Effect of processing parameters on the properties of transglutaminase-treated soy protein isolate films

Abstract

The effects of processing parameters, including the applied amount of microbial transglutaminase (MTGase), the pH of film-forming solution, air-drying temperature, as well as the additional pre-incubation, on the properties of MTGase-treated soy protein isolate (SPI) films were investigated. The treatment with low concentration of MTGase (4–10 units per gram of SPI, U g^− 1) significantly increased the tensile strength (TS) values of SPI films, while high concentration of MTGase (over 20 U g^− 1) resulted in significant decrease in the TS values (P ≤ 0.05). The elongation at break (EB) values of corresponding films gradually decreased, and the contact angle values persistently increased with the enzyme concentration. At alkaline pH range, the TS and EB values of MTGase-treated SPI films were significantly higher than that at pH 7.0 (P ≤ 0.05). Meanwhile, the contact angle values significantly decreased with increasing pH from 7.0 to 10.0 (P ≤ 0.05). The TS, EB and contact angle values of MTGase-treated films gradually but insignificantly decreased with increasing the air-drying temperature from 18 to 50 °C (P > 0.05). The properties of MTGase-treated films were also affected by the additional pre-incubation of film-forming solutions with MTGase before casting. Furthermore, the aggregation of SPI or its components induced by MTGase has been proved to mainly account for the influence of processing parameters on the properties of SPI films (MTGase-treated). Thus, low concentration of enzyme, alkaline pH range and low air-drying temperature, at which conditions the MTGase-induced aggregation of SPI in film-forming solutions could be in some extent inhibited or delayed, might facilitate the improvement of the properties of SPI films by MTGase, especially the mechanical and surface hydrophobic properties.

Industrial relevance

The development of biodegradable protein film has attracted a lot of attention worldwide. The enzymatic cross-linking induced by transglutaminase has been confirmed to improve mechanical and surface hydrophobic properties of cast films from most of food proteins, including soy proteins. Results of this study show that, the improvement of properties of cast films of soy proteins by transglutaminase treatment is largely dependent upon many processing parameters, e.g., enzyme concentration, the pH of film-forming solution and temperature.

Introduction

Edible films from biopolymers including polysaccharide and protein have attracted a lot of attention recently, due to increasing demands of consumers for both higher quality and longer shelf life of foods in combination with environmental needs for reduction in quantity of disposable packaging wastes (Chen, 1995). The films cast from food proteins, including soy proteins, corn zein, wheat proteins, cotton seed proteins, pea proteins, peanut proteins, sunflower proteins, gelatin, casein and sodium caseinate have been widely investigated (Carvalho and Grosso, 2004, Gennnadios et al., 1993, Guéguen et al., 1998, Jangchud and Chinnan, 1999, Orliac et al., 2002, Orliac et al., 2003, Parris and Coffin, 1997, Rhim et al., 2000, Sanchez et al., 1998, Schou et al., 2005). Most of protein films have good barrier characteristics against gas, organic vapor and oil as compared to synthetic films. However, the poor mechanical characteristics and the high water permeability of protein films limit their application as a packaging material. An effective approach to improve mechanical strength of protein films is the cross-linking technique by chemical or enzymatic methods. Many chemical cross-linking agents including formaldehyde, glyoxal, gossypol and glutaraldehyde have been applied to improve the mechanical and other properties of protein films (Ghorpade et al., 1995, Marquié et al., 1995, Marquié et al., 1997, Rhim et al., 1998, Rhim et al., 2000). Meanwhile, the possible toxicity makes it questionable to use such modifying agents in foods (Galietta, Di-Gioia, Guilbert, & Cuq, 1998). Thus, the application of enzymatic cross-linking to improve the properties of protein films seems to be more potential.

The enzymes that have been shown to cross-link proteins include transglutaminase (TGase; EC.2.3.2.13), lipoxygenase, lysyl oxidase, polyphenol oxidase and peroxidase. TGase is a kind of enzyme, which can catalyze the covalent cross-linking reactions between proteins to form high molecular weight (MW) biopolymers. This kind of enzyme has been confirmed to effectively modify some selected properties of many proteins, such as α_s1-casein (Motoki, Aso, Seguro, & Nio, 1987), whey proteins (Mahmoud and Savello, 1993, Yildirim and Hettiarachchy, 1997), soy protein isolates (SPI) or 11S globulins (Tang et al., 2005, Yildirim and Hettiarachchy, 1997), gelatin (Lim, Mine, & Tung, 1999), deamidated gluten (Larré, Desserme, Barbot, & Guéguen, 2000) and pectin-soy flour (Mariniello et al., 2003), including emulsifying activities, hydration ability, thermal properties, and gel or film-forming ability. Based on these findings, this enzymatic cross-linking has also been applied to improve the properties of protein films. Generally, the cross-linkage by TGase improves the tensile strength (TS) of protein films, while it decreases the elongation at break (EB) and solubility properties. In some cases, e.g. SPI and deamidated gluten films, the TGase treatment also significantly increased the surface hydrophobicity of films (Larré et al., 2000, Tang et al., 2005). However, there are a lot of problems involved in this cross-linking technique to be investigated, before this technique is commercially applied to the film industry. For example, it is uncertain that this cross-linking by TGase can improve the properties of cast films from various food proteins, and that for a certain protein, there are optimal processing parameters to obtain the best effect of the improvement.

The improvement in the properties of protein films by TGase seems to be dependent upon the types of substrate protein and some processing parameters, such as applied enzyme amount. In our previous study, we have confirmed that a low enzyme level (4 U g^− 1) of microbial transglutaminase (MTGase) resulted in the significant increases in TS and surface hydrophobicity of SPI films (Tang et al., 2005). It is unknown whether the treatment with a high enzyme level of MTGase (e.g., 20 U g^− 1) would also result in similar improvements in properties of SPI films. In the SPI solutions with protein concentrations less than 3%, it was observed that the cross-linking by MTGase leads to the coagulation of SPI components, and the coagulation is mainly dependent upon the enzyme concentration (Tang et al., 2006). Thus, it is expected that the improvement in the properties of cast SPI films by MTGase treatment is also affected by the enzyme concentration, since the coagulation or aggregation would decrease the mechanical properties of protein films.

The first objective of this work was to investigate the effect of enzyme concentration on the modification of SPI films by MTGase, and confirm the above assumption. The second objective was to investigate other processing parameters, such as pH of the film-forming solution, drying temperature, and the additional reaction before casting on the properties of TGase-treated SPI films. Last, we also tried to confirm the occurrence of aggregation reaction of SPI induced by TGase.