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    Topic review

    Subcritical-Water Extraction of Natural Products

    Submitted by: Yu Yang

    Definition

    Subcritical water refers to high-temperature and high-pressure water, but below water's critical point of 374 °C and 218 atm. A unique and useful characteristic of subcritical water is that its polarity can be dramatically decreased with increasing temperature. Therefore, subcritical water can behave similarly to methanol or ethanol. This makes subcritical water a green extraction fluid used for a variety of organic species. 

    1. Introduction

    Among the various new green extraction and separation technologies developed recently, subcritical water extraction (SBWE) is the most promising one. Subcritical water refers to the liquid water at temperature and pressure below its critical point (Tc = The pressure of the subcritical water must be higher than the vapor pressure at a given temperature to keep water in the liquid state. With the increase of temperature, the physical-chemical properties of subcritical water change drastically.

    2. Compounds Extracted by Subcritical Water

    2.1. Flavonoids

    Flavonoids, also known as bioflavonoids, are widely found in plants and berries. They are important natural compounds in human diets. They have been used to prevent and treat cardiovascular diseases. In addition, they have strong antioxidant activities and antibacterial activities. When high-flavonoid apples were fed to healthy mice, decreases in some inflammation markers were reported [1].
    Generally speaking, flavonoids belong to phenols. Since they are widely investigated, they can be presented separately from phenols. Flavonoids have the general structure of a 15-carbon skeleton by connecting two benzene rings with a heterocyclic ring. The basic nucleus is 2-phenylchromone. Flavonoid compounds are usually poorly soluble in ambient water and most organic solvents. Table 1 summarizes SBWE of flavonoids from plant materials.
    Table 1. SBWE of flavonoids.
    Samples Medicinal Parts Compounds Extracted Extracts Activity Extraction Conditions Analytical Methods Other Extraction Methods (Solvent, Ratios of Yields) Ref.
    Panax ginseng C.A. Meyer stem
    leave
    TP and flavonoids antibacterial 110 and 165 °C, 15 min
    190 °C, 10 min
    TEM, UV heating (water 95.4%; ethanol 91.3%) [2]
    Chamomilla matricaria L. flowers TP, TF, 18 polyphenolic compounds, apigenin antioxidant, enzyme inhibitory activity 65–210 °C, 5–60 min
    1:30–1:100 g/mL
    TLC, UV, HPLC-MS   [3]
    Allium cepa onion wastes quercetin-4′-glycoside, quercetin, etc.   40–160 °C, 5 min, 5 MPa,
    1–10 mm, pH 3.0–7.0
    LC-MS/MS HPLC-UV convention (methanol and hydrochloric acid 94.3%) [4]
    Crocus sativus L. stigmas TP, dodecane, γ-terpinene, tetradecane, etc. antioxidant (DPPH, FRAP), antibacterial 100–180 °C, 10–30 min,
    1:10 g/mL
    GC/MS, UV-vis   [5]
    Saururus chinensis, etc. skin, leave, peel, etc. quercetin, isorhamnetin, kaempferol, isoquercitrin, etc.   10 MPa, 110–200 °C, 5–15 min HPLC   [6]
    Camellia sinensis leaves epigallocatechin gallate   80–120 °C, 3–7 min,
    40–60 mL/g
    HPLC convention (water 87.6%) [7]
    Origanum vulgare L. leaves TP, flavanone, flavone, flavanol antioxidant (DPPH, TEAC, ABTS) 10.34 MPa, 30 or 15 min
    25–200 °C
    HPLC-
    DAD, UV
      [8]
    orange peels reducing sugar, TP, pectin, hesperidin, narirutin antioxidant (DPPH, FRAP) 110–150 °C, 10–30 mL/min
    10 MPa
    HPLC,
    UV-vis
    Soxhlet (ethanol 79.2%), shaker (ethanol 250%), UAE (ethanol 114%) [9]
    orange peels flavones, 7-hydroxyflavone   100–150 °C, 0.5 mL/min GC-FID UAE (methylene chloride) [10]
    Citrus unshiu Markovich peels rutin, naringin, hesperidin, naringenin   0.5–14 MPa, 5–15 min, 100–190 °C HPLC   [11]
    Allium cepa L. peels TP, TF, quercetin antioxidant (DPPH, TBA, FTC) 110 and 165 °C, 15 min, p < 3.4 MPa HPLC, UV heating (ethanol 153%; water 45.6%) [12]
    Hippophae rhamnoides leaves TP, TF, isorhamnetin, kaempferol, quercetin antioxidant, cytotoxicity 25–200 °C, 15 min,
    10.34 MPa
    HPLC, UV, FM maceration (water 21.3%), Soxhlet (ethanol 64.6%) [13]
    Allium cepa L. peels TP, TF, kaempferol, quercetin antioxidant (DPPH) 170–230 °C, 3 MPa, 30 min,
    pH 2–10
    HPLC, UV-vis heating (ethanol 26.7%) [14]
    Achillea millefolium L. herbal dust TP, TF, HMF, chlorogenic acid antioxidant (DPPH, TEAC, ABTS) 120–200 °C, 10–30 min
    0–1.5% HCl, 3 MPa
    HPLC, UV-vis   [15]
    Curculigo latifolia root TP, TF, pomiferin, etc. antioxidant (DPPH, ABTS, TEAC) 100–200 °C, 10 MPa
    30–120 min, 0.5 mL/min
    LC-MS, UV   [16]
    Citrus unshiu peels hesperidin and narirutin   110–190 °C
    3–15 min
    HPLC   [17]
    Glycine max okara genistin, daidzin, genistein, daidzein   100–200 °C, 5 min, 2–5 MPa, 10–30 g/mL HPLC Soxhlet (methanol, 108%) [18]
    onion skins quercetin, quercetin-4′-glucoside   100–190°C, 5–30 min,
    9–13 MPa
    HPLC convention (methanol, 92.8%) [19]
    Puerariae lobata root puerarin, daidzin, daidzein
    3-methoxypuerarin
      100–200 °C, 15–75 min
    1:10–1:25 g/mL
    HPLC reflux (ethanol 91.6%), UAE (water 95.9%) [20]
