DEHYDRATION OF APPLE AND TOMATO PUREES INTO FLEXIBLE LAMINATES. HEAT AND MASS TRANSFER STUDIES AND PRODUCT CHARACTERISTICS
Daniela Moreno Semhan1,2, Elisa Albertario1,2 and Sergio A. Giner1,2,3 1. CIDCA-UNLP-CONICET-CIC. Calle 47 y 116 (B1900 AJJ) LaPlata, Argentina, E-mail: email@example.com 2. Departamento de Ingeniería Química, Facultad de Ingeniería-UNLP 3. Comisión de Investigaciones Científicas, Provincia de Buenos Aires.
Keywords: fruit, vegetables, gelation, dehydration, puree, pectins ABSTRACT Food laminates are novel products formed during dehydration of fruits or vegetables purees. For convenience, the initial, fluid-likeproduct is preformed as a thin layer in a tray dryer and once the product has lost some of its original water content, it begins to retain its shape, becoming a gel-like, flexible material. Apple own pectin are responsible for gelation while, in tomatoes, gelation was assisted by pregelatinized starch. To test drying kinetics, formulations were dehydrated at air temperatures between 40-80°C to a finalmoisture content corresponding to a water activity below 0.7, for microbial stability. There was considerable shrinkage during drying, modelled by considering that volume reduction follows the volume of water evaporated. The experimental drying and heating curves were used in the energy balance to determine heat transfer coefficients (unsteady-state method), which were comparable to those fromexisting correlations. Observed drying rates showed two falling rate periods, one with mixed control ("wet zone") and another with strict internal control ("dry zone"). Diffusive models were proposed for each stage to determine diffusion coefficients, resulting higher in the "wet zone". Heat transfer coupling was also solved and predictions were good and can assist equipment design. Surface,CIE-scale product color was measured along storage and a first-order kinetics fitted to results to predict shelf life. INTRODUCTION Food laminates are novel products based on fruits or vegetable purees which, upon dehydration in heated air, form a gel-like, flexible structure that retains its shape, which can be dehydrated further to reach
microbial stability (Bell, 1994). This opens analternative route in drying research because most previous efforts were aimed at preserving an existing structure (Barbosa-Cánovas & Vega-Mercado, 1996) often a cellular tissue. In fact, only freeze-drying keeps substantially original product characteristics and can resist comparison with properties from the fresh. Osmotic drying has been presented as an alternative to heated air drying to improveproduct quality because solutes are incorporated slowly from a hypertonic solution while the product dehydrates. Spiess (2001) has indicated that solution recovery and process economics must be improved to permit wide application of osmotic drying in industry. In laminate formation, a formulation is prepared with solutes added before drying to develop a new gel-like structure by the action of naturalpectins present in fruits, or a gelation agent such as starch in vegetables while solutes concentrate during heated air drying. A thin, plane plate shape is selected to reduce dehydration time. The objective is to keep a new network created for as long as possible during storage at ambient temperature. During lamination by dehydration, fruits are exposed to milder treatment than in jams or jelliesand therefore their nutritional properties are better preserved. Laminates can be used for breakfast, as candy, sweet snack, in sandwiches, and even to prepare soft drinks, while vegetable laminates- notably tomato- can be used in sauces for pizza or pasta dishes, among other applications (Bell, 1994). No research seems to have been carried out on this line of products and sorption isotherms...