Bioactive compounds in potato peels, extraction methods, and their applications in the food industry: a review

ABSTRACT

The potato is a tuber with high global demand and importance in the human diet. It is used by food industries to manufacture processed products (French fries, chips, and purees), and consequently, significant quantities of potato peel are generated as a by-product. Potato peels are rich in compounds like anthocyanins, glycoalkaloids, phenolic compounds, and flavonoids. These bioactive compounds are associated with good health because they protect the body’s cells from oxidative processes and have antioxidant, anti-inflammatory, antitumor, and anticancer properties. Hence, new methods to recover these compounds and their application to food matrices are being explored. This review aims to raise awareness of the value of potato peels as a source of different phenolic compounds and the need to recover them. The review highlights the most recent research on the use of this by-product in different foods to add value to the final composition or improve technological and nutritional properties.

KEYWORDS:

• Potato peels
• bioactive compounds
• phenolic compounds
• food applications
• by-product
• side stream

1. Introduction

The potato (Solanum tuberosum L.) is was first domesticated in the Andes Mountains of South America, where over 4,000 varieties exist (CIP, 2023). After wheat, rice, and corn, it is the 4th most important agricultural product worldwide and is an important part of the human diet (Benkeblia, 2020). Annual increases in potato consumption have prompted the food industry to process potatoes for manufacturing new products such as French fries, chips, and frozen and mashed potatoes. Consequently, these sectors generate large amounts of potato peel (PP), which accounts for nearly 10% of total waste and between 15% and 40% of the tuber, depending on the peeling methods (Rodríguez-Martínez et al., 2021). Currently, industries are looking for new ways to salvage this waste and utilize its constituents (Akyol et al., 2016; Sabeena Farvin et al., 2012).

The study of food waste and by-products as a source of different bioactive compounds has been a topic of great interest in recent years because it has been demonstrated that they have antimicrobial and antioxidant properties. Potato by-products are promising raw materials for sustainable production and future applications (Rodríguez-Martínez et al., 2021). However, when potatoes are processed they are peeled do not use to recover these by-product, generating a significant quantities of residues which could lead to environmental and and sanitation problems (Xu et al., 2022). Traditionally, PP was used to produce low value animal feed, fertilizer and biogas; however, the important content of phenolic compounds were wasted (Javed et al., 2019). In order to justify the recover of this residue, extraction of bioactive compounds from PP has been studied, and different authors affirm that this is an optimal way to valorize residues that are usually discarded (Sampaio et al., 2020).

There is a great interest in using bioactive compounds from natural sources as functional ingredients to protect cells from oxidative damage and reduce the risk of degenerative diseases associated with oxidative stress. Previous research has shown that PP is a good source of dietary fiber, phenolic acids, flavonoids, glycoalkaloids, anthocyanins (in coloured varieties) essential amino acids, vitamins (C, B₁, B₂, B₃), and minerals (calcium, phosphorus and iron) (Akyol et al., 2016; Joshi et al., 2020). Several extraction techniques, including the use of green solvents, have been employed over time, with each one improving specific aspects such as efficacy, speed, and sustainability, among others, to recover bioactive compounds as much as possible. Economic viability is another important aspect to be considered in extraction methods to ensure that the compound can be recovered quickly and efficiently (Apel et al., 2020). With the pass of the time and the improvement of the extraction techniques, the yield of total phenolic compounds extracted was increasing by creating more permeation, maceration and tissue exposure to the solvent (Joshi et al., 2020). The most commonly employed methods are solid-liquid extraction and ultrasound-assisted extraction due to their simplicity and operability. Relatively newer methods employ other solvents or technologies, such as supercritical fluids, microwaves, pulsed electric fields, or pressurized liquids. However, these new methods have some limitations, such as the lack of experimental data and models to describe the obtained curves (Xu et al., 2022). The PP extracts have been used in several sectors, such as food, pharmaceuticals, healthcare, biotechnology, animal feed, renewable energy, etc. The objective of these industries is to identify ways to utilize the bioactive compounds present in the peels to produce by-products with significant economic and sustainable benefits. However, the development of industrial processes to use this waste is currently too lengthy and requires substantial investments (Javed et al., 2019).

This review aims to help re-evaluate the use of PP and summarizes recent research on its nutritional value, constituent compounds, and potential applications in the food industry.

2. Proximate composition of potato peel

The potato is a well-known tuber worldwide, and its place of origin, Peru, has over 4,000 varieties of native potatoes, with most of them growing in the Andes and varying in size, color, and shape. There are also over 180 varieties of wild potatoes that are bitter to eat and highly resistant to pests, diseases, and weather conditions. Therefore, the composition of the potato will vary depending on agronomic and environmental factors (CIP, 2020; Kumari et al., 2017). Table 1 shows the proximate compositions (g/100 g) of PPs determined in different studies.

Table 1. Proximal composition (g/100 g) of potato peels (FW) from different varieties.

Varieties	Moisture	Total carbohydrates	Proteins	Fats	Ash	Reference
Puma Makin	72.7	21.3	4.4	0.4	1.2	Bellumori et al. (2020)
Leona	76.8	18.3	3.2	0.6	1.1
Yawar Manto	74.4	21.6	3.4	0.2	1.0
Añil	66.2	29.7	2.7	0.1	1.3
Sangre de Toro	61.9	33.3	3.4	0.4	1.0
Qequrani	72.4	22.8	3.3	0.1	1.4
Spunta	82.5	12.2	2.0	0.1	0.8	Jiménez et al. (2009)
Collareja	75.9	17.8	2.5	0.1	1.2
Señorita	81.1	12.6	2.3	0.4	1.1
Imilla Negra	70.2	22.6	1.9	0.3	1.7
Imilla Colorada	73.7	20.5	2.5	0.5	1.3
Runa	74.9	19.1	1.6	0.3	0.9
N.D	84.3	9.5	2.6	0.15	0.8	Zhivkova (2021)
N.D	83.3–85.1	8.7–12.4	1.2–2.3	0.1–0.4	0.9–1.6	Javed et al. (2019)

According to the analysis of proximate composition, carbohydrates are the most abundant compounds in PP, with levels ranging from 8.7 to 33.0 g/100 g FW (fresh weight). Besides, other studies reported most carbohydrates under the form of polysaccharides and a minor amount under the form of simple sugars, Choi et al. (2016) found fructose in PP ranging from 433 to 683 mg/100 g DW (dry weight), glucose from 566 to 723 mg/100 g; and, to a lesser extent, sucrose, ranging from 290 to 427 mg/100 g.

The protein content of PPs could be variable, according to Table 1 it could be from 1.2 to 4.4 g/100 g of fresh sample; similar results were obtained by Calliope et al. (2018), who evaluated forty-four genotypes of Andean South American potatoes and obtained a wide range of 1.9 to 4.8 g/100 g. The proteins in PP, despite their low content, have an important biological value, and their recovery requires their addition to high-consumption food products (bread, cakes, pastas, etc.) (Calliope et al., 2018). Maxwell et al. (2019) evaluated the use of Saccharomyces cerevisiae for increasing crude protein content via solid-state fermentation and nitrogen supplementation with ammonium sulfate and urea salts. The results showed significant protein enrichment with both supplements, with ammonium sulfate being more effective when used with Saccharomyces cerevisiae at 30°C, pH of 5.5, for 3 days, and humidity levels at 40%. Additional studies are required to discover new supplements and potential food applications.

In terms of lipids and ash, the content of both in PPs is extremely low, and there have not been many studies on them yet. The lipid fraction can include components such as long-chain fatty acids, alcohols, triglycerides, and sterols (Javed et al., 2019). Moreover, unsaturated fatty acids, such as omega-3 and omega-6, have been found in small amounts (Wu et al., 2012).

