Food physics insight: the structural design of foods

Abstract

Understanding food materials from the classical realm of physics including soft condensed matter physics has been an area of interest especially in the structural design engineering of food products. The contents of this review would help the reader in understanding the thermodynamics of food polymer, structural design principles, structural hierarchy, steps involved in food structuring, newer structural design technologies, and structure measurement techniques. Understanding the concepts of free volume would help the food engineers and technologists to study the food structural changes, manipulate process parameters and, the optimum amount of nutraceuticals/ingredients to be loaded in the food matrix. Such understanding helps in reducing food ingredient wastage while designing a food product.

Introduction

The term “food physics” might appear an interesting combination of two immiscible entities that are flocked together, to anybody who is unused to this field. Understanding food physics requires the understanding of food and physics individually. A food material is referred to as any substance obtained from plant or animal origin that provides nutrients to the body to support life. However, physics is a science that is related to the properties of matter and the energies related to them. We observe food physics in our routine life, but without indeed knowing it. A couple of physicists and food technologists have been able to say or highlight this abnormal relationship (Vilgis and Limbach, 2016). Food physics does not include the concept of chemical response to elucidate the food system, whereas, it fairly considers that the atoms/molecules stay unaltered but act differently under different conditions. It can be examined amid the foam aeration, material flow, during estimation of water or moisture content, and in the melting and evaporation phenomenon. Unit operations involved in food processing or manufacturing such as size reduction, extraction, leaching, cooking, etc., can be explained by understanding and applying the physics fundamentals (such as energy exchanges and heat and mass transfers). In addition, upon food consumption, destruction and unjamming of the food into pieces by the application of forces encompass the fundamentals/basics of applied mechanics (Chen, 2009).

Physics administers the food system, where its components such as carbohydrates, fats, and proteins along with the water, provide a structure to the food materials. It helps in understanding the food framework at the sub-atomic level and helps in foreseeing the relationship between the texture, structure, and flavor of the food product. Mechanism supporting the food breakdown during the digestion process and then release of the compounds/nutrients from the food matrix, their bio-accessibility, and bioavailability at a particular site of the gastro-digestive system have facilitated food research including formulation planning, designing, and creation of novel food products (Ubbink and Kruger, 2006). All such aggregated information shows the significance of the basics of physical science and its usage in food structuring.

Nonetheless, a food material has been regarded as a soft condensed matter by a few physicists (Mezzenga et al., 2005), where the particles and atoms interact to frame denser solids and fluids because their attractive energies are stronger than the thermal energy. In these matters, the mesoscale assemblies (air/gas bubbles, amphiphiles, colloidal particles, and polymers) are in the dispersed phase and the complex fluids act as a continuous phase. Properties of (consumable/edible) soft matters (SMs) at mesoscale mostly don't rely upon the chemical properties of the compounds. In any case, their properties vary with length scale and are influenced by externally applied fields such as mechanical forces or stresses, which gives them a soft delicate appearance.

Development of a food product at a large scale involves a series of experiments carried out at laboratory and pilot scales by manipulating various independent and dependent parameters to get optimized food products. Numerous works of literature are available on the development of food gels and biodegradable films, enrichment, and fortification of food products (Thakur et al., 2017; Chong et al., 2018; Kojic et al., 2019; Singh and Banerjee, 2013). Most of these processes are optimized using mathematical tools including RSM (response surface methodology), ANN (artificial neural network regression), and GP (Gaussian process) and optimized values fall within the predetermined boundary conditions which many a time fails to explain the variability among the responses. Further, these tools fail to quantify the changes that would occur at the molecular levels. In many food product manufacturing processes, the required nutrient/biomolecule/plasticizer are added into the food matrix without even knowing the amount of the free volume available in their matrix. This may lead the food products with the uneven (poor/high) loading of the molecules per unit area of the food matrix. To overcome these limitations, knowledge of free volume and the use of some sophisticated software such as molecular dynamics simulation would help in scanning the process at its molecular level and can predict the various desired responses at the length and time scale.

Thus, understanding food structural designing (FSD) necessitates intensive information on phase change, equilibrium, interior properties of the material, available free volume, and changes taking place in the food framework or food structure concerning the changes in the environment such as temperature and moisture content. This review will highlight the fundamentals of food physics based on thermodynamics and the steps involved in the food structural designing process including 3D and 4D printing technologies for novel food structure, and the techniques to examine the structural morphology of the food matrix.

