Starch microemulsions are colloidal systems consisting of dispersed starch particles in a continuous phase of oil and water stabilized by an emulsifier. In a starch microemulsion, the starch particles are typically less than 100 nanometers in size and are evenly dispersed throughout the emulsion. The emulsion can be used as a food ingredient to provide a range of functional properties, such as improved texture, stability, and reduced fat content.
Microemulsions are stable mixtures of water, oil, surfactant, and cosurfactant that are used in a variety of fields, including oil recovery, pharmaceuticals, and chemical engineering. Microemulsions have unique properties that allow them to solubilize immiscible liquids and have an ultralow interfacial tension. They are different from macroemulsions, which are non-transparent, less stable, and require occasional homogenization with agitation. Microemulsions are thermodynamically stable and can form spontaneously with the right composition of water, amphiphile, and oil.
Starch is a natural polymer that is commonly used as a thickener, gelling agent, bulking agent, and water-retention agent in food and industrial applications. However, native starch has limitations, such as poor processability and solubility. Starch can be modified using physical, chemical, or enzymatic treatments to enhance its properties and improve its industrial applications.
Starch microemulsions are colloidal dispersions of oil droplets (discontinuous phase) in an aqueous medium (continuous phase), stabilized by the presence of starch nanoparticles (SNPs) and a surfactant. These emulsions have several properties that are important for their stability and functionality.
- Stability: Starch microemulsions require a greater amount of surfactant than emulsions to achieve thermodynamic stability. Various factors, including polymer type and concentration, volume fraction of the dispersed phase, excipients, stirring rate, temperature, homogenization speed, and surfactant type and amount, affect the stability of the microemulsion. Stability is evaluated based on visual inspection, droplet size distribution, and microscopic images.
- Temperature and stability: Starch microemulsions are thermodynamically stable and form spontaneously under the right conditions. However, some energy input (in the form of gentle mixing, stirring, or heating) facilitates their formation because there are kinetic energy barriers that must be overcome or mass transport limitations that retard their formation.
- Particle size and stability: The emulsifying ability of starch granules seems inversely proportional to their size. When the diameters of SNPs were 150–700 nm, the mean droplet size of an emulsion stabilized by the SNPs increased from 29.33 to 48.76 μm.
- Ions and stability: Generally, as the salt concentration increases, there is a greater tendency for the particles of a microemulsion to aggregate because the stabilizing repulsive forces are weakened by the increase in the electrolyte concentration. However, NaCl concentrations of 0–90 mM had no obvious effect on the stability of a Pickering emulsion stabilized with cornstarch-based nanoparticles.
- Oil concentration and stability: A marked increase in the oil fractions of corn, tapioca, sweet potato, and waxy cornstarch nanoparticles was observed after 1 month in storage. When the oil fractions were 0.6 and 0.75, the internal oil phase accounted for a great proportion of the emulsified phase, which approached or even exceeded the critical oil fraction for closely packed emulsion systems, and thus easily underwent phase inversion; oiling off occurred after 1 day in storage.
- Kinetic instability: The internal contents of starch microemulsion droplets are known to be exchanged between droplets, typically on a millisecond time scale. They diffuse and collide, and if the collisions are sufficiently violent, then the surfactant film may rupture, facilitating droplet exchange. Therefore, the droplets are kinetically unstable.
- Interfacial Tension: Starch microemulsions form spontaneously under the right conditions. In simple aqueous systems, the formation of starch microemulsions is dependent on the surfactant type and structure. The ultralow interfacial tension between the oil and water phases is one of the most fundamental properties of microemulsions.
- Percolation Phenomena: Starch microemulsions can display percolation phenomena at specific volumetric fractions of water. According to the percolation theory, phase transformation from reverse structures (W/O) to normal-type systems (O/W) through the emergence of bicontinuous systems and other aggregates may occur as the aqueous content of the system increases. This is generally accompanied by an increase in the system’s electrical conductivity.
- Rheological Behavior: The flow behavior of starch microemulsions is influenced by the volume fraction of the dispersed phase and the type and amount of surfactant. These emulsions display a shear-thinning behavior and exhibit pseudoplasticity. The storage modulus is higher than the loss modulus, indicating
Different techniques for preparing starch microemulsions, including solvent evaporation, coacervation, and emulsion cross-linking techniques, along with their advantages and disadvantages.
