J. M. Rodríguez Patino

Structural and morphological characteristics of β-casein monolayers at the air–water interface

Brewster angle microscopy (BAM) coupled with surface pressure (π)–area (A) isotherms was used to visualize and determine structural characteristics of β-casein monolayers at the air–water interface as a function of subphase pH. The measurements were performed at 20°C. From the results of π–A isotherms, it can be concluded that β-casein monolayers at the air–water interface adopt two different structures. The monolayer structure was more condensed on acidic aqueous solutions. From BAM images, the domains that the residues of β-casein molecules adopt at the air–water interface appeared to be of uniform reflectivity, suggesting homogeneity in thickness and film isotropy. A method was applied to measure the relative reflectivity and relative film thickness. The relative reflectivity versus surface pressure plots reflects the surface equation of state for the spread β-casein monolayer, and is particularly sensitive of the existence of transitions in β-casein monolayers with compression. The relative film thickness increased with the film compression and was a maximum at the collapse point. At the same surface pressure, the relative film thickness is independent of the pH, but the surface density is higher at pH 5 than at pH 7, as a consequence of the more compact packing of β-casein residues on acidic subphases.

Effect of Hydrocarbon Chain and pH on Structural and Topographical Characteristics of Phospholipid Monolayers

Structural characteristics (structure, elasticity, topography, and film thickness) of dipalmitoyl phosphatidylcholine (DPPC) and dioleoyl phosphatidylcholine (DOPC) monolayers were determined at the air-water interface at 20 degrees C and pH values of 5, 7, and 9 by means of surface pressure (pi)-area (A) isotherms combined with Brewster angle microscopy (BAM) and atomic force microscopy (AFM). From the pi-A isotherms and the monolayer elasticity, we deduced that, during compression, DPPC monolayers present a structural polymorphism at the air-water interface, with the homogeneous liquid-expanded (LE) structure; the liquid-condensed structure (LC) showing film anisotropy and DPPC domains with heterogeneous structures; and, finally, a homogeneous structure when the close-packed film molecules were in the solid (S) structure at higher surface pressures. However, DOPC monolayers had a liquid-expanded (LE) structure under all experimental conditions, a consequence of weak molecular interactions because of the double bond of the hydrocarbon chain. DPPC and DOPC monolayer structures are practically the same at pH values of 5 and 7, but a more expanded structure in the monolayer with a lower elasticity was observed at pH 9. BAM and AFM images corroborate, at the microscopic and nanoscopic levels, respectively, the same structural polymorphism deduced from the pi-A isotherm for DPPC and the homogeneous structure for DOPC monolayers as a function of surface pressure and the aqueous-phase pH. The results also corroborate that the structural characteristics and topography of phospholipids (DPPC and DOPC) are highly dependent on the presence of a double bond in the hydrocarbon chain.

Orientational changes in dipalmitoyl phosphatidyl glycerol Langmuir monolayers.

Dipalmitoyl phosphatidyl glycerol (DPPG) as Langmuir monolayers at the air/water interface was investigated by means of surface pressure measurements in addition to Brewster angle microscopy (BAM) during film compression/expansion. A characteristic phase transition region appeared in the course of surface pressure-area (pi-A) isotherms for monolayers spread on alkaline water or buffer subphase, while on neutral or acidic water the plateau region was absent. This phase transition region was attributed to the ionization of DPPG monolayer. It has been postulated that the ionization of the phosphatidyl glycerol group leads to its increased solvation, which probably provokes both a change in the orientation of the polar group and its deeper penetration into bulk phase. Film compression along the transition region provokes the dehydration of polar groups and subsequent change of their conformation, thus causing the DPPG molecules to emerge up to the interface. Quantitative Brewster angle microscopy (BAM) measurements revealed that along the liquid-expanded to liquid-condensed phase transition the thickness of the ionized DPPG monolayer increases by 4.2 A as a result of the conformational changes of the ionized polar groups, which tend to emerge from the bulk subphase up to the surface.

