Food Structure and Functionality

By Charis M. Galanakis

Food Structure and Functionality helps users further understand the latest research related to food structuring and de-structuring, with an emphasis on structuring to achieve improved texture, taste perception, health and shelf-stability. Topics covered address food structure, nanotechnology and functionality, with an emphasis on the novel experimental and modeling approaches used to link structure and functionality in food. The book also covers food structure design across the lifespan, as well as design for healthcare and medical applications. Dairy matrices for oral and gut functionality is also discussed, as is deconstructing dairy matrices for the release of nutrient and flavor components.

This book will benefit food scientists, technologists, engineers and physical chemists working in the whole food science field, new product developers, researchers, academics and professionals working in the food industry, including nutritionists, dieticians, physicians, biochemists and biophysicists.

• Covers recent trends related to non-thermal processes, nanotechnology and modern food structures in the food industry
• Begins with an introduction to the structure/function of food products and their characterization methods
• Addresses biopolymer composites, interfacial layers in food emulsions, amyloid-like fibrillary structures, self-assembly in foods, lipid nano-carriers, microfluidics, rheology and function of hydrocolloids
• Discusses applications and the effects of emerging technologies on process, structure and function relationships

• Language: English
• Publisher: Academic Press
• Release date: Nov 17, 2020
• ISBN: 9780128214640

Book preview

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Preface

Understanding the relationship between food structure and specific sensory attributes, as well as digestion and transport of food components within the gastrointestinal tract is necessary to design food products with specific functionalities, for example, healthy foods, foods with an increased feeling of satiety, food with increased protein, or reduced-fat content among others. However, with the recent advantages in food processing (e.g., nonthermal technologies and nanoencapsulation), new developments, data, and state-of-the-art come up in the field. Nowadays, modern food scientists and technologists often deal with new product development and functional foods, and thereby an essential guide connecting food structure and functionality is needed.

Food Waste Recovery Group provides insights into the food and environmental science and technology sectors, and this line is publishing in several books. The latter deals with food waste recovery technologies, biobased products, and industries, the valorization of food processing by-products (e.g., from olive, grape, cereals, coffee, and meat), sustainable food systems, saving food efforts, innovations strategies in the food and environmental science, innovations in traditional foods, nutraceuticals, and nonthermal processing. The group has also prepared textbooks for innovative food analysis, shelf-life, and food quality, personalized nutrition, nonalcoholic drinks, and customized guides for specific food components like carotenoids, dietary fiber, glucosinolates, lipids, polyphenols, and proteins.

Following the above considerations, the current book covers food structure and functionality, focusing on food products with tailored properties toward end-user preferences (e.g., improved texture, shelf stability, taste perception, and health). The ultimate goal is to support the scientific community, food engineers, food scientists, and food technologists who aspire to develop industrial and commercialized applications in the food industry. The book fills the gap of transfer knowledge between academia and industry, covering issues coming up to the table over the last decade with the development and applications of emerging and nonthermal technologies in the food industry.

The book consists of 12 Chapters. Chapter 4 provides an overview of the most important aspects that govern the composition, structure, and mechanical properties of interfaces in food emulsions. The main theoretical concepts and experimental methods existing traditionally to study fluid interfaces are revised. Likewise, the characteristics of the pendant drop film balance are discussed before providing a detailed analysis of the interfacial properties of adsorbed layers.

Chapter 2 deals with the rheology, structure, and sensory perception of hydrocolloids. Hydrocolloids have long been used as food additives due to their influence on texture, rheology, structure, and mouthfeel of food products. Also, their use as functional ingredients in foods is currently on the rise due to their beneficial health effects. Starch and cellulose derivates and gums are mostly used in food formulations. The different ways of modification of starch and cellulose are described as well as resulting changes in physicochemical and rheological properties more suitable for usage in food are also discussed.

In Chapter 3, the fundamental principles of microfluidics and their feasibility to process food materials and design food structures are thoroughly covered and discussed. Microfluidics has been studied as a rapid detection tool for food-safety related applications for a long time. Recently, more and more interests have been attracted to use microfluidics as microreactors to assemble biopolymers and to achieve desired functionalities. Attributed to the accurate flow control and unique bottom-up mechanism, the properties of the assembled biopolymer structures could be well-controlled.

High-pressure processing (HPP) has a decisive role in nonthermal food pasteurization being, among others, the most successful nonthermal processing technology, which has motivated both academia and industry to develop novel applications for HPP and new food products. HPP can lead to the development of new foods with improved texture aspects due to the cold gelation, a process that leads to the formation of proteins’ networks with a stable thermodynamic confirmation allowing a phase transition from liquid to solid. In Chapter 4, the effects of HPP on animal and plant-based proteins are presented and discussed. Important fundamental features of proteins, such as three-dimensional conformation, techno-functional properties, and other applications, are highlighted.

