Pressurized Hot Water Extraction: Promising Green Extraction Technology
Authors: Bibwe Bhushan, Manoj Kumar Mahawar, Kirti Jalgaonkar, Dukare Ajinath and Pankaj Kannaujia
Horticultural Crop Processing division, ICAR-CIPHET, Abohar

The effective sample preparation and extraction procedures are regarded as a vital step in the analysis of solid sample. Classical sample preparation techniques that rely on extraction with solvents are liquid–liquid extraction (LLE), sonication, soxhlet extraction etc. However, these traditional methods are error-prone, time consuming with low extraction efficiency and also require large volume of harmful organic solvents. With the amount of hazardous waste needing to be decreased worldwide, the focus today is on automated and environmentally friendly extraction techniques that utilise a minimal amount of organic solvent. Such type of extraction techniques includes, supercritical fluid extraction (SFE), microwave-assisted extraction (MAE), pressurized liquid extraction (PLE) and pressurized hot water extraction (PHWE).

Water is possibly the most abundantly and naturally available non-toxic liquid on the Earth necessary for life, so its possibility to be used as a solvent has almost negligible environmental impact considering its production, use, transportation and recycling. In order to reduce the usage of organic solvents, PHWE can be one of the feasible green solvent extraction methods as it utilizes pressurized water as extractant (extracting solvent) at elevated temperature and controlled high pressure conditions. PHWE extraction technique uses liquid water at temperatures well above the atmospheric boiling point (100°C), but below its critical point (374°C, 22.1 MPa). PHWE also called as superheated water extraction, subcritical water extraction or hot liquid extraction. Water has an extremely high relative static permittivity (dielectric constant, ε). It has been demonstrated by scientists that with increasing water temperature at enough pressure, the dielectric constant (ε) of water (80 at 25oC) decreases to 27 at 250 oC and 50 bar, which falls between those of methanol (ε = 33) and ethanol (ε = 24) at 25oC to maintain it in liquid state. The polarity of water can be varied close to those of alcohols and allows it to dissolve a wide range of medium and low polarity analytes. PHWE has steadily become an efficient and low cost method for extraction of bioactive, essential and nutritional compounds from plants and food materials. PHWE can also be used for removal of organic contaminants in food stuff and also to extract different classes of pesticides and herbicides from soil and sediment samples to allow for remediation. It is dynamic extraction method in which larger volume of fluid is continuously refreshed during the whole process of extraction and extraction efficiency is strongly depends on the temperature and flow rate.

Comparison of different extraction techniques:

Traditional technique Advanced extraction technique
Extraction technique Soxhlet PHWE SFE PLE MAE
Extraction time 4-48 h 5-30 min 30-90 min 2-20 min 30-60 min
Extracting solvents Acetone, Hexane, Acetone-hexane, dichloromethane, toluene, methanol Water CO2 Acetone, hexane, acetone-dichloromethane Acetone, hexane
Solvent consumption (ml) 300 2-5 8-50 15-40 25-50

Parameters affecting PHWE process:

  1. Temperature: Temperature is one of the important factor which could affect the extraction efficiency and selectivity in PHWE. With increase in temperature, both dielectric constant (ε) and Polarisability of water decreases which are relatively high at ambient conditions, e.g, for liquid water, dielectric constant of 80 at 25oC decreases to 27 at 250oC and 50 bar.
  2. Pressure: The effect of adjusting pressure could change the phases of water. Moderate pressures such as 15 bar at 200oC and 85 bar at 300oC are required to maintain the liquid phase of water. Pressure is usually varied from 10 to 80 bars to maintain water in its liquid phase at extraction temperature and often has little effect on the extraction efficiency of PHWE than temperature.
  3. Dynamic or static extraction mode: PHWE can be performed in either static or dynamic mode. Static mode of PHWE is very simple, inexpensive as it doesn’t use high pressure pumps. The dynamic extraction mode, of PHWE provided much improved yield and cleaner extract compared to static process. The process mainly governed by both extraction time and flow rate.
  • Advantages of PHWE process:
  • It reduces the consumption of organic solvents
  • It is environmentally compatible technique
  • Water is easily available, non-flammable, non-toxic and can be recycled or disposed with minimal environmental problems
  • It is a low cost technique
  • Limitations of PHWE process:
  • Some degradation reactions can occur in the product (mainly Maillard and caramelization reactions if carbohydrates-proteins are present)
  • Need for development of industrial equipment Conclusion PHWE utilises very cheap, abundantly available and non-toxic liquid water as an extractant which is environment friendly can be recycled or disposed off with certain treatments. In PHWE technique, yield increases as the temperature increases as a result of increased solubility and decreased static permittivity and polarisability. PHWE gives higher extraction yield than the other extraction method with a short extraction time with less or negligible environmental impact. PHWE could be a suitable technique for scale up to handle larger matrix sizes for industrial applications.

About Author / Additional Info:
I am working as a scientist at ICAR-Central Institute of Post Harvest Engineering and Technology, Abohar (ISO 9001:2015 institute).