The Application of the Drop Volume Technique to Measurements of the Adsorption of Proteins at Interfaces.

A new procedure for the application of the drop volume technique to measurements of the rate of adsorption of proteins at interfaces has been developed. The mode of adsorption of the proteins lysozyme, β-lactoglobulin and bovine serum albumin (BSA) at the air-water interface has been measured with the drop volume method and has been compared to measurements with the Wilhelmy plate technique. Due to surface enlargement of the drop throughout the process of the surface tension decay, slower kinetics of the adsorption process is obtained by the drop volume method compared to the Wilhelmy plate technique, and the proteins investigated were differently sensitive to this surface expansion. The adsorption process of the proteins has been evaluated in terms of different rate-determining processes. Different intermediate states between the native and the denatured forms have been observed

The Interfacial Behaviour of Three Food Proteins Studied by the Drop Volume Technique.

The adsorption behaviour of three food proteins, a soy protein isolate, a sodium casein‐ate and a whey protein concentrate, at the air‐water interface has been studied by the drop volume method. The kinetics of surface tension decay were evaluated in terms of different rate‐determining steps at different ionic strengths and concentrations. This analysis indicates the following characteristics concerning the surface behaviour of the protein systems studied. The soy proteins diffuse slowly to the interface compared to the other proteins, probably due to the large particle size of the association complex of soy proteins. For the soy proteins, diffusion is slower in distilled water than in 0.2M‐NaCl solution and spreading of molecules at the interface is most easily performed in 0.2M‐NaCl solution. The whey proteins diffuse quickly to the interface, which is in accordance with their aqueous association; mainly small molecular complexes. Diffusion is slower and spreading easier in distilled water than in 0.2M‐NaCl solution. Although the caseinate has a complex quaternary structure, like the soy proteins, it has a very different surface behaviour. The diffusion step is rapid at concentrations above 10−3 wt % and contributes to a large extent to the interfacial tension decay, especially when the caseinate is dispersed in 0.2M‐NaCl solution. At a concentration of 10−3 wt % and below, the rate of the diffusion step is slowed down drastically, with an accompanying drop in the surface activity of the protein. This type of surface behaviour can be explained if the migration of the caseinates to the interface takes place via the casein monomers in the bulk phase

Functional Characterization of Protein Stabilized Emulsions: Emulsifying Behaviour of Proteins in a Valve Homogenizer.

Protein stabilised emulsions have been prepared in a valve homogeniser incorporated into a recirculating emulsification system, where the power input and number of passes have been varied. The food proteins studied were a soy‐bean protein isolate, a whey protein concentrate (WPC) and a sodium caseinate. The emulsions obtained were characterized in terms of particle size distribution and amount of protein adsorbed on to the fat surface (protein load). Generally, the final fat surface area of the emulsions obtained increases more as a function of power input than as a function of number of passes. Distribution width, cs, decreases mostly with increasing power supply and number of passes, but at the highest power input cs increases. The protein load on the fat globules is largely determined by the fat surface area and by the type of protein adsorbed. The soy proteins give a high protein load and the caseinates give a low protein adsorption at small fat surface areas created. This relation is reversed at large surface areas of the fat globules. The relation between percentage protein adsorbed from bulk as a function of surface area suggests that the caseinates mainly cover the newly created interface by adsorption from the bulk, whereas the soy proteins fulfil this task mostly by spreading at the interface. Salt addition to 0.2M‐NaCl enhances protein adsorption at the fat globule interface in the case of soy protein and caseinate, but for the whey proteins protein load is higher in distilled water

The Adsorption Behavior of Proteins at an Interface as related to their Emulsifying Properties

The emulsifying properties of proteins have been a subject concern for those dealing with functional properties of proteins. The studies so far have been restricted to two main approaches: emulsifying capacity and emulsion stability measurements. The former measures the maximum oil addition until inversion or phase separation of the emulsion occurs, whereas the latter measures the ability of the emulsion to remain unchanged. variety of empirical methods has been used, which makes it difficult to compare the results obtained by different authors. Not only methods of measurement vary, but also the way the emulsions are prepared, which strongly influences the properties of emulsions formed (1). As protein stabilized emulsions are usually very stable and the adsorption of proteins at interfaces can be considered as mainly irreversible, the emulsifying properties of the proteins during the emulsification process become increasingly important in determining the properties of the emulsion formed

Functional Characteristics of Protein Stabilized Emulsions: Emulsifying Behaviour of Proteins in a Sonifier.

