Mechanical anisotropy in pectin films

Mechanical characterisation of hydrocolloid-based biopolymer films remains highly variable across studies, limiting meaningful comparison between formulations and obscuring structure-function relationships. This study systematically evaluates the influence of testing geometry on mechanical descriptors by comparing uniaxial tensile and biaxial puncture protocols using 4% (w/w) high-methoxy (HM) pectin films plasticised with 5% (w/w) glycerol as a model system. Films were prepared under controlled casting and humidity conditions, and mechanical behaviour was assessed using extensive replication (n = 35 per protocol) to quantify reproducibility and methodological variability. Uniaxial testing produced predominantly linear stress-strain responses suitable for determining intrinsic material constants, including Young's modulus and tensile strength however, rupture-related descriptors showed substantial variability, with coefficients of variation exceeding 40%. In contrast, biaxial testing generated nonlinear membrane deformation behaviour but yielded improved reproducibility, with average failure stress, strain, and stiffness consistently exhibiting coefficients of variation below 15%. Analysis of anisotropy indices further demonstrated that mechanical performance differed under multidirectional loading, indicating that isotropic behaviour cannot be assumed for pectin-based films. These findings show that mechanical descriptors for HM pectin-glycerol films depend on testing geometry and deformation mode. Combining uniaxial and biaxial approaches provides complementary insight and supports clearer interpretation of hydrocolloid film mechanics.