Chapter 5 addresses the way electric fields and electromagnetic waves exert an effect on protein’s structure and functionality and, consequently, its impact on human health. It starts with background regarding electric fields and the electromagnetic spectrum, followed by the key technologies based on these principals, namely pulsed electric fields, ohmic heating, microwaves, and radiofrequency. Then, their use as an alternative process for protein extraction is reported regarding their mechanism and results are also found. Finally, the impact of the structural change of the protein on human health was addressed, covering the relationship between structure and protein digestibility and allergenicity.

Chapter 6 provides an overview of microscopy and spectroscopy techniques used to analyze the microstructure (structure) of food powders. With the knowledge of the powder structure, it is possible to propose the design conditions for quality products with better stability. The development of the structure of the powders will depend on the process used for their elaboration, among those of more significant industrial applications are the fragmentation (grinding), pulverization, lyophilization, drum drying, and spray drying, the latter being the most widely used.

Chapter 7 describes solid lipid nanoparticles and nanostructured lipid carriers, which are lipid-based nanocarriers used for encapsulating lipophilic compounds, such as carotenoids, polyphenols, essential oils, and some vitamins. First, the formulation, production process, and applications of solid lipid nanoparticles for encapsulating bioactive food components are discussed. Then, the discussion focuses on nanostructured lipid carriers. Finally, issues regarding safety are mentioned, and a summary of the current regulatory framework in various parts of the world is presented.

Chapter 8 summarizes the recent studies on the utilization of novel technologies in starch modification and their effect on the physicochemical, structural, rheological, and functionality of modified starch. The effect of these strategies on structure–function relationships of starch is also reviewed. The results provide useful information for the application of the novel method in starch-based industries. Moreover, these methods can be used for the designing of tailor-made products contained starch and their derivatives.

Chapter 9 presents current knowledge about the fabrication of different types of protein-based nanostructures, reviewing the leading strategies and techniques to fabricate a variety of nanostructured proteins. Encapsulation, retention, protection, and release of bioactive compounds in protein-based nano-delivery systems are discussed. The effect of manufacturing variables on functional and physicochemical properties of the nanostructured proteins and bioavailability of encapsulated bioactives are also denoted, whereas the particular emphasis is given on the interactions between the bioactive ingredients and nanostructured proteins.

Chapter 10 revises the current knowledge of plant protein properties, their interactions with other polymers (proteins or carbohydrates) in food systems, the structure of complexes formed by these interactions at interfaces, and the applications of these complexes. The properties of polymers, environmental conditions, and processing technologies can affect interactions of plant proteins with other polymers and the applications of their complexes in the food industry.

Food-derived compounds have been explored as active components for the management of type 2 diabetes mellitus. One of the primary bioactivity mechanisms is through the inhibition of α-glucosidase, which is involved in the release of glucose from dietary carbohydrates during digestion in the gastrointestinal tract. Chapter 11 describes the regulation of carbohydrate digestion during normal and abnormal conditions, and the α-glucosidase inhibitory activity of food-derived compounds, including peptides, phenolic compounds, terpenoids, and nonstarch polysaccharides.

In Chapter 12, the role of various components of food in the pathobiology of metabolic syndrome is highlighted with particular attention to their role in the metabolism of essential fatty acids, an essential but often ignored component of food, and its impact on metabolic syndrome and cancer.

Conclusively, the current book is assisting food scientists, technologists, engineers, biochemists, and physical chemists working in the whole food science field as well as new product developers, researchers, academics, and professionals working in the food industry with food applications and food processing. University Libraries and Institutes could also use it all around the world as ancillary reading in under-graduates and post-graduate level multi-discipline courses dealing with food science and technology and food processing.

I would like to thank and acknowledge one by one all the authors for their fruitful collaboration and their dedication to editorial guidelines and timeline. I am fortunate to have had the opportunity to work together with international experts from Argentina, Brazil, Iran, Ireland, Mexico, Portugal, Serbia, Spain, the Republic of Korea, Turkey, and the United States. I would also like to thank the acquisition editor Megan Ball, the book managers Laura Okidi, and Lena Sparks, as well as all the colleagues from Elsevier’s production team, for their assistance during the preparation of this book. Last but not least, please note that those collaborative efforts to provide an integral book end up to a manuscript of hundreds of thousands of words that may contain gaps or errors. Suggestions, comments, and even criticism are always welcome. So please do not hesitate to contact me to discuss any relevant issues.

