FLUIDIZATION BEHAVIOR OF FRENCH FRIES DURING PRE-FRYING OPERATIONS

摘要

Deep frying involves submerging of food products in hot oil,thus drying the product while giving it a distinctive taste.Pre-frying is one of the most common unit operations in the catering and food processing industry.These industries are continuously exploring how to improve the pre-frying process with respect to product quality and energy usage.Critical to the pre-frying process is how the oil is contacted with the product.A higher local turbulence increases the heat transfer coefficient by decreasing the oil film thickness.Additionally,the total heat transfer is increased by fluidizing the product due to a larger oil-product interface.In this research project the fluidization of French fries is investigated.The main issue is the influence of fluidization on the final product quality.Another criterion is that the fluidization velocity should not be too high with respect to possible safety issues and transport problems in the pre-frying unit.The fluidization behaviour of French fries and subsequent product quality distribution is studied using a cross-flow laboratory fryer setup.This fryer includes transparent walls for visualization and an external pump and heating unit to regulate oil flow and temperature.Image analysis software is used to analyse the movement of the French fries in the frying unit (e.g.orientation and mixing).Oil and water contents of the French fries are determined by Soxhlet extraction and oven drying respectively.Besides the experimental work,a numerical model is constructed for the pre-frying process.Computational Fluid Dynamics (CFD) software is used to describe oil flow,while the Discrete Element Method (DEM) is adopted to describe the dynamic behaviour of the solid phase (i.e.the French fries).The combination of modelling and experimental work should provide insight in creating an economically and environmentally more viable process with a uniform product quality.