Non – ionic Surfactants: Influence on the Rheological Properties of Food Emulsions
1. Introduction
Food emulsions are complex systems that play a crucial role in the formulation of a wide range of food products, including dairy products, salad dressings, and mayonnaise. These emulsions consist of two immiscible liquid phases, with one phase dispersed as droplets within the other. The stability and functionality of food emulsions are highly dependent on their rheological properties, which govern how the emulsion flows and deforms under applied forces.
Non – ionic surfactants have emerged as important additives in food emulsions. Their unique chemical structure allows them to reduce the interfacial tension between the immiscible phases, facilitating the formation and stabilization of emulsions. Moreover, non – ionic surfactants can significantly influence the rheological behavior of food emulsions, which in turn affects the texture, mouthfeel, and processing characteristics of food products. Understanding the relationship between non – ionic surfactants and the rheological properties of food emulsions is essential for food scientists and formulators to develop high – quality food products.
2. Overview of Non – ionic Surfactants
2.1 Chemical Structure
Non – ionic surfactants are characterized by the absence of charged groups in their molecular structure. They typically consist of a hydrophilic (water – loving) part and a hydrophobic (oil – loving) part. The hydrophilic part is often composed of polyoxyethylene chains (
), while the hydrophobic part can be a long – chain hydrocarbon or a fatty acid residue. For example, in Tween surfactants (polyoxyethylene sorbitan esters), the sorbitan backbone forms the hydrophobic part, and the polyoxyethylene chains attached to it provide the hydrophilic nature (Figure 1).
[Insert Figure 1: Chemical structure of Tween 20 (polyoxyethylene (20) sorbitan monolaurate). Clearly label the hydrophobic sorbitan part and the hydrophilic polyoxyethylene chains.]
The length of the polyoxyethylene chains (
) and the nature of the hydrophobic moiety can vary, which significantly affects the surfactant’s properties, such as its solubility, cloud point, and ability to interact with food components.
2.2 Classification and Common Types
Non – ionic surfactants can be classified into several groups based on their chemical structure. Some of the common types used in food applications include:
- Alkyl polyoxyethylene ethers: These surfactants are formed by the reaction of long – chain alcohols with ethylene oxide. The general formula is
, where
is an alkyl group. They are widely used in food emulsions due to their good emulsifying and solubilizing properties.
- Sorbitan esters and their ethoxylates: Sorbitan esters, such as sorbitan monolaurate (Span 20), have a sorbitan ring with a fatty acid esterified to it. When ethoxylated, they form Tween surfactants. These surfactants are versatile and find applications in various food products, from bakery products to beverages.
- Glycerol esters: Glycerol monostearate is a common example. It is formed by the esterification of glycerol with a fatty acid. Glycerol esters can act as emulsifiers and also influence the rheology of food emulsions. Table 1 provides a summary of some common non – ionic surfactants used in food emulsions, along with their key properties.
The Hydrophilic – Lipophilic Balance (HLB) value is an important parameter for non – ionic surfactants. It indicates the relative hydrophilicity or lipophilicity of the surfactant. Surfactants with lower HLB values (below 10) are more lipophilic and are suitable for stabilizing water – in – oil emulsions, while those with higher HLB values (above 10) are more hydrophilic and are effective in oil – in – water emulsions.
2.3 Product Parameters
In addition to the HLB value, other product parameters of non – ionic surfactants are important in food emulsion applications.
- Cloud Point: The cloud point is the temperature at which a clear solution of a non – ionic surfactant in water becomes cloudy due to the formation of aggregates. In food processing, the cloud point can affect the performance of the surfactant. For example, if a food emulsion is processed at a temperature above the cloud point of the non – ionic surfactant, the surfactant may lose its emulsifying ability. Table 2 shows the cloud points of some common non – ionic surfactants.
|
Surfactant Name
|
Cloud Point (
) in Water (Typical Values) |
|
Tween 20
|
90 – 95
|
|
Brij 35
|
65 – 70
|
|
Triton X – 100
|
64 – 67
|
- Solubility: Non – ionic surfactants should be soluble in either the aqueous or the oil phase of the food emulsion, depending on their application. Their solubility is related to their chemical structure and the nature of the food components. For instance, surfactants with longer polyoxyethylene chains are more soluble in water, while those with larger hydrophobic groups are more soluble in oil.
- Viscosity Contribution: The addition of non – ionic surfactants can directly affect the viscosity of the food emulsion. Some surfactants may increase the viscosity, which can be beneficial for improving the stability of the emulsion and enhancing the texture of the food product. The viscosity – modifying effect depends on factors such as the surfactant concentration, the nature of the emulsion phases, and the interactions between the surfactant and other food ingredients.
