3.1. Package characterization
The tensile properties of the films are shown in Table 1. These parameters are useful to identify and characterize flexible films. Multilayer films A–C were more resistant than monolayer film D. Film B was the most rigid film because there was less extension in this film when compared to the other films. There are several hypotheses for this behavior, e.g. manufacturing process of film B promotes stronger interactions between chains or even cross-links the chains, which reduces the flexibility of the material (Martucci & Ruseckaite, 2010). With regard to surface properties,the multilayer films were smoother than the monolayer film. Fig. 1 shows the inner surface of the analyzed films obtained by AFM. The white points are characterized by high ridges, and the dark points represent depressions. By analyzing the roughness values (Table 2), differences were noted among the surfaces of the tested films. For example, film D showed the most suitable surface for bacterial adhesion because it had higher values for Ra and Rq. In addition, film roughness can protect microorganisms from UV radiation in the sanitation step of packaging, which reduces treatment efficacy and consequently product shelf life. Film D also had a higher average of irregular points (Rz), which indicated regions that were difficult to access by the sanitation agents and are consequently the most suitable surface for bacterial adhesion and contamination of the product after processing. Fig. 2 shows the cross-section of the different packaging materials, which allowed the thickness of each layer composing the films to be observed. The thickness of each layer is important to determine barrier properties of packaging, which are generally related to film thickness. The multilayer materials had similar thickness values among them, and these values were higher than the thickness values found for the monolayer film. These data suggest that the multilayer films have better mechanical and barrier properties than the monolayer film. The differences in oxygen and light barrier properties are presented inTables 3 and 4, respectively. Multilayermaterials A and B presented a greater oxygen barrier value than multilayer C. However, package C and A proved to be a good light barriers, probably due to the black pigment which covers them. Monolayer package D showed a poor oxygen and light barrier, which was an expected result due to its lower thickness and pigmentation as compared to the other packaging films. Package A was preferred in view of the light, oxygen and mechanical protection. However, for the application in pasteurized milk, the multilayer packages were similar to the monolayer ones (see below). The price of the former may be at least 3 times greater than the monolayer films. Since package did not have great influence on the shelf life of pasteurized milk, the least expensive one can be chosen to avoid a hike in milk price to the final consumer.