Aloe vera Cuticle: A Promising Organic Water-Retaining Agent for Agricultural Use

Abstract

Water is an important resource for both human and environmental survival. However, due to current human practices, we are facing a serious crisis in accessing water. Thus, solutions must be explored to optimize the use of this resource. In the search for an organic water-retaining agent for agricultural use, the techno-functional properties of Aloe vera (Aloe barbadensis Miller) cuticle, an agro-industrial residue generated after gel extraction, were evaluated. The residue was dried and ground. The effects of particle size (180 µm and 250 µm), temperature (10 °C, 20 °C, 30 °C, and 40 °C), and pH (4.5, 6.0, and 7.0) on the solubility and water-holding capacity (WHC) of the obtained product (i.e., hydrogel) were then evaluated. The treatment with the highest WHC was selected and compared with the WHC of a commercial synthetic polyacrylamide gel widely used in agriculture. The effects of KNO₃ and Ca(NO₃)₂ at different concentrations (10 g L⁻¹, 20 g L⁻¹, 30 g L⁻¹, and 40 g L⁻¹) on the WHC of the gels were assessed. Particle size, temperature, and pH interactions had statistically significant effects on solubility, while the WHC was affected by particle size × temperature and pH × temperature interactions. The highest product solubility (75%) was obtained at the smallest particle size (i.e., 180 µm), pH 4.5, and 20 °C. Meanwhile, the highest WHC (18 g g⁻¹) was obtained at the largest particle size (i.e., 250 µm), pH 6.0, and 20 °C. This optimized gel kept its WHC across both salts and their concentrations. In contrast, the commercial gel significantly decreased its WHC with salt concentration. The product elaborated with A. vera cuticle could have bioeconomic potential as a water-retention agent for agricultural use, with the advantage that it is not affected by the addition of salts used for plant fertilization.

1. Introduction

Water is a key resource for humanity and is also fundamental for maintaining the natural balance of the planet. However, factors derived from human activities have brought us to a point where access to water is extremely difficult [1]. In the last couple of years, many cities all around the world have faced water restrictions as drinking water supplies run low. So, it could be said that we are already living in a water crisis.

One of the most water-intensive human practices is agriculture. Agricultural irrigation alone accounts for ~70% of global freshwater withdrawals and ~90% of consumptive water use worldwide [2], so it is imperative to seek sustainable agricultural solutions to alleviate the severe water crisis we are going through.

Proper water supply and distribution in crop soil improves the photosynthetic capacity of plants, thus improving their physiological and productive performance [3,4,5]. Water stress in crops can improve their productivity due to the evolution of plant mechanisms responding and adapting to environmental stress at different phenological stages [6]. However, when this water deficit extends over time, abiotic stress originates in the plant [7], which has a greater impact on plant development, generating a negative response in crops [8,9]. Plants subjected to water stress show a low production of photosynthetically active leaf areas [10], a loss of turgor, and affectations in stem growth, plant height, root system, and biomass development [3]. Plants extract water from the soil; however, if water is absorbed and retained in the soil, it is not possible to capture it efficiently and the plant dies [6]. Incorporating water-retaining agents into crop soil generates positive effects on plant growth, allowing higher yields [11]. In addition, the use of water-retention agents can reduce irrigation requirements by up to 25% [12].

Currently, there are water-retention agents used in agriculture (e.g., hydrogels) [13] that are added to the soil to mitigate the water demand of crops that suffer from water stress [8]. Most commercial hydrogels are synthetic products based on acrylate and acrylamide. These polyacrylamides have a remarkable water absorption and retention capacity [3]. However, in the environment, through mechanical, thermal, chemical, photocatalytic, and/or biodegradation pathways, polyacrylamide is transformed into oligomers and acrylamide [14]. The latter has been identified as a cytotoxin, neurotoxin, and carcinogen, and has the potential to induce alterations in the peripheral and central nerves, impair fertility, and cause genetic defects [14,15].

