Development of edible composite film from konjac glucomannan and whey protein

Abstract

The aim of this work was to study the effect of incorporation of whey protein isolate (WPI) on the physical and mechanical properties of konjac glucomannan (KGM) based films. Effects of drying rate and storage condition on physical properties of edible composite film from KGM-WPI were also investigated. Solutions of KGM and/or WPI at 1:0, 0.8:3.4, 0.6:3.6, 0.4:3.8 and 0:4.2 g KGM :g WPI/ 100 g solution were prepared. Glycerol (GLY) was used as a plasticizer at ratio of 1.5 and 1.8 g/ 100 g solution. The casting solutions were dried by using tray dryers at 50 ℃ for 15 h. Results showed that incorporating EPI into KGM film, significantly increased L and decreased a and b values (p  0.05). Total color difference showed that WPI and KGM-WPI blend films were not visually different from commercial colorless polypropylene film. Incorporated WPI proportionally increased transparency and water insolubility with decreased tensile strength of KGM-based films. Nevertheless, WPI did not improve water vapor barrier of KGM-WPI films. Overall, the range of GLY in this study did not significantly affect properties of the films (p  0.05). WPI and blend film with the highest concentration of WPI could be heat sealed at 175 ℃. According to differential scanning calorimetry study, onset temperature of the melting of WPI and blend films was around 157-160 ℃ and enthalpy of melting from 5.6 J/g to 19.0 J/g. Formulation of the blend films did not significantly affect onset temperature, peak temperature and enthalpy of melting (p  0.05). However, peak temperature and enthalpy of melting of WPI films were obviously higher than those of the blend films. Microscopic study showed the heterogeneous structure of the blend films. Higher WPI concentration induced the formation of smaller clusters of whey protein molecules. To investigate the effect of drying process, the blend formula with 0.4, g KGM, 3.8 g WPI and 1.5 g GLY in 100 g casting solution was selected on the basis of significantly better functional properties. The casting solution of selected formula was completely dry at 50 ℃ using tray dryer by either fast (velocity 3.04  0.67 m/s and 19% RH, 3 h drying time) or slow (velocity 0.92  0.15 m/s and 18% RH, 15 h drying time) drying rates. The result showed that the drying rate did not significantly affect overall color of KGM-WPI film (p  0.05). The fast drying method significantly improved transparency and mechanical properties (p  0.05). without impairing solubility or water vapor permeability of the blend film. Slowly dried film had significantly higher onset temperature of melting of those films was not significantly different (p  0.05). Fast drying induced the formation of larger whey protein clusters in the film matrix. To study the effect of storage condition on KGM-WPI films properties, the optimized blend film (0.4 g KGM, 3.8 g WPI and 1.5 g GLY / 100 g casting solution) film was stored at 50% RH in three different temperatures (4, 25 and 35 ℃) for 24 days. There was no significant increase in total color difference of blend film at all controlled storage time (p  0.05). Although the changes in tensile strength as a result of storage time and temperature were less pronounced than % elongation and elastic modulus, overall mechanical properties were gradually improved over storage. Solubility and water permeability did not significantly change over time at all controlled storage conditions (p  0.05). Microstructure of all aged films was relatively similar to that of the freshly prepared film. Overall, the storage stability of the KGM-WPI blend film exhibited in all three simulated commercial conditions for 24 days indicate a potential feasibility for future market use.