1. Introduction
Caesalpinia coriaria is a plant found in the Pacific coastal plains of Central America, commonly called “cascalote” or “divi-divi”, and it is used to obtain firewood, charcoal, poles, and beams. This plant produces pods 3–7 cm long, with a green color when unripe and dark brown when ripe. According to Palma García [
1], one cascalote tree can produce up to 150 kg of pods, which indicates a large production.
These pods are commonly used in leather tanning and fodder and are traditionally used as a treatment for infectious skin problems [
2]; however, most of the pods produced are underutilized and, consequently, not fully exploited. Additionally, economic activity derived from harvesting has declined. Cascalote pods are astringent due to their high content of phenolic compounds—important molecules that are of great interest in health studies, mainly due to their biological activities such as antimicrobial, antioxidant, and anti-inflammatory [
3,
4]. The main phenolic compounds that have been reported in cascalote fractions with biological activity are gallic acid, ethyl gallate, stigmasterol, tannic acid, and corilagin [
2,
5].
Phenolic compounds are commonly recovered using solvents and several extraction techniques, such as maceration, microwave, and ultrasound, among others; however, their use involves expensive equipment as well as environmental pollution and toxicological safety concerns regarding the use of solvents [
6,
7]. As a biotechnological alternative, the use of fermentative or enzymatic methods for the assisted extraction of biomolecules from vegetal matrices has been reported, which includes solid-state fermentation (SSF) [
8].
SSF does not require the use of solvents and promotes high yields and the easy recovery of bioactive compounds. During the SSF process, polyphenols could be biosynthesized or biotransformed into simpler molecules using the activity of microbial enzymes [
9,
10], which degrade the wall cell components, thus increasing the extraction of free and bound polyphenols [
11]. Recently, SSF has been successfully used to obtain bioactive compounds from rambutan peel [
12], orange peel [
13], pomegranate husk [
14], and pineapple waste [
15], among others; however, the use of SSF in cascalote pods as a substrate to obtain bioactive compounds has not been studied. Based on responsible production and consumption, this work aimed to develop an eco-friendly bioprocess, SSF, for the extraction, recovery, and identification of compounds with antioxidant capacity from cascalote pods.
4. Discussion
A culture medium is characterized by a mixture of nutrients that, in adequate concentrations and under optimal physical conditions, allows microbial growth and metabolic processes to occur [
14]. The obtained results (
Table 2) are in the range of the values reported in the literature for moisture (3%), protein (3.85–5.34%), ash (1.87–2.58%), fat (0.19–3.35%), and carbohydrate (71.62–88.82%) content [
1,
23].
In the SSF process, WAC, CHP, and maximum moisture are important parameters for the evaluation of the vegetal material’s suitability as a solid support for fungal growth and metabolite production. WAC indicates the amount of water that can be absorbed by the substrate, and it is related to the hydroxyl groups present on it and allows additional water interactions by hydrogen bonding [
12,
24]. Thus, a high WAC value is convenient for the SSF process because moisture content can be modified, thus allowing microbial growth. Cascalote pods presented a WAC value of 2.97 g gel/gdw, which is in the range reported (2.16–3.4 g gel/gdw) for agro-industrial byproducts considered as suitable supports for SSF such as grape waste [
25], corn cobs [
26], candelilla stalks [
26], rambutan peels [
12], and fig byproducts [
27]. The CHP represents the water linked to the support that cannot be used for the biological functions of the microorganism. Microbial growth is promoted at low CHP values since high values affect it in a big proportion of the water linked to the substrate; therefore free water content is low [
15]. The CHP of the cascalote pods was 3.75%, which is lower than those reported for fig byproducts (4.63%), sugarcane bagasse (9.46%), pomegranate peel (10.13%), coconut husk (16%), corn cobs (27%), grape waste (53.33%), pineapple waste (55.6%), and mango seeds (56.5%)—materials successfully used as substrate supports in SSF [
11,
14,
15,
25,
26,
27]. Additionally, cascalote pods could be used as a substrate in the SSF process at high moisture levels due to the maximum moisture level that they obtained (79.33%); however, it is recommended to work at a moisture level below 70% in order to avoid oxygen transfer limitations, particle agglomeration, and bacterial growth [
12,
28]. Based on their physicochemical characterization, cascalote pods are suitable for use as a substrate support for SSF.
