Characterization of biodegradable films prepared with hake proteins and thyme oil

Abstract

Hake protein biodegradable films containing different thyme oil levels (0.025, 0.05, 0.1 and 0.25 ml oil/g protein) were prepared and their physical, mechanical and antioxidant properties were studied. Dried proteins were solubilized at pH 11 with NaOH and glycerol (59% of protein content) was added as plasticizer.

The addition of thyme oil levels reduced both the film thickness and water vapor permeability. Films were homogeneous and transparent with a yellowish color. The optical properties of films were not generally affected by the thyme oil addition. Any clear trend between the mechanical properties of biodegradable films and thyme oil added was observed. Hake protein films exhibited some antioxidant activity, which was improved by the addition of 0.25 ml of thyme oil/g of protein.

Introduction

Packaging is used to protect the product from surroundings and to maintain the food product quality. Many materials are employed for the preparation of packaging but the majority is made from plastics. Synthetic plastic packaging has come into widespread use thanks to its good mechanical properties and effectiveness as a barrier to oxygen and water. However, synthetic packaging films represent a serious ecological problem due to their non-biodegradability. As a consequence, in recent years, packaging research has focused more on biodegradable and/or edible films made from natural polymers. Such polymers may be protein, lipid or polysaccharide-based and their chemical nature determines the physical properties of the resulting films. Among these materials proteins from different sources have been extensively employed because of their relative abundance, film-forming ability and nutritional qualities. Myofibrillar and sarcoplasmic fish proteins have been also successfully used as reported by Cuq et al. (1995) on the preparation of edible packaging films from sardine meats. Transparent and flexible edible films were also made from blue marlin myofibrillar proteins (Shiku et al., 2003). The properties of films produced from surimi from threadfin bream (Prodpran and Benjakul, 2005) and bigeye snapper (Chinabhark et al., 2007) were also evaluated.

These films are generally good barriers against oxygen but poor barriers against water vapor due to the hydrophilic character of proteins. Thus, several lipids (fatty acids, waxes, oils, essential oils) have been tested in order to improve the barrier properties of these films. Essential oils improve the water barrier properties of films and also their food-protective function due to the antimicrobial and/or antioxidant properties of these oils. Essential oils or extracts from cinnamon, garlic, ginger, oregano, rosemary or thyme (Pranoto et al., 2005, Goméz-Estaca et al., 2009, Hosseini et al., 2009, Atarés et al., 2010a, Atarés et al., 2010b) are by far the most used ones. These oils are rich in volatile and nonpolar phenolic compounds, which are significant contributors to their antioxidant properties.

Fish processing particularly the production of fish fillets and fish portions generates a substantial amount of by-products with protein content similar to that of fish muscle. Thus, the recovery of these proteins and their utilization to prepare biodegradable films represents a valuable alternative for the upgrading of by-products from fish processing industry.

The objective of this work was to prepare and characterize biodegradable films made with dried fish proteins recovered from Cape hake by-products incorporated with different levels of thyme oil.