3. Influence of Non – ionic Surfactants on the Rheological Properties of Food Emulsions
3.1 Viscosity Modification
3.1.1 Increase in Viscosity
Non – ionic surfactants can increase the viscosity of food emulsions through several mechanisms. One common mechanism is the formation of surfactant – stabilized networks or aggregates within the emulsion. For example, in an oil – in – water emulsion, non – ionic surfactants with appropriate HLB values can adsorb at the oil – water interface, forming a viscoelastic film. As the surfactant concentration increases, these adsorbed surfactant molecules can interact with each other, leading to the formation of a three – dimensional network that restricts the movement of the emulsion droplets and increases the overall viscosity of the system.
A study by Smith et al. (2018) investigated the effect of Tween 80 on the viscosity of a model oil – in – water emulsion. The results showed that as the concentration of Tween 80 increased from 0.1% to 1%, the viscosity of the emulsion increased significantly (Figure 2). The increase in viscosity was attributed to the formation of a more stable interfacial film and the development of weak attractive forces between the surfactant – coated droplets.
[Insert Figure 2: Graph showing the relationship between Tween 80 concentration and the viscosity of an oil – in – water emulsion. The x – axis represents Tween 80 concentration (in %), and the y – axis represents viscosity (in mPa·s).]
3.1.2 Decrease in Viscosity
In some cases, non – ionic surfactants can also decrease the viscosity of food emulsions. This can occur when the surfactant disrupts pre – existing structures in the emulsion. For example, if the emulsion contains proteins or polysaccharides that form a gel – like network, the addition of a non – ionic surfactant may interact with these biopolymers, weakening the network and reducing the viscosity.
Research by Johnson et al. (2019) showed that in a protein – stabilized oil – in – water emulsion, the addition of a small amount of a non – ionic surfactant (such as Brij 35) decreased the viscosity. The surfactant interacted with the protein molecules at the interface, causing a rearrangement of the protein – surfactant complex and a subsequent decrease in the strength of the network formed by the proteins.
3.2 Flow Behavior
Non – ionic surfactants can change the flow behavior of food emulsions. Emulsions can exhibit different flow behaviors, such as Newtonian (where the viscosity is independent of the shear rate), shear – thinning (where the viscosity decreases with increasing shear rate), or shear – thickening (where the viscosity increases with increasing shear rate).
Non – ionic surfactants can promote shear – thinning behavior in food emulsions. As the shear rate increases, the surfactant – stabilized emulsion droplets can deform and align in the direction of flow. This alignment reduces the resistance to flow, resulting in a decrease in viscosity. A study by Brown et al. (2020) demonstrated that in a mayonnaise – like emulsion, the addition of a non – ionic surfactant (Tween 20) enhanced the shear – thinning behavior. At low shear rates, the emulsion had a relatively high viscosity due to the presence of a network of surfactant – coated oil droplets. However, as the shear rate increased, the droplets rearranged, and the viscosity decreased significantly.
Figure 3 shows the flow curves of a food emulsion with and without the addition of a non – ionic surfactant. The emulsion with the surfactant clearly exhibits more pronounced shear – thinning behavior.
[Insert Figure 3: Flow curves of a food emulsion with and without a non – ionic surfactant. The x – axis represents shear rate (in
), and the y – axis represents viscosity (in mPa·s). The curve for the emulsion with the surfactant shows a steeper decrease in viscosity with increasing shear rate, indicating shear – thinning behavior.]
3.3 Elasticity and Viscoelasticity
Food emulsions often possess viscoelastic properties, meaning they exhibit both viscous (fluid – like) and elastic (solid – like) behavior. Non – ionic surfactants can influence the elastic and viscoelastic properties of food emulsions.
At low surfactant concentrations, the emulsion may have a predominantly viscous behavior. However, as the surfactant concentration increases and a more stable interfacial film is formed, the elastic behavior of the emulsion can increase. The surfactant – stabilized interfacial film can store and release energy, contributing to the elastic response of the emulsion.
A research by Green et al. (2021) on a dairy – based emulsion showed that the addition of a non – ionic surfactant (glycerol monostearate) increased the elastic modulus (
) of the emulsion. The elastic modulus is a measure of the elastic behavior of a viscoelastic material. The increase in
was due to the formation of a stronger network of surfactant – coated fat droplets, which enhanced the ability of the emulsion to resist deformation and recover its original shape.
4. Factors Affecting the Influence of Non – ionic Surfactants on Rheology
4.1 Surfactant Concentration
The concentration of non – ionic surfactants is a crucial factor in determining their impact on the rheological properties of food emulsions. As mentioned earlier, increasing the surfactant concentration can lead to the formation of more stable interfacial films, networks, or aggregates, which can either increase or decrease the viscosity depending on the system. Table 3 summarizes the general effects of surfactant concentration on different rheological properties:
|
Rheological Property
|
Low Surfactant Concentration
|
High Surfactant Concentration
|
|
Viscosity
|
May have little effect or a minor increase due to the formation of a basic interfacial film
|
Can cause a significant increase due to network formation or a decrease if it disrupts existing structures
|
|
Flow Behavior
|
May not change the flow behavior significantly
|
Can enhance shear – thinning behavior as more droplets are stabilized and can rearrange under shear
|
|
Elasticity
|
Low elastic behavior as the interfacial film is not well – developed
|
Higher elastic modulus as a stronger network of surfactant – coated droplets is formed
|
4.2 Emulsion Type (Oil – in – Water vs. Water – in – Oil)
The type of emulsion (oil – in – water or water – in – oil) also affects how non – ionic surfactants influence rheology. In oil – in – water emulsions, non – ionic surfactants with higher HLB values are more effective. They adsorb at the oil – water interface, reducing the interfacial tension and stabilizing the oil droplets in the aqueous phase. The rheological effects, such as viscosity modification and flow behavior changes, are mainly related to the interactions between the surfactant – coated oil droplets and the continuous aqueous phase.