    Coriandrum sativum seeds TP, TF antioxidant (DPPH) 100–200 °C, 10–30 min
    3–9 MPa
    UV   [21]
    Citrus unshiu peels flavanones, polymethoxy-Flavones, etc. anticancer, cardioprotectives 120–180 °C, 1.0–2.0 mL/min, 5.0 ± 0.1 MPa GC, HPLC, convention (methanol 75.0%; ethanol 41.6%; acetone 17.2%) [22]
    Phlomis umbrosa whole part TP, TF, iridoids glycosides antioxidant (DPPH, ABTS) 110–200 °C, 10 MPa, 1–25 min HPLC,
    ESI-MS
    convention (ethanol; methanol; water) [23]
    Actinidia deliciosa peels TP, TF, antioxidant (DPPH, ABTS, FRAP) 120–160 °C, 0–30 min, 3 MPa,
    pH 2–5.5
    UV-vis, pH convention (ethanol 81.9%) [24]
    Scutellaria baicalensis root baicalin, baicalein, wogonin,
    wogonoside
      110–160 °C, 10–90 min,
    20–100 mesh
    HPLC HRE (ethanol 93.0%) [25]
    Citrus unshiu pomaces TP, polymethoxylated flavones, sinensetin, etc. antioxidant (DPPH, TP) 25–250 °C, 10–60 min,
    0.1–5.0 MPa
    HPLC, UV   [26]
    citrus unshiu peels hesperidin, narirutin, prunin, naringenin, sinensetin, etc. antioxidants (DPPH, FRAP), enzyme 145–175 °C, 15 min
    5 MPa, 0.75–2.2 mL/min
    HPLC 2M HCl extraction 42.9%; 2 M NaOH extraction 38.9% [27]
    citrus unshiu peels hesperidin and narirutin   110–200 °C, 5–20 min,
    10 ± 1 MPa
    HPLC, MS/MS convention (ethanol 56.4%; methanol 35.8%; water 6.2%) [28]
    palatiferum Radlk. leaves TP, TF, protein, saponin, sugar, apigenin, kaempferol antioxidants (DPPH, FRAP, ABTS), 110–270 °C, 15 min, 8 MPa
    1:70 g/mL
    HPLC, UV convention (water 77.7%; methanol 32.8%), Soxhlet (ethanol 43.7%) [29]
    Glycyrrhiza uralensis Fisch. root TP, TF, liquiritin, flavanone, isoflavone antioxidants (DPPH, ABTS) 80–320°C, 2–100 min, 7.0 MPa, 1:30 g/mL, pH 3–11 HPLC,
    MS/MS, UPLC
    UAE (water 20.6%; ethanol 44.9%), MAE (water 25.6%; ethanol 63.8%) [30]
    Tagetes erecta L. flower residues TP, TF, 5-HMF, reducing sugar, free amino acids antioxidants (DPPH, ABTS) 80–260 °C, 15–90 min
    1:20–1:60 g/mL,120 rpm
    HPLC-DAD, UV leaching (water 9.4%; methanol 69.9%; ethanol 68.8%; acetone 94.0%), UAE (water 9.9%; methanol 69.8%; ethanol 64.3%; acetone 87.6%) [31]
    Daucus carota leaves polyphenols, luteolin   110–230 °C, 0–114 min, 4 MPa UV, PLC   [32]
    Matricaria chamomilla L. flowers TP, TF, apigenin-7-O-glucoside, etc. antimicrobial, cytotoxic activity 200 °C, 40 min, 1:50 g/mL UHPLC, HESI-
    MS/MS, UV
    Soxhlet (ethanol 129%), MAE (ethanol 117%), UAE (ethanol 104%) [33]
    Silybum murianum L seeds taxifolin, silychristin, silydianin, and silybin   75–250 °C, 40–60 min, 12.5 MPa, 0.1–0.5 mm HPLC convention (ethanol 101%; water 43.6%) [34]
    Echinacea purpurea L. flowers TP, TF antioxidant
    (TEAC, ABTS)
    103.4–216.56 °C, 3 MPa, 5.86–34.414 min UV-vis   [35]
    Humulus lupulus pellets TP, desmetylxanthohumol, prenylflavonoids, etc. anti-inflammatory 50–200 °C, 30 min, 10 MPa HPLC,
    MS/MS
    convention (hexane 17.2%; ethanol 105%) [36]
    Kunzea ericoides leaves TP, TF, 5-HMF, quercetin, catechin, syringic acid, etc. antioxidant
    (DPPH, FRAP)
    150–210 °C, 0–40 min
    15–35 g/mL, 4 MPa
    HPLC, UV convention (ethanol 37.5%) [37]
    Pistacia atlantica subsp. mutica hull TP, kaffesaure, ethyl vanillin, flavanomarein, etc. antioxidant (DPPH), reducing power 110–200 °C, 30–60 min,
    10–50 g/mL
    HPLC-DAD, UV HWE (85 °C 42.8%) [38]
    Satureja hortensis L. whole part TP, TF, rosmarinic acid, rutin, quercetin, etc. cytotoxic, antibacterial 140 °C, 30 min
    4 MPa, 1:20 g/mL
    HPLC-PDA, UV maceration (ethanol 57.2%), Soxhlet (ethanol 18.4%), UAE (ethanol 69.2%), MAE (ethanol 81.3%) [39]
    Urtica dioica L. leaves TP, TF, twenty-seven compounds cytotoxic, antifungal, antimicrobial 125 °C, 30 min, 3.5 MPa,
    1:30 g/mL
    UHPLC-HESI-MS/MS UAE (water 48.5%), MAE (water 100%) [40]
    Chamomilla recutita R. flowers 2 flavonoids, 4 esters, 1 amino acid, 11 phenols, etc.   150 or 200 °C, 5.0 ± 0.1 MPa,
    1.7 mL/min, 40 min
    UV, HPLC, GC-MS   [41]
    Glycine max okara TP, gallic acid, syringic acid, ferruric acid, etc. antioxidant (ABTS, DPPH, FRAP) 150 °C, 4 MPa, 5–275 min
    20 mg/mL
    UV, HPLC   [42]
    Carménère grape pomace flavanols, stilbenes, and phenolic acids   90–150 °C, 5 min, 10 MPa, 15–50% glycerol UPLC-MS   [43]
    Zingiber officinale root TP, TF, four macro- and five microelements antioxidant (OH·, ABTS, TRP, etc.) 80–180 °C, 1 h, 5MPa,
    1:10 g/mL
    UV-vis, ICP-MS convention (water, 62.5%) [44]
    Momordica foetida leaves quercetin, kaempferol, isorhamnetin   100–300 °C, 5 mL/s
    6.9± 1.4 MPa psi
    UHPLC-q-TOF-MS   [45]
    Ko and coworkers have investigated the relationship between flavonoid structure and SBWE. They found that flavonoids with an OH side chain were optimally extracted at lower temperatures than O-CH3 and H side chains. The optimal temperatures of the glycoside forms are lower than that of the less polar aglycones [6]. Turner et al. found that different glycosidic compounds may be converted by their respective aglycones in less than 10 min reaction time in water from onion waste [46]. Similar results were obtained by Nkurunziza et al. [18] and Zhang et al. [20] who investigated the extraction of four flavonoids from okara and from Puerariae lobata, respectively.

