Raw material

The cladodes from N. cochenillifera were manually and randomly harvested in the Valley region of Tulancingo, Hidalgo, Mexico. The cladodes were used at commercial maturity, considering the following morphometric variables: 25 cm length, 6 cm width, and 1 cm thickness.

Sample preparation

Three batch cladodes with spines were selected according to the provisions in the Codex Alimentarius (^{Codex, 2017}). First, spines were removed manually, and cladodes were washed with distilled water and disinfected with a NaClO solution (4.5 mg/L). Next, cladodes were cut into strips (10 cm long, 1 cm wide), then dried at 45 ºC for 48 h in a convection oven (3489M-1, Barnstead International, Dubuque, Iowa, US), ground in a mill (UDY Cyclone Sample Mill), and sieved through a 100-mesh screen (US). Finally, powder samples stored in airtight polyethylene bags at room temperature and darkness until analysis.

Proximal analysis

Moisture (method 934.06), total soluble solids (method 932.12), protein (method 978.04), ashes (method 940.26), and lipids (method 950.54) were quantified using the official methods in AOAC (2012). Total carbohydrates were calculated by difference (equation 1), while energetic value was estimated using the equation (2) proposed by ^{Nabil et al. (2020)}.

Dietary fiber

Dietary fiber (DF) was assessed by the enzymatic-gravimetric method with modifications by ^{Mañas and Saura-Calixto (1995)}. First, samples (0.5 g) mixed with 25 mL of phosphate buffer (0.08 M, pH 6) was heated at 100 °C for 5 min, and then treated with 25 µL of thermostable α-amylase (A-3306, Sigma Chemical Co., St Louis MO, US) at 100 °C/35 min, after that, samples were cooled at ambient temperature. Furthermore, the pH of the solution was adjusted at 7.5 with NaOH (1N); next, 50 µL of protease (Sigma Chemical Co., St Louis MO, US Sigma, P-5380 at 50 mg/mL of phosphate buffer) were added, and samples incubated in a water bath at 60 °C/35 min, and cooled at ambient temperature. The pH was adjusted at 4.5 with HCl (1N), and 150 µL of β-amyloglucosidase (Sigma Chemical Co., St Louis MO, US Sigma, A-9913) were added, and the solution was heated again at 60 °C/35 min. Finally, the solution was centrifuged using a high-speed centrifuge (BKC-TH21RL, Biobase, Shandong China) at 3000 x g, 25 ºC, for 15 min, to separate the soluble and insoluble fractions. The supernatants collected from the centrifugation were dialyzed (D-9652, 33 mm, 12400 Da, Sigma Aldrich) against deionized water for 24 h. The supernatants were dialyzed against water to prevent the loss of soluble dietary fiber (SDF). The dialysates containing SDF (17 mL) were acid hydrolyzed (12 M H2SO4) and incubated in water bath at 100 °C for 90 min. The SDF content was calculated using the method proposed by ^{Englyst and Cummings (1988)}, using 3,5 dinitrosalicylic acid and glucose as standard at 530 nm. On the other hand, residues (insoluble dietary fiber, IDF) previously oven-dried were subject to acid hydrolysis (3 mL of 12 M H2SO4 in a water bath at 37 °C for 1 h), followed by the addition of 33 mL of distilled water and kept in a water bath (100 °C for 90 min). Finally, the solution was centrifuged (6000 x g, 25 ºC, 15 min) and supernatant was recovered. The IDF content was quantified using DNS reagent at 530 nm, as previously described. The remaining residues of IDF were quantified as Klason lignin (KL) by gravimetric method. IDF was calculated as NSP plus KL, while the total dietary fiber content was the sum of SDF and IDF.

pH

The pH was assessed with a digital potentiometer (HI 2211 PH/MV, HANNA) calibrated before the measurement with reference buffer solutions (pH 4.0 and 7.0), according to ^{AOAC (2012}), based on the immersion of an electrode in the solution. The sample (1 g) was mixed in 10 mL distilled water for analysis, and the reading was recorded.