The mineral content in PP is typically between 0.9% and 1.6% (DW) (Gaudino et al., 2020), in which important minerals would be found, such as zinc, potassium, sodium, calcium, iron, and magnesium (Dusuki et al., 2020). Bellumori et al. (2020) found lower values of mineral content in the PPs, ranging from 1.0% to 1.4%, in which potassium was the most abundant in their samples, and having a range of 12,418 to 17,388 µg/g DW. Similarly, Zhivkova (2021) reported the content of different mineral (DW) present in PP, potassium as the predominant with a content in of 4959 µg/g, followed by sulfur (390 µg/g), phosphorus (378 µg/g), and other minerals in minor concentrations (magnesium, calcium, iron).

3. Bioactive compounds

3.1. Phenolic compounds

PPs are an excellent source of phenolic compounds since almost 50% of these molecules are situated in the peel and adjoining tissues (Rodríguez-Martínez et al., 2021). These compounds are responsible for functions such as ultraviolet (UV) protection, pigmentation, disease resistance, and plant defence against pathogens (Al-Weshahy et al., 2013; Manach et al., 2004; B. Singh et al., 2020). Phenolic acids are compounds of major interest in PP; hence, several authors have used different extraction methods to quantify total polyphenols, with solvents including methanol, ethanol, and acetone being the most common. Potato contains polyphenols consisting of phenolic acids, flavonoids, and anthocyanins, which will be described in the following sections.

3.1.1. Phenolic acids

Phenolic acids are the main group of phenolic compounds in PPs. They are divided into cinnamic acids (chlorogenic acid, caffeic acid, p-coumaric acid, ferulic acid, and their derivatives) and hydroxybenzoic acids (vanillic acid, protocatechuic acid, gallic acid, p-hydroxybenzoic acid, and their derivatives) (Rodríguez-Martínez et al., 2021). Other compounds that were found in the PPs are syringic acid, coniferyl alcohol, and coniferyl aldehyde, which are involved in the plant’s defence mechanism against pathogen attack (Oertel et al., 2017; Rasheed et al., 2022). Table 2 summarizes the concentration of various phenolic acids present in different PPs, with chlorogenic acid being the most abundant phenolic acid, with concentrations ranging from 8.3 to 601.3 mg/100 g DW. The substantial variation in chlorogenic acid content can be attributed either to the genotype or extraction method. Martínez-Inda et al. (2023) used a mixture of ethanol:water (96:4 v/v) with a solid:liquid ratio of 1:100 (w/v); after extraction, the solvent was removed by rotatory evaporation, thereby increasing the percentage of chlorogenic acid and other phenolic acids, and obtaining 601.3 mg chlorogenic acid/100 g DW sample of PP. In contrast, Lakka et al. (2020) reported a concentration of 8.3 mg chlorogenic acid/100 g DW sample of PP, using a batch stirred-tank solid-liquid extraction process with hydroxypropyl β-cyclodextrin. The comparison showed that a mixture of ethanol:water is ideal for obtaining high yields of phenolic compounds. Also, the content of caffeic acid was very low compared to the other studies shown in Table 2, except for ferulic acid.

Table 2. Phenolic acids present in peels of different potatoes varieties (mg/100 g) (DW).

PP variety	Chlorogenic acid	Caffeic acid	p-coumaric acid	Ferulic acid	Gallic acid	Reference
N.D (~)	601.3 ± 0.19	129.7 ± 0.08	3.1 ± 0.002	15.2 ± 0.01	N.D	Martínez-Inda et al. (2023)
HeiJingang	90.5 ± 0.15	69.7 ± 0.01	15.70 ± 0.04	2.9 ± 0.01	19.1 ± 0.02	Makori et al. (2022)
Shepody	43.5 ± 0.05	27.2 ± 0.01	N.D	N.D	11.2 ± 0.05
Favorita	73.8 ± 0.30	46.9 ± 0.27	N.D	N.D	17.2 ± 0.05
Kexin No. 1	61.1 ± 0.04	44.8 ± 0.96	N.D	N.D	13.3 ± 0.03
Maris Piper (~)	387	92	N.D	N.D	N.D	De Andrade Lima et al. (2021)
Spunta (~)	8.3 ± 0.54	8.9 ± 0.28	N.D	10.8 ± 3.52	N.D	Lakka et al. (2020)
Fianna	346.0 ± 2.14	332.5 ± 3.67	N.D	3.2 ± 0.05	233.5 ± 9.78	Silva-Beltrán et al. (2017)

N.D: Not determined.

Chlorogenic acid is the main phenolic acid present in PPs; its concentration usually ranges from 35.21% to 81.87% of the total phenolic compounds (Rasheed et al., 2022). In the case of caffeic acid, only the colored varieties contain concentrations of up to 19.7% (Valiñas et al., 2017). Furthermore, different researchers found that potatoes with colored peels contain 3–4 times the concentration of phenolic acids as those with uncolored peels (Ben Jeddou et al., 2017; Brahmi et al., 2022; Valiñas et al., 2017), which may be related to the method of extraction, conditions, and employed solvents (Sampaio et al., 2020). These phenols could be recovered and used to develop new value-added products, maximizing the potato’s potential (Navarre et al., 2009).

Phenolic acids have been deeply studied, and the results have shown that these compounds may have significant health benefits, particularly the chlorogenic and caffeic acids found in higher concentrations in the PPs. According to Tajik et al. (2017), chlorogenic acid has potent antioxidant, antidiabetic, anti-inflammatory, and antibacterial properties. Other studies have reported effects related to the treatment of overweight by improving glucose tolerance (Rodriguez de Sotillo et al., 2006) and increasing lipid metabolism, thereby inhibiting cholesterol synthesis and reducing cholesterol absorption (Kumar et al., 2020). Mature potatoes have a high level of ethyl acetate fraction, making the inhibition of α-glucosidase enzymes and glucose uptake more efficient; hence, the anti-obesity effect of potatoes should be considered (Arun et al., 2015).

Caffeic acid at a concentration of 10 mg/100 g per day helps reduce oxidative damage to the brain exposed to methotrexate (2 mg/100 g) that is administered either as part of therapy for cancer or other chronic diseases (W. Shen et al., 2012). Moreover, it has been demonstrated that caffeic acid has remarkable activity against hepatocarcinoma (fatal liver cancer) via its antioxidant and chelating activity with metals such as iron and copper and inhibiting the formation of free radicals (Espíndola et al., 2019). Caffeic acid also has cardiovascular benefits due to its vasorelaxant properties. Its derivatives slow the heart rate, thereby lowering the risks of high blood pressure and hypertension (Silva & Lopes, 2020).

3.1.2. Anthocyanins

These compounds are responsible for the color of the PPs, their anthocyanins are water-soluble and stable in acidic environments, as well as in neutral or slightly alkaline environments (Rytel et al., 2021). Anthocyanins have been found in plants with different chemical structure due to terminal modifications (glycosylation, acylation, methylation, etc); however, in nature there are six anthocyanidins (sugar-free counterparts of anthocyanins) widely distributed which are peonidin, petunidin, cyanidin, malvidin, delphinidin, and pelargonidin (Bvenura et al., 2022; Rytel et al., 2021; Valiñas et al., 2017).