    2.2. Polyphenols

    Polyphenols, also known as polyhydroxyphenols, are a structural class that is mainly natural, by the presence of more than one phenolic unit and being deprived of nitrogen-based functions. Many fruits, vegetables, herbs, tea leaves, nuts, and algae contain high levels of naturally occurring phenols. It has been reported that polyphenols can resist oxidation [47]. As shown in Table 2, extractions of polyphenols can be carried out either using a sole solvent such as water, methanol, ethanol or a mixture of solvents such as ethanol-water and methanol-water-formic acid.
    Table 2. Subcritical water extraction of polyphenols.
    Samples Medicinal Parts Compounds Extracted Extracts Activity Extraction Conditions Analytical Methods Other Extraction Methods (Solvent, Ratios of Yields) Ref.
    Allium ursinum L. leaves TP, TF, 5-HMF, catechin, p-cumaric, ferulic acids, etc. antioxidant (DPPH, ABTS), Millard products 120–200 °C, 10–30 min, 0–1.5% HCl, 1:10 g/mL HPLC-DAD   [47]
    Terminalia chebula fruits TP, allic acid, corilagin ellagic acid antioxidant (ABTS) 120–220 °C, 2–4 mL/min, 4 MPa TLC, UV, MS, NMR, HPLC Soxhlet (water 74.5%; ethanol 46.3%), HWE (water 46.3%) [48]
    Lycium ruthenicum Murr. fruits total anthocyanin, seven anthocyanins antioxidant (ABTS, DPPH) 110–170 °C, 30–90 min, 1–3 min/L HPLC, UPLC-MS UAE (water 59.8%; methanol 81.1%) [49]
    Punica granatum L. peels TP, TF, punicalin, etc.   100–220 °C, 5–30 min, 3.0 MPa UV-vis, HPLC MAE (water 121%; ethanol 146%) [50]
    Castanea sativa shells tannins, phenolic acids, flavonoids, anthocyanins antioxidant (DPPH, FRAP, ABTS) 51–249 °C, 6–30 min UV-vis,
    LC/ESI-MS
      [51]
    Salvia officinalis L. by–products TP, TF antioxidant (DPPH, TEAC, reducing power) 120–220°C, 10–30 min, 3 MPa, 0–1.5% HCl UV maceration (water 59.9%) [52]
    Pistacia vera L. hulls gallotannin, anacardic acid, etc. antioxidant (ABTS, FRAP) 110–190 °C, 6.9 MPa, 4 mL/min HPLC-ESI/MSn UAE (methanol 83.9%)) [53]
    Zingiber officinale pulp and peel 6-gingerol, 6-shogaol antioxidant (FRAP) 10 MPa, 110–190 °C, 5–40 min HPLC convention (methanol 114%; water 77.1%) [54]
    Sorfhum bicolor L. bran TP, oligomeric procyanidins, taxifolin, taxifolin hexoside antioxidant (DPPH, ABTS), antiproliferative 110–190 °C, 5–40 min, 1:10–1:50 g/mL HPLC, ESI-MS/MS heating (water 74.9%) [55]
    Nelumbo nucifera seed epicarp TP, proanthocyanidin dimers, trimer, cyanidin, etc. antiproliferation effect (MTT) 100–180 °C, 5–25 min, 1:20–1:60 g/mL, 1–5‰ NaHSO3 HPLC-ESI-MS, UV HWE (water 33.9%) [56]
    German chamomile flowers 9 phenolic acids and derivatives antioxidant, cytotoxic, enzyme 100 °C, 1–9 MPa, 30 min UHPLC-DAD, MS/MS   [57]
    Fagopyrum tataricum grains phenols, 13 phenolics, 4 flavonoids, 3 anthocyanins antioxidant (TEAC, CAA and FRAP), cytotoxicity 220 °C, 60 min, 5 MPa, 1:60 g/mL HPLC-MS, UV UAE (water 83.5%) [58]
    A. uva–ursi herbal dust TP, TF antioxidant (DPPH, reducing power) 120–220 °C,3 MPa, 10–30 min, 0–1.5% HCl UV maceration (water 38.5%; ethanol 69.5%) [59]
    Hippophaë rhamnoides L. seed residue TP, TF, proanthocyanidins antioxidant (DPPH) 80–180 °C, 15–90 min, 1:10–1:50 g/mL, 6 MPa UV convention (water 19.6%; methanol 104%; ethanol 80.0%) [60]
    grape (Croatina) pomace TP, TF antioxidant (DPPH) 100–140 °C, 8–15 MPa, 1–2 mL/min UV convention (water 5.3%; ethanol 7.87%) [61]
    Matricaria chamomilla L. flowers polyphenolic compounds, etc. antioxidant, cytotoxic, enzyme inhibitory 65–210 °C, 30 min, 4.5 MPa UHPLC-ESI-MS/MS, UV   [62]
    Nelumbo nucifera seedpods TP, TF, proanthocyanidin dimer, isoquercetin, etc. antioxidant, antiproliferative (HepG2) 100–180 °C, 30–70 mL/g, 5–25 min, 1–6‰ NaHSO3 UV-Vis, HPLC, ESI-MSn HWE (water 91.4%) [63]
    Vitis vinifera L. grape pomace catechins, flavonols, tannins, proanthocyanidins, etc. antioxidant (DPPH, ABTS) 40–120 °C, 10 min, 10.34 MPa, 10–40% NADES UV, HPLC-ESI-MS   [64]
    sweet chestnut bark TP, tannins, ellagic and gallic acids, ellagitannins, etc. antioxidant (DPPH) 150–250 °C, 10–60 min, 10–30 mL/g, 4.5 MPa UV-Vis, HPLC   [65]
    Symphytum officinale root TP, TF antioxidant (DPPH), enzyme inhibitory 120–200 °C,10–30 min, 0–1.5% HCl UV, ELISA UAE (methanol 2.5%; ethanol 17.4%); maceration (methanol 4.4%; ethanol 29.8%) [66]
    Pinot Nero grape skins TP   80–120 °C, 2 h,10 MPa, 2–5 mL/min UV-Vis   [67]
    Coffea arabica L. spent coffee grounds TP, caffeoylquinic acid, feruloylquinic acid, etc. antioxidant (DPPH, ABTS) 160–180 °C, 35–55 min, 14.1–26.3 g/L HPLC-ABTS+, MS, UV   [68]
    Curcumalonga L. rhizomes curcumin, demethoxycurcumin   120–160 °C, 6–22 min, 1–2.5 MPa HPLC-UV, SEM   [69]
    Curcuma longa L. rhizomes α-phellandrene, curcumin, β-caryophyllene, trans-β-farnesene, β-bisabolene, γ-curcumin, etc.   90–150 °C, 1–4 mL/min, 2 MPa, 0.5–1.5 mm GC/GC-MS,
    GC -FID
    HD (80.7%), Soxhlet (n-hexane 1.2-fold) [70]
    Curcuma longa L. rhizomes curcumin, demethoxycurcumin, bisdemethoxycurcumin   110–150 °C, 1–10 min, 0.5–10 MPa HPLC convention (ethanol, 1.13-fold) [71]
    Curcuma longa L. rhizomes curcumin, demethoxycurcumin, bisdemethoxycurcumin   90–250 °C, pH 1.0–5.5 5.0 MPa, 0.5 mL/min HPLC, UPLC, LC-MS Soxhlet (acetone, 1.17-fold) [72]
    2.3. Organic Acids
