Table 2. Extraction and Analysis of Phenolic Compounds Obtained from Plant Peels and Husks^a.

reference	source	best extraction conditions	extraction solvent	chromatographic analysis	TPC/TFC	phenolic compounds
Nayak et al. (2015)66	Citrus sinensis peel	MAE	water–acetone (51:49, v/v)	HPLC-DAD, C18 column (4.6 mm × 250 mm, 5 μm), 30 °C, 254, 289, 520, 300, and 700 nm, 6:94 (v/v) acetic acid in water, 100% ACN, 40 min	12.20 mg GAE/g	chlorogenic acid, catechin, rutin, gallic acid, p- coumaric acid, caffeic acid, and ferulic acid
		T: 80 °C
		t: 122 s
		Po: 500W
		F: 2.45 kHz
		25 mL/g
Wang et al. (2018)67	red orange peel	UAE	water–ethanol (85:15, v/v)	HPLC-DAD-ESI-M2, reverse- phase amide column (15 cm × 4.6 mm), 25 °C, 320 nm, water in ACN, 40 min	n-d	tangeretin and nobiletin
		T: 50 °C
		t: 40 min
		Po: 150 W
		F: 20 kHz
		20 mL/g
Kaderides et al. (2019)68	pomegranate peel	MAE	water–ethanol (50:50, v/v)	HPLC UV–vis, reverse-phase column (250 mm × 4.6 mm, 5 μm), 30 °C, 254 and 280 nm, 5% acetic acid in water, and CAN, 12 min	199.4 mg GAE/g	phenolics acids, flavonoids, catechins, and procyanidins
		T: 30 °C
		t: 4 min
		Po: 600W
		F: 2.45 kHz
		60 mL/g
Mei et al. (2020)69	Xanthoceras sorbifolia husks	UAE	water–ethanol (60:30, v/v)	HPLC UV–vis, ODS-C18 column (250 mm × 4.6 mm, 5 μm), 25 °C, 0.1% formic acid and ACN, 20 min	23.16 mg GAE/g	gallic acid, protocatechuic acid, epicatechin, rutin, rutinoside, and quercetin
		T: 40 °C
		t: 40 min
		F: 100 kHz
		100 mL/g
Figueroa et al. (2021)39	avocado peel	MAE	water–ethanol (36:64, v/v)	HPLC-ESI-TOF/GTOF-MS, C18 column (150 nm × 4.6 nm, 1.8 μm), 25 °C, formic acid in water and ACN, 55 min	72.04 mg GAE/g	flavonoids, catechins, procyanidins, and phenolic acids
		T: 130 °C
		t: 39 min
		Po: 850 W
		30 mL/g
Dewi et al. (2022)70	cacao pod husk	MAE	water–ethanol (50:50, v/v)	HPLC-VWD, reverse-phase column (250 mm × 4.6 mm, 5 μm), 30 °C, 30 °C, 280 nm, 0.1% of orthophosphoric acid in water, and ACN, 50 min	100.4 mg GAE/g	catechin, quercetin, epicatechin, gallic acid, coumaric acid, and protocatechuic acid
		T: 50 °C
		t: 5 min
		Po: 120 W
		F: 2.45 kHz
		80 mL
Estrada-Gil et al. (2022)24	rambutan peel	UAE + MAE	water (1:16, m/v)	HPLC-MS, reverse-phase (150 mm × 2.1 mm, 3 μm), 30 °C, 245, 280, 320, 550 nm, formic acid and ACN, 60 min	176.38 mg GAE/g	geraniin, corilagin, and ellagic acid
		UAE
		t: 20 min
		F: 25 kHz
		MAE
		T: 70 °C
		t: 5 min
		F: 2450 kHz
Anticona et al. (2022)71	citrus peel	UAE	water–ethanol (50:50, v/v)	HPLC-UV, C18 column (250 mm × 4.6 mm, 5 μm), 280 nm, formic acid in water and ACN, 80 min	n-d	catechin, caffeic acid, vanillic acid, quercetin, naringenin, apigenin, and hesperidin
		T: 35–40 °C
		t: 30 min
		Po: 400 W
		F: 20 kHz
		45 mL
Wang et al. (2023)38	brocade orange peels	UAE	water– methanol– DMSO (1:4:5)	UHPLC-Q-TOF-MS, C18 column (4.6 mm × 100 mm, 3.5 μm), 35 °C, formic acid and methanol, and HPLC-DAD, Aq-C18 column, same gradient, 35 °C	24.97 mg GAE/g	caffeic acid, sinapic acid, ferulic acid, narigin, hesperidin, luteolin, rutin, sinensetin, nobiletin, and tangeretin
		T: 26 °C
		t: 30 min
		Po: 60 W
		F: 40 kHz
		17.6 mL/g
Gómez-Urios et al. (2022)72	orange peel	magnetic stirring	choline chloride and glycerol ratio of 1:2 with 25% water	HPLC UV–vis, C18 column (250 nm × 4.6 nm, 5 μm), 280 nm, 5% acid formic in water and 40% in ACN, 80 min	903 mg GAE/g	ascorbic acid, flavonoids, and polyphenols
		T: 45 °C
		t: 20 min
		10g/mL
Maimulyanti et al. (2023)34	coffee husk from west Java, Indonesia	magnetic stirring	choline chloride and proline ratio of 1:1with 50% water	HPLC-PDA, reverse phase column (250 mm × 4.6 mm, 5 μm), 30 °C, 327 nm, 50 mM acid in distillate water and 50 mM acid in ACN, 10 min	10.07 mg GAE/g	chlorogenic acid
		T: 80 °C
		t: 30 min
		Po: 200 W
		F: 34 kHz
		10 mL/g
Strieder et al. (2024)44	Arabic coffee husk: green coffee beans 60:40 (w/w)	PLE	water	HPLC-PDA, Kinetex C18 column (100 mm × 4.6 mm, 2.6 μm), 50 °C, 270 and 325 nm, 0.1% acetic acid in water and in ACN, 9 min	n-d	chlorogenic acid and caffeine
		T: 125 °C
		P: 150 MPa
		flow: 2 mL/min
		50 mL/g

^aUAE: ultrasound-assisted extraction; MAE: microwave-assisted extraction; PLE: pressurized liquid extraction; HAE: homogenate-assisted extraction; TPC: total phenolic content; TFC: total flavonoid content; (UH)PLC: (ultrahigh) performance liquid chromatography; DAD/PDA: photodiode-array detector; ESI-M2: electrospray ionization coupled mass spectrometry; (Q)TOF: quadrupole time-of-flight; MS: mass spectrometry, VWD; variable wavelength detector; T: temperature; RT: room temperature; t: extraction time; SM: sample mass; P: pressure; Po: Power; A: amplitude; F: frequency; TPC: total phenolic content; TFC: total flavonoid content; GAE: gallic acid equivalent; n-d: nondetermined; ACN: acetonitrile.