In potatoes, the content of each anthocyanidin varies depending on the color of the peel. For example, those with red peels have primarily acylated glycosides of pelargonidin in the range of 20 to 200 mg/100 g FW. In the purple varieties, petunidin and pelargonidin acylated glucosides reach levels of 200 to 500/100 g (Lachman & Hamouz, 2005; Peña & Restrepo, 2013). The results of the identification of certains anthocyanidins in colored PPs (purple and red varieties) are shown in Table 3 being the most detected: petunidin, pelargonidin, peonidin, and malvidin. Makori et al. (2022) found peonidin and pelargonidin in all PP samples, but only malvidin was detected in the HeiJingang potato, suggesting that this anthocyanin is only found in the purple varieties of potatoes. Similarly, pelargonidin was found only in HeiJingang and Favorita potatoes, at concentrations of 7.7 and 0.2 mg/100 g DW, respectively. Bellumori et al. (2020) evaluated the content of anthocyanidins in different PPs; petunidin was the major anthocyanin present in concentrations ranging from 22.1 to 4.3 mg/100 g DW. However, malvidin was not detected in any potato, and pelargonidin was detected in Yawar Manto (6.2 mg/100 g DW) and Sangre de Toro varieties (12.4 mg/200 g DW) only. Sampaio, Petropoulos, et al. (2021) research phenolic compounds in ten coloured potato peels, they could identify up to six peaks of acylated anthocyanidin in red (Rosemary, Red Cardinal, and Red Emmalie) and purple potato varieties. Petunidin and malvidin were detected in all samples of PPs, whereas peonidin was detected only in the red-colored potatoes. The principal anthocyanidin in the purple varieties of potato was petunidin, with concentrations ranging from 37.8 to 107.6 ± 0.6 mg/100 g DW. Also, malvidin was detected in all purple-colored potatoes except for Shetland Black. Overall, the authors found that purple-skinned potatoes, with a total anthocyanin content of 1.3 mg/100 g DW, had a higher content of anthocyanins in their peels than red-skinned potatoes.

Table 3. Anthocyanidins compounds present in peels of different potatoes varieties (mg/100 g) (DW).

PP variety	Petunidin	Pelargonidin	Peonidin	Malvidin	Reference
HeiJingang	32.3 ± 0.22	7.7 ± 0.03	26.1 ± 0.10	6.3 ± 0.44	Makori et al. (2022)
Shepody	0.4 ± 0.10	N.D	1.9 ± 0.0	N.D
Favorita	5.5 ± 0.01	0.2 ± 0.01	2.2 ± 0.01	N.D
Kexin No. 1	0.9 ± 0.03	N.D	2.8 ± 0.01	N.D
Puma Makin (~)	4.3 ± 0.71	N.D	3.1 ± 0.23	N.D	Bellumori et al. (2020)
Leona (~)	12.14 ± 0.36	N.D	4.5 ± 0.13	N.D
Yawar Manto (~)	N.D	6.2 ± 3.94	8.2 ± 0.2	N.D
Anil (~)	22.1 ± 0.75	N.D	5.2 ± 0.17	N.D
Sangre de Toro (~)	N.D	12.4 ± 1.8	N.D	N.D
Qequrani (~)	N.D	N.D	N.D	N.D
Rosemary (~)	N.D	27.7 ± 0.06	24.8 ± 0.4	N.D	Sampaio, Petropoulos, et al. (2021)
Red Cardinal (~)	N.D	9.9 ± 0.2	43.8 ± 0.13	N.D
Red Emmalie (~)	N.D	5.8 ± 0.06	21.2 ± 0.03	20.5 ± 0.1
Purple (~)	107.6 ± 0.6	N.D	N.D	29 ± 0.2
Kefermarkter Blaue (~)	81.8 ± 1.2	N.D	N.D	21.6 ± 5.4
UACH 0917 (~)	37.8 ± 0.41	1.6 ± 0.02	N.D	86.8 ± 0.68
Salad Blue (~)	92.9 ± 0.9	N.D	N.D	45.8 ± 0.1
Blaue aus Finnland (~)	76.2 ± 0.8	N.D	N.D	22.1 ± 0.2
Shetland Black (~)	50.3 ± 0.9	2.3 ± 0.04	45.9 ± 0.52	N.D
Violetta (~)	61.4 ± 0.41	3.9 ± 0.002	N.D	21.1 ± 0.1

N.D: Not determined.

Different studies have found that the anthocyanin concentration in PP is usually higher than that in the pulp, which can be more than ten times higher. Albishi et al. (2013) determined that the purple potato skin contained 6.8 ± 4.03 mg/100 g (DW) of anthocyanin, while the pulp contained 0.6 ± 0.17 mg/100 g (DW). This indicated that the peel contained 10.69 times more anthocyanins than the pulp. Yin et al. (2016) evaluated the peels and pulps of colored potatoes from China and determined that the average anthocyanin content in the peel was 15.34 times higher than in the pulp. This confirms that the anthocyanin content is usually higher in the peels, the research is helpful for further studies for the recovery of PP.

Anthocyanins can be used to replace synthetic colorants with natural ones; however, the natural anthocyanins can be altered due to the different processes to which they are subjected in the food industry, if they undergo thermal treatments (such as baking, frying, or cooking) they can degrade (Rasheed et al., 2022). Tierno et al. (2015) observed a decrease in the anthocyanin content of cooked potatoes from a initial content of 253 ± 1.88 mg equivalents of cyanidin 3-O-glucoside per 100 g of DW (mg CGE/100 gr) to 208 ± 3.33 mg CGE/100 g after cooking, possibly due to its retention in the water. T. Jiang et al. (2019) investigated the degradation of anthocyanins in purple sweet potato peels at 90°C at different pHs ranging from 3.0 to 7.0; after 24 h of heating, the anthocyanin content was found to be at its lowest at pH 3, indicating an obvious degradation. The degradation of anthocyanins usually follows a first-order reaction kinetics, occurring more rapidly at low pH. In addition, the reduction in anthocyanins caused by changes in the pH of the solution could cause the hue to shift from purple/red to brown.

3.1.3. Flavonoids

Flavonoids are secondary metabolites composed of two benzene rings attached to a heterocyclic pyran ring. They can be found in fruits, vegetables, and different food crops, and play an essential role in plant responses such as the coloring of flowers to attract insects or protection against biotic and abiotic sensors of the environment (N. Shen et al., 2022). Important biological properties, such as antioxidant, antimicrobial, and cardioprotective, are possessed by these compounds. Consumption of these compounds improves brain and heart health, strengthens the immune system, and protects against oxidative stress (Forni et al., 2021; Samtiya et al., 2021). Some well-known flavonoids are flavonols, flavanols, and flavanones, such as quercetin, catechin, and hesperetin, respectively (Yalcin & Çapar, 2017), according to a botanical taxonomic classification made by Peterson and Dwyer (1998) where they determined the potential flavonoid content in one hundred ninety foods, it can be classified as low (0.01–3.99 mg/100 g), medium (4.0–9.9 mg/100 g) and high (>10.0 mg/100 g); the botanical classification is an useful way to assert the presence of flavonoids in food but still imprecise. Moreover, estimating a recommended daily intake of these compounds is difficult because of their variable distribution in vegetables and because their bioavailability is affected by different factors such as molecular weight, glycosylation, metabolic conversion, and interaction with the microbial flora (Thilakarathna & Rupasinghe, 2013).

Potato varieties with colored peels have twice the flavonoid content compared to the white varieties, with catechin being the most abundant compound, ranging from 0 to 204 mg/100 g DW (Akyol et al., 2016). Quercetin, rutinose, and rutin are also present but in smaller amounts. Meanwhile, the flavonol content of white varieties is usually higher. Therefore, the concentration of these phenolic compounds is highly dependent on the color of the peels and the potato cultivars (Rasheed et al., 2022).