    In general, organic acids in natural products are widely distributed in the leaves, roots, and fruits of the plants. The synthetic organic acids through chemical synthesis, enzymatic catalysis, and microbial fermentation are not discussed in this review. Organic acids are mostly soluble in water or ethanol and exhibit acidic properties, but they are difficult to dissolve in other organic solvents. It is generally believed that aliphatic organic acids have no special biological activity, but some natural organic acids such as citric acid, malic acid, tartaric acid, ascorbic acid, etc. have antibacterial, anti-inflammatory, hypoglycemic, antioxidant, and immune regulation effects. Depending on the organic acid in free state or in salt form, the extraction solvents could be water, dilute alkaline solution, diethyl ether, petroleum ether and cyclohexane, and other lipophilic organic solvents. A summary of recent studies on SBWE of organic acids are shown in Table 3.
    Table 3. Subcritical water extraction of organic acids.
    Samples Medicinal Parts Compounds Extracted Extracts Activity Extraction Conditions Analytical Methods Other Extraction Methods (Solvent, Ratios of Yields) Ref.
    Panax ginseng Meyer root TP, chlorogenic acid, caffeic acid, gallic acid, etc. antioxidant (DPPH, ABTS, FRAP, HRS) 100–240 °C, 15 min,
    4–9 MPa, 200 rpm
    HPLC, UV   [73]
    Helicteres isora L.   hexadecanoic acid, octadecnoic acid, heptadecen-8-carbonic acid etc. antibiofilm, antioxidant,
    antimicrobial, antienzymatic
    160 °C, 30 min, 1 MPa, 1: 30 g/mL GC-MS, UV   [74]
    XiLan olive fruit olive dreg TP, chlorogenic acid, gallic acid, syringic acid, etc. antioxidant (ABTS, DPPH, reducing power) 100–180 °C, 5–60 min, 1:20–1:60 g/mL LC-MS-IT-TOF, UV convention (methanol 3.2%; ethanol 0.6%; DMK 0.9%) [75]
    Camellia oleifera Abel. seeds free fatty acids (palmitic acid, stearate, oleic acid, etc.), tea saponin antioxidant (DPPH) 60–160 °C,2–7 MPa,
    5–60 min, 1:3–1:25 g/mL
    GC-MS, FT-IR Soxhlet (petroleum ether 100%), cold pressed (100%) [76]
    sunflower seeds (Natura) dehulled seeds total proteins, total carbohydrates, TP antioxidant capacities 60–160 °C, 5–120 min,
    3 MPa, 1:10–1:30 g/mL
    GC-FID, UV-Vis, HPLC Soxhlet (hexane 67.3%) [77]
    cottonseed (Egypt) cottonseed linoleic acid, palmatic acid, oleic acid, myristic acid   180–280 °C, 5–60 min, 1:2–2:1 g/mL GC-FID, heating (hexane 89.5%) [78]
    green coffee (Robusta Uganda) beans chlorogenic acid   130–170 °C, 40–90 min, 0–30 % ethanol HPLC convention (ethanol 66.7%) [79]
    Nannochloropsis gaditana   fatty acids, omega-3, omega-6, lipid   156.1–273.9 °C, 6.6–23.4 min, 33–117 g/L GC-FID, SEM Soxhlet (n-hexane 100%) [80]
    Saccharina japonica   gallic, caffeic, vanillic, syringic, chlorogenic, p-hydroxybenzoic acids, etc. antioxidant (DPPH, ABTS, total antioxidant (FRAP) 100–250 °C, 5 min, 5 MPa, 0.25–1.00 M ILs HPLC, UV convention (DMK 0.2%; DCM 0.3%; Et2O 0.8%; IL 1.6%) [81]
    Haematococcus pluvialis   p-hydroxybenzoic acid, gallic acid, siringic acid, vanillic acid, etc. antioxidant (ABTS, TEAC), antimicrobial activity 50–200 °C, 20 min,
    10 MPa
    HPLC-DAD-MS, SEM, GC-MS   [82]
    Momordica charantia fruits TP, gallic acid, gentisic acid, chlorogenic acid antioxidant (ABTS) 130–200 °C, 10 MPa, 2–5 mL/min HPLC, UV Soxhlet (methanol 4.9%), UAE (methanol 4.0%) [83]
    Morus nigra L., Teucrium chamaedrys L., Geranium macrorrhizum L., Symphytum officinale L. leaves, flowers TP, chlorogenic acid, gallic acid, vanillic acid, etc. antioxidant, antifungal, antibacterial, cytotoxic 60–200 °C, 30 min, 1 MPa, 1:40g/mL HPLC-DADUV   [84]
    Prunus avium L., Prunus cerasus L. stems 3 alcohols, 10 organic acids, etc. antioxidant, antiproliferative 150 °C, 30 min, 2 MPa GC-MS, UV   [85]
    Castanea sativa nuts ellagic acid, feru lic acid, gallic acid, etc. antioxidant 120–135 °C,
    15–60 min
    HPLC   [86]
    Solanum tuberosum potato peel TP, gallic acid, caffeic acid, chlorogenic acid, protocatechuic acid, etc.   100–240 °C,
    30–120 min, 6 MPa
    HPLC, UV convention (methanol 1.6%; ethanol 2.0%) [87]
    Actinidia deliciosa pomace TP, chlorogenic acid, protocatechuic acid, etc. antioxidant (DPPH, FRAP, ABTS) 170–225 °C
    10–180 min, 5 MPa
    UV, HPLC, pH   [88]
    hypnea musciformis   chlorogenic, protocatechuic, and gallic acids, TP, TF, etc. antioxidant (DPPH, ABTS), emulsify 120–270 °C, 10 min,
    1:50–1:150 g/mL
    pH, UV, HPLC   [89]
    Carica papaya L. papaya
    seeds
    TP, 18 phenolic acids, 20 flavonoids, 1 stilbene, etc. antioxidant (DPPH, β-carotene bleaching) 70–150 °C, 10 MPa, 1–40 min, 4 mL/min LC-ESI-MS/MS, UV Soxhlet (water 37.1%) [90]
    Zingiber officinale ginger
    rhizome
    12 sugars, 8 diols, 4 phenolic acids, etc. antimicrobial, cytotoxic 150 °C, 1 h, 1:10 g/mL HPLC-ESI-TOFMS heating (water) [91]
    Chlorella sp. microalgae   TP, caffeic acid, ferulic acid, p-coumaric acid antioxidant (DPPH) 100–250 °C,
    5–20 min
    UV, SEM, HPLC   [92]
    Vitis vinifera vine-canes TP, flavonoids, phenolic acids, flavonols antioxidant, antiradical 125–250 °C, 50 min HPLC, UV   [93]
    Cinnamomum Cassia Blume cinnamon coumarin, cinnamic acid, cinnamaldehyde, cinnamyl alcohol, etc.   110–130°C, 20–60 min, 2–4 MPa, 1:10 g/mL HPLC  
    The use of SBWE was explored for the extraction of gallic acid, chlorogenic acid, caffeic acid, ferulic acid, vanillic acid, and coumaric acid from various matrices. Inevitably, some other active components such as phenolics [73], flavonoids [45], proteins [73], lipids, peptides, amino acids, and other organic compounds were often coextracted. Švarc-Gajić et al. [85] have used SBWE for the extraction of alcohols, organic acids, sugars, and other organic compounds from both sweet and sour cherry stems, finding the chemical compositions of the two samples similar. Harun et al. [80] reported lipid extraction with a relatively high content of eicosapentaenoic acid from Nannochloropsis gaditana, finding 237 °C and 14 min to be the optimum extraction conditions.

