Interactions of polysaccharides with β-lactoglobulin spread monolayers at the air–water interface

In the present work we have studied the static (film structure and elasticity) and dynamic characteristics (surface dilatational properties) of β-lactoglobulin (βLG) monolayers spread at the air–water interface in the presence of polysaccharides in the aqueous phase, at 20 °C and at pH 7. The measurements were performed on a fully automated Wilhelmy-type film balance. As polysaccharides with interfacial activity we have used propylene glycol alginates (PGA). To evaluate the effect of the degree of PGA esterification and viscosity, different commercial samples were studied-kelcoloid O (KO), kelcoloid LVF (KLVF) and manucol ester (MAN). Xanthan gum (XG) and λ-carrageenan (λC) were studied as non-surface active polysaccharides. The results reveal a significant effect of surface active and non-surface active polysaccharides on static—when the polysaccharide was added in the subphase the π-A isotherms shifted to higher surface pressure values as the time increased-and dynamic—the presence of polysaccharide in the aqueous phase decreased the surface dilatational modulus of a pure β-lactoglobulin monolayer-characteristics of β-lactoglobulin monolayers. To explain the observed effects three phenomena were taken into account: (i) the ability of the polysaccharide to adsorb at the interface by it-self and to increase the surface pressure, (ii) the interfacial complexation of the polysaccharide with the adsorbed protein and (iii) the existence of a limited thermodynamic compatibility between the protein and polysaccharide, depending on the protein-polysaccharide system.     

Effect of ageing on different egg yolk fractions on surface properties at the air–water interface

The aim of this study was to evaluate the impact of egg ageing on the surface properties of whole and fractionated yolk at the air–water interface. Eggs were stored at 4°C for 24 h, 1 week and 3 weeks after laying. A laboratory scale fractionation process was then applied at each ageing time. Egg yolk was separated into two fractions, plasma composed of low‐density lipoproteins (LDLs) and livetins, and granules formed by high‐density lipoproteins (HDLs), phosvitins and LDLg (g = granule). Moreover, recombined plasma and granules fractions were investigated to highlight a potential synergic effect on surface properties. Results have shown the main contribution of LDLs on surface properties of yolk and an improvement of granules surface properties when they are disrupted. Moreover, ageing affected surface properties differently depending on the considered fractions. Broken LDLs and disrupted granules could explain this observed behaviour. Recombined fractions showed different compression isotherms at the air–water interface than whole yolk.     

Hydroxypropylmethylcellulose–β-lactoglobulin mixtures at the oil–water interface. Bulk,interfacial and emulsification behavior as affected by pH

In this work we have studied the bulk, interfacial and emulsification behavior of a surface-active polysaccharide, hydroxypropylmethylcelulose (HPMC) and β-lactoglobulin (βlg) mixtures at two pHs, 3 and 6.     

Protein–polysaccharide interactions at fluid interfaces

Protein–polysaccharide interactions find many applications in food engineering and new food formulations. This review article describes recent research on the effect of protein–polysaccharide interactions on the properties of air–water and oil–water interfaces, as affected by their behaviour in the bulk phase. The interfacial behaviour of protein–polysaccharide mixtures exhibiting associative (i.e., net attractive) interactions as well as their performance in food emulsions and foams has been the subject of several reviews in the last decade. Much less attention has been paid to the interfacial behaviour of protein–polysaccharide mixtures exhibiting unfavourable interactions. Thus we are concerned here with the interfacial behaviour of both kinds of mixtures.     

