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. 2024 Nov 27;34(7):1581–1588. doi: 10.1007/s10068-024-01758-y

Conversion of rice husks into antioxidative and prebiotic biomaterials using subcritical water pretreatment and cellulase incubation under high-pressure

Chang-Geun Son 1, Jong Won Lee 1, Mi-Yeon Lee 2,✉, Geun-Pyo Hong 1,✉
PMCID: PMC11929643  PMID: 40129712

Abstract

This study investigated the combined effects of subcritical water (SW, 170 °C) pretreatment and cellulase incubation under high-pressure (HP, 200 MPa) on the hydrolysis efficiency of rice husks (RH) and explored the antioxidative and prebiotic activities of the RH hydrolysates. The SW pretreatment disintegrated the cell wall of RH, increasing the brix and reducing sugar content. RH hydrolyzed by cellulase exhibited greater antioxidative activity than that before enzymatic treatment. Particularly, the antioxidative activity of RH hydrolysates incubated under HP was higher than those incubated under atmospheric pressure (AP), regardless of SW pretreatment. Although SW pretreatment reduced the prebiotic potential of RH hydrolysates, RH hydrolyzed under HP without pretreatment exhibited greater prebiotic potential for Lactobacillus rhamnosus GG (LGG) than that hydrolyzed under AP incubation. The results demonstrated that SW pretreatment and HP incubation have potential applications for producing bioactive substances using RH, and optimization of the process parameters warrants further exploration.

Keywords: Rice husk, High-pressure incubation, Subcritical water, Antioxidative activity, Prebiotic potential

Introduction

Rice is classified as one of the four major staple crops harvested worldwide. Reportedly, the world’s annual rice production is ~ 600 million tons, generating > 120 million tons of rice husk (RH) byproduct (Zhang et al., 2020). By 2030, RH production is expected to exceed 200 million tons (Kaur and Ubeyitogullari, 2023). However, most RH becomes industrial waste owing to the lack of recycling or upcycling techniques, releasing gas pollutants into the environment and causing economic and environmental problems (Kaur and Ubeyitogullari, 2023; Zhang et al., 2020). Meanwhile, RH is a good source of bioactive phytochemicals such as phenolic acids and flavonoids (Kaur and Ubeyitogullari, 2023), which provide various biological benefits (Karimi et al., 2014). Particularly, RH is considered a potential biomass for producing fermentable sugar or bioethanol because cellulose and hemicellulose make up the majority of RH (Abaide et al., 2019).

Conventionally, enzymatic processes are the only way to produce bioactive compounds from the biomass, and various cellulases are available commercially. However, there are various hurdles involved in the enzymatic conversion of RH into bioactive compounds. Despite the high hydrolytic efficiency of cellulases, the high cost of the commercial enzymes acts as an economic barrier, limiting the enzymatic conversion of the agro-industrial byproduct (Weiss et al., 2013). In recent, it has been shown that high pressure (HP) improves the activity of enzymes involved in the hydrolysis of organic compounds (Eisenmenger and Reyes-De-Corcuera, 2009; Ma and Mu, 2016; Son et al., 2023). In particular, it was reported that moderate pressurization (100–300 MPa) has benefits for the hydrolysis of proteins or polysaccharides, and it is believed that the impact of HP on enzyme activity results from structural changes in enzymes and substrates (Ma and Mu, 2016). From this perspective, HP-assisted enzymatic processes may be a promising technique to not only improve the hydrolysis efficiency of lignocellulosic biomass but also enhance economic viability by reducing the amount of enzyme added.

On the other hand, the recalcitrant nature of RH, due to the presence of lignin, is the other factor that hinders overall enzymatic activity (Zoghlami and Paës, 2019), and physical or chemical pretreatment was essential to exposure substrates for the selected enzymes. Nevertheless, existing pretreatments show limitations such as reduced substrate yield and insufficient weakening of lignin, requiring an alternative pretreatment technique.