    2.4. Glycosides

    Glycosides are compounds in which sugars or sugar derivatives are bound to another type of non-sugar substance (also called aglycones, ligands or substituents). Glycosides are linked by an O- N-, S-, or C-glycosidic bond between a sugar and a non-sugar component, which are widely found in the root, stems, leaves, flowers, and fruits of plants. Most glycosides are colored crystals, and generally a little bitter.
    Glycosides extracted by SBWE have been proven to have antioxidant activities and tyrosinase inhibitory activity, as shown in Table 4 [94][95][96][97][98][99][100][101][102][103]. Gao et al. [95] has performed SBWE of phenolic compounds from pomegranate seed residues at 80–280 °C. The results showed that TP increased with the rise of extraction temperature from 80 °C to 220 °C and decreased from 220 °C to 280 °C. At 80–220 °C, the breakage of the bonds led to the increase of TP, however, a higher temperature caused the phenolics to degrade. In addition, they compared SBWE with leaching and UAE using water (room temperature) and organic solvents namely methanol, ethanol, and acetone. TP and antioxidant capacities of SBWE (120 °C) were not as high as organic solvents; however, with respect to the extraction time (2 h for leaching vs. 30 min for SBWE) and toxicity, subcritical water is more acceptable. Meng and Cheng [100] have studied 13 phenolic compounds and 20 inorganic elements of Erigeron breviscapus. They also have found similar results, as the glycosides are not stable at a high temperature and with a long extraction time. For example, scutellarein and apigenine are the aglycones of corresponding acutellarin and apigenin 7-glucuronide, when at high temperature glycosidic bonds become unstable and begin to decompose to its glycone and aglycone. Haznedaroglu et al. [98] have optimized the parameters such as temperature, extraction time, and flow rate. Temperature and extraction time were found as the most effective parameters for TP and total flavonoids while extraction time and flow rate for anthocyanin contents. In addition, temperature and time were the leading parameters for the effectiveness of extracts on tyrosinase inhibition.
    Table 4. Subcritical water extraction of glycosides.
    Samples Medicinal Parts Compounds Extracted Extracts Activity Extraction Conditions Analytical Methods Other Extraction Methods (Solvent, Ratios of Yields) Ref.
    Phaleria macrocarpa fruits mangiferin   323–423 K, 1–7 h,
    0.7–4.0 MPa
    HPLC, LC-MS convention (water 69.6%; ethanol 34.1%; methanol 108%), HRE (water 85.7%; ethanol 60.8%; methanol 115%), Soxhlet (water 86.1%; ethanol 55.8%; methanol 113% methanol) [94]
    Punica granatum L. pomegranate seed TP, kaempferol
    -3-O-rutinoside
    antioxidant (DPPH, ABTS) 80–280 °C, 5–120 min, 1:10–1:50 g/mL, 6.0 MPa HPLC-DAD, UV, HPLC-ABTS+ leaching (water 40.6%; methanol 79.7%; ethanol 41.7%; acetone 45.5%), UAE (water11.3%; methanol 20.6%; ethanol 18.9%; acetone 15.2%), Soxhlet (methanol 71.4%; acetone 39.7%) [95]
    Teucrium montanum L. aerial parts rutin, naringin, epicatechin, etc. antioxidant (DPPH, FRAP) 60–200 °C, 30 min,
    1–10 MPa, 1:10 g/mL
    HPLC-PDA, UV   [96]
    Paeonia lactiflora root albiflorin, paeoniflorin   100–260 °C, 10–60 min,10–40 mL/g HPLC reflux (water 83.5%), UAE (ethanol 77.8%) [97]
    Morus nigra L. fruits TP, TF, cyanidin 3-glucoside, etc. 40–80 °C, 20–60 min, 2–6 mL/min, 15 MPa tyrosinase inhibitory activity UPLC-DAD-ESI-MS/MS shaker (ethanol:water 116%), UAE (ethanol:water:TFA 134%) [98]
    Stevia rebaudiana leaves TP, stevioside,
    rebaudioside A
    antioxidants (DPPH) 100–150°C, 30–60 min, 23 MPa, 1:10 g/mL HPLC-UV, UV   [99]
    Erigeron breviscapus whole parts scutellarin, 20 inorganic elements, etc. antioxidant (DPPH) 120–140 °C, 5–15 min, 150–420 um HPLC, HPLC-MS reflux (methanol 86.1%; ethanol 84.8%) [100]
    Mangifera indica L. leaves quercetin3-d-glucoside, mangiferin antioxidant (DPPH) 100 °C, 4 MPa, 10 g/min, 3 h UV, HPLC SCCO2 (20% methanol 18.7%) [101]
    Crocus sativus L. stigmas picrocrocin, safranal, crocin   5–15 min, 105–125 °C GC-MS, UV, HPLC   [102]
    Glycyrrhiza uralensis Fisch licorice
    root
    TP, glycyrrhetic acid, glycyrrhizin, liquiritin antioxidant (DPPH, reducing power) 50–300 °C, 10–60 min,
    0.002–5 MPa
    HPLC, UV-Vis   [103]
    2.5. Carbohydrates
    Carbohydrates is a very common term that include sugars, starch, and cellulose, which are an important class of organic compounds widely distributed in nature. The saccharides are divided into four groups: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. As shown in Table 5, carbohydrates extracted by SBWE possess antioxidants [104], antimitotic [105], and growth inhibitory effects [106].
    Table 5. Subcritical water extraction of carbohydrates.
    Samples Medicinal Parts Compounds Extracted Activity/Mixtures Extraction Conditions Analytical Methods Other Extraction Methods (Solvent, Ratios of Yields) Ref.
    Lycium barbarum berries total sugar content antioxidant (FRAP, TEAC), immunomodulatory 1:30 g/mL, 110 °C, 5 MPa HPGPC HWE (water 71.5%), UAE (water 89.9%), UWE (water 132%) [107]
    sunflower sunflower heads galacturonic acid, pectin   10–50 min, 2–8 mL/g, 100–140 °C, 0.2–1 MPa TG/TGA, DSC, UV−vis, FTIR, HPSEC, NMR   [108]
    Aronia melanocarpa chokeberry stems 1 amino acid, 8 alcohols, 11 sugars, 2 fatty acids, etc. antioxidant (DPPH), enzyme inhibitory activity 130 °C, 3.5 MPa, 20 min, 1:20 g/mL GC-MS   [109]