A comparison of the interfacial behaviour of three food proteins adsorbed at air—water and oil—water interfaces

The adsorption behaviour of three food proteins—a soya protein isolate, a sodium caseinate and a whey protein concentrate—at a soya bean oil-water interface has been studied by the drop volume method. The interfacial behaviour has been compared with that at an air—water interface. The kinetics of surface tension decay were evaluated in terms of different rate-determining steps at different ionic strengths and concentrations of the proteins. The ranking order with respect to the surface activity of the proteins adsorbed at an air—water interface was the same as that at a soya bean oil—water interface. In the high concentration range the surface activity of the proteins was higher at an air-water interface than at a soya bean oil-water interface, whereas thereverse was found in the low concentration range. In general, the adsorption of the proteins was more diffusion controlled at an air-water interface than at a soya bean oil-water interface; this suggested that proteins were less folded at the soya bean oil-water interface. A comparison of the rates of the diffusion controlled steps for the proteins at air-water and soya bean oil-water interfaces indicated that the solvation energy gained when caseinates adsorb at the soya bean oil interface was enhanced compared with the other two proteins. This indicated an enhanced loop formation of the caseinate molecules in the oil phase when adsorbing at this interface, as compared with the air-water interface.     

Sequential adsorption of egg-white proteins at the air–water interface suggests a stratified organization of the interfacial film

The aim of this work was to get further insight into the formation of the interfacial film by ovalbumin and lysozyme in equimolar binary solutions, as well as the organization of these proteins inside the interfacial film. Our approach consisted in the sequential adsorption of the two proteins, with or without previous replacement of the sub-phase with protein-free buffer before injection of the second protein.     

Interfacial and foaming properties of soy protein and their hydrolysates

The objective of the work was to study the impact of soy protein hydrolysis on foaming and interfacial properties and to analyze the relationship between them. As starting material a sample of commercial soy protein isolate was used (SP) and hydrolysates were produced by an enzymatic reaction, giving hydrolysates from 0.4% to 5.35% degree of hydrolysis (DH).In this contribution we have determined foam overrun (FO), stability against liquid drainage and foam collapse, and the apparent viscosity of foams produced by a whipping method. The surface properties determined were the adsorption isotherm and surface dilatational properties of two hydrolysates (2 and 5.35% DH, H1 and H2 respectively).The hydrolysis of soy proteins increased the surface activity at bulk concentrations where SP adopts a condensed conformation at the monolayer. At concentrations where it adopts a more expanded conformation a very low degree of hydrolysis (H1) also promoted the enhancement of surface activity. However, at 5.35% degree of hydrolysis (H2) the surface activity decreased. Moreover, H2 presented lower surface activity than H1 at every bulk concentration.The hydrolysis increased the elastic component of the dilatational modulus and decreased phase angle of films at bulk concentrations below that corresponding to the collapse of SP monolayer (2% bulk protein).SP hydrolysis increased foam overrun and the stability against drainage that could be related to increased surface activity of protein hydrolysates. However, the collapse of foams was promoted by hydrolysis and could be ascribed to a decrease of the relative viscoelasticity (higher phase angle) of surface films.The results point out that a low degree of hydrolysis (2–5%) would be enough to improve the surface activity of SP, decrease foam drainage and maintaining a considerable viscoelasticity of the surface films to retard foam collapse.     

Antibacterial activity of Ginseng (Panax ginseng C. A. Meyer) stems–leaves extract produced by subcritical water extraction

Antibacterial activities of ginseng extracts produced from ginseng by‐products, stems and leaves, using subcritical water extraction (SWE) at 110, 165 and 190 °C, were evaluated and compared with those of ginseng extracts prepared by hot water and ethanol extraction. The ginseng stems–leaves extract produced by SWE at 190 °C contained the greatest concentration of phenolics (98.4 mg GAE g^?1 of extract). All ginseng extracts inhibited the growth of Bacillus cereus, Salmonella enteritidis, Escherichia coli O157:H7 and Listeria monocytogenes. Among four strains, B. cereus was more sensitive to the ginseng extract by SWE at 190 °C than other bacteria. Cell membranes of bacteria were disrupted by the addition of SWE ginseng extract, observed by transmission electron microscopy (TEM), with the release of cellular contents. These findings provided evidence about the potential utilisation of ginseng stems and leaves by using an environmental friendly extraction process, SWE, to produce ginseng extract for the inhibition of bacteria growth.     

