A cooked mashed potato is cooled on trays in a chilling unit with refrigerated air at 2 C blown over the product surface at high velocity. The depth pf the product is 30mm and the initial temperature is 95 C. The product has a thermal conductivity of 0.37 W/ (m K), specific heat of 3.7 kJ/kg K and density of 100 kg/m^3. Assuming negligible resistance to heat transfer at the surface, calculate product temperature at the center after 30 min.