Color attributes

The color was quantified using a colorimeter (CR-410, Konica Minolta, Sensing Inc., Osaka, Japan) previously calibrated. Results were expressed according to the rectangular coordinate system L*a*b*.

Apparent density

The apparent density was determined using the method reported by ^{Kaur and Singh (2005)}. Density was calculated in a graduated cylinder previously tared with 10 mL of the sample, and the sample weight was calculated as grams per volume unit (g/mL).

Water solubility index and water absorption capacity

The water solubility index (WSI) and the water absorption capacity (WAC) analysis were performed following the methodology proposed by ^{Anderson et al. (1969)}, with minor modifications. The sample (2.5 g) was homogenized with 30 mL distilled water, kept at 30 ºC in a water bath for 30 min, and then centrifuged using a high-speed centrifuge (BKC-TH21RL, Biobase, Shandong China) at 3000 x g/30 min. The supernatant was decanted and oven-dried (3489M-1, Barnstead International, Dubuque, Iowa, US) at 90 ºC for 24 h. Additionally, the pellet weight was recorded, and WSI and WAC were calculated with equation 3 and 4, respectively. The results of WSI are expressed in percentage, and WAC as g of retained water per g dry weight.

And

Oil absorption capacity

Oil absorption capacity (OAC) was assessed following the methodology by ^{Anderson et al. (1969)} with minor modifications. The sample (2.5 g) was mixed with 30 mL common corn oil in a 50-mL conical tube, shaken in a vortex for 1 min and then centrifuged using a high-speed centrifuge (BKC-TH21RL, Biobase, Shandong China) at 3000 x g/30 min, and the supernatant was decanted. The pellet weight was recorded, the OAC was calculated using the equation (5), and results expressed as g of oil retained per g of sample.

Swelling capacity

The swelling capacity (SC) was determined using the method described by ^{Robertson et al. (2000)}. The sample (100 mg) was hydrated with 10 mL distilled water in a calibrated cylinder (1.5 cm diameter) at room temperature. Then, after equilibration (18 h), the volume occupied by sample was recorded and expressed as volume per g dry weight.

Total soluble polyphenols, hydrolyzable polyphenols, and condensed tannins quantification

According to the methodology by ^{Pérez-Jiménez et al. (2008)}, an organic-aqueous extraction was performed to quantify the different compounds. The sample (0.5 g) was mixed with 20 mL methanol-acidified water solution (0.8 % HCl 72.8 g/L). The mixture was stirred at room temperature at a moderate speed (10 × g for 1 h) in a shaker (Heidolph Rex 2, Heidoplh Instruments, Schuwabach, Germany) and then centrifuged at 8000 × g for 10 min at 4 °C. Next, the recovered supernatants and the residues were mix with acetone-water solution (20 mL, 80:20 v/v), stirred for 1 h at room temperature and centrifuged under described conditions. Both supernatants were combined to measure soluble phenols (at 750 nm) by the ^{Montreau (1972)} procedure, using the Folin-Ciocalteu reagent, and the results expressed as milligrams of gallic acid equivalent per 100 g dry weight (mg EAG/100 g DW), with a standard curve of gallic acid. The residues of the extraction were used to quantify non-extractable polyphenols (NEP) made up of hydrolyzable polyphenols (HP) and condensed tannins (CT). The determination of HP was according to the method proposed by ^{Hartzfeld et al. (2002)}. The residues obtained from the organic-aqueous extraction were hydrolyzed with 20 mL methanol/H2SO4, 90:10 (v/v) at 85 ºC for 20 h, and then centrifuged using a high-speed centrifuge (BKC-TH21RL, Biobase, Shandong China) at 3000 x g at 25 ºC for 15 min, to assess HP in the supernatants using the Folin-Ciocalteu reagent as previously described. The CT were analyzed in the extraction residues, which were hydrolyzed with 10 mL butanol-HCl-FeCl3 (100 ºC, 3 h) and centrifuged using a high-speed centrifuge (BKC-TH21RL, Biobase, Shandong China) at 8000 x g, for 10 min, to recover supernatants, and absorbance was measure at 555 nm, according to ^{Reed et al. (1982)}. CT were calculated using a standard curve of proanthocyanidins from Mediterranean carob pods (Ceratonia siliqua L.), and the CT content expressed as mg/g DW. The quantification of total phenolic compounds was determined with the sum of soluble polyphenols (TSP) and hydrolysable phenols. A multi-mode microplate reader (Synergy HT, Biotek Instruments Inc., Winooski, Vermont, US) was used to quantify soluble and hydrolysable phenols.

