In the food sector, fruits and vegetables have been processed to produce new products in large numbers. The majority of the by-products are regarded as wastage. Tomatoes are among the most famous crops around the world having yearly output exceeding 14 million tonnes, nearly 86 percent of which is converted into multiple products. 5-10 % of tomato pomace is produced during industrial tomato processing (mainly peels and seeds). This massive volume of by-product (0.6-1.2 million tonnes annually) is usually discharged straight in landfills, resulting in a serious pollution problem. Similarly, the massive volume of grape pomace produced are generally kept as a by-product as animal feeds or delivered directly to landfill as garbage, resulting in loss of value and serious environmental burden.
In the years 2018-2019, worldwide lemon (Citrus lemon) productivity is estimated to cross 8.2 million metric tonnes, with about 30% of it being processed and resulting in a huge amount of lemon peel as trash. Citrus peel is a rich source of terpenoids (limonene, g-terpinene, β-pinene and βmyrcene), which become important for its aromatic taste & antibacterial effects that are used in a wide range of commercial items, which include alcoholic as well as non-alcoholic drinks, sweets and ice creams. As a result, waste management seems to have become a significant issue for the processing industries. However, these wastes contain beneficial phytochemical compounds that are extensively available in plants and also have numerous health and nutrition benefits for humans, including mitigating the risk of neurodegenerative disorders by decreasing oxidative stress and restricting macromolecular oxidation, avoiding cardiovascular disease.
Anthocyanins, flavonols, phenolic acids and stilbenes are among the antioxidant compounds contained in grape pomace as polyphenols. Furthermore, the anthocyanins obtained from plants also have bright colours and excessive water solubility and are regarded as alternative natural colourants in the food business. The extraction of phenolic compounds offers a lot of opportunity for valourization. As a result, recovering polyphenols from garbage or pomace to be used in value-added products is gaining prominence. Phenolic can be detected in either free or bound format in pomace or industrial by-products (seed/skin). However, the recovery of bound phenolic is challenging, since the cell wall structure inhibits release and perhaps to retain free phenolic compounds. As a result, the extraction efficiency of phenolic compounds can be influenced significantly by the extraction method adopted.
Some extraction techniques such as maceration and Soxhlet extraction needs a longer time for the extraction process which could decrease the quality of extracts. The conventional methods for phenolic compounds extraction are liquid-liquid extraction or solid-liquid extraction. However, there are two main concerns with these processes: intensive solvent use (e.g., methanol, ethanol, acetone, etc.) and relatively low extraction efficiency.
Besides, conventional extraction methods are not effective in extracting bound phenolics from plant matrix. In order to improve the extractability of bound phenolics, alkaline or acid hydrolysis is normally used, which is not an environment-friendly approach either, due to large demand for strong acid/alkaline as well as operation at extremely high temperatures (up to 80-95°C for acid hydrolysis). The bound phenolics could also be liberated through microwave pre-treatment due to the heat generated. However, its operation not only needs high power input as it is energy-intensive, but also has risks of organic solvent explosion. Furthermore, excessive exposure to microwave can cause considerable degradation of phenolic compounds, due to extremely high temperature (>100°C) at the hot spots within the treated sample. Therefore, a green and effective alternative extraction method is an emerging need. Ultrasound-assisted extraction, microwave-assisted extraction, pressurized liquid extraction and supercritical fluid extraction are non-conventional extraction techniques from plants. These techniques can reduce extraction time and solvent consumption. Cold plasma is one of the modern technologies that is used in our work for the increasing extraction rate. The important difference between cold plasma as compared to other extraction methods is that cold plasma is used for pre-treatment before carrying out the process of extraction, whereas the other methods are usually directly used for the extraction.
Cold plasma (CP) is a cutting-edge novel non-thermal technology method that has recently gained popularity in the food processing sector. It is a novel and break-through approach for safety and preservation of food products with minimal influence on food quality. The CP comprises ions, electrons, ultraviolet photons, atoms, molecules and free radicals. In general, these active agents can decompose covalent bonds and produce many chemical reactions such as surface etching (creation of pores/tissue damage), depolymerization (formation of new compounds through starch breakdown), cross linking (cleavage of C-OH polymeric chains for adding new C-O-C bond by eliminating water).