Solvent Evaporation: In solvent evaporation, a polymer and a solvent are dissolved in a volatile organic solvent to form a homogenous solution. The solution is then dispersed in an aqueous phase containing a surfactant to form an emulsion. The organic solvent is then evaporated, which leads to the precipitation of the polymer and the formation of microemulsions.
Coacervation: Coacervation involves the phase separation of two oppositely charged polymers. One polymer is dissolved in water, while the other is dissolved in an organic solvent. When the two solutions are mixed, they undergo phase separation, resulting in the formation of two distinct liquid phases. The polymer-rich phase forms droplets that are stabilized by the surfactant molecules present in the system, leading to the formation of microemulsions.
Emulsion Cross-Linking Techniques: In emulsion cross-linking techniques, a polymer solution is emulsified in a continuous phase containing a cross-linking agent. Cross-linking agents can be added either to the continuous phase or the dispersed phase. The cross-linking reaction leads to the formation of a stable network structure that entraps the dispersed phase, leading to the formation of microemulsions.
These methods can be used to prepare starch microemulsions with varying properties such as particle size, surface charge, and stability.
Applications in non-food industries
Starch microemulsions have several potential applications in non-food industries, including:
- Drug delivery: Starch microemulsions can be used as a type of controlled drug delivery system, with advantages such as tailored drug release, stability, and lower dosing frequency. They can also be used for targeted drug delivery by exploiting the pH sensitivity, thermal sensitivity, or magnetism of the drug carrier.
- Metal ion adsorbents: Starch microemulsions have potential utility in adsorbing heavy metal ions from aqueous solutions, making them useful for wastewater treatment.
- Topical hemostasis: Starch microemulsions have been approved and used as topical hemostasis agents, maintaining hemostasis during surgery and coadministration of cytotoxic drugs in the treatment of malignancies.
- Embolization agents: Starch microemulsions, such as absorbable gelatin powder and prolamine, have been used as embolizing agents in the treatment of nonresectable primary and secondary liver tumors.
The advantages of starch microemulsions, such as their thermodynamic stability, spontaneous formation, easy scale-up, large interfacial area, nanosized droplets, isotropy, and low viscosity, make them applicable to many fields of science and technology.
Applications in food industry
Starch microemulsions have a wide range of potential applications in the food industry. Here are some examples:
- Carriers for bioactive food components: Starch microemulsions can act as carriers for water-insoluble functional ingredients, such as vitamins, polyunsaturated fatty acids, carotenoids, and flavor compounds. The ingredients can be incorporated into oil droplets where they are isolated from the external environment by the interface and the water phase, improving their physicochemical stability, water-dispersing capacities, and bioaccessibility.
- Delivery systems for natural antimicrobial compounds: Starch microemulsions can be used as a delivery system for natural antimicrobial compounds, such as nisin, ε-poly-l-lysine, and thymol, to prolong their efficacy and prevent the contamination of fresh-cut produce by foodborne pathogens. The emulsions retain much greater antimicrobial activity during storage and inhibit pathogenic bacteria much more strongly than the non-emulsion, aqueous formulations.
- Fat replacers: Starch microemulsions can be used as fat replacers in low-fat and reduced-calorie foods, such as salad dressings, mayonnaise, and cheese spreads, to improve their texture and mouthfeel.
- Coating agents: Starch microemulsions can be used as coating agents to improve the sensory properties, such as crispness and crunchiness, of fried foods, such as chicken nuggets and french fries.
- Edible films and coatings: Starch microemulsions can be used to prepare edible films and coatings for food packaging and preservation, to improve the mechanical properties and barrier properties of the films, and to provide a matrix for the incorporation of bioactive compounds.
- Emulsifiers and stabilizers: Starch microemulsions can be used as emulsifiers and stabilizers in a wide range of food products, such as baked goods, dairy products, and confectionery, to improve their texture, stability, and shelf life.
Overall, starch microemulsions offer a promising platform for the development of novel functional foods with desirable health benefits and organoleptic properties.