Potential Hydrothermal-Humification of Vegetable Wastes by Steam Explosion and Structural Characteristics of Humified Fractions

Abstract

In this work, steam explosion (SE) was exploited as a potential hydrothermal-humification process of vegetable wastes to deconstruct their structure and accelerate their decomposition to prepare humified substances. Results indicated that the SE process led to the removal of hemicellulose, re-condensation of lignin, degradation of the cellulosic amorphous region, and the enhancement of thermal stability of broccoli wastes, which provided transformable substrates and a thermal-acidic reaction environment for humification. After SE treatment, total humic substances (HS), humic acids (HAs), and fulvic acids (FAs) contents of broccoli samples accounted for up to 198.3 g/kg, 42.3 g/kg, and 166.6 g/kg, and their purification were also facilitated. With the increment of SE severity, structural characteristics of HAs presented the loss of aliphatic compounds, carbohydrates, and carboxylic acids and the enrichment of aromatic structures and N-containing groups. Lignin substructures were proved to be the predominant aromatic structures and gluconoxylans were the main carbohydrates associated with lignin in HAs, both of their signals were enhanced by SE. Above results suggested that SE could promote the decomposition of easily biodegradable matters and further polycondensation, aromatization, and nitrogen-fixation reactions during humification, which were conducive to the formation of HAs.

Principle and Application of Steam Explosion Technology in Modification of Food Fiber

“…There have been studies using steam explosion to hydrolyze rutin glycosidic bonds to prepare quercetin, process turmeric to extract and prepare diosgenin, and combine steam explosion and solid-state fermentation to convert Polygonum cuspidatum resveratrol glycosides to prepare resveratrol. In recent years, the technology of steam explosion pretreatment has been investigated for DF modification from a wide range of feedstocks, including wheat bran, grapefruit peel, okara, sugarcane bagasse, pineapple peel, citrus pomace, and vegetable waste [ 60 , 61 , 62 , 63 , 64 , 65 , 66 ].…”

“…Liu et al [ 90 ] used SE pretreatment to process sweet potato vine with different moisture content, and found that the hemicelluloses shoulder of manually dried feedstock samples’ derivative thermal gravimetric (DTG) curve almost disappeared after SE pretreatment, and the height of the maximum peak increased 12.5% and moved to a higher temperature due to the degradation of hemicelluloses and the related lifting of cellulose. In the study of Sui et al [ 66 ], the authors investigated SE processing on the deconstruction of wasted broccoli stems. The results showed that the SE process led to the removal of hemicelluloses, re-condensation of lignin, degradation of the cellulosic amorphous region, and enhancement of the thermal stability of broccoli wastes.…”

Humic Substances Derived From Biomass Waste During Aerobic Composting and Hydrothermal Treatment: A Review

“…HA is a typical kind of HS that can be easily prepared from various biomass wastes via hydrothermal technology ( Shu et al, 2016 ; Wang J. et al, 2017 ; Chang & Huang, 2018 ; Yang et al, 2019 ; Malhotra & Garg, 2020 ; Sriramoju et al, 2020 ; Sui et al, 2021 ). Chang & Huang (2018 ) produced a value-added nutrient solution containing 17% of HA in HT by using sugarcane exocarp as the raw material.…”

“…As for other types of biomass waste, similar phenomena can be observed. For instance, Sui et al (2021 ) used broccoli waste as the raw stock for hydrothermal humification to investigate the influence of the reaction temperature on the degree of humification. The humification rate calculated by the content of FA and HA increased from 0.16 to 0.25 as the reaction temperature increased from 184 to 220°C.…”

“…Reaction time is another key parameter for the conversion of biomass waste into HA. Sui et al (2021 ) reported that the humification rate increased from 0.19 to 0.22 as the reaction time increased when using broccoli waste as a feedstock (entry 11, Table 4 ). Similar to the reaction temperature, it needs to control the reaction time for a good yield of HA because excessive energy input will lead to degradation and transformation of HA.…”