Subcritical water (SW) has been investigated as a potential technique to replace the traditional enzymatic conversion process of biomass. Various advantages of SW technology have been reported, including the extraction of nonpolar compounds and the hydrolysis of organic compounds from lignocellulosic biomasses (Abaide et al., 2019; Draszewski et al., 2021; Lee et al., 2024). For RH, recent literature indicated that SW at ~ 150 °C could weaken the lignin structure, and hydrothermal hydrolysis of hemicellulose underwent at ~ 200 °C (Lee et al., 2024). At higher than 200 °C, SW could hydrolyze cellulose, and the RH hydrolysates provided various physiological activities. Nevertheless, generation of thermal derivatives of sugars could potentially produce toxic compounds at > 200 °C which has not been thoroughly investigated (Lee et al., 2024; Prado et al., 2016). For this reason, most SW conversion technology has been tested in a limited temperature range of 150–200 °C, but these temperatures caused only limited hydrolysis of the tested biomass. Conversely, SW at these temperature ranges can potentially be utilized as a pretreatment technique for enzymatic processes.

Despite the potential effectiveness of the techniques, there have been no investigations applying SW as a pretreatment for enzymatic processes or exploring the effects of HP-assisted enzymatic hydrolysis technology. Therefore, this study investigated the effects of SW pretreatment and HP-assisted enzymatic process on the hydrolysis efficiency of RH and explored the antioxidative activity and prebiotic potential of RH hydrolysates for use as food biomaterials.

Materials and methods

Materials

RH was obtained from a local farm (Gaya Industrial, Goryeong, Gyeongbuk, Korea) and dried at 65 °C overnight using a hot-air dryer (HB-501S, Hanbaek scientific Co., Bucheon, Gyeonggi, Korea). The dried RH was gently ground for 1 min using a commercial blender (HR3571/90, Philips CO., Ltd., Amsterdam, Netherlands). The RH particulates were packaged in a low-density polyethylene bag and kept at − 40 °C until use (within 3 d). A commercial cellulase (Cellulast® 1.5 L) was purchased from Novozymes (Bagsvaerd, Denmark), and a type-cultured human gut probiotic strain, Lactobacillus rhamnosus GG (LGG), was kindly donated by the Food Microbiology and Microbiome Laboratory, Sejong University (Seoul, Korea). All chemicals used in the analyses were obtained from a local supplier and were analytical grade.

SW pretreatment and scanning electron microscopy

To compare the impact of SW pretreatment temperature, RH was subjected to a lab-assembled SW device, as described in our previous study (Lee et al., 2024), at the Biopolymer Research Center for Advanced Materials (Seoul, Korea). A total of 25 g of the dried RH was suspended in 475 mL of distilled water to formulate 500 mL of 5% (w/v) suspension. An aliquot of 120 mL was placed in a reactor (working volume of 130 mL). After sealing the reactor tightly, it was heated using a 600 °C electronic heater until the temperature inside reached 150 °C (27–31 min of heating) and 170 °C (33–37 min of heating), respectively. The pressure inside the reactor was built by the vaporization of the suspension, reaching 0.69 MPa at 150 °C and 0.87 MPa at 170 °C. After completing the SW pretreatment, the reactor was immersed in a 4 °C water bath for ~ 20 min to cool the suspension to ~ 40 °C. The treated suspension was centrifuged at 3000×g for 15 min at ambient temperature, and the precipitates were dried at 105 °C for 24 h. The untreated suspension was used as a control, and the precipitates were prepared via centrifuging and drying using the same procedure described above. The morphology of the dried precipitates was compared using a scanning electron microscope (TM4000Plus, Hitachi High-Technologies, Tokyo, Japan) at the Biopolymer Research Center for Advanced Materials (Seoul, Korea). Both a semiconductor backscattered electron detector and a secondary electron detector were used for the morphology analysis of the precipitates with an accelerating voltage of 15 kV and magnifications of × 200 and ×500.

Preparation of RH hydrolysates

RH was suspended in distilled water to create 5% (w/v) suspension. The RH suspension was subjected to different hydrolysis procedures, as outlined in Fig. 1. Half of the suspension underwent SW pretreatment at 170 °C under the same conditions described above and labeled SW. The other half did not undergo SW pretreatment and was labeled NP (nonpretreatments). Each suspension was further divided into two groups for enzymatic hydrolysis, one under atmospheric pressure (AP) and the other under 200 MPa (HP). After adding 100 units/g of the cellulase, each group was split into six low-density polyethylene bags and sealed. The AP group was incubated in a 50 °C chamber for 0, 6, 12, 18, 24, and 48 h at atmospheric pressure, whereas the HP group was pressurized using an HP device as described in our previous study (Son et al., 2023) at the Biopolymer Research Center for Advanced Materials (Seoul, Korea). The temperature inside the HP chamber was maintained at 50 °C, and the HP treatments were performed for 0, 6, 12, 18, 24, and 48 h at 200 MPa. After incubation, the samples were immersed in 100 °C hot water for 10 min to stop the cellulase reaction and centrifuged at 10000×g for 15 min at ambient temperature. The supernatant was used to characterize the RH hydrolysates. To ensure experimental replication, all procedures described above were repeated thrice with a new batch of RH.