Charis M. Galanakis¹, ², ³, ¹Research & Innovation Department, Galanakis Laboratories, Chania, Greece, ²College of Science, King Saud University, Riyadh, Kingdom of Saudi Arabia, ³Food Waste Recovery Group, ISEKI Food Association, Vienna, Austria

Chapter 1

Structure and functionality of interfacial layers in food emulsions

Julia Maldonado-Valderrama, Teresa del Castillo-Santaella, María José Gálvez-Ruiz, Juan Antonio Holgado-Terriza and Miguel Ángel Cabrerizo-Vílchez,    University of Granada, Granada, Spain

Abstract

This chapter provides an overview of the most important aspects that govern the composition, structure, and mechanical properties of interfaces in food emulsions. The main theoretical concepts and the main experimental methods existing traditionally to study fluid interfaces are revised. The characteristics of the pendant drop film balance owing to its versatility and acceptance to work at liquid interfaces are discussed. Then, a detailed analysis of the interfacial properties of adsorbed layers considering individual systems, mixtures, as well as different methods of formation and devices is provided. This knowledge is then applied mechanisms to control stability via interfacial design. The relationships between interfacial magnitudes and physical properties of emulsions are also denoted before discussing the digestion of emulsified systems and the advanced design of food products with tailored functionality.

Keywords

Emulsion; interfacial tension; dilatational rheology; monolayer; interactions; stability; digestion

1.1 Introduction

Emulsions are dispersions of one liquid (dispersed phase) in another liquid (continuous phase) (Berton-Carabin and Schroën, 2015, 2019; Bos and Van Vliet, 2001; Langevin, 2000). A critical factor in the emulsification process is the formation of an interfacial layer, which protects the entity of the droplets of the dispersed phase. The process of emulsion formation involves the creation of large amounts of interfacial area, that is, regions of high free energy. Accordingly, emulsions are thermodynamical, unstable systems.

On the one hand, in order to form and disperse the droplets, we need to apply (mechanical) energy to the system. On the other hand, emulsions tend to separate owing to the high free energy of the free interfacial area of the dispersed droplets. Emulsifiers can decrease the free energy of the system by decreasing the interfacial tension of the dispersed droplet. In this way, the amount of work required to form the emulsion is reduced, and the interfacial layer promotes a physical stabilization of droplets. The properties of these interfacial layers, which protect emulsion droplets, play a fundamental role in food emulsions.

Emulsifiers are amphiphilic molecules, which have two parts: one hydrophilic and one hydrophobic (Hamley, 2007). Different amphiphiles differ in their molecular structure, being proteins, lipids, biosurfactants, or biopolymers. A characteristic feature of amphiphiles is their tendency to migrate spontaneously to nonpolar interfaces and reduce the interfacial tension. Then, the specific properties of the interfacial layer protecting the dispersed phase droplets will be strongly dependant on the molecular structure of the amphiphile, their intermolecular associations, and the interaction with the adjacent phases. The thickness, morphology, and mechanical properties of the interfacial layer depend on the nature of the emulsifier. Also, the characteristics of the interfacial film will determine not only the properties of the emulsion system by affecting its stability but also its texture and, ultimately, its digestibility (Berton-Carabin and Schroën, 2019). This chapter focuses on oil-in-water emulsions stabilized by proteins as one of the leading food emulsifiers displaying functional and nutritional properties (Hoffman and Reger, 2014). It looks into the fundamentals of protein interfacial layers and their mixture with different systems and how this can influence the behavior of emulsified systems.

fluid interfaces with an emphasis on the pendant drop tensiometer. The next section contains an in-depth analysis of the interfacial properties of adsorbed layers considering individual systems, mixtures, and different methods of formation. This fact leads on to some applications of interfacial design in food systems, which are revised in Section 4.4, the stability of emulsions, and Section 4.5, interfacial digestion development of emulsions with tailored digestibility. Section 4.6 summarizes and provides a future perspective where the challenge of connecting the interfacial picture and the behavior of emulsions and the digestibility of the corresponding systems is highlighted.

1.2 Interfacial magnitudes, systems, and methodologies

1.2.1 Surface and interfacial tension

Surface tension results from the difference of forces acting on the molecules. The latter is located at the surface of a liquid concerning those molecules located in bulk. The liquid phase has more molecules than the air phase. This fact leads to a net repulsive force acting at the surface and a gradual decrease in density (Fig. 1.1). The surface tension of a liquid (γ) can be defined by the work done to create an area (ΔA) of surface or the force per unit length associated with this process (Hamley, 2007):

(1.1)

Figure 1.1 Surface tension results from the difference of forces that apply to molecules located at the surface.

Similarly, the interfacial tension between two immiscible liquids 1 and 2 is a consequence of differences in the intermolecular forces acting at the interface and is also responsible for the free energy of the interface. However, there are three types of interactions acting at the interface that should be considered:

1. Homomolecular interactions in liquid 1

2. Homomolecular interactions in liquid 2

3. Heteromolecular interactions between liquids 1 and 2.

Accordingly, the following relation is derived:

(1.2)

where γ1 and γ2 and stands for the surface tension of liquids 1 and 2, respectively, γ12 stands for the interfacial tension between liquids 1 and 2 and W12 depends on the degree of intermolecular interaction of the two liquids—partial solubility, polarity, and density.