Recently, Cheng et al. [14] provided an overview of the occurrence, degradation pathways, toxicity, and risks associated with polyacrylamide in the environment. In this review, the authors stated that acrylamide poses significant environmental and health risks. Its high-water solubility and low absorption in sludge and sediment increase its potential for groundwater contamination. These authors also claimed that polyacrylamide’s extensive use in agriculture raises concerns due to its persistence in the environment and potential to release harmful acrylamide. While polyacrylamide is generally low in toxicity to humans due to its large polymer size, it can still cause mild irritation upon contact with the skin, eyes, or respiratory organs. Meanwhile, acrylamide poses serious ecological threats, affecting aquatic life and soil organisms like earthworms. So, despite their benefits in soil amendment, the risks associated with commercial hydrogels, especially at high concentrations, necessitate cautious use and responsible application practices to minimize ecological and health impacts.

Commercial hydrogels also face technical problems that limit their application. Water quality is an important factor in the use of polyacrylamide hydrogels. When the hydrogel is reconstituted with water that has different characteristics to deionized water (e.g., chlorinated water or rainwater), the water-holding capacity (WHC) of the commercial hydrogel falls considerably [3]. Also, the application dose of the hydrogel has relevant effects on plants. High doses of hydrogel can negatively affect plant growth and development by altering root development and biomass. Doses per plant higher than those recommended might generate adverse effects such as growth retardation and rotting in the root system. Therefore, it is very important to evaluate the dosage for each crop. Furthermore, the addition of nutrients to the soil produces an increase in ionic strength, which causes hydrogel collapse. Finally, when synthetic hydrogels are applied in the field and interact with the soil, they come into contact with fertilizer solutions containing monovalent cations such as K and N, and divalent cations such as Fe²⁺, Ca²⁺, or Mg²⁺, which reduce the capacity of the polyacrylamide network to absorb and retain water by up to 70% and 90%, respectively, in both cases losing its functionality [16].

The cuticle (also known as rind) of Aloe barbadensis Miller (better known as Aloe vera) is an agro-industrial waste product of gel extraction that represents roughly 20–30% of the weight of the A. vera leaf [17]. This cuticle is usually used for composting; yet, it has 85 g protein kg⁻¹ dry matter [18] and has high Ca and P contents [19], so it could display useful techno-functional properties as an organic water-retaining agent.

The A. vera gel business is expected to grow at a compound annual growth rate (CAGR) of roughly 9% until 2025; thus, an increase in the generation of cuticles as waste from this activity should be expected on a global scale at the same rate [20]. Therefore, the objective of this research was to study the cuticle of A. vera as a potential organic water-retaining agent. First, the solubility and WHC of a gel made from A. vera cuticle were evaluated at different particle sizes, temperatures, and pH conditions. Then, the effect of the addition of agricultural nutritional salts at different concentrations on the WHC of the product elaborated with A. vera cuticle (at optimal conditions of particle size, temperature, and pH) and that of a commercial synthetic polyacrylamide gel of widespread agricultural use was evaluated.

2. Materials and Methods

2.1. Production of the Organic Agent

A. vera cuticles from a company in Jamundí (Valle del Cauca, Colombia), which processes 100 tons of A. vera leaf per month to obtain gel, were used. Cuticles were washed with drinking water and disinfected by immersion with sodium hypochlorite solution at 200 ppm for 10 min. The material was then cut with a stainless-steel knife into 1.5 × 1.5 cm cubes and frozen at −50 °C for 24 h (New Brunswick™ Innova^® Freezers, USA). Subsequently, the samples were freeze-dried (Freezone 6 Plus, Labconco, USA) at a condenser temperature of −50 °C and 12 Pa for 24 h up to 3.5% moisture. Then, the dried samples were ground (M 20 Universal mil, IKA, China) and sieved (Ro-Tap, WS Tyler, USA) to 180 µm and 250 µm.

2.2. Measurement of Water-Holding Capacity (WHC) and Water Solubility

The effects of particle size (180 µm and 250 µm), temperature (10 °C, 20 °C, 30 °C, and 40 °C), and pH (4.5, 6.0, and 7.0) on the WHC and water solubility of the organic agent produced in the previous step were evaluated.