3.2. Glycoalkaloids

Glycoalkaloids are secondary metabolites that protect potato plants against microorganisms, viruses, and insects because of their toxicity toward these agents. The main glycoalkaloids present in PPs are α-solanine and α-chaconine, and their ingestion cannot exceed 3–5 mg/kg of body weight due to the risk of colic and other stomach problems such as diarrhea, vomiting, and gastroenteritis (Alves-Filho et al., 2018; Rodríguez-Martínez et al., 2021). The Table 4 shows research of glycoalkaloid extraction from PP, Friedman et al. (2017) found an α-chaconine concentration in the range of 424–1297 µg/g DW in the PPs of conventional potatoes and 610–2830 µg/g DW in organic potatoes, also reported the α-solanine concentration to be 215–412 µg/g DW in conventional peels and 239–750 µg/g DW in organic peels. For its part, Jin et al. (2018) evaluated three PP of different species where Shepody and Atlantic varieties had a final content of total glycoalkaloids of 6.71 and 71.17 µg/g DW, respectively which are lower compared the other studies of Table 4 possibly due to environmental and storage-related factors. Hence, in industrial processes, it is better to employ varieties with a lower content of glycoalkaloids because a higher content could be toxic for human consumption. Finally, Eraso-Grisales et al. (2019) found a content of total glycoalkaloids of 213,4 µg/g DW in Ratona Morada variety, while Apel et al. (2020) reported a final content of total glycoalkaloids of 766.3 µg/g in Rooster variety.

Table 4. Concentration of glycoalkaloids (α-chaconine and α-solanine) in potato peels (DW).

Potato variety	α-chaconine (µg/g)	α-solanine (µg/g)	Total (ug/g)	Reference
Conventional gold	670 ± 130	253 ± 44	920 ± 140	Friedman et al. (2017)
Conventional red	1297 ± 56	412 ± 24	1709 ± 61	Friedman et al. (2017)
Conventional Russet	424 ± 30	215 ± 43	639 ± 52	Friedman et al. (2017)
Organic gold	2830 ± 370	750 ± 120	3580 ± 390	Friedman et al. (2017)
Organic red	610 ± 110	239 ± 34	850 ± 120	Friedman et al. (2017)
Organic Russet	1180 ± 110	374 ± 54	1550 ± 120	Friedman et al. (2017)
Shepody	5.06 ± 0.33	1.65 ± 0.94	6.71	Jin et al. (2018)
Russet Burbank	143.16 ± 1.87	231.46 ± 10.29	374.62	Jin et al. (2018)
Atlantic	51.80 ± 0.58	19.37 ± 0.46	71.17	Jin et al. (2018)
Ratona Morada	105,6 ± 0.13	107,8 ± 0.26	213,4 ± 0.24	Eraso-Grisales et al. (2019)
Rooster	324.9 ± 4.08	441.4 ± 17.8	766.3 ± 29.48	Apel et al. (2020)

Despite their toxicity, glycoalkaloids are phytochemicals widely studied in clinical applications since they could have antitumor, anticancer, and anti-inflammatory properties depending on the dose and the conditions of use (Benkeblia, 2020). α-Solanine, at concentrations of 30 µM or below, can inhibit the growth of multiple cancer cells and enhance apoptosis in human JEG-3 choriocarcinoma cells (Gu et al., 2021). α-Solanine has shown anticancer activity against leukemic cell lines (THP-1 and MV4–11) in vitro (Zhang et al., 2021), as well as induction of apoptosis and inhibition of proliferation of malignant cells in urinary bladder cancer (Dong et al., 2022), suppression of SGC-7901 cells that promote gastric cancer (T. Li et al., 2022), and inhibition of the proliferation of human colorectal cancer cells and tumor growth (Yan et al., 2020). The other glycoalkaloid, α-chaconine, has been studied as a potential anti-inflammatory molecule. Lee et al. (2015) reported the suppression of α-chaconine by different inflammatory mediators such as COX-2 and pro-inflammatory cytokines (IL-1β, IL-6, and TNF-α). H. Li et al. (2023) found a concentration of α-chaconine that was 1.8 times higher than α-solanine in PPs. They also reported the antifungal activity of α-chaconine against Fusarium sulphureum, which killed the fungus’ mycelium and reduced the activities of induced peroxidase and superoxide, leading them to the conclusion that glycoalkaloids can be used to develop natural fungicides.

Other associated benefits include hormone production, benefits in the treatment of malignant skin tumors, and potent activity against liver cancer cells (Friedman, 2006; Hossain, Aguiló-Aguayo, et al., 2015). However, the extraction of glycoalkaloids is not a simple process because many factors need to be considered, such as the high extraction solution-to-sample ratio and the extraction medium. The latter must be acidic for the glycoalkaloids to be extracted and to avoid possible hydrolysis. Longer ultrasound extraction times, multiple extractions (at least three times), and the interaction between temperature and time will allow a good extraction percentage to be obtained (Apel et al., 2020; Jin et al., 2018).

It is well known that PP has an important content of glycoalkaloids with a promising potential for clinical applications; however, for food applications this should be accurately done due for its toxicity which could affect human health. The report of the European Food Safety Authority (EFSA, 2020) mention that the recommended intake has to be up 1 mg/kg of total glycoalkaloids per kg of body weight per day in humans, a concentration with no observed affected effect level in human body. More research of glycoalkaloids application in food systems at different concentration, along with in vivo assays should be developed in order to see the feasibility of applications on an industrial scale.

3.3. Dietary fiber (DF)

DF includes the remnants of edible plants consisting of carbohydrate polymers and analogs, that are resistant and not absorbed by the human small intestine and pass to the large bowel, where they are finally fermented by the gut microbiota (He et al., 2022; Rezende et al., 2021),

Usually, DF is divided into two groups: the first one is water-soluble DF (SDF) like pectin, β-glucans, galactomannans, fructans, agar, and inulin (Pathania & Kaur, 2022). This fiber aids the digestive tract as a source of probiotics for the gut microbiota, thus increasing intestinal mobility (Al-Weshahy & Rao, 2012). Also, it can form gels that help in the removal of cholesterol, which is particularly useful for patients with diabetes. Other benefits include the lowering of blood pressure, a reduction in inflammation and lipid levels, weight loss, and blood glucose control (Popoola-Akinola et al., 2022). The other group, i.e. the water-insoluble dietary fibers, comprises hemicellulose, cellulose, and lignin (Pathania & Kaur, 2022). This type of fiber doesn’t dissolve in water and passes through the gastrointestinal tract relatively intact as a low-viscosity mixture, hence, it is not a source of calories (Villanueva Flores, 2019).

Despite their resistance to digestion, DFs are important for human health due to their several health-related benefits, most of them related to weight and heart health. Studies mention that DF has positive effects in the prevention of several diseases, like diabetes and cardiovascular diseases. Also, the regular consumption of DF delays the absorption of cholesterol and triglycerides, regulating the metabolism of blood lipids and lipoproteins. Finally, it promotes satiation and lowers calorie intake, resulting in a reduction in body weight (An et al., 2022; Popoola-Akinola et al., 2022). In food applications, DF enhances the functional and technological properties like water-holding, oil-holding, and glucose adsorption capacities as well as the viscosity of different foods; this could be affected by hydrophilic and lipophilic groups in the DF (He et al., 2022; Q. Ma et al., 2022). Additionally, it is widely used in different food fields, such as meat and bakery, to be used as a partial replacement for fats due to its satiating property and lower caloric content (Sharma et al., 2016). Lastly, DF is used as a prebiotic in dairy products to increase the gut microbiota in the human digestive tract (Popoola-Akinola et al., 2022).