Antioxidant activity

The antioxidant capacity of the organic aqueous extracts was quantifying 2,2’-azino bis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS•+) radical cation scavenging activity, analyzed using a modified version of the methodology by ^{Re et al. (1999)}. The radical cation was prepared by mixing 19.3 mg of ABTS•+ (7 mM) in 5 mL of potassium persulfate (2.45 mM) and stored (12 - 16 h) in the dark at room temperature under magnetic stirring. The ABTS•+ solution was adjusted with phosphate buffer (0.1 M, pH 7.4) to an absorbance of 0.7 (± 0.02) at 734 nm. The Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2carboxylic acid) was used as a standard and methanol as a blank. Sample (30 μL) was mixed with 255 µL ABTS•+ (30 ºC for 7 min). The reduction in absorbance was measured at 734 nm. The scavenging of 1,1-diphenyl-2-picrylhydrazyl (DPPH•) radicals was performed according to ^{Prior et al. (2005)} with some modifications. The Trolox sample or standard (30 μL) reacted with 200 µL DPPH• solution (190 µM, using methanol as dissolvent) while absorbance was measured at 517 nm after 10 min. The ferric reducing antioxidant power assay (FRAP) was carried out following the methods by ^{Benzie and Strain (1996)}, with modifications. The FRAP solution, 10:1:1 (v/v/v) of sodium acetate buffer (0.3 M, pH 3.6), 10 mM TPTZ (2,4,6-tripiridyls-triazine), and 20 mM iron chloride hexahydrate, was tempered to 37 ºC before mixing with the samples. The extract of the sample (24 µL) or Trolox standard was added to a 96-well microplate, then mixed with 180 µL FRAP solution using a multichannel dispenser, and the absorbance measured at 595 nm after 30 min. A multi-mode microplate reader (Synergy HT, Bio-Tek, Instrument Inc., Winooski, Vermont, US) was used to quantify the antioxidant activity of described methods. All the methods of antioxidant capacity and results were expressed as mmol Trolox equivalent per g dry weight (mmol TE/g DW).

Carotenoid content

The sample carotenoids were determined according to ^{Ortega et al. (2013)}. Two g of sample were mixed with 7 mL of acetone:petroleum ether 80:20 (v/v) and 0.5 g of MgCO3, then homogenized for 1 min before centrifuged using a high-speed centrifuge (BKC-TH21RL, Biobase, Shandong China) at 15000 rpm for 30 minutes at 4 °C. The supernatant was placed in a separator funnel adding 15 mL of NaCl (20%), the aqueous phase drained, and the organic layer diluted to 10 mL with petroleum ether. Absorbance was measured at 448 nm using a UV/Vis spectrophotometer (Jenway model 6705). The quantification was performed using a β-carotene calibration curve, and the results expressed as mg β-carotene per 100 g dry weight.