2.2.1. Water-Holding Capacity (WHC)

For WHC measurements, in 50 mL Falcon tubes, 0.25 g of the dried and ground organic agent was added at the selected particle size (180 µm or 250 µm). Then, 25 mL of distilled water was added at a previously adjusted pH (4.5, 6.0, or 7.0) and temperature (10 °C, 20 °C, 30 °C, or 40 °C). The mixture was stirred and left at room temperature for 1 h. Then, the tubes were centrifuged at 436 RCF for 10 min (Eppendorf 5804R, Germany), the supernatant was discarded, and the pellet was weighed. WHC was calculated as the amount of water retained by the pellet (g g⁻¹ dry weight), according to the method previously described by Hassan et al. [21] and Robertson et al. [22], as shown in Equation (1):

$$\mathrm{Waterholding} \mathrm{capacity} \left(\mathrm{WHC}\right)={\displaystyle \frac{\mathrm{Pellet} \left(\mathrm{g}\right)- \mathrm{Dried} \mathrm{organic} \mathrm{agent} \left(\mathrm{g}\right)}{\mathrm{Dried} \mathrm{organic} \mathrm{agent} \left(\mathrm{g}\right)}}$$

(1)

2.2.2. Water Solubility

Regarding the organic agent’s solubility in water, the method described by Serna-Cock et al. [23] was followed. For this, 0.25 g of dried and ground organic agent was added to 50 mL Falcon tubes at the selected particle size (180 µm or 250 µm). Then, 25 mL of distilled water was added at a previously adjusted pH (4.5, 6.0, or 7.0) and temperature (10 °C, 20 °C, 30 °C, or 40 °C). The tubes were then placed on a shaking plate at 1150 rpm for 5 min (PC420D, Corning, USA) and centrifuged at 1744 RCF for 5 min. Afterward, 12.5 mL of the supernatant was taken and transferred to a Petri dish previously dried in an oven at 105 °C for 5 h (UN 110, Memmert, Germany). The solubility percentage was calculated by weight difference according to Equation (2):

$$\mathrm{Solubility} \mathrm{in} \mathrm{water} (\%)={\displaystyle \frac{\mathrm{Solids} \mathrm{in} \mathrm{the} \mathrm{supernatant} \left(\mathrm{g}\right)}{\mathrm{Supernatant} \left(\mathrm{g}\right)}}\times 100\%$$

(2)

2.3. Comparison with a Commercial Polyacrylamide Hydrogel

Both, potassium (KNO₃) and calcium (Ca(NO₃)₂) nitrate are fertilizers widely used in agriculture [24]. So, the effects of different concentrations (0 g L⁻¹, 10 g L⁻¹, 20 g L⁻¹, 30 g L⁻¹, and 40 g L⁻¹) of KNO₃ and Ca(NO₃)₂ solutions on the WHC of both the organic agent previously described in this study and a commercial inorganic water-retaining agent were evaluated.

In the case of the organic agent, the treatment with the highest WHC in Section 2.2.1. was assessed. Thus, in the knowledge of the maximum WHC of the organic agent, 1 g of the organic agent was mixed with 1 mL of the saline solution. As for the inorganic water-retaining agent, a commercial cross-linked polyacrylamide copolymer (94% w·w⁻¹) was used. This commercial agent was assessed considering its datasheet; so, 0.1 g of dry product was mixed with 50 mL of saline solution. For both organic and inorganic agents, the mixture was stirred and left at room temperature for 1 h. Then, the tubes were centrifuged at 436 RCF for 10 min, the supernatant was discarded, and the pellet was weighed. WHC was calculated as the amount of water retained by the pellet (g g⁻¹ dry weight) according to the method previously described in Section 2.2.1 [21,22] and Equation (1).

2.4. Experimental Design and Statistical Analysis

First, the effects of the factors (and their levels) of particle size (180 µm and 250 µm), temperature (10 °C, 20 °C, 30 °C, and 40 °C), and pH (4.5, 6.0, and 7.0) on WHC and water solubility were evaluated using a 2 × 4 × 3 completely randomized multifactorial design. Second, the effects of the factors (and their levels) of type of water retainer (organic agent and commercial polyacrylamide), type of salt solution (KNO₃ and Ca(NO₃)₂), and concentration (10 g L⁻¹, 20 g L⁻¹, 30 g L⁻¹, and 40 g L⁻¹) on WHC were analyzed using a 2 × 2 × 4 multifactorial design.

The experiments were repeated three times. Analyses were performed in triplicate for each replicate (n = 3 × 3). Means and standard deviations were calculated for all data. To determine the difference between treatments, an analysis of variance (ANOVA) was used with a 95% confidence level (p ≤ 0.05) and the DUNCAN test was used to compare the means between treatments using RStudio version 2024.04.2+764.

