Aloe vera Gel: Comparison
Please note this is a comparison between Version 1 by Chunpeng Wan and Version 2 by Vivi Li.

Edible coating gels developed from the Aloe vera plant have been used as a traditional medicine for about 3000 years. Aloe vera contains approximately 110 potentially active constituents from six different classes: chromone and its glycoside derivatives; anthraquinone and its glycoside derivatives; flavonoids; phenylpropanoids and coumarins; phenylpyrone and phenol derivatives; and phytosterols and others. Apart from medicinal uses, Aloe gels have an important role in food preservation as edible coatings. They provide an edible barrier for atmospheric gases and moisture and help to reduce the respiration and transpiration of fresh produce, which helps to preserve its postharvest quality.

  • Aloe vera
  • chemical constituents
  • antimicrobial activity
  • postharvest storage
  • biodegradable
  • edible coating

1. Introduction

Food quality mainly refers to three attributes: external (size, color, appearance, etc.), internal (taste, color, juicy, texture, seedless, etc.), and hidden (food safety and nutritional contents). External and internal quality attributes were of greatest importance to consumers for many years. However, since the occurrence of food-derived health problems has begun to increase, consumers have started to pay more attention to the hidden quality attributes of fresh produce and are asking for food to be free of chemical residues [1]. Fungicides and other agrochemicals are of great importance in controlling postharvest diseases and have crucial role for the preservation of the postharvest quality, but misuse and/or excessive use of them might cause negative impacts on human health [2][3][2,3]. There is an increasing effort in postharvest studies to develop natural preservatives and antimicrobials to extend the storage duration of foods without chemical preservatives [4][5][4,5]. So far, many storage techniques and natural preservatives have been developed to extend the postharvest life of foods. The currently utilized natural preservatives are chitosan [6][7][6,7], essential oils [8][9][8,9], propolis extract [10], plant extracts [11][12][11,12], edible coatings [13][14][13,14], and organic salts [15]. Among these, edible coatings have been receiving more attention in recent years due to their potential for developing edible packaging materials [16]. Weight loss, changes in textural quality, changes in chemical structure, and microbial pathogens (mostly fungus) are the most important postharvest problems for foods [17][18][17,18].
Aloe belongs to the family of Xanthorrhoeaceae, which consists of about 420 species, and has been used as a traditional medicine for about 3000 years [19]. The perennial plant known as Aloe vera is Aloe barbadensis Miller, which is a well-known pharmaceutical herb that has long been used in traditional Chinese medicine for the treatment of various diseases. It is widely distributed in the semitropical regions and cultivated in many provinces of China.

2. Chemical Constituents of Aloe vera

The two-main class active constituent of the Aloe vera plant extract are chromone and anthraquinone and its glycoside derivatives, alongside others such as phenylpyrone derivatives, flavonoids, phenylpropanoids, coumarins, phytosterols, naphthalene analogs, lipids, and vitamins.

2.1. Chromone and its Glycoside Derivatives

Approximately 29 chromone derivatives were isolated and identified from Aloe vera (Table 1Figure 1). Aloesin (1, formerly called aloeresin B), aloeresin A (23), isoaloeresin D (13) and aloeresin E (9) are the most significant active constituents of Aloe vera. Three aloediols (7, 8, and 9) were isolated and identified from Aloe vera, but the absolute configuration has not yet been determined.
Figure 1. Chemical structure of chromone and its glycoside derivatives from Aloe vera.
Table 1. Chromone and its glycoside derivatives isolated and identified from Aloe vera.
No Constituents Molecular Formula Exact Mass References
1 aloesin C19H22O9 394.1264 [20][
]
[
37
]

2.3. Flavonoids

Approximately 13 flavonoids and their glycoside derivatives were isolated and identified from Aloe vera (Table 3Figure 3), including three types; namely flavone (62–67), flavonol (68–72), and flavan-3-ol (73,74).
Figure 3. Chemical structure of flavonoids from Aloe vera.
Table 3. Flavonoids isolated and identified from Aloe vera.
No Constituents Molecular formula Exact Mass References
62 apigenin 106C15H1021][22,23]
O5 270.0528 148.0524[38][40] 2 neoaloesin A C19
[40H22O9 9394.1264 [22][24]
] 418.1264 [27] 63[29] luteolin 3 8-C
]
90
dihydrocoumarin ethyl ester
C
2
5
H
26O7 438.1679 [41][43]

