Mechanical property stability of soy protein isolate films plasticized by a biological glycerol-based polyester and application in the preservation of fresh-cut apples

Soy protein isolate (SPI) film is a newly developed material used for packaging. However, its application has long been quite limited due to its poor mechanical property stability during storage. In our study, a biological glycerol-based polyester (bio-poly glycerol: adipic acid = 1:2 (BGP I), 1:1 (BGP II), and 2:1 (BGP III); w/w) is used as a plasticizer to enhance mechanical properties. These films were evaluated for tensile strength (TS), elongation at break (EB), water vapor permeability (WVP), moisture content (MC), glycerol emigration rate, oxygen permeability (OP), and contact angle (CA) and the morphologies of the films were visualized with a scanning electron microscope (SEM) and an atomic force microscope (AFM) for 60days at 25 degrees C. The TS of SPI-BGP I, SPI-BGP II, and SPI-BGP III films increased by 12.94%, 18.22%, and 7.93%, respectively, and the EB increased by 23.63%, 37.27%, and 20.33%, respectively, compared to the films plasticized by glycerol. The stability of the WVP, MC, glycerol emigration rate, and OP of the films also increased, and a higher CA indicated a higher hydrophobicity in the films. The SEM and AFM images indicated that the films with BGP could form better cross-links, so the inner structure of the film was still relatively complete and compact after storing for a period of time. The application experiments showed that the SPI-BGP films had a good control effect on the browning reaction, effectively prohibiting the growth of microorganisms in fresh-cut apples and extending the shelf life for at least 23days compared to the control SPI films. Thus, the addition of a biological glycerol-based polyester greatly broadens the applications for a SPI film. Practical applicationsThe present study shows the mechanical property stability of SPI films plasticized by a BGP. The study also shows the application of BGP-SPI films in the preservation of fresh-cut apples. From the application point of view, this work could facilitate the application of SPI films on fresh-cut fruits and various foods. Moreover, the study is helpful in understanding the cause and the mechanism of mechanical properties' decline of SPI films during storage as well as providing help for other researchers in this field.