Integral valorization of fruit waste from wine and cider industries

Journal of Cleaner Production 242 (2020) 118486

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Integral valorization of fruit waste from wine and cider industries
Soria a, Rosa Rodriguez b, Daniela Salvatori a, Paula Sette a, Anabel Fernandez b, Jose a ,*
n Mazza Germa a b


n y Desarrollo en Ingeniería de Procesos, Biotecnología y Energías Alternativas, PROBIEN (CONICET-UNCo), Neuqu
Instituto de Investigacio en, Argentina Instituto de Ingeniería Química - Facultad de Ingeniería (UNSJ) - Grupo Vinculado al PROBIEN (CONICET-UNCo), San Juan, Argentina

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 November 2018 Received in revised form 22 August 2019 Accepted 17 September 2019 Available online 18 September 2019

Agro-industrial waste disposal is becoming a key issue as a consequence of the increasing number of byproducts generated. Within this context, this work proposes an integral, effective and sustainable use of biowaste from regional food industries as a drive towards cleaner production. Apple and grape waste from cider and wine industries was valorized by extracting bioactive compounds and pyrolyzing/gasifying the residual solid to obtain value-added chemical products, such as biochar. The extraction conditions were as follows: a solvent/fruit ratio of 2/1 (w/w), an extraction temperature of 348 K and an extraction time of 1.25 h. The extracts were analysed for soluble solids, sugars, acids, total polyphenol content, antiradical activity and superficial colour. The grape stalk extract presented the highest polyphenol content (1,758 ± 24 g gallic acid/100 kg) and antioxidant capacity (156 ± 6 g gallic acid/100 kg). The liquid fractions can be used to develop functional ingredients. After the extraction process, the remaining solid fraction was subjected to pyrolysis or gasification at low heating rates (10, 15 and 20 K/ min). The resulting biochars presented low to moderate surface areas (0.9e266 m2/g) and low H/C ratios (high aromaticity). The obtained biochar can be used for the production of activated carbon to be used in fuel applications and as soil structure reinforcement. The proposed methods allowed for the optimization of waste reclamation with two applications: the development of ingredients for the nutraceutical and food industries and the production of biochar and renewable energy. © 2019 Elsevier Ltd. All rights reserved.

Handling Editor: Kathleen Aviso Keywords: Agro-industrial waste Bioactive compounds recovery Pyrolysis Gasification Biochar

1. Introduction Increasing concern for environmental pollution related to high amounts of industrial waste has been noted in recent years. In addition, the final disposal of this waste has become increasingly expensive. In particular, in the Northern Patagonia region of Argentina, fruit production (mainly apples, pears and grapes) is one of the most developed activities. Fruits that do not meet the requirements for the fresh market are destined to industries generating approximately 360 kt/y of organic waste, which represents approximately 60% of total processed fruit. The waste discarded in juice, cider, wine and sparkling wine production is an important ~ eiro source of phenolic compounds with antioxidant capacity (Din García et al., 2009), inhibitors of digestive enzymes and thickeners (e.g., pectin, dietary fibre and other antioxidant compounds) ~ ellas, 2007). Considering the environmental (Llobera and Can

* Corresponding author. E-mail address: [email protected] (G. Mazza). https://doi.org/10.1016/j.jclepro.2019.118486 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

hazards related to the accumulation of solid residues with high organic content, it is appealing to consider the waste of pears, apples and grapes as potential substrates for obtaining products with high added value. Bioactive compounds can be recovered from waste by using adequate extraction methods. Operational conditions of the extraction, such as the type of solvent and process duration and temperature, determine the performance and quality of the compounds obtained (Pinta
c et al., 2018). New technologies have been explored for more efficient extraction, such as the use of ultrasound, microwaves, and pulsed electric fields, which are commonly expensive, difficult to scale up, or energy intensive (Okolie et al., 2019) taking into account both the economic and environmental cost-benefit. Hydrothermal treatment appears as an eco-friendly technology with several advantages, such as the absence of organic solvents and related corrosion problems, and the fact that it is easy to operate and cost effective (Sepúlveda et al., 2018). Moreover, some studies reported higher bioactive content after hot-water extractions compared to solvent extraction (Kabir et al., 2015).