1.2.2 Adsorption and dilatational rheology of interfacial layers

water interfaces. Amphiphilic molecules expose their hydrophobic part to the nonpolar environment and the hydrophilic part into the polar phase (water). Hence, adsorption of amphiphiles to nonpolar interfaces is an entropic effect resulting partly from the energetically unfavorable interaction of hydrocarbon chains and water. When free amphiphiles expose their hydrophobic parts to water, they disrupt some hydrogen bonds in water imposing a locally ordered structure, which is highly energetically unfavorable owing to decreased entropy of water molecules. Conversely, amphiphiles located at the interface reduce the contact between hydrophobic part and water, and hence, the entropy increases as the disordered structure of water recovers. Therefore the adsorption of amphiphiles is a spontaneous process entropically driven. This process also decreases the interfacial tension owing to the occupation of regions of high free energy, that is, interfacial area. The decrease of the interfacial tension is very dependent on the molecular structure of the amphiphile as the interfacial area is occupied. The interfacial tension decreases rapidly from the value of the clean interface (γ0) as the concentration of amphiphiles in the bulk solution increases until reaching a plateau. This limiting value reflects the saturation of the interface, where the chemical potential of amphiphiles is independent of the concentration. At this concentration value, the amphiphilic molecules can self-assembly into different structures such as micelles, multilayers of another type of aggregates, which are structures entropically favored. The shape of the adsorption isotherm (surface/interfacial tension vs bulk concentration) contains further information on the packing density of the interfacial layer, the thickness, and the intermolecular associations (Hamley, 2007).

Adsorption dynamics of amphiphiles generally involves three adsorption regimes (Maldonado-Valderrama et al., 2005a). The first regime implies the diffusion of amphiphiles from bulk solution onto the subsurface layer. A negligible change of interfacial tension characterizes this regime. The second regime implies the adsorption of amphiphiles onto the interface, and this region is characterized by a rapid and steep change of the interfacial tension. In the final regime, the interfacial tension reaches a stable value, meaning that a pseudo equilibrium is attained between the interfacial layer and the bulk solution. The rate of change of the interfacial tension, the appearance of these regimes, and the final value achieved depend on the nature of the amphiphile as regards its size, hydrophobicity, structure, and shape.

The dilatational rheology of interfacial layers provides further information on the structure and mechanical properties of interfacial films. This technique applies an oscillatory perturbation to the interfacial layer by compressing and expanding the interfacial area and records the response of the interfacial tension to the deformation (Langevin, 2000). Owing to the deformation of the interface, the surface concentration changes, hence affecting the interfacial tension of the film. For soluble surfactants, the Gibbs equation predicts that the surface concentration Γ is associated with the bulk concentration c by Eq. (1.3), where k is the Boltzmann constant, and T is the absolute temperature:

(1.3)

Hence, changes in the interfacial tension are linked to the changes in the surface concentration, and interfacial elasticities and viscosities as magnitudes of excess within the interfacial layer can be defined. By analogy with bulk systems, the compression elasticity can be described as a function of the interfacial area A:

(1.4)

Moreover, for an insoluble monolayer, Eq. 1.4 reads:

(1.5)

The compression viscosity η can be defined as the imaginary part of the complex modulus describing the linear response to a sinusoidal deformation of frequency ω:

(1.6)

where E is the dilatational modulus, E′ is the storage modulus, and E′′ is the loss modulus. Also, ε is the dilatational elasticity, and η is the dilatational viscosity of the interfacial layer. The response of the interfacial layer to the applied deformation is usually studied at different oscillation frequencies. The dilatational response of soluble surfactants depends strongly on the frequency because of the vital coupling of interfacial and bulk events. This question has been studied in detail by Lucassen-Reynders et al. (2010). This model assumes that upon compression of an interfacial layer, some surfactant molecules solubilize into the bulk as a response to the gradient in interfacial tension in order to restore the equilibrium surface concentration. Similarly, upon expansion of the interfacial layer, some surfactant molecules adsorb back onto the interface as a response to the gradient in interfacial tension in order to restore the equilibrium surface concentration. Two extreme cases are depending on the oscillation frequency. On the one hand, at very low oscillation frequencies, the interfacial layer always has time to reach equilibrium, and there is no gradient of interfacial tension and no resistance to the deformation so that ε=η=0. On the other hand, at very high oscillation frequencies, the interfacial layer has no time to adapt to the deformation, and it behaves as an insoluble monolayer [Eq. (1.5)]. The response to the deformation is purely elastic, and η=0. In the intermediate frequencies, the response of the interfacial layer to deformation has both elastic and viscous components [Eq. (1.6)].

1.2.3 Experimental methods to study interfacial properties of amphiphiles

The most common ways of measuring interfacial tension experimentally using commercial equipment are classified according to the measuring method used (Hamley, 2007).

1. Methods based on direct measurement of the force using microbalance: Wilhelmy Plate or Du Noüy Ring. These techniques measure the force required to detach a glass plate or a ring from a liquid using the microbalance. If the liquid wets the ring/plate, the detachment force is proportional to the interfacial tension.

