Water sorption and mechanical properties of cassava starch films and their relation to plasticizing effect
Introduction
Biopolymer films and coatings from polysaccharides, proteins and lipids, formulated either with one or more components, have potential to control mass transfer and thus extend food shelf life (García et al., 1999, García et al., 2000; Parris, Coffin, Joubran, & Pessen, 1995). As being totally degradable, biopolymers could contribute to reducing the amount of plastic wastes. In addition, these polymers are obtained from renewable sources unlike the synthetic polymers (Souza & Andrade, 2000).
The first studies about the use of starch in biodegradable food packaging were based on substituting part of the synthetic matrix by starch (below 10%), however, the main difficulties found were attributed to chemical incompatibility of starch with synthetic polymers (Griffin, 1977). Recently, many works dealt with the addition of plasticizers to pure starch-based materials to overcome film brittleness caused by high intermolecular forces (Bader and Göritz, 1994, García et al., 2000, Lourdin et al., 1995; Mali et al., 2002, Mali et al., 2004; Souza & Andrade, 2000).
Plasticizers increase film flexibility due to their ability to reduce internal hydrogen bonding between polymer chains while increasing molecular space. The most effective plasticizers will generally resemble most closely the structure the polymer they plasticize, thus, the most commonly plasticizers used in starch-based films are polyols, such as sorbitol and glycerol. They avoid cracking of the film during handling and storage (Gontard, Guilbert, & Cuq, 1993), affect gas, water vapor and solute permeabilities (Banker, 1966) and sorption characteristics.
The moisture content in starch films can also affect significantly the physical and barrier properties of starch-based films; due to their inherent hydrophilic nature, starch films tend to absorb large quantities of water at elevated relative humidity (RH) conditions. Although there are studies focused on sorption properties of edible films, little information is available on the effect of type of plasticizer on sorption characteristics of these films (Cho & Rhee, 2002).
Water sorption isotherm equations are useful for predicting water sorption properties of hydrophilic films; they provide little insight into the interaction of water and film components. There are several mathematical models to describe water sorption isotherms of food systems materials, but no one gives accurate results throughout the whole range of water activities, or for all types of foods systems (Al-Muhtaseb, McMinn, & Magee, 2004). Labuza (1980) attributed this to the fact that the water is associated with the food matrix by different mechanisms in different water activity regions. Models available in the literature to describe moisture sorption isotherm can be divided into several categories; kinetic models based on a monolayer (BET model), kinetic models based on a multilayer and condensed film (GAB model), semi-empirical (Ferro-Fontan, Henderson and Halsey models) and empirical models (Smith and Oswin models) (Al-Muhtaseb et al., 2004). In more recent years, the Guggenheim, Anderson and de Boer (GAB) isotherm equation has been widely used to describe the water sorption behavior of foods (Bizot, 1984) and according to Al-Muhtaseb et al. (2004), GAB parameters are more representative in explaining the hydration process in foods and foodstuffs.
The objectives of this study were to evaluate the effect of plasticizer type and concentration on kinetic and static moisture sorption characteristics and on mechanical properties of cassava starch films.
Section snippets
Materials
Cassava starch (19% amylose) was provided by Hiraki Industry (São Paulo, SP, Brazil). Glycerol and sorbitol were purchased from Synth (Diadema, São Paulo).
Films preparation
Cassava starch films were prepared by casting employing 3 g of starch/100 g of filmogenic solution; three types of plasticizer (glycerol, sorbitol, and 1:1 mixture of glycerol and sorbitol) were added at three levels (0, 20 and 40 g/100 g of starch). For each experiment, the quantity of filmogenic solution poured onto the acrylic plates was
Moisture adsorption kinetics of cassava starch films
Moisture adsorption curves of cassava starch films are shown in Fig. 1. Moisture adsorption was more rapid at the initial stages of the storage and lower amounts of water were adsorbed as time increased. Then, the moisture content of cassava starch films reached a plateau indicating that they became equilibrated with storage RH. Moisture equilibrium time was influenced by storage RH and plasticizer type and concentration; films stored at 90% needed more time to reach the equilibrium and films
Conclusions
Hydrophilicity of plasticizer and its concentration were found to be important factors in determining the moisture affinity of cassava starch films. Peleg and GAB models were useful to fit moisture adsorption rates and sorption isotherm data, respectively. Glycerol films adsorbed faster and more water during its storage comparing to sorbitol films.
The presence of plasticizers resulted in lower stress and Young's Modulus values; glycerol films were more affected in its mechanical properties,
Acknowledgements
The authors wish to thanks the financial support provided by CAPES (ProDoc), CYTED and CNPq.
References (30)
- et al.
Water sorption isotherms of starch powders. Part 2: Thermodynamic characteristics
Journal of Food Engineering
(2004) Film coating, theory and practice
Journal of Pharmaceutical Science
(1966)- et al.
Sorption characteristics of soy protein films and their relation to mechanical properties
Lebenmittel Wissenchaft und Technology
(2002) - et al.
Modeling the effect of glycerol on the moisture sorption behavior of whey protein edible films
Journal of Food Engineering
(2000) - et al.
Plasticization and mobility in starch–sorbitol films
Journal of Cereal Science
(1999) - et al.
Influence of equilibrium relative humidity and plasticizer concentration on the water content and glass transition of starch materials
Polymer
(1997) - et al.
Microstructural characterization of yam starch films
Carbohydrate Polymers
(2002) - et al.
Barrier, mechanical and optical properties of plasticized yam starch films
Carbohydrate Polymers
(2004) - et al.
Effect of glycerol on behavior of amylose and amylopectin films
Carbohydrate Polymers
(2002) - et al.
Mechanical, water vapor barrier and thermal properties of gelatin based edible films
Food Hydrocolloids
(2001)