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Most works concerning the extraction of bioactive compounds from agri-food residues focus on the extraction process and discard the remaining solid. This by-product, rich in lignocellulosic compounds, can be used for obtaining fuels and energy using thermochemical treatments, such as pyrolysis and gasification (Di Blasi, 2009). Biowaste can be considered a neutral CO2 source, with low nitrogen and sulfur contents, resulting in lower sulfur and nitrogen oxide emissions (Biagini et al., 2008). Pyrolysis and gasification of biomass produce three main products: biochar, liquid (tar and water) and non-condensable gases with yields that depend on the operating conditions and feedstock properties. Syngas and tar have a high LHV (lower heating value) and can be used for energy recovery. Tar can also be further upgraded to renewable transportation fuels. Biochar is a low-value product that has received increasing attention due to a wide range of potential applications. It can be used as fuel, precursor of activated carbons, or as a fertilizer replacement, offering an advanced option for biological sequestration of carbon (Qian et al., 2015). The interest of the production of biochar by gasification is still relatively novel, but recent reported results are promising (Fryda and Visser, 2015), particularly since it can be used to both improve soil characteristics and catalyse hydrogen production. Therefore, the use of biochar in these valuable applications could provide economic benefits to biorefinery plants, including those that use gasification or pyrolysis processes. The biorefinery approach contributes to a cleaner production, which is related to the concept of a circular economy (Sousa-Zomer et al., 2018). Considering that it promotes the systems where services and products are traded in cycles (closed loops), the 3R approach of “reduce, recycle and reuse” is the guiding concept. In this way, this work proposes an integral, effective and sustainable use of biowaste from wine and cider industries by applying: 1) an environmentally friendly extraction to obtain antioxidant-rich ingredients and 2) a slow pyrolysis and gasification of waste to produce chemicals and fuels, particularly biochar generation. This twostep sequence leads to an almost-zero-waste condition based on sustainable techniques. 2. Materials and methods A combination of a selection of techniques and processes for the integral valorization of fruit waste is comprehensively presented. Experimental set-up and analytical and statistical methods are described throughout the following subsections. 2.1. Materials Fruit waste was collected from industries located in the Río Negro province (Argentine Patagonia). Humberto Canale winery located in the city of General Roca provided grape waste (var. Sauvignon Blanc), and La Flor S.A. cider industry from Cipolletti City provided green apple waste (var. Granny Smith). Three kinds of waste were considered: green apple pomace (AP), grape marc waste (GM) and grape stalk waste (GS) (Fig. 1). After collection, the biowaste samples were immediately frozen and stored at 251 K until use. 2.2. Waste characterization The three biowaste samples (AP, GM and GS) were characterized according to the Official Methods of Analysis of the Association of Official Agricultural Chemists (AOAC, 2000): moisture (925.09), soluble solids (932.12) (Pal e Pocket, ATAGO CO. LTD, Tokyo, Japan), acidity (945.26), and pH (945.27). Water activity (aw) was determined using an electronic dew point water activity metre (Aqualab

Series 3 TE, Washington, USA). The methods used for total phenolic content and antiradical activity determinations are described below in Section 2.5. 2.3. Extraction process of bioactive compounds An aqueous extraction process (Fig. 1a) was performed on the waste samples, following the methodology reported by Garrido Makinistian et al. (2019) with the optimal temperature-time condition defined by these authors. After the extraction process, two very different by-products were obtained: 1) liquid extracts containing bioactive compounds and 2) a solid fraction, rich in lignocellulosic material. Both products were characterized to predict their usefulness. 2.4. Processing of the solid fraction The residual solid fraction (RSF) obtained after the extraction process (Fig. 1a) was treated following the steps presented in Fig. 1b: samples were dried under forced air convection at 333 K (z10% relative humidity and air speed ¼ 1.5 m/s) using an air convection oven model Venticell 111- Standard (MMM Medcenter Einrichtungen GMBH, Munich, Germany). The three sets obtained after drying (AP, GM, GS) were ground and passed through a sieve (sieve ASTM-USA, mesh 30). Particulate samples with a 500 mm size were used for the thermochemical treatments. 2.5. Chemical and functional properties of raw waste and aqueous extracts Chemical determinations were performed on methanolic extracts, which were prepared as follows: 0.001 kg of sample was homogenized in 25 mL of absolute methanol and constantly mixed using a magnetic stirrer for 1200 s prior to filtration. Extracts were prepared by triplicate. A T60 UVeVisible spectrophotometer (PG Instruments, Leicestershire, United Kingdom) was used for analytical assessments. 2.5.1. Total phenolic content (TPC) TPC was determined using the FolineCiocalteu reagent according to Sette et al. (2017). A calibration curve was generated using gallic acid as the standard. The results were expressed as gallic acid equivalents (GAE) per 100 kg sample (waste or extract) on a dry basis (d.b.). 2.5.2. Antiradical activity (AA) AA was evaluated using the bleaching method of the radical 1,1diphenyl-2-picrylhydrazyl (DPPH$) according to Sette et al. (2017). A calibration curve was generated using gallic acid as the standard. The results were expressed as GAE per 100 kg sample (waste or extract) on a dry basis (d.b.). 2.5.3. Sugars and acids content Glucose, fructose, sucrose and organic acids (citric, malic, tartaric, succinic, lactic, ascorbic and acetic) were assessed in the three waste extracts. Twenty microliters of extract was injected in an Agilent 1260 HPLC (Agilent Technologies, USA) equipped with an automatic injector (ALS) and two detectors: a refractive index detector (RID, sugar detection at 328.15 K) and a diode array detector (DAD, organic acid detection). Separation was achieved using a Hiplex H column (300  7.7 mm, 8 mm particle size, Agilent Technologies, USA) at 348.15 K, and chromatograms were recorded at 214 nm. The mobile phase was composed of 0.001 M H2SO4 (isocratic) and the flowrate was 0.4 mL/min. Standard compounds were used to build calibration curves in aqueous solutions with high