Quantification of phenolic acids by HPLC

The identification of phenolic acids was performed in an HPLC system (1260 Infinity, Agilent Technologies, Waldbron, Germany) equipped with photodiode-array detection and a C18 reversed phase column (particle size 5 µm, diameter 4.6 mm, and length 250 mm, Thermo Scientific®, Sunnyvale, California, US). The mobile phases were 2% acidified water with acetic acid (eluent A) and acidified water (0.5% acetic acid): methanol 10:90 (eluent B). The standards and extracts of the sample were analyzed using a 0% B gradient; 0−35 min, 35% B; 35−55 min, 75% B; 55−60 min, 100% B; and 60−70 min, 0% B, at a flow rate of 0.4 mL/min. The peak areas were detected at 280 and 320 nm for the sample and calibration curve (0.5−300 µg/mL) of gallic, ferulic, chlorogenic, p-coumaric, syringic, and neochlorogenic acids (Sigma-Aldrich, Inc., St. Louis, Missouri, US), were used to identify and quantify the polyphenolic compounds (^{Jiménez et al., 2014}).

Statistical analysis

All results were expressed as mean values ± standard deviation. Data were analyzed using Sigma-Stat version 11. All the analytical measurements were done in triplicate on a same batch of nopal flour, nine determinations were made.

Proximal analysis

Table 1 shows the nutritional composition of the N. cochenillifera flour. The moisture content (4.6%) is adequate for storage, which reduces the enzymatic reactions and/or presence of microorganisms (Rodríguez-García et al., 2007). Moreover, the lipid (1.20%) and protein (6.44%) contents were lower than those reported in Opuntia ficus-indica flour (lipid 2.3% and protein 8.76%), while the total carbohydrates (75.16%) and ashes (17.2%) in N. cochenillifera were higher than those reported (^{Nabil et al., 2020}) in Opuntia ficus-indica (74.27% carbohydrates and 11.90% ashes respectively). Additionally, the caloric contribution of cladodes flour was of 337 kcal/100 g, while higher values has been reported for commercial flours from corn and wheat (397.68 kcal/100 g and 384.2 kcal/100 g, respectively) (^{Kavitha and Parimalavalli, 2014}).

The consumption of dietary fiber associates to beneficial effects on consumers’ health (^{Holscher, 2017}). The total dietary fiber content in N. cochenillifera flour was 18.41% (16.92% insoluble and 1.49% soluble), similar to that reported by ^{Stintzing and Carle (2005)}, who found the average content of fiber in dehydrated Opuntia was 18%. Additionally, reports show that the amount of soluble fiber decreases as cladode ages, while insoluble fiber increases (^{Nabil et al., 2020}; ^{Rodríguez-García et al., 2007}). The low soluble fiber content in N. cochenillifera may associate to the drying temperature (45 ºC) during cladodes flour processing, which is the same as that of the glass transition of mucilage, promoting soft-flexible to rigid state and solubility changes, leading to possible hydrolysis (^{Ventura‐Aguilar et al., 2017}). According to the World Health Organization, the recommended fiber dietary intake (RDI) is an average of 25 g/day for both men and women; therefore, adding 15 g of N. cochenillifera flour to food products could contribute to at least 10% of the RDI. Subsequently, N. cochenillifera flour is a potential source of dietary fiber and its consumption could prevent and control non-transmissible chronic diseases (^{Bchir et al., 2014}). Furthermore, previous reports show that both soluble and insoluble nopal fiber has therapeutic effect on irritable bowel syndrome when nopal when consumed at a 20 g/day dose (^{Remes-Troche et al., 2021}). In this context, it is feasible the use N. cochenillifera flour as a functional ingredient in products with low caloric and high dietary fiber contents.

Physicochemical properties

Table 2 shows pH, total soluble solids, and color attributes of cladodes flour that showed a pH of 5.1, consistent with pH reported by Sáenz and Berger (2006), who found pH values from 4.0 to 7.0 in Opuntia flours. The total soluble solids in the flour were 6.9 ºBrix, possibly associated with N. cochenillifera soluble sugar (glucose) content (^{Nabil et al., 2020}). On the other hand, the color parameters if luminosity (*L), reddening (a), and yellowing (b) of cladodes flour were 67.62, −3.47, and 18.2, respectively, indicating that the flour exhibited a yellow, pale green color, similar to that reported in O. ficus-indica powder (Sáenz et al., 2010). The N. cochenillifera cladodes flour apparent density was 0.6 g/mL, similar to the reports on spineless O. ficus indica f. inermis cladodes powders (^{Ayadi et al., 2009}). This parameter relates to dietary fiber content; insoluble fiber content (mostly insoluble cellulose and hemicellulose) or soluble fiber fraction (mainly mucilages and pectins), which could modify powder density (Nuñez-López et al., 2013).

