Part 16: Refining of sunflower oil on a small scale

By Theresa Siebert, Petrus Britz, Pr Eng and Agrelek

The processing of small quantities of sunflower seed (between 50 and 100 kg) on a daily basis can be classified as small-scale oil processing. The main role players in this category are farmers or entrepreneurs in rural communities that seek to add value to their own sunflower crop.

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

The technology applied to press this quantity of seed is mostly very simple and therefore the oil yield is not as high as with the more sophisticated and expensive screw expellers. Small-scale processors usually start with the cheaper, manually operated equipment. There are several methods of producing sunflower oil on small scale:

1. The boiling method
2. The water-assisted methods
3. Batch presses using low pressure
4. Ram presses using low pressure

Boiling method of producing sunflower oil on a small scale

This method has been used in African countries at a household level to extract oil from pounded sunflower kernels. This method is time-consuming and strenuous, and has an oil extraction efficiency of around 40%.

The harvested seeds are cleaned and purified from foreign matter such as sticks and stones by screening the seeds with a set of sieves. In some instances, the seeds may be dehulled. Dehulling is achieved through friction between the seeds and between the seeds and the surface of a rotating drum. Dehulling need not be complete since the husks aid the separation of the oil from the paste.

The sunflower seeds are then pounded into coarse, gritty flour. The seeds are usually grounded manually unless the local mill is accessible and affordable. The flour is then mixed with a small quantity of water in a pot to form a paste, which is heated. Boiling water is added, and the mixture is stirred and brought to the boil. The heat causes the oil to leach from the cells. The mixture is allowed to cool to allow the oil to gather at the top from where it is scooped off. This method uses a lot of water. However, it is a useful method for rural households with limited resources who need small quantities of oil at a very low cost.

Water-assisted methods using low pressure to produce sunflower oil on a small scale

Sunflower seeds are harvested and cleaned as described above. Dehulling is optional. The seeds are roasted over an open fire to aid the oil extraction. A perforated drum filled with seed is placed over the fire on supports, and hand-rotated by means of a handle. The rotating action promotes even heating. Devices known as seed scorchers are also available to provide a greater degree of temperature control during roasting. These devices are also capable of handling substantially larger quantities of raw material. (Illustrations and specifications of such devices are found in “Oil processing: Food Cycle Technology Source Books” – see list of references).

The roasted seeds are then milled into a fine powder that is mixed with a small quantity of water to form a paste containing 11 to 14% of water by weight. The paste is stirred continuously while the water is added. Mixing continues until the oil emerges spontaneously from the mass and settles at the bottom of the mixing basin. The mass is gently squeezed by hand to remove as much oil as possible. The mass is formed into balls and fried in some of the oil. This removes even more oil from the balls. The fried balls are pounded to flour and used in cooking.

The overall oil extraction efficiency can be as high as 70%. The process requires only household utensils, water and labour.

Small batch presses using low pressure to produce sunflower oil

This is the oldest method of extracting oil from oil seeds. This process was developed to process one batch of seed at a time. Cleaned and heated (roasted) seeds are wrapped in a bag or cloth and pressed between two plates. Screws and levers apply pressure.

Hydraulic plate presses are also available. (Illustrations and specifications of such devices are available in “Oil processing: Food Cycle Technology Source Books” – see list of references).

The screw press is manually operated, and the seeds are pressed slowly by a plunger (round steel plate) which is forced down by a screw onto the bagged seeds in a perforated cylinder. Capacities of screw presses average at around 15 kg per batch. The oil extraction efficiency depends on the maximum pressure applied to the seed, the duration of the maximum pressure period, the rate at which the pressure is applied and the temperature and viscosity of the oil.

The hydraulic press can be manually or power driven. Pressure is exerted by a hydraulic device such as a lorry jack, taking care that the hydraulic fluid does not come into contact with the seed. Hydraulic presses generate high pressures and therefore require very rigid frames. These high pressures generate heat, which increases the oil yield.