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Fig. 1. Processing of the fruit waste.

linearity for all standards (r2 > 0.999): D-(þ)-glucose (Sigma Aldrich), D-()-fructose (Sigma Aldrich), sucrose (Merck), citric acid (Cicarelli), L-(þ)- tartaric acid (RP ACS), L-malic acid (Sigma Aldrich), succinic acid (Merck), L-lactic acid (Megazyme), L-ascorbic acid (Sigma Aldrich), acetic acid (Cicarelli) and glycerol (Sigma Aldrich). 2.6. Superficial colour of the waste extracts Superficial colour was determined by photocolorimetry using a handheld colorimeter (Minolta Co, model CR400, Japan). Colour functions were calculated for illuminant C at a 2 standard observer and in the CIELAB uniform colour space. Measurements were performed using glass vials containing enough powder to reach a height of 1 cm, and ten replicates were used. A white cylindrical cup was used to cover the vial and standardize the measurements. L*, a* and b* values were obtained. The L* value represents colour lightness (0 ¼ black and 100 ¼ white), the a* scale indicates the chromaticity axis from green () to red (þ) and the b* axis ranges from blue () to yellow (þ).

2.7.1. Lignin, cellulose and hemicellulose contents The lignin, cellulose and hemicellulose contents of the different RSFs were determined according to the American Society for Testing and Materials standards (ASTM D1106-56, ASTM D1103-60 and ASTM D1103-60). 2.7.2. Density The absolute density was calculated according to the ISO 135032 (2016) standard using an Accupyc 1330 pycnometer (Micromeritics Instruments, Norcross, GA, USA). 2.7.3. Particle morphology The microstructural characteristics of the dry powders were analysed by scanning electron microscopy (SEM) using a Zeiss microscope Supra 40 (Carl Zeiss, Oberkochen, Germany). The samples were placed in an aluminium support using conductive carbon double-sided adhesive tape, and then coated with gold nanoparticles using a sputter coater (Cressington Scientific Instruments 108). Images were taken with the detector within the lens, using an acceleration voltage of 3.00 kV. 2.8. Macro-TGA of RSF

2.7. Chemical and functional properties of residual solid fraction (RSF) Lignin, cellulose and hemicellulose contents, density and particle morphology of RSFs were evaluated prior to thermal treatments.

The macro-TGA experiments were carried out in a 5 cm ID and 100 cm height stainless steel reactor as described by Fernandez et al. (2019). Three heating ramps, 10, 15 and 20 K/min, up to final temperature of 1,173 K, were assayed in this study. A mass of 5 g RSF with particle sizes ranging from 0.212 to 0.250 mm was used. In the

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pyrolysis experiments, the flow rate of nitrogen was set at 100 mL/ min. Gasification experiments were carried out under an air/steam atmosphere. The steam flow rate was 0.17 mL/min, setting the steam/biomass ratio at 2.5 (Fernandez et al., 2019) and the equivalent ratio (ER) at 0.1. 2.9. Biochar characterization The elemental analysis was accomplished in a Carlo Erba EA 1108 elemental analyser (Carlo Erba Instruments, Milan, Italy), according to ASTM D5373. The carbon, hydrogen, nitrogen and sulfur contents were directly determined, while oxygen was calculated by difference. The ASTM standards (D1102e84 ASTM, 2001; E872 - 82 ASTM, 1998) were used to determine the proximate analysis. The specific surface area was calculated using a multilayer BET adsorption model with N2 at 77 K (Nova 2200, Quantachrome Instruments, Florida, USA). The biochar characterization also included the evaluation of typical properties such as recalcitrance potential (R50), carbon sequestration potential (CS), persistence in soil (in terms of the mean residence time, MRT), aromaticity factor (f a Þ, stable C mass fraction (SCF) and the high heating value (HHV). The calculation of these parameters is described in Section SI-1 of the Supporting Information, Appendix A. 2.10. Assessment of the integral valorization yield (hI) Aiming to assess the yield of the integral valorization, partial yields on a dry basis for the extraction process (hE) and for the thermochemical process (hT,G for gasification and hT,P for pyrolysis) were calculated, as well as the yield of the polyphenol compounds present in the extracts (hTPC,E). Yields were defined as weight proportions with respect to the initial waste in all cases. Then, the integral yields for pyrolysis (hI,P) and gasification (hI,G), were calculated as follows:

hI;P ¼ hE þ hT;P hI;G ¼ hE þ hT;G

(1) (2)

where, hE ¼ mass of extracts (d.b.)/initial waste mass (d.b.),

hT,P ¼ mass of char issued from pyrolysis (d.b.)/initial waste mass (d.b.) and hT,G ¼ mass of char issued from gasification (d.b.)/initial waste mass (d.b.)

