1. Introduction

The Agaves are plants commonly known as “magueys”, since ancient times they have had a strong relationship with Mexican culture and history [1]. They were sacred to the Aztec population of prehispanic Mexico and in the Nahuatl language, were named Metl . Agave (Agavaceae) is endemic to the American continent, with a distribution extending from the southern United States (with two disjunct species in Florida) to northern South America, including the Caribbean islands [2]. The genus contains approximately 210 species: 159 are present in Mexico (75% of the total) and 129 are endemic to the Mexican territory, representing 61% of the world’s species [3]. These data are constantly changing as new species are discovered.

Agave is mainly used for the production of distilled (spirits) and non-distilled alcoholic beverages, including Tequila, Mezcal, Bacanora, Raicilla, Sotol and Pulque, all of which have special connections to Mexican history and culture, and contribute to the Mexican economy [4]. Four main species dominate the agave market, namely Agave tequilana , Agave salmiana , Agave angustifolia , and Agave fourcroydes [1]. Their cultivation has been exponentially expanded because of their minimal water and maintenance requirements [5].

Currently, the agave has several alternative uses, including the whole plant and certain by-products. The whole plant is used as a living fence, improving the soil pH, increasing the concentration of phosphorus and potassium, and avoiding erosion, whereas the different by-products have a wide range of applications. In this review, we summarize the main uses for the re-valorization of leaves, bagasse, and fibers that otherwise are considered waste for the agave industry. Particular emphasis is placed on the processing methods for agave by-products and their transformation into high-value products.

2. Chemical Composition of the Main Agave By-Products

The overall composition of agave is cellulose, hemicellulose, lignin, and pectin in fluctuating amounts according to the variety. The plant is a source of other molecules, such as glucose, sucrose, and fructose, fructans, gums, saponins, and phenolic compounds. Table 1 summarizes the chemical composition of several by-products from agaves. Given the importance of the tequila industry in Mexico, the predominance of A. tequilana by-products and information about their composition is found in the literature. Cellulose, mainly comprising glucans, is the main component of agaves, followed by hemicellulose (mainly xylans), and lignin. Cellulose is a valuable material since it can be hydrolyzed to obtain simple sugars (monosaccharides) that serve as substrates for fermentation and bioethanol production. Nanoparticles and bioplastics made with cellulose have also been obtained from leaves and bagasse [39,40]. Crystallinity is an important feature of cellulose that might be undesirable depending upon the application. A high crystallinity index makes cellulose less digestible and hard to convert into simple sugars by enzymatic or chemical methods [22]. Cellulose from leaves seems to have higher crystallinity than other tissues [39], although nanocrystals with a high crystallinity degree have been obtained from bagasse using an adequate pretreatment [39,40]. Fourier transform infrared spectroscopy (FTIR) experiments have shown that the leaves have a degree of crystallinity of 50.1% [41], proportional to the hydrogen bond formation. Moreover, the leaves have slightly higher concentrations of lignin than other by-products, increasing the recalcitrance of these tissues, which necessitates a pretreatment for delignification before utilization as substrates for biofuel of methane production.

There are some differences in the composition of leaves and bagasse; it is known that bagasse contains higher amounts of simple sugars and xylan [26], whereas leaves and stalks are richer in lignin. Spines represent an interesting source of hemicellulose, since their content is comparable to that of cellulose [35]; other valuable compounds identified in spines include flavonoids, condensed tannins, and monolignol subunits. Although scarce studies are published on the chemical composition of foliar cuticle, all reported water, cutin and cutan as its main components [51].

3. Main Applications of Agave By-Products

Recently, agave fructans have been employed as functional ingredients for granola bars [56], and encapsulating material for bioactive compounds, given their high degree of polymerization [80].

Regarding agave bagasse, isolated polysaccharides from A. salmiana bagasse were used to elaborate and stabilize indomethacin nanoemulsions, and to improve the cellular uptake in a human dermal fibroblast model [72].

Cellulose crystallinity is another important feature of agave fibers, in this regard, a higher crystallinity index has been related to better mechanical properties for composites, although some studies have not found an improvement of these properties when fibers crystallinity is increased [9].

Lastly, hybrid composites containing pine sawdust, agave bagasse and HDPE were fabricated in a twin-screw extruder [16]. Results showed that the inclusion of agave bagasse increased the flexural and tensile strength, whereas the pine was helpful in reducing water absorption and conferring stability to the composites.

4. Processes and Technologies for the Treatment of Agave By-Products

Alkaline delignification with dilute solutions of NaOH (2%) has been tested on agave bagasse prior to the enzymatic step for saccharification [19]. A higher amount of reducing sugars was obtained with the addition of alkali, specifically, the glucose content was significantly increased compared with an acid pretreatment.

Regarding enzymatic hydrolysis, this method is useful for releasing simple sugars after the chemical pretreatment. The saccharification process is driven by the enzymatic action on the agave fibers to break down complex carbohydrates into simple sugars, which in turn, serve as a substrate for fermentation to produce biofuels such as bioethanol, or anaerobic digestion to produce methane and hydrogen.

Microwave technology has been considered an environmentally friendly method, and suitable for scaling up for industrial applications. With this technique by-products from different agave varieties have been processed to obtain activated carbon, bioactive compounds, and simple sugars. Finally, agave leaves have proven to be good microwave absorbers for anechoic chambers, blocking electromagnetic interferences [103]. Usually, polyurethane is used for this purpose, but it may release toxic gases and represents an environmental problem when discarded. Agave leaves are better absorbers than other wastes tested for this purpose, which makes this by-product a low-cost and eco-friendly material.

The supercritical fluids (SCF) technology has been explored as a pretreatment for the saccharification of agave bagasse and for bioactive molecules extraction, as previously mentioned. Increments up to 40% in the simple sugars content were observed with SCF applied to A. tequilana bagasse [104]. The saccharification yield was highly dependent on the hydration level; at low values of CO 2 the diffusivity was higher, resulting in higher damage to the agave structure. Furthermore, the combined treatment of US and SCF had a good potential for the extraction of saponins from A. salmiana bagasse, preserving the antioxidant capacity of these bioactive molecules [59,61]. Micellar extraction with ionic surfactants has been reported as an alternative method for saponin extraction [30] with high yield (89%), compared with other techniques such as US, but with low recovery for phenolic compounds.

5. Conclusion

Agaves are a rich source of by-products with a wide range of potential applications. Rather than considering these by-products as waste, current reports have shown their value as biofuels, materials for nanocomposites, and functional ingredients. Particularly, the leaves and bagasse from the agave industry, are lignocellulosic materials that can be converted into digestible sugar for bioethanol, methane, and hydrogen production. Most of the by-products have a significant content of saponins, that are valuable biomolecules with application in the food industry (foaming, emulsification, bitterness) and in the pharmaceutical sector (antimicrobial, anti-cancer and anti-obesity activity).

This entry is adapted from the peer-reviewed paper 10.3390/polysaccharides2030044