Functional properties

The WSI relates to the presence of soluble molecules in the flour. The cladodes flour WSI was 5.4; powdered spineless cladodes have shown a higher WSI as compared against spine-covered cladodes (^{Ayadi et al., 2009}). The WAC is a property linked to water retention within a matrix, which largely depends on the physicochemical nature of the flour fiber fraction; it directly affects the technological and functional properties of the flour. The cladodes flour WAC value was 11.4 g of water/g DW, which relates to the water retention capacity due to hydrophilic components in its matrix. N. cochenillifera showed a higher WAC value (11.4 g of water/g DW) than that reported in Opuntia (8 g of water/g DW) (^{Ayadi et al., 2009}; ^{Nuñez-López et al., 2013}). Furthermore, its WAC was higher than those reported for flours made from apples, pears, and carrots; which showed WAC ranging from 1 to 8 g of water/g DW (^{Ahmad et al., 2016}).

On the other hand, the OAC is a relevant parameter to evaluate the hydrophobic nature of the particles that constitute the fiber fraction in the flour (^{Ventura‐Aguilar et al., 2017}). N. cochenillifera powder showed an OAC of 2.05 g of oil/g DW, comparable to the same OAC value in flours obtained from vegetable sources such as apple and pear (^{Bchir et al., 2014}).

In addition, the SC relates to the dietary fiber content since carbohydrates interact with free polar groups through hydrophilic bonds, retaining them in a matrix. Cladodes flour showed an SC of 25 mL/g DW, which suggests that its components can absorb water due to its capacity to form matrices similar to those of a gel. In addition, these properties promote a low diffusion and a higher absorption of lipid compounds or sugars (^{Nuñez-López et al., 2013}).

These characteristics directly affect the cladodes flour technological and functional properties. They could induce beneficial effects in health by significantly increase chyme viscosity in a dose- or amount-dependent manner. This fact leads to a beneficial regularity by altering fecal viscosity since this property determines the speed at which a substrate transits through the large intestine (^{McRorie Jr and McKeown, 2017}).

Biocompound contents and antioxidant capacity

Table 3 shows the soluble and hydrolyzable polyphenols, tannins, carotenes, and antioxidant activity. The N. cochenillifera flour total phenol content was 207.95 mg GAE/100 g DW, slightly higher than the 180 mg GAE/100 g DW reported by ^{Gallegos-Infante et al. (2009)} in O. ficus-indica cladodes flour. The difference between these values may relate to the drying process, since previous work showed that different airflow rates and temperatures, modify total phenols (^{Medina-Torres et al., 2011}). The content of hydrolyzable polyphenols in N. cochenillifera flour was 647.99 mg GAE/100 g DW. The hydrolyzable forms include a larger proportion of the polyphenols present in N. cochenillifera flour than the soluble polyphenols, as previously shown.

On the other hand, tannins are high-molecular-weight polyphenols beneficial to health. Their monomer structure classifies them into two groups: hydrolysable tannins formed by phenolic acids and condensed tannins (^{Chung et al., 1998}). The latter showed values of 3.55 mg/g DW in N. cochenillifera, which depends on the maturity of the cladodes (^{Figueroa-Pérez et al., 2018}). Another activity evaluated in Carotenes, is their antioxidant activity, since they are very efficient physical quenchers of singlet oxygen and scavengers of other reactive oxygen species (^{Fiedor and Burda, 2014}). The N. cochenillifera cladodes flour carotene content (4.36 mg β-carotene/100 g DW) was higher than that reported for Opuntia cladodes (3.79 mg/100 g DW) subjected to heat treatments. There are reports showing that the extraction of nopal carotenes depends on the presence of mucilage, since these compounds can form complexes with pectin, thus the use of high temperatures for their extraction are necessary (^{Sáenz et al., 2010}, ^{Saenz, 2006}; ^{Jaramillo-Flores et al., 2003}).

