The objective of peeling is to remove unwanted or inedible material from vegetable raw materials. This improves the appearance and taste of the final product. During peeling, peeling losses need to be minimised by removing as little of the underlying food as possible but still achieving a clean peeled surface.
Field of application
Peeling is applied on an industrial scale to fruits, vegetables, roots, tubers and potatoes.
Description of techniques, methods and equipment
Various methods for peeling exist: flash steam peeling, knife peeling, abrasion peeling, caustic peeling and flame peeling.
a) Steam peeling
Steam peeling is carried out either as a batch-wise or a continuous process. The raw materials (roots, tubers) are treated in a pressure vessel and exposed to high-pressure steam (180°C to 200°C). In the case of tomatoes, the temperature may be lower (120 -130°C). The high temperature causes a rapid heating and cooking of the surface layer (within 15 to 30s). The pressure is then instantly released, which causes flashing-off of the cooked skin. The continuous steam peeler is a pipe with a screw inside. The steam is feed direct into the pipe (generally at lower pressure than the batch process) and the product is heated during the residence time (adjustable). Most of the peeled material is discharged with the steam. Any remaining traces are sprayed off with water.
b) Knife peeling
The materials to be peeled (fruits or vegetables) are placed onto a rotating disc and pressed against stationary or rotating blades to remove the skin. Knife peeling is mostly used for citrus fruits, as the citrus skin is easily removed and the fruit suffers little damage.
c) Abrasion peeling
The material to be peeled is fed onto abrasive rollers or fed into a rotating bowl which is lined with an abrasive. The abrasive surface removes the skin, which is then washed away with water. The process is normally carried out at ambient temperature.
d) Caustic peeling
The material to be peeled is passed through a dilute solution (1 to 2%) of sodium hydroxide. This treatment softens the skin, which can then be removed by high-pressure water sprays. A new development in caustic peeling is dry caustic peeling. The material is dipped in a 10 % sodium hydroxide solution. The softened skin is then removed by rubber discs or rollers. A drawback of caustic peeling is that it causes decolourisation of the product.
e) Flame peeling
A flame peeler utilises a conveyer belt to transport and rotate the material through a furnace heated to temperatures above 1000°C. The skin (e.g. paper shell, root hairs) is burned off and then removed by high-pressure water sprays. Flame peeling is used, for example, for peeling onions.