Extraction of bioactive compounds from coffee roasting by-product with sub-critical water

Coffee is an important economic crop. The coffee brew is a popular drink consumed around the world due to its wide variety of flavors that can be adapted. In 2016, the world coffee bean production was about 10 million tons. Thailand has about 5-6 billion tons of production, representing 0.05 percent of the world during the year 2012 - 2016. In the year 2016, the market value was approximately 39,000 million baht and the year 2017 has increased to 40,000 million baht, which is likely to increase continuously. This has corresponded with the data regarding the business expansion of coffee shop operators that are likely to expand as well.

Considering the production process, coffee roasting processing generates waste or by-products from the roasting step, known as coffee silverskin (CS) which represent approximately 4.2% (w / w) of green coffee beans. CS has a relatively stable physical property owing to its relatively low humidity of about 5 - 7% of the roasted coffee roasting plant. Therefore, coffee roasting factories generally dispose of CS through burning in order to recover energy as heat for the boiler. This method of waste management may affect ambient air quality and trigger global warming situation. This research aims to restore the value of the use of agro-industrial waste, i.e. CS, through the extraction of high valuable substances such as phenolic compounds, which contain antioxidants or anti-aging properties. These phenolic substances can be used further in the food industry, medicine, and cosmetics. An example of further research application is the development of facial mask sheets as we have a PolyCam research group under our Chemistry Society for Healthy Society (ChGSH) Cluster, which is capable of producing Bio-scaffolding film, modified natural rubber as the main component.

Coffee silverskin (CS) is a part of fresh coffee beans, which become a waste or by-product from the roasting process of coffee beans. It is estimated that the amount of CS is approximately 4.2 percent by weight of coffee beans. General practices for CS waste management for the coffee roasting industry are the recycling process in the form of energy, feedstocks, and fertilizer or soil conditioner due to its high carbon content [12, 13].

At present, there have been many researchers studying the use of residue or by-products from the agro-industry as raw materials in the extraction process. This extract generally contains natural antioxidants, which can be used further to replace synthetic chemicals in either food, pharmaceutical, and cosmetic industries. Few studies have reported total phenolic compounds in CS, which have been found to have relatively high antioxidant activity, including chlorogenic acid (CGA) and caffeic acid (CA) [13-17]. In addition, one research has reported that CA and CGA also have anti-inflammatory effects, anti-bacterial, anti-viral, and anti-tumor in relatively high amounts.

Caffeic acid (3,4-dihydroxycinnamic) or CA is the secondary metabolite obtained from the derivative of cinnamic acid, also known as phenylpropanoids, which can be found in plant tissues. For the CGA found in CS, there are 3 isomers including 3-caffeoylquinic acid (3-CQA), 4-caffeoylquinic acid (4-CQA), and 5-caffeoylquinic acid (5-CQA). It is reported that coffee beans, Robusta and Arabica varieties, contain CGA contents between 1-6% (dry weight of CS). However, it is reported that the process of roasting fresh coffee beans, that require high heat, may result in the loss of CGA up to 42-99% of total CGA in CS.

Extraction of total phenolic compounds can be conducted by several methods. Conventional extraction with maceration technique is a method that helps obtain a large quantity and good quality of crude extracts, which can be further extracted to obtain bioactive compounds. Factors affecting the quality of the extracts by this technique are temperature, mixing intensity to achieve thorough heat transfer, and extraction time. The limitation of extraction with the maceration technique involves the use of a large number of volatile solvents. This may result in higher chemical costs, generating chemical waste from the extraction process. These solvents may remain in intermediate products if not removed properly.

Subcritical water extraction, one of the hydrothermal techniques, is the extraction under high temperature and pressure by using water as solvent. This technique allows the structure of the raw material to expand more and allows the extract to release from the biological matrix through a dissolving mechanism into the solution (water). This extraction method helps obtain large quantities of crude extracts and reduce extraction process duration compared with the conventional extraction methods. In addition, it helps reduce the solvent elimination step as its remaining could become waste from the extraction process or may be left in intermediate products. However, it is necessary to operate hydrothermal extraction with optimum temperature and pressure because using too high a temperature may cause deterioration/breakdown of the active ingredients. Factors affecting hydrothermal extraction efficiency include temperature and pressure.

In 2016, one had study the extraction of phenolic compounds from fresh coffee beans from 5 regions using hydrothermal techniques. The results showed that at 180 ° C, 30 bar pressure could extract total phenolic compounds up to 144.4 ± 4.2 (g GAE/g raw coffee bean). In the same year, one research has studied the extraction of phenolic compounds from the spent coffee ground (SCG) using autoclave hydrothermal techniques. It is reported that at 120 ^oC the obtained crude extract and total phenolic compounds were 32.92 (mg/g SCG) and 19.17 (mg GAE/g SCG), respectively. In 2018, one research has reported the optimum temperature for extracting phenolic compounds from fresh coffee beans with hydrothermal techniques is 157.2 ^oC. The obtained extract contained 6.75 mg GAE/g green coffee bean. 3-o-caffeoylquinic acid, which is one of the isomers of chlorogenic acids (CGAs), may be degraded through the effect of hydrolysis reaction with a constant rate of decay of approximately 3.4 × 10^-4 min. ^-1) at 200 ° C.

From previous research, it can be seen that hydrothermal techniques are highly effective in extracting phenolic compounds from coffee beans and coffee grounds. However, there has been no study on the use of this technique in the extraction of phenolic compounds from coffee silverskin (CS) with hydrothermal technique especially the optimization study of this technique.