Physicochemical characteristics of food lipids and proteins at fluid-fluid interfaces

In this paper we are concerned with adsorption, structure, morphology, and dynamic properties of food dairy proteins (β-casein, caseinate, and whey protein isolate (WPI)) and water-insoluble lipids (monopalmitin and monoolein) at the air-water and oil-water interfaces. Combined surface chemistry (surface film balance and dynamic tensiometry) and microscopy (Brewster angle microscopy (BAM)) techniques have been used to determine the adsorption, structure, morphology, relative film thickness, relaxation phenomena, and dilatational rheological characteristics of emulsifiers (proteins and lipids) at fluid-fluid interfaces, including the effect of temperature. The derived information shows that protein and lipid type and temperature affect the interfacial characteristics. The nature of emulsifier (protein or lipid) interactions at the interface has an important role in their physicochemical characteristics. Important functional differences have been established between proteins and lipids, and between globular (WPI) and disordered (β-casein and caseinate) proteins.

Rheological Characteristics of a Detergent Slurry Containing Pentasodium Triphosphate

The purposes of the present work are to analyze the rheological characteristics of a detergent slurry containing pentasodium triphosphate (TPP) and to study the influence that some factors exert on the rheological consistency of this slurry. Experimental results were achieved using a Brabender Plastograph and have been analyzed according to approximate methods to convert torque-rheometer data into fundamental rheological units.From the experimental results it can be inferred that: (i) The systems studied exhibit plastic rheological behavior at steady shear and a progressive decrease in consistency by increasing shear rate; (ii) an increase in the yield value and apparent viscosity of the slurry are produced by increasing the concentration of nonaqueous substances or the phase I/phase II TPP ratio; and by decreasing the temperature or the hydrotrope content; (iii) the effect that temperature and concentration exert on the rheological consistency of the slurry may be quantified according to the usual equations found in the references.

Analysis of pentasodium triphosphate hydration kinetics

The present work analyzes the effect of various factors on the hydration of pentasodium triphosphate. The experimental method is based on application of the hydration test. Technical-quality products with different proportions of phase I and phase II have been used. The variables studied are phase I/phase II ratio, initial temperature, particle size, stirring rate and composition of the slurry (presence of hexahydrate crystals and water hardness). The results have been discussed according to a kinetics model that includes a series of stages of a physical nature (dissolution of anhydrous salt and the crystallization of the hexahydrate), as well as of a chemical nature (solvation of the ions in solution). Crystallization of the hexahydrate may be the controlling stage in the process.

Rheokinetic analysis of the hydration of pentasodium triphosphate in a detergent slurry

The main objective of this research was to analyze, from a rheokinetic point of view, the hydration of pentasodium triphosphate (TPP) in a detergent slurry with a standard composition. A torque rheometer (Brabender Plastograph) with a roller-type measuring system was used. The influence of temperature, TPP quality, system agitation and concentration of dry substances on the total torque of the samples has been studied. From the experimental results it can be concluded that all of these parameters, as well as the reaction time, influence the consistency of the samples. The kinetics of the process are controlled by diffusional resistances.

Surfactants in Food, Pharmaceutical, and Cosmetic Industries

The fat phase of cow’s milk consists mainly of triglyceride globules surrounded by phospholipid bilayer membrane binding integral proteins. In homogenized milk and foods incorporating dairy products, the lipid/water interface may also adsorb casein and whey proteins. To better understand the surface behavior of whey proteins in foods, we studied the interaction of mixtures of phosphatidylglycerol/ phosphatidylcholine in monolayers with serumalbumin, beta-lactoglobulin, and alphalactalbumin adsorbed from solution. UV and CD spectroscopy was used to determine the quantity and conformation of proteins in the monolayers. The effect of pH and calcium ion on lipid-protein interaction was also determined. The binding of calcium and proteins to the phospholipids was competitive and depended on the solution pH and isoionic pH of the protein. An electrostatic mechanism will be discussed.