Effect of fatty acids and ‘Brazilian elemi’ on composite films based on gelatin

Abstract

Composite edible/degradable films produced with hydrocolloids and lipids can result in better functionality than films produced with the components, especially with respect to their barrier properties. Of the lipids, waxes produce the best water vapor barrier properties, but produce fragile/brittle films. The problem of incorporating lipids into a hydrocolloid in a homogenous way has still to be solved. The objective of this research was the incorporation of ‘Brazilian elemi’, a highly hydrophobic resinous exudate of the botanical family Burseraceae, into gelatin films, using a blend of stearic and palmitic acids to dissolve the elemi, and subsequent emulsification of the filmogenic solution using triacetin as plasticizer. Films with the addition of acids, the blend and the blend and the elemi presented better water vapor barrier properties as compared to the gelatin/triacetin film. The mechanical resistance decreased with the addition of the lipids and the opacity and soluble matter increased. The confocal laser scanning microscopy showed the size distribution of the lipid drops and their localization in the matrix. They were not homogeneously incorporated despite the improvement in the water barrier property and the fact that the films appeared to be homogenous and malleable. The analyses by differential scanning calorimetry showed additional melting points besides that characteristic of the protein rich fraction, representing melting of the part of the lipid not incorporated into the filmogenic matrix. Dynamic mechanical calorimetry also showed the presence of more than one glass transition temperature (T_g), indicating the occurrence of phase separation.

Introduction

The development of natural biopolymeric packaging and the possibility of association with synthetic materials has increased the amount of research on biodegradable/edible films and coatings (Kamper & Fennema, 1996).

Usually films made from polysaccharides or proteins show good mechanical properties, but are sensitive to moisture. On the other hand, films made from lipids show good water vapor barrier properties, but are opaque, only slightly flexible and brittle (Guilbert, 1986).

Therefore, composite films made from both hydrocolloids and lipids may have an advantage. Composite films or coatings show a heterogeneous structure, that is, they are composed of a continuous matrix with some inclusions such as lipid globules in the case of an emulsion, or solid particles in the case of non-soluble substances (Debeaufort, Quezada-Gallo, & Voilley, 1998).

An extensive study of the effect of different lipids (fatty acids, beeswax, paraffin and vegetable oils) in emulsified and bilayer composite films showed that the water vapor permeability (WVP) was highly dependent on fatty acid chain length and degree of saturation. When using saturated lipids, decreases in the hydrophobic chain length resulted in increases in WVP. The best barrier properties were obtained with films containing fatty acid blends and covered with beeswax or paraffin, although they were quite brittle (Kamper and Fennema, 1984a, Kamper and Fennema, 1984b). Recently, Morillon, Debeaufort, Bond, Capelle, and Voilley (2002) published an extensive review of the factors involved in the water permeability of edible films based on lipid incorporation.

Pommet, Redl, Morel, and Guilbert (2003) produced films from gluten and saturated fatty acids with different carbon chain lengths as plasticizers, using a counter-rotating batch mixer to improve the incorporation of the acids into the protein matrix. The authors concluded that the homogeneous incorporation of acids only occurred up to a chain length of 10 carbons, longer chains (C12, C14 and C16) being only partially incorporated. Despite the longer chain fatty acids being incompletely incorporated, the WVP decreased with increase in chain length throughout the whole range under study (C6 to C16).

‘Brazilian elemi’ is one of the common names for a highly hydrophobic resinous oil, which exudes from the bark of various species of the botanical family Burseraceae, especially the genus Protium (Sussunaga, 1996). Basically, it consists of terpenoid substances in a mixture of volatile (monoterpenes) and fixed (triterpenes) elements (Ramos, Siani, Tappin, Guimarães, & Ribeiro, 2000), partially soluble in cold alcohol and water insoluble (Costa, 1994). A mixture of α- and β-amirin is predominant in the fixed fraction of the resin, as compared to the other terpenoids (Maia et al., 2000, Sussunaga, 1996). The high viscosity of elemi associated with its resistance and hydrophobicity make it appropriate for processes aimed at controlling water absorption (Mors & Rizzini, 1966).

The objectives of this research were to study the incorporation of the elemi into a gelatin film using a blend of palmitic and stearic acids, and to evaluate the physico-chemical characteristics of the resulting films, all of which contained triacetin as the plasticizer. Films containing the individual fatty acids and the blend, without the addition of elemi, were also characterized. The films were evaluated for their water vapor and oxygen permeability (OP), soluble matter, mechanical resistance, opacity, morphology, and their thermal characteristics as evaluated by DSC and DMTA. The elemi used was chemically characterized by gas-chromatography/mass spectroscopy (GC-MS) and ¹³C nuclear magnetic resonance (¹³C NMR).