Mechanisms of Food Additives, Treatments, and Preservation Technology 309
- Flash steam peeling. The surface of the plant material is heated by high-
pressure steam (1.5 MPa) in a rotating pressure vessel for 15 to 30 sec.
Then the pressure is released, which causes steam to form under the skin
and the surface of the food “flashes off.” - Knife peeling. The skin is removed by pressing the surface of fruits or
vegetables against blades. Various systems use rotating fruits and vegeta-
bles and stationary blades or rotating blades and stationary plant materials. - Abrasion peeling. Fruits or vegetables are fed onto carborundum rollers
or into rotating bowls lined with carborundum. The abrasive surface
removes the skin, which is washed away by water. - Caustic peeling. Fruits or vegetables are treated with a heated solution of
sodium hydroxide (1 to 2% at 100 to 120°C), then the softened skin is
removed by high-pressure water sprays. When dray-caustic peeling is
used, the food is dipped in 10% sodium hydroxide solution and the skin
is removed with rubber discs or rollers. - Flame peeling. A conveyor belt carries and rotates onions through a
furnace heated to 1,000°C; the outer layers and root hairs are burned off
and charred skin is removed by high-pressure water sprays.
The ideal peeling method would be gentle hand-peeling with a sharp knife.
Industrial-scale knife peeling is also gentler than other methods, and losses during
high-pressure peeling can also be relatively low when the regimen is sufficiently
optimized. Product losses during the flash peeling of vegetables are about 8 to 18%,
compared to about 25% in the case of abrasion peeling (Laurila and Ahvenainen,
2002). The choice of the peeling method also has other consequences. Abrasion-
peeled potatoes should usually be treated with browning inhibitors, whereas water
washing is enough for knife-peeled potatoes. Abrasion, steam peeling, and caustic
peeling disturb the cell walls of vegetables, thus enhancing the possibility of micro-
bial growth and enzymatic changes. Peeling often affects the appearance and the
shelf life of the produce and is not suitable for minimally processed products.
Minimizing of mechanical damage is also important for cutting and shredding.
These must be performed with knives or blades as sharp as possible and made from
stainless steel to avoid dissolving metals in processed material that can affect enzymatic
and chemical reactions, and subsequently the product quality. Similarly, quality reten-
tion depends on the degree of cell breakage and release of tissue fluid. Slicing machines
must be installed solidly to avoid vibrations. All parts of the machine in contact with
plant material should be regularly cleaned and disinfected (Fellows, 2000b).
The effect of wounding may be minimized by careful selection of appropriately
mature fruit. Recently, the beneficial effect of 1-methylcyclopronene (1-MCP) treatment
on climacteric fruit was described (Jiang and Joyce, 2002). 1-MCP applied on whole
fruit or after cutting can block ethylene binding because of their similar chemical
structure. When applied before coring and slicing of apples, ethylene production was
reduced during 10 days of storage; firmness was retained and superficial scald reduced.
Wounding stress during peeling, cutting, or shredding is also affected by the
distribution of different types of cells in fruit or other parts of the processed plant
(root, leaf, etc.). In fruits such as apples or pears the core and adjacent tissues have