# Critical Review of Techniques for Food Emulsion Characterization
## 3 Techniques for Emulsion Characterization
### Thermal Properties Analysis  
Thermal characteristics of emulsions play a pivotal role in delineating their stability and functionality. These properties encompass heat capacity, thermal conductivity, and heat transfer coefficient. Various factors influence the thermal properties of emulsions, including the thermal attributes and composition of both the continuous and dispersed phases (e.g., surfactant concentration, presence of ions and additives), as well as droplet size and size distribution [157]. Smaller droplets, characterized by a higher surface area-to-volume ratio, facilitate improved heat transfer within the emulsion [158], while larger droplets impede heat transfer, resulting in diminished thermal conductivity. Nonetheless, the overall thermal properties of the emulsion hinge significantly on the relative thermal conductivity of the dispersed phase in comparison to the continuous phase [157]. The effective medium theory offers a means to estimate the thermal conductivity of emulsions. In the case of spherical dispersed particles, such as droplets in water-in-oil emulsions, this theory simplifies to the Maxwell-Garnett equation (Equation (2)):  
\[\frac{k}{k_{0}}=\frac{k_{D}(1+2\alpha)+2k_{0}+2\phi[k_{D}(1-\alpha)-k_{0}]}{k_ {D}(1+2\alpha)+2k_{0}-\phi[k_{D}(1-\alpha)-k_{0}]} \tag{2}\]  
where \(k_{0}\) represents the continuous phase thermal conductivity, \(k_{D}\) is the thermal conductivity of the emulsified water, \(\phi\) is the water volume fraction, \(k\) is the effective thermal conductivity of the emulsion, and \(\alpha\) is the interfacial resistance and droplet size [159]. The highest thermal conductivity is exhibited by nanoemulsions and nanofluids containing droplets/particles smaller than 100 nm with substantially higher bulk thermal conductivity than the continuous phase [158]. Modifying the concentration of surfactant in the emulsion can be used to modulate its melting point, heat capacity, and thermal conductivity by altering interfacial interactions. In general, higher surfactant concentrations enhance the thermal conductivity of emulsions by preventing the formation of larger droplets and increasing heat transfer [158]. There are several methods to measure the thermal properties of emulsions. One of the simplest methods is the hot plate technique, where a sample of the emulsion is placed on a heated surface and the temperature gradient across the sample is measured over time [119]. The hot plate method is relatively simple to perform, requires minimal equipment, and is suitable for both liquid and solid emulsion samples. However, it may not accurately reflect the thermal properties of emulsions since it does not consider factors such as heat transfer through convection or the presence of other components in the emulsion [119]. Another method for analysis of emulsion thermal properties, known as the transient hot wire technique [160], involves inserting a thin wire probe into the emulsion sample and passing an electrical current through it. The wire serves both as an electrical heating element and a resistance thermometer. The thermal conductivity of the sample is measured based on the changes in the wire temperature and heat generation. The alteration in hot wire temperature is detected using a Wheatstone bridge. The voltage imbalance across the bridge is recorded by a data acquisition system over time. The transient hot wire technique, although accurate in measuring thermal conductivity, may not be suitable for highly viscous emulsions due to the risk of damage to the emulsion structure during probe insertion. A method that can provide more detailed information about phase transitions and the heat capacity of emulsions is differential scanning calorimetry (DSC). DSC measures the heat exchange and temperature changes associated with the phase transitions in the emulsion, or polymorphic transition of crystals, over a range of temperature and time [5; 161].