Adsorption of β-Lactoglobulin variants A and B to the air–water interface

Summary The adsorption of proteins to interfaces is a vital and complex process for the formation and stabilization of multiphase food systems (emulsions and foams). The process of protein adsorption is generally understood only at the phenomenological level, as the complexity of protein unfolding during adsorption is very difficult to predict and model. By comparing proteins with very similar structures, it is possible to attribute observed changes in adsorption behaviour. The A and B genetic variants of β-lactoglobulin (β-lg) differ by only two amino acids (Asp-64, Val-118 in A, and Gly-64, Ala-118 in B), thus making them ideal candidates for this type of comparison. In this study we monitored the surface behaviour of β-lg A and B, measuring the surface tension and surface dilational modulus of adsorbed protein, and the compression behaviour of spread protein films. At pH 7, variant B lowered the surface tension and increased the surface dilational modulus more rapidly than variant A. Raising the pH to 7.8 should increase the level of dissociation into monomers. Indeed, this was confirmed by the rate of adsorption, which increased in both cases. Also, the surface tension of both variants was much lower than at pH7. Variant B was less sensitive to the change of pH than A. Regardless of pH, after 3 h adsorption the difference between the variants in surface tension or surface dilational modulus was negligible. The differences in surface behaviour between the variants are discussed in terms of interactions between monomers at and with the interface, and the dimer : monomer equilibrium in the bulk solution.     

Adsorption dynamics and surface activity at equilibrium of whey proteins and hydroxypropyl–methyl–cellulose mixtures at the air-water interface

In this work the competitive behaviour of whey protein concentrate (WPC) and three well-characterized hydroxypropyl–methyl–celluloses (HPMCs), so-called E4M, E50LV and F4M, with different interfacial properties, were studied by measurement of the dynamics of adsorption and surface pressure isotherms. Such differences may be attributed to differences in the molecular weight and degree and molar substitution among the HPMCs. Thus, E4M having the highest molecular weight showed the highest surface activity among the HPMCs. The lower surface activity of F4M, in comparison to E50LV, may be attributed to its lower degree of total substitution. The experiments were performed at constant temperature (20 °C), pH 7, ionic strength 0.05 M and variable concentrations of the components.The differences observed between mixed systems were according to the relative bulk concentration of biopolymers and molecular structure of HPMC. In the presence of E4M a strong competence for the interface can be observed at short adsorption time. As E4M is more surface active than WPC, the replacement of E4M at the interface by WPC resulted in lower surface pressure. The mixture approached E4M behaviour at longer adsorption time. An additive or synergistic behaviour was observed for celluloses with lower surface activity (E50LV and F4M) at the lowest WPC and HPMC concentrations in the aqueous phase. A depletion mechanism in the vicinity of the interface would predominate in the presence of E50LV and F4M over the competence for the interface, giving rise to a cooperative behaviour. When WPC saturated the interface and the HPMC concentration in the aqueous phase was low enough, WPC dominated the surface pressure at long-term adsorption.     

Effect of food preparation on the structure and metabolic responses to a monostearin–oil–water gel-based spread