Table 1 Physicochemical and functional properties of the three types of fruit waste: grape marc (GM), grape stalks (GS) and apple pomace (AP). Properties

GM

GS

AP

aw Water Content (% w.b.) pH Acidity (%, d.b.) TPC (g GAE/100 kg d.b.) AA (g GAE/100 kg d.b.)

0.957 ± 0.007a 61.6 ± 1.2b 4.62 ± 0.12b 3.2 ± 0.4a 1,140 ± 23b 269 ± 12a

0.962 ± 0.003a 59.2 ± 0.3a 4.23 ± 0.12a 3.3 ± 0.3a 1,537 ± 31c 971 ± 60b

0.976 ± 0.004b 85.2 ± 0.6c 4.3 ± 0.7a 3.9 ± 0.2a 650 ± 35a 245 ± 33a

Means within columns with a different lowercase superscript letter are significantly different (p < 0.05). The values are expressed as the mean ± standard deviation (n ¼ 3). (d.b. ¼ dry basis; w.b. ¼ wet basis; aw ¼ water activity).

Table 2 Physicochemical properties of waste extracts obtained from apple pomace (AP), grape marc (GM) and grape stalks (GS). Properties

GM

GS

AP

aw pH Density (g/mL) Brix (%) Acidity (%) Glucose (g/kg) Fructose (g/kg) Sucrose (g/kg) Tartaric acid (g/kg) Malic acid (g/kg)

0.985 ± 0.002a 4.14 ± 0.02b 1.024 ± 0.004b 5.33 ± 0.06b 0.23 ± 0.02b 15.308 ± 0.048c 15.137 ± 0.039b nd 1.754 ± 0.032a 1.14 ± 0.06b

0.987 ± 0.001a 4.12 ± 0.02b 1.021 ± 0.003b 7.073 ± 0.064c 0.34 ± 0.02c 8.251 ± 0.072b 9.063 ± 0.042a nd 1.834 ± 0.044b 0.865 ± 0.032a

0.987 ± 0.001a 3.48 ± 0.02a 1.016 ± 0.002a 4.27 ± 0.12a 0.17 ± 0.02a 6.519 ± 0.272a 14.165 ± 0.199b 1.736 ± 0.057 nd 1.313 ± 0.019c

Means within columns with a different lowercase superscript letter are significantly different (p < 0.05). The values are expressed as the mean ± standard deviation (n ¼ 3). nd: not detected.aw ¼ water activity.

Table 3 Characterization of the RSFs from apple pomace (AP), grape marc (GM) and grape stalks (GS). Properties

GM

GS

AP

aw Water content (%) Neutral detergent fibre (%) Acid detergent fibre (%) Lignin (%) Hemicellulose (%) Cellulose (%) Density (g/cm3)

0.222 ± 0.002a 3.346 ± 0.005a 41.7 ± 0.7b 36.46 ± 0.18b 29.8 ± 0.8c 5.2 ± 0.7a 6.6 ± 0.5a 1.4 ± 0.2a

0.214 ± 0.003a 4.93 ± 0.05c 43.2 ± 0.5b 37.1 ± 0.3b 24.9 ± 0.6b 6.1 ± 0.5b 12.2 ± 0.4c 1.41 ± 0.03a

0.287 ± 0.001b 4.34 ± 0.02b 38.9 ± 1.4a 28.39 ± 0.6a 18.1 ± 0.5a 10.6 ± 0.8c 10.3 ± 0.4b 1.47 ± 0.12b

Means within columns with a different lowercase superscript letter are significantly different (p < 0.05). The values are expressed as the mean ± standard deviation (n ¼ 3). (aw ¼ water activity).

2.11. Statistical analysis With the exception of superficial colour, the determinations involved three replicates (n ¼ 3), and the results were expressed in terms of the mean and standard deviation of the mean (SD). Analysis of variance (ANOVA) was accomplished to establish the presence or absence of significant differences. Multiple comparisons were performed using the Tukey test to establish significant differences between means at a significance level set at p < 0.05 (letters used as superscripts in Tables 1e3). All statistical analyses were carried out using the data analysis software system STATISTICA 8.0 (StatSoft, Inc., Tulsa, OK, USA). 3. Results and discussion This section is divided into three parts to present the results obtained: 1) physicochemical characterization of the as-received

residues samples, 2) analysis of the extraction process stage, and 3) analysis of the thermochemical treatments of the remaining solid waste.

3.1. Waste characterization Table 1 shows properties that are relevant to usage of extracts from wastes, such as polyphenol content and antioxidant capacity, as well as other parameters often used to characterize materials from vegetable sources including water content, water activity and acidity, among others. Regarding the bioactive compounds, the GS stands out from the rest of the biowaste samples, since significantly higher values of polyphenol and antioxidant capacity were recorded for this sample. These results are in accordance with those obtained for the

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polyphenol content of stalks (Alonso et al., 2002) and pomace ~ ellas, 2007) from the winemaking process. The TPC (Llobera and Can values obtained in the AP samples fit the data values reported by ~ eiro García et al. (2009). The AP sample also had a high huDin midity, similar to that of the parenchymatic tissue of fresh apples (var. Granny Smith) (Salvatori et al., 1998).