The content of DF in PP can be close to 50% of the dry base. Some polysaccharides, such as pectin and starch, can be extracted using different methods (chemical, enzymatic, enzyme-chemical, membrane separation, and microbial fermentation) (C. L. Li et al., 2019). The properties of PP are very similar to those of other commercial dietary fibers, and it can be used as an ingredient in food production (Durmaz & Yuksel, 2021). Some studies propose using PP as a food ingredient since SDF can form a gel with water and could therefore be an option to replace commercial gums. Diantom et al. (2020) evaluated tomato pulps and concentrates in which potato fiber was added and compared them against the use of xanthan gum. This could be a viable alternative to replace xanthan gum and reduce the need for additives because the products were more red and had a higher apparent viscosity.

4. Extraction methods for bioactive compounds from PP

The selection of the appropriate extraction method and the conditions under which it is performed are crucial to recovering bioactive compounds as much as possible (Akyol et al., 2016). PPs have been reported to contain up to ten times more bioactive compounds than potato flesh. However, this could vary according to the variety of potatoes, geographical location, and peel color (Palos-Hernández et al., 2022).

Usually, organic solvents like ethyl acetate, acetone, methanol, and ethanol are most commonly employed. Studies have reported a good yield of extraction; however, they are considered toxic and not recommendable in the food sector, except for ethanol (Gaudino et al., 2020). In this section, conventional and innovative extraction methods will be presented. A summary of the methods of extraction of bioactive compounds from PPs and the associated studies are shown in Table 5.

Table 5. List of employed methods in the extraction of total phenolic compounds in potato peels and their obtained results.

Techniques	PP variety	Targeted compound	Extraction conditions	Result	Reference
SLE	N.D	Total phenolic compounds	Ethanol; 80% v/v; 150 min; 1 g/30 mL	~2.0 ± 8.64 mg GAE/g	Brahmi et al. (2022)
		Total flavonoids	Ethanol; 80% v/v; 150 min; 1 g/30 mL	0.2 ± 0.76 mg QE/g
	Rooster	Glycoalkaloids	Water: acetic acid: sodium metabisulphite; 76.6 min; 83.5°C	~0.7 ± 29.48 mg/g	Apel et al. (2020)
	Ratona morada	Total phenolic compounds	Acetone; 70% v/v; 15 min.	6.5 ± 25.93 mg GAE/g	Benavides et al. (2020)
			Methanol; 70% v/v; 15 min.	6.2 ± 24.52 mg GAE/g
	Curiquinga	Total phenolic compounds	Acetone; 70% v/v; 15 min.	8.7 ± 50.21 mg GAE/g	Benavides et al. (2020)
			Methanol; 70% v/v; 15 min.	8.0 ± 40.36 mg GAE/g
	Gala	Total phenolic compounds	Ethanol 96%	28.4 mg GAE/g	Samotyja (2019)
	Jazzy	Total phenolic compounds	Ethanol 96%	40.5 mg GAE/g	Samotyja (2019)
	Sumi	Total phenolic compounds	Acetone	38.9 ± 5.55 mg GAE/g	J. Kim et al. (2019)
			Methanol:water (80:20, v/v)	24.0 ± 0.66 mg GAE/g	J. Kim et al. (2019)
NADES	Bologna	Total phenolic compounds	Previous solid CO2 Cryomaceration Ethanol/water 10% v/v, 27°C	3.9 ± 0.02 mg GAE/g	Venturi et al. (2019)
	N.D	Sugar production	choline chloride – glycerol; 3 h; 1:16 w/v; 115°C	217 kt/yr (total sugars)	Procentese et al. (2018)
UAE	Rooster	Glycoalkaloids	20 min; 85°C; amplitude of 100%	~0.5 ± 24.5 mg/g	Apel et al. (2020)
	Ratona Morada	Total phenolic compounds	Acetone; 70% v/v; 154 W; 37 kHz; 50 min; 50°C	17.7 ± 27.13 mg GAE/g	Benavides et al. (2020)
	Curiquinga	Total phenolic compounds	Acetone; 70% v/v; 154 W; 37 kHz; 50 min; 50°C	19.1 ± 20.72 mg GAE/g	Benavides et al. (2020)
	Lady Claire	Total phenolic compounds	Methanol 80%; 33 kHz; 30–900 min; 30–45°C	4.2 ± 0.01 mg GAE/g	Kumari et al. (2017)
	Lady Rosetta	Total phenolic compounds	Methanol 80%; 42 kHz; 30–900 min; 30–45°C	7.6 ± 0.79 mg GAE/g	Kumari et al. (2017)
	N.D	Steroidal alkaloids	Methanol; amplitude of 61 lm; 17 min	~1.1 ± 1.02 mg/g	Hossain et al. (2014)
PLE	Ratona Morada	Glycoalkaloids	Methanol: chloroform (2:1 v/v); 1 hour; 80 bar; 80°C	2.4 mg/g	Eraso-Grisales et al. (2019)
	N.D	Steroidal alkaloids	Methanol 30–90%; 40–80°C; 1500 psi; 5 min	0.002 mg/g	Hossain, Rawson, et al. (2015)
	Pinta-Boca	Anthocyanins	Ethanol 5% v/v; pH 2.6; Flow rate 50 g/min; 100 bar; 80°C	~1000 mg anthocyanins/g	Cardoso et al. (2013)
	Pinta-Boca	Total phenolic compounds	Ethanol 5% v/v; pH 2.6; Flow rate 50 g/min; 200 bar; 80°C; 2 min.	~300 mg phenolic total/g	Cardoso et al. (2013)
SFE	Maris Piper	Total phenolic compounds	CO2; 80°C; 350 bar; MeOH 20%; flow rate 18 g/min	37% of the total phenolic acid content and 82% of the caffeic acid originally present in this matrix	De Andrade Lima et al. (2021)
	Pinta-Boca	Anthocyanins	Ethanol + CO2 (5%v/v); pH 2.6; 100 bar; 65°C	~900 mg anthocyanin/g	Cardoso et al. (2013)
PEFAE	Desiree	Total phenolic compounds	1 kV/cm and at 5 kJ/kg; ethanol 52%; 230 min; 50°C	~1.1 mg GAE/g	Frontuto et al. (2019)
	Lady Claire	Steroidal alkaloids	0.75 kV/cm; 600 μs	~1.8 mg/g	Hossain, Aguiló-Aguayo, et al. (2015)
MAE	N.D	Flavonoids	Ethanol; 70% v/v; liquid to solid 20:1; 700 E; 70°C; 25 min.	6.5 ± 0.06 mg GAE/g	Z. Jiang et al. (2018)
	Diacolcapiro	Total phenolic compounds	Ethanol; 50% v/v; sample:solvent 1:20 g/ml; 6 min.	7.6 ± 0.096 mg GAE/g	Chaves Morillo et al. (2016)
	Russet Burbank	Total phenolic compounds	Methanol 65% v/v; extract sample-solvent ratio 30% v/v; 85°C; 15 min,	3.7 mg GAE/g	A. Singh et al. (2014)

SLE: Solid liquid extraction/NADES: natural deep eutectic solvents/UAE: Ultrasound assisted extraction/PLE: Pressurized liquid extraction/SFE: Supercritical fluid extraction/PEFAE: Pulsed electric field assisted extraction/MAE: Microwave assisted extraction.