2.5. Phenylpyrone and Phenol Derivatives

Approximately three phenylpyrone derivatives (91–93), one triglucosylated naphthalene derivative named aloveroside A (94), and one 1-methyltetralin derivative feroxidin (95) were isolated and identified from Aloe vera (Table 5Figure 5). Nine phenol derivatives (96–104) and vitamin C (105) were also isolated from Aloe vera.
Figure 5. Chemical structure of phenylpyrone and phenol derivatives from Aloe vera.
Table 5. Phenylpyrone and phenol derivatives isolated and identified from Aloe vera.
No Constituents Molecular Formula Exact Mass References
91 aloenin A C19H22O10[42]
410.1213 [42][44] cycloartanol C30H52O 428.4018 [43][45] C15H10O6 286.0477 [39 76 p-coumaric C9H8O
1073 164.0473]
92 aloenin B 24-methylene-cycloartanolC34H C3138[41]
OH5217O[38] 718.2109[40 440.4018]
[33] [43][42][35,44] [45] -glucosyl-(R)-aloesol C19H24O9 396.142
32 6′-O-acetyl-aloin A C23H24O10[20][22]
460.1369 [ 64 77isovitexin C21
108 lophenolH20O C28H481027] 432.1056[29] caffeic acid C9H8O[ 4
934 p-coumaroyl aloenin C28H28O39][41]
180.0423 [38][40]
12 556.1581 O[33 400.3705][35] [43][45 8-C-glucosyl-7-methoxy-(R)-aloesol C20H26O
33 6′-O-acetyl-aloin B C239 H24O410.1577 6510 isoorientin C21H20O460.1369[20][22]
] 11[27][29] 448.1006 [39][41]
78 ferulic acid C10H10O4 194.0579 672.2265[38] [33][35] 5 8-
109 24-methyl-lophenolC-glucosyl-(S)-aloesol C19H24O9 396.142
34 10-hydroxyaloins A C21H22[23][25]
[40]
94 aloveroside A C30H40O17 C29H50O 414.3862 [43][45] O10 434.1213 [26][30][28,32] 66 saponarin C27H30O15 594.1585 95 feroxidin C11H14O3 194.0943 [
110 24-ethyl-lophenol C30H52O 428.401830][32] 6 8-C-glucosyl-7-methoxy-(S)-aloesol C20H26O9 410.1577 [23][24][25,26]
[39][41]
79 sinapic acid C11H12O5 224.0685 [38][40]
[43][45] 35 6710-hydroxyaloins B lutonarinC21H22O C
8027H 5-p-coumaroylquinic30O16 C1610 96H434.1213 18O8 1-(2,4-dihydroxy-6-methylphenyl) ethanone C9H10O3[26][30][28, 338.100232] [39][41] 166.0630 [30 7 8-C-glucosyl-7-O
][32] 36-methylaloediol aloinoside AC20
97 p-anisaldehydeH26O10 C27H426.1526 32O13[20][23 C8H][22,25]
564.1843 8O2[27][29] 136.0524 [30][32] 8 8-glucosyl-(2’-O-cinnamoyl)-7-O-methylaloediol A C29H32O12 572.1894
37 aloinoside B C27H32[25][27]
98 salicylaldehydeO13 C564.1843 7H6[27][29] O241] 122.0368 [30][32] 9 8-glucosyl-(2’-O-cinnamoyl)-7-O-methylaloediol B C29H32O12 8572.1894 [
318.0376
83 caffeoylshikimic C16H16O8 336.0845[38][40]
[39][41] 71 quercitrin C21H20O11 448.1006 [38][40]
72 rutin C27H30O16 610.1534 [38][40]
73 catechin C15H14O6 290.0790 [38][40]
74 epicatechin C15H14O6 290.0790 [38][40]