3.2. Extraction of bioactive compounds Cebulj et al. (2017) observed that the apple skins of different varieties presented higher contents of sugars, organic acids and phenolic compounds compared to flesh. Peixoto et al. (2018) also observed in grapes that seed, skin and pomace contained the highest phenolic content. During industrial pressing, only a fraction of these compounds will transfer into the juice, while the rest will be retained in the biowaste (Oszmianski et al., 2011). Characterization in terms of sugar content and organic acids (Table 2) showed that sugar-rich extracts, containing mainly fructose and glucose and with a considerable amount of acids, were obtained. As expected, malic acid was detected in all cases, and tartaric acid was present in the extracts from the grape waste samples. In GS extracts, the presence of a certain amount of sugars is due to the impregnation of the material by grape juice during the destemming operation (Spigno et al., 2013) (Table 2). GM extract presented the highest content of glucose and fructose because this fruit is richer in sugars and because the marc from white wine making processes did not undergo fermentation preserving residual sugars. Although both grape and apple fruits are composed of sucrose, it was only detected in AP extracts, probably as a consequence of hydrolysis during processing. Samples containing the highest phenolic content, such as GS extracts (Fig. 2a), also showed the highest antiradical activity (Fig. 2b). AP extract exhibited the lowest antiradical activity (AA)

Fig. 2. Total phenolic content (TPC) (a) and Antiradical Activity (AA) (b) of the extracts obtained from the different fruit waste. Apple pomace (AP), grape marc (GM) and grape stalks (GS). (w.b. ¼ wet basis).

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value (24.2 ± 4 g GAE/100 kg) among all the samples, in line with this extract exhibiting very low TPC content (6.5 ± 0.7 g GAE/ 100 kg). The absence of a clear correlation between the phenolic content of GM and the AA value suggests that factors other than polyphenol compounds are involved in the antioxidant potency in terms of the ability to reduce free radicals. Indeed, the degradation products of phenolic compounds can also have AA that is sometimes higher than that of the initial phenolic compounds (Murakami et al., 2004). Additionally, polyphenols have been found to be synergists; that is, the antioxidant effect is enhanced when they are combined (Phan et al., 2017). Regarding the significant level of phenolic content and AA detected in extracts from grape stalks, some studies have demonstrated that leaves and stems of grapevine represent an ideal biomass feedstock to obtain phenolic compounds (Eftekhari et al., 2017). Grape extracts presented a particularly high polyphenol level and AA (Fig. 2) compared to the corresponding initial biowaste (Table 1); only the AP extract exhibited a lower TPC value. The observed increase could be due to the release of phenolic and antioxidant compounds as a consequence of the operating conditions of the extraction process. All soluble phenolic compounds are accumulated in the cell vacuole. Free compounds are easy to extract, but when phenolic acids and several other components exist as insoluble bound complexes, they are not as easy to extract, since they are coupled to cell wall polymers through ester and glycosidic links. Therefore, the total polyphenol content determined in waste samples would not account for the phenolic acids bound to the cell wall of the fruit tissue. However, when the extraction process is applied, the integrity of cell walls can be affected and make the phenolic compounds in waste fruits more accessible for extraction. This phenomenon could explain the higher polyphenol amount observed in the extracts compared to that in both grape waste samples. From a nutritional standpoint, it can be concluded that compared with commercial fruit juice as a polyphenol source, the intake through biowaste extracts would be similar, if not higher. The consumption of commercial apple juice provides approximately 15e60 g of polyphenols/100 kg (Ignat et al., 2011), while commercial grape juice provides between 111.2 and 343.3 g TPC/ 100 kg of juice (Yuan and Baduge, 2018). The GM and GS extracts obtained in this work had a TPC higher than 250 g/100 kg extract (Fig. 2), indicating the high potential of these products as a source of natural phenolic compounds. For applications in foods such as beverages, jelly or concentrates, it is also important to consider the colour of the extracts evaluated through the chromatic parameters (Fig. 3). Positive a* and b* parameters were obtained in all cases, with the extracts generated from AP having the highest b* value and the lowest a* value, consistent with the more yellowish tone also illustrated in Fig. 3c. Regarding luminosity, these samples were also the lightest. These chromatic characteristics make it possible to use AP extracts, for example, for the development of light preparations. Grape extracts exhibited darker colours ( a*), thus being suitable for polyphenol enrichment of dark beverages or other foods without changing the colour to a great extent. Considering the presence of natural carbohydrates and organic acids (Table 2), the extracts could also be considered for the preparation of natural syrups with antioxidant potential. For example, for their application in nutraceutical tablets or medicinal capsules, the extracts must be purified and dried and further combined with other compounds according to the requirements for these products. If a powdered product is developed for use in composite food development, such as cereal, fruit bars or candies with antioxidants, no purification of the extracts would be needed prior to the drying process.

