To investigate as deeply as possible the underlying mechanisms
of paper-coating by proteins, this study aims to understand how
the structural, surface and transfer properties of wheat gluten
(WG) coated paper could be influenced by the nature of the paper
used. For this purpose, a surface treated paper (TP) and an untreated
paper (UTP) were compared. UTP displayed a higher level
of protein penetration than TP (61% against 43%) resulting in a better
penetration of WG-coating into the depth of paper. This led to
the formation of a significant junction zone resulting in a interpenetrated
structure for WG–UTP, suggesting a composite-like structure,
while a bi-layer one was obtained for WG–TP as clearly
evidenced by optical and scanning electron micrographs of the
materials cross-section. Through static contact angle and air permeation
measurements, both the hydrophilicity and roughness of
paper were demonstrated to favour the penetration of the WGcoating
solution into paper whereas its air porosity acted against.
The level of penetration of the WG layer into paper (and the resulting
junction zone) appeared to influence greatly the subsequent
gas permeability (toward O2 and CO2) of WG-coated papers, and
to a minor extent their water vapour permeability (WVP) and surface
properties (wettability and grease resistance). Scanning electron
microscopy observations performed onto the specimens
surface revealed fracture zones in WG–TP but not in WG–UTP.
These local disruptions in the coated layer led to permeable zones
altering the overall gas barrier performances of the WG–TP without
affecting its WVP. Due to the relative humidity gradient set
by the WVP measurement, the plasticization of the WG layer
would occur resulting in a swelling of the polymeric matrix. As a
consequence, the fracture zones would be reduced and the surface
integrity of the WG-layer partially restored.