4.1. Solid-liquid extraction (SLE)

Nowadays, conventional SLE methods are the most commonly employed techniques to recover important bioactive compounds. This method is simple and doesn’t require substantial investments, but it consumes considerable quantities of solvents, and the extraction times may be extended (Gaudino et al., 2020). The yield of extraction will depend on different factors, such as the solvent, the material-solvent ratio, time, temperature, and even the particle size (Rodríguez-Martínez et al., 2021). For this method, several solvents are employed, of which the most common are methanol, ethanol, or acetone, but in most cases, these solvents are diluted with distilled water to have higher bioactive compound extraction yields.

This method is based on the principles of diffusion and osmosis, enabled by maceration of the PPs. The process requires constant agitation to avoid rapid saturation of liquid around the solid (Naviglio et al., 2019). It is ideal for thermolabile samples since they do not require high temperatures but it takes too long to extract the compounds (Pai et al., 2022).

The SLE method uses large amounts of toxic solvents that are not safe for food samples, and there is a risk that the compounds we want to recover will be lost due to high temperatures and prolonged times (Rodríguez-Martínez et al., 2021). Moreover, the method is not always scalable for industrial applications due to the long contact time between solids and liquids, since large-scale productions need high-yield extractions in a short time (Naviglio et al., 2019). Since this represents a challenge for different industries, some recent studies have considered looking for a sustainable solvent that is easy to adopt and produces safer outcomes.

Natural deep eutectic solvents (NADES) have gained more attention due to their safe nature. They have been applied in SLE to study the yield and quality of extraction of important compounds in food matrices. A deep eutectic solvent is a liquid consisting of two or three non-safe compounds like non-toxic quaternary ammonium salts, amines, sugars, alcohols, and carboxylic acids (Palos-Hernández et al., 2022). These compounds are linked by intramolecular hydrogen bonds to create a mixture with a lower melting point than its components (Gaudino et al., 2020; Rodríguez-Martínez et al., 2021). The employment of NADES has different advantages: low costs, versatility, simple preparation, and low toxicity (Palos-Hernández et al., 2022). However, studies of the employment of NADES in PP for the extraction of bioactive compounds are very few. Procentese et al. (2018) employed choline chloride – glycerol and choline chloride – ethylene glycol (NADES) for fermentable sugar production from PPs with a final yield of 217 kt yr⁻¹, obtaining a new way to valorize this by-product together with other food wastes. Venturi et al. (2019) extracted potato phenolic compounds by solid CO₂ cryomaceration followed with SLE extraction with 10% ethanol/water, obtained a final content of 3.9 mg GAE/g (DW); although the content was minor compared to other studies, possessed an efficient antioxidant activity to keep the qualitative parameters of fresh-cut apple. The study resumes the efficience use of NADES to avoid the use of chemicals and thus, a possible use for foods. More studies are needed to understand how NADES could be useful for the extraction of phenolic compounds in PPs.

On the other hand, new and “green” methods of extraction have been appearing, either using more eco-friendly solvents, shorter extraction times, high pressures, or employing different and modern equipment. These alternative extraction methods will be described below.

4.2. Ultrasound-assisted extraction (UAE)

A non-conventional extraction method that has been of interest to different researchers is UAE since this method is rapid and easy to operate, non-thermal, and doesn’t need a toxic solvent. The food industry employs this technique more often than conventional extraction methods due to its low frequency and power intensity (Ampofo & Ngadi, 2022; Sengar et al., 2022). The method employs sonic waves of >20 kHz (20 to 2000 kHz). Its effectiveness depends on the propagation of the ultrasound pressure waves, which results in cavitation phenomena: production of bubbles and high temperature will violently break cell walls, releasing the contents of the cell into the extraction medium, which is the extraction solvent in the majority of cases (Ebringerová & Hromádková, 2010).

The employment of this method has some advantages: the sonication increases the surface area between the solid and liquid phases, improving the extraction and yield process. Moreover, the consumption of solvent and process duration is much lower than SLE and there is a lesser risk of losing important bioactive compounds due to degradation or denaturation since it is a non-thermal process (Rodríguez-Martínez et al., 2021; Sengar et al., 2022). Some of the advantages of the employment of UAE in food matrices are the improvement of quality and physicochemical characteristics, the improved bioavailability of compounds, and the inhibition of the enzymatic browning process (Yusoff et al., 2022).

The UAE is a promising technology with significant industrial applications in the future; however, several factors must be considered to recover important molecules from food matrices. In the case of PPs, most of the studies employ UAE to recover the total phenolic content and glycoalkaloids. However, the important factors to be considered to avoid losing the target compounds are the temperature (no more than 50°C), power (no more than 40 kHz), and extraction solvent. The UAE extraction is also used for pretreatment to obtain bioethanol from PP since it has sufficient amounts of hydrolyzable starch and fermentable sugars. A study carried out by Suresh et al. (2018) used UAE to obtain ethanol from PP by HCl and enzymatic hydrolysis. Both extracts were incubated in a Yeast Extract-Peptone-Dextrose media and inoculated with S. cerevisiae for the fermentation process. The authors concluded that ultrasonic treatment is an effective way to obtain bioethanol, and the yield process could be better at different treatment conditions, but this will need more research.

4.3. Supercritical fluid extraction (SFE)

The technique employs supercritical fluids that exceed their specific critical pressure and temperature, possess properties of both liquids and gases, and can be regulated by variations in pressure and temperature (Pagano et al., 2021). The method operates in supercritical conditions, i.e. at very high temperatures and pressures; this makes the process more efficient, and the gas-like solvent can penetrate the food matrix easily (Pai et al., 2022).

The most commonly used fluid is carbon dioxide (CO₂), due to its safe nature, low toxicity, low critical temperature and pressure (31.6°C; 7.386 MPa), and minimal damage to thermosensitive bioactive compounds (Pai et al., 2022; Rifna et al., 2021). Other advantages of CO₂ include higher penetration into the sample, the possibility of different pressure-temperature combinations for a better extraction process, and the recyclability of CO₂ as the extraction process has no harmful effects on the environment (Alara et al., 2021).

Nevertheless, for efficient extraction, the method is limited to working with dry raw materials and organic co-solvents. Also, CO₂ has a low polarity, which limits the efficiency of the extraction of organic compounds (Rodríguez-Martínez et al., 2021). SFE with CO₂ has been employed in oil removal processes to replace n-hexane and recover the major content of free fatty acids. Using the lowest CO₂ conditions (40°C and 13.8 MpA), King et al. (2001) could extract large amounts of free fatty acids (FFA) from Vernonia galamensis seeds. This confirmed the selective impact of temperature and pressure on the FFA content. On the other hand, CO₂ is also used to extract carotenoids from fruits and vegetables. A study conducted by De Andrade Lima et al. (2018) optimized the extraction of carotenoids from carrot peels using supercritical CO₂; the optimal conditions for the extraction were a temperature of 59°C, a pressure of 349 bar, and the use of 15.5% ethanol as a solvent with a final yield of 86.1%.

In the particular case of PP, few studies have reported outcomes of the application of SFE using this tuber or its peels. De Andrade Lima et al. (2021) recovered phenolic compounds from PP using SFE-CO₂, and the optimal extraction conditions were 80°C, 350 bar, MeOH of 20%, and a flow rate of 18.0 g/min with a final recovery of 37% of which caffeic acid recovery alone was 82%. Despite the efficiency of the extraction method, the authors suggested using mixtures of water and organic solvents and co-solvents to enable the recovery of specific phenolic acids like caffeic or chlorogenic acids, which are the most abundant in PPs.