Fig. 1.

Fig. 1

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Experimental design and sampling procedures depending on pretreatment and enzymatic hydrolysis of rice husks

Brix and reducing sugar content of RH hydrolysates

The brix of RH hydrolysates was measured in triplicate using a refractometer (PAL-α, Atago Co. Ltd., Tokyo, Japan). The reducing sugar content of RH hydrolysates was also measured in triplicate using the dinitrosalicylic acid (DNS) assay (Gonçalves et al., 2010) with D-glucose (Sigma Aldrich Co., St. Louis, MO, USA) as the standard.

Simple sugar compositions of RH hydrolysates

To compare the effects of SW pretreatment and HP incubation on cellulase activity, the simple sugar composition of RH hydrolysates incubated for 0 and 24 h was estimated. Before analysis, all the samples were filtered through a 0.45-μm polyvinylidene fluoride syringe filter (No. 6779-1304, Whatman, Maidstone, UK), and 10 μL of each filtrate was applied to the chromatography. Compositions of simple sugars were analyzed in duplicate using capillary ion chromatography (ICS-5000, Dionex, Sunnyvale, CA, USA) equipped with two serially connected CarboPac columns (SA10 and SA10G, Dionex) and a pulsed amperometric detector (Dionex). The oven temperature was set to 32 °C, and a constant elution rate of 0.6 mL/min was maintained with 0.1 M sodium hydroxide as the mobile phase. Monosaccharide standard (Sigma Aldrich Co.) consisting of arabinose, galactose, rhamnose, glucose, xylose, fructose, and mannose were used to identify the detected simple sugars.

Antioxidant activity of RH hydrolysates

The total phenolic content (TPC) of the hydrolysates was measured in triplicate using the method of Noreen et al. (2017). TPC was calculated from a standard curve prepared using gallic acid and expressed as mg gallic acid equivalent (GAE)/mL.

The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity of the hydrolysates was determined in triplicate following the method of Oh et al. (2024) with minor modifications. The sample hydrolysates were mixed with an equal volume of 0.2 mM DPPH in ethanol and reacted in a dark room at 37 °C for 30 min. The absorbance of the reaction mixture was measured at 517 nm, with distilled water serving as the control. The DPPH radical scavenging activity was calculated using the following equation:

DPPHradicalscavengingactivity%=A0-A1A0×100

where A0 is the absorbance of the control, and A1 is the absorbance of the sample.

The 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid; ABTS) radical inhibition activity of the samples was measured in triplicate using the method of Guo et al. (2023) with minor modifications. The ABTS radical stock was generated by mixing 7 mM ABTS solution with an equal volume of 2.4 mM potassium persulfate. The mixture was kept in a dark room at 30 °C for 16 h to complete the reaction and then diluted with distilled water until the absorbance at 734 nm was 0.70 ± 0.01. For analysis, 100 μL of the sample was added to 3 mL of the ABTS stock solution and reacted at 30ºC for 15 min. Absorbance at 734 nm was measured, and the ABTS radical inhibition activity of RH hydrolysates was calculated using the same equation as for DPPH radical scavenging activity.

Prebiotic potential

The prebiotic potential of RH hydrolyzed for 24 h was determined in duplicate using LGG and compared with the untreated RH control. The pH of all samples was adjusted to 7.0 using 0.1 M NaOH and sterilized in an autoclave at 121 °C for 15 min. A probiotic stock solution was prepared by inoculating LGG into presterilized saline to 3-log levels, and 1 mL of the stock was added into 9 mL RH hydrolysates. The mixture was cultured at 37 °C for 24 h. After serial dilution of the cultures to the 109 level, 1 mL of each diluted LGG culture was spread on Petri-film (lactic acid bacteria count plate, 3 M, Saint Paul, MN, USA). The Petri-films were further incubated at 37 °C for 48 h, and plates with 30–300 colonies were counted. The microbial count was expressed as log CFU/mL.

