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Aloe vera Gel: History

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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]. 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]. 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], essential oils [8,9], propolis extract [10], plant extracts [11,12], edible coatings [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].

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 1, Figure 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	[22,23]
2	neoaloesin A	C₁₉H₂₂O₉	394.1264	[24]
3	8-C-glucosyl-(R)-aloesol	C₁₉H₂₄O₉	396.142	[22]
4	8-C-glucosyl-7-methoxy-(R)-aloesol	C₂₀H₂₆O₉	410.1577	[22]
5	8-C-glucosyl-(S)-aloesol	C₁₉H_24O9	396.142	[25]
6	8-C-glucosyl-7-methoxy-(S)-aloesol	C₂₀H₂₆O₉	410.1577	[25,26]
7	8-C-glucosyl-7-O-methylaloediol	C₂₀H₂₆O₁₀	426.1526	[22,25]
8	8-glucosyl-(2’-O-cinnamoyl)-7-O-methylaloediol A	C₂₉H₃₂O₁₂	572.1894	[27]
9	8-glucosyl-(2’-O-cinnamoyl)-7-O-methylaloediol B	C₂₉H₃₂O₁₂	572.1894	[27]
10	C-2′-decoumaroyl-aloeresin G	C₂₀H₂₄O₈	392.1471	[22]
11	aloeresin E	C₂₉H₃₂O₁₀	540.1995	[26]
12	isoaloeresin D	C₂₉H₃₂O₁₁	556.1945	[26,28,29]
13	iso-rabaichromone	C₂₉H₃₂O₁₂	572.1894	[25]
14	8-[C-β-D-[2-O-(E)-cinnamoyl] glucopyranosyl]-2-[(R)-2-hydroxypropyl]-7-methoxy-5-methylchromone	C₂₉H₃₂O₁₀	540.1995	[30]
15	aloeresin D	C₂₉H₃₂O₁₁	556.1945	[22,30]
16	rabaichromone	C₂₉H₃₂O₁₂	572.1894	[22]
17	allo-aloeresin D	C₂₉H₃₂O₁₁	556.1945	[22]
18	aloeresin K	C₃₁H₃₄O₁₂	598.205	[29]
19	aloeresin J	C₃₀H₃₄O1₁	570.2101	[29]
20	8-C-glucosyl-noreugenin	C₁₆H₁₈O₉	354.0951	[27]
21	4’-O-glucosyl-isoaloeresin DI	C₃₅H₄₂O₁₆	718.2473	[27]
22	4’-O-glucosyl-isoaloeresin DII	C₃₅H₄₂O1₆	718.2473	[27]
23	aloeresin A	C₂₈H₂₈O₁₁	540.1632	[23]
24	7-O-methyl-aloeresin A	C₂₉H₃₀O₁₁	554.1788	[23,31]
25	9-dihydroxyl-2’-O-(Z)-cinnamoyl-7-methoxy-aloesin	C₂₉H₃₀O₁₂	570.1737	[31]
26	6′-O-coumaroyl-aloesin	C28H28O12	556.1581	[32]
27	7-methoxy-6′-O-coumaroyl-aloesin	C29H30O12	570.1737	[33]
28	aloeveraside B	C₂₈H₂₈O₁₂	556.1581	[32,34]
29	aloeveraside A	C₂₉H₃₀O₁₂	570.1737	[32,34]