Based on some studies, the CP could be employed in some of the industries, including the food industry for microbiological decontamination of fruits and vegetables and enzyme inactivation and seed germination in agricultural industry. Furthermore, it has been revealed that some phenolic compounds extraction such as diosmetin in Valerianella locusta leaves and polyphenols in blueberry juice were increased by using CP. The TPC of green tea leaves was slightly increased by the CP treatment jet. The shallow penetration depth is desirable in various cases. A shallow penetration depth protects the majority of the nutrients within the food, while allowing the surface to be completely decontaminated in food structure which has more surface-to-volume ratio.
Cold plasma, also known as non-thermal plasma contains electrons of a much higher temperature than heavy particles (ions and neutrals). Since cooling of ions and uncharged molecules is more effective than energy transfer from electrons, the gas temperature of cold plasma remains low in a state of thermodynamic non-equilibrium. Therefore, cold plasma has been an emerging processing technology with various potential applications for temperature-sensitive biological materials. Furthermore, cold plasma can be operated at atmospheric pressures or low pressures. In addition to low energy cost, atmospheric cold plasma is an affordable technique, as it does not require costly reaction chamber to regulate pressure and temperature conditions.
There are several cold plasma generation systems in which Dielectric Barrier Discharge (DBD) and jet plasmas are the most widely used systems for food applications (Pankaj, Wan, & Keener, 2018). These two types of plasmas can generate highly active species (atoms, ions, electrons, free radicals, photons, etc.) by discharging the gaseous medium.
Mechanism of extraction:
Cold plasma is an affordable processing technique and has been widely used for modification of various material surfaces, including increasing surface roughness and surface area through etching process, modulating wettability of polymer surfaces, etc. Based on the research studies, the cold plasma used for modifying the surface structure by creating raptures on its cell wall structure, results in increasing its surface hydrophilicity.
For food applications, cold plasma has been used for commercial sterilization, in which plasma reactive species attack the cell envelope of bacteria, inducing its disruption and resulting in cell leakage. Cold plasma also has the capability of manipulating surface characteristics of food products. The surfaces of fresh lettuce and white grape became more hydrophilic after plasma treatments, due to degradation of the cuticle layer, which consists of hydrophobic cuticular and epicuticular waxes. The cellular damages and surface modifications caused by plasma indicate its potential for decreasing the resistance to diffusion of internal molecules and increasing the extractability of hydrophilic compounds, which can facilitate phenolic recovery from biomass.
Plasma, a quasi-neutral ionized gas causes surface etching of the peel, which helps in rupturing the oil glands present below the cuticles and facilitates easy leaching of oil. Due to these reactive species, cold plasma has been applied for modifying biological materials, in terms of surface rupture, wettability and roughness, which are the characteristics closely related to the tendency of the migration of their containing substances from interior to surface during extraction. SEM was carried out to evaluate morphological changes of green tea leaves treated by the nitrogen CP. The untreated sample showed a normal and smooth undamaged surface, whereas the treated sample revealed fissures, cell ablation and ruptures of the surface. The roughness and granularity of the surface increased after the nitrogen CP. The results proved that highly rich reactive nitrogen species interacted with the surface of green tea during plasma treatment, in the way of cell wall physical bombardment. Water can easily penetrate to this surface and increase the extraction rate of the TPC through fissures and holes formed. The unique characteristics of cold plasma described above highlight its potential for agricultural by-products treatment in order to assist in the extraction of their bioactive compounds.
Cold plasma treatment process:
The different types of plasma generation system used in food industry are Dielectric barrier discharge (DBD), Plasma jet (PJ), Microwave (MW) plasma systems and Radio frequency (RF) plasma systems. The DBD plasma generated between two electrodes contain insulating material by way of electrical discharge. Plasma jet systems are capacitive coupled devices where gas is passed among two coaxial electrodes with high velocity, producing miniature “plasma fires” that typically originate in the radio frequency band. CD system is point-to-plate geometries (a strongly curved electrode positioned in the opposite direction of a flat electrode) and cylindrical arrangements.
RF-CP system create electromagnetic fields by providing RF currents flowing in coils or antennas, which are either submerged in plasma or isolated from it by a dielectric barrier. In MW plasma system, the electrons absorb the microwave energy, resulting in an increase in kinetic energy and inelastic collisions releasing energy as photons of UV and visible light which leads to ionization processes. For example, the tomato pomace treated with CP was generated by Dielectric Barrier Discharge (DBD) operated at the input voltage of 120 V (AC) at 60 Hz. Two 15.24-cm-diameter circular aluminum electrodes were respectively attached to the top and bottom dielectric barriers, creating a fixed gap of 5.2 cm for plasma generation. The plasma treatment, pomace powder (0.5 g) was homogeneously distributed at the bottom of a glass petri dish (8 cm diameter) covered with parafilm. The high voltage atmospheric cold plasma (HVACP) applied on the samples were exposed between two electrodes under 60-kV cold plasma for 5, 10 and 15 minutes and kept for further analysis.
