Most kinds of edible vegetable oil in our daily life are hot pressed, which means roasting the oilseeds before pressing, while cold pressing is without roasting. The oilseeds must be evenly roasted. Their water content, temperature, and structure should meet the requirement of oil pressing. The effect has a direct influence on the smooth progress of the whole process of oil production, the oil yield, and the quality of oil as well as oil cake. By roasting the oilseeds, the oil yield can be largely increased, and the oil yield of hot pressing is twice that of cold pressing.
How Does Roasting Increase the Oil Yield?
1. The cell absorbs water and dilates so that the cell wall breaks, and the oil can come out.
2. The protein in the oilseeds is solidified and denatured by the high temperature, so that the oil is separated from the protein, and gather into larger oil drops from fine particles. The thoroughly denatured protein promotes a higher oil yield.
3. The plasticity of oilseeds is changed so that to meet the requirement of oil pressing.
4. With the rising of temperature, the velocity of oil decreases, which is to the advantage of oil pressing.
Why Should Oilseeds Be Roasted before Pressing by Oil Expeller?
However, the crude oil that by pressing the roasted oilseeds should be refined before it’s edible.
The reasons are as follows:
1. The content of phosphatide increases, which makes the color of oil deeper, and the oil foams and sediments appear when heating. So the crude oil needs degumming.
2. The pigments in the oil increase, so that the color of the oil is deeper, and bleaching is needed.
3. The acid value of oil increases, so neutralization is required.
On the other hand, the crude oil by cold pressing has a shallow color and there are few impurities in it. It does not need to be refined. Filtering it can reach the standard of edible oil. Cold pressing is suitable for small breed oilseeds or top grade health care oil for which heating is not good, such as olive oils, flaxseed oil, and safflower oil. Oilseeds that have a high content of oil are mostly cold pressed, and the denaturation of protein is small. Peanut and sesame which contains rich oil are often hot pressed to get a nice flavor.
Another point that deserves to note is that the fat-soluble vitamins in oilseeds suffer a great loss during hot pressing, and the nutritional value of oil cake is also degraded.
The drum type oilseeds roasting machine is suitable for small and medium-sized oil plants. It makes use of the vapor created by the oilseeds to evaporate and cook the seeds to soften the plant cellulose and prevent the oil seeds from carbonization caused by high temperature. The motive power is optional to electricity, diesel, coal, etc. It can be used to roast cottonseeds, soybeans, sesame, peanuts, sunflower seeds and rapeseeds etc.