Assessment of the effect of autohydrolysis treatment in banana’s pseudostem pulp

Highlights

• Autohydrolysis of post-fibre extraction pulp from banana pseudostem was studied.

• A high concentration of malto-oligosaccharides was obtained in the liquors.

• Solid fraction presented an improved enzymatic digestibility for further valorization.

• Two-stage autohydrolysis improved oligosaccharides production and solid digestibility.

Abstract

Banana’s pseudostem pulp (BPP) is a potential by-product obtained in the mechanical fiber extraction of banana’s pseudostem. Its chemical characterization revealed to have an interesting composition, with a high polysaccharides content and low content in lignin, which makes it particularly relevant for the biorefinery’s biochemical platform. Autohydrolysis pretreatment, studied under isothermal (140 °C) and non-isothermal conditions (140–220 °C), yielded oligosaccharides, mainly gluco-oligosaccharides, as the main soluble products. The highest oligosaccharides production (24 g/100 g raw material) was obtained at a severity factor of 2.3. Autohydrolysis pretreatment effectively disrupted the structure of the material, inducing an improvement of the enzymatic digestibility from 73% for the raw material up to 90% for the most severe conditions. Two stage autohydrolysis, with increasing severity, was also studied, allowing to obtain a higher amount of oligosaccharides (32 g/100 g raw material) and higher digestibility of the remaining solid (up to 97%).

Introduction

In recent years, there has been an increasing trend towards more efficient utilization of agro-industrial residues. Banana is cultivated over 130 countries, being the second largest produced fruit, after citrus (Mohapatra et al., 2010), and its residues are available around the world (Gabhane et al., 2014), including in Europe. Global production reached a record of 114 million tons in 2017 (FAO). Canary Islands is the largest banana producer region in the European Union, with 0.4 million tons of bananas produced each year (ASPROCAN).

As banana plants only bear fruit once in its lifecycle, once they have been harvested the plant is cut, producing significant amounts of agricultural residues. For each ton of fruit harvested, around four tons of lignocellulosic wastes are generated, among which 75% consists of banana plant pseudostem (Souza et al., 2014). This by-product is sometimes processed into low-grade animal feed by local farmers and has been used to produce various handcrafts, eating utensils, food wrapping, etc. (Santa-Maria et al., 2013); however in most cases it is usually left in the plantation, producing wastes accumulation and having no nutritional value for the soil. An interesting strategy to manage these wastes is the development of new applications, which could also represent an interesting income for banana producers, thus boosting the regional economy (Oliveira et al., 2007), particularly in the Canary Islands, where banana crop is an essential socio-economic pillar.

Mechanical fiber extraction is one of the most relevant alternatives proposed for the valorization of the pseudostem (Saraiva et al., 2012). This material contains 90% of moisture, 0.6% of fiber and 9.4% of pulp (Benítez et al., 2013). Banana fiber has high strength, lightweight, low elongation and shiny appearance, among other textile qualities (Sengupta et al., 2019) and it has been proven in different applications like composite materials (Ortega et al., 2013). Fiber extraction also produces important amount of a lignocellulosic by-product, banana’s pseudostem pulp (BPP), which is the raw material in this study, and whose characterization and exploitation has not been yet explored in the literature. The use of the entire pseudostem and not only the fiber would improve the economic balance of fiber production, making it more attractive and launching its industrial production. On the other hand, these results could be used in abaca (Musa textilis) plantations, whose only product is fiber (over 80 000 tons/year) (Ortega et al., 2013).

Liquid hot water (LHW) treatment, or autohydrolysis, is among the most promising fractionation technologies for lignocellulosic biomass-based biorefineries, presenting the attractive that uses just compressed hot water for biomass treatment (Carvalheiro et al., 2016). This process is considered to be the most appropriate choice for the selective separation of hemicelluloses (Moniz et al., 2014), reason why it has been applied primarily to biomass rich in this component. Besides, autohydrolysis is suitable for oligosaccharides (OS) production (Moniz et al., 2014), which are receiving substantial attention due to their functional properties and health benefits as active ingredients in functional foods (Carvalho et al., 2013). The type of oligosaccharide that can be obtained will depend on the raw material and on the operation conditions. On the other hand, autohydrolysis allows the recovery of cellulose and lignin in a solid phase in advantageous conditions (altered surface and improved digestibility) for further processing.

The considerable amount of non-structural glucans in BPP turns it into a suitable candidate for hydrothermal treatments, despite its low hemicellulose content (Guerrero et al., 2017). The effect of the LHW treatment demonstrated for other raw materials encourages the study of this feedstock as a strategy of upgrading BPP in the biorefinery’s context. Some references applying hydrothermal treatments to agricultural waste from banana crops have been found. One of them (Santa-Maria et al., 2013) compared autohydrolysis and steam explosion (hydrothermal treatment using steam instead of liquid water), for the three main lignocellulosic residues of banana plants (leaves, pseudostem and rachis). Other authors (El-Zawawy et al., 2011, Kamdem et al., 2015) evaluated steam explosion treatment on different combined morphological parts of banana plant, while Guerrero et al. (2017) also evaluated steam explosion pretreatment of banana lignocellulosic biomass (rachis and pseudostem) but using an acid as catalyst. However, these studies mainly focused on improving biomass digestibility for bioethanol production, not paying any attention to oligosaccharides production in the liquid fraction. Moreover, no studies dealing with hydrothermal treatment of post-fiber extraction wastes from banana pseudostem have been found.

On the other hand, two-stage autohydrolysis has been proposed in literature for different goals. For example, it can be used as an alternative to a previous ethanol extraction in order to remove easily extractable compounds (as waxes, low molecular weight phenolics and soluble inorganic compounds), facilitating the purification of the liquor obtained in the second stage (Charalampopoulos and Rastall, 2009). Furthermore, the removal of these extractives can improve the oligosaccharide yields attained after hydrothermal processing (Alves-Ferreira et al., 2019). On the other hand, Lee et al. (2010) used a combination of a mild pretreatment to extract hemicelluloses followed by a harsher pretreatment to increase the enzymatic digestibility of the residue as a strategy for the full recovery of sugars from Coastal Bermuda grass. Agricultural waste from banana crops usually presents high contents in extractives, reason why two consecutive autohydrolysis becomes an interesting option.

The goal of this work is to study the production of OS by application of hydrothermal pretreatment on BPP. For this, isothermal and non-isothermal autohydrolysis at different final temperatures (140–220 °C) on BPP were performed. The effect of the treatment in the recuperation of OS in the liquid fraction, as well as the impact on the enzymatic digestibility and composition of the solid fraction, was evaluated. Moreover, a pretreatment composed of two consecutive autohydrolysis of increasing severity was also evaluated with the objective of obtaining a higher amount of oligosaccharides.