Statistical analysis

A completely randomized design was adopted to evaluate the main effects (hydrolysis conditions). Mean values and standard deviations were calculated from data collected from experiments repeated thrice, and one-way analysis of variance was performed using SPSS software (v.25, IBM Inc., Armonk, NY, USA). Duncan’s multiple range test was conducted when the main effects were significant (p < 0.05).

Results and discussion

Morphology of SW-pretreated RH

The surface morphology of SW-pretreated RH is shown in Fig. 2. The untreated control exhibited an intact surface morphology characterized by a smooth and regular appearance. However, the surface of RH pretreated with SW lost its morphological integrity, with more changes in the surface morphology occurring intensively when 170 °C SW was applied compared with 150 °C SW. For hydrolysis of lignocellulosic biomass, the destruction of cell walls is a key step before enzymatic actions, and SW has been reported as an effective technique for the pretreatment of lignocellulosic biomass compared with ultrasounds, which is generally applied before the enzymatic hydrolysis of biomass (Dzah et al., 2020). The high enthalpic contribution of SW enabled the loosening of the lignin structure and promoted the cleavage of hemicellulose and cellulose fibers (Lee et al., 2024). It was believed that SW-mediated partial destruction of the cell wall could assist enzymatic action by generating a greater surface area of RH and allowing easier access of cellulase to the substrates (Dzah et al., 2020; Zhang et al., 2020). Following the changes in surface morphology, 170 °C SW was employed in this study as the pretreatment condition for the enzymatic hydrolysis of RH.

Fig. 2.

Fig. 2

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Surface morphology of untreated rice husks (A, D) and those pretreated with subcritical water at 150 °C (B, E) and 170 °C (C, F) at a magnification of ×200 (A–C) and ×500 (D–F), respectively. Scale bars indicate 100 μm

Brix and reducing sugar of RH hydrolysates

The brix of RH hydrolysates was affected by pretreatment, but the hydrolysis condition did not influence the brix of the RH hydrolysates (Fig. 3A). Irrespective of incubation time, the brix of SW groups was twice greater than those of NP groups (p < 0.05). Lignin has been reported as the main barrier involved in the hydrolysis of lignocellulosic biomass, and its degradation or solubilization is a mandatory procedure for the enzymatic hydrolysis of RH (Dos Santos et al., 2019). Lee et al. (2024) found that the hydrothermal degradation of lignin was manifested by SW at 200 °C, after the hydrothermal hydrolysis of hemicellulose and cellulose into oligomers and monomers intensely proceeded with SW temperature. For cellulase action, HP tended to increase the brix of RH hydrolysates compared with AP treatments, but the differences in brix between HP and AP treatments were not significantly different. This could be explained by the enzyme used in this study being an endo-type cellulase, leading to a dominance of oligomers in RH hydrolysates, which would rarely affect the brix of the hydrolysates.

Fig. 3.

Fig. 3

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Brix (A) and reducing sugar content (B) of rice husk hydrolysates. Treatment codes indicate nonpretreatment (NP), subcritical water pretreatment (SP), cellulase incubation at atmospheric pressure (AP), and at high-pressure (HP), respectively. Vertical bars indicate standard deviations (n = 9). Means with different lowercases among incubation times (a–f) and uppercases among treatments (A–D) were significantly different (p < 0.05)

The impact of HP on cellulase action was clearly demonstrated by the reducing sugar content of RH hydrolysates (Fig. 3B). Initially, the reducing sugar content of the RH suspension was 0.23 mg/mL, but it increased to 0.64 mg/mL after SW pretreatment (p < 0.05). During the enzymatic reaction, the reducing sugar content of the RH hydrolysates increased significantly with incubation time, and the HP treatments exhibited greater reducing sugar content compared with the corresponding AP treatments (p < 0.05). This result indicates that both SW pretreatment and HP-assisted enzymatic hydrolysis were effective in hydrolyzing RH. Reportedly, HP affects the solubility and structure of substrates, promoting better enzymatic hydrolysis of biomass (Kim and Han, 2012). Although the mechanisms involved in HP-assisted enzymatic hydrolysis are not clearly understood, it is believed that moderate HP (100–300 MPa) causes structural modifications of organic compounds, making enzymes more accessible to substrates (Cabrera et al., 2014). Therefore, HP could enhance enzymatic activity and accelerate hydrolysis reactions (Ma and Mu, 2016), potentially improving the productibility of biomaterials from RH. The results demonstrate that SW pretreatment combined with HP-assisted enzymatic hydrolysis has potential applications in overcoming the recalcitrance of lignocellulosic biomass for cellulase action.

