Supercritical CO2 Extraction

Supercritical Fluids and Applications of Supercritical CO₂ in Extracting Flavors and Bioactive Compounds in Food Processing Technology

Part 1: Introduction

(See detailed equipment specifications)

Science and technology are constantly evolving, bringing new discoveries and innovations aimed at improving human life. The development of new processing methods, novel products, and alternative raw materials is among the strategic directions in the food technology sector today.

Advanced techniques for food processing and preservation continue to emerge. Among these, supercritical fluid extraction (SFE) has gained increasing attention for its ability to produce high-purity, non-toxic extracts from natural sources, while also minimizing environmental pollution and avoiding the use of harmful chemicals.

A supercritical fluid (SCF) is a physical state of a substance at temperature and pressure conditions above its critical point. Among various SCFs, carbon dioxide (CO₂) is the most commonly used due to its favorable characteristics: abundance, low cost, non-flammability, and chemical inertness.

This extraction method ensures the preservation of active compounds, offers high selectivity, and is completely safe for both operators and end users. Solvent properties of supercritical fluids can be adjusted by varying temperature and pressure to match different extraction conditions.

Using this approach, numerous natural compounds—including enzymatic systems in plants—have been successfully extracted, which would be difficult or impossible using traditional methods.

The supercritical CO₂ extraction (SC-CO₂) technique was first developed in the United States, followed by Russia, and is now actively being adopted by China and India due to its exceptional advantages. The method has successfully extracted valuable antiviral and anticancer compounds (against HIV, HPV, HBV, etc.) from medicinal plants.

Vietnam has begun to adopt this advanced method, promising major progress in extracting valuable bioactive compounds from the nation’s rich medicinal resources. In 2009, the Vietnam Academy of Science and Technology, with support from the Russian Academy of Sciences, imported a Russian-made supercritical CO₂ extraction system.

Although CO₂ is often criticized for its role as a greenhouse gas, advances in science and technology have revealed its potential as a highly useful industrial solvent. Researchers are exploring CO₂ in a new light—as a supercritical solvent for extracting desired compounds from plants and other industrial materials.

In laboratory research, CO₂ has long been used for analyzing pesticide residues and polychlorinated biphenyls (PCBs) in food, water, and soil samples. In the food industry, CO₂ has been employed for cleaning spices, brewing yeast, and decaffeinating coffee.

At the supercritical state, CO₂ exists between a gas and a liquid—having liquid-like density that allows it to dissolve organic compounds, making it an excellent alternative to chlorinated solvents.

Since ancient times, humans have extracted essential oils from natural sources—most commonly rose oil from petals for perfumes. Over time, various extraction methods have evolved: pressing, water distillation, organic solvent extraction, steam distillation, microwave-assisted extraction, ultrasonic extraction, and most recently, supercritical CO₂ extraction—used to isolate Taxol (an anticancer compound from the bark of Taxus brevifolia).

While any substance can theoretically reach a supercritical state (as defined by its phase diagram), only a few—like CO₂ and water—are practical due to their moderate critical temperature and pressure.

At these conditions, matter exhibits both gas-like diffusivity and liquid-like density, making supercritical fluids powerful and tunable solvents for selective extraction.

Part 2: Principle of Supercritical Fluid Extraction

When CO₂ is raised above its critical temperature (31°C) and critical pressure (73.8 bar), it becomes a supercritical fluid. In this state, CO₂ exhibits both the solvating properties of a liquid and the diffusion characteristics of a gas.

It can dissolve and extract target compounds from solid, liquid, or gaseous matrices. After extraction, reducing the pressure below the critical point allows CO₂ to revert to the gas phase, leaving behind the purified extract without solvent residues.

By tuning temperature and pressure, CO₂ can selectively extract different compounds based on their solubility profiles.

Comparison of SC-CO₂ with Conventional Extraction Methods

Advantages:

• High-quality extracts: Essential oils retain natural color and aroma; bioactive compounds remain intact.
• No residual solvent left in the product.
• High concentration of active compounds in the extract.
• Environmentally friendly and non-toxic process.
• High-tech and safe method for processing natural products.
• Rapid mass transfer due to low viscosity of CO₂, leading to shorter extraction times.
• CO₂ is abundant and inexpensive, being a byproduct of other chemical industries.
• Chemically inert—does not react with the extracted substances.
• Non-flammable and non-explosive.
• Non-corrosive and leaves no toxic residues upon evaporation.
• Excellent solvent for organic compounds, both solid and liquid, and volatile aromatic substances.
• Selective solvency, excluding heavy metals, with tunable parameters for targeted extraction.

Supercritical CO₂ Properties:

• Low surface tension
• High diffusivity and mobility
• Low viscosity
• Liquid-like density
• Easily tunable solvency with temperature and pressure

Disadvantages:

Despite its advantages, SC-CO₂ extraction is still limited mainly to laboratory and pilot scales. The main challenges are high equipment cost, strict safety requirements, and operation at elevated temperature and pressure.

Part 3: Applications of Supercritical CO₂ in Extracting Flavors and Bioactive Compounds in Food Processing

Currently, SC-CO₂ extraction is widely used for isolating bioactive compounds in the food, pharmaceutical, cosmetic, and natural products industries.

Germany was the first country to establish an industrial-scale SC-CO₂ plant for coffee decaffeination, built by HAG A.G. in 1979.

Applications

• Caffeine Removal in Coffee and Tea
  The beverage industry increasingly requires products with reduced caffeine content due to health concerns.
  SC-CO₂ extraction effectively removes caffeine, reducing levels to below 0.1%, while preserving flavor compounds.
• Taxol Extraction from Pacific Yew (Taxus brevifolia)
  Since the 1960s, the U.S. National Cancer Institute identified Taxol as a potent anticancer agent.
  Among various extraction methods, SC-CO₂ provides the highest yield and purity.
• Palm Oil Extraction
  Malaysia, one of the largest palm oil producers and exporters, has demonstrated the superiority of SC-CO₂ over traditional organic solvent extraction.
  This method not only improves oil purity and color but also reduces CO₂ emissions, supporting environmental initiatives like the Kyoto Protocol.
• Other Industrial Applications
  SC-CO₂ is used for extracting hop compounds for brewing and pharmaceuticals, with production capacities exceeding 10,000 tons per year in Germany.
  It’s also used for producing low-fat, cholesterol-free foods and functional food ingredients.
• Cosmetics and Biotechnology
  SC-CO₂ efficiently extracts premium essential oils (lavender, cedarwood, vetiver, jasmine, grapefruit) for perfume and flavor industries.
  Extracted oils retain their natural aroma and high purity.
  It’s also used to isolate polyphenols and antioxidants from green tea, turmeric, and ginger for use in anti-aging creams, moisturizers, and oral care products.
• Pharmaceutical Applications
  SC-CO₂ is employed to extract medicinal compounds that enhance health or treat diseases from herbal sources.
• Emerging Applications
  Beyond food and pharmaceuticals, SC-CO₂ is being explored in rare-earth and radioactive material separation, particularly in Belgium, highlighting its potential across diverse industrial sectors.

📘 Excerpt from: Department of Food and Beverage Technology,
Faculty of Food Technology,
University of Agriculture and Forestry, Ho Chi Minh City, Vietnam.