Food as a soft condensed matter

A food material is perceived principally as a three-dimensional matrix possessing both continuous and dispersed phases. The simplest form of food material is biological tissues. Tissues contain cells and these cells have cytoplasm as a continuous phase where polymers (proteins, fats, and carbohydrates), and salts act as a dispersed phase. Soft condensed matter as food is classified based on a dispersed component which is either gas (G), liquid (L), or solid (S) (Fig. 1). These soft matters are very sensitive towards the external thermal fluctuations, stresses, and magnetic and electric fields (van der Sman, 2012a). The physics behind soft matters helps food scientists to know the key structural elements for designing new food materials. Soft matter occurs in between the two extremes: fluid and solid states and possess high viscosity compared to simple liquids and accompany viscoelastic properties. Organic molecules with the macro-and supramolecular entities act as the building block of these matters and are bound by weak interaction forces (Kleman and Laverntovich, 2007).

Fig. 1.

Soft matter classification of dispersions and their examples. Adapted and

Modified from van der Sman and van der Goot (2009)

Manufacturing of the food products undergoes a series of unit operations, which requires a series of energy exchanges. The understanding of the various forces involved in every single step involves the considerate knowledge of the basic concepts of thermodynamics concerning the food polymers.

Thermodynamics of food polymers

Fundamentals of thermodynamics are widely used in the study of synthetic polymers for many years. Since food is also a biopolymer (polymer from biological origin), these theories can also be applied in understanding food polymers. Thermodynamics deals with the study of the exchange of energy between the system and its surroundings, and among the components of the food system. When the food is processed, there are series of energy exchanges causing the thermal effects, for example, heating, cooling, initiation, and activation of any reaction. Many phenomena concerned with the food materials such as their processing, storage, degradation, loading, and unloading of the nutrients can be explained by the help of thermodynamics including chemical kinetics. Furthermore, the interactions among the molecules can be easily manipulated, controlled, and predicted if once its molecular makeup is known (Aguilera and Stanley, 1999). Thermodynamics provides insight and related useful information regarding in which direction a system would initiate and what would be the influence of the processing variables (such as temperature, pH, solute: solvent concentration) on the system.

Gibbs free energy (G) is an important parameter in thermodynamics, required for understanding the phases occurring at the equilibrium (Seiffert, 2020). It is expressed as:

$$\mathrm{\Delta }{G}_{\text{mix}}=\mathrm{\Delta }{H}_{\text{mix}}-T\mathrm{\Delta }{S}_{\text{mix}}$$ 1

where, ΔG, ΔH, ΔS, T, and mix stand, respectively, for Gibbs free energy, enthalpy, entropy, temperature, and mixing. For an ideal solution where the molecular interaction is the same and so volume change after mixing and ΔH_mix are zero and ΔG_mix depends only on the entropic term. However, for the regular solution, ΔH_mix becomes infinite and form the following equation:

$$\frac{\mathrm{\Delta }{G}_{\text{mix}}}{N}=\mathrm{\Delta }{H}_{\text{mix}}+RT\left(x_{1}ln{x}_1+x_{2}ln{x}_2\right)$$ 2

where, x represents the molar fraction, 1 is solvent, 2 is solute, N represents the total number of moles, R is gas constant, T represents the absolute temperature, and the combined term “RT” is thermal energy related to the intermolecular bonding.

Food thermodynamics can be understood by Flory Huggins’s theory (FHT). FHT is a lattice model based on the dissimilarities among the molecules based on their size. Each lattice site is either occupied by the solvent molecule or by the polymer (Aguilera and Stanley, 1999). FHT assumes that there is no change in volume upon mixing the polymer and solvent. Such a basis can be employed in understanding the phase behaviours of the polymer mixture (van der Sman and Meinders, 2011). It deals with the free energy of the polymer solution as:

$$\mathrm{\Delta }{G}_{\text{mix}}=RT\left(x_{12}{\mathrm{\varnothing }}_1{\mathrm{\varnothing }}_2+{\mathrm{\varnothing }}_{1}ln{\mathrm{\varnothing }}_1+\frac{{\mathrm{\varnothing }}_2}{x}ln{\mathrm{\varnothing }}_2\right)$$ 3

where, Φ represents the volume fraction of the solvent (1) and solute (2). Both the Eqs. (2 and 3) have the same terms, the only difference is the molar fraction in Eq. 2 has been replaced with the volume fraction in Eq. 3. The enthalpy term in Eq. 3 is an interaction parameter between solvent (1) and solute (2) and is termed as Flory Huggins Interaction parameter (FHIP) as:

$$x_{12}=\frac{\mathrm{\Delta }{H}_{\text{mix}}}{RTN{\mathrm{\varnothing }}_2}$$ 4