    Lentinus edodes fruit bodies hetero–polysaccharides, xylose, mannose, etc. antioxidant (OH·, DPPH, ABTS) 120–160 °C,30–50 min, 0.033–0.05 g/mL UV-vis, SEM, GC, GPC, FT-IR   [110]
    Lentinus edodes fruit bodies l-rhamnose, d-arabinose, d-xylose, d-mannose antioxidant (ABTS), growth inhibitory effect 100–150 °C, 10–30 min, 5 MPa FT-IR, UV-Vis, AFM, GC, HP SEC-MALLS   [106]
    Lentinus edodes fruit bodies polysaccharides, rhamnose, arabinose, xylose, etc. antioxidant (DPPH, reducing power) 140 °C, 40 min, 1:25 g/mL, 5 MPa GC, FT-IR, AFM, SEM   [111]
    Lentinula edodes fruit bodies TCC, total β-glucan, chitin HMGCR, immuno-
    modulatory
    200 °C, 11.7 MPa, 15–60 min GC-MS, HPSEC, NMR UAE (water 65.2%), HWE (water 32.3%), SPE (water 33.0%) [112]
    Grifola frondosa fruit bodies total polysaccharide, total protein antioxidant (DPPH, reducing power) 100–230 °C, 2–4 min, 20–100 mesh, 5 MPa FT-IR, SEM HWE (water ~87.8%) [113]
    Sagittaria sagittifolia L. fruit bodies polysaccharides antioxidant (DPPH, ABTS, reducing power) 150–190°C, 12–20 min, 1:20–1:40 g/mL, pH 7–9 FT-IR, 1H and 13C NMR, UV HWE (water 55.8%) [114]
    Sagittaria sagittifolia L. fruit bodies l-rhamnose, d-arabinose, d-xylose, d-mannose antioxidant,
    immuno-modulatory
    170°C, 16 min HPLC, GC, SEM, IR, AFM, HPSEC-MALLS HWE (water 75.6%); UAE (water 96.1%) [115]
    Sagittaria sagittifolia L. fruit bodies α-pyranose polysaccharide, β-pyranose polysaccharide immuno-stimulatory 1 MPa, pH 7,170 °C, 16 min, 30:1 mL/g IR, GC-FID, UV, HPSEC, AFM   [116]
    Cordyceps militaris fruit bodies total sugars, protein and uronic acid   180 °C, 13 min, pH = 8,
    21 mL/g
    IR, GC, AFM, GPC-MALLS   [117]
    wheat bran monosaccharide, etc. antioxidants (DPPH) 160–180°C, 5–60 min HPAEC-PAD, SEC   [104]
    Saccharina japonica   fucoidan, fucose, glucose, galactose, mannose, etc. antioxidant, antimitotic
    anti-proliferative
    100–180 °C, 5–15 min, 2–8 MPa FTIR, TGA, UV-Vis convention (0.05 M HCl 100%) [105]
    Citrus grandis L. pomelo peel pectin   90–120 °C, 3–10 MPa HPSEC-MALLS   [118]
    Theobroma cacao L. cacao pod husks xylose, arabinose, etc.   121 °C, 30 min, 10.3 MPa FT-IR, GC-FID, SEM convention (4% citric acid 76.1%) [119]
    Kappaphycus alvarezii. A   ĸ-carrageenan, glucose, 3,6-anhydrogalactose, etc. antioxidant (DPPH, ABTS) 60–180°C, 5 MPa, 5 min FTIR, TGA, XRD convention (water 94.3%; water with IL 101%) [120]
    Pseuderanthemum palatiferum leaves TCC, monosaccharides anticoagulant, antioxidant 150–200°C, 5–10 mL/min HPLC, GPC, NMR, UV convention (0.1 M NaOH 48.8%) [121]
    wheat bran TCC, reducing sugar, arabinose, xylose, etc. antioxidant, α-amylase inhibitory 140 °C, 5 MPa, 30 min SEC-MALLS, FT-IR, DLS, DSC, UV SBWE (water with citric acid 97.6%); UWE (water with citric acid 103%) [122]
    Lycium barbarum L. fruits polysaccharides antioxidant (O2·, OH·, DPPH) 5 MPa, 25 mL/g, 110 °C, 1 h UV HWE (water 86.2%); UAE (water74.9%); UWE (water 111%) [123]
    Cocos nucifera L. defatted coconut mannose, galactosamine, xylose, rhamnose, etc. antioxidant,
    hypoglycaemic, adsorption
    1:10–1:50 g/mL, 10–50 min,
    120–200 °C, 20–100 mesh
    HPLC, XRD, TGA, DTGA, SEM, FT-IR   [124]
    okara   polysaccharides, TP, TF antioxidant (ABTS, DPPH) 1:30 g/mL, 160–230 °C, 10 min UV   [125]
    Saccharina japonica   polysaccharide, fucoidan, alginate antioxidant (ABTS, DPPH, FRAP) 100 –150 °C, 1–5 MPa, 1:30–1:50 g/mL IR, DSC, TGA, 1HNMR, HPLC, HPSEC-ELSD   [126]
    Passiflora edulis fruit peel pectic polysaccharide, mannose, glucose, etc. antioxidant (DPPH) 100–160 °C, 5.64–7.94 min, 10–30% ethanol HPLC, UV, viscometer   [127]
    Chlorella vulgaris, Sargassum vulgare, Sargassum muticum, Porphyra spp., Cystoseira abies–marina, Undaria pinnatifida and Halopitys incurvus, Rosmarinus officinalis L., Thymus vulgaris, Verbena officinalis microalgae, algae, leaves sugar, TP, melanoidins antioxidant (ABTS, O2¯) 100–200 °C, 20 min, 10 MPa UV   [128]
    rice bran bran protein, TCC, TP antioxidant (DPPH) 120–250 °C,0.5–5 mL/min UV, UV-Vis   [129]
    Nizamuddinia zanardinii   TCC, rhamnose, xylose, arabinose, fucoidan, fucose antioxidant, anticancer, macrophage, etc. 425 rpm, 10–30 min, 90–150 °C, 0–40 mL/g, 0.75 MPa, 1500 W FT-IR, GC-MS, SEM, UV, HPSEC-MALLS-RI   [130]
    Dendrobiumnobile Lindl. stems polysaccharide, arabinose, galactose, glucose, etc. antioxidant (OH·, ABTS) 0.5–1.5 MPa, 5–20 min 120–160 °C, 1:25 g/mL UV−vis, GPC, HPLC, HPAEC   [131]
    Ecklonia maxima   TP, polysaccharide, sulphate, and alginate antioxidant (ABTS) 100–180 °C, 5–30 min, 10–50 mL/g, 4 MPa UV, elemental analysis, ICP-MS convention (70% ethanol 0%; 0.05 M HCl 20.1%) [132]
    Vitis vinifera grape pomace glucose, fructose, galactose, arabinose, mannose, etc. antimicrobial, antioxidant (DPPH) 170–210 °C, 10 MPa, 5–10 mL/min HPLC, UV   [133]
    green coffee beans spent coffee grounds carbohydrates, phenolics antioxidant, antibacterial 150–220 °C, 7 MPa, 10 mL/min, HPLC, UV,   [134]
    Tamarindus indica seed TP, xyloglucan antioxidant (DPPH) 100–200°C, 1:20 g/mL SEC, UV convention (water 74.6%) [135]
    Mentha arvensis leaves carbohydrates, apocynin antioxidant (DPPH) 180–260 °C, 1:20 g/mL, 5 min HPLC, GC-MS, UV,   [136]
    2.6. Essential Oils, Alkaloids, Quinones, Terpenes, Lignans, and Steroids
    Subcritical water has also bee used to extract essential oils, alkaloids, quinones, terpenes, lignans, and steroids from plant and other materials. A summary of SBWE of essential oils, alkaloids, quinones, terpenes, lignans, and steroids can be found in Table 6.