4.4. Microwave-assisted extraction (MAE)

This is a relatively new technology, that involves non-contact energy transfer from electromagnetic equipment. MAE uses a microwave oven that will irradiate energy in the form of microwaves (300 MHz to 300 GHz), which will be absorbed by polar materials and then transformed into thermal energy by two mechanisms: ionic conduction and dipole rotation (Khadhraoui et al., 2021; Pattnaik et al., 2021). The internal moisture begins to evaporate, resulting in high pressure on the cell walls, producing a mechanical rupture and the release of bioactive compounds (Pai et al., 2022).

An advantage of this technique is that the rupture of cell walls will allow the extraction of the bioactive compounds more quickly, reducing extraction duration (Gil-Martín et al., 2022). There are important parameters to consider for MAE. The first one is to use only polar solvents since their dielectric constant is higher than those of non-polar solvents and they can easily absorb energy (Pattnaik et al., 2021). The second crucial parameter is the temperature, which can be modulated during the process, and finally, the power should be considered, as excessive irradiation may cause overheating and could lead to undesirable reactions such as isomerization or degradation of the product (Kapadia et al., 2022; Lefebvre et al., 2021).

The method presents several advantages compared to conventional extraction; MAE has been widely applied for the extraction of bioactive compounds in different by-products, especially in plant foods. However, there are limited studies documenting the usefulness of MAE in PPs, compared with SLE or UAE techniques. It is necessary to conduct more research about the usefulness of this method in PPs and determine the suitable conditions to apply this technique in this by-product.

4.5. Pulsed electric field assisted extraction (PEFAE)

This is a non-conventional extraction method that works with electric pulses to break cell membranes, releasing bioactive compounds. This permeabilization can be performed at moderate electric fields (<10 kV/cm) or low specific energies (<10 kJ/Kg) (Gaudino et al., 2020). The food is inserted in the treatment chamber between two electrodes, these apply electric pulses for short seconds (could be ranging from milli to nanoseconds). Cells exposed to the electric field accumulate charges in the membrane surface, generating transmembrane potential on the cell surface. Consequently, cells get stressed and form pores that facilitate the release of the targeted compounds (electroporation) (Pattnaik et al., 2021). This method is less destructive than other methods which break cell walls (like UEA), allowing a selective extraction and preserving the color, flavor, vitamin C content, etc. (Khadhraoui et al., 2021).

This mechanism has a positive effect on the extraction process because of the increment of mass transfer of intracellular components to the solvent medium. Besides, PEFAE don’t employ high temperatures in their process and the concentration of solvent is minimal facilitating the accessibility to the desired compounds (Bocker & Silva, 2022), this has an environmental impact due to its higher extraction yield with lower consumption of energy (Pattnaik et al., 2021). Hossain, Aguiló-Aguayo, et al. (2015) applied PEFAE and pulsed light pretreatment on the SLE extraction of steroidal alkaloids from PPs and compared with samples treated with SLE only in order to evaluate the efficacy of PEFAE, reporting a final recovery of steroidal alkaloids of 1.8 mg/g dried PPs applying PEFAE treatment, 99% higher than SLE extraction alone.

Likewise, Frontuto et al. (2019) employed PEFAE to intensify the extractability of phenolic compounds, obtaining 1.1 mg GAE (gallic acid equivalent)/g FW PP. The High Performance Liquid Chromatography (HPLC) analyses of the obtained extracted showed that chlorogenic and caffeic acids were the most abundant compounds in the extracts and, compared to extract with SLE only, there was no significant degradation due to PEFAE application and concluding the effectiveness of the methods in extraction and reducing of solvent consumption and process time.

However, the main drawback is the high cost of investment to acquire the equipment and the difficulty to use it and it should be combined with other technologies to recover the compounds of interest (Khadhraoui et al., 2021). Moreover, this technology has been thought to be applied in homogeneous liquids since the application in solid foods could incorporate air into the product, causing undesirable reactions or even damages in the equipment (Bocker & Silva, 2022). PEFAE is expected to be industrially applied in thermolabile foods, together with SLE or UAE it may have positive effects in the extraction of sensible compounds such as antioxidant or colorants (H. S. Kim et al., 2022).

4.6. Pressurized liquid extraction (PLE)

In a recent non-conventional, “green” extraction method that employs high pressures to heat the extraction solvent over its boiling point to obtain a high-yield extraction (Lefebvre et al., 2021), the pressurized solvents remained in a liquid state, even above their boiling points (Eraso-Grisales et al., 2019). The mass transfer, solubility, and leaching rate of the targeted compound are increased due to high temperatures, reducing the extraction times and consumption of solvent in contrast with conventional methods (Gaudino et al., 2020; Rifna et al., 2021).

Some advantages of this technique are higher extraction rates, good mass transfer, shorter extraction times, and lower solvent consumption (Akyol et al., 2016). The use of high temperatures enhances extraction efficiency by decreasing the viscosity of the solvent, leading to its better penetration into the matrix containing the compounds of interest (Eraso-Grisales et al., 2019). However, there is a risk that the extracted bioactive molecules could deteriorate following PLE since the use of high pressures could result in their degradation. It is recommended that this technique be employed for quality control analysis and that a means of industrial application in the food, nutraceutical, and pharmaceutical sectors be sought (Rifna et al., 2021).

The PLE method has been employed in the extraction of flavonoids, anthocyanins, and saccharide molecules from different food matrices (Lefebvre et al., 2021). Research conducted by Eraso-Grisales et al. (2019) reported the extraction of α-solanine and α-chaconine glycoalkaloids in the Ratona Morada potato variety, where α-chaconine was the most abundant glycoalkaloid with optimal extraction conditions of 80 bar and 80°C. Hossain, Rawson, et al. (2015) recovered steroidal alkaloids from PP (1.92 mg/g dried PP) and compared it with SFE extraction (0.98 mg/g dried PP) with optimal conditions of 80°C in 89% methanol. Cardoso et al. (2013) compared the SFE extraction with the PLE extraction of anthocyanins in PP to see the recovery efficacy of each method, with PLE extraction 1000 mg anthocyanin/g extract was recovered, in contrast to SFE, which resulted in the recovery of 900 mg/g extract. Moreover, SFE employs CO₂ as a cosolvent for increasing the yield of extraction but is not suitable for the extraction of polar analytes. Further studies in agro-industrial wastes are necessary, and with this, possibilities to employ this method at industrial scales must be evaluated, which could be of great interest to the food, pharmaceutical, and nutraceutical industries.

5. Applications in the food industry

5.1. As a source of dietary fiber

This concept of using PP as a source of DF is one of the most popular among industries because its recovery represents an increase in income and cost reduction. Azizi et al. (2021) manufactured PP snacks at an industrial level. The obtained product had less than 10% fat, and 8.3% fiber, and was well-accepted by consumers. Namir et al. (2022) prepared a functionally fiber-enriched pasta with the partial replacement of wheat flour with PP at different particle sizes. The outcome was an increase in the DF of 7–21 times more than pasta made with wheat flour alone and improved cooking properties and texture. The sensory evaluation revealed that the PP fiber-enriched pasta had a favorable acceptability in terms of color, taste, flavor, texture, and overall acceptability, all of which received greater than seven points on a nine-point hedonic scale. Due to the pasta’s fiber content, which made it darker than the control, the color criterion was the only one to obtain an average of five points in some samples.

Badr and El-Waseif (2018) studied the effects of PP flour as a 25%, 50%, 75%, and 100% fat substitution in meatballs. The results indicated that the fiber content in the meatballs increased significantly. The control (0% PP flour) had a fiber percentage of 1.12%, while the other assessments with 25%, 50%, 75%, and 100% of PP flour showed fiber percentages of 2.03%, 2.82%, 3.67%, and 4.45%, respectively. Furthermore, the high DF content of PP meal increased the water-holding capacity of the meatballs from 1.31 g/g (control) to 3.12–5.87 g/g. Recently, Durmaz and Yuksel (2021) fried wheat chips with different concentrations of PP flour, reducing the fat content from 40% to 10% while also increasing protein concentrations.