This represents, the ratio of the energy involved in interaction and that of thermal energy. The positive and negative values of $x_{12}$ represent repulsion and attraction among the molecules, respectively (Marsac et al., 2006). Here, the repulsion can be regarded as the immiscibility of two molecules and attraction as their miscibility (Aguilera and Stanley, 1999). Thus, depending upon the nature of the interaction, the phase behaviors can be predicted by FHT. The repulsive and attractive interactions are, respectively, represented by upper critical solution temperature (UCST) and lower critical solution temperature (LCST). Complete miscibility may occur among the polymer (solute) and solvent below LCST and above UCST. These interactions influence the morphological and structural properties and, thus, the food product makeup. For instance, Marsac et al., (2006) developed the thermodynamic approach to better understand the factors to determine the polymer's ability to stabilize the pharmaceutical ingredients. The crystallization of the drugs in the polymer system led to reduced apparent solubility, decreased bioavailability, and also compromise the rate of dissolution. To get the solution to this problem, they estimated the FHIP among the polymer and the drug. Using this, the miscibility was predicted for all the compositions and the composition with the best thermodynamic stability was considered. However, reports on the application of such concept in the development of functional foods or any other fortified /nutrient blended food products are meagre.

Few papers have employed the understanding of these phenomena for the development of food products. For instance, van der Sman (2007) used the soft matter fundamentals during the cooking of meat to understand the heat and mass transfer phenomenon by employing the Flory Rehner theory (extension of FHT). The theory was also used to understand the mass transfer phenomenon in the swelling and shrinkage behaviors or structural integrity of the gels and the water holding capacity of the various food products such as meats, jams, jellies, and vegetables (van der Sman, 2007, 2012b, 2015; van der Sman and Meinders, 2013). With this understanding, one can easily predict the amount of water or other solvents that can be held by the food material, such as gels, without compromising the food quality and structural integrity.

For the development of the new food product, it is also important to understand the basic mechanism and the steps involved in its design (discussed briefly in the following sections).

Structural designing of the food products

Food structure is a three-dimensional network that not only supports its components but also holds its weight when the external force is applied to it. The structuring of the food items is like the architecture of the building structures. Food materials employ its components such as fat, protein, and carbohydrates as the construction material, and some prefabricated materials such as hydrocolloids and lecithin for designing and fabrication of the food products. Similarities between designing of buildings and food products are shown in Fig. 2.

Fig. 2.

Similarities between the designing of buildings and food structures

Food microstructure relies on three characteristic features: (a) presence of a distinct domain, (b) organization of the spaces between these distinct domains, and (c) presence of some type of interaction among them. A simple example of this can be a hydrogel, where the network of polymer is arranged in such a way that it forms a crosslinked structure, and the spaces or voids are filled by the water molecules. Food structure derives the sensory properties of the food product and is often used as the delivery vehicle for the transport of the drug or nutraceutical to the specific site when the product is ingested (Ubbink and Kruger, 2006).

The novel food system can be developed by using the knowledge and understanding of the traditional concepts of processing and fundamental knowledge of physics. All the soft matter is thermodynamically unstable and can thus lose its structure when exposed to the external environment or with the time scale. Its structure must be stabilized by modifying its surface properties to bring it to its jammed state. Therefore, food structuring design is becoming an important research direction, because of its applications in ensuring bioavailability of the functional molecules present in the foods, and in the sustainable use of raw agricultural materials which provide building blocks for food structure.

Assembling of the food structure

Depending upon the basis of the electron arrangement, the food molecules can be divided into polar and non-polar molecules. The presence of charge on the molecules determines their adhesiveness. For the adhesiveness or stickiness of the molecules, there must be an attractive force between them. Otherwise, no attraction would result in impossible stabilization and structuring of the food products. Forces that connect the molecules or assemble the food ingredients are known as molecular glues. These molecular glues result from the types of forces among the molecules such as hydrophobic, van der Waals, electrostatic, and hydrogen bonding (McClements et al., 2009). These forces help in binding the molecules together. For example, in the case of yogurt products, there is a need to control the occurring electrostatic forces among the protein molecules. Casein (milk protein) has a negative charge and, thus, causes repulsion among its molecules and destabilization of the structure. The addition of enzyme to the milk converts the lactose (milk sugar) into lactic acid and the casein molecules losses their charge, thus facilitating the linkage among its molecules via van der Waals and hydrophobic forces. Thus, it forms the three-dimensional network of the coagulated proteins, gives a silky and delicate yogurt texture.