After the Cold plasma treatment, the treated sample was subjected to extraction process to extract phenolic compounds or essential oil. Different authors followed different extraction methods. For instance, the green tea leaves treated were added into distilled water (100:1 ml/g) and placed in a dark and shaking water bath at 80°C for 30 minutes. Grape pomace (0.4 g) was well mixed with 10 ml of 50% (v/v) ethanol, then stirred continuously using an incubator shaker at 150 RPM. After incubation at room temperature for 2 hours, the mixture was centrifuged at 2000×g for 15 minutes. The supernatant was collected and stored at -80°C. Tomato pomace powder (0.3 g) was placed in a 50-ml centrifuge tube and well mixed with 12 ml of 50% (v/v) ethanol under vortex for 30 seconds. The mixture was stirred using an incubator shaker at 150 RPM at room temperature for 15 minutes before centrifugation at 2000 × g for 15 minutes. The supernatant was collected and concentrated under nitrogen flow. The concentrated extract was then freeze-dried and reconstituted with 3 ml of 50% ethanol. For essential oil extraction, hydro distillation is one of the methods of extraction. The treated lemon peel powder was taken in a round bottom flask and added with distilled water in the ratio of 1: 10, which was then distilled at 100°C in the Clevenger apparatus to extract the essential oil.
Application in the Food Industry for extraction:
Cold plasma has been widely considered as a green technology in the field of food processing, including in the applications for microbial decontamination and inactivation of unwanted enzymes, oil hydrogenation, structure and functionality of proteins, starch and lipids modification in food products. To the best of our knowledge, cold plasma has rarely been applied as pre-treatment to enhance nutrients extraction and its effect on polysaccharide extraction is still unknown.
Hydrophilic compounds can facilitate phenolic recovery from biomass. However, to the best of our knowledge, current application of cold plasma towards nutrients extraction has only been explored by one study on essential oil. Only few researches have been carried out in the extraction of bioactive compounds like TPC and Essential oil, which are discussed further. The nitrogen DBD atmospheric cold plasma at 15 W of generation power for 15 minutes (optimum conditions) caused an increase in the TPC and antioxidant capacity of green tea leaves by 41.14% and 41.06% respectively, as compared to the untreated sample. The increase in antioxidant activity of green tea after applying the CP might be due to increase in the catechin (by 103.12%), which has been proven to have antioxidant properties, owing to its chemical structure. However, a significant decrease in the TPC (16.83%) and antioxidant capacity (21.93%) was observed after the air cold plasma at the generation power of 5 W and treatment time of 5 minutes. Cell ablation and ruptures of the green tea leaf surface were observed after nitrogen DBD cold plasma. Although, no changes were observed in the green tea leaves colour parameter after the nitrogen cold plasma, a significant decrease in L* parameter of treated leaves was found. Based on the obtained results, the DBD cold plasma source used in this study could decrease the negative effects of reactive oxygen species on green tea, since nitrogen was used as the CP forming gas.
Similarly, the cold plasma applied to modify grape processing by-products aims to enhance bioactive compounds recovery. The results prove that HVACP pretreatment is a promising method to increase the extraction efficiency of phenolic compounds by changing surface properties of grape pomace. Disruption of cell structure as well as formation of hydrophilic surface of grape pomace were observed after being exposed to HVACP, resulting in higher extraction yields of phenolic compounds. The non-monotonous increases in TPC and TAC suggest potential phenolic oxidation associated with plasma reactive species. HVACP treatment also improved the antioxidant capacity of grape pomace extracts by producing different phenolic profiles with a higher concentration of anthocyanins. We found that HVACP treatments disrupted the cell wall structure of tomato pomace with formation of more hydrophilic surfaces, resulting in enhanced phenolic compounds extraction. Moreover, HVACP treatments enhanced the antioxidant capacity of tomato pomace extracts by changing their concentration profiles of phenolic compounds. In conclusion, the developed HVACP pre-treatment method successfully improved the phenolic compounds extraction from tomato pomace in terms of extraction efficiency and antioxidant capacity of extracts. The results of this study can serve as the groundwork for process optimization in order to maximize the phenolic compounds extraction yield and facilitate the design of pilot-scale plasma facility. In the meantime, further exploration is needed to gain more insights into the mechanisms of plant cell wall rupture caused by cold plasma and the interactions between phenolic compounds and plasma reactive species at molecular or atomic level.
