser fraction pass through more rollers.

Particles are separated according to size by scalping, dusting and grading. Sieves are used, the most common being the plansifters which are flat, rectangular, framed sieves suspended within chests on flexible canes in such a way that they can gyrate.

Metal detection and inspection of sunflower meal

The meal is then passed over magnets to remove ferrous metals and metal detectors to detect non-ferrous metals that may have been deposited by the processing equipment. The meal is also inspected to ensure that the products adhere to the minimum predetermined quality standards required for further processing and eventually end use.

Packaging of sunflower meal

Packaging is defined as the containment of a food product in a protective barrier that prepares goods for transport, distribution, storage, retailing and end use. The product must be packaged as soon as possible to prevent the absorption of moisture that can cause spoilage. The product requires packaging that protects it against moisture absorption and insect infestation.

Legislation

The processor should be familiar with and adhere to the laws and regulations that are applicable to the products manufactured to ensure the safe and legal marketing of oilseed products.

Next month we shall take a closer look at sunflower meal.

Published with acknowledgement to the ARC – Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.