Innovative techniques such as; 3D and 4D printing for assembling food structures are discussed in the separate sub-section.

Role of plasticizer in structural designing of food

A plasticizer can be regarded as any substance which when incorporated in a material influences its flexibility, workability, or dispensability (Vieira et al., 2011). For a plasticizer to have excellent compatibility, it should have a low degree of branching as branching deteriorates its compatibility with the polymer due to steric restriction effects. Plasticizer position itself between the polymers chains where it affects the interaction among the polymers, thereby, increases molecular mobility, flexibility, and free volume (Vieira et al., 2011).

Water acts as an excellent plasticizer due to its unique structure and can be easily removed by dehydration processes. Water is a common constituent present in all food products and biopolymers and plays an important role in a phase transition, physical, thermal, electrical, and some other related properties. The presence of water affects the food texture (Wollny and Peleg, 1994), functioning of nutrients (Lawal, 2004), the activity of enzymes (van Erp et al., 1991), and other properties such as food stability, quality, and safety (Hayta and Aday, 2015). The amount of water present in food products varies over a large range. Depending upon the processing conditions, water may be present in any of its physical states, which means that water can undergo phase transitions among its phases and can also affect the phase transition of other components. The presence of water in the product influences the protein denaturation (Vanzi et al., 1998), starch gelatinization (Slade & Levine, 1993), and also the state transitions in amorphous food components (Matveev et al., 2000). Freezing of water involves the formation of ice resulting from the crystallization of pure water present in the food product and is important in determining the process efficiency and quality of the food products (Petzold and Aguilera, 2009). The degree of ice formation in food products depends upon the freezing temperature. Amorphous food components are miscible in water and thus the transition temperature decreases with an increase in the water content (Lourdin et al., 1997). Therefore, the variation in water content and temperature may lead to changes in the physical state of the food products such as stickiness or crystallization affecting the food structure.

Role of the molecular size in structural designing of food

A food product is made up of atoms and molecules characterized by their molecular size on a length scale. Food structural elements cover a large scale range from nano- to milli-meters. Prominent nano-scaled entities are liposomes, micelles, ternary structures of polymer complexes, and entanglement in the gel network. Micro scaled entities (micro length scale) are oil or water droplets in milk, sugar, or ice crystals in the milk (Michel & Sagalowicz, 2008). These structures are not only different in their size but also possess different thermodynamic properties. The functionality and properties of food products are closely related to their length scales. Figure 3 shows the characteristic length scale in food with some examples. At the lower extreme, the food products are macroscopic, while at the upper end, a food product is composed of small molecules and atoms, characterized by their length scale at a molecular level. The macroscopic levels are developed during the processing of the food ingredients.

Fig. 3.

The characteristic length scale of food items with their respective examples. Adapted and

modified from Ubbink et al., (2008)

Based on their characteristic length scale, the biopolymer interactions can be divided into four structural levels: super macromolecular, macromolecular, sub-macromolecular, and macroscopic levels. In other words, we can say that there are four levels for the conformational potential (CP) of food macromolecules and out of these, three are at microscopic levels and one is at the macroscopic level of food structure. CP means the ability of the biopolymers to form an intermolecular junction zone (a region in inter-biopolymer complexes, where, segments of the macromolecules are joined together). It gives the required properties to the food systems such as rheological, structural, and other physicochemical properties. The size of the junction zone, strength, and nature of the inter-macromolecular forces are important aspects of the structural hierarchy of food products.

The submacromolecular level of structure formation depends upon the monomer size. This includes the interaction between the functional groups present, for example, the formation of disulfide, hydrogen, and hydrophobic bonds of inter-and intramolecular types. This level includes both the enzymatic and chemical modification of biopolymers, posing changes in structure–property, such as Maillard reactions (Tolstoguzov, 1996). The macromolecular level of structure formation is related to the size of macromolecules and involves conformational changes, variations in size and shape of individual macromolecules, association, dissociation, and aggregation of the macromolecules. However, the supermacromolecular level relates to the colloidal dimensions. It involves interaction among the macromolecular aggregates and the development of three-dimensional networks (gels). The last level macroscopic relates to the development of various macroscopic structures by involving macromolecules such as lipids, which form a separate system phase. However, the supermacromolecular, macromolecular, and submolecular food structural elements are developed by the non-specific interaction of the biopolymers and take place simultaneously, leading to form the non-equilibrium structure of the food systems. All four levels of structure formation are closely related and have a great influence on the properties of food products (Tolstoguzov, 1996).