2. Methods based on measurements of the capillary pressure: maximum bubble pressure and growing drop devices. Herein, air (liquid) is flown continuously through a capillary immersed in a liquid (air). The pressure required to form a bubble (drop) is directly proportional to the surface tension of the liquid and inversely proportional to the capillary diameter.

3. Methods based on the analysis of different capillary gravity forces: the capillary rise and drop volume.

a. Capillary rise is based on Jurin’s law; the surface tension of a liquid is related to the rise in a capillary against gravity, the diameter of the capillary, the adhesion of the liquid onto the capillary walls, and the density difference between liquid and air.

b. The drop volume technique is based on the weight of falling drops of a liquid from a capillary. The surface tension of drops at the point of detachment from a capillary is proportional to their weight.

liquid interfaces by immersing the droplet in a liquid media.

1.2.3.1 Pendant drop tensiometer: Dinaten

The pendant drop technique provides an accurate measurement of the interfacial tension of liquid–fluid interfaces. The pendant drop techniques feature many advantages compared with other interfacial tension devices since it requires substantially smaller quantities of material to produce drops and enhanced control of experimental conditions.

The pendant drop technique is based on the analysis of the shape of a pendant drop of liquid suspended from a capillary inside a fluid environment (another liquid or air). The shape of the pendant drop exhibits a curvature that depends on the competition of the gravity and interfacial tension. This curvature can be described theoretically by the Young–Laplace equation of capillarity:

(1.7)

where R1 and R2 are the central radii of curvature, γ is the interfacial tension of the interface, ΔP0 is the reference pressure at z=0 (drop apex), Δρ is the density difference between both fluids, g is the gravity acceleration, and z is the vertical coordinate.

The value of the interfacial tension can be calculated by analyzing the shape and the profile of the pendant drop acquired by a camera. The measurement procedure comprises two stages: (1) extraction of the pendant drop profile from the image and (2) application of an optimization algorithm, which compares the extracted drop profile with the profiles generated by the Young–Laplace equation until the best fit is achieved. When the fitted Young–Laplace solution is found, the liquid–fluid interfacial tension γ as well as the drop volume V and the drop surface area A are then obtained indirectly.

Different procedures were proposed in the literature, which differs on the image processing techniques applied to filter, detect, and extract the drop profiles as well as the optimization algorithm used to obtain the best fit to Young–Laplace solution (Hoorfar and Neumann, 2006; Berry et al., 2015). Among others, axisymmetric drop shape analysis (ADSA) methods are considered to date the most accurate and versatile since it can be applied to different drop disposition (Hoorfar and Neumann, 2006; Cabrerizo Vilchez et al., 1999).

In general, an experimental setup of a pendant drop tensiometer is composed of six components: a lighting subsystem, an image acquisition component, the environmental control setup, the liquid flow control device, the antivibration setup, and, finally, the processing unit. A schematic view is outlined in Fig. 1.2.

Figure 1.2 At the bottom, a schematic view of a pendant drop tensiometer is shown. At the top, a schematic of the improvements—coaxial double capillary and subphase multiexchange device—included in the pendant drop tensiometer.

The lighting subsystem contains a light source and a diffuser, which provides uniform illumination of the drop. The image acquisition component consists of a video camera equipped with an objective mounted on a microscope standby through a custom-made adapter. In our laboratory, the video camera (Pixelink) can capture digitized images with a resolution of 1280×1024 pixels with 256 gray levels. The environmental control setup consists of a glass cuvette (Helma) inserted into a temperature-controlled cell placed on top of a three-axis micropositioner, which allows the handling of the setup in any direction. Then, the pendant drop is formed inside a cuvette. The liquid flow control device is a microinjector or a syringe that pushes the solution or liquid, manually or automatically, at a specified flow rate into a Teflon capillary until the formation of a pendant drop is formed at the tip. Finally, the antivibration setup includes all instrument components in order to avoid vibrations that can affect the measurement.

The processing unit is a computing device that allows the planning, control, and monitoring of the whole experiment with the assistance of a Windows-integrated program (Dinaten) (Laplace solutions that fit better to the experimental drop profiles by using ADSA. As outputs, the drop volume V, the interfacial tension γ, and the interfacial area A are achieved concerning time.

1.2.3.2 Pendant drop film balance for dynamic studies

An essential improvement to the standard pendant drop tensiometer was developed at the beginning of the 20th century to provide experimental studies of penetration, sequential adsorption, multilayers, desorption kinetics, reversibility of adsorption, and enzymatic degradation of interfacial layers (Maldonado-Valderrama et al., 2015). These improvements are the coaxial double capillary and the subphase multiexchange device.

