Protein-LMWE interactions

Protein-LMWE interactions at the air-water interface have been studied by tensiometry (Patino et al., 2003). Prom these experiments it has been observed that the interfacial characteristics of mixed proteins and LMWE at air-water interfaces depend at least on the way in which these surface active compounds are adsorbed/spread to the interface (Figure 14.2). 

The existence of protein-LMWE interactions depends on the interfacial composition and on the protein/LMWE ratio. In general, the surface activity of the mixed films is determined by the LMWE as the surface pressure of the mixed film is the same as the LMWE equilibrium spreading pressure, and the monolayer is not saturated by the protein. However, the protein determines the surface activity of mixed films as the protein saturates the monolayer. In the intermediate region there exists coexistence of protein and LMWE at the interface. 

However, it was observed that t)q)ical food reagents (salts, ethanol, or sugars) in the aqueous phase play a role in soluble LMWE micellisation and protein-LMWE interactions and, as a consequence, on the interfacial characteristics of the mixed films (Patino et al., 2003). 

From a systematic study focused on fhe tt-A isofherm of protein-LMWE mixed monolayers (including fhe application of fhe additivity rule on miscibility and the quantification of inferacfions between monolayer components by excess free energy ( if has been concluded that, at a macroscopic level, these compounds form a pracfically immiscible monolayer at the air-water interface, af tt < Tlf At higher tt the collapsed protein is displaced from the interface by LMWE (monoglycerides, phospholipids, etc.). The existence of low profein interactions in disordered proteins ((3-casein and caseinate) facilitates the protein displacement by LMWE from fhe air-water interface. However, the lower surface acfivify of unsafurafed-LMWE explains the fact that this lipid has a lower capacity than saturated-LMWE for protein displacement. 

The results with biosurfactant (protein and LMWE) monolayers indicate that the dilatational modulus is not only determined by the interactions between spread biosurfactant molecules (which depend on the surface pressure or surface density), but that the structure of the spread molecule also plays an important role (Nino et al., 2003). 

The orogenic displacement mechanism is a consequence of the low level of interaction between proteins and LMWE at the air-water interface. This model has been shown to work for a range of proteins with different secondary and tertiary structures and different t)q)es of LMWE (monoglycerides and phospholipids). 

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