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Fig. 3. CIELAB coordinates b* versus a* (a) and lightness (L*) (b) of AP extract (:), GM extract (-) and GS extract (C) (c) Images of the obtained extracts from grape marc (GM), grape stalks (GS) and apple pomace (AP).

3.3. Use of RSF for pyrolysis and gasification The chemical composition of the three RSFs of the present study is an important topic to be considered when the thermal treatment is to be selected, but it also determines the type of fuel or energy by-product that will be primarily generated. From Table 3, it can be observed that the highest hemicellulose and lignin contents were present in AP and GM, respectively, while GS had the highest cellulose content (12.19%). The contents of cellulose, hemicellulose and lignin of AP, GS and GM were similar to those reported by Saffe et al. (2019). A preliminary qualitative observation of the particle size and shape, as well as surface features from micrographs obtained by SEM (Fig. 4), allows comparing the three RSF samples to anticipate the potential applicability of these solids. In Fig. 4a, it can be observed that the solid AP presented a more homogeneous particle size. The particles were smaller than those of GM and GS (Fig. 4c and e), which exhibited more irregular structures. Thus, the apple pomace RSF looked adequate for mixing dry ingredients or for thermochemical treatments in fluidized bed units. 3.3.1. Pyrolysis Fig. 5 shows the weight loss and the weight loss rate (DTG) curves obtained during the pyrolysis process. Thermal decomposition occurred in three stages. During the first step, the average weight loss was approximately 12% for GM (Figs. 5a), 10% for GS (Figs. 5b) and 14% for AP (Fig. 5c). It is related to water evaporation and free sugar decomposition or volatilization, taking place in the temperatures range of 300e520 K for GM, and up to 540 and 560 K for GS and AP, respectively (Nolasco and Massaguer, 2006). The greatest weight loss was observed in the second stage achieving 37% for GM, 42% for GS and 54% for AP. It is most likely due to the degradation of some low-molecular-weight components, such as hemicellulose and cellulose (Volker and Rieckman, 2009) and it occurred in the range 560e750 K for GM, 540e830 K for GS and 520e800 K for AP. The AP exhibited the highest mass loss, which could be a consequence of the higher fraction of cellulose and hemicellulose. The weight loss and corresponding temperature ranges for the third stage were 27% (750e1,173 K) for GM, 33%

(830e1,173 K) for GS and 26% (800e1,173 K) for AP. It can be attributed to the decomposition of substances with more complex structures, such as lignin. Lignin decomposition can also take place throughout a wider range of temperatures, as reported by Yang et al. (2007). The additional peaks observed for GS (Fig. 5b) at approximately 998 K could indicate triglyceride decomposition, as stated by Casazza et al. (2016). The highest total mass loss (94%) during pyrolysis corresponded to AP (Fig. 6a). Such a behaviour can be explained by the higher contents of cellulose and hemicellulose and the lower lignin fraction compared with GM and GS (Table 3) as reported by Wang et al. (2018). When performing TGA experiments on cellulose, hemicellulose and lignin samples, the authors found that the highest weight loss occurred for cellulose. Additionally, the heating rate had a significant influence on the biochar yield, although no defined trend was observed (Fig. 6a), which is in agreement with the observations reported by Zhao et al. (2018). A comparison with unextracted GM and GS samples (Fernandez et al., 2016) was performed. The authors found that the maximum weight loss at 10 and 15 K/min occurred at lower temperatures (500e600 K) when compared to the results obtained in this work. A lower weight loss was observed during the first stage of pyrolysis for the RSFs from GM and GS, which may be due to the removal of compounds other than polyphenols, such as a certain fraction polysaccharides in the waste matrix (Casazza et al., 2016). Moreover, the biochar yields obtained for the RSF samples were higher than those obtained from the as-received samples (Casazza et al., 2016). 3.3.2. Gasification Three stages were also observed for gasification (Fig. 7). The first stage, also related to water evaporation and free sugar volatilization, produced a 14% weight loss for GM (Figs. 7a), 13% for GS (Figs. 7b) and 14% for AP (Fig. 7c). It occurred between 300 and 620 K (GM), 300 and 630 K (GS) and 300 and 580 K (AP). Greater weight loss (40% for GM, 40% for GS, and 60% for AP) took place in the second stage between 610 and 900 K for GM, 590 and 830 K for GS and 520 and 770 K for AP. The last weight loss observed was related to the decomposition of more complex compounds and occurred between 800 and 1,173 K for GM, 700 and 1,173 K for GS

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Fig. 4. Scanning electron microscope micrographs of the solid waste powders: (a, b) AP; (c, d) GM; (e, f) GS.

