Abstract
Water activity is considered an important factor in assessing the stability of food. Understanding the relationship between water activity and equilibrium moisture content (moisture sorption isotherm) benefits food processing in terms of modeling of drying and estimation of shelf life. In addition, glass transition helps to quantify molecular mobility which helps in determining the stability of food. The aim of this study was to determine the moisture sorption isotherm and thermal characteristics of freeze-dried tuna. These characteristics will help in determining the monolayer moisture and glassy state of the product, at which food is considered most stable. Moisture sorption isotherm at 20°C and thermal characteristics (over a wide temperature range i.e. from -90 to 250 °C) of freeze-dried tuna flesh were measured. Isotherm data were modeled by BET (Brunauer-Emmett-Teller) and GAB (Guggenheim-Anderson–De Boer) models. The GAB and BET monolayer water values were determined as 0.052 and 0.089 g g-1 dry-solids (dry-basis), respectively. In the case of samples at moisture contents above 0.10 g g-1 (wet basis), DSC (Differential Scanning Calorimetry) thermograms showed two-step state changes (i.e. two glass transitions), one exothermic peak (i.e. molecular ordering) and another endothermic peak (i.e. solids-melting). However, the sample at moisture content of 0.046 g g-1 showed three-step state changes (i.e. three glass transitions). The multiple glass transition could be explained by the natural heterogeneity of tuna flesh and inhomogeneity due to molecular incompatibility of the different compositions. The moisture content did not affect the first glass transition temperature nor the exothermic peak (p>0.05), whereas the third glass transition temperature decreased (i.e. plasticized) with increasing moisture content (p<0.05). The solids-melting peak temperature decreased, and enthalpy increased with decreasing moisture content (p<0.05).
Highlights
Water interacts with different ingredients present in foods
The BET and Guggenheim–Andersen–de Boer (GAB) monolayer water values were 0.089 and 0.052 g g−1 dry-solids, respectively, and samples at BET monolayer water could be considered most stable since deteriorative chemical reactions are minimal (Rahman & Hamed Al-Belushi, 2006; Sablani et al, 2007)
The GAB model is useful for predicting water activity over the complete range of moisture up to 0.90
Summary
Water interacts with different ingredients present in foods. This interaction is important in determining the physical, chemical and microbial stability of the foods (Bhandari & Howes, 1999; Rahman, 2006; Sablani, Kasapis, Rahman, AlJabri, & Al-Habsi, 2004). It has been observed that the water activity concept is not sufficient to determine stability of food, glass transition has been proposed to quantify molecular mobility (Delgado & Sun, 2002; Karel, Buera, & Roos, 1993; Karmas, Buera, & Karel, 1992; Sablani et al, 2004; Slade & Levine, 1988, 1991). It is important to measure the thermal characteristics and sorption isotherm in order to determine efficient processing and storage stability of dried tuna (Roos & Karel, 1991). Harnkarnsujarit, Kawai, and Suzuki (2015) measured the glass transition and water sorption isotherm of water extract from freeze-dried tuna Most probably, they attempted to simplify the complex intact structure into a sample of only soluble components. These characteristics could be used to determine monolayer moisture and glassy state, which is considered the best stability during processing and storage
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