Interfacial characteristics of β-casein spread films at the air–water interface |
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Authors: | Ma Rosario Rodr guez Ni o, Cecilio Carrera S nchez,Juan M.Rodr guez Patino |
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Affiliation: | Departamento de Ingenierı́a Quı́mica, Facultad de Quı́mica, Universidad de Sevilla, C/Prof. Garcı́a González, s/n, 41012 Seville, Spain |
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Abstract: | Casein is well known to be a good protein emulsifier and β-casein is the major component of casein and commercial sodium caseinate. This work studies the behaviour of β-casein at the interface. The interfacial characteristics (structure and stability) of β-casein spread films have been examined at the air–water interface in a Langmuir-type film balance, as a function of temperature (5–40°C) and aqueous phase pH (pH 5 and 7). From surface pressure–area isotherms (π–A isotherms) as a function of temperature we can draw a phase diagram. β-Casein spread films present two structures and the collapse phase. That is, there is a critical surface pressure and a surface concentration at which the film properties change significantly. This transition depends on the temperature and the aqueous phase pH. The film structure was observed to be more condensed and β-casein interfacial density was higher at pH 5. β-Casein films were stable at surface pressures lower than equilibrium surface pressure. In fact, no hysteresis was observed in π–A isotherms after continuous compression-expansion cycles or over time. The relative area relaxation at constant surface pressure (10 or 20 mN m−1) and the surface pressure relaxation at constant area near the monolayer collapse, can be fitted by two exponential equations. The characteristic relaxation times in β-casein films can be associated with conformation–organization changes, hydrophilic group hydration and/or surface rheology, as a function of pH. |
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Keywords: | Air–water interface β-Casein Monolayer Surface film balance Relaxation phenomena |
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