    Table 6. SBWE of essential oils, alkaloids, quinones, terpenes, lignans, and steroids.

    Samples

    Medicinal parts

    Compounds Extracted

    Extraction Conditions

    Methods

    Other Extraction Methods (Solvent, Ratios of Yields)

    Ref.

     

    Essential oils

     

    Thymbra spicata L.

    leaves

    α-thujene, α-pinene, terpinen-4-ol, p-cymene, γ-terpinene, 1-carvone, thymol, carvacrol, etc.

    100–175 °C, 1–3 mL/min, 2–9 MPa, 30 min

    GC-TOF/MS, GC-FID

     

     

    [12]           

    Aquilaria malaccensis

    leaves

    butanal, cyclopentanone, acetoxyacetone, benzaldehyde, acetophenone, creosol,etc.

    100–271 °C, 1–34 min, 0.08–0.22 g/mL

    GC-MS, SEM, FT-IR

    HD (95.4%)

    [22]

    Mentha piperita L.

    peppermint

    leaves

    TP, menthone, menthol, eriocitrin, etc.

    40–160 °C, 10.3 MPa, 1–30 min

    GC-MS, FID, HPLC

    convention (methanol 53.2%)

    [28]

    Coriandrum sativum L.

     

    coriander seeds

    thujene, sabinene, pinene, myrcene, cymene, limonene, ocimene, terpinene, terpinolene, etc.

    100–175 °C, 1–4 mL/min, 0.25–1 mm, 2 MPa, 20 min

    GC-FID, GC-MS

    HD (1.54-fold), Soxhlet (hexane 1.4-fold)

    [174]

    Coriandrum sativum L.

     

    coriander seeds

    3,4-dimethoxycinnamic acid, coumaric acid, sinapic acid,cis-and trans-linalooloxides, linalool, etc.

    100–200°C, 10–30 min, 3–9 MPa

    HPLC-MS/MS, GC-MS

     

    [175]

    Kaempferia galangal L.

    rhizome

    ethyl-p-methoxycinnamate, d-limonene, eucalyptol, tridecane, camphor, borneol, tetradecane, etc.

    120 °C, 10 MPa, 30 min

    GC-MS

    HD (82.3%), UWE (100%)

    [176]

    Piper betle

    leaves

    4-allyl resorcinol, chavibetol

    2 MPa, 10–90 min, 50–250 °C, 0.25–1 mm, 1–4 mL/min

    HPLC-UV

    convention (water 92.2–111%; methanol 96.6–110 %)

    [177]

    Aquilaria malaccensis

    leaves

    nonacosane, triacontane, pentadecanal, 9-octadecenal, (Z)-, tetradecanal, tetrapentacontane, guaiacol

    100–271 °C, 1–34 min

    GC/MS, SEM, BET

     

    [178]

    laurel

    leaves

    α-phellandrene, β-pinene, 1,8-cineole, borneol, nona-3,7-dienol, isobornyl acetate, γ-terpineol, etc.

    15 min, 50–200 °C, 1.5–15 MPa, 0.5–5.0 mL/min

    GC-MS, GC-FID

     

    [179]

    Citrus hystrix

    leaves

    linalool, isopulegol, neoisopulegol, citronellal, 4-terpineol, citronellol, geraniol, menthoglycol, etc.

    120–180 °C, 5–20 g/mL, 5–30 min

    GC-MS

    HD (28.2%)

    [180]

    Coriandrum sativum L.

     

    coriander

    seeds

    α-pinene, β-pinene, camphor, methylchavicol, γ-terpinene, linalool, geraniol, carvacrol, etc.

    100–200 °C, 1:10 g/mL, 2 MPa, 20 min

    GC-MS, GC-FID

    HD (27.0%), Soxhlet (DCM 6.5-fold), SCCO2 (4-fold)

    [181]

    Lavandula L.

    lavender flowers

    a-thujene, a-pinene, camphene, sabinene, pinene, myrcene, hexylacetate, terpinene, limonene, etc.

    125 °C, 3 MPa, 30 min

    GC-MS, FID

    HD (1.2-fold), US-HD (1.3-fold), NaCl-HD (1.3-fold)

    [182]

     

    Alkaloids

    Sophora

    Ait.

    root

    cytisine, matrine, sophoridine, sophocarpine, oxymatrine

    70–190 °C, 5–14 min, 4.0–13.8 MPa

    CE

    ASE (ethanol 78.1%)

    [16]

    black tea brick

    leaves

    theophylline, epicatechin gallate, caffeine, etc.

    120–180 °C, 7–42 min, 6–18 mL/min

    HPLC

     

    [46]

    Symphytum officinale L.

    root

    lycopsamine, echimidine, lasiocarpine, symviridine

    60–120 °C, 40 min

    HPLC, LC-MS, MSn

    HRE (methanol 2.8-fold)

    [183]

    hydrastis canadensis

    root

    hydrastine, berberine

    100–160 °C,1–10 MPa, 5–60 min, 0.5–1.5 mL/min

    HPLC-DAD

    reflux (methanol 90.8%), UAE (methanol 106%)

    [184]

    cocoa

    shells

    TP, theobromine, theophylline, caffeine, epicatechin, etc.

    120–220 °C, 15–75 min,

    1:10–1:30 g/mL

    HPLC, UV

     

    [185]

    Musaceae, Beta vulgaris

    peels

    dopamine, total betacyanin, betaxanthin

    150°C, 5 min, 3 MPa, 1:20 g/mL

    HPLC, UV-Vis

    infusion (100%), decoction (1.2-fold), maceration (97.4%), UAE (101%), MAE (50.3%)

    [186]

    Coffeea

    arabica, C. arabica, C. canephora var. robusta,

    C. canephora var. robusta

    coffee silver skin

    total sugar, reducing sugar, protein, TP, caffeine, HMF, etc.

