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Article

The Properties of Pectin Extracted from the Residues of Vinegar-Fermented Apple and Apple Pomace

by
Asli Muslu Can
1,2,
Rusen Metin Yildirim
1 and
Ayse Karadag
1,*
1
Department of Food Engineering, Faculty of Chemical and Metallurgical Engineering, Yildiz Technical University, Davutpasa, 34210 Istanbul, Turkey
2
Department of Food Technology, Istanbul Gelisim Higher Vocational School, Istanbul Gelisim University, 34310 Istanbul, Turkey
*
Author to whom correspondence should be addressed.
Fermentation 2024, 10(11), 556; https://doi.org/10.3390/fermentation10110556
Submission received: 26 September 2024 / Revised: 18 October 2024 / Accepted: 30 October 2024 / Published: 31 October 2024
(This article belongs to the Section Fermentation for Food and Beverages)
Browse Figures
Versions Notes

Abstract

:
In this study, both apple slices and apple pomace, the by-product of apple juice processing, were subjected to vinegar fermentation. The pectins extracted from the solid residue of vinegar-fermented apple slices (FAP) and apple pomaces (FAPP) were compared to the pectin extracted from non-fermented apple pomace (AP). All samples were classified as high-methoxyl pectins, and vinegar fermentation increased pectin extraction yield. FAP, which has a lower degree of methylation, also exhibited lower thermal stability. The changes in the pectin structure were dependent on both fermentation and the state of the raw material used to produce the vinegar. Compared to AP, the proportion of homogalacturonans (HGs) increased, and the proportion of rhamnogalacturonan I (RG-I) decreased in FAP, providing higher linearity, whereas in FAPP, the RG-I region became more dominant with reduced linearity. The molecular weight distribution of samples showed that pectin from vinegar-fermented sources changed the average molecular weights and mass fraction ratios of two peaks (1255 and 340 kDa) eluted from AP. In FAP, the mass fraction of the first peak (1294 kDa) increased from 35 to 89%, whereas in FAPP, the mass fraction of the second peak (478 kDa) increased to 91%. FAPP showed greater viscosity and a more noticeable shear-thinning behavior. G′ and G″ in FAPP were also higher than those of AP and FAP at the same concentrations (5%, 7%, and 10% w/v). This study found that applying vinegar fermentation to apple slices and apple pomaces altered the structural and rheological properties of the extracted pectins. FAP and FAPP could be suitable food additives when certain properties are required.

1. Introduction

Pectin, a complex polysaccharide located in the primary cell wall and intracellular layer of higher plants, is particularly abundant in fruits such as apples, oranges, and lemons. Its complex structure and primary composition of α (1–4) glycosidic bonds linking D-galacturonic acid residues make it a valuable ingredient in food design because of its stabilizing, glazing, gelling, and thickening properties. Pectin’s widespread use as a functional ingredient has resulted in a 6.5% annual increase in the global pectin market, requiring the exploration of new pectin sources [1].
Apple pomace and peels derived from various processed apple products such as juice, cider, vinegar, and wine have been reported as rich and versatile sources of commercially available pectin [2]. There have also been some attempts to utilize apple pomace in the production of vinegar, cider, and alcoholic beverages or flavorings [3]. Vinegar can be defined as a condiment made from various sugary and starchy materials by alcoholic and subsequent acetic fermentation. Apple vinegar is industrially produced from extracted apple juice and apple cider, whereas traditional processing uses sliced apples as the raw material, generating by-products that could be further valorized [4].
Recent studies suggest new strategies to enhance the value of fruit pomace by fermentation, including the modification of dietary fibers with improved nutritional and technological functions [5]. For example, when the pomace of Rosa roxburghii fruit was fermented with Bacillus natto, it showed better water retention, swelling, oil absorption capacity, and higher soluble dietary fiber content due to the degradation of cellulose and hemicellulose [6]. Meanwhile, a few studies have investigated how fermentation affects the composition and characteristics of pectin extracted from fruit pomace. When Xu et al. [7] compared the structural properties of the pectins extracted from the pomaces of unfermented and wine-fermented apples and grapes, the structural properties of the pectins significantly differed by yeast fermentation, and the pectins from fermented sources had much smoother surfaces, fewer crystalline fractions, higher proportions of amorphous structures, and lower viscosities. In the study of Jiang et al. [8], pectin extracted from yeast-fermented hawthorn wine pomace had a lower molecular weight and more elastic and solid properties compared to that extracted from steeped pomace. In a further study, the pectins extracted from the residues of apple wine and vinegar provided higher foaming stability, hygroscopicity, and lower oil absorption and emulsification properties compared to those obtained from non-fermented apples [9].
Unlike the limited previous studies related to pectins extracted from fermented sources, in this study, vinegar fermentation was applied to both sliced apples and apple pomaces, and the solid residues of both vinegars were used to extract pectins. The composition, structural features, and technological and functional properties of the pectins derived from vinegar-fermented apple sources were assessed and compared to those of pectins obtained from non-fermented apple sources to elucidate how vinegar fermentation affected the properties of the extracted pectin when soluble sugar-rich and -deficient substrates were utilized.

