Special issue on The Physics of Food

This review discusses the (soft matter) physics of food. Although food is generally not considered as a typical model system for fundamental (soft matter) physics, a number of basic principles can be found in the interplay between the basic components of foods, water, oil/fat, proteins and carbohydrates. The review starts with the introduction and behavior of food-relevant molecules and discusses food-relevant properties and applications from their fundamental (multiscale) behavior. Typical food aspects from 'hard matter systems', such as chocolates or crystalline fats, to 'soft matter' in emulsions, dough, pasta and meat are covered and can be explained on a molecular basis. An important conclusion is the point that the macroscopic properties and the perception are defined by the molecular interplay on all length and time scales.

In this article, we present food perception across a range of time and length scales as well as across the disciplines of physics, chemistry and biology. We achieve the objective of the article by presenting food from a material science angle as well as presenting the physiology of food perception that enables humans to probe materials in terms of aroma, taste and texture. We highlight that by using simple physical concepts, one can also decipher the mechanisms of transport that link food structure with perception physiology and define the regime in which physiology operates. Most importantly, we emphasise the notion that food/consumer interaction operates across the biological fluid interface grouped under the terminology of mucus, acting as a transfer fluid for taste, aroma and pressure between food and dedicated receptors.

The first simulation study of the crystallisation of a binary mixture of triglycerides using molecular dynamics simulations is reported. Coarse-grained models of tristearin (SSS) and tripalmitin (PPP) molecules have been considered. The models have been preliminarily tested in the crystallisation of pure SSS and PPP systems. Two different quenching procedures have been tested and their performances have been analysed. The structures obtained from the crystallisation procedures show a high orientation order and a high content of molecules in the tuning fork conformation, comparable with the crystalline α phase. The behaviour of melting temperatures for the α phase of the mixture SSS/PPP obtained from the simulations is in qualitative agreement with the behaviour that was experimentally determined.

Chocolate blooming, one of the major problems in the confectionery industry, is the formation of visible white spots or a greyish haze on the surface of chocolate products due to large sugar or fat crystals on the surface. This leads to aesthetic changes and deterioration of taste and thus large sales losses for the confectionery industry due to consumer complaints. Chocolate blooming is often related to migration of lipids or sugar molecules to the chocolate surface, where they recrystallize with an associated polymorphic change of crystal structure on the surface. The wetting behaviour from contact angle measurements gives further insight into surface properties and is needed to determine surface energies and to evaluate possible migration mechanisms and preferred pathways. Therefore, an equilibrium contact angle is needed which is not directly accessible and is influenced by surface texture and interaction between solid and test liquid. In this study, the surface of cocoa butter and conventional chocolates was characterized by measuring the contact angle with the sessile drop protocol. The influence of roughness, test liquid and pre-crystallization of the samples as well as the storage temperature were investigated. In case of no pre-crystallization, a change in surface properties due to storage at 20 °C was detected, whereas samples stored at 30 °C showed the same wetting behaviour as fresh samples. This is associated with polymorphic transformation from thermodynamically less stable crystals to more stable configurations.

From a physico–chemical point of view, most food is soft matter. Usually, these systems are complex in the sense that they combine multiple ingredients with a wide range of structural length scales and dynamics on various time scales. Amongst these systems, foams belong to the well studied but less well understood systems. Particle stabilized foams are very common in food systems. As a model system, we produced aqueous foams from silica nanoparticle dispersions. The silica nanoparticles were hydrophobized by the in situ adsorption of short-chain alkyl amines of chain length C₅ to C₈ to render them surface active. We determined the role of the particles in stabilizing the produced foams. It is shown that the depletion of the bulk silica concentration during the foam formation can be quantified by precise density measurements. In the case of nanoparticle aggregation, more particles are trapped in the foam and will form a network in the foam channels. Diffusing wave spectroscopy was used to study the different time and length scales of the composite system. We find that it is possible to obtain the size of the particles within the foam by two different approaches. Additionally, the dynamics of the foam network is analyzed and it is confirmed that the formation of an aggregated particle network within the foam is responsible for a deceleration of the foam structure evolution.

