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LANGOVA JITKA, JAISUT DONLUDEE, THUWAPANICHAYANAN RATIYA, PHOWONG CHAROTORN, STENCL JIŘI, JOVANOVIC DANIJELA, FRYC JIŘI, VLASEK VACLAV: Modelling the moisture sorption isotherms of Roselle (Hibiscus sabdariff a L.) in the temperature range of 5–35 °C. Acta Universitatis Agriculturae et Silviculturae Mendelianae Brunensis, 2013, LXI, No. 6, pp. 1769–1777 Water sorption tests of Roselle (Hibiscus sabdariff a L.) carried out under laboratory conditions are presented together with mathematical analyses of the moisture sorption isotherms (MSI’s). Moisture equilibrium data for adsorption and desorption of water from Roselle powder were investigated at near ambient air temperatures in the range of 5 and 35 °C and water activity (Aw) ranging from 0.11 to 0.97. The manometric method has been used for water sorption tests. Models for MSI’s are exponential equations. Coeffi cients of determination are 0.998 and 0.996 (for adsorption and desorption at 5 °C, respectively), 0.998 and 0.999 (for adsorption and desorption at 20 °C, respectively), and 0.998 and 0.999 (for adsorption and desorption at 35 °C, respectively). The equilibrium moisture content (EMC) of Roselle samples increased with an increase of Aw at a constant temperature both for adsorption and desorption. Adsorption curve equates to desorption curve at higher temperatures of tests carried out. Critical values of EMC of samples tested corresponding to the Aw equal to 0.6 were between 13.401% moisture content wet basis (MC w.b.) and 15.934% MC (w.b.) for moisture adsorption and desorption, respectively. These values are useful for storing conditions optimisation from point of view microorganisms grow and structural changes analyses. Crystal structure changes were observed during adsorption and desorption in the microscope, too. It was found out glass transition in dependence on the water content of samples tested. water activity, equilibrium moisture content, hysteresis, prediction, grow of microorganisms, glass transition Roselle (Hibiscus sabdariff a L.) belongs to the family Malvaceae. It originated in India and it is widely distributed in tropical and subtropical regions as a potential new food crop of considerable economic potential. Roselle is a short-day annual erect shrub and can grow to a height of 1–3 m, depending on the species. Its calyces have been suggested for the production of so drinks, Roselle tea, jam, juices, and natural food colourants (Chen et al., 2005; Chang et al., 2012). The extracts of calyces and leaves of Roselle show a signifi cant antihyperlipidemic activity (Ochani and D’Mello, 2009). Roselle calyx (Fig. 1) is usually harvested at high MC (w.b.), 85%. Therefore, drying is an important postharvest treatment prior to reduce the MC and to increase the shelf life. Moreover, the main purpose of drying the products is to minimize packaging requirements, to prolong the shelf life and to reduce shipping weights (Vengaiah and Pandey, 2007). Tab. I shows the chemical composition of Roselle calyces (Babalola et al., 2001). 1770 J. Langova, D. Jaisut, R. Thuwapanichayanan, Ch. Phowong, J. Stencl, D. Jovanovic, J. Fryc, V. Vlasek In food industry hot air drying is probably the most widely used drying technique since it is easy to operate. However, many disadvantages of hot air drying have been identifi ed, including oxygen-rich nature, which leads to quality degradation in terms of physical and nutritive qualities (Feng and Tang, 1998; Bai et al., 2002). One way to alleviate the adverse eff ect of hot air drying is a pretreatment of the product, either physically or chemically (Gornicki and Kaleta, 2007). Acid pretreatment is method which can to improve the quality of product through inactivation of enzymes and texture modifi cation. The colour of product can also be maintained due to the chelating properties of some acids (Branen et al., 2002). Eff ect of ascorbic acid pretreatment on the colour of apple cubes was studied by Zhu et al. (2007). It was found that ascorbic acid was eff ective in slowing down the rate of enzymatic browning. Water sorption properties of the Roselle were tested, because this surface layer is in direct contact with the near ambient air during storage and moreover it creates a natural consumable package. MC plays an important role in food quality: it infl uences texture and porosity, physical, chemical, and microbial properties. Values of Aw were measured from the viewpoint of the food safety (Beuchat, 1981), too. It is necessary to determine how sensitive the EMC of the food is to temperature changes. Many other similar plants have been investigated from the aspect of moisture sorption characteristics, e.g. green instant tea powder (Sinija and Mishra, 2008), black and green tea leaves (Ghodake et al., 2007), Chenopodium ambrosioides L. leaves (Jamali et al., 2006), Mentha crispa L. leaves (Park et al., 2002), saffl ower (Carthamus tinctorius L.) petals and tarragon (Artemisia dracunculus L.) (Kaya and Kahyaoglu, 2007), and bitter orange leaves (Mohamed et al., 2005). Critical Aw also exists below which no microorganisms can grow. For most foods, this is in the range 0.6–0.7. In general, dehydrated foods have Aw’s less than 0.6; semi moist foods, such as cereal grains, raisins, syrups and intermediate moisture pet foods, usually have Aw between 0.62–0.92. Greater Aw values than 0.92 have cheeses, jams, jellies, meat, and fi sh. Therefore, with knowledge of the MSI, we can predict the maximum moisture that the food can be allowed to gain during storage. It means that higher Aw’s can be allowed if other factors such as pH, salt content, antimicrobial agents, and temperature are taken into consideration, e.g. Stencl (1999). The principal methods for determination of Aw are gravimetric, manometric, and hygrometric. The gravimetric method is the most common type of sorption test. It is possible to obtain MC changes of samples continuously or periodically using a static system, usually a closed jar containing saturated salt solutions or sulphuric acid solutions which give a certain equilibrium relative humidity (ERH), or a dynamic system, circulated air with a constant fl ow rate (Stencl and Homola, 2000). Principle of the manometric method is measurement of partial pressure of water vapour in the air above the sample. The hygrometric method is direct method based on knowledge of relationship between MC and Aw of the sample tested (Iglesias and Chirife, 1982). 1: Fresh Roselle (Saeed et al., 2008) I: Chemical composition of dried Roselle calyces

Modelling the moisture sorption isotherms of Roselle (Hibiscus sabdariffa L.) in the temperature range of 5-35 °C