and 720 and 1,173 K for AP, with weight loss values of 22%, 17% and 24% for GM, GS and AP, respectively. Comparing the pyrolysis and gasification results, more significant weight loss took place during the latter process. The second stage of gasification began at higher temperatures than those for pyrolysis, probably due to the highly complex chemical reaction mechanisms exhibited in gasification (Jarosz and Małecki, 2014). Even if the reaction of char with steam commonly produces higher weight loss than that corresponding to the third stage of pyrolysis, the mass loss observed in our experiments was quite similar for both pyrolysis and gasification. This may be caused by the pretreatment step (extraction). The biochar yields did not seem to follow the same trend, when increasing heating rate, as that of pyrolysis (Fig. 6b). The lignincellulose-hemicellulose interactions, along with the influence of

the steam/air atmosphere, may produce complex chemical reaction mechanisms, which impact in biochar yield. 3.4. Biochar characterization The composition of biochar produced at different heating rates during both pyrolysis and gasification processes of the three residual solid fractions is presented in Table S1 (Appendix A, in the Supporting Information). The three biochars exhibited different behaviours, illustrating the heterogeneous nature of the material. It can be noted that char from AP contained the highest carbon content (>74 wt%) as well as hydrogen content, followed by GM, while GS displayed the lowest content of carbon (<65 wt%). In contrast, biochar from GS presented the highest oxygen content, regardless of the heating rate and thermochemical process. The

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Fig. 5. Weight loss and derivatives curves for (a) grape marc, (b) grape stalk and (c) apple pomace at different heating rates (pyrolysis).

Fig. 6. Residual biochar (%) after the pyrolysis (a) and gasification (b) for apple pomace (AP), grape marc (GM) and grape stalk (GS) at different heating rates.

P. Sette et al. / Journal of Cleaner Production 242 (2020) 118486

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Fig. 7. Weight loss and derivatives curves for (a) grape marc, (b) grape stalk and (c) apple pomace at different heating rates (gasification).

nitrogen for the three biochar samples was in the range of 1e1.95%, which is similar to chars obtained from vegetables. Moreover, sulfur was not detected since it was eliminated during pyrolysis. On this basis, the biochar could be suitable as a fuel and for the production of activated carbons. The results of Table S1 indicate that an increase in the heating rate enhanced the carbon content for the three biochars. In contrast, the presence of the oxygen decreased by using higher heating rate. Onay (2007) reported a similar trend for the pyrolysis of safflower seed. The effect of the heating rate on oxygen content
s et al., can be attributed to CO and CO2 generation (Ayala-Corte 2019). Moreover, it was observed that the effect of heating rate on elemental composition was more important in GS than in GM or AP. No significant changes were detected when pyrolyzing or gasifying the AP or the GM. Nevertheless, the pyrolyzed grape stalk biochar had a lower content of carbon than the corresponding gasified RSF, in agreement with observations from Qian et al. (2013).

Based on the theoretical concepts presented by Xiao et al. (2016), the H/C and O/C ratios were calculated to first estimate the degree of aromaticity and stability of the biochar samples. It is found that the three biochar samples presented a low H/C ratio which indicates a high aromaticity (Ronsse et al., 2013). The low values of the H/C ratio derive from the cleavage of weak bonds inside the biochar matrix at high temperatures (Demirbas, 2004). The influence of the heating rate was not significant, and the H/C ratio seemed to be more dependent on the type of RSF than on other factors. In addition, the thermochemical process did not considerably influence the H/C ratio. The effect of the heating rate on the O/C ratio was not significant for the biochar derived from pyrolysis/gasification of AP and GM but it has a remarkable effect on the material derived from GS. Additionally, it is worth noting that the biochar from GM and AP had the lowest O/C ratio, enhancing the stability in comparison to the GS biochar. Table S2 summarizes the main parameters for the characterization of the biochar, as mentioned in Section 2.9. The R50 values of

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P. Sette et al. / Journal of Cleaner Production 242 (2020) 118486

Table 4 Yields of the extraction (hE) and thermochemical (hT,P,hT,G) steps and overall yield (hI,P,hI,G) obtained with apple pomace (AP), grape marc (GM) and grape stalks (GS). Yield

AP

GM

GS

hE (%) hTPC,E(%)

43.60 9.52 0.56 3.96 2.03 5.32 1.85 0.41 0.13 47.56 45.63 48.92 45.45 44.01 43.73

14.40 58.67 0.86 7.78 18.36 25.31 23.79 8.32 26.81 22.18 32.76 39.71 38.19 22.72 41.21

