Pharmacological Activity of Chemical Compounds of Potato Peel Waste (Solanum tuberosum L.) in vitro: A Scoping Review

Introduction

Potato (Solanum tuberosum L.) is a member of the Solanaceae family of short-lived tuber plants, exhibiting varying shapes that range from round to oval. Additionally, the color of potatoes varies depending on the variety. Worldwide, there are approximately 5000 varieties of potatoes, and Indonesia alone has 34 types, including Granola, Atlantic, Cipanas, and Margahayu, among others.^1,2 Apart from being a source of 80% carbohydrates and one of the most consumed types of food by the world population, potatoes contain essential compounds crucial for maintaining good health. Notably, the composition of the compound depends on variety and growing conditions. They are rich in flavonoids and vitamin C, which function as antioxidants.^3–6 Additionally, potatoes contain anthocyanin, which functions as an antioxidant, anti-inflammatory, and antimicrobial. Potatoes are used as traditional herbal medicine in Indonesia, but there are no references written about them.⁷

Despite the general practice of discarding potato peels and utilizing only the inner part, a recent study has reported that potato peels are rich in polyphenol compounds, surpassing the content found in the flesh. Understanding the pharmacological properties of these compounds could open up new avenues for utilizing potato peels in food and medicine, promoting more sustainable and beneficial practices. This study aims to conduct an extensive review of various publications that investigate the in vitro content of potato peel compounds and their pharmacological potencies in the last ten years, so that it can provide information for further research because there have been no articles reviewing in vitro potato peel research before.

Materials and Methods

Protocol and Eligibility Criteria

This scoping review was conducted following the Preferred Reporting Items for Scoping Reviews and Meta-Analyses (PRISMA) method (Figure 1). The research questions were formulated using the Population, Intervention, Comparison, and Outcome (PICO) method. Specifically, the population of interest was focused on potato peels; the intervention under investigation involved compounds derived from potato peels; the comparison was made against a placebo; and the primary outcome of interest was the pharmacological activity of these compounds.

The inclusion criteria for this scoping review included research articles published within the last 10 years (2012–2022), in vitro research involving potato peel compounds, articles indexed in Indonesian or English, and English full-text availability. Based on these inclusion criteria, clinical research studies with only abstracts available were excluded, as they may not provide enough comprehensive data on the specific in vitro content and pharmacological activity of potato peel compounds.

Article selection and data extraction, according to the inclusion criteria, were carried out meticulously to ensure the integrity and comprehensiveness of the scoping review by two reviewers independently. The results of their evaluations were recorded in a table for further analysis. Journal articles were obtained through electronic searches as well as manual searching. The data obtained went through several stages of review. In the preliminary stage, abstracts were selected based on the relevance of the title and abstract to the research topic. The second stage involved further selection based on the content of the articles and adherence to the inclusion and exclusion criteria. Finally, the selected data from the eligible articles was extracted and processed for analysis.

Information Resources and Search

The article search strategies employed a comprehensive approach to gather relevant research articles. Relevant keywords such as “potato” or “Solanum tuberosum” and “peel” or “skin” were used, and advanced search techniques with “AND” and “OR” operators were utilized to refine the search results. The search was focused on electronic resources, including Scopus, ScienceDirect, EBSCOhost, and Portal Garuda. To ensure up-to-date information, the search was limited to articles published between 2012 and 2022. Portal Garuda, an online library in Indonesia, played a crucial role in the search as it houses research conducted in Indonesia and publishes it in Science and Technology Index (SINTA)-accredited journals. Articles sourced from Portal Garuda were considered to be from reputable journals, as SINTA is a journal accreditation issued by the Indonesian Ministry of Research and Technology.

Results

The PRISMA (Figure 1) diagram illustrates the systematic article selection process for the scoping review. Initially, a total of approximately 4773 articles were obtained from four search engines: Scopus, ScienceDirect, EBSCOhost, and Portal Garuda. After undergoing screening and selection based on inclusion and exclusion criteria, a total of 14 articles were deemed suitable for a review. Subsequently, the 14 selected articles were thoroughly read and analyzed to extract the relevant data. This data was then organized and recorded in Table 1.

Figure 1 Flowchart of Preferred Reporting Items for Scoping Reviews and Meta-Analyses (PRISMA).

Table 1 Characteristic Distribution of Articles on Potato Peel

Among these articles, 12 utilized potato peel extract preparations in their research, while two articles used potato peel fraction preparations. Additionally, six articles provided information on the specific varieties of potato peel used in the research, whereas the remaining articles referred to potato peels without specifying the variety. The majority of researchers focused on utilizing potato peel extract for its antioxidant and anti-inflammatory properties. Some studies also examined additional aspects such as antioxidant and antibacterial or antioxidant and antiviral properties. Moreover, anti-aging and anti-hyperglycemic properties were explored in certain studies, along with anti-viral effects.