Filamentous fungi are the most used microorganism in SSF, and they have great potential to release bioactive compounds from vegetal matrices [
8]. Fungi can assimilate a wide variety of carbon sources, thus being able to synthesize, degrade, and transform several phenolic compounds and other aromatic compounds [
29,
30]. During SSF, fungi produce several enzymes that degrade the cell wall components, resulting in an enhanced phenolic compound extraction [
15].
A. niger GH1 has been previously reported as a potential fungus for the degradation of lignocellulosic materials and the release of phenolic compounds [
11,
12,
14]. In this study, the use of
A. niger GH1 increased the TPC release 1.67-fold and the AA 2.17-fold as compared to the other
Aspergillus strains evaluated.
The TPC release was positively correlated with AA (0.95;
p < 0.01), confirming that the increase in AA values was due to the increase in TPC release during the SSF process. This correlation is in accordance with the previous studies published by Paz-Arteaga et al. [
15], Jericó-Santos et al. [
31], and Buenrostro-Figueroa et al. [
14] on the SSF of pineapple, tamarind, and pomegranate byproducts, which reported correlation values between TPC and DPPH of 0.63, 072, and 0.86, respectively. Increments in the TPC released are associated with the fungal enzymes produced during the microbial growth phase. These enzymes (xylanases, pectinases, proteases, and glucosidase, among others) are responsible for the oxidative degradation of lignin and the breakdown of the links between the cell wall matrix and phenolic compounds, which results in their release [
15,
32].
Exploring the effects of nutritional and physical parameters in SSF influences microbial growth and their metabolic processes. Moisture plays an important role in fungal growth. SSF requires the close control of water content; depending on the microbial strain used, a specific moisture content is needed to ensure growth and metabolite production [
33]. In this study, TPC release was favored at high levels of moisture (60%), similar to the results obtained by Buenrostro-Figueroa et al. [
27] in the SSF of fig byproducts using
A. niger GH1.
Magnesium is necessary for fungal nutrition, and it has an important role in hyphae development. An optimal concentration of this mineral improves the sporulation rate, thus promoting an efficient enzyme synthesis and consequently, fungal biomass proliferation and phenolic compound release [
14,
34]. Using
A. niger GH1 and pomegranate husk to obtain ellagic acid and total phenolic compounds by SSF, Sepulveda et al. [
34] and Buenrostro-Figueroa et al. [
14] found that increments in MgSO
4 levels improved the release of ellagic acid and total phenolic compounds, respectively. Furthermore, those authors reported that the best phenolic compound release was attained at high levels of KH
2PO
4 and KCl. In the present study, the same KH
2PO
4, KCl, and MgSO
4 levels were used; however, a contrary effect was observed. This may be due to the substrate support (cascalote pods) itself containing sufficient amounts of minerals (Mg, P, and K) needed for both microbial growth and TPC release. The addition of higher amounts of minerals could have affected the enzyme production [
35]. The cell wall of
A. niger contains carbohydrates, proteins, ash, and lipids [
36]. Increases in protein and lipid content are related to the fungal biomass present in fermented cascalote pods. During fungal growth, the available nutrients are used to synthesize lipids as mycelium components. Reduction in carbohydrate and fiber contents are associated with fungal growth since these components are used by the fungi as a carbon source for its development and production of cell wall-degrading enzymes [
15,
32].
Out of all the treatments evaluated, the best values for the response variables (TPC and AA) were obtained in treatment 6, which may have been due to these conditions (30 °C, 60% moisture, and a concentration of mineral salts in the medium (g/L) of: KH
2PO
4: 1.52, NaNO
3: 7.65, MgSO
4•7H
2O: 1.52, and KCl: 1.52) promoting the better growth and metabolic processes of
A. niger GH1. SSF substantially improved the amount of phenolic compounds released from cascalote pods as well as the antioxidant activity. This is explained by the fact that several enzymes with important hydrolytic activities participate in SSF, inducing the release of phenolic compounds from polymeric matrices [
15,
37].
