DEVELOPMENT OF POLYLACTIC ACID–WHEY PROTEIN ISOLATE FILM AS NOVEL OXYGEN BARRIER PACKAGING FILM

Abstract

This research aims to develop a novel eco-friendly multi-layer barrier film fabricated entirely from biopolymeric materials, by employing mechanical strength and moisture barrier of poly(lactic acid) (PLA) films and oxygen barrier enhancement of glycerol (GLY) plasticized whey protein isolate (WPI). The effect of glycerol content (WPI:GLY 1:0.25, 1:0.4 and 1:0.67) on physical and barrier properties of PLA/WPI/PLA films were evaluated and compared with those of three-layer structures made of linear low-density polyethylene (LLDPE/WPI/LLDPE). The results showed that composite structures of PLA/WPI/PLA and LLDPE/WPI/LLDPE can be obtained by simple solution-casting process. The transparent WPI layer could successfully be formed thinly between two base layers of corona discharge-treated PLA or LLDPE films. The resulting multi-layer films, both PLA/WPI/PLA and LLDPE/WPI/LLDPE showed no visible optical change and possessed good flexibility and layer adhesion. The range of GLY contents used in this study was sufficient to prevent the films from cracking and curling. Transparency at 600 nm (T600) of multi-layer films were significantly reduced (p ≤ 0.05) comparing to their parental substrates as a result of discontinuous layers. The resulting total color difference (∆Eab) was lower than the detectable threshold by the human eye. This indicated that the composite structures were not visibly different from their single substrates. PLA enhanced tensile strength of the composite structures. PLA/WPI/PLA films exhibited a higher tensile strength (TS) but lower percentage of elongation (%E) compared to highly-extensible LLDPE/WPI/LLDPE films. The oxygen permeability of the three-layer structures were significantly reduced (p ≤ 0.05). However, the water vapor permeability of the structure relied mainly on the base films. Overall properties of the three-layer films tended to depend on the intrinsic trait of the substrate films more than the GLY content. To examine shelf stability of PLA/WPI/PLA structure, the multi-layer structure with WPI:GLY = 1:0.4 was stored in the simulated commercial storage temperature; 4, 25 and 35°C, at 50% relative humidity (%RH), for 21 days, along with single-layer WPI and PLA substrates. The multi-layer film showed no visible optical change and maintained good handling ability and layer adhesion throughout the storage test. It was found that transparency of PLA/WPI/PLA film gradually decreased over time, corresponding to the increasing trends of ∆Eab. Non-enzymatic Maillard browning of whey proteins and trace reducing sugar in the middle-layer and molecular re-arrangement were hypothesized to slowly cause changes in transparency and color of the film. Storage at 35 °C caused PLA/WPI/PLA film to become stronger and less extendible. On the other hand, the multi-layer structure showed lower TS and higher %E over storage at 4 and 25 °C. Oxygen and water vapor barrier abilities of PLA/WPI/PLA gradually improved over time as evident by the decreased permeabilities, especially at 35 °C. Overall, the PLA/WPI/PLA structure generally had properties in between its parental substrates. The changes in properties of laminate structure can be empirically fitted with either zero or first order reaction kinetics, with overall R2 ≥ 0.85. Although such changes were temperature dependent, they did not follow Arrhenius behavior. The shelf life extension of oxygen-sensitive baby formula by using PLA/WPI/PLA pouch was also assessed at 4, 25 and 35 °C, 50% RH for 19 days, in comparison with PLA pouch. The result showed that PLA/WPI/PLA pouch did not improve color stability of packaged baby formula. The aw and moisture content of packaged baby formula indicated that PLA/WPI/PLA pouch provided equal protection against ingress water vapor to PLA packaging. The result of conjugate diene and carbonyl compound of baby formula concluded that WPI-enhanced composite structure could effectively retard autoxidation of lipid in dry food. In conclusion, the results suggested a novel multi-layer barrier film made entirely from biodegradable materials; PLA/WPI/PLA, could work successfully as a “green” oxygen barrier alternative. The results also indicated that PLA/WPI/PLA film could be stored and used to package dry food properly at 4-35 °C, 50% RH, for extended period of inventory or storage time.