Peanut kernel contains 40-50% of oil, which is often extracted by pressing of peanut kernels. The pretreatment process, therefore, must through cleaning, shelling and kernel shell separation process. The subsequent process is similar to soybean hot pressing method. Due to the high oil content and soft nature of peanut kernel, prepressing – extraction or double pressing method is often used. With the development of peanut protein extraction process, the kernel after shelling and separation still need to be dried and peeled. Peanut skin, also named “red skin” with content of about 2.5 to 3.5%, is also a kind of by-product used in medicine.
Peanut kernel pressing technology and process:
1. Shelling and kernel shell separation
Requirement for shelling: no shell in the kernel (or shell content is less than 2-4%), kernel content in shell is lower than 0.5%.
Shelling equipment: currently, knife cage shelling and kernel shell separation equipment are used.
2. Softening, flaking and cooking
Peanut kernel has soft texture, generally do not need to soften. Sometimes, it can be flaked only after proper heat treatment (adjusting moisture). The flake thickness should be less than 0.5mm when being used for hot pressing. Prevent sticking of roller in flaking process.
Generally, cooking is adjusted according to the extract conditions of hot pressing (with 1-2% of moisture and temperature about 130°C). Flake of slightly oily with the color of dark yellow is preferred. Increase the moisture (about 4%) and lower the temperature (107-121°C) appropriately in prepressing to reduce the rate of protein denaturation.
3. Single pressing method
The following should be noticed when using the screw oil press:
(1) According to the measured calculation, the actual compression ratio of pressed material is 4-4.5, which is larger than soybean obviously, so the compression ratio should be 10 or so when using the screw oil press.
(2) Oil discharge gap of the press cage should be broadened and lengthened appropriately. The oil outlet position can be appropriately in advance under the premise of no oil leakage in the oil feeding end.
(3) The inner diameter of pressing chamber should not be too large. The pressing bolt shaft structure must be beneficial to the axial thrust, strictly prevent bonding or rotation of flakes along with the axis when pressing. Cakes can be mixed in pressing when necessary in order to improve the pressure of pressing chamber.
4. Double pressing method
In the double pressing process, different devices (hydraulic machine or screw press) can be used. Only 76-78% of oil is considered to be extracted in the first pressing to make the residual oil in the cake after second pressing up to 12-14%. The cake after secondary pressing can also be extracted by solvent extraction again or used as feed. However, most factories use single prepressing to make the residual oil dropped to 12-14%.
Peanut oil cold pressing process
Peanut meal contains high protein content without antinutritional factors, thus it is a good protein resource. But the high temperature in ordinary peanut prepressing extraction process and production process of superfine fragrant peanut oil would cause overheating denaturation of peanut protein and make its nutritional value as feed reduced greatly due to the loss of effective amino acids in peanut protein although the protein content in peanut meal is high. At the same time, overheating denaturation of protein makes its functionality used as edible protein significantly reduced and the color is deep, thus it cannot be applied in the food industry. This would not only lead to the waste of valuable high-quality oil on peanut protein resources, but also greatly reduces the added value of peanut oil production.
Peanut oil cold pressing technology takes high quality fresh peanut as the raw material through drying (moisture content of 4% – 5%), red skin removing (removal rate of more than 98%), crushing and conditioning before low temperature cold pressing (residual oil is below 7% after twice pressing, the cake outlet temperature is not more than 80°C). National standard first class peanut oil can be obtained after removing impurities in pressed crude oil through cold filter.
Through coarse grinding, fine grinding and superfine grinding of peanut cake, peanut protein powder can be obtained with particle size within 80μm (200 mesh) and nitrogen solubility index NSI of greater than 65%. It has white color and inherent fragrance of peanut and can be used as edible grade peanut protein powder.
Peanut oil cold pressing process description:
Cleaning and grading. Cold pressing requires that the peanut should be fresh, full with uniform size, no empty grain, enzyme rotten, metamorphism or damage by worm, etc. Therefore, the raw material must be strictly cleaned and graded by combining machinery and manual work (vibratory cleaning sieve, specific-gravity stoning machine, magnetic separation machine, conveying picker, etc.). The total impurity content after cleaning is less than 0.1%.
Drying. Reduce the moisture below 5% with low temperature drying technology, and then use cold air for cooling to lower the temperature of the peanut skin quickly and make the red skin separated better. In order to reduce the amount of protein denaturation, the temperature of raw material in the drying process should be controlled under 75°C for about an hour. Then, cooling with cold wind to room temperature after drying.
Red skin and germ removing. Complete removing of peanut red skin is the indicator guaranteeing appearance quality of peanut protein and peanut oil, also the process affecting its application. Peanuts would be broken under extruding and shearing action of rubber roller, then the red skin and germ would be separated by air separation. Raw material is required to be crushed to 2-4 parts with the red skin removing rate of above 90%.
Conditioning. Put peanut kernels after crushing and removing of red skin in layer type conditioning pot for conditioning treatment of the moisture and temperature and make peanut kernel has suitable pressing moisture ((6%-8%) and temperature (50-55°C). Improve the pressing yield efficiency under the premise of guaranteeing low protein denaturation.
Pressing. To reduce the protein denaturation, the oil press should be used for cold pressing with strong screw conveying and thin layer material pressing. Residual oil content of cake after first pressing is 13% to 15% with cake temperature of not exceed 65°C and moisture of about 8%. The residual oil content of the second pressed cake is less than 6% and the cake outlet temperature is not more than 80°C.
Peanut oil refining. After cooling the crude oil obtained by double pressing, use the centrifugal filter to filter slags.
Superfine grinding: after coarse grinding, the pressed cake would be crushed by the superfine crusher again. The particle size after crushing is 5.1505m as the peanut protein powder product.