Simple sugar compositions of RH hydrolysates

To estimate the hydrolysis activity of cellulase, the simple sugar compositions of RH hydrolysates incubated for 24 h were compared with those before incubation (Fig. 4). For untreated RH suspensions, small amounts of glucose (0.05 mg/mL) and xylose (0.03 mg/mL) were detected. Although arabinose, galactose, and mannose were also detected in suspension, their amounts were negligible (< 0.01 mg/mL). As the cellulase used in this study belongs to 1,4-β-glucanase, glucose is a key indicator of enzymatic activity. For the NP group, glucose content increased to 0.99 and 1.07 mg/mL under AP and HP conditions, respectively (p < 0.05). Additionally, the amounts of other detected simple sugars also increased after cellulase incubation, with xylose exhibiting a profound increase. As xylan, a polysaccharide comprising xylose residues, is the main composition of RH hemicellulose (Rennie and Scheller, 2014), the action of cellulase would be attributed to the yield of xylose.

Fig. 4.

Fig. 4

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Simple sugar compositions in rice husk hydrolyzed for 24 h. Treatment codes indicate nonpretreatment (NP), subcritical water pretreatment (SP), and cellulase incubation at atmospheric pressure (AP) and at high-pressure (HP), respectively. Abbreviations for simple sugars are arabinose (Ara), galactose (Gal), rhamnose (Rha), glucose (Glu), xylose (Xyl), and mannose (Man), respectively

Alternately, RH pretreated by SW showed higher glucose (0.62 mg/mL) and xylose (0.68 mg/mL) compared to untreated RH (p < 0.05), and arabinose (0.39 mg/mL) as well as trace amounts of galactose (< 0.01 mg/mL) and rhamnose (0.02 mg/mL) were detected. According to Lee et al. (2024), hemicellulose was more susceptible to hydrothermal processes compared with cellulose, accounting for the large generation of arabinose and xylose in the RH hydrolysates after SW pretreatment. After the cellulase reaction, the glucose content in the SW group increased to 1.54 and 1.73 mg/mL under AP and HP conditions, respectively. Moreover, the action of cellulase yielded greater amounts of arabinose (0.41–0.45 mg/mL) and xylose (1.52–1.53 mg/mL) under both AP and HP incubations (p < 0.05), but there was no significant difference in the amounts of these sugars between AP and HP treatments. Following the glucose contents estimated in this study, it is clear that both SW pretreatment and HP incubation improved cellulase activity in hydrolyzing RH. Cellulose was an appropriate substrate for the action of cellulase, whereas cellulase could also affect the hydrolysis of hemicellulose to a limited extent. As the hydrolysis of cellulose and hemicellulose improved the solubility of RH, the results also supported the brix and reduced sugar content of RH hydrolysates.

Antioxidant activities of RH hydrolysates

Three estimated antioxidative activities of RH hydrolysates exhibited a similar pattern of changes during enzymatic hydrolysis, with SW pretreatment being the main contributor affecting the antioxidative activity of RH hydrolysates rather than the hydrolysis conditions (Fig. 5). Before the cellulase reaction, RH pretreated by SW showed significantly greater antioxidative activities than nonpretreated RH (p < 0.05). Upon cellulase reaction, the antioxidative activity of RH hydrolysates gradually increased; however, NP groups that reacted with cellulase for 48 h showed lower antioxidative activities than SW-pretreated RH without cellulase reaction. It is well reported that RH is rich in phenolic acids and flavonoids, and an organic solvent is essential to extract the nonpolar compounds. As a polar solvent, water could not extract nonpolar compounds, which explained the low antioxidative activities of NP groups before enzymatic reaction. Conversely, the dielectric constant of water decreases with temperature, enabling SW to act like an organic solvent (Carr et al., 2011). This nature of SW results in greater TPC, thereby manifesting higher DPPH and ABTS radical scavenging activities in RH.