Biopolymer hydrogel particles formed by electrostatic complexation of proteins and polysaccharides have various applications within the food and other industries, including as delivery systems for bioactive compounds, as texture modifiers, and as fat replacers. The functional attributes of these electrostatic complexes are strongly influenced by their morphology, which is determined by the molecular interactions between the biopolymer molecules. In this study, electrostatic complexes were formed using an amphoteric protein (gelatin) and an anionic polysaccharide (pectin). Gelatin undergoes a helix-to-coil transition when heated above a critical temperature, which impacts its molecular interactions and hydrogel formation. The aim of this research was to study the influence of thermal annealing on the properties of hydrogel particles formed by electrostatic complexation of gelatin and pectin. Hydrogel particles were fabricated by mixing 0.5 wt% gelatin and 0.01 wt% pectin at pH 10 (where both were negatively charged) at various temperatures, followed by acidification to pH 5 (where they have opposite charges) with controlled acidification and stirring. The gelation (${{T}_{\text{g}}}$) and melting temperature (${{T}_{\text{m}}}$) of the electrostatic complexes were measuring using a small amplitude oscillation test: ${{T}_{\text{g}}}=26.3$ °C and ${{T}_{\text{m}}}=32.3$ °C. Three annealing temperatures (5, 30 and 50 °C) corresponding to different regimes ($T<{{T}_{\text{g}}}$, ${{T}_{\text{g}}}<T<{{T}_{\text{m}}}$, and $T>{{T}_{\text{m}}}$) were selected to control the configuration of the gelatin chain. The effects of formation temperature, annealing temperature, and incubation time on the morphology of the hydrogel particles were characterized by turbidity, static light scattering, and microscopy. The results of this study will facilitate the rational design of hydrogel particles with specific particle dimensions and morphologies, which has important implications for tailoring their functionality for various applications.

Colloid stability is of high importance in a multitude of fields ranging from food science to biotechnology. There is strong interest in studying the stability of small particles (of a size of a few nanometres) with complex surface structures, that make them resemble the complexity of proteins and other natural biomolecules, in the presence of oppositely charged nanoparticles. While for nanoparticles with homogeneously charged surfaces an abrupt precipitation has been observed at the neutrality of charges, data are missing about the stability of nanoparticles when they have more complex surface structures, like the presence of hydrophobic patches. To study the role of these hydrophobic patches in the stability of nanoparticles a series of negatively charged nanoparticles has been synthesized with different ratios of hydrophobic content and with control on the structural distribution of the hydrophobic moiety, and then titrated with positively charged nanoparticles. For nanoparticles with patchy nanodomains, the influence of hydrophobic content was observed together with the influence of the size of the nanoparticles. By contrast, for nanoparticles with a uniform distribution of hydrophobic ligands, size changes and hydrophobic content did not play any role in co-precipitation behaviour. A comparison of these two sets of nanoparticles suggests that nanodomains present at the surfaces of nanoparticles are playing an important role in stability against co-precipitation.

Understanding how solid fats structures come about in edible oils and quantifying their structures is of fundamental importance in developing edible oils with pre-selected characteristics. We considered the great range of fractal dimensions, from 1.91 to 2.90, reported from rheological measurements. We point out that, if the structures arise via DLA/RLA or DLCA/RLCA, as has been established using ultra small angle x-ray scattering (USAXS), we would expect fractal dimensions in the range ~1.7 to 2.1, and ~2.5 or ~3.0. We present new data for commercial fats and show that the fractal dimensions deduced lie outside these values. We have developed a model in which competition between two processes can lead to the range of fractal dimensions observed. The two processes are (i) the rate at which the solid fat particles are created as the temperature is decreased, and (ii) the rate at which these particles diffuse, thereby meeting and forming aggregates. We assumed that aggregation can take place essentially isotropically and we identified two characteristic times: a time characterizing the rate of creation of solid fats, ${{\tau}_{\text{create}}}(T)\equiv 1/{{R}_{S}}(T)$, where ${{R}_{S}}(T)$ is the rate of solid condensation (cm³ s⁻¹), and the diffusion time of solid fats, ${{\tau}_{\text{diff}}}\left(T,{{c}_{S}}\right)=\langle {{r}^{2}}\rangle /6\mathbb{D}\left(T,{{c}_{S}}\right)$, where $\mathbb{D}\left(T,{{c}_{S}}\right)$ is their diffusion coefficient and $\langle {{r}^{2}}\rangle$ is the typical average distance that fats must move in order to aggregate. The intent of this model is to show that a simple process can lead to a wide range of fractal dimensions. We showed that in the limit of very fast solid creation, ${{\tau}_{\text{create}}}\ll {{\tau}_{\text{diff}}}$ the fractal dimension is predicted to be that of DLCA, ~1.7, relaxing to that of RLCA, 2.0–2.1, and that in the limit of very slow solid creation, ${{\tau}_{\text{create}}}\gg {{\tau}_{\text{diff}}}$, the fractal dimension is predicted to be that obtained via DLA, ~2.5, relaxing to that of RLA, 3.0. We predict that, given a system which satisfies our model assumptions and which can either be cooled rapidly or cooled slowly to yield fractal dimensions ${{D}_{\text{rapid}}}$ and ${{D}_{\text{slow}}}~$ then ${{D}_{\text{rapid}}}\leqslant {{D}_{\text{slow}}}$. This is supported by both rheological [1] and USAXS measurements [2, 3] even though the latter models do not conform to the assumptions of those presented here.