    180–270 °C, 10 min, 1.0–5.3 MPa

    HPLC, UV

    convention (0.1 M HCl 96.6%; 0.1 M NaOH 1.5-fold)

    [187]

     

    Quinones

    Rheum tanguticum

    root

    damnacanthal

    33–67 min, 100–200°C, 1.4–4.6 mL/min,

    HPLC, NMR, HSCCC

     

    [17]

    Garcinia mangostana Linn

    mangosteen pericarps

    TP, xanthone

    120–160 °C, 1–10 MPa, 5–60 min, 10–30% DES

    UV-vis, FT-IR, SEM

     

    [188]

    Phaleria macrocarpa

    mahkota dewa fruits

     mangiferin

    4.0 MPa, 5 h, 50–150 °C

    HPLC

     

    [189]

    Lithospermum erythrorhizon

     

    root

    shikonin, acetylshikonin, β-dimethylacrylshikonin, etc.

    40–60 mesh, 120 °C, 5 MPa

    UV, HPLC-ELSD

    SCCO2 (86.3%), Soxhlet (ethyl acetate 95.4%), UWE (1.4-fold)

    [190]

    Morinda citrifolia

    root

    alizarin

    4 MPa, 150 and 220 °C, 1.6–4 mL/min

    RP-HPLC-UV

     

    [191]

    Morinda citrifolia

    root

    1,2-dihydroxyanthraquinone, alizarin

    110–220 °C, 2–6 mL/min

    UV-Vis

    ethanol (3 d)

    [192]

    Morinda citrifolia

    root

     

    4 MPa, 150–200 °C, 2–6 mL/min

    UV-Vis

    convention (ethanol 81.16%), Soxhlet (ethanol 97.94%), UAE (ethanol 79.62%) SWBE (96.41%)

    [193]

     

    Terpenes

    Hedyotis diffusa Willd.

    whole

    plants

    ursolic acid

     

    120–200 °C, 10–50 min,

    20–40 mL/g, 0.6–3.0 MPa

    HPLC-ESI-TOF-MS

    maceration (ethanol 58.8%), HRE (ethanol 78.4%), UAE (ethanol 90.4%), MAE (ethanol 74.9%)

    [13]

    Centella asiatica

    whole

    plants

    asiatic acid, asiaticoside

    100–250°C, 10–40 MPa, 5h

    HPLC, DLS

     

    [14]

    basil, oregano

    leaves

    limonene, citronellol, etc.

    100 and 150 °C, 10 min

    GC-FID

     

    [48]

    Ganoderma lucidum

    fruits

    ganodermanon-triol, ganoderic acids, lucidumol

    100–200 °C, 5–10 MPa, 5–60 min

    HPLC, SEC-UV, SEM, MALDI-TOF

     

    [194]

    Orostachys japonicus

    stems,

    leaves

    triterpene, camellia, etc.

    110–260 °C, 5–20 min, 10 MPa

    HPLC-MS

     

    [195]

    Betula pendula

    birch bark

    betulinic acid

    160–200 °C, 10–30min, 10 MPa

    HPLC

     

    [196]

    Inula racemose

    plants

    igalan, soalantolactone, alantolactone

    23.2–56.8 min, 1.3–4.7

     mL/min, 129.5–230.5 °C

    HPLC, 1H-NMR 13C-NMR, MS

    Soxhlet (ethanol 100%), UAE (ethanol 70.36%), SCCO2 (76.06%)

    [197]

    Semen richonsanthis

    seeds

    3,29-dibenzoylkarounidiol, polysaccharides

    80–160 °C, 5.0–30.0 min

    HPLC, UV, SEM

     

    [198]

    Cucurbita pepo

    pumpkin peel

    14 carotenoid compounds

    120 °C, 3 h, 5 MPa

    UV, HPLC

    SCCO2 (75.4%)

    [199]

    Betula pendula

    birch bark

    sesquiterpenes, steroids

    10 min, 100–200 °C

    LC, GC/MS, NMR

     

    [200]

    S. rebaudiana

    Bertoni leaves

    steviol glycosides, tannins, chlorophyll A

    100–160 °C, 5–10 min,

    10.34 MPa, 1:3 g/mL

    HPLC, UV, UV/Vis

     

    [201]

     

    Lignans

    Linum usitatissimum L.

    flaxseed

    SDG lignan, phenolics, flavonoids

    160–180 °C, 5–60 min, 10 MPa

    HPLC-MS/MS, UV

     

    [41]

    Sesamum indicum L.

    sesame seeds

    lignans, TP, flavonoids, flavonols

    140–220 °C, 8–14 MPa, 0–95% ethanol, 0–75 min

    UV

     

    [42]

    Linum usitatissimum L.

    flaxseed

    total fat content, SDG lignan

    120–180 °C, 15–90 min, 10–13.8 MPa

    HPLC-MS/MS, UV

     

    [43]

    Sinopodophyllum hexandrum

    root

    podophyllotoxin

    12 mL/g, 3 MPa, 2ml/min, 120–240 °C

    HPLC

     

    [202]

     

    Steroids

    Pfaffia

    glomerata, Amaranthaceae

    ginseng root

    sugar, fructooligosaccharides, beta-ecdysone

    80–180 °C, 5–15 min, 2–12 MPa

    HPLC-ELSD, HPLC

     

    [24]

    Panax ginseng C.A. Meyer

    ginseng root

    TP, maltol, panaxadiol, panaxatriol

    150–200 °C, 5–30 min, 100 MPa

    HPLC, UV

    convention (water 32.6%; methanol 24.1%; ethanol 18.7%)

    [25]

    Panax ginseng C.A. Meyer

    ginseng root

    total ginsenosides, total sugar,

    1-oleanane ginsenosides, etc.

    120–200 °C, 20 min, 1:20 g/mL, 6.0 MPa

    FT-IR, UV, UFLC-MS/MS

    heating (water, 30.9%; ethanol 94.4%)

    [26]

    grapevine

    root,

    wood, cane

    E-piceid, E-piceatannol, E-resveratrol, E-parthenocissin, etc.

    100–190 °C, 5–30 min, 10 MPa

    LC-DAD/ESI-IT, Q-TOF, NMR

    ASE (116% for cane; 103% for wood; 1.5-fold for root)

    [203]

    Withania somnifera L

    root leaves

    TP, withanoside IV V, withaferin A, withanolide A, B

    100–200 °C, 10–30 min, 10 MPa

    HPLC, UV

    maceration (water 31.7%), Soxhlet (ethanol 39.2%), MAE (methanol 45.8%)

    [204]

    Acanthophyllum glandulosum

    root

    saponin

     

    121 °C, 0.15MPa,

    15 min, pH 4–9

    FT-IR, UV-vis, HPLC

     

    [205]

    Vaccaria segetalis

    cowcock seed

    vaccarosides, segetosides

    125–175 °C, 15–180 min

     

    USE (methanol 46.8%; water 27.9%; ethanol 5.2%)

    [206]

     
     

     

    The entry is from 10.3390/molecules26134004

    References

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