3. Results and Discussion

3.1. Water-Holding Capacity (WHC) Assessment

Table 1. Water-holding capacity (WHC) of the organic water-retaining agent as a function of particle size, temperature, and pH.

Particle Size (µm)	Temperature (°C)	pH	WHC (g g−1)	Particle Size (µm)	Temperature (°C)	pH	WHC (g g−1)
180	10	4.5	8.7 ± 0.4	250	10	4.5	15.6 ± 0.4
		6.0	7.9 ± 0.6			6.0	15.7 ± 0.6
		7.0	7.3 ± 0.7			7.0	15.1 ± 0.4
	20	4.5	7.0 ± 0.7		20	4.5	17.9 ± 0.9
		6.0	6.8 ± 0.8			6.0	18.0 ± 0.3
		7.0	6.4 ± 0.9			7.0	17.5 ± 0.5
	30	4.5	6.8 ± 1.5		30	4.5	16.6 ± 0.4
		6.0	5.6 ± 2.4			6.0	17.1 ± 0.3
		7.0	7.5 ± 2.1			7.0	16.6 ± 0.8
	40	4.5	7.5 ± 0.8		40	4.5	15.9 ± 0.4
		6.0	6.3 ± 2.6			6.0	16.1 ± 0.6
		7.0	7.6 ± 2.2			7.0	17.0 ± 0.4

shows the WHC of the organic agent produced from A. vera cuticles in this study as a function of the factors (and their levels) of particle size (180 µm and 250 µm), temperature (10 °C, 20 °C, 30 °C, and 40 °C), and pH (4.5, 6.0, and 7.0).

According to Table 1, WHC tended to increase with larger particle sizes. The ANOVA showed statistically significant differences (p ≤ 0.05) in WHC with the interactions of particle size × temperature (p = 0.0001) and pH × temperature (p = 0.0161), as can be seen in Figure S1. In Table 1, the highest WHC (i.e., 18 g g⁻¹) is found in the interaction effect between the particle size of 250 µm and the temperature of 20 °C. Cellulose, hemicellulose, lignin, and some phenolic compounds contained in the cuticle of A. vera can interact with water at the molecular level due to the presence of hydroxyl (-OH) functional groups in their structure. At a temperature of 20 °C, water molecules had moderate kinetic energy, which means that these water molecules could move sufficiently to interact effectively with the -OH groups of the cuticle, thus forming hydrogen bonds and therefore being retained in the gel’s structure. At higher temperatures (i.e., 30 °C and 40 °C), water molecules had a higher kinetic energy, which could have hindered their effective interaction with the hydroxyl groups of the cuticle, resulting in a lower water-holding capacity at these temperatures [25].

The ANOVA revealed that particle size had a significant influence (p ≤ 0.05) on the WHC of the organic agent. At the largest particle size (i.e., 250 µm), the WHC was more than twice the WHC at 180 µm. Decreases in fiber size may break down the natural porous structure of fibers, thus decreasing the connectivity between the pores, and hence, impacting the WHC. Larger particles may interact differently with water, as they can clog pores, penetrate deeply, and be retained to a greater extent in the particle matrix [16,17].

The same interaction and effect on WHC were found by Ahmed et al. [26], who reported a considerable decrease in the WHC of commercial water chestnut (Trapa natans) flour at lower particle sizes. Those authors attributed a reduction in WHC at the smallest particle size to the presence of lesser amounts of fiber, starch, and protein in the structure, which resulted in a lower water absorption capacity. These results also agree with the studies of Serna-Cock et al. [23], who found that, with freeze-dried powders obtained from the peels of three mango varieties, the particle diameter of 250 µm showed higher WHC values than that of 180 µm.

3.2. Water Solubility Evaluation

Table 2. Water solubility of the organic water-retaining agent as a function of particle size, temperature, and pH.