Fig. 5.

Fig. 5

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Total phenolic content (A), DPPH radical scavenging activity (B), and ABTS radical inhibition activity (C) of rice husk hydrolysates. Treatment codes indicate nonpretreatment (NP), subcritical water pretreatment (SP), cellulase incubation at atmospheric pressure (AP) and at high-pressure (HP), respectively. Vertical bars indicate standard deviations (n = 9). Means with different lowercases among incubation times (a–f) and uppercases among treatments (A–D) were significantly different (p < 0.05)

Irrespective of pretreatment, the antioxidative activities of RH hydrolysates gradually increased with incubation time (p < 0.05). For RH without pretreatment, RH hydrolyzed by HP exhibited greater antioxidative activities than those hydrolyzed by AP (p < 0.05), but the impact of HP incubation was not observed when pretreated RH was applied to cellulase. Again, the result implies that HP enhances the extraction efficiency of phenolic compounds. According to Pourali et al. (2010), phenolic compounds in RH are present in the cell wall, bonded to hemicellulose and cellulose. The action cellulase gradually degraded the RH cell wall; hence, the cellulase promotes the release of embedded phenolic compounds (Ma and Mu, 2016). As seen by reducing sugar content and simple sugar composition, cellulase-induced disintegration of RH cell walls was efficiently manifested under HP incubation, leading to higher antioxidative activities than AP. Conversely, cell walls were damaged by SW pretreatment, allowing the extraction of phenolic compounds before enzymatic hydrolysis. These impacts of SW probably compensated for the advantage of HP incubation in pretreated RH.

The hydrolysis of polysaccharides could reportedly enhance antioxidant activity owing to the exposure of reducing hydroxyl groups (Hu et al., 2021), and this was responsible for the gradual increase in the antioxidative activities of RH hydrolysates with incubation time. Following the results, it was suggested that SW has a potential advantage in producing antioxidants from RH compared with HP incubation. However, the impact of HP incubation on the antioxidative activities of RH could be controversial, as HP incubation was tested only at 200 MPa, warranting further exploration.

Prebiotic potential

Eventually, RH hydrolysates incubated for 24 h were selected to compare their prebiotic potential of RH hydrolysates (Fig. 6). The prebiotic potentials of the RH hydrolysates were affected by SW pretreatment and hydrolysis condition, whereas the influence of SW pretreatment and HP incubation showed an opposite impact on the LGG count. Originally, inoculated LGG in the sample hydrolysates was at a 2-log level, and the LGG count in the untreated control increased to 3.99 log CFU/mL after 24 h. For NP groups, RH hydrolyzed by HP showed 6.97 log CFU/mL, significantly greater than the 6.54 log CFU/mL of AP treatment (p < 0.05). A major problem in the microbial fermentation of lignocellulosic biomass is the crystalline structure and high degree of polymerization of the substrates, requiring hydrolysis to make the substrate more accessible to microorganisms (Zoghlami and Paës, 2019). From this perspective, the results showed that RH could be available for prebiotic products in the food industry, and HP incubation has the potential for the mass production of prebiotic materials.

Fig. 6.

Fig. 6

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Counts of Lactobacillus rhamnosus GG (LGG) inoculated in rice husk hydrolyzed for 24 h. Treatment codes indicate nonpretreatment (NP), subcritical water pretreatment (SP), and cellulase incubation at atmospheric pressure (AP) and at high-pressure (HP), respectively. Vertical bars indicate standard deviations (n = 6). Means with different lower cases among treatments (a–e) were significantly different (p < 0.05)

Alternately, limited growth of LGG was observed in RH pretreated by SW. Although HP incubation resulted in a greater LGG count of 3.86 log CFU/mL compared with 3.14 log CFU/mL of AP treatment (p < 0.05), the LGG count in the SW groups was generally lower than that of the control (p < 0.05). Similar results were reported by Lee et al. (2024), suggesting that the production of an antimicrobial agent was likely involved in the SW pretreatment. In general, it has been reported that simple sugars are converted into thermal derivatives such as 5-hydroxymethylfurfural (HMF) and furfural with increasing SW temperature, and HMF is known to adsorb onto the surface of microorganisms, inhibiting their growth (Chai et al., 2013; Ricke et al., 2020). Meanwhile, it was also reported that the SW condition of 170 °C employed in this study did not produce such thermal derivatives (Lee et al., 2024). Alternatively, SW pretreatment favored the generation of Maillard products, which would inhibit the growth of LGG (Nooshkam et al., 2020). Therefore, it was suggested that an additional process to separate proteins from RH prior to SW pretreatment was required not only to prevent the generation of melanoidins but also to facilitate the conversion of RH into a prebiotic material, which warranted further exploration.

