Review
Edible microgel as a texture modifier
2021 年 27 巻 5 号 p. 687-693
詳細
2021 年 27 巻 5 号 p. 687-693
Microgels are defined as discrete gel particles with sizes ranging from several micrometers to tens of micrometers. Although they are recognized as an impurity that appears during polymer synthesis, microgels have attracted attention mainly as a rheology control agent for paints. However, they are also expected to be used as a texture modifier in the field of food science. Specifically, they can be used as a fat analogue in processed foods. In this review, based on their classification, research examples of the different methods used for producing microgels are given. Moreover, as an example, a study on agar microgels obtained by applying an emulsifying system to low-oil mayonnaise is described in detail.
The proper definition of a gel is difficult to agree upon. However, in a narrow sense it can be defined as a cross-linked polymer containing a solvent in its network structure and exhibiting elasticity as a macroscopic property. Gels can be prepared in a variety of ways. As a typical method, polyacrylamide gel, which is often used in protein analysis, will be described as an example. A randomly cross-linked gel is generated when a small amount of methylene-bis-acrylamide is added to acrylamide as a cross-linking agent and thermal radical polymerization is started using an appropriate polymerization initiator. This gel has the same shape as the polymerization vessel. That is, the monomer solution loses its fluidity and becomes a gel when the monomer is stretched around the polymerization vessel in a three-dimensional network structure. Polyacrylamide is a so-called chemical gel in which polymer chains are cross-linked by covalent bonds; however, the same applies to physical gels formed by physical interactions.
Microgels are defined as discrete gel particles with a size ranging from several micrometers to tens of micrometers and are well-known in the field of polymer science (Saunders and Vincent, 1999). Interesting features of microgels include 1) a large specific surface area, 2) a solvent swelling and deswelling ability, and 3) rheological properties of dispersion. Regarding 1), it is conceivable to apply the substance to an adsorption carrier by utilizing its vast surface. Furthermore, if the swelling / deswelling ability can be controlled, application to material recovery systems can be considered. As an example, a study was conducted on the swelling behavior of a cross-linked polymethacrylic acid gel whose surface is modified with polyethylene oxide by an organic solvent (Kaneda and Vincent, 2004). This idea can be applied to the recovery of harmful oil-soluble components in the environment. The rheological characteristics of microgel dispersions are the main theme of this review. Such characteristics are of great interest and various microgel applications are extending into different industries. Here, the rheological characteristics of microgel dispersions will be described first.
Examples of industrial applications of microgels include its use as a rheology control agent for paints (Raquois et al., 1995), cosmetics, and personal care products (Kaneda, 2017). These industrial products require a high degree of rheology characteristics control. An example of a microgel used as a rheology control agent is Carbomer or Carbopol (Ketz et al., 1988; Lochhard, 1993; Gutowski et al., 2012), which cross-links slightly with polyacrylic acid and is an extremely popular material as a rheology control agent for personal care products and cosmetics (Kaneda, 2017).
One of the characteristics required when using a microgel as a rheology control agent is its flow characteristics. This is the so-called shear-thinning behavior. In general, concentrated colloidal dispersions show shear thinning, and many studies have been conducted on ideal model systems using hard particles, and many empirical formulas describing their fluid characteristics have been reported (Larson, 1999). However, the rheology of dispersions of soft gel particles still has many unsolved problems. In other words, without a systematic understanding, it is necessary to research individual cases. As such, many studies have been reported, including on the flow behavior of microgel suspensions (Shewan and Stokes, 2015), the effects of the deformability of microgel particles (Frith and Lips, 1995; Berli and Quemada, 2000), yielding behaviors (Kaneda and Sogabe, 2005; Kaneda, 2006), the effect of microgel size (Hahn et al., 2015), shear thickening and jamming (Frith et al., 1996), and micro rheology by dynamic light scattering (Basu et al., 2014). However, display of such flow characteristics peculiar to each system indicates the potential to express unique rheological characteristics, i.e., textures, through their utilization.
When considering microgels used in the field of food science, it is essential that they must be edible. That is, polysaccharides and proteins with gelling ability will be targeted. Specifically, this includes agar and carrageenan, which are a type of low-set gel, and egg white, which is a high-set gel. Specific methods must be employed to obtain microgel particles of a few micrometers from these ingredients. In general, there are two methods for preparing gel particles, i.e., top-down and bottom-up methods. The former is a method of refining the bulk gel, and the latter is a method of reducing the size at the stage in which the gel network is formed. Concrete research examples are introduced in the following section.
As previously mentioned, it is necessary to control the size of the gel network in the preparation of microgels. Assuming that the microgels are applied to industrial products, they must be manufactured with good reproducibility. In this section, we first describe microgels produced by applying mechanical force. Specifically, we will explain microgels prepared by shearing, crushing and atomization methods.