2.2. Anthraquinone and its Glycoside Derivatives

Approximately 32 anthraquinones and their glycoside derivatives were isolated and identified from Aloe vera (Table 2, Figure 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	C₂₁H₂₂O₉	418.1264	[29]
31	aloin B	C₂₁H₂₂O₉	418.1264	[29]
32	6′-O-acetyl-aloin A	C₂₃H₂₄O₁₀	460.1369	[29]
33	6′-O-acetyl-aloin B	C₂₃H₂₄O₁₀	460.1369	[29]
34	10-hydroxyaloins A	C₂₁H₂₂O₁₀	434.1213	[28,32]
35	10-hydroxyaloins B	C₂₁H₂₂O₁₀	434.1213	[28,32]
36	aloinoside A	C₂₇H₃₂O₁₃	564.1843	[29]
37	aloinoside B	C₂₇H₃₂O₁₃	564.1843	[29]
38	7-hydroxyaloin A	C₂₁H₂₂O₁₀	434.1213	[23]
39	7-hydroxyaloin B	C₂₁H₂₂O₁₀	434.1213	[23]
40	7-hydroxy-8-O-methylaloin A	C₂₂H₂₄O₁₀	448.1369	[23,28]
41	7-hydroxy-8-O-methylaloin B	C₂₂H₂₄O₁₀	448.1369	[23,28]
42	6′-malonylnataloin A	C₂₄H₂₄O₁₂	504.1268	[23]
43	6′-malonylnataloin B	C₂₄H₂₄O₁₂	504.1268	[23]
44	homonataloside B	C₂₈H₃₄O₁₄	594.1949	[23]
45	elgonica dimer A	C₃₆H₃₀O₁₄	686.1636	[29,35,36]
46	elgonica dimer B	C₃₆H₃₀O₁₄	686.1636	[29,35,36]
47	aloindimer A	C₄₂H₄₂O₁₈	834.2371	[29]
48	aloindimer B	C₄₂H₄₂O₁₈	834.2371	[29]
49	aloindimer C	C₄₂H₄₂O₁₈	834.2371	[29]
50	aloindimer D	C₄₂H₄₂O₁₈	834.2371	[29]
51	aloe-emodin-11-O-rhamnoside	C₂₁H₂₀O₉	416.1107	[32]
52	chrysophanol	C₁₅H₁₀O₄	254.0579	[37]
53	emodin	C₁₅H₁₀O₅	270.0528	[32,37]
54	physcione	C₁₆H₁₂O₅	284.0685	[37]
55	aloe-emodin	C₁₅H₁₀O₅	270.0528	[32,37]
56	nataloeemodin	C₁₅H₁₀O₅	270.0528	[23]
57	aloesaponarin I	C₁₇H₁₂O₆	312.0634	[38]
58	aloesaponarin II	C₁₅H₁₀O₄	254.0579	[38]
59	madagascine	C₂₀H₁₈O₅	338.1154	[39]
60	3-Geranyloxyemodin	C₂₄H₂₄O₅	392.1624	[39]
61	rhein	C₁₅H₈O₆	284.0321	[37]