Eventually, SW pretreatment was effective in modifying the structure of the substrate, making RH more accessible for cellulase activity, and HP enhanced the activity of cellulase, thereby improving the hydrolysis efficiency of RH. As a result, the RH hydrolysates exhibited various physiological activities, suggesting their potential application as functional food ingredients.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant Nos. 2022R1A6A1A03055869; 2022R1I1A1A01065657) and by the Ministry of Science and ICT (Grant No. RS-2024-00341861). The authors acknowledge Prof. Hakdong Shin (Sejong University, Seoul, Korea) for kindly providing the type-culture probiotic strain (LGG).

Funding

Funding was provided by Ministry of Education (2022R1A6A1A03055869, 2022R1I1A1A01065657) and Ministry of Science and ICT, South Korea (RS-2024-00341861).

Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Footnotes

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Mi-Yeon Lee, Email: leemy@konkuk.ac.kr.

Geun-Pyo Hong, Email: gphong@sejong.ac.kr.

References

  1. Abaide ER, Ugalde G, Di Luccio M, de FPM Moreira R, Tres MV, Zabot GL, Mazutti MA. Obtaining fermentable sugars and bioproducts from rice husks by subcritical water hydrolysis in a semi-continuous mode. Bioresource Technology. 272:510-520 (2019) [DOI] [PubMed]
  2. Cabrera E, Muñoz MJ, Martín R, Caro I, Curbelo C, Díaz AB. Alkaline and alkaline peroxide pretreatments at mild temperature to enhance enzymatic hydrolysis of rice hulls and straw. Bioresource Technology. 167:1-7 (2014) [DOI] [PubMed] [Google Scholar]
  3. Carr AG, Mammucari R, Foster NR. A review of subcritical water as a solvent and its utilisation for the processing of hydrophobic organic compounds. Chemical Engineering Journal. 172:1-17 (2011) [Google Scholar]
  4. Chai WM, Liu X, Hu YH, Feng HL, Jia YL, Guo YJ, Zhou HT, Chen QX. Antityrosinase and antimicrobial activities of furfuryl alcohol, furfural and furoic acid. International Journal of Biological Macromolecules. 57:151-155 (2013) [DOI] [PubMed] [Google Scholar]
  5. Dos Santose AC, Ximenes E, Kim Y, Ladisch MR. Lignin-enzyme interactions in the hydrolysis of lignocellulosic biomass. Trends in Biotechnology. 37:518-531 (2019) [DOI] [PubMed] [Google Scholar]
  6. Dzah CS, Duan Y, Zhang H, Authur DA, Ma H. Ultrasound-, subcritical water- and ultrasound assisted subcritical water-derived tartary buckwheat polyphenols show superior antioxidant activity and cytotoxicity in human liver carcinoma cells. Food Research International. 137:109598 (2020) [DOI] [PubMed] [Google Scholar]
  7. Draszewski CP, Bragato CA, Lachos-Perez D, Celante D, Frizzo CP, Castilhos F, Tres MV, Zabot GL, Abaide ER, Mayer FD. Subcritical water hydrolysis of rice husks pretreated with deep eutectic solvent for enhance fermentable sugars production. The Journal of Supercritical Fluids. 178:105355 (2021) [Google Scholar]
  8. Eisenmenger MJ, Reyes-De-Corcuera JI. High pressure enhancement of enzymes: A review. Enzyme and Microbial Technology. 45:331-347 (2009) [Google Scholar]
  9. Gonçalves C, Rodriguez-Jasso RM, Gomes N, Teixeira JA, Belo I. Adaptation of dinitrosalicylic acid method to microtiter plates. Analytical Methods. 2:2046-2048 (2010) [Google Scholar]
  10. Guo H, Fan L, Ding L, Yang W, Zang C, Guan H. Separation and purification of antioxidant peptide from fermented whey protein by Lactobacillus rhamnosus B2-1. Food Science of Animal Resources. 43:10-24 (2023) [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Hu TG, Zou YX, Li EN, Liao ST, Wu H, Wen P. Effects of enzymatic hydrolysis on the structural, rheological, and functional properties of mulberry leaf polysaccharide. Food Chemistry. 355:129608 (2021) [DOI] [PubMed] [Google Scholar]