A fluid gel is a gel particle obtained by forming a gel while applying shear deformation. The concept of a fluid gel was proposed by Norton (Norton et al., 1999; Norton et al., 2000; Norton et al., 2006) and is a technique for obtaining gel particles by gelling an aqueous solution of a polysaccharide with gelling ability while applying a shearing force. The size of the gel particles obtained using this method depends on the concentration of the gelling agent, solvent conditions and shear load conditions. It has been reported that gel particles having a size of several micrometers to several tens of micrometers can be obtained by gelling an agar gel while applying shear under various conditions. The shape of these gel particles is irregular according to optical microscopy observations, and there is a large particle size distribution. This method can be applied to systems using agar (Norton et al., 1999) and carrageenan (Garrec et al., 2013), which are low-set gels. Moreover, it can be applied to whey proteins that form high-set gels (Moakes et al., 2015). Detailed studies have been conducted on the growth of gel networks under shear using gelatin, and the results showed that the size of microgels formed under shear depends strongly on the shear conditions (Djaboirov et al., 2000).
We would like to consider the reason why microgels of a certain size can be obtained under shear deformation. There is a lower critical value for the concentration of polymer chains (e.g., polysaccharides) constituting the gel network, which is called the critical gelation concentration. This concentration is considered to be approximately the same as the overlapping concentration of the polymer. However, under shearing, it is considered that a small polymer network is a discrete structure, not a network that runs through the system. Tokita (1989) investigated in detail the dynamics of milk gel formation by acid aggregation and found that the gelation process can be explained based on percolation theory. That is, if the polymer chains collide with each other at a certain frequency, a small polymer network is formed even if the polymer concentration is lower than the critical gelation concentration. It is thought that this small polymer network becomes the core of the microgel and reacts with another unreacted polymer under shear to grow into a microgel of a certain size.
We can also obtain microgels by crushing the bulk gel. Kaneda and Yanaki (2002) succeeded in obtaining a paste-like microgel dispersion with good reproducibility by shearing a bulk gel prepared from agar with low molecular weight and low gel strength using a homo-mixer. The microgel particles obtained using this method are also approximately several tens of micrometers, and their irregular shape is similar to the above-mentioned fluid gel. It has been systematically determined that the flow characteristics of this dispersion depend on the volume fraction of microgel particles and the agar concentration (hardness of the bulk gel). Such basic data are useful when exploring how much viscosity and yield stress are required in the final product.
An interesting study on the formation of microgels using atomization has been reported (Perrechil et al., 2011). In this method, microgel particles are obtained by spraying a hot aqueous solution of carrageenan with an atomizer and recovering the atomized carrageenan solution in a KCl solution. It has been reported that the microgel particles obtained using this method change from granular to spheroid because the hot aqueous carrageenan solution sprayed becomes spherical by surface tension. However, its size is still on the order of tens of micrometers.
A series of studies have been reported on the preparation of microgels by squeezing an aqueous sodium alginate solution, which is well known to gel with calcium ions, into a calcium chloride solution from an injection needle (Bokkhim et al., 2016). The gel particles obtained using this method are on the order of millimeters in size and are described as "gel beads". However, it is believed that fine microgel particles can be formed by properly devising the gelation mechanism of alginate.
A shearing method applied to microgel formation has an advantage in that it can be prepared through a relatively simple process; however, the size of microgel particles ranges from approximately several tens of micrometers to several hundreds of micrometers, and the shape is irregular. If we can obtain a finer and well-shaped microgel, it may be a good material for basic scientific research on the rheology of deformable particle dispersions. Moreover, it can be applied as a rheology control agent for cosmetics, personal care products, and foods. This section describes microgels formed using emulsions.
Unilever's research group succeeded in obtaining a microemulsion by dispersing and emulsifying an aqueous agar solution in oil to obtain a W/O emulsion and cooling it (Adams et al., 2004). Using this method, completely spherical microgel particles can be obtained; the size of the particles is several micrometers to several tens of micrometers, which is smaller than that of the shearing method. When considering food applications, the relationship between the concentration (volume fraction) of microgel particles and the flow characteristics is important from the viewpoint of texture adjustment. To study the flow behavior of the spherical microgel particles, they separated the microgels from the oil phase and re-dispersed the microgels in water after the washing procedure, and they then conducted systematic rheological studies using the microgels.
The important point is that the microgel particles are deformable fine particles. A systematic experiment was conducted using deformable microgels (synthesized acrylic polymer microgels) of different hardness to characterize their rheological behavior using theoretical and empirical equations (Fridrikh et al., 1996). It was clarified that the steady flow viscosity of the harder microgel dispersion was higher than that of the softer ones. Moreover, it was also shown that the elastic modulus measured in the linear region of the closed packed deformable microgel dispersion reflects the elastic modulus of each microgel particle.
