What Impact Does Temperature Have On The Ultrasonic Extraction Process?

While the cavitation effect inherent to ultrasonication generates localized, instantaneous high temperatures, the overall temperature of the system is determined by temperature control settings.
Temperature is a critical process parameter in ultrasonic extraction; it can enhance extraction efficiency but also degrade heat-sensitive target substances (such as pigments, alkaloids, flavonoids, volatile oils, and polysaccharides).

 

**Beneficial effects of rising temperature**
1. **Increased solvent solubility and diffusion rate**
As temperature rises, solvent viscosity decreases and molecular diffusion accelerates. This allows the solvent to penetrate disrupted plant cells more easily and dissolve active ingredients faster, thereby increasing extraction rates and yields within a certain temperature range.

 

2. **Reduced solvent surface tension**
Lower surface tension facilitates the formation of ultrasonic cavitation bubbles; moderate heating enhances the cavitation effect, accelerating the disruption of cell walls.

 

3. **Softening of plant tissues**
For hard medicinal materials (such as roots and rhizomes), moderate heating softens the tissue, aiding the release of active ingredients.
Generally, extraction efficiency for most plants is optimal within the 40–60°C range.

 

**Negative effects of excessive temperature (Key risks)**
1. **Degradation and inactivation of heat-sensitive active ingredients**
• Natural pigments (anthocyanins, chlorophyll): High temperatures cause oxidative decomposition, leading to reduced color value or fading;
• Alkaloids and volatile oils: Prone to decomposition or loss through volatilization at high temperatures;
• Flavonoids, polyphenols, and some polysaccharides: Oxidation occurs at high temperatures, reducing the content of target products.

 

2. **Weakened cavitation effect**
At excessively high temperatures, the solvent's internal vapor pressure rises. While cavitation bubbles form in large numbers, they tend to collapse prematurely or ineffectively, reducing the intensity of cavitation-induced impacts and diminishing the actual cell-disruption efficiency.
In aqueous systems, the cavitation effect declines significantly when the temperature exceeds 70°C.

 

3. **Increased dissolution of impurities**
High temperatures cause the dissolution of large amounts of unwanted substances-such as proteins, gums, and tannins-resulting in a turbid extract and complicating subsequent filtration and purification steps.

 

4. **Increased solvent volatilization**
Organic solvents like ethanol suffer significant losses due to evaporation at high temperatures, which also poses safety risks.
Disadvantages of excessively low temperatures
At excessively low temperatures, solvent viscosity is high and molecular diffusion is slow; solute solubility is low, resulting in a slower extraction rate and a lower extraction yield within the same timeframe.