Incompatibility among the food polymers

While developing a stable food product, there are various challenges faced by a researcher. A food product is a complex material made up of either protein, carbohydrates, fat, or water. Any dissimilarity in these components can make the product unstable. Besides these components, the other processing factors such as pH and salt concentration strongly affect their interaction, for example in the case of food gels and emulsion (Dickinson, 2003). A poor interaction leads to incompatibility among the polymers and thus disturbs the structure of the product. The strength of compatibility and incompatibility is dependent upon the extent of their attractive and repulsive forces. Thus, there comes a need to have an understanding of the interaction parameter. It can be made understood theoretically by examining the behaviors of Flory Huggins’s interaction parameters ($x$). A positive value of $x$ represents the repulsion and thus immiscibility among the polymers and a negative value represents attraction and miscibility. Similarly, a positive value of ΔG_mix means the polymers are not thermodynamically compatible and try to form a single-phase mixed solution (Higgins et al., 2010). This means that the molecule of each polymer prefers to be surrounded by its other molecules. This situation brings thermodynamical instability or incompatibility. This situation calls for the application of the free volume (FV) concept which are based on the interaction among the particles. The repulsive forces among the particles lead to the formation of the free volume. Any desired new particle or molecules can be inserted into these free volumes (van der Sman, 2012a). There is another type of volume known as excluded volume (EV) around every particle where no new particle or molecule can be incorporated. These volumes are shown in Fig. 4.

Fig. 4.

Free volume (FV) and excluded volume (EV) around hard spheres. Adapted and

modified from van der Sman, (2012a)

Free volume (FV)

Every polymer possesses a free volume; however, its extent depends upon the size of its molecules. FV results due to the uneven or non-uniform packing of the molecules, instability of their densities, and topographical hindrances among the molecules. FV can be determined experimentally using confocal microscopy and theoretically by understanding the Widom insertion (WI) method (Dullens et al., 2006; Sastry et al., 1998). FV is also driven by the temperature and moisture content of the polymer. For example, at the glass transition temperature (GTT), the FV is mainly zero. This means that the FV is minimum at the higher densities and low moisture, and the phenomenon is called as anti-plasticization. Similarly, when the water concentration is high, FV becomes large.

Excluded volume (EV)

Excluded volume results due to the steric exclusion effects. When the food components can occupy a large volume in the system, then they try to exclude the other components by occupying the same volume. It increases with the increase in the effective volume of the molecules/particles and with an increase in their molar concentration (McClements et al., 2009). The high molecular weight of the molecules, their unfolding, and poor interaction lead to a decrease in compatibility and EV development. EV results in the unjamming/destabilization of the polymeric suspension. Also, an increase in EV is inversely proportional to FV (Schellman, 2003). This understanding can help in food structural designing by knowing the amount of space occupied by the biopolymer solution.

Process to determine the free volume (FV)

Flory Huggins theory can be used to determine the free volume by relating it with glass transition temperature and water activities. Glass transition temperature defines the molecular mobility within the food matrix. At the glassy state/jammed state, molecular mobility is at its lowest or ceased, whereas, at the rubbery state mobility is at its highest. Both states are related to the water activities of the food matrix. An extended form of the Flory Huggins theory (Vrentas and Vrentas, 1994) consider two main parameters namely, $x$ and, T_g which can be measured independently at water activity.

The Flory Huggins theory states that (van der Sman and Meinders, 2011),

$$\frac{ln{a}_w}{\mathit{RT}}=ln\left(1-\mathrm{\varnothing }\right)+\left(1-\frac{1}{N}\right)\mathrm{\varnothing }+x{\mathrm{\varnothing }}^2$$ 5

$$\text{Or}\frac{ln{a}_w}{\mathit{RT}}=N\left[ln\frac{\mathrm{\varnothing }}{N}-\left(1-\frac{1}{N}\right)\left(1-\mathrm{\varnothing }\right)+x{\left(1-\mathrm{\varnothing }\right)}^2\right]$$ 6

where, $\mathrm{\varnothing }$ represents the volume fraction of the dissolved polymer, T is absolute temperature, N = v_s/v_w i.e., the ratio of molar volume of solute and water, respectively, and R is gas constant.