2. Materials and Methods

2.1. Materials

Around 50 kg of golden apples were purchased in September 2023, at their harvest time. Once in the lab, ruptured, injured parts were discarded; the samples were washed with water and blotted gently. Around 40 kg of apples were processed by a high-speed centrifugal juicer (Vestel Enerjik, KMS600, Manisa, Turkey) to obtain apple pomace. For extracting non-fermented apple pectin (AP), half of the pomace was dried for 9 h at 55 °C in an air-circulating oven, and the other half was subjected to vinegar fermentation. The apples (~10 kg) were washed with pure water and then chopped into cubes (1.5 × 1.5 cm) and mixed with water (25 L), honey (50 g), chickpeas (15 g), and fermented apple vinegar (500 mL) in glass jars. Then, alcohol fermentation was carried out under anaerobic conditions at 30 °C for 7 days, and jars were stirred daily. The acetic acid fermentation was further continued under aerobic conditions at 30 °C for 23 days. The same procedures were applied to the previously obtained apple pomace. The solid residues of apple vinegar and apple pomace vinegar were filtered and dried (55 °C for 9 h) for further extraction of fermented pectins (FAP and FAPP, respectively). The dried powders, ground by a high-speed grinder to pass through mesh no. 50, were kept in sealed aluminum packages at −20 °C until use.
D-mannose (Man), D-ribose (Rib), L-rhamnose (Rha), D-galacturonic acid (GalA), D-glucose (Glc), D-galactose (Gal), L-arabinose (Ara), D-xylose (Xyl), and L-fucose (Fuc), 1-phenyl-3-methyl-5-pyrazolone (PMP), PMMA (polymethylmethacrylate) standards, D2O (Deuterium oxide), Folin–Ciocalteau reagent, bovine serum albumin (BSA), m-hydroxyl biphenyl, trifluoroacetic acid (TFA) hydrochloric acid (HCl), NaOH, acetone, chloroform, ethanol, salts, and solvents were obtained from Sigma-Aldrich Ltd. (Steinheim, Germany).

2.2. Pectin Extraction

The extraction procedure of Yılmaz et al. [10] was followed with slight modifications. Briefly, dried powders were mixed with deionized water (1:35, w/v), and the pH was adjusted to 2 using a 1 mol/L HCl solution. After 3 h of extraction at 90 °C, the mixture was centrifuged at 13,416× g for 10 min at 4 °C. The supernatant was concentrated to half of its initial volume with a rotary evaporator at 40 °C under vacuum, mixed with four volumes of ethanol (96%, v/v), and kept at 4 °C overnight for precipitation. The pectin separated by centrifugation (2147× g for 30 min) was washed with ethanol three times and finally with acetone and dried at 55 °C in an air-circulating oven, then ground. The pectin yield (%) was calculated as the mass of dried pectin obtained per 100 g of dry powder.

