Heat transfer coupled with enzyme inactivation in the processing of pre-cooked frozen cruciferous vegetables

J. PEREZ; M. V. SANTOS; A. CALIFANO; N. ZARITZKY

Montreal

Congreso; 17th IUFoST World Congress of Food Science and Technology; 2014

Department of Food Science, University of Guelph & Advanced Foods and Materials Canada, Guelph, ON

Vegetables are a great source of essential nutrients in the human diet and there is a strong demand of these frozen products in the market. Brussels sprouts (Brassica oleracea) is a cruciferous vegetable that contains important antioxidant compounds.Peroxidase (POD) and lipoxygenase (LOX) are enzymes commonly found in vegetables; they are often utilized as an index for pre-cooking adequacy, since their inactivation increases the shelf-life of vegetables during frozen storage. The objective of the present work was to determine adequate processing conditions to produce high quality precooked frozen cruciferous vegetables. A finite element numerical code to solve the transient heat transfer partial differential equations during precooking and freezing stages was developed using Matlab Language. The following aspects were considered: a) the irregular shape of the product; b) the enzyme inactivation kinetics during the pre-cooking stage, c) the freezing process that is a highly non-linear mathematical problem, due to the abrupt changes in the thermo-physical properties which are temperature dependent.The heat transfer coefficients of the equipment involved in each stage were experimentally determined and incorporated into the numerical model; digital photographs were used for the mesh generation. The specific heat of the vegetable, the initial freezing temperature and the latent heat of melting of the vegetable were measured using Differential Scanning Calorimetry (DSC).Enzymes inactivation kinetics of POD and LOX during precooking process at different temperatures were experimentally determined at different temperatures (70-95ºC) and coupled to the heat transfer code in order to determine the optimal processing conditions.Numerical simulations were validated with experimental results; Brussels sprouts were submitted to pre-cooking in water at controlled temperatures and frozen in an air blast freezer. Time-temperature curves in the product and in the external fluid were recorded using thermocouples. A very good agreement between numerical simulations of the heat transfer processes and experimental time-- temperature curves was observed (temperature percent relative error