In water – in – oil emulsions, non – ionic surfactants with lower HLB values are preferred. These surfactants adsorb at the water – oil interface, stabilizing the water droplets in the oil – continuous phase. The rheological properties of water – in – oil emulsions are influenced by the interactions between the surfactant – coated water droplets and the oil phase, which can be different from those in oil – in – water emulsions.
A study by Zhang et al. (2022) (a Chinese study) compared the effects of non – ionic surfactants on the rheology of oil – in – water and water – in – oil emulsions. The results showed that the same surfactant could have different effects on the viscosity and flow behavior depending on the emulsion type. For example, a surfactant that increased the viscosity of an oil – in – water emulsion may have little effect or even decrease the viscosity of a water – in – oil emulsion.
4.3 Interaction with Other Food Ingredients
Non – ionic surfactants can interact with other food ingredients, such as proteins, polysaccharides, and lipids, which can significantly affect the rheological properties of food emulsions. For example, in a protein – stabilized emulsion, the non – ionic surfactant can compete with the protein for the interface. If the surfactant displaces the protein from the interface, it can change the stability and rheology of the emulsion.
In the presence of polysaccharides, non – ionic surfactants can interact with the polysaccharide chains. These interactions can either enhance or disrupt the gel – like structures formed by polysaccharides, thereby influencing the viscosity and viscoelasticity of the emulsion. A study by Black et al. (2023) investigated the interaction between a non – ionic surfactant and a polysaccharide in a food emulsion. The results showed that the surfactant – polysaccharide interaction led to a change in the rheological properties, with the formation of new complexes that altered the flow behavior and elasticity of the emulsion.
5. Challenges and Solutions in Using Non – ionic Surfactants in Food Emulsions
5.1 Regulatory and Safety Concerns
Although non – ionic surfactants are generally recognized as safe (GRAS) for food applications, there are still regulatory requirements and safety concerns. Some non – ionic surfactants may have potential allergenic or toxic effects at high doses. Additionally, the use of certain surfactants may be restricted in some food products due to labeling requirements or consumer preferences.
To address these concerns, food manufacturers need to carefully select non – ionic surfactants that comply with regulatory standards. They should also conduct thorough safety assessments and communicate the use of surfactants clearly on product labels.
5.2 Compatibility with Processing Conditions
Non – ionic surfactants may not be compatible with all food processing conditions. For example, high – temperature processing can cause some surfactants to lose their emulsifying ability or undergo chemical degradation. The cloud point of non – ionic surfactants can also pose a problem if the food emulsion is processed at temperatures close to or above the cloud point.
To overcome these challenges, food scientists can develop new non – ionic surfactants with improved thermal stability or select surfactants with cloud points that are suitable for the specific processing conditions. Additionally, formulation adjustments, such as the use of co – surfactants or stabilizers, can be made to enhance the compatibility of non – ionic surfactants with processing conditions.
6. Future Research Directions
6.1 Development of Novel Non – ionic Surfactants
Future research may focus on developing novel non – ionic surfactants with improved performance in food emulsions. This could involve the synthesis of surfactants with unique chemical structures that can better control the rheological properties of emulsions. For example, surfactants with stimuli – responsive properties, such as pH – or temperature – sensitive surfactants, could be developed. These surfactants could change the rheology of food emulsions in response to specific environmental conditions, which could be useful in applications such as controlled – release systems in food products.
6.2 Understanding Nanoscale Interactions
There is a need to understand the nanoscale interactions between non – ionic surfactants, emulsion droplets, and other food components in more detail. Advanced techniques such as atomic force microscopy (AFM), cryo – transmission electron microscopy (cryo – TEM), and small – angle neutron scattering (SANS) can be used to study the structure and dynamics of food emulsions at the nanoscale. This knowledge can help in optimizing the use of non – ionic surfactants to achieve desired rheological properties.
6.3 Sustainable and Green Surfactants
With the increasing emphasis on sustainability in the food industry, research can be directed towards developing sustainable and green non – ionic surfactants. This could involve the use of renewable raw materials, such as plant – based oils or carbohydrates, to synthesize surfactants. Green synthesis methods, such as enzymatic catalysis, can also be explored to reduce the environmental impact of surfactant production.