2.3. Flavonoids

Approximately 13 flavonoids and their glycoside derivatives were isolated and identified from Aloe vera (Table 3, Figure 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	C₁₅H₁₀O₅	270.0528	[40]
63	luteolin	C₁₅H₁₀O₆	286.0477	[41]
64	isovitexin	C₂₁H₂₀O₁₀	432.1056	[41]
65	isoorientin	C₂₁H₂₀O₁₁	448.1006	[41]
66	saponarin	C₂₇H₃₀O₁₅	594.1585	[41]
67	lutonarin	C₂₇H₃₀O₁₆	610.1534	[41]
68	kaempferol	C1₅H₁₀O₆	286.0477	[40]
69	quercetin	C₁₅H₁₀O₇	302.0427	[40]
70	myricetin	C₁₅H₁₀O₈	318.0376	[40]
71	quercitrin	C₂₁H₂₀O₁₁	448.1006	[40]
72	rutin	C₂₇H₃₀O₁₆	610.1534	[40]
73	catechin	C₁₅H₁₄O₆	290.0790	[40]
74	epicatechin	C₁₅H₁₄O₆	290.0790	[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 4, Figure 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	148.0524	[42]
76	p-coumaric	C₉H₈O₃	164.0473	[40]
77	caffeic acid	C₉H₈O₄	180.0423	[40]
78	ferulic acid	C₁₀H₁₀O₄	194.0579	[40]
79	sinapic acid	C₁₁H₁₂O₅	224.0685	[40]
80	5-p-coumaroylquinic	C₁₆H₁₈O₈	338.1002	[41]
81	chlorogenic	C₁₆H₁₈O₉	354.0951	[40]
82	5-feruloylquinic	C₁₇H₂₀O₉	368.1107	[41]
83	caffeoylshikimic	C₁₆H₁₆O₈	336.0845	[41]
84	5-p-cis-coumaroylquinic	C₁₆H₁₈O₈	338.1002	[41]
85	3-(4-hydroxyphenyl) propanoic acid	C₉H₁₀O₃	166.063	[32]
86	methyl 3-(4-hydroxyphenyl) propionate	C₁₀H₁₂O₃	180.0786	[32]
87	7-demethylsiderin	C₁₁H₁₀O₄	206.0579	[32]
88	feralolide	C₁₈H₁₆O₇	344.0896	[33,36]
89	dihydrocoumarin	C₂₂H₁₈O₇	394.1053	[43]
90	dihydrocoumarin ethyl ester	C₂₅H₂₆O₇	438.1679	[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 5, Figure 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	410.1213	[44]
92	aloenin B	C₃₄H₃₈O₁₇	718.2109	[35,44]
93	p-coumaroyl aloenin	C₂₈H₂₈O₁₂	556.1581	[35]
94	aloveroside A	C₃₀H₄₀O₁₇	672.2265	[35]
95	feroxidin	C₁₁H₁₄O₃	194.0943	[32]
96	1-(2,4-dihydroxy-6-methylphenyl) ethanone	C₉H1₀O₃	166.0630	[32]
97	p-anisaldehyde	C₈H₈O₂	136.0524	[32]
98	salicylaldehyde	C₇H₆O₂	122.0368	[32]
99	p-cresol	C₇H₈O	108.0575	[32]
100	pyrocatechol	C₆H₆O₂	110.0368	[42]
101	gentisic acid	C₇H₆O₄	154.0266	[40]
102	gallic acid	C₇H₆O₅	170.0215	[40]
103	vanillic acid	C₈H₈O₄	168.0423	[40]
104	syringic acid	C₉H₁₀O₅	198.0528	[40]
105	ascorbic acid	C₆H₈O₆	176.0321	[40]

2.6. Phytosterols and Others

Five phytosterols (Table 6, Figure 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) [45]. Some polar and nonpolar lipids, as well as prostanoids, were also isolated from Aloe vera leaves [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
106	cycloartanol	C₃₀H₅₂O	428.4018	[45]
107	24-methylene-cycloartanol	C₃₁H₅₂O	440.4018	[45]
108	lophenol	C₂₈H₄₈O	400.3705	[45]
109	24-methyl-lophenol	C₂₉H₅₀O	414.3862	[45]
110	24-ethyl-lophenol	C₃₀H₅₂O	428.4018	[45]

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 [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. [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. [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 [50]. Sitara et al. [51] conducted a comprehensive study regarding the antifungal activity of Aloe vera gel at three different doses against five plant pathogenic fungi: A. niger, Aspergillus flavus, Alternaria alternata, Drechslera 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 gladiolorum, Fusarium oxysporum f.sp. gladioli, Heterosporium pruneti, and Penicillium gladioli were reported to vary between 80 and 100 μL/mL, depending on the fungal species [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 [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 [54]. Navarro et al. [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 stolonifer, B. cinerea, and P. digitatum. Aloe 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 [56,57], citrus fruits [58,59], raspberry fruits [60], blueberries [61], strawberries [62], and ready-to-eat pomegranate arils [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 [64,65].

In a different study [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. [67] and Cellini et al. [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 [69] and against influenza A virus replication by Li et al. [70].

This entry is adapted from the peer-reviewed paper 10.3390/agronomy9120831

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