Based on the above results, SSF was successfully carried out by
A. niger GH1 on cascalote pods, with 12 h as the best time for TPC release. This is the first report to be conducted for cascalote pods under the conditions described. Using extraction by maceration with methanol, Sánchez et al. [
38] reported values of 21.71 mg/gdw for condensed tannins and 32.06 mg/gdw for total tannins. Rojas-Morales et al. [
39] reported values for condensed and total tannins of 13 and 34 mg/gdw, respectively. A total condensed tannins content of 7.72 mg/gdw was reported by Pineda-Peña et al. [
40]. The values for condensed and total polyphenols released from cascalote pods in the present work are 120–521% and 265–288% higher than the values previously reported [
38,
39,
40].
In addition, the cascalote pods exhibited strong antioxidant activity by reducing the agents for ferric ions and scavenging free radicals. According to the ABTS assay, SSF provided an increase of up to 93% in the AA of the extract (498.46 mgTE/gdw) as compared to the value before SSF (258.18 mgTE/gdw). Ethanolic extract from cascalote was evaluated against DPPH and ABTS antioxidant assays [
41]. At 500 mg/L, the inhibition rate was higher than 90% in both cases. Based on the strongly positive and highly significant correlation between phenolic compound content and activity in antioxidant assays (DPPH, ABTS, and FRAP), the increase in AA can be attributed to the amount and type of phenolic compounds released by the SSF. These results show the feasibility of obtaining TPC with AA from vegetal matrices via SSF in comparison to chemical processes or the use of commercial enzymes.
The RP-HPLC-ESI-MS of cascalote extracts showed that the main compounds were 5-O-galloylquinic acid, ellagic acid derivatives, corilagin, gallagyl-hexoside, lagerstannin B derivative, geraniin, and ellagic acid. There have been no reports indicating the identification of phenolic compounds obtained from cascalote pods by SSF; however, some of these compounds have been previously reported from the hydroalcoholic extracts of
C. coriaria [
40,
42]. In addition, other compounds, such as ethyl gallate, methyl gallate, gallic acid, tetragalloylglucose, pentagalloylglucose, valoneic acid dilactone, digalloylshikimic acid, and other phenolic compound derivatives, have been cited [
2,
4,
43].
From the nine compounds detected, seven of them increased in concentration at 12 h of SSF (in terms of units of absorbance), with 5-
o-galloyquinic acid, corilagin, geraniin, and ellagic acid having values 12.19-, 2.34-, 5.11-, and 7.72-fold higher than those obtained at 0 h of SSF. The lagerstannin B derivative was only found at 12 h of SSF, while gallagyl-hexoside was only detected at 0 h. The differences between the phenolic profile and absorbance values might be due to the phenolic compounds being in free or conjugated form (esterified). During SSF, these bonds are broken by the action of microbial enzymes, which facilitates the partial or complete release of the phenolic compounds, therefore improving their solubility or producing new molecules [
15].
Identified polyphenol molecules in cascalote pods have different biological activities, such as antioxidant effects, hepatoprotective effects [
41], anthelmintic effects [
39,
44], arginase inhibitory activity [
45], antimicrobial activity [
4,
43,
46], and gastroprotective effects [
40]. These benefits provide a wide range of possible applications in the food, pharmaceutical, and cosmetic industries.
Accurate data related to cascalote production were not found; however, a yield of 150 kg of pods per tree has been reported [
1]. Using the bioprocess described here, up to 18.6 kg of TPC per tree of cascalote pods could be obtained (124 kg of TPC/ton). Considering the commercial prices on the Sigma-Aldrich website (
https://www.sigmaaldrich.com/US/en/life-science/sigma-aldrich. accessed on 31 August 2023) for corilagin (SKU: 75251), geraniin (SKU: PHL80994), and ellagic acid (SKU: PHL89653) (594 USD/10 mg, 501 USD/10 mg, and 271 USD/50 mg, respectively), the SSF extraction process represents a profitable and sustainable alternative for the acquisition of valuable compounds with industrial applications. These results confirm that SSF permits the recovery of larger amounts of high-value molecules through a process that involves a shorter amount of time. With particular focus on the highly desired circular economy model, an alternative to the diversification of cascalote pods has thus been reported.