2.4. Phenylpropanoids and Coumarins

Approximately 12 phenylpropanoid acids and their ester derivatives (75–86), and four coumarins (87–90), were isolated and identified from Aloe vera (Table 4Figure 4).
Figure 4. Chemical structure of phenylpropanoids and coumarins from Aloe vera.
Table 4. Phenylpropanoids and coumarins isolated and identified from Aloe vera.
No Constituents Molecular Formula Exact Mass References
75 cinnamic acid C9H8O2
610.1534
[
39
][41]
68 kaempferol
81 chlorogenicC15H10O C16H186 O286.0477 9[ 354.095138][40]
[38][40] 69 quercetin C15H10O7 302.0427
82 5-feruloylquinic C17H20O9[38 368.1107][40] [39 38 7-hydroxyaloin A C
99 p-cresol21H22O1025 C7H][ 434.121327]
][ 8O[21][23] 108.0575 [30][32] 10 C-2′-decoumaroyl-aloeresin G C20H24O
398 7-hydroxyaloin B C21H22O392.1471 [
8420][22]
10 434.1213 [21][23] 5-p-cis-coumaroylquinic C16H18O8 338.1002 [39
100 pyrocatechol C6H6O2][41] 110.0368 [40][42] 11 aloeresin E C29H32O10 540.1995
40 7-hydroxy-8-O-methylaloin A C22H24O10[24][26]
448.1369
101 gentisic acid C7H6O4[21][26][23,28]
85 3-(4-hydroxyphenyl) propanoic acid C9 154.0266 [38 12 isoaloeresin D
H10O3 166.063 [30][32] ][40] 41 7-hydroxy-8-O-methylaloin BC29H32O11 556.1945
86[ C22H24O methyl 3-(4-hydroxyphenyl) propionate10
102 gallic acid C7H6O524][26][27][26, 170.021528,29]
C448.1369 10H12O3[21] [38][40[26][23,28] 180.0786 [30][] 13
32] 42iso-rabaichromone 6′-malonylnataloin AC29H32O12 572.1894 C24H24O12[23 504.1268][25]
[
87 7-demethylsiderin C11H
103 vanillic acid C810O421][23] 206.0579 H8O4[30][32] 168.0423 [38][40] 14 8-[C-β-D-[2-O-(E)-cinnamoyl] glucopyranosyl]-2-[(
88R)-2-hydroxypropyl]-7-methoxy-5-methylchromone C29H32 feralolideO10
104540.1995
43 6′-malonylnataloin B C24H syringic acid24O C18H1612 C9H10[28] O7504.1268[30]
O5[21] 344.0896[23] [31] 198.0528[34][33, [36] 38][40] 15 aloeresin D C29H32O11 556.1945
44 homonataloside B C
8928H34O14 dihydrocoumarin C22H18[20
105 ascorbic acidO C6H87594.1949] O6[[28][22,30]
21][23] 176.0321 [38][40] 16 rabaichromone C29H32O12 572.1894 [20][22]
394.1053 [41][43 elgonica dimer A C36H30O14 686.1636 [27][33][34][29,35,36] 17 allo-aloeresin D C29
46 elgonica dimer BH32O11 C36H3556.1945 [20][22]
0O14 686.1636 [27][33][34][29,35,36] 18
47aloeresin K C31H aloindimer A C4234O12 H598.205 42O18[27][29]
834.2371 [27][29] 19 aloeresin J C30H34O11 570.2101 [27][29]
20
45
48 aloindimer B C42H42O18 834.2371 [27][29] 8-C-glucosyl-noreugenin C16H18O9 354.0951 [
49 aloindimer C C42H42O1825][27]
834.2371 [27][29] 21 4’-O-glucosyl-isoaloeresin DI C35H42O16 718.2473
50 aloindimer D C42H

2.6. Phytosterols and Others

Five phytosterols (Table 6Figure 6) were isolated from Aloe vera gel, including cycloartanol (106), 24-methylene-cycloartanol (107), lophenol (108), 24-methyl-lophenol (109), and 24-ethyl-lophenol (110) [43][45]. Some polar and nonpolar lipids, as well as prostanoids, were also isolated from Aloe vera leaves [44][46]. Chemical investigation of the major constituents in Aloe vera leaves revealed moisture, ash, fiber, protein, lipids, minerals, organic acids, free sugars, and polysaccharides. Glucose, fructose, and sucrose were the main free sugars. Oxalic, L-Malic, isocitric, lactic, acetic, isocitric, lactone, citric, and fumaric acid were the main organic acids.
Figure 6. Chemical structure of phytosterols from Aloe vera.
Table 6. Phytosterols isolated and identified from Aloe vera.
No Constituents Molecular Formula Exact Mass References
[
25
]
[
27
]
42
O
18
834.2371
[
27
]
[
29
]
22 4’-O-glucosyl-isoaloeresin DII C35H42O16 718.2473
51 aloe-emodin-11-O-rhamnoside C21H20O9[25] 416.1107[27]
[30][32] 23 aloeresin A C28H28O11 540.1632 [21][23]
24 7-O-methyl-aloeresin A C29H

2.2. Anthraquinone and its Glycoside Derivatives

Approximately 32 anthraquinones and their glycoside derivatives were isolated and identified from Aloe vera (Table 2Figure 2). The isomers of aloin A (30) and aloin B (31), two anthraquinone glucosides, are the most abundant active constituents of Aloe vera. However, chrysophanol (52), emodin (53), physcione (54), aloe-emodin (55) are four major anthraquinone aglycones. Six anthraquinone dimmers (45–50) were also identified from Aloe vera.
Figure 2. Chemical structure of anthraquinone and its glycoside derivatives from Aloe vera.
Table 2. Anthraquinone and its glycoside derivatives isolated and identified from Aloe vera.
No Constituents Molecular Formula Exact Mass References
30 aloin A C21H22
]
O9 418.1264 [27][29]
52
chrysophanol
C
15
H
10
O
4
254.0579
[
35
]
[
37]
30O11 554.1788
53 emodin C15H10O5[21][29][23 270.0528,31]
[30][35][32,37] 25 9-dihydroxyl-2’-O-(Z)-cinnamoyl-7-methoxy-aloesin C29H30O12 570.1737
54 physcione C16H12O5[29][ 284.068531]
[35][37] 26 6′-O-coumaroyl-aloesin
27 7-methoxy-6′-O-coumaroyl-aloesin C29H30O12
31 aloin B C21H55 aloe-emodinC28H28O12 C15556.1581 H10O5[ 270.052830] [30][35][32 570.1737 [31][33]
22O[32,37]
56 nataloeemodin C15H10O5 270.0528 [21][23] 28 aloeveraside B C28H28O12 556.1581 [30][32][32,34]
57 aloesaponarin I C17H12O6 312.0634 [ 29 aloeveraside A C29H aloesaponarin II30O12 C15H10O4 254.0579
70 myricetin C15H 570.1737 [30][32][32,34]
36
]
[
38
]
[
36
]
[
38
]
59
madagascine
C
20
H
18
O
5
338.1154
[
37
]
[39
]
10
O
58
60
3-Geranyloxyemodin
C
24
H
24
O
5
392.1624
[
37
]
[
39
]
61
rhein C15H8O6 284.0321 [35