  12. Karimi E, Mehrabanjoubani P, Keshavarzian M, Oskoueian E, Jaafar HZ, Abdolzadeh A. Identification and quantification of phenolic and flavonoid components in straw and seed husk of some rice varieties (Oryza sativa L.) and their antioxidant properties. Journal of the Science of Food and Agriculture. 94:2324–2330 (2014) [DOI] [PubMed]
  13. Kaur S, Ubeyitogullari A. Extraction of phenolic compounds from rice husk via ethanol-water-modified supercritical carbon dioxide. Heliyon. 9:e14196 (2023) [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Kim D, Han GD. High hydrostatic pressure treatment combined with enzymes increases the extractability and bioactivity of fermented rice bran. Innovative Food Science and Emerging Technologies. 16:191-197 (2012) [Google Scholar]
  15. Lee JW, Nam J, Lim W, Lim TG, Hong GP. Subcritical water mediated hydrolysis of rice husk lignocellulose and upcycling applications of the hydrolysates. Food and Bioprocess Technology. In press (2024) [Google Scholar]
  16. Ma M, Mu T. Modification of deoiled cumin dietary fiber with laccase and cellulase under high hydrostatic pressure. Carbohydrate Polymers. 136:87-94 (2016) [DOI] [PubMed] [Google Scholar]
  17. Nooshkam M, Varidi M, Verma DK. Functional and biological properties of Maillard conjugates and their potential application in medical and food: A review. Food Research International. 131:109003 (2020) [DOI] [PubMed] [Google Scholar]
  18. Noreen H, Semmar N, Farman M, McCullagh JS. Measurement of total phenolic content and antioxidant activity of aerial parts of medicinal plant Coronopus didymus. Asian Pacific Journal of Tropical Medicine. 10:792-801 (2017) [DOI] [PubMed] [Google Scholar]
  19. Oh YN, Choi HY, Kim YB, Hong SG, Kim HY. Effect of paprika powder on the antioxidant capacity of emulsion-type sausage. Food Science of Animal Resources. In press (2024) [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Pourali O, Asghari FS, Yoshida H. Production of phenolic compounds from rice bran biomass under subcritical water conditions. Chemical Engineering Journal. 160:259-266 (2010) [Google Scholar]
  21. Prado JM, Lachos-Perez D, Forster-Carneiro T, Rostagno MA. Sub- and supercritical water hydrolysis of agricultural and food industry residues for the production of fermentable sugars: A review. Food and Bioproducts Processing. 98:95-123 (2016) [Google Scholar]
  22. Rennie EA, Scheller HV. Xylan biosynthesis. Current Opinion in Biotechnology. 26:100-107 (2014) [DOI] [PubMed] [Google Scholar]
  23. Ricke SC, Dittoe DK, Richardson KE. Formic acid as an antimicrobial for poultry production: A review. Frontiers in Veterinary Science. 7:563 (2020) [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Son CG, Choi EJ, Ryu WB, Hong GP. Effect of high-pressure incubation on the hydrolysis efficiency and characteristics of strong and floury rice hydrolysates. Korean Journal of Food Science and Technology. 55:378-383 (2023) [Google Scholar]
  25. Weiss N, Börjesson J, Pedersen LS, Meyer AS. Enzymatic lignocellulose hydrolysis: Improved cellulase productivity by insoluble solids recycling. Biotechnology for Biofuels. 6:5 (2013) [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Zhang Y, Li T, Shen Y, Wang L, Zhang H, Qian H, Qi X. Extrusion followed by ultrasound as a chemical-free pretreatment method to enhance enzymatic hydrolysis of rice hull for fermentable sugars production. Industrial Crops and Products. 149:112356 (2020) [Google Scholar]
  27. Zoghlami A, Paës G. Lignocellulosic biomass: understanding recalcitrance and predicting hydrolysis. Frontiers in Chemistry. 7:874 (2019) [DOI] [PMC free article] [PubMed] [Google Scholar]

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