2.3. Chemical Composition and Physicochemical Properties of Pectin Samples

The soluble protein content of pectin samples (AP, FAP, and FAPP) was determined according to Bradford [11] using BSA as the standard. The calibration curve for the BSA was prepared between 0.1 and 0.01 mg/mL. The total phenolic content (TPC) was determined with the Folin-Ciocalteu (FC) reagent according to the method described by Singleton et al. [12] Results were given as mg gallic acid equivalents (GAE)/100 g dw (dry weight). The degree of methylation (DM) of pectin samples was determined by using the titrimetric method of Al-Amoudi et al. [13] The color attributes of samples were measured using a chromameter (Konica Minolta CR-400, Tokyo, Japan). They were expressed as L* (whiteness/darkness), a* [redness (−)/greenness (+)], and b* [yellowness (+)/blueness(−)].

2.4. Monosaccharide Composition of Pectin Samples

Monosaccharide composition was determined by the HPLC method of pre-column derivatization with PMP as reported by Chen et al., Dai et al., and Karadag et al. [14,15,16] A total of 25 mg of pectin was hydrolyzed with 2 mL of trifluoroacetic acid (TFA) at 120 °C for 4 h in a vacuum-sealed tube. The excess TFA was eliminated under a stream of N2, cleaned with methanol three times (2 mL), and the solvent was evaporated to dryness. Following 5 min of centrifugation at 2800× g, the dried hydrolyzed pectin was dissolved in 2 mL of ultrapure water, and the supernatant was used for subsequent derivatization. A 400 µL aliquot of the supernatant or monosaccharide standard mixture was derivatized with 400 µL of NaOH (0.6 mol/L) and 800 µL of PMP (0.5 mol/L) solution in a water bath at 70 °C for 100 min. After cooling to room temperature, the mixture was neutralized with 0.3 mol/L HCl. After the mixture was completely dry, it was extracted three times using 1.5 mL of water and 1.5 mL of chloroform. The chloroform layer was discarded, and the filtered aqueous layer (0.22 μm) was analyzed using the Shimadzu HPLC system (LC-20AD pump, SIL-20A HT autosampler, CTO-10ASVP column oven, DGU-20A5R degasser, and CMB-20A communications bus module) coupled to a diode array detector—SPDM20A DAD (Shimadzu Corp., Kyoto, Japan). Chromatographic conditions were as follows: ACE 5-C18 (4.6 mm × 250 mm, 5 μm, GLSciences, Tokyo, Japan) column, a diode array detector (250 nm), the mobile phase was the mixture of 0.1 mol/L phosphate buffer (pH 6.7) and acetonitrile (83:17, v/v), the flow rate was 1 mL/min and the column temperature was 30 °C. The calibration curve built for monosaccharide standards across the 0.025–0.25 mg/mL concentration range was used for the quantification.

2.5. FTIR, NMR Spectroscopy and XRD Analyses

FTIR spectra of pectin samples were analyzed using an ATR-FTIR spectrophotometer (Bruker Tensor 27, Bremen, Germany) with OPUS software (Version 7.2). The DM was also calculated by FTIR [7], as the average ratio of the peak area at 1740 cm−1 (COO-R) to the total peak area of 1740 cm−1 (COO-R) and 1640 cm−1 (COO−).
Pectin samples were fully dissolved in 99.9% D2O and scanned 256 times at 25 °C using an Agilent 400 MHz NMR spectrometer (Agilent NMR 400/54/ASC, Santa Clara, CA, USA) [17].
XRD analysis was performed using an X-ray diffractometer (Malvern Panalytical Empyrean MultiCore, Almelo, The Netherlands). XRD patterns were acquired with a Cu-K α radiation source and a diffraction angle (2θ) between 5° and 80° with the time per step set as 1 min and step size as 1° 2θ [18].

