Qua trinh truyen nhiet va truyen khoi trong chien chan khong khoai tay (The heat and mass transfer processes during the vacuum frying of potato chips)

A heat and mass transfer were two processes which occurred during vacuum frying potato chips. The heat transfer process included the convection of heat from the surface to the product and the conduction of heat into the product. | The beginning of the frying period, the water in the potato chips evaporates quicker under vacuum conditions and it is less vigorous. The molecules oil will move into the materialabout 10-13% compared with the final product. This oil contributes to increase the temperature of the potato slices and decrease the hydrogen bonding of water with hydrophilic particles, allowing the separation of water occurred more easily. The vacuum pressure level affect the oil content of potato chips fried. Higher vacuum levels lead the pores on the surface of potatoes (due to loss of the characteristic hidrophilicity of raw potatoes) to develop rapidly. During this period, the capillary pressure is negligible (Moreira, CastellPerez, &Barrufet, 1999), so there is no driving force for the oil to flow into the chip’s pores. Vacuum pressure affects the speed at which moisture leaves the potato chips. The rate of evaporation and osmotic of oil will affect the shape and the size of the product. The rate of evaporation is increase, the volume of cell is increase, the material will be expands. After frying, the potato chips undergo the cooling period the surface oil that adhered to the surface of the potato chips into the pores of the chips (Moreira &Barrufet, 1995).If the absorption rate of oil is fast enough to occupy position where the water has evaporated, the final product will be porous and brittle. If the absorbed rate of oil is slow, the final product will reduce volume, the texture will be hard brittle and less porous. During vacuum frying, two process has approximately the same speed so the final product is often little change in size, good crispness and porous. The heat causes the cell volume increases, the water release will increase the concentration of dry material, the cell structure is destroyed, the oil absorbs into the cell.