Lecithin is one of the most used food additives in a wide varying range of food products. It has great beneficial effects on rheological properties and prolongs the shelf-life of foods. Lecithins have a varying molecular composition due to environmental influences. To characterize the molecular components of lecithin, the interactions between different phospholipid head groups and a sucrose crystal surface were examined by performing non-equilibrium molecular dynamics simulations on the example of chocolate conching. Pulling simulations were used to detach six different lecithin molecules from a sucrose crystal. Forced detachment allows characterization of the strength of the interaction between the sucrose crystal and individual phospholipids, as it models the reversed process of lecithin adsorption during chocolate conching. The required work for the detachment of 1,2-dilinoleoyl-phosphatidylcholine (DLPC), 1-palmitoyl-2-linoleoyl-phosphatidylcholine (PLPC), 1,2-dilinoleoyl-phosphatidylethanolamine (DLPE), 1-palmitoyl-2-linoleoyl-phosphatidylethanolamine (PLPE), 1,2-dilinoleoyl-phosphatidylinositol (DLPI), and 1-palmitoyl-2-linoleoyl-phosphatidylinositol (PLPI) from the (1 0 0) sucrose crystal surface was calculated. Molecules with the phophatidylinositol head group (DLPI, PLPI) were shown to require the biggest detachment work, followed by phospatidylcholine molecules (DLPC, PLPC) with medium detachment work and phosphatidylethanolamine molecules (DLPE, PLPE) with the lowest detachment work. The different aliphatic chains seem to have no impact on head group detachment. Furthermore, the influence of different adsorption states of DLPC and PLPE molecules were examined. It was shown that the required detachment work correlates with the number of hydrogen bonds between the phospholipids and the sucrose.

We study the dimerisation of beta-lactoglobulin (type A) at concentrations between 10 mg ml⁻¹ and 200 mg ml⁻¹ at fixed pH 3 and an ionic strength of 1.2 M and show that the degree of dimerisation can be determined from the coherent change in the ATR FTIR spectrum due to changes in folding induced by the dimerisation. This allows us to determine the IR spectrum of monomeric BLG and the dimerisation constant for which we find a value of K =1.84 (±0.5) · 10² M⁻¹. Furthermore, we show that including self-crowding effects at high concentrations accounts for the concentration dependence of the apparent dimerisation constant.

The food industry has discovered that oleosomes are beneficial as carriers of bioactive ingredients. Oleosomes are subcellular oil droplets typically found in plant seeds. Within seeds, they exist as pre-emulsified oil high in unsaturated fatty acids, stabilised by a monolayer of phospholipids and proteins, called oleosins. Oleosins are anchored into the oil core with a hydrophobic domain, while the hydrophilic domains remain on the oleosome surface. To preserve the nutritional value of the oil and the function of oleosomes, microencapsulation by means of spray drying is a promising technique. For the microencapsulation of oleosomes, maltodextrin was used. To achieve a high oil encapsulation efficiency, optimal process parameters needed to be established. In order to better understand the mechanisms of drying behind powder formation and the associated powder properties, the findings obtained using different microscopic and spectroscopic measurements were correlated with each other. By doing this, it was found that spray drying of pure oleosome emulsions resulted in excessive component segregation and thus in a poor encapsulation efficiency. With the addition of maltodextrin, the oil encapsulation efficiency was significantly improved.

Astringency is one of the predominant factors in the sensory experience of many foods and beverages ranging from wine to nuts. The scientific community is discussing mechanisms that explain this complex phenomenon, since there are no conclusive results which correlate well with sensory astringency. Therefore, the mechanisms and perceptual characteristics of astringency warrant further discussion and investigation. This paper gives a brief introduction of the fundamentals of oral tribology forming a basis of the astringency mechanism. It discusses the current state of the literature on mechanisms underlying astringency describing the existing astringency models. The review discusses the crucial role of saliva and its physiology which contributes significantly in astringency perception in the mouth. It also provides an overview of research concerned with the physiological and psychophysical factors that mediate the perception of this sensation, establishing the ground for future research. Thus, the overall aim of the review is to establish the critical roles of oral friction (thin-film lubrication) in the sensation of astringency and possibly of some other specific sensory features.

In this article, we present food perception across a range of time and length scales as well as across the disciplines of physics, chemistry and biology. We achieve the objective of the article by presenting food from a material science angle as well as presenting the physiology of food perception that enables humans to probe materials in terms of aroma, taste and texture. We highlight that by using simple physical concepts, one can also decipher the mechanisms of transport that link food structure with perception physiology and define the regime in which physiology operates. Most importantly, we emphasise the notion that food/consumer interaction operates across the biological fluid interface grouped under the terminology of mucus, acting as a transfer fluid for taste, aroma and pressure between food and dedicated receptors.