Recebido em 17/8/07; aceito em 14/3/08; publicado na web em 26/8/08The moisture sorption isotherms of Chilean papaya were determined at 5, 20, and 45 ºC, over a relative humidity range of 10-95%. The GAB, BET, Oswin, Halsey, Henderson, Smith, Caurie and Iglesias-Chirife models were applied to the sorption experimental data. The goodness of fit of the mathematical models was statistically evaluated by means of the determination coefficient, mean relative percentage deviation, sum square error, root-mean-square error, and chi-square values. The GAB, Oswin and Halsey models were found to be the most suitable for the description of the sorption data. The sorption heats calculated using the Clausius-Clapeyron equation were 57.35 and 59.98 kJ·mol -1 , for adsorption and desorption isotherms, respectively.Keywords: Chilean papaya; sorption isotherms; isosteric heat.
INTRODUCTIONThe Chilean papaya, Vasconcellea pubescens, which grows in a temperate climate regime, is a distant relative of the tropical papaya Carica papaya L.1,2 Most of the Chilean papayas come from the (irrigated) region around La Serena, Chile (ca.30 ºS) which has a Mediterranean-type climate, typically overcast mornings and evenings, and an annual relative humidity normally over 80%.3 The Chilean papaya is very different from the tropical papaya in that it is much smaller, and is consumed whole, only after cooking, with the seeds removed. It has an edible yield of 46%, a sugar content of about 5% and is high in papain content. 4 Exogenous sugar is usually added to its commercialized products, since this papaya is commercialized mainly in canned and dried (candied) forms.Water activity within a food is the amount of water available for chemical and biochemical reactions (i.e. lipid oxidation, enzymatic, and Maillard reactions), its participation in microbial development, and as a parameter useful in predicting the shelf life of the product. 5 In addition, water activity (a w ) measured within a product provides a good indicator of its shelf-life. The isotherm of a product, for a constant temperature, relates the equilibrium moisture content to the thermodynamic water activity, since in equilibrium, the latter is equal to the relative humidity of the air around the product. 6 The isotherms are important for the analysis and design of various food transformation processes such as drying, mixing, and storage; and to predict changes in food stability and aid in the selection of suitable packaging materials.
7Various empirical and semi-empirical equations have been proposed to predict the behaviour of sorption isotherms of foods and foodstuffs. The equation proposed by Guggenheim, Anderson and de Boer usually called GAB, is very useful for foods and has been recommended by the European project COST 90. 8 The GAB equation is based on the Brunauer, Emmett and Teller usually called BET adsorption theory, 9 which gives a physical explanation for the parameters of the equation.
10The monolayer moisture content (X m ) for the GAB and BET models is related to f...