3. Antimicrobial Activity of Aloe vera

Aloe vera plant extracts have antimicrobial characteristics that kill microorganisms (including bacteria (antibacterial activity), fungi (antifungal activity), and viruses (antiviral activity)) or stop their growth. Fruit decay is an important parameter influencing the postharvest quality of fresh produce. Previous studies have shown that the use of Aloe vera gel as an edible coating has positive effects on the prevention of fruit decay and microbial spoilage. The inhibitory effects of Aloe vera gel on the growth of mycelium (Penicillium digitatum and Aspergillus niger) was reported by Nabigol and Asghari [45][47], who performed a range of laboratory tests. They suggested that the inhibition of the mycelium growth rate increased with gel concentration. The 500 mL/L dose of Aloe vera gel was found to cause 100% inhibition of P. digitatum and 64% of A. niger. According to the findings of Kator et al. [46][48], 20%, 60%, and 100% concentrations of Aloe vera gel are effective in preventing the occurrence of decay in tomato fruits for seven days of storage. However, these authors suggested that a 100% concentration has significantly higher effects, and the positive impact may continue for 16 days of storage. Benitez et al. [47][49] reported that Aloe vera gel provides higher efficacy for the prevention of mesophilic bacteria and yeasts and molds than alginate and chitosan for kiwifruit slices. In another study, the shelf life of guava was reported to be increased by about one more week with the application of an Aloe vera gel coating, due to the fact that the edible coating prevents microbial growth [48][50]. Sitara et al. [49][51] conducted a comprehensive study regarding the antifungal activity of Aloe vera gel at three different doses against five plant pathogenic fungi: A. nigerAspergillus flavusAlternaria alternataDrechslera hawaiensis, and P. digitatum. The highest test dose (0.35%) of Aloe vera gel was reported to completely inhibit the growth of Drechslera hawaiensis and Alternaria alternata. In another study, the minimum fungicidal concentrations of Aloe vera against Botrytis gladiolorumFusarium oxysporum f.sp. gladioliHeterosporium pruneti, and Penicillium gladioli were reported to vary between 80 and 100 μL/mL, depending on the fungal species [50][52].
Previous studies have also shown that the combination of Aloe vera gel with some homogenizers, such as glycerol starch (0.15 g), improves the efficacy in controlling fungal decay and weight loss in cherry tomatoes [51][53]. The specific mechanism of action is still unknown, but it is known that saponins, acemannan and anthraquinone derivatives, which are found in Aloe vera, have antibacterial activity [52][54]. Navarro et al. [53][55] performed a study with Aloe vera gel alone or in combination with thymol on nectarines and reported that the Aloe vera gel alone is more efficient in prevention of the decay caused by Rhizopus stoloniferB. cinerea, and P. digitatumAloe vera gel coatings were previously tested against decay and found to significantly lower counts for molds, yeast, and mesophilic aerobics in different fruits and vegetables, including tomatoes [54][55][56,57], citrus fruits [56][57][58,59], raspberry fruits [58][60], blueberries [59][61], strawberries [60][62], and ready-to-eat pomegranate arils [61][63]. The preharvest application of Aloe vera gel treatment was also previously tested and found to be effective in postharvest storage; specifically, it was found to reduce the decay incidence of table grapes [62][63][64,65].
In a different study [64][66]Aloe vera leaf gel was found to inhibit the growth of two bacteria: Shigella flexneri and Streptococcus progenes. The antibacterial activities of A. vera gel was also reported by Wang et al. [65][67] and Cellini et al. [66][68] against Heliobacter pylori. Moreover, antiviral activities of A. vera have also been of interest to many researchers, wherein its positive influence has been reported against herpes simplex virus (HSV) type 2 strains by Zandi and Rastian [67][69] and against influenza A virus replication by Li et al. [68][70].