Particle Size (µm)	Temperature (°C)	pH	Solubility (%)	Particle Size (µm)	Temperature (°C)	pH	Solubility (%)
180	10	4.5	65.2 ± 3.4	250	10	4.5	72.8 ± 4.5
		6.0	51.2 ±1.8			6.0	65.3 ± 5.0
		7.0	49.9 ± 4.4			7.0	54.9 ± 3.5
	20	4.5	75.3 ± 1.6		20	4.5	65.1 ± 2.9
		6.0	49.7 ± 4.9			6.0	45.1 ± 1.3
		7.0	56.7 ± 3.9			7.0	53.1 ± 2.6
	30	4.5	63.8 ± 12.8		30	4.5	71.1 ± 4.7
		6.0	51.9 ± 4.3			6.0	42.7 ± 1.3
		7.0	50.1 ± 3.3			7.0	45.7 ± 5.1
	40	4.5	59.4 ± 3.1		40	4.5	45.4 ± 2.2
		6.0	50.4 ± 4.8			6.0	46.4 ± 3.7
		7.0	52.4 ± 1.8			7.0	47.5 ± 2.9

shows the solubility in water of the organic agent produced from A. vera cuticles as a function of particle size, temperature, and pH.

According to Table 2, the water solubility patterns were not as consistent as those analyzed in the previous section. The ANOVA reported an interaction effect (p ≤ 0.05) among particle size, temperature, and pH, as can be seen in Figure S2. The highest solubility value (i.e., 75%) was found with the interaction particle size of 180 µm, 20 °C, and pH 4.5. At neutral and near neutral (i.e., 6.0) pHs, a decrease was observed. Thus, Table 2 shows that higher solubility values were observed at the lowest pH. This solubility behavior is analogous to that reported for the Inga paterno plant [27].

The water solubility of the molecules that make up the natural agent of this work were key in the ability of the material to form a gel, in addition to affecting the water absorption and mechanical properties of the final product [28]. The compounds contained in A. vera cuticle, such as fiber, alkaloids, tannins, flavonoids, sterols, triterpenes, mucilages, and other compounds, have different affinities and capacities to interact with water [29]. Polysaccharides, such as cellulose and hemicellulose, have hydroxyl (-OH) groups that could have formed hydrogen bonds with the water molecules, which could have favored their solubility. In addition, phenolic compounds, such as tannins and flavonoids, may have formed water-soluble complexes with the water due to their chemical structures and their ability to dissociate in aqueous solutions [30]. Phenolic acids, for example, may have been ionized at different pH ranges, which may have affected their solubility. At pH 4.5, phenolic acids (such as caffeic, ferulic, and chlorogenic acids) may have been predominantly in their ionized form, which may have facilitated their dissolution in water [31,32].

In addition, the pH of the medium could have influenced the surface charge of the compounds contained in the cuticle and, therefore, their interactions with water [33]. This may have occurred because, at acidic pHs, the carboxyl groups (-COOH) in these phenolic acids tend to lose a proton (H⁺), becoming their respective anions [34]. For example, caffeic acid can lose a proton from the carboxyl group to form the caffeate anion. These anions are more soluble in water than their non-ionized forms due to their negative charge, which facilitates their interaction with water molecules and their dissolution in aqueous solutions [35]. Thus, at pH 6.0 and pH 7.0, the medium could have been less acidic, and the carboxyl groups may have been less ionized. Therefore, it is possible that fewer phenolic acids would have been in dissociated form compared to at pH 4.5, which may have resulted in the lower solubility of some phenolic compounds at pHs closer to neutral [36].

The results of this research in terms of solubility also agree with those obtained by Serna-Cock et al. [23], who obtained a higher solubility at the smallest particle size, obtaining values higher than 70% for the particle size of 180 µm and less than 60% for 250 µm. These authors attributed this result to the fact that the smaller the particle size, the higher the dissolution rate.

3.3. Performance Comparison against a Commercial Product

The organic gel that showed the highest WHC was obtained at the largest particle size (i.e., 250 µm), pH 6.0, and 20 °C (see Table 1). This section is devoted to comparing the WHC behavior of this organic A. vera cuticle agent with a commercial cross-linked polyacrylamide copolymer in relation to different concentrations of salts used in agriculture as fertilizers. Accordingly, Figure 1 shows the effect of different concentrations of aqueous solutions of KNO₃ and Ca(NO₃)₂ on the WHC of the gel produced in this work and the commercial one.

