(1) Cooling
After roasting, peanuts must be rapidly cooled to remove residual heat. This step is crucial for ensuring the final quality of the peanut butter. In continuous roasting systems, rapid cooling is often achieved in the latter section of the oven using forced cold air. This process not only reduces the temperature but also further lowers the moisture content. The cooling causes the kernels to contract and their skins (testae) to crack, which facilitates the subsequent removal and separation of the skins.
(2) Decortication (Skin Removal)
- Dry Decortication: This method typically involves heating the shelled peanuts to about 138°C for approximately 25 minutes. The specific roasting temperature and time must be carefully controlled, based on factors such as peanut variety, moisture content, kernel size, and maturity, to achieve optimal skin removal.
- Process Flow: After cooling, the peanuts are continuously fed through a skinning machine. Different types of machines employ different skin removal mechanisms.
- New Method: A newer technique utilizes high-pressure air streams within a hopper to rapidly spin the peanuts, causing them to rub against each other and frictionally remove the red skins. This advanced equipment can effectively remove the skins from both roasted and raw (unroasted) peanuts.
- Belt-Type Skinner: If a belt-type skinner is used, peanuts pass continuously through the gap between a rotating corrugated rubber belt and a corrugated rubber plate. The gentle friction between the surfaces strips off the skins, which are then carried away by an air stream into a collection bag. The adjustable gap minimizes kernel breakage and powder generation. This method can effectively separate the peanut germ (heart) and is suitable for producing half-kernel, blanched peanut pieces.
- Abrasive Roller Skinner: Another common type is the abrasive roller skinner, which operates differently from the belt-type. Here, roasted and cooled peanut kernels pass against a rotating abrasive roller (e.g., a 50cm long sand-coated cylinder). The friction against this abrasive surface removes the skin.
- While these abrasive rollers are highly efficient and new rollers with sharp grit remove skins quickly, they can also abrade the surface of the kernels themselves. This generates more peanut powder, reduces the smoothness of the cotyledon surface, and increases product loss. However, for peanut butter production, this surface abrasion has a negligible impact on the final product quality.
By-Products and Yield:
- During processing, fat migrates and is absorbed by the red skins, increasing their fat content to 26%-30%. These skins, being high in oil (which is prone to oxidation) and containing nutrients like protein, are often diverted for use in animal feed.
- The peanut germ is rich in fat, protein, and other nutrients, making it a potential ingredient for high-value nutritional products. However, due to its bitter taste, the germ is typically removed during peanut butter processing and used for other purposes.
- The combined processes of heating and skin removal result in a total weight loss of approximately 12%. This loss is comprised of about 4% skins, 3% germ, and at least 3% moisture.