The coaxial double capillary system is a setup where the standard capillary tip has been substituted by an arrangement of two coaxial capillaries so that the outer and inner capillaries are connected independently to each of the channels of a double microinjector. Each microinjector can operate independently, allowing the modification of the interfacial area by changing the drop volume as well as the exchange of the dropped content by through-flow (Cabrerizo Vilchez et al., 1999). Consequently, the subphase exchange of the pendant drop can be carried out by simultaneously extracting liquid from the outer capillary and injecting the new solution at the same flow rate through the inner one. This procedure allows a subphase replacement of the bulk phase with no disturbance of the interface, preserving the drop volume and the surface area throughout the whole subphase exchange. Experimentally, the subphase is completely exchanged when the total volume exchanged is 200% of the original volume (Maldonado-Valderrama et al., 2015). The addition of the coaxial double capillary system and the double microinjector into the pendant drop tensiometer enable a new Langmuir-type film balance, which allows the performance of noninvasive kinetic studies of adsorption of bilayers, desorption of soluble amphiphiles, and penetration into adsorbed films and interfacial reactions (Cabrerizo Vilchez et al., 1999; Maldonado-Valderrama et al., 2003, 2004a, 2007, 2015; Torcello-Gómez et al., 2012).

Furthermore, the addition of a subphase multiexchange device (Fig. 1.2) to the instrumental setup allows extending the penetration and reaction studies to multiple layers. This device provides multiple liquid flow control of the pendant drop, allowing the cleaning of the device and capillaries, the preparation of the pendant drop, the multisubphase exchange, and the control of the drop pressure and interfacial area, among others. In our laboratory, the subphase multiexchange device consists of two independent microinjection systems (PSD/3 syringe pumps; Hamilton Company) with nine vias valves where each of the two syringes are connected to eight independent channels. Again, the regular capillary tip is substituted by the arrangement of two coaxial capillaries but now connected to the two channels of the eight port-valve microinjectors. Each syringe can operate independently and enable the automatic, noninvasive, and complete exchange of the subphase of the drop changing several times the bulk content in each subphase exchange (Maldonado-Valderrama et al., 2015). This implementation allowed further studies of multilayers, desorption, surface interactions, and simulation of the in vitro digestion process at interfaces (Maldonado-Valderrama et al., 2010a, 2014; Santaella et al., 2014; Bellesi et al., 2018; Del Castillo-Santaella et al., 2015, 2016a, 2018, 2019; Maldonado-Valderrama, 2019; Torcello-Gómez et al., 2013, 2014a).

1.2.4 Langmuir film balance

One of the essential methods to characterize two-dimensional states of insoluble amphiphiles is the Langmuir Film balance. This device provides the surface pressure–area isotherms (π–A), where the surface pressure is the reduction in surface tension due to the presence of the monolayer concerning the surface tension of a clean interface γ0:

(1.8)

The surface pressure is usually measured using a Wilhelmy plate dipped into the aqueous subphase. An insoluble substance (lipids, denatured proteins, and particles) is spread from a volatile solvent to ensure a uniform monolayer. The total area of the monolayer can be compressed through a moveable barrier. The shape of the π–A isotherm depends on the lateral interactions between molecules (Fig. 1.3). This fact, in turn, depends on the molecular structure and lateral interactions at the interface. The surface Gibbs elasticity of the monolayer (compressibility modulus) is obtained directly from the π–A isotherms using Eq. (1.9) and coincides with the limiting elasticity at high oscillation frequencies.

(1.9)

Figure 1.3 area isotherms (π–A) of lipids, a schematic diagram of the state of the monolayer as a function of the lateral compression.

The molecules in a monolayer can be arranged in some routes depending on the packing state and the lateral forces between them (Martín-Molina et al., 2019). Lipid monolayers undergo different structural regimes as the laterally compressed and forced to interact. They display the following behavior regimes as the lateral pressure increases: gaseous, liquid expanded, liquid condensed, and collapse (Fig. 1.3). Gaseous display a π close to 0, meaning that molecules practically do not interact. Liquid expanded films provide a moderate increase of the surface pressure, meaning that the molecules begin to interact, and these films show low values of the limiting Gibbs elasticity. In liquid-expanded films, the π–A plot extrapolates at π=0 providing the lift-off area. Liquid condensed films are characterized by a sharp increase of the π as molecules interact further and begin to be close-packed. Liquid condensed films are characterized by significantly higher values of the limiting Gibbs elasticity. The transition between different states can display a coexistence region characterized by a planar region of π between liquid regimes (expanded and condensed). In liquid condensed films, the π–A plot extrapolates to π=0 providing the limiting molecular area. This value is close to the molecular cross-section and provides an estimation of the molecular area of the molecule. On further compression, the film collapses in a three-dimensional state, and we can measure the collapse pressure of the monolayer. These compression regimes occur similarly with other types of amphiphiles, where analysis of the π–A isotherms provides information on the lateral interaction within the interfacial layer (Del Castillo-Santaella et al., 2016b; Maldonado-Valderrama et al., 2005b, 2017; Gálvez-Ruiz, 2017; Rodríguez-Patino et al., 2007; Yang et al., 2020).