The ANOVA yielded statistically significant differences (p ≤ 0.05) between the WHC of the A. vera organic agent and the commercial polyacrylamide water-retaining agent for the effect of the addition of the two salts and their different concentrations. The ANOVA also suggested that the WHC of the organic agent was affected (p > 0.05) neither by the presence of salts nor by the different concentrations of the salts, while the WHC of the polyacrylamide hydrogel was drastically affected (p ≤ 0.05) by the type of salt and by increasing its concentration.

Figure 1 shows that the WHC of the A. vera cuticle remained stable across both salt treatments and their concentrations (p > 0.05), with slight fluctuations within the range of 17.3 g g⁻¹ to 17.7 g g⁻¹, which is proof of its versatility, as it maintained its capacity to retain water at different concentrations, regardless of the salt used. In contrast, the commercial cross-linked polyacrylamide copolymer showed significant variations (p ≤ 0.05) in WHC (1 g g⁻¹ to 38.5 g g⁻¹) between the two salts, with notable reductions as the concentration of both salts increased (see Figure 1).

As can be seen in Figure 1, when comparing the WHC values between KNO₃ and Ca(NO₃)₂ for the A. vera cuticle, there were no differences (p > 0.05) between the two salts at the same concentration. Meanwhile, the commercial product showed the highest WHC when KNO₃ was used. However, this synthetic product reduced its WHC by half over the range of concentrations studied. Finally, when the commercial product was tried with Ca(NO₃)₂, the WHC was the lowest of all the experiments, at all concentrations tested, which demonstrates its low versatility. In addition, it is worth noting the biodegradable nature of the product of this work.

The previous good results of the biodegradable agent could be explained by the fact that the polysaccharides and mucilages contained in A. vera cuticle can form stable complexes with K and Ca²⁺, preventing their interference with the gel structure. In addition, the presence of chelating compounds such as flavonoids and terpenoids can help inactivate or sequester metal ions, preventing them from interfering with the interactions that maintain the gel structure. In polyacrylamide gel, salts can compete with water molecules for binding sites in the porous gel structure, and as a result, some water molecules that were initially retained in the gel structure may be released to form aqueous solutions with the salt ions. This may lead to a decrease in the amount of water retained in the gel, resulting in a reduction in its WHC. Unlike A. vera cuticle, which contains a wide variety of natural compounds such as flavonoids, terpenoids, and other compounds with protective properties, polyacrylamide hydrogel may lack these compounds. These protective compounds can help maintain gel stability in the presence of salts by forming complexes or interactions with the ions present in the salts, thus protecting the gel structure from destabilization [37].

4. Conclusions

Particle size reduction influenced the WHC of the organic water-retaining agent produced. The dried and ground A. vera cuticle with a particle size of 250 µm had a remarkable capacity to retain water, reaching up to roughly 18 times its weight in water under specific conditions of pH (i.e., 6.0) and temperature (i.e., 20 °C).

Compared to polyacrylamide-based hydrogels, A. vera cuticle offered significant advantages by retaining its water-retention capacity even after the addition of different concentrations of calcium and potassium salts. For field application, the organic product can be added directly as a dry powder or as a gel. The choice between organic and synthetic agents would depend on factors such as cost, availability, and the specific requirements of the application. Nevertheless, the A. vera-based product is biodegradable. Therefore, these findings benefit the achievement of the following sustainable development goals (SDGs): health and well-being (SDG 3), clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11), responsible consumption and production (SDG 12), climate action (SDG 13), life below water (SDG 14), and life on land (SDG 15) [38].

The Colombian industry alone currently creates approximately 6000 tons per year of A. vera cuticle, produced by the agro-industry to obtain A. vera gel. Therefore, the use of A. vera cuticle could be a sustainable and effective alternative to improve water retention in agricultural soils and to decrease water stress in crops with high water demand. However, it is recognized that future research is needed to deepen our understanding of the behavior of A. vera cuticle in different types of soils and the mechanisms responsible for its water-retention and -release capacities, as well as to explore its potential application in other fields of science and technology.

5. Patent

Superintendencia de Industria y Comercio (SIC) from Colombia

Resolution No. 43658 from 31 July 2023

File reference NC2019/0009648

IPC: A61F 13/00, C08J 3/00, C08J 3/075, A61L 15/00

Effective from 5 September 2019, to 5 September 2039.

Holder: Universidad Nacional de Colombia

Title: “Composición absorbente de agua que comprende partículas secas de la cutícula de Aloe barbadensis Miller”.