At the same time, we established a method for synthesizing a polyacrylamide-based microgel using a W/O microemulsion polymerization system to produce a spherical microgel from a water-soluble monomer (Kaneda et al., 2004). Unlike physical gels made of polysaccharides such as agar, microgels synthesized by this method can consist of extremely “soft” microgel particles made of acrylamide-based copolymers. We can adjust the flexibility by changing the co-monomer ratio and the hardness by changing cross-link densities that make up the gel network. The microgel synthesized by this method is utilized as a rheology modifier for cosmetics (Kaneda, 2017).
Subsequently, we developed a method for producing microgels with high reproducibility by a W/O emulsification system in which an edible gelling agent such as agar was applied (Suzawa and Kaneda, 2010), and systematically studied its rheological properties. Although the Unilever group studied the aqueous dispersion of a microgel, which was separated from the oil phase, we attempted to study the oil dispersion in an actual application in the food industry.
Although analysis of the flow characteristics of the microgel dispersion is an extremely important issue, there are some challenges. It is technically difficult to measure the steady flow viscosity of a heterogeneous system such as a microgel dispersion with good reproducibility. Moreover, although there are many empirical formulas for analyzing the non-linear flow curve of such a heterogeneous system, the physical meaning of the characteristic parameters obtained is unknown.
On the other hand, since the origin of the mechanical response under small deformation is predicted to be due to the molecular-level structure in the system, it is possible to consider the structural property correlation from such results. It is well known that careful rheological studies in such linear regions have made a significant contribution to the polymer physics field.
Rheological characteristics analysis in the linear region is specifically used to analyze the frequency dependence of the dynamic modulus, that is, the mechanical spectrum. In general, however, it is difficult to analyze the mechanical spectrum of a concentrated colloidal system using a general linear viscoelastic theory. We therefore focused on the weak-gel model, which is one of the theoretical models that can quantitatively evaluate the rheological properties of concentrated colloidal systems.
The concept of the weak-gel model was described by Bohlin (1980), and has been modified (Gabliele et al., 2001). When considering a sample that can be regarded as a concentrated colloidal system such as food, it is thought that these colloidal particles are cooperatively rearranged when the sample is deformed. Specifically, the complex elastic modulus and frequency can be described using a single equation.
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where, G * is the complex elastic modulus, ω is the angular frequency, and Af and z are the fitting parameters, which are gel strength and coordination number, respectively. The coordination number (z) is a particularly important parameter. It is assumed that the target sample is composed of colloid-sized particles, and these particles are considered to be cooperatively rearranged when the sample is deformed. Since the macroscopic mechanical response of the entire system appears as a result of the mechanical interactions between each particle, the coordination number is the quantification of the cooperativeness. Intuitively, the higher this value, the higher the spatial density of the particles. We investigated the changes in the parameters when the volume fraction and hardness of the microgel particles were systematically changed using the agar microgel oil dispersion described above. As a result, we found that the coordination number (z) shows good correlation with the volume fractions of microgels. It was also clarified that the gel strength (Af), which corresponds to the value of the complex elastic modulus at ω = 1 rad/s, depends on the hardness of agar microgel particles (Kaneda, 2018). These results indicate that this theoretical model can be applied to the rheological characterization of microgel dispersions.
This model has been used in recent years to quantify the rheological properties of various foods including apple puree (Roversi and Piazza, 2016), cheese (Meza et al., 2012), cake batters (Meza et al., 2011), and tofu gel (Nicole et al., 2016).
It is thought that microgel food applications can be developed by exploiting the unique rheological characteristics of a material. One possibility is to use a microgel as a substitute raw material for fats and oils in concentrated O/W emulsions. Mayonnaise is a good research subject as an incredible ultra-high internal phase ratio O/W emulsion with an internal oil phase of 65% or more. In such an emulsion, the internal oil phase can no longer take a spherical shape, but is deformed into a polygon and densely packed structure. Elasticity is developed in the system by the close contact of the interfaces and it depends on the volume fraction of the inner phase as described by the following empirical equation (Princen and Kiss, 1986).
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Where, G is the shear modulus, Γ is the interface tension, a is the Sauter mean droplet radius, φ is the volume fraction of the droplet, and φ0 is the critical volume fraction at which G collapses to zero. That is, a sufficient oil phase above φ0 is required to develop the unique texture of mayonnaise.