Discussion

The analysis of potato peel content revealed the presence of phenolic compounds, flavonoids, and anthocyanins, with higher levels of flavonoids observed in potato peels compared to the flesh itself. The reviewed articles predominantly reported on the antioxidant activity of potato peel extract preparations.^8–16 The high antioxidant activity of potato peels can be attributed to the presence of vitamin C, flavonoids, polyphenols, and anthocyanins.^4–6 Vitamin C is an antioxidant that produces collagen, which is vital for maintaining skin health and facilitating wound healing by stimulating fibroblast formation. Increased fibroblast activity can accelerate the wound healing process.¹⁷ The polyphenolic compound in potato peels can help prevent cell mutations that may lead to malignancy, while the antioxidant in potato peels can counteract the effects of free radicals.¹⁶ Flavonoids, as reported in the research, exhibit antioxidant, anticarcinogenic, antibacterial, anti-inflammatory, anti-allergic, analgesic, and antiviral properties.^12,18 Flavonoids act as antioxidants by neutralizing reactive oxygen species (ROS). Free radicals suppress the level of antioxidants, which leads to oxidative instability, known as oxidative stress. This condition is characterized by an increase in lipid peroxidation, as measured by the marker malondialdehyde, due to an excess of lipid peroxidase. Flavonoids exhibit antioxidant activity by binding to free radical ions and preventing the formation of singlet oxygen. With multiple phenol groups (–OH and aromatic groups) and double bonds, flavonoids can counteract free radicals by donating one –OH hydroxyl group.¹⁴ The mechanism can occur directly or indirectly. In indirect action, flavonoids donate one hydroxyl ion to bind to free radicals and neutralize their toxic effects. Indirectly, flavonoids increase the expression of endogenous antioxidants by activating nuclear factor-erythroid 2 related factor 2 (Nrf2) genes, which are involved in the synthesis of endogenous antioxidant enzymes such as superoxide dismutase (SOD).¹⁴

Potato peel chemical compounds also have anti-inflammatory properties.^8,18–20 Anti-inflammatory activity is the second most prominent after antioxidants. Several researchers have conducted studies simultaneously on anti-inflammatory and antioxidant effects, carried out by Wahyudi (2020) and Sampaio (2021). These studies reported that potatoes contain anthocyanin compounds besides flavonoids, which act as both antioxidants and anti-inflammatory agents. In other studies, anthocyanins have been found to contribute to the inhibition of collagen aging by preventing cell damage, promoting vascularization, initiating DNA synthesis, and playing a role in fibronectin and fibroblast synthesis, suggesting their potential in the wound healing process.^7,18,21 The anti-inflammatory mechanism of flavonoids involves the inhibition of the cyclooxygenase and lipoxygenase pathways, as well as the inhibition of histamine and leukocyte accumulation, which are crucial in overcoming inflammation and allergic symptoms. Flavonoids can also suppress the secretion of arachidonic acid and lysosomal enzymes.^22–25 According to al-Kayri (2022), flavonoids target the NF-κB, MAPK, ERK, and Akt pathways to reduce inflammation and oxidative stress. Furthermore, anthocyanins and flavonoids have been shown to reduce proinflammatory cytokines such as TNF-α, IL-6, IL-8, IL-1β, IL-17, and IFN-γ.^22,25,26

The antibacterial activity of potato extract has been investigated in studies conducted by Amanpour (2015), Gebrechristos (2020), and Helal (2020). Potato peel extract has been shown to effectively inhibit the growth of Propionibacterium, gram-positive and gram-negative bacteria, and Candida albicans.^10,16,27 Amanpour’s research (2015) specifically indicates that potato peel extract has a greater impact on gram-positive than gram-negative bacteria.²⁸ Flavonoids, anthocyanins, solasodine, and solanidine in potato peel extract exhibit strong antimicrobial and antioxidant properties. Silva’s study (2017) found that derivatives of phenolic compounds, namely chlorogenic and caffeic acids, have a role in antiviral and antimicrobial activity.^12,16 Phenolic derivatives of potato peel extract, such as quercetin, interfere with the processing of retroviral (RV) polyproteins by RV proteases, which are essential for the inactivation of RNA polymerase. Consequently, these compounds can interfere with viral replication through multiple mechanisms. Caffeic and chlorogenic acids inhibit viral capsids, and this activity may stem from the presence of free hydroxyl and ester groups.^12,16 The higher effectiveness of antimicrobial activity against gram-positive bacteria can be attributed to the differences in cell wall structure, with gram-negative bacteria possessing a layered cell wall compared to gram-positive bacteria.¹⁶

Another benefit of potato peel extract is its anti-aging activity. The extract activates PI3K/Akt and MAPK/ERK signaling through TGF-β receptors, which then stimulate type I collagen synthesis by fibroblasts.²⁹ Previous studies have also reported that ascorbic acid, various phenolic compounds, and flavonoids stimulate type I collagen synthesis in the dermis.²⁹

Glycoalkaloid compounds and flavonoids present in potato peels also possess antihyperglycemic properties.⁶ Quercetin, a type of flavonoid, plays a role in maintaining the normal function of pancreatic beta cells, thereby controlling fasting blood sugar levels and postprandial hyperglycemia. Additionally, quercetin can also inhibit the α-glucosidase enzyme and inhibit glucose transport by GLUT2. GLUT2 (Glucose Transporter 2) is responsible for carrying glucose from the digestive tract into the bloodstream, and inhibiting this transporter helps regulate blood sugar levels.^14,27 Arun (2015) reported that ethyl acetate extract from potato peel increased glucose absorption in the blood through the GLUT4 transporter in skeletal muscle and adipose cells.¹⁴

Conclusion

Potato peels are rich in potential compounds that offer various health benefits, particularly in young potato peels. Flavonoids, phenolic compounds, and glycoalkaloids have been extensively studied for their antioxidant and anti-inflammatory activities, primarily in the form of ethanol extracts. Further research is needed to understand the specific mechanism of action for each compound in potato peels and explore their potential for other pharmacological activities beyond antioxidation and anti-inflammation.