The N. cochenillifera cladodes flour antioxidant activity, assessed with DPPH•, FRAP, and ABTS•+, was 15.28, 20.97, and 51.31 mmol TE/g DW, respectively. The highest antioxidant activity value may relate to the N. cochenillifera cladodes flour high polyphenols content. In contrast, the low tannins and carotenes content may influence the lowest value, reported previously, showing that polyphenols significantly correlate with ABTS•+ assays, while tannins with DPPH• assays (^{Figueroa-Pérez et al., 2018}). Additionally, the antioxidant capacity shown with FRAP could be mainly due to the tannins and carotenes content, since these compounds are very efficient at controlling singlet oxygen, and phenolic compounds can sequester free radicals and electron transfer (ABTS•+) better (^{Koracevic et al., 2001}; ^{Pertuzatti et al., 2014}).

Polyphenol profile

The N. cochenillifera cladodes flour phenolic compounds (gallic, ferulic, chlorogenic, p-coumaric, syringic, and neochlorogenic) content detected by HPLC, are shown in Table 4 and Figure 1. According to soluble polyphenols chromatographic profile, multiple unidentified peaks are shown, which could correspond to phenolic compounds that have been reported by ^{Figueroa-Perez et al. (2018)}, who identified 15 phenolic acids compounds in nopal cladodes at different maturity stages, from which only 11 were previously identified in commercial and wild Opuntia spp. (^{Guevara-Figueroa et al., 2010}). Considering the diversity of phenolic compounds reported in other nopal varieties, the unidentified peaks in the N. cochenillifera phenolic profile, may include salicylic, protocatechuic, 4-Hydroxybenzoic, vanillic, caffeic, synaptic, eucomic and chlorogenic acids (^{Astello-García et al., 2015}; ^{Figueroa-Pérez et al., 2018}; ^{Guevara-Figueroa et al., 2010}; ^{Kolniak-Ostek et al., 2020}; ^{Mata et al., 2016}; ^{Missaoui et al., 2020}). Therefore, further studies are necessary to identify N. cochenillifera phenolic compounds.

Figure 1 Soluble polyphenols chromatographic profile of N. cochenillifera cladodes flour.
Figura 1. Perfil cromatográfico de polifenoles solubles de la harina de cladodios de N. cochenillifera. 

On the other hand, the most abundant phytochemicals identified were gallic, ferulic and chlorogenic acids. The gallic acid content (2708.49 µg/g) indicated in this study was higher than that found by ^{Jun et al. (2013)}. However, the same author reported that ferulic, chlorogenic and p-coumaric acid contents were higher than the data obtained in this study (796.73, 179.70 and 80.58 µg / g, respectively). This difference in the phenolic acid content is likely due to factors such as the cladodes maturity stage, species, geographic conditions, and other environmental conditions (^{Moussa-Ayoub et al., 2014}).

^{Figueroa-Pérez et al. (2018)} observed different polyphenol profiles in nopal (O. fiscus-indica) cladodes collected at different maturity stages, with association to health beneficial effects. For example, chlorogenic acid relates with antidiabetic effects by delaying intestinal glucose absorption and inhibiting hepatic gluconeogenesis, while gallic acid has shown beneficial effects in the prevention and/or management of various disorders, attributed to its antioxidant potential (^{Kahkeshani et al., 2019}; ^{Ong et al., 2012}). These results suggest that N. cochenillifera cladodes flour may provide antioxidant properties and offer effective protection from free radicals, and support that N. cochenillifera could be a promising source of natural antioxidants.