Effect of processing parameters on the properties of transglutaminase-treated soy protein isolate films
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 αs1-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.
Section snippets
Materials
Commercial soy protein isolates (SPI, 85.2% protein) were obtained from Wonderful Technol. Co. Ltd. (Shandong Province, China). The commercial microbial transglutaminase (MTGase) was obtained from Changshou Biological Co. Ltd. (Jiangsu province, China), and stored in the freezer (− 20 °C) before use. The preparation and purification of this enzyme solution was according to the method of Tang et al. (2005). The enzymatic activity of TGase was measured by the colorimetric procedure (Folk & Cole,
Effects of enzyme concentration
Table 1 shows the influence of enzyme concentration on the properties of MTGase-treated SPI films, including mechanical characteristics (TS and EB) and surface hydrophobic properties. The treatment with low enzyme concentration (4–10 U g− 1) of MTGase led to significant increase in TS and decrease in EB relative to the control (non-treated) (P ≤ 0.05; Table 1). This is consistent with that of a previous study (Tang et al., 2005). In the MTGase-treated films, the TS and EB values gradually
Conclusions
The properties of SPI cast films, especially the TS and the hydrophobicity, could be modified by the transglutaminase treatment. However, this work indicated that the modification of properties of SPI films by MTGase was in a various extent dependent upon many processing parameters, including the enzyme concentration, the pH of film-forming solutions and the air-drying temperature. The influence of those processing parameters on the MTGase-treated SPI films could be accounted for the
Acknowledgments
This work is part of the research projects of the Chinese National Natural Science Fund (serial number: 20306008 and 20436020), sponsored by the NSFC. The author also gratefully acknowledges the financial support from the Natural Science Fund of Guangdong Province, China (serial number: 05006525).
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