The Flory Huggins interaction parameter, ($x$), can be calculated experimentally from the temperature coefficient of volume fraction (Singh and Sharma, 2014) as

$$x=\left[\mathrm{\varnothing }{\left(1-\mathrm{\varnothing }\right)}^{-1}+Nln\left(1-\mathrm{\varnothing }\right)+N\mathrm{\varnothing }\right]{[2\mathrm{\varnothing }-{\mathrm{\varnothing }}^{2}N-{\mathrm{\varnothing }}^2{T}^{-1}{\left(\frac{d\mathrm{\varnothing }}{\mathit{dT}}\right)}^{-1}]}^{-1}$$ 7

where, $\left(\frac{d\mathrm{\varnothing }}{\mathit{dT}}\right)$ is the value of slope obtained from the plot of volume fraction against temperature.

Here, the volume fraction of the polymer $\mathrm{\varnothing }$ represents the polymer fraction in the rubbery state and can be expressed as

$$\mathrm{\varnothing }=\frac{\left(1-\xi \right)y_s/{\rho }_s}{\frac{y_w-y_{w,x}}{{\rho }_w}+\left(1-\xi \right)y_s/{\rho }_s}$$ 8

where, ξ is the degree of crystallinity, ρ_w and ρ_s are densities of water and polymer, respectively, y_s = 1-y_w, y_s, and y_w represents the mass fractions of polymer and water, respectively and y_w,x is the amount of structural water and can be calculated as

$$y_{w,x}=\frac{1}{3}\frac{M_w}{M_u}\xi y_s$$ 9

where M_w and M_u represent the molar weight of water and anhydrous glucose, respectively. The reason for incorporating factor 1/3 is that it represents the ratio of structural water molecules to the glucose molecules in one lattice unit cell.

However, the water activity following the Flory Huggins free volume theory (van der Sman and Meinders, 2010) is

$$\frac{ln{a}_w}{\mathit{RT}}=ln\left(1-\mathrm{\varnothing }\right)+\left(1-\frac{1}{N}\right)\mathrm{\varnothing }+x{\mathrm{\varnothing }}^2+F\left(\mathrm{\varnothing }\right)$$ 10

The above equation is similar to Eq. 5 with the additional free volume term i.e., $F\left(\mathrm{\varnothing }\right)$.

where,

$$F\left(\mathrm{\varnothing }\right)=0ifT\ge T_g$$ 11

$$F\left(\mathrm{\varnothing }\right)=M_w{y}_w^2\frac{\mathrm{\Delta }{C}_{p,w}}{\mathit{RT}}\frac{d{T}_g}{d{y}_s}\frac{T-T_g}{T_g}ifT<T_g$$ 12

where, ΔC_p,w is a change in the specific heat capacity at glass transition of pure water, and is a well-known quantity in the literature. The term T_g is moisture content dependent glass temperature of the water/polymer mixture and can be obtained via Gordon Taylor theory or Couchman and Karasz theory:

Couchman-Karasz theory as

$$T_g=\frac{y_w\mathrm{\Delta }{C}_{p,w}{T}_{g,w}+y_s\mathrm{\Delta }{C}_{p,s}{T}_{g,s}}{y_w\mathrm{\Delta }{C}_{p,w}+y_s\mathrm{\Delta }{C}_{p,s}}$$ 13

where, T_g,s, and T_g,w are the glass temperatures of the dry polymer and pure water, and the term ΔC_p,s represents the difference in the specific heat across the glass transition.

Gordon Taylor equation:

$$T_g=\frac{y_w{T}_{g,w}+k{y}_s{T}_{g,s}}{y_w+k{y}_s}$$ 14

Thus, by substituting the above values, the free volume term can be calculated from Eq. 10.

Structural design principles

In the food domain, there are a variety of components, some are hydrophilic, and some are lipophilic which are needed to be delivered in the edible form (McClements et al., 2009). These compounds are mostly added and delivered in the aqueous system to increase their stability, bioactivity, and palatability. However, the addition of these compounds possesses lots of technical challenges because the food polymer shows a variety of the characteristics such as charge, molar mass, hydrophobicity, and flexibility of chains. The biopolymer can undergo a transition from one conformation to another or one aggregation state to another, by altering the processing environment such as temperature, pH, and ionic strength. Understanding the molecular forces, interaction among the polymers, their conformations, and the mechanism behind the assembly of the components in the food system plays an important role (McClements, 2009; McClements et al., 2009). Following are some design principles that form the novel food structures:

Phase separation

When two dissimilar materials are mixed, then they can either be completely miscible and form a single-phase system or can separate to form the different phases. This phenomenon in the system is driven by the type and strength of the interaction among the molecules. separation of the phases can occur due to the strong thermodynamic unfavorable interaction, for example, electrostatic repulsion and steric exclusion, among the different polymers (McClements, 2009; McClements et al., 2009).