Widespread recognition of the negative health effects of trans and saturated fats has prompted research to develop alternative structures that can structure liquid oils into semi-solid plastic pastes for food applications. We have recently developed and described the physical chemical properties of a unique a monostearin–oil–water gel (MAG) that achieves this goal. Furthermore, ingestion of this MAG in the form of a margarine-like spread resulted in beneficial suppression of blood lipid and insulin responses in humans compared to compositionally-equivalent controls lacking the MAG structure. However, the integrity of this novel structure and its salubrious metabolic effects have not been previously evaluated under food processing conditions. The purpose of the current study was to evaluate the integrity of the MAG when applied to toasted bread and when mixed with a warm pasta meal, and to evaluate the metabolic effects over 6 h following ingestion of both types of meals with MAG compared to compositionally-equivalent unstructured oil preparations. MAG structure was maintained in the toast study, but the pasta meal destroyed the MAG structure. Triglyceride, free fatty acid, and insulin responses were suppressed in the MAG trial compared to the unstructured oil trial of the toast study, whereas there was no difference in the responses of these variables between the two trials in the pasta study. The results demonstrate for the first time that the metabolic effects of MAG depend on maintenance of the MAG structural integrity throughout the food processing procedures leading up to ingestion. Thus, this study demonstrates the utility of MAG as a trans- and saturated fat-free vehicle to structure and deliver liquid oil as a semi-solid plastic paste, and establishes processing limits to the integrity of the additional beneficial metabolic effects of this novel structure.     

Characterization of deep‐fat frying in a wheat flour–water mixture model using a state diagram

BACKGROUND: Crispness is an important characteristic to be controlled in deep‐fat fried products. The physical state of food polymers influences the development of cellular structure and textural qualities of fried food. Glassy state is believed to play an important role in the mechanical properties of low‐moisture food. Therefore, an understanding of the physicochemical phenomena in the development of fried food structure using a state diagram of the frying process is discussed. RESULTS: Wheat flour models containing 400 and 600 g kg^?1 initial moisture content were fried in frying oil at 150 °C for 1–7 min. Thermal properties of wheat flour, structure alteration and textural properties of fried samples were evaluated. The porous structure continuously enlarged when the sample was in the rubbery state. As the frying time was prolonged, the state of the product became glassy due to a decrease in water content, resulting in the ceasing of porous enlargement. Conclusion: The results revealed that physicochemical changes during frying influence the alteration of microstructure and quality of fried food, and the state diagram could be applied to explain the formation of microstructure during the frying process and used as a decision‐making tool to choose the proper conditions to provide desirable qualities in fried food. Copyright © 2007 Society of Chemical Industry     

Water activity,water glass dynamics,and the control of microbiological growth in foods

Water is probably the single most important factor governing microbial spoilage in foods, and the concept of water activity (a_w) has been very valuable because measured values generally correlate well with the potential for growth and metabolic activity. Despite some drawbacks (e.g., solute effect), the concept of a_w has assisted food scientists in their effort to predict the onset of food spoilage as well as to control food‐borne disease hazards in food products. In the last decade the concept of a_w has been challenged. It has been suggested that reduced‐moisture food products (e.g., low and intermediate) may be nonequilibrium systems and that most of them are in the amorphous metastable state, which is very sensitive to changes in moisture content and temperature. It has been proposed that the glass transition temperature T_g (temperature at which the glass‐rubber transition occurs), is a parameter that can determine many product properties, the safety of foods among them. The concept of water dynamics, originating in a food polymer science approach, has been suggested instead of a_w to better predict the microbial stability of intermediate‐moisture foods. The usage of a_w to predict microbial safety of foods has been discouraged on the basis that (1) in intermediate‐moisture foods the measured water vapor pressure is not an equilibrium one, and because a_w is a thermodynamic concept, it refers only to equilibrium; and (2) the microbial response may differ at a particular a_w when the latter is obtained with different solutes.

This review analyzes these suggestions on the basis of abundant experimental evidence found in the literature. It is concluded that nonequilibrium effects (e.g., inability of water to diffuse in a semimoist food) appear to be in many cases slow within the time frame (food's shelf life) of the experiments and/or so small that they do not affect seriously the application of the a_w concept as a predictor of microbial stability in foods.

The claims that a food science polymer approach to understanding the behavior of aqueous sugar glasses and concentrated solutions may be used to predict the microbial stability of food systems is not substantiated by experimental evidence. This approach does not offer, at the present time, a better alternative to the concept of a_w as a predictor of microbial growth in foods.