By contrast, in recent years, health hazards owing to excessive calorie intake have been remarkable, and research on reducing calories in foods has been active. For mayonnaise, reducing the amount of fat and oil will reduce the total calories; however, at the same time, the consistency will also be drastically affected. We studied standard mayonnaise and low calorie mayonnaise (products with calories reduced by 75%) on the market in terms of their rheological characteristics (Kaneda and Takahashi, 2011). No significant differences were observed in the steady flow viscosities between these samples; however, a large difference was observed in the analysis results of the viscosity growth behavior. This means that the mechanical response at the beginning of deformation and before reaching a steady flow is significantly different. Specifically, stress overshoot was observed in the standard mayonnaise, while it was not observed in the low calorie mayonnaise. The cause of the overshoot observed in the standard mayonnaise was thought to be jamming of densely packed oil droplets. That is, it has been suggested that the steady flow viscosity measured as a quality control parameter at food manufacturing sites lacks a strong correlation with texture. Therefore, other parameters are needed to consider the texture of foods. Observation of the viscosity growth behavior described above is one of the useful methods for this purpose.
A study attempted to utilize the crushed type microgel mentioned earlier as a fat analogue (Li et al., 2014); however, we studied the effect of spherical microgel particles as a fat analogue in mayonnaise (Kaneda and Shibata, 2020). The details of the study are described as follows.
The overall design of this study is shown in Fig. 1. Full-mayonnaise (F-mayo) is a full-fat mayonnaise and half-mayonnaise (H-mayo) contains only half the amount of oil. A 50% agar microgel oil dispersion was prepared using a W/O emulsification system, and a model mayonnaise (A-mayo) was prepared using this as an oil phase. The final oil content of A-mayo was reduced by half. X-mayo (xanthan added mayonnaise) is made by dispersing xanthan in the continuous phase of H-mayo.
Schematic illustration of the model mayonnaise preparation procedures. (reproduced from Kaneda and Shibata 2020 with permission from the Society of Rheology, Japan)
The dynamic modulus for these samples was measured in the linear region. The measurement results were analyzed and evaluated using a weak-gel model.
Fig. 2 shows the results of the frequency dependence of the complex modulus of these samples. The complex modulus of H-mayo, which was simply halved in terms of the oil content, was dramatically lower than that of F-mayo; however, the value of A-mayo was restored to almost the same level as that of F-mayo. By contrast, the value of X-mayo was lower than that of F-mayo. As an aside, the steady flow viscosity of both A-mayo and X-mayo showed almost the same apparent viscosity as F-mayo. As is clear from Fig. 2, all samples behave according to the power law; therefore, these data were analyzed using the weak-gel model (Eq.1). Here, the result of the coordination number z, which is one of the weak-gel model parameters, is specifically described.
Frequency dependence of the complex modulus for the model mayonnaise. The circles, squares, inversed triangles, and triangles denote F-mayo, A-mayo, X-mayo and X-mayo, respectively. (reproduced from Kaneda and Shibata 2020 with permission from the Society of Rheology, Japan)
Fig. 3 shows the value of the coordination number z for each sample. Looking at F-mayo as a reference, the z value of H-mayo, which contains half the oil content, drops to exactly half that of F-mayo. A-mayo recovers to almost the same level as F-mayo; however, X-mayo, which showed almost the same steady flow viscosity as F-mayo, remains at the same level as H-mayo. This result indicates that xanthan gum, which is a linear polymer dispersed in an aqueous phase, does not form a structure capable of generating internal stress under shear deformation. This means that there are fewer mechanical interactions with oil droplets than in A-mayo. By contrast, it was found that the microgel particles generate internal stress owing to close contact with the oil droplets and cause mechanical interactions between the particles. These results suggest that the microgel particles can be used as a fat analogue material for emulsion-type foods
The coordination number (z) obtained by analysis with weak-gel model (Eq.1) for the mayonnaise model. (reproduced from Kaneda and Shibata 2020 with permission from the Society of Rheology, Japan)
Since microgels derived from polysaccharides can be regarded as having almost zero calories, their application as a substitute ingredient for fats and oils is highly anticipated. As an example, the formulation in oil-reduced mayonnaise was described in detail; however, it is thought that microgels can be widely used as a fat analogue in other products (butter, margarine, chocolate, etc…).
On the other hand, its use as a starch analogue is also of interest. For example, custard and potage soup can be considered to exhibit characteristic rheological properties owing to gelatinized starch granules, which can be considered as a microgel. When replacing starch with microgels made of polysaccharides for the purpose of lowering the glycemic index, the texture can be adjusted by preparing and blending a microgel of the desired size and hardness as described in this review.
As another option, microgels made of protein have recently been attracting attention as a fat substitute material (Moakes et al., 2015). Elderly people tend to have lower protein intake, indicating the need to develop foods that allow them to consume sufficient protein without resistance. The application of protein microgels to foods could be one solution to this problem.
To widely utilize edible microgels industrially, it is essential to establish a technology for producing microgels at low cost and with good reproducibility. Further research is expected to be conducted in the future.
Acknowledgements I would like to thank Editage (www.editage.com) for English language editing.
Conflict of interest There are no conflicts of interest to declare.