These results demonstrate that the developed HVACP pre-treatment can successfully advance the extraction of bioactive compounds by not only increasing their yield but also improving their nutritional quality. HVACP can be a green approach, in terms of reduced chemical use, to effectively enhance the extraction efficiency of fenugreek galactomannan and modify its functional properties, thereby facilitating more diverse applications in both food and polymer industries.
The developed HVACP pre-treatment technology is a promising method for valourizing tomato processing by-products with high nutritional values. More treatments are required to be tested in order to provide scientific insights into the mechanisms and kinetics of cold plasma-induced surface modification and bioactive oxidation in plant materials. Furthermore, other operating parameters of cold plasma, such as working gas and voltage have to be studied to determine the optimal treatment for phenolic compounds extraction.
The findings of this study can serve as the groundwork to scale up the HVACP pre-treatment for valourizing more agricultural by-products with high nutritional values, while mitigating associated environmental burdens. Additionally, it has great potential for recovering other bioactives from different agriculture wastes, which can be utilized as functional food ingredients and nutraceuticals. To ensure that this process is affordable and also add profits, reviewing capital and operational costs would not be sufficient, but would actually require a comprehensive techno-economic analysis to be performed.
Table 1: Extraction of bioactive compounds by Cold Plasma technology
1. Bao, Y., Reddivari, L., & Huang, J.-Y. (2020a). Development of cold plasma pretreatment for improving phenolics extractability from tomato pomace. Innovative Food Science \& Emerging Technologies, 65, 102445.
2. Bao, Y., Reddivari, L., & Huang, J.-Y. (2020b). Enhancement of phenolic compounds extraction from grape pomace by high voltage atmospheric cold plasma. LWT, 133, 109970.
3. Faria, G. Y. Y., Souza, M. M., Oliveira, J. R. M., Costa, C. S. B., Collares, M. P., & Prentice, C. (2020). Effect of ultrasound-assisted cold plasma pretreatment to obtain sea asparagus extract and its application in Italian salami. Food Research International, 137, 109435.
4. Keshavarzi, M., Najafi, G., Ahmadi Gavlighi, H., Seyfi, P., & Ghomi, H. (2020). Enhancement of polyphenolic content extraction rate with maximal antioxidant activity from green tea leaves by cold plasma. Journal of Food Science, 85(10), 3415-3422.
5. Mehta, D., Yadav, K., Chaturvedi, K., Shivhare, U. S., & Yadav, S. K. (2022). Impact of Cold Plasma on Extraction of Polyphenol From De-Oiled Rice and Corn Bran: Improvement in Extraction Efficiency, In Vitro Digestibility, Antioxidant Activity, Cytotoxicity and Anti-Inflammatory Responses. Food and Bioprocess Technology, 1–15.
6. Pragna, C. H., Gracy, T. K. R., Mahendran, R., & Anandharamakrishnan, C. (2019). Effects of microwave and cold plasma assisted hydrodistillation on lemon peel oil extraction. International Journal of Food Engineering, 15(10).
7. Rashid, F., Bao, Y., Ahmed, Z., & Huang, J.-Y. (2020). Effect of high voltage atmospheric cold plasma on extraction of fenugreek galactomannan and its physicochemical properties. Food Research International, 138, 109776.
8. Rezaei, S., Ghobadian, B., Ebadi, M.-T., & Ghomi, H. (2020). Qualitative and quantitative assessment of extracted oil from Camelina sativa seed treated by dielectric-barrier discharge cold plasma. Contributions to Plasma Physics, 60(9), e202000032.
9. Zhang, X.-L., Zhong, C.-S., Mujumdar, A. S., Yang, X.-H., Deng, L.-Z., Wang, J., & Xiao, H.-W. (2019). Cold plasma pretreatment enhances drying kinetics and quality attributes of chili pepper (Capsicum annuum L.). Journal of Food Engineering, 241, 51–57.
About the Authors:
Sangeetha Karunanithi*, Gnana Moorthy Eswaran, Proshanta Guha & Prem Prakash Srivastav
Department of Agricultural & Food Engineering,
Indian Institute of Technology, Kharagpur,
West Bengal – 721302, India.
Email ID: email@example.com
The views/opinions expressed by authors on this website solely reflect the author(s) and do not necessarily reflect the views/opinions of the Editors/Publisher. Neither the Editors nor the Publisher can be held responsible and liable for consequences that may arise on account of errors/omissions appearing in the Articles/Opinions.