Advantages of these presses are:

• Small batch presses are simple and work well
• They produce a good quality product
• They are easy to repair
• Purchase and operating costs are low
• They do not require trained operators
• These presses can be used in remote areas to satisfy the market for locally produced oil
• Few resources are required for an operation of this type

The disadvantages are:

• Equipment is very labour intensive
• Oil yield is relatively low

Ram press using low pressure to produce sunflower oil on a small scale

This is a continuous process used to extract oil from sunflower seeds. Projects are underway to introduce these presses, together with their supporting technology, to rural communities in Southern Africa.

The seeds are compressed with a piston. The size of the piston determines the quantity of seed to be processed with each piston stroke. Compression takes place in the expeller cage. The cage comprises of a number of square bars welded together with a gap of about 0,5 mm between the bars. The crude oil emerges from the cage through these gaps. The cage is sealed with an adjustable choke in the shape of a cone. The gap between the cage and the cone is adjusted by screwing or unscrewing the cone. This is done to adjust the pressure inside the cage. The oil cake exits the expeller cage through the gap between the cage and the cone.

For the seed to flow into the cylinder, the handle, with a length of 1 to 1,5 m, has to be lifted to its upper position. Pressure is applied when the handle is pushed downwards. For maximum pressure, the piston has to be pushed to its lowest position. Oil may be produced at a rate of 7 to 15 kg of seeds per hour. The quantity of clear oil obtained is in the order of 20 to 25 litres per 100 kg of seed. The oil that emerges from the cage is crude and contains seed debris. It has to be filtered with a gravity filter, or the crude oil may be left for three days for the solids to settle.

Clarification of the oil can be improved by treatment with charcoal or by filtering oil through a cloth.

Advantages of these presses are:

• Ram presses are simple and work well
• A good quality product is produced
• The equipment has been designed and manufactured to be durable
• They are easy to repair
• Purchase and operating costs are low
• These presses may be used in remote areas to satisfy the local market
• Various models and sizes are available
• Few resources are required for an operation of this type
• Local materials will suffice
• They produce cold-pressed oil that may be sold at a higher price as a health product

Disadvantages are:

• The process is very labour intensive
• Throughput capacity is low
• Oil yield is relatively low

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 into 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.

Packaging of crude sunflower oil

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 crude oil has a very limited shelf life and is susceptible to oxidation that causes rancidity. Packaging of the oil in clean and resealable containers that are grease proof, moisture proof and have an airtight seal, increases the shelf life and market value of the product.

Assistance to small-scale oilseed farmers

Most farmers, especially those located in the rural areas, are not really familiar with the idea of adding value. Special assistance must be provided for these rural farmers. Governments, NGOs and aid organisations should focus on enabling small-scale farmers and other entrepreneurs in Africa to increase the profitability of their enterprises, reduce the outflow of capital from their communities and improve human and animal nutrition and quality of life by producing edible and other oils and seed cakes presently unknown, unobtainable, unaffordable or originating predominantly from outside their communities.”

The objective, inter alia, should be to develop logical pathways and assisting communities in deciding on the best combination of, for example cropping practices (if they wish to grow their own seed), marketing (for both the seedcake and oil), pressing schedule, et cetera.

Training programmes are of great importance, especially in the rural areas. The introduction of new technology should always be accompanied by training. Should this not be the case, one could easily end up with the total failure of a potentially successful business. One of these failed attempts could lead to the final termination of this entire concept of adding value.

Legislation

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

Next month we shall look at the general production 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.

LITERATURE SOURCES

1. Anon. 1993. Oil Processing: Food Cycle Technology Source Books. Intermediate Technology Publications.
2. Fellows, P. 1997. Traditional foods: Processing for profit. London: Intermediate Technology Publications, Ltd.
3. Fellows, P & Hampton, A. 1992. Small-scale food processing: A guide to appropriate equipment. London: Intermediate Technology Publications.