Sequential subcritical water process applied to orange peel for the recovery flavanones and sugars

https://doi.org/10.1016/j.supflu.2020.104789Get rights and content

Highlights

  • Subcritical water process to obtain flavanones and sugars from orange peel.

  • The maximum flavanone yield (24.4 mg/g) was observed at a flow rate of 10 mLmin–1.

  • In hydrolysis step the main products were glucose, xylose, arabinose, and fructose.

  • Pectin was extracted during hydrolysis temperatures (200, 225, 250 °C).

Abstract

The economically viable orange peel biorefinery can be based on a single process capable of extraction and hydrolysis. One of the major barriers in the OP biorefinery concept is the lack of an efficient separation technology for the complete removal of valuable components from the biomass. To address this issue, a new two-step hydrothermal process for sequential removal flavanones and sugars was evaluated. In the first stage, flavanones are extracted by subcritical water at 150 °C. The second stage consists of hydrolyzing the residual biomass at temperatures greater than 200 °C. The maximum flavanones yield (24.4 mg/g OP) was observed at a flow rate of 10 mL min–1. In the hydrolysis step, the main products were glucose, xylose, arabinose, and fructose, with small amounts of HMF and furfural. Spectroscopic and thermal analysis provided information on the bulk composition of the residual biomass and pectin extracted during hydrolysis temperatures above 200 °C.

Introduction

A wide variety of biomass residues are generated every year by the agro-food industry. Many of these wastes can be converted to valuable food, chemical, and energy products instead of being discarded. Citrus peels are an especially relevant example of an under-utilized waste resource due to their composition and volume produced every year. In 2017, the world's orange production was around 73 million tons. Brazil alone produced 17 million tons, making it one of the world’s largest orange and orange juice producers [1].

Processing fresh fruit consists of several steps, starting with extraction of juice followed by the extraction of essential oil as the most important [1]. The bagasse (about 50 % of the fruit) contains peel (60–65 %), internal tissues (30–35 %) and seeds (0–10 %) and has high levels of pectin, proteins, soluble sugars, hemicelluloses and cellulose fibers [2,3].

Bagasse is not only carbohydrate and protein constituents, but also has an attractive source of bioactive compounds, including phenolic-based flavonoid compounds. Flavonoid concentrations in citrus fruits are greater in the peel and seeds than the pulp, meaning that more compounds will be retained in the peel than the juice. Flavonoids have received special attention because of their reported benefits to human health, including prevention and treatment of cancer, cardiovascular diseases, among others associated to anti-inflammatory and antioxidant effects [4,5]. Two flavonoids are found in high concentration in OP: hesperidin and narirutin [6,7]. As much as 1.02 × 105 tons of hesperidin can be obtained from Brazil’s annual Orange production, representing value of USD 2704.5 trillion [8]. Flavonoids are thermally labile [7] and their recovery requires mid-polar solvents, such as ethanol or methanol [3,6]. Use of these solvents contributes to waste and inefficient processes, since the solvents are only partially recovered and recovery comes at the cost of increased process energy consumption.

On the other hand, soluble and insoluble carbohydrate content of OP make it a potential feedstock that can be used for obtaining fermentable sugars with potential for production of second generation bioethanol or other chemicals such as pectin used in the food industry. OP contains less lignin than other biomass types, which makes it a promising target for enzymatic hydrolysis and fermentation [9,10]. To maximize the efficiency of the enzymatic step, a pretreatment stage is required to open the lignocellulose matrix and remove hemicellulose. Different pretreatments, such as steam explosion [11], acid pretreatment [12], and high voltage electrical discharges pretreatment [9], have been studied for the production of biofuels from OP. However, these pretreatments can be costly and energy intensive and have negative environmental impacts. “Green technologies” and innovative techniques, such as subcritical water, can be used to reduce the use of hazardous solvents, contribute to environmental preservation, and decrease the energy consumption in these processes involved in the reuse of the OP.

Water exists in a subcritical state at temperatures between its boiling point and critical point (100 °C at 1 bar and 374.1 °C at 221 bar), at sufficient pressure to maintain the liquid state [13,14]. Under those conditions, the physicochemical properties of water, in particular its dielectric constant, could vary considerably with increasing temperature [15,16]. As a result, water shifts from being a polar solvent to a non-polar one as temperature increases. This makes subcritical water a potential multi-purpose extraction/hydrolysis solvent. When subcritical water extraction is performed at temperatures ≤150 °C, it acts as mid-polar solvent, capable of extracting phenolic compounds [17,18]. At temperatures ≥150 °C, water increasingly acts as a mid-polar solvent that is also capable of hydrolyzing hemicellulose and cellulose to yield simple sugars and sugar oligomers [19,20].

Based on these considerations, subcritical water has promise for sequential extraction of valuable flavonoids followed by hydrolytic reaction of hemicellulose and cellulose sugars. The challenge is adjusting the treatment temperature to balance the solvent properties of water, in order to extract flavonoids, free sugars and degradation rates biopolymer hydrolysis. Accordingly, this work describes a process for tuning the hydrothermal reaction temperature to satisfy these requirements. OP was subjected to flow-through subcritical water treatments over a range of temperatures from 150 °C to 250 °C for the extraction of flavanones and hydrolysis of complex carbohydrates. The water flow rate was varied from 10 to 30 mL min–1 to examine the effects of thermodynamics, mass transport, and residence time on yields and product degradation. The results obtained in this study demonstrate the potential of an OP biorefinery based on subcritical water extraction and hydrolysis to produce a range of valuable products [21,22].

Section snippets

Raw material

OP was supplied by CPKelco located in Limeira, SP, Brazil. The OP was received dry and was ground in a knife mill (Marconi, model MA 340, Brazil) coupled to an induction motor operating at 3800 rpm. The size reduced OP was separated by sieving using grandest sieves with a mesh of 16, 24, 32, 48, 80 and 100 Tyler series. The final mean diameter was 303 μm.

Orange peel characterization

Table 1 provides composition data describing the main constituents of the OP used in this study. Elemental analysis indicates that OP consists mainly of carbon (42 wt%) and oxygen (51 wt%), along with hydrogen (6 wt%) and nitrogen (1 wt%). Sulfur was present at concentrations less than instrumental detection limits (<1 wt%). The moisture content was 8.71 ± 0.03 wt%, sufficient to impact negatively the energy balance of dry thermochemical valorization using technologies such as incineration,

Conclusions

The sequential hydrothermal process to obtain of high added value products as flavanones and sugars were studied in an OP biorefinery concept. The highest yields of hesperidin and narirutin (22.9 ± 0.7 and 1.9 ± 0.2 mg/g OP respectively) were obtained at 150 °C and 10 mL min–1. Extraction times and S/F ratios need to be optimized during the dynamic process. Optimal sugars yields were observed at 200 °C, reaching values of 7.14 ± 0.95 % and 13.44 ± 1.00 % of arabinose and glucose, respectively,

Declaration of Competing Interest

The authors declare that there are no conflicts of interest.

Acknowledgements

The authors acknowledge financial support from the São Paulo Research Foundation – FAPESP (2011/19817-1, 2018/05999-0, 2018/14839-4, 2016/04602-3 and 2018/14582-5). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior - CAPES- Finance code 001. MAR is thankful to Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq for the productivity grant (303568/2016-0). The WPI contribution was supported by the U.S. NSF (CBET 1554283).

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