Protein stabilized emulsions made up of 40% soybean oil by weight and protein dispersions of 2.5% (w/w) protein content have been prepared in an ultrasonic device. The emulsifying apparatus was incorporated into a recirculating system, where power input and number of passes were varied. The food proteins studied were a soy bean protein isolate, a whey protein concentrate (WPC), and a sodium caseinate. The emulsions obtained were characterized in terms of droplet size distribution and amount of protein adsorbed per unit fat surface area (protein load). The results were compared to the fat surface area and the protein load of emulsions made in different ways in a valve homogenizer. Flocculated emulsions, such as the types stabilized with soy protein and WPC (0.2–7), produced either in a sonitier or in a valve homogenizer, have larger fat surface areas and broader size distributions than nonflocculating systems. In the sonifier, overprocessing in terms of fat surface area occurs at high power inputs for all protein stabilized emulsions. Sonified emulsions have smaller droplet size and a broader spectrum of globule size than valve homogenized emulsions made with the same power input respectively, and number of passes. The protein load is largely determined by the protein/fat surface area ratio and by the type of protein used as an emulsifier, irrespective of the emulsifying conditions (emulsifying apparatus, intensity, and time). But the latter are dominating with regard to the final droplet size distribution of the emulsions, where the choice of protein is of minor importance. The relation between percentage protein adsorbed from the bulk phase as a function of the fat surface area suggests that the caseinates are mainly adsorbed from the bulk phase to the newly created interface up to a surface area of 8–9 m2/ml emulsion. For the soy protein (0–7) and WPC (0.2–7) stabilized emulsions spreading of already adsorbed molecules becomes more favorable than adsorption of proteins from the bulk at fat surface areas as low as ∼ 3 m2/ml emulsion. This change in the way of covering the interface occurs for WPC (0–7) stabilized emulsions at surface areas of about 4 m2/ml emulsion

Engineering processes in meat products and how they influence their biophysical properties.

Food engineering aspects of cooking of meat products in relation to their biophysical properties, such as water- and fat-holding, have been reviewed. Moreover, some of the new emerging, mild cooking technologies, such as high pressure and electro-based heating (radio frequency cooking and ohmic heating) have been discussed in relation to the biophysical properties of the meat products treated. The holding of the bulk water (about 70% of the muscle weight) was discussed, arguing capillary forces to be one of the dominating mechanisms for this holding, whereas the losses of water and fat (the flow) within the meat are governed by Darcy's law. If we compare the fat-holding in beef burgers and emulsion sausages (frankfurter type) beef burgers lose much larger part of the fat than the emulsion sausages and for the former the fat losses increase with fat content. For emulsion sausages, however, fat losses are independent of fat content and the properties of the fat and the protein matrix are more interrelated. It has been shown experimentally during double sided pan frying of beef burgers that the pressure driven water loss (up to 80% of the water loss) is a substantially more important mechanism governing the water loss than the evaporation losses occurring at the surface crust. Fat losses increased significantly with fat content and were not influenced to any large extent by the cooking temperature and were in the form of drip. By using processing technologies such as high pressure and/or electro-based heating (radio frequency cooking and ohmic heating) a more homogenous heating can be achieved, the reason being volumetric heating. In comparison with conventional heating shorter cooking times were obtained and with smaller temperature gradients lower water- and fat-losses occurred and the yield can be substantially improved. High pressure processing (100-1000MPa) is a preservation technology that allows the reduction of the microbial load at low or moderate temperature. The highest potential application for meat products might be to pressurise finally sealed packages of contaminated sliced high value salami and ham products as the colour of those products can resist high pressure. Ohmic heating is based on the passage of electrical current through a food product having an electrical resistance. For radio-frequency (RF) cooking and micro-wave heating the food product forms a dielectric media between the two electrodes and the heating is caused by the internal friction of the polar molecules. The used frequencies for RF-cooking are lower in the MHz range than for micro-wave heating being in the GHz range

Effects of heat on meat proteins - Implications on structure and quality of meat products

Globular and fibrous proteins are compared with regard to structural behaviour on heating, where the former expands and the latter contracts. The meat protein composition and structure is briefly described. The behaviour of the different meat proteins on heating is discussed. Most of the sarcoplasmic proteins aggregate between 40 and 60 ° C, but for some of them the coagulation can extend up to 90 ° C. For myofibrillar proteins in solution unfolding starts at 30-32 ° C, followed by protein-protein association at 36-40 ° C and subsequent gelation at 45-50 ° C (cone. > 0.5% by weight). At temperatures between 53 and 63 ° C the collagen denaturation occurs, followed by collagen fibre shrinkage. If the collagen fibres are not stabilised by heat-resistant intermolecular bonds, it dissolves and forms gelatine on further heating. The structural changes on cooking in whole meat and comminuted meat products, and the alterations in water-holding and texture of the meat product that it leads to, are then discussed. © 2005 Elsevier Ltd. All rights reserved

A Surface Tension Apparatus According to the Drop Volume Principle

A new construction of a drop volume apparatus has proved to be successful in determining the interfacial tension for a variety of pure liquids as well as for solutions having surface tensions, which come to equilibrium quickly. Temperature dependence of the surface tension can easily be recorded even at elevated temperatures, which has been shown for water