Influence of sugar composition on water sorption isotherms and on glass transition in apricots

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Abstract

Desorption isotherms of fresh and osmotically treated (70%, 30 °C) apricots have been measured at 30, 45 and 60 °C by the static gravimetric method. A differential scanning calorimeter was used to determine the Tg of samples equilibrated with several water activities. The osmotic pretreatment affected the shape of the desorption isotherms because of biopolymer binding at low activities values and dissolution of sucrose at high activities values. At 45 °C, isotherms of fresh and sucrose impregnated apricots are identical. At 60 °C, sucrose impregnation depressed water activity, while at 30 °C the opposite effect is observed. Evolution of moisture content at the first saturation layer expresses these effects of sucrose impregnation and temperature. Peleg model fitted the best experimental desorption isotherms of the fresh and osmotically treated apricots (0.990  r2  0.999, 0.005  S  0.045). A strong plasticizing effect of water on the Tg was found, with a great reduction in this value with increase in water activity.

Highlights

► Effect of sugar (sucrose) enrichment on water sorption isotherms and on glass transition in apricots. ► Osmotic pretreatment affected the sharp of the desorption isotherms. ► Strong plasticizing effect of water on the Tg with a great reduction in this value with increase in water activity.

Introduction

The knowledge of sorption data is important for many aspects related to food technology like prediction of microbiological, enzymatic and chemical stability, selection of packaging materials, designing of drying and concentration processes as well as the choice of adequate storage conditions. Water activity (aw) of food products should not exceed 0.6 to extend product shelf life. Therefore, it is essential to study sorption characteristics of dried or semi-dried products in the range of water activity varying from 0 to 0.6. Sorption isotherms can then be used to predict the shelf life of packed moisture-sensitive products.

Structure and composition of the food material play basic role on the shape of sorption isotherms whose experimental determination is necessary to simulate complex systems such as food. Other factors such as temperature are also important to influence its shape. Temperature affects mobility of water molecules and corresponding dynamic equilibrium (Al-Muhtaseb et al., 2004a, Al-Muhtaseb et al., 2004b). For these reasons many reported work data about the sorption isotherms of different products and some recompilations can be found in literature (Ahmed et al., 2005, Goula et al., 2008, Syamaladevi et al., 2009).

The analysis of sorption isotherms coupled to glass transition temperature, Tg, should provide an approach to the role of water in food. The glass transition of a polymer is described as a transition from a flexible rubber to a hard brittle solid. Tg is a terminology to characterize the transition from a liquid to a glassy state (or inversely) of a substance. Below Tg, molecular mobility is extremely slow because of high viscosity of food matrix (Slade and Levine, 1991). At temperatures above Tg, a drastic decrease of viscosity (about 102 Pa s per 10 °C) causes an extensive increase of molecular mobility. Such changes in viscosity may have dramatic effects on physical state of low-moisture foods. Foodstuffs with low-moisture contents and Tg value above the storage temperature can be considered stable. However, a slight increase in moisture content significantly reduces Tg. Therefore, moisture sorption isotherms and Tg data provide critical values for the water activity and moisture content at room temperature (Khalloufi et al., 2000, Roos, 1995).

Water and soluble solids (sugars) are the main fruit components. In drying processes such as air-drying, an amorphous state, which is a non-equilibrium state, usually occurs (Roos, 1995). Amorphous materials may change from solid glassy state to liquid-like rubbery state by increasing molecular mobility while Tg is reached by increasing temperature. The importance of Tg of amorphous food materials for processing and storage stability has been recognized and emphasized by Slade and Levine (1991). As Tg is dependent on water content, a change from rubbery to glassy state can also occur as a consequence of decrease in product water contents during its processing or storage.

The aim of the present work was to evaluate the influence of sucrose impregnation of apricot on its desorption isotherm and on glass transition temperature of apricots slices. The effect of temperature on desorption isotherm of apricots was also examined. Modeling of sorption isotherms using models selected from literature was studied (GAB and Peleg models). The Gordon-Taylor (1952) and Roos (1987) models were used to describe the glass transition temperature variation versus equilibrium moisture content and water activity.

Section snippets

Raw material

Fresh apricots (Royal flam) purchased from a local market (Massy, France) at their commercial maturity, were cut to thin slices of approximately 2 mm.

Osmotic dehydration

Apricots were osmo-dehydrated at 30 °C in sucrose syrups at 70% (w/w) with agitation (125 rpm) during 10 and 65 min. The fruit/syrup ratio was fixed at 1/20. After osmotic treatment, samples were carefully rinsed to remove adhering sucrose solution and quickly blotted with tissue paper. Each osmotic dehydration experience was repeated three times.

Equilibrium experiments

Results and discussion

Fig. 1 shows the water loss (WL) and the solid gain (SG) of apricots slices after osmo-dehydration in sucrose (30 °C, 70%) during 10 and 65 min. Sugar exchanges, during osmo-dehydration, modified sugar composition of apricot (Table 2). Comparing the variation of the principal chemical constituents of osmo-dehydrated apricots with respect to the initial dry fruit weight, at atmospheric pressure, total sugar intake increased with the osmosis time. The increases of total sugars reflected the trend

Conclusion

The moisture desorption isotherms of fresh and osmo-dehydrated apricots follow a sigmoid isotherm curve typical of the type III, according to BET classification. The desorption isotherm curves were significantly affected by the pre-treatment sucrose solution time and equilibrium temperature. In the sorption isotherms of fresh and osmo-dehydrated apricots, the temperatures effects are dependent on aw level. For aw lower than 0.65–0.77, the equilibrium moisture content decreases with increasing

Acknowledgements

The authors gratefully acknowledge ‘the Tunisian Ministry of Higher Education, of Scientific Research and Technologies and by the French Institute of cooperation (IFC)” for providing financial support and Pr MRAD Abdelkarim for his proof reading of the paper in English.

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