Self-assembly

Structural elements or mesoscopic structures are sometimes bonded by sufficiently poor attractive forces. This causes alterations in their positions and the elements thus try to reorient to find the favourable self-assembled configurations. Self-assembly of the polymers in a system can occur when there is a balance between the repulsive and attractive forces (van der Sman, 2012a). The self-assembled structures are thermodynamically stable and can become unstable if their molecules or components degrade or else react with the external environment (Michel and Sagalowicz, 2008).

The self-assembly of the molecules can be of two types: spontaneous self-assembly and directed self-assembly. In the former, the food components assemble or align themselves in such a way that it forms a well-defined structure to minimize the free energy of the system. This can occur under the appropriate environmental conditions. This type of self-assembly includes microemulsion, vesicles, and micelles. In the latter type, self-assembly does not occur spontaneously, however, it requires controlled environmental conditions (such as temperature, ionic strength, time, and pH), to form a metastable structure (van der Sman, 2012a).

Food structural designing (FSD) process

The food structural designing process involves the use of the external fields to form new structures by the transitions and stabilization. This process demands a careful balance among various forces. It can be divided into three steps: (a) destruction of the native structure of food materials, (b) formation of new food structure, and (c) arresting or jamming the formed food structure (de Kruif et al., 1995; van der Sman and van der Goot, 2009). The transformation involved in the FSD can be understood by employing the following steps:

Destabilization of the native phase and formation of new phases

During the process of food structuring, the native or old food structure is often destroyed or destructed by applying external forces such as temperature, pH, enzymes, and incorporation of external crosslinkers. Destabilization of the food structure is a common practice in dairy food plants, where the stability of milk colloids is destroyed to obtain yogurt, ice cream, or cheese. Food colloids possess a short range of interactions, and they can be made stabilized by incorporating interactions among the molecules (van der Sman and van der Goot, 2009). The food colloids can aggregate to form new phases after the destabilization by phase transitions or aggregations. Aggregation, depending on the strength of interaction potential (IP), can be of two types: flocculation and coagulation. If the IP is more than the thermal fluctuation, then it causes coagulation, and if less then leads to flocculation. Sometimes, the new phases are also developed by biochemical means, for instance, the carbon dioxide is a product of the fermentation by the yeast which results in the formation of bubbles in the beverages, and bakery products. Such information could help in the formation or development of new phases.

Stabilization and jamming of the newly formed food structure

These newly formed structures are required to be preserved until they are consumed. It can be done by converting the food system to its jammed state. Jammed/arrested states are often known as glassy states (or gel states). In this state, an immobilized percolating structure is formed by the mesoscopic structures (van der Sman, 2012a). When the food materials are in the arrested form, they are considered shelf-stable. Though, they are vulnerable to slow deterioration upon storage and are referred to as slow dynamics (Cipelletti and Ramos, 2005). The system is said to be in slow dynamics when the thermal fluctuations are very small, and the dispersed elements present are unable to escape the cage formed by the surrounding dispersed elements. The unjamming of the structure can be achieved by either increasing the temperature above its glass transition temperature, by manipulating the volume fraction by diluting or by concentrating the solute, or by applying any mechanical stress. This shows that the food system is created and stabilized in the manufacturing and designing process.

These steps can be easily understood by taking an example of the formation of a hydrogel. Figure 5 shows the hydrogel as a nutraceutical delivery system and its destruction upon consumption or as a response to the stimuli. In the first step, the starch and water were mixed and upon heating the starch gelatinizes to form a gel, where the drug or nutraceutical can be loaded depending upon its mesh size and equilibrium swelling. This hydrogel when comes in the presence of some stimuli, destructs to yield the loaded nutraceutical/drug.

Fig. 5.

Food structuring process: Basic steps involved in food structuring of the starch hydrogel as a nutraceutical delivery system

Recent technologies involved in food structural designing

The recent development in the structural designing of the food products include 3D and 4D printing technologies. These technologies are known as additive manufacturing relies upon the computer-aided design (CAD) and produces limitless shapes and structures (Sanei and Popescu, 2020). It stacks the printing inks (flowable food ingredients) layer over the layer to create a product as per the preset graphics (Chen et al., 2022). However, the next generation of printing techniques (4D printing) allows the use of smart or stimuli responsive material for printing various food structures that could alter their attributes (color, shape, texture, flavor, and nutrition content) in response to the specific stimuli (heat, light, pH, water, etc.) over time scale (Zhao et al., 2021). Another advantage of 4D printing is its compatibility with the concept of “flat packaging”, where the deformed food products can attain their shapes upon the response to the stimuli. This can also reduce the storage space, packaging, and transportation space (Teng et al., 2021).

