Ultrasonic Extraction of Caffeine
Coffee – made from roasted coffee beans – is a very popular drink that is worldwide consumed. Besides to its vitalizing impact if consumed as a stimulant drink, the compounds of coffee are of interest for the food, pharmaceutical (e.g. in pain relievers) and cosmetic industry as used as valuable additives in various products. This applies especially for the caffeine (1,3,7-trimethylxanthine) and the antioxidants, which are known for their positive effects on the human health. Coffee contains, amongst others, phenolic diterpenes such as cafestol and kahweol, and ascorbic acids, which are known for their antioxidative activity. Epidemiological studies suggest that coffee’s ingredients may have a preventive effect on several chronic diseases, including type 2 diabetes mellitus, Alzheimer’s disease, Parkinson’s disease, and liver diseases such as cirrhosis and hepatocellular carcinoma.
Ultrasonics is a well known and proven tool for many different applications in various industries. A very successful application is the ultrasonic extraction. Thereby, the ultrasonic cavitation effects on the cell material causing cell disruption and the release of the intracellular matter.
High Power Ultrasound
To ensure a more easy understanding of the ultrasound assisted extraction procedure, the effect of ultrasound in liquids must be explained.
Ultrasound – introduced in liquids – causes locally very extreme effects. When sonicating liquids at high intensities, the sound waves that propagate into the liquid media result in alternating high-pressure (compression) and low-pressure (rarefaction) cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high-pressure cycle. This phenomenon is termed cavitation. During the implosion very high temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity. [Suslick 1998] By these extreme forces sonolysis occurs, cell walls are disrupted, and intracellular material is extracted.
By the use of sonication also the operating temperature can be reduced, allowing the extraction of temperature-sensitive components. Compared with other novel extraction techniques such as microwave-assisted extraction, the ultrasound apparatus is cheaper and its operation is easier. Furthermore, the ultrasonically assisted extraction can be used with any solvent, such as the Soxhlet extraction, for extracting a wide variety of natural compounds. [Wang et al. 2006] If necessary, explosion-proof systems for industrial process capacities are available.
A substantial advantage of ultrasonics is the influence on the most important processing parameters: amplitude, time, temperature, pressure, and viscosity. Thereby, the extraction process can be optimized to ensure that the structure of the extracts does not become damaged.
Ultrasonic Extraction of Coffee Compounds
Ultrasound-assisted extraction is a common method utilized to isolate bioactive substances from plant material [Dong et al. 2010]. Regarding coffee beans, the caffeine and antioxidant phenolic compounds might be the most valuable compounds for extraction due to their wide applications in pharma and food industry. But also flavonoids, chlorogenic acid and protocatechuic acid are extracts which are used as additives.
Using traditional extraction methods such as the liquid-liquid extraction in solvents, generally the extraction efficiency increases with increasing extraction temperatures. This causes often damages and quality loss of the extract as temperature affects the stability of the phenolic compounds.
The ultrasound-assisted solid-liquid extraction has been shown as an effective and time-saving extraction method. The highly powerful ultrasonic forces provide the necessary energy for the extraction, so that less or even none solvents are needed. The temperature can be well controlled as the sonicated batch or flow cell reactor can be efficiently cooled (or heated if necessary). For the extraction processes with solvents, Hielscher Ultrasonics provides ATEX and FM certified explosion-proof ultrasonic systems, too.
Due to the intense extraction forces of ultrasound, also the already spent coffee ground (coffee waste) is still a raw material rich in extractable compounds. As coffee waste material is cheap and available in large amount, it is an ideal raw material for the extraction of the remaining active compounds. Although the content of caffeine and other components in coffee waste is lower than in unused coffee powder, still a large amount remains and is extractable. To release these compounds from the coffee ground, the full influence on the processing parameters becomes especially significant. High-power ultrasound is able to extract high amounts of active compound within a short processing time.
Caffeine can be claimed the most commonly consumed stimulant. As caffeine is not only consumed by drinking brewed coffee, caffeine extract is used in the industry to treat other product with caffeine as additive. Thereby it becomes possible to create stronger coffee or to formulate soft drinks (e.g. cola), energy drinks or other food (e.g. chocolate).
But caffeine is not only used as additive in the food production, it is an important active compound in pharmaceuticals as well. A common application of caffeine extract is e.g. the admixture in drugs for headache and migraine or in pain relievers.
To extract caffeine, the main alkaloid in coffee, ultrasonication is a suitable method. Wang and his colleagues [Wang et al. 2011] found that only a short extraction time is needed to reach a saturated state, if ultrasonication is used. This means that ultrasound a very efficient and time saving technique for gaining caffeine.
Aromatic and flavour compounds
The volatile coffee compounds are the most valuable fraction of the roasted coffee bean and imparts coffee its unique flavour and fragrance. The quality of soluble coffee can be substantially improved by the addition of aroma-absorbed coffee oils to the coffee powder.
A comparison study, examinating the extraction of phenolic compounds from strawberries, showed that ultrasonic extraction causes less degradation of phenolics and is a much faster extraction process compared with other extraction methods including solid-liquid, subcritical water and microwave-assisted method. [Herrera et al. 2005]
The study of Wang and his colleagues [Wang et al. 2011] shows that low frequency, high power ultrasound is more efficient for the extraction of coffee flavours. Especially for 4-Tridecanone and 2-Methoxy-3-Methylpyrazine, they found the ultrasonic extraction more easy and efficient technique obtaining very high extraction yields. Further, it is shown that the temperature must be controlled as coffee flavor components are very volatile at high temperatures. They achieved good extraction results in the temperature range between 35~65 °C under relatively short-time ultrasonic irradiation.
The results achieved by ultrasonically assisted extraction are also suitable for the extraction of tea compounds (e.g. green tea leafs). A study by Xia et al. showed a significantly higher content of tea polyphenols, amino acid and caffeine in ultrasonically treated tea infusions than in those obtained by conventional extraction. This results in improved results during an organoleptic evaluation that turned out: the sensory quality of tea infusion with ultrasonic-assisted extraction was better than that of tea infusion with conventional extraction. [Xia et al. 2005]
The ultrasonically assisted extraction is an efficient, time-saving and controllable method for the extraction of active compounds from coffee. The most interesting and valuable compounds are caffeine, and the antioxidants such as phenolic diterpenes (cafestol, kahweol), and ascorbic acids. The main advantages of the ultrasound-assisted extraction are based on the influence and control over the ultrasonic extraction parameters.
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