2.6. Molecular Weight Distribution

The molecular weights (Mw) were analyzed using gel permeation chromatography (GPC) using a Tosoh EcoSEC HLC-8320-RI-UV GPC system (Tosoh Bioscience, Tokyo, Japan) with HQ-806 M and HQ-807 M columns and a WYATT HELEOS-II MALS DynaPro DLS detector. The samples were solubilized with phosphate-buffered saline and filtered through a 0.45 μm membrane filter (Millipore Co., Milford, MA, USA). A flow rate of 0.5 mL/min, column temperature of 25 °C, and injection volume of 100 μL were used as the experimental conditions. A PMMA (polymethyl methacrylate) standard was used to draw the calibration curve, which was plotted between 800 Da and 1,600,000 Da (r2 > 0.999).

2.7. Morphology of Pectin Samples

Morphology of pectin samples (AP, FAP, and FAPP) was determined according to Wang et al. [19]. SEM images were used to examine the surface patterns of pectin grains. The pectin samples were put on adhesive-coated aluminum stubs and sputter-coated three times with a thin layer of gold at a working distance of 50 mm while operating under vacuum at 40 mA for 100 s. The images were taken using an SEM (QUANTA FEG-250, Hillsboro, OR, USA) at 200, 2000, and 10,000× magnifications.

2.8. Thermal Analysis

Thermal behavior of pectin samples was examined by thermogravimetric (TG), differential thermal analysis (DTG) (Exstar 6300 TG/DTA, Seiko Instruments SII, Tokyo, Japan), and differential scanning calorimetry (DSC) (TA Q100, Thermo DSC Q2, New Castle, DE, USA). Aluminum pans used to hold the samples were heated from 25 °C to 600 °C for TG/DTG and 10 °C to 500 °C for DSC analysis under a nitrogen atmosphere. The empty pan was used as the reference.

2.9. Rheological Analysis

Pectin solutions prepared at 5%, 7%, and 10% w/v were mixed in a magnetic stirrer at room temperature for 18 h. Steady and dynamic shear measurements for pectin solutions were performed using a stress–strain-controlled oscillatory and rotational rheometer (Anton Paar, MCR 302, Graz, Austria) equipped with a Peltier heating system.

2.9.1. Steady Shear Properties

Shear rate was controlled in measurements that were performed in the shear rate range of 1–100 s−1 at 25 °C using a parallel plate configuration (diameter 50 mm, gap 0.5 mm). Each measurement was repeated three times. The apparent viscosity of the sample was determined as a function of shear rate. The relation between shear rate and shear stress was explained by the Ostwald de Waele model based on the following equation (Equation (1)):
σ = K(γ) n
σ; shear stress (Pa), K; consistency coefficient (Pa.sn), γ shear rate (s−1) and n; flow behavior index (dimensionless).
Temperature sweep test was performed at 0.5 Pa within a temperature range of 5–70 °C. Measured η70 values versus temperature data were fitted to the following Arrhenius equation (Equation (1)):
η70 = η0e(Ea/RT)
η70 is the apparent viscosity at a shear rate of 70 s−1 (Pa.s), η0 is the Arrhenius equation constant (Pa.s), Ea is the activation energy (kJ mol−1), R is the universal gas constant (kJ mol−1 K −1) and T is the temperature (K).

2.9.2. Dynamic Viscoelastic Properties

A linear viscoelastic region (LVR) was established by performing an amplitude sweep test in the strain range of 0.1–100% at 10 rad/s at 20 °C using a parallel plate arrangement. A frequency sweep test was performed at the determined 0.5% strain within the LVR at 25 °C in the range of 0.1–100 rad/s. The results were calculated using the power-law model based on the following equation (Equation (3)):
G′ = K′(ω)n′; G″= K″(ω)n″
where G′ (Pa) is the storage modulus, G″ (Pa) is the loss modulus, ω is the angular velocity value (1/s), and K′ (Pa.sn) and K″ (Pa.sn) values indicate the consistency index values, and