There are three main paradigms on which 4D relies: (a) Self actuation, which is associated with the changes in the size of the objects or materials, such as thermal expansion; (b) Self-assembly, which is the basis of 4D printing and it relates to the adjustment in the smart structure itself upon stimulation, for instance, self-assembly of the small micro sized smart particles that could change their pattern upon stimulation; and (c) Self-supporting structure of the printed polymeric material that could support the growth of organic cells and tissues.

Both the above techniques allow the benefits of tailoring the food by adding specific ingredients and functional components or by eliminating certain ingredients (Zhao et al., 2021). However, these techniques are limited to the printing of low viscosity food materials that can be ejected smoothly and easily at a specific rheological parameter such that the product would maintain its shape after its ejection.

Structure measurement techniques

Various dynamics involved in food structural development during the production process (Aguilera, 2006; Aguilera et al., 2000), structural variations while storage and breakdown upon consumption (Lucas et al., 2002) and digestion process (Armand et al., 1996), are not well understood till date. Many methodologies have been developed to understand the structural changes occurring over time scale. Such techniques are the electron, optical and atomic force microscopy that enables one to achieve an accurate image of the food product.

Both direct and indirect approaches are available to measure the structure of food materials. The examples of the former are tomography, scanning electron microscopy (SEM), optical (OM), and confocal microscopy (CFM) while the latter is spectroscopy or mechanical response tomographical techniques such as magnetic resonance imaging (MRI) and X-ray tomography which allows full 3D reconstruction of sample structure (Ubbink et al., 2008). These techniques are limited in resolution and slow in acquisition time. Another disadvantage of X-ray tomography is that the image contrast depends highly on electron density difference. Optical microscopy also possesses this limitation of the resolution, however, the structure of the order of 1 μm can be imaged provided food sample to be transparent. This technique can even be used under highly controlled conditions i.e. when the special sample stages are used. For example, changes occur when the sample is cooled, or they are prone to mechanical deformation on the application of tensile stresses. The ultrastructure of food can be studied by electron microscopy particularly transmission electron microscopy. Electron microscopic technique provides microscopic images with a high resolution (mainly by SEM microscopy). Environmental scanning electron microscopy enables the sample analysis at a required relative humidity. Atomic force microscopy allows mapping of the three-dimensional surface structure with high resolution and determines the elastic properties and interaction forces of the surfaces. It has been widely used to image the structure and the functional properties of biopolymers (Gunning et al., 2004) and biological systems. A new technique, positron annihilation lifetime spectroscopy (PALS) has been recently introduced. This technique is used to study the molecular structure and packing of crystalline and amorphous carbohydrate matrices (Kilburn et al., 2004). PALS does not provide topographical information directly, but it gives quantitative detail about the molecular holes and voids present between molecules in the dense matrix. These structural techniques help in better understanding the transitional and structural changes occurring in the food system.

Conclusion

Foods are considered biopolymers, therefore, theories including principles of thermodynamics used in studying synthetic polymers may be used in understanding food polymers. Food structuring is a complex problem that can be well understood from the concept of soft matter physics. Structural designing of the food material has become essential in the development of novel food products based on consumer preferences, the bioavailability of the functional compounds, and the nutritional requirement. Application of thermodynamics involving Flory Huggins theory (and its extensions) helps in the quantification of free and expanded volumes of the food matrix for food formulation containing nutraceuticals. Advances in food structuring using computer added design such as 3D and 4D printing has helped in designing various shapes and smart food products. Further, molecular dynamic simulation (MDS) could further open new untravelled path of food physics where one can visualize the effect of the chemical bonding, their energies to predict the engineering properties of food materials, and in fabricating the novel food products.

Abbreviations

EV

Excluded volume

FHT

Flory Huggins theory

FHIP

Flory Huggins interaction parameter

FSD

Food structural designing

FV

Free volume

LCST

Lower critical solution temperature

UCST

Upper critical solution temperature

Author contributions

Manab Bandhu Bera designed the contents of article and Palak Mahajan performed the literature search and article writing. All authors drafted and/or critically revised the work.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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Conflict of interest

All the authors that they have no conflict of interest.

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We declared that we followed the ethical rules and good scientific practices as mentioned in Journal of Food Science and Technology Author Guidelines.

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