It is also recognized that a_w has several limitations and should be always used carefully, and this must include precautions regarding the possible influences of nonequilibrium situations. This aspect may be summarized by simply saying that anyone who is going to employ the term water activity must be aware of the implications of its definition.     

Improving the functional properties of soy glycinin by enzymatic treatment. Adsorption and foaming characteristics

In this contribution we have determined the effect of limited enzymatic hydrolysis on the interfacial (dynamics of adsorption and surface dilatational properties) and foaming (foam formation and stabilization) characteristics of a soy globulin (glycinin, fraction 11S). The degree of hydrolysis (DH=0%, 2%, and 6%), the pH of the aqueous solution (pH=5 and 7), and the protein concentration in solution (at 0.1, 0.5 and 1 wt%) were the variables studied. The temperature and the ionic strength were maintained constant at 20 °C and 0.05 M, respectively. The rate of adsorption and surface dilatational properties (surface dilatational modulus, E, and loss angle) of glycinin at the air–water interface depend on the pH and DH. The adsorption decreased drastically at pH 5.0, close to the isoelectric point of glycinin, because of the existence of a lag period and a low rate of diffusion. The interfacial characteristics of glycinin are much improved by enzymatic treatment, especially in the case of acidic aqueous solutions. Hydrolysates with a low DH have improved functional properties (mainly foaming capacity and foam stability), especially at pH close to the isoelectric point (pI), because the native protein is more difficult to convert into a film at fluid interfaces at pH≈pI. The foam capacity depends on the rate of diffusion of protein to the interface and is much improved by the enzymatic treatment. Foam stability correlates with surface pressure and, to a minor extent, with surface dilatational modulus at long-term adsorption with few exceptions.     

The adherence of Salmonella Enteritidis PT4 to stainless steel: the importance of the air-liquid interface and nutrient availability

Biofilm formation on stainless steel by Salmonella enterica serovar Enteritidis PT4 during growth in three different nutritious conditions was studied. The ability of micro-organisms to generate biofilms on the stainless steel surfaces was studied for a total period of 18 days at 20 degrees C, under three different experimental treatments: (i) growth medium (tryptone soy broth) was not refreshed (no further nutrients were provided) during the incubation period, (ii) growth medium was renewed every 2 days and (iii) growth medium was renewed every 2 days and at the same time the planktonic cells from the old medium were transferred to the new fresh medium. It was found that biofilms developed better and a higher number of adherent cells (ca. 10(7) cfu/cm(2)) were recovered when the organism was grown in periodically renewed nutrient medium than when the growth medium was not refreshed. Regardless of the availability of nutrients, biofilm development was better (range 2-3 logs greater) when coupons were not totally covered by the growth medium and part of the surface was exposed to the air-liquid interface, than when coupons were submerged in the medium. The results suggest that existence of air-liquid interface and adequate nutrient conditions provide the best environment for Salmonella Enteritidis PT4 biofilm formation on stainless steel. The possible role of stationary phase planktonic cells in biofilm development by sessile/attached microbial cells is also discussed.     

Salt uptake and water loss in hams with different water contents at the lean surface and at different salting temperatures

The salt uptake homogeneity is crucial in assuring quality in dry-cured hams. The aim of this study was to evaluate the effect of the water contents at the lean surface before salting and of the temperature during salting on the salt uptake. Pieces of loin stored at 3 °C for 3 days before salting absorbed less salt through a surface that has been dried during storage. A group of raw hams were subjected to different pre-salting storage times (0, 3 and 6 days) and another group subjected to different set room temperatures during salting (− 1.0, 0.5 and 4.0 °C). The duration of storage before salting and the temperature during salting had a negative and a positive effect on the average salt absorption, respectively. The most important effects appeared after 6 days of storage and at 4 °C. No significant differences in salt uptake homogeneity were found between storage times and between salting temperatures.