41.00 317.9 0.59 9.90 5.43 14.25 7.41 17.94 23.06 50.90 46.43 55.25 48.41 58.94 64.06

xRSF ¼ wtRSF/wtdry

hT,P (%) hT,G (%) hI,P (%) hI,G (%)

raw waste

10 K/min 15 K/min 20 K/min 10 K/min 15 K/min 20 K/min 10 K/min 15 K/min 20 K/min 10 K/min 15 K/min 20 K/min

the biochars were lower than 0.50, which is similar to uncharred plant biomass. Additionally, there were no significant differences between the R50 values for the biochar from pyrolysis and from gasification. Low values of CS were obtained, regardless of the thermochemical process, although the highest value of CS corresponded to biochar from gasification of the grape marc RSF. Additionally, no significant differences were observed for the MRT values, which were between 1253 and 1701 years. The aromaticity (fa) varied between 0.87 and 0.98 and between 0.74 and 0.88 for biochar from pyrolysis and gasification, respectively. The obtained SCF values ranged between 0.81 and 0.86 for pyrolysis and between 0.77 and 0.85 for gasification. The low H/C ratio and high aromaticity values resulting from the high thermal processes (>500  C), would indicate that these biochars could not be suitable for the removal of inorganic pollutants. On the contrary, the obtained biochars would be adequate for the remediation of organic compounds from water and soil (Oliveira et al., 2017). On the other hand, the high stability of the biochars suggests their potential to reinforce soil structure. Finally, it should be noted that the HHV values of biochar derived from pyrolysis and gasification of the three RSFs were higher than those of biochars generated from unextracted biomass (Fernandez et al., 2016). The influence of the heating rate and RSF type on the biochar surface area (SA) is presented in Fig. S1a and S1b for pyrolysis and gasification, respectively. As the heating rate increased from 10 to 20 K/min, the SA decreased. It is believed that the volatiles are produced faster, which may lead to an intra- and inter-particle accumulation of volatiles, thus increasing the chance of pore blockage by carbon deposits (Angın, 2013). A decrease in SA with heating rate was also reported previously (Mui et al., 2010). The highest surface area was obtained for biochar from the apple pomace RSF for both pyrolysis and gasification processes. However, the biochar SA for the gasified RSF was considerably lower than the value obtained during pyrolysis. The gasification tends to provoke the breakage of the biochar structure and it may cause carbon soot deposition which partially obstructs the voids. Both processes contribute to the decrease in SA. Compared with activated carbons, the biochars exhibited much lower SA, which may limit their use as sorbents for removing contaminants. 3.5. Yield of the integral valorization Although GS and AP wastes presented higher yield values regarding the extraction process (hE) compared to GM waste, the effective ingredient yield in terms of polyphenols was significant in both grape extracts. In particular, the extraction method applied to the GS samples succeeded in recovering polyphenol compounds of

the initial waste (Table 4). The same trend was observed when analysing the overall yields. As the heating rate increased, the overall yield was the highest for GS samples. Thus, GS waste seems to be the most convenient for the integral valorization processes proposed in this work.

4. Conclusions In this work, the combination of an aqueous extraction of phenolic compounds, as a pretreatment, followed by thermochemical processes has been proposed for the valorization of grape and pomace waste from regional industries. It was found that aqueous extracts, especially those obtained from grape stalks usually not considered in extractive processes, presented high polyphenolic content and antioxidant capacity. They even surpass those of commercial fruit juice, suggesting their potential as functional ingredients in foods or for nutraceutical development. The TGA results indicated that the pretreatment caused the maximum weight loss to occur at higher operating temperatures, producing a slight decrease in the solid biochar yield. Based on the low nitrogen and sulfur contents along with the small surface areas of the biochars, they can be used as fuel for energy purposes as well as a raw material for the production of activated carbons. Additionally, the high stability of the biochar enables their consideration as an agent to reinforce soil structure. Finally, the overall yield analysis revealed grape stalk waste as the most promising waste for the valorization approach proposed in this work. This work provides a strategy to achieve a high valorization of fruit residues, promoting the development of combined sustainable processes while minimizing waste disposal. This approach could have a positive economic and environmental impact, contributing to cleaner production in line with the concept of a circular economy. Future work can focus on the use of new green extraction technologies in order to improve the antioxidant potential of the extracts. Additionally, bearing in mind that this work dealt with the biochar production, a further study on operating conditions to upgrade gas and tar fractions is envisaged.

Declarations of interest The authors have no conflicts of interest to report.

P. Sette et al. / Journal of Cleaner Production 242 (2020) 118486

Acknowledgements The authors appreciate the support of the following Argentine institutions: Universidad Nacional del COMAHUE, Argentina (PIN No. 04/I223 and 04/L007); CONICET (National Scientific and Technical Research Council, Argentina) PUE PROBIEN 22920150100067; ANPCyT (National Agency for Scientific and Technological Promotion, Argentina) e MINCyT (PICT No. 2014e2078); and CONICET and SECITI e San Juan (PIOeNo. 15020150100042CO). Anabel Fernandez has a Doctoral Fellowship from CONICET. Paula Sette has a Post-doctoral Fellowship from ANPCyT. Germ
an
Soria are Research Members of Mazza, Daniela Salvatori and Jose CONICET. The authors also thank L. Bajda, M. Amaro and R. C. Maturano (PROBIEN, CONICET-UNCo) for their technical assistance in the HPLC analysis and waste physicochemical characterization. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.118486. References
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