5.2. In bakery production

Ben Jeddou et al. (2017) demonstrated how adding PP flour to cakes improves their nutritional, technological, and textural properties. The flour obtained from different PPs had a significant impact on water retention capacity and fat absorption. The cakes made with this flour also received good sensory acceptance by different consumers, demonstrating PP’s potential for use in the preparation of high-fiber cakes.

PP flour can be used to fortify foods such as bread and thus reduce the formation of toxic components such as acrylamide. Crawford et al. (2019) prepared quinoa flatbreads by adding PP flour (Umatilla Russel variety) and peel powders from other fruits, vegetables, and mushrooms. The results showed that adding 5% of PP flour to the flatbreads reduced the acrylamide content from 487 μg/kg to 367 μg/kg; the same was true for the other samples to which the other flours were added. The authors suggest further research into the benefits of incorporating these by-products to improve physicochemical properties and reduce unwanted compounds.

Curti et al. (2016) developed a flour-based bread with 0.4% potato fiber extracted from the peel. The fiber improved the texture and affected several parameters, including water activity and moisture content. According to the authors, the use of PP fiber should be further studied to understand how it interacts with other bread components. Jacinto et al. (2020) elaborated on gluten-free bread with the addition of PP flour, there were no significant differences in the proximate composition, but in the sensory acceptance, bread with 5% of the alternative flour had higher sensory acceptance compared to other bread and the control. The author concluded that the addition of alternative flour is a viable alternative for the elaboration of bread with sustainable products and with a similar proximate composition to commercial gluten-free bread.

Recently, Akter et al. (2023) prepared cakes with the addition of different ratios of wheat flour to PP flour (PPF) (100:0, 96:4, 94:6, and 92:8). The cakes with PPF presented a higher height, volume, and weight than the control cakes (100% wheat flour). The cake with 4% PP flour in it scored the highest on the sensory evaluation, suggesting that PPF can be used to prepare cakes with better physical and organoleptic qualities.

5.3. As a source of natural antioxidants

Oxidation of fats and oils in foods causes not only rancid odors and off-flavors but also nutritional quality reduction and loss of nutrients, such as unsaturated fatty acids, due to the formation of toxic compounds (Al-Weshahy & Rao, 2012). For many years, different food industries have used synthetic antioxidants such as beta-hydroxy acids and butylhydroxytoluene to prevent oxidation, especially in oils and fats, by lowering peroxide and p-anisidine concentrations. However, because these are natural antioxidants, they must be used in higher concentrations, and the search for new applications continues (Abebaw, 2020; Javed et al., 2019).

Amado et al. (2014) used a PP extract to stabilize soybean oil against accelerated oxidation processes, evaluated by peroxide and p-anisidine values which are indexes of fatty matter stability, founding that after 15 days of storage at 60°C the concentrations of peroxide and p-anisidine were reduced. Likewise, Franco et al. (2016) added PP extracts to soybean oil at three different concentrations: 14.01, 20.37, and 31.94 ppm. The samples containing the PP extracts demonstrated lower peroxide index values and greater resistance to oxidative processes than the control (without the extract).

It has been demonstrated that PP can be used to increase antioxidant concentrations in foods other than fats. Fradinho et al. (2020) extracted the bioactive compounds from the Agria variety PPs via subcritical water extraction and converted the resulting liquid to a gluten-free paste. The antioxidant activity was evaluated using the The 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•+) radical cation-based method; the total concentration of phenolic compounds was nine times higher than that in the control. A remarkable finding was that the paste developed a coffee-like aroma, possibly due to the chlorogenic acid in the PP being converted to caffeic acid during storage. Also, Brahmi et al. (2022) found that yogurt with PP increased their antioxidant activity and phenolic compounds, also increased the consistency and overall quality. The study concluded that PP is a viable alternative to be used as an ingredient in yogurt formulation with good results in physicochemical properties.

Furthermore, since PP extracts contain antioxidant compounds, they can be used in different foods to slow down the oxidation processes. Sabeena Farvin et al. (2012) used PP extracts to delay oxidation in lipids and proteins in minced horse mackerel at 2.4 and 4.8 g/kg to protect the fish’s beneficial compounds. Likewise, Saeed et al. (2022) compared the stabilizing activity of PP extract against the synthetic antioxidant butylated hydroxyl anisole (BHA) in sunflower oil after 60 days of storage. The iodine value (the best indicator for the level of unsaturated fatty acids) in the control sample ranged from 110 to 99 (day 60), while the addition of BHA and PP extract (3200 ppm) could decrease the iodine value from 110 (day 0) to 75 (day 60) and 82 (day 60), respectively. This demonstrates that PP extract can be used to reduce the use of synthetic antioxidants and contribute to industrial waste management.

5.4. In food coatings

Because of its high content of phenols, recent studies also seek to use this by-product to produce protective films. Rommi et al. (2016) used PP and glycerol to create casings with properties similar to those made solely from starch. The coating with PP and the addition of starch had a similar appearance to the starch coating but with better barrier properties, such as moisture resistance and oxygen transmission. Similarly, Gebrechristos et al. (2020) prepared edible starch-based coating film with PP extract concentrations of 5%, 10%, and 20% to evaluate the antimicrobial and antioxidant activities of the film. The results showed that 1,1-diphenyl-2-picrylhydrazyl (DPPH) activity levels were found to be 22.4%, 34.52%, and 55.12%, respectively, and the concentration of polyphenols in the coating increased as the concentration of PP extract in the film increased. The films also demonstrated a release of between 1000 and 2200 mg GAE/100 g of film, which is equal to a 24%–55% reduction of growth of Escherichia coli, Salmonella enterica, and Staphylococcus aureus.

On the contrary, Y. Ma et al. (2022) manufactured a film with PP and chitosan nanoparticles that were used to cover cheese slides to study the change in pH and peroxide levels during storage. The pH of the samples covered with PP film and chitosan did not change, indicating that the cheese’s rancidity was minimal or absent. It also had the lowest peroxide value when compared to the samples not covered with the film. This indicated that the coating aided in the retardation of oxidation in the cheese samples and demonstrated the antioxidant potential of PPs when used as a food coating. Lopes et al. (2021) added PP extract to a starch-based coating to coat-smoked goldfish fillets. The coating allowed the fillets to retain their color while also providing greater rigidity and juiciness than samples packed in simple plastic.

6. Conclusions

There is a wide variety of potatoes that differ in shapes and colors and are marketed for either local or mass consumption (such as in restaurants), and their peels are different too. In the next few years, it is expected that the demand for this tuber and novel food products derived from it will increase both locally and abroad. Consequently, large amounts of PP will be generated, and the food industry will have to be prepared on how to recover or reuse the PPs. PP is a by-product with substantial potential because it is a good source of fiber and contains different bioactive compounds that have been shown to have health benefits. Furthermore, researchers have begun to use PP, either as flour or extract, in different types of food and the development of coatings. These studies have yielded positive results for their products, improving the physicochemical properties of foods, including their bioactive compound content and shelf life. Therefore, we suggest seeking industrial applications, either in the food industry or in other fields, to promote the revaluation of the PP and the development of functional and sustainable products.

Acknowledgments

This work was supported by Universidad San Ignacio de Loyola (Lima-Peru).

Disclosure statement

No potential conflict of interest was reported by the author(s).