Physical properties of inulin and inulin–orange juice: Physical characterization and technological application

Highlights

• The physical properties of pure inulin and inulin–orange juice systems were studied.

• Glass transition temperature decreases as water was adsorbed.

• Crystallization was induced by water adsorption and microstructure was observed.

• Overall appearance was related with morphology developed by microstructure.

• Results may be useful in predicting shelf stability of food products.

Abstract

In this work two systems based on a carbohydrate polymer were studied: inulin as model system and inulin–orange juice as complex system. Both system were stored at different water activity conditions and subsequently characterized. Water adsorption isotherms type II were fitted by the GAB model and the water monolayer content was determined for each system. From thermal analyzes it was found that at low water activities (a_w) systems were fully amorphous. As a_w increased, crystallinity was developed. This behavior was corroborated by X-ray diffraction. In the inulin–orange juice system, crystallization appears at lower water activity caused by the intensification of the chemical interaction of the low molecular weight species contained in orange juice. Glass transition temperature (T_g), determined by modulated differential scanning calorimeter, decreased with a_w. As water is adsorbed, the physical appearance of samples changed which could be observed by optical microscopy and effectively related with the microstructure found by scanning electron microscopy.

Introduction

Inulin is a storage carbohydrate found in many plants, vegetables, fruits and cereals. The chemical structure is composed by linear chains of fructose molecules joined by β-(2-1) glycosidic bonds terminated by an α-d-(1-2)-glucopyranoside ring group (glucose) (André et al., 1996a, André et al., 1996b, Pitarresi et al., 2012). Regularly, linear chains have a length of 3 to 60 units of fructose with molecular weight ranging from 3500 to 5500 Da (Derycke & Vandamme, 1984). Industrially, inulin is extracted from chicory root (Cichorium intybus) and is used as an ingredient in foods, contributing to the improvement of the product and providing a beneficial effect for humans (Kawai, Fukami, Thanatuksorn, Viriyarattanasak, & Kajiwara, 2011).

Inulin partially exists in the amorphous state, which can be reached by a rapid change from an equilibrium state into a non-equilibrium state. The non-equilibrium state can be reached by removing the dispersion media i.e. water, or by rapid cooling at temperatures below the melting temperature. Amorphous state is induced in processes as cooling, extrusion and dehydration. Accordingly to Chiou and Langrish (2007), the amorphous state is beneficial in powder products since shelf life is longer. In this state, high viscosity is presented and chemical and biochemical reactions are generally restricted, achieving greater food stability. However, amorphous systems are considered as meta-stable states which are susceptible to undergo crystallization or structural relaxation in order to reach the equilibrium condition. Change velocity between the non-equilibrium and equilibrium states depends on factors such are temperature, time and water content (Roos, 1995, Slade and Levine, 1988). Several studies have been performed with the aim of analyzing the stability of inulin during storage in different fields such as pharmaceutical and food industry (André et al., 1996a, André et al., 1996b, Dan et al., 2009, Glibowski and Pikus, 2011, Kawai et al., 2011, Pitarresi et al., 2012, Ronkart et al., 2006, Ronkart et al., 2009, Zimeri and Kokini, 2003). In these fields, inulin has been employed as an excipient, additive, technological agent, thickener, emulsifier, gelling and sugar substitute. Based on those studies, for an amorphous material such as inulin, the most important parameter affecting the product stability is the glass transition temperature (T_g). T_g is a second order transition defined as the change between the glassy and rubbery states, observed during the variation in temperature and influenced by the water content (Rhaman, 2010, Sablani et al., 2010).

From the physical stability point of view, the most common problems arising during the handling of amorphous powders are: cohesiveness, stickiness and agglomeration, crystallization of sugars, loss of volatile compounds and loss of crunchiness (Adhikari et al., 2001, Khalloufi et al., 2000, Labuza and Hyman, 1998). Recently, the concepts of water activity (a_w) and glass transition temperature have been used together for evaluating the storage stability of inulin. Results have shown that inulin is more stable when stored at temperature below its T_g and at a_w near the monolayer level. Nevertheless, in most of these studies inulin has been analyzed as a model system. Kawai et al. (2011) studied the effect of water content, molecular weight and crystallinity of inulin on the T_g. Zimeri and Kokini (2003) determined T_g for inulin–water mixtures and inulin with different water contents. Ronkart et al., 2006, Ronkart et al., 2009 studied the T_g from inulin with polymerization degrees of 10 and 30, respectively. On the other hand, food products deviate from the model systems due to the interactions among the different added ingredients and thus are considered as complex systems.

Therefore, in this work we present the physical characterization of a model and a complex systems, inulin and inulin–orange juice, respectively. The study contributes to a better understanding of the role of the physical properties of the systems in the stability of food products during storage. The proposed methodology is useful in the science and industry fields. For this purpose, dried powders were first saturated with different water contents and representative samples were characterized by thermal analysis, X-ray diffraction, electron microscopy and optical microscopy. Results are discussed in terms of water content, chemical interactions, thermal properties and microstructure.