1.3 Interfacial properties of food emulsifiers

1.3.1 Interfacial tension and dilatational rheology

1.3.1.1 Adsorption of individual systems: proteins, surfactants, and polysaccharides

The characteristics of adsorbed layers depend fundamentally on the type of surfactant used (Berton-Carabin and Schroën, 2015, 2019; Lam and Nickerson, 2013; Bouyer et al., 2012). Common types of food emulsifiers are proteins, surfactants, biopolymers, and polysaccharides, and recently, particles or microgels should also be considered in food science and technology. All these compounds differ notably in their chemical structure. Therefore the properties of their adsorbed layers are very different.

1.3.1.1.1 Proteins

chemical conformation, the amino acid composition/sequence, the size of the molecule, the presence of charges and structural bonds, and the shape of the molecule. The interfacial properties of food proteins will also be affected by external factors, such as temperature, pH, presence of an electrolyte, and other reactive compounds in food (Rodríguez-Patino and Pilosof, 2011).

Protein adsorption occurs in three steps or adsorption regimes: diffusion, adsorption, and interfacial reorganization. First, the protein diffuses from the bulk solution to the subsurface layer by diffusion and convection. The subsurface is a layer directly adjacent to the fluid interface; in this regime, the interfacial tension barely changes, and it is also known as lag time or induction period. Second, proteins are adsorbed onto the fluid interface, showing the hydrophobic residues to the nonpolar phase; in this regime, the interfacial tension changes very drastically as the interface fills with protein molecules. Finally, the adsorbed protein unfolds and rearranges its segments at the interfacial layer in a final regime characterized by a minor change in interfacial tension. In general, the rate of change of interfacial tension increases as a function of the concentration in bulk solution, and this rate is strongly affected by the pH and the type of protein (Maldonado-Valderrama et al., 2005a).

Globular, compact, and large proteins such as β-lactoglobulin or soy globulins diffuse slower than flexible and small hydrophobic proteins such as β-casein. Changes in pH can also increase the protein diffusion rate onto the interface by promoting the unfolding of the native structure and hydrophobicity. The induction period is only observed at pH close to the isoelectric point and low protein concentrations (Bos and Van Vliet, 2001; Rodríguez-Patino and Pilosof, 2011). Concerning the dilatational elasticity of protein layers, it can be extremely complex, depending on the confirmation of the protein. Proteins form immobile, viscoelastic interfacial films with non-Newtonian behavior (Wilde et al., 2004). The dilatational response of protein layers originates from the formation of loops and tails, interfacial unfolding, gelation, multilayer formation, and other relaxation phenomena (Bos and Van Vliet, 2001; Lucassen-Reynders et al., 2010). Proteins adsorb irreversibly onto fluid interfaces forming cohesive layers depending on the intra- and intermolecular interactions at the interface. In general, globular proteins (soy and β-lactoglobulin) form very cohesive layers reaching high dilatational elastic moduli (80–100 mN/m), while flexible proteins (caseins) form looser interfacial networks with lower dilatational elasticity (10–20 mN/m). Also, the type of oil can importantly affect the adsorption behavior (Santiago et al., 2008; Maldonado-Valderrama et al., 2004b) and the dilatational response by tuning the intermolecular interactions (Maldonado-Valderrama et al., 2010b, 2005c).

1.3.1.1.2 Low-molecular-weight surfactants

Surfactants with low-molecular-weight form soluble and mobile interfacial layers upon adsorption. Surfactants adsorb very quickly onto fluid interfaces and create loose layers owing to the lack of intermolecular bindings and internal structure (Wilde et al., 2004). They make up fluid interfaces with a solid surface lateral diffusion coefficient, hence showing low values of dilatational elasticity at the accessible oscillations frequencies with the pendant drop device (<10 mN/m) (Langevin, 2000). Also, surfactants adsorb reversibly onto nonpolar interfaces and desorb from the interface upon depletion of the bulk solution (Maldonado-Valderrama et al., 2015).

1.3.1.1.3 Polysaccharides

There are various types of polysaccharides used in food emulsions, which display different interfacial activity (Bouyer et al., 2012; Rodríguez-Patino and Pilosof, 2011). Most high-molecular-weight polysaccharides (pectin, xanthan, carrageenan, Arabic gum, and guar gum) have a negligible interfacial activity, which can be altered by the presence of impurities. There are also surface-active polysaccharides such as cellulose derivatives. These adsorb at the interface as the bulk concentration increases and can adopt expanded or condensed structures depending on the type of derivative and backbone composition. Molecular differences between derivatives such as degree of substitution, molecular weight, or molar substitution provide different resulting viscoelasticities of interfacial films. The highest viscoelasticity values have been obtained with more hydrophobic cellulose derivatives (Rodríguez-Patino and Pilosof, 2011). Another type of surface-active polysaccharides is produced by different degrees of esterification of sugar (sucrose and saccharose) derivatives. These adsorb onto fluid interfaces but do not develop strong intermolecular bonding, leading to interfacial films with low dilatational elasticity.

1.3.1.1.4 Poloxamers

Polymeric surfactants such as poloxamers adsorb irreversibly onto fluid interfaces and display a complex dilatational response. Poloxamers can absorb flat at the interface or form loops tails into the subphase. These adsorption regimes are reflected in dilatational moduli, showing maxima and minima as the bulk concentration increases (Pérez-Mosqueda et al., 2013; Torcello-Gómez et al., 2014b).

1.3.1.1.5 Particles

The use of particles as emulsion stabilizers (Pickering emulsions) has increased in recent years (water interfaces and provide substantially thicker films than other emulsifiers, owing to the larger size of particles compared to surfactant or proteins (Berton-Carabin and Schroën, 2019; Dickinson, 2009). Particle layers display very high viscoelasticity and are affected by two types of interactions: interactions between particles and bulk and interactions at the interface.

Furthermore, the presence of lateral attractive capillary forces, owing to the presence of fluid around the particles, also contributes to the high elasticity of the interfacial layer. The particles can be made with different substances. For instance, inorganic particles are based on silica and calcium carbonate. Carbohydrate particles are made from starch, cellulose, cocoa, chitin, and chitosan. Protein particles can be formulated from different protein sources such as dairy or plant proteins, and using several ways, provided they display high hydrophobicity. In general, food-grade particles provide interfacial tension values, which decrease with the bulk concentration and provide final values similar to some proteins that usually stabilize emulsions (Tzoumaki et al., 2011). Protein-based microgel particles are also beginning to be used in food applications (Murray, 2019). The interfacial properties of microgel particles resemble that of protein in many aspects. Microgels expose hydrophobic residues onto nonpolar interfaces and form loops and tails at the interfacial layer. The conformational change undergone by microgel particles at interfaces depends on the balance between the affinity of the microgel particle for the interface and the internal resistance to deformation, which is determined by the cross-linking density, swelling state, and interaction with neighboring microgels in the interface (Maldonado-Valderrama et al., 2017; Yang et al., 2020). There are still very few works on the dilatational response of protein microgels, and this is a promising area of research (Murray, 2019).

1.3.1.2 Mixed emulsifier layers obtained by simultaneous adsorption

The interfacial distribution of emulsifiers at fluid interfaces is importantly influenced by the competitive adsorption of different species, their interfacial interactions, and also their interaction in bulk. The interfacial tension and the dilatational rheology of complex interfaces have practical applications to determine the characteristics of food emulsions and foams. Dilatational rheology implies a change in the interfacial area produced by deformation. This technique is sensitive to the natural softness or hardness of the molecules adsorbed at the interface, intermolecule interactions, and adsorption. It is, therefore, a useful magnitude to characterize intermolecular bonding on mixed adsorbed layers (Bos and Van Vliet, 2001; Langevin, 2000; Maldonado-Valderrama and Rodríguez-Patino, 2010; Murray, 2002; Wilde, 2000). Also, different emulsifiers, such as proteins, surfactants, biopolymers, or particles, display the very different dilatational response, and the interaction between different species can be interpreted by evaluating the dilatational properties of mixed layers.

1.3.1.2.1 Proteins and surfactants

Simultaneous adsorption of proteins and surfactants depends fundamentally on the nature of the surfactant (lipid, polymeric, nonionic, and ionic). The case of mixed layers formed by protein and nonionic linear surfactants is a simple case, which has been thoroughly analyzed in the literature (Wilde et al., 2004). This interaction results in a competitive adsorption phenomenon, which is affected by weak hydrophobic interactions between protein and surfactant that can result in alterations of the protein structure. Hence, at the first instants of the adsorption process, the rate of adsorption of emulsifiers is proportional to their bulk concentration. The more concentrated species adsorbs first, and it can be later displaced. Surfactants displace proteins from the interfacial layer via the orogenic mechanism (Wilde et al., 2004). Highly surface-active surfactants adsorb into defects in the interfacial layer and compress the protein layer. Thus surfactant domains grow and eventually displace the protein into the subphase (Wilde et al., 2004). Consequently, surfactant molecules may dominate the interface above a specific concentration surfactant.

In the case of mixed layers formed by protein and ionic surfactants, the interaction mechanisms are affected mainly by electrostatic interactions. These dominate the formation of complexes until the surfactant ions compensate for the available charges in the protein molecule. These electroneutral complexes can display then higher surface activity compared with the native protein. However, hydrophobic interactions become more critical upon further increase of the surfactant concentration, increasing the hydrophilicity and reducing the interfacial activity of the complex. Again, the competition within the adsorbed layer results in a layer composed of surfactant molecules adsorbed earlier, which cannot be displaced by less surface-active complexes. Finally, above the critical micelle concentration (CMC) of the surfactant, the adsorption layer is mainly formed by surfactant molecules. This process can be followed by the evolution of the dilatational rheology of mixtures of proteins and nonionic surfactants and ionic surfactants as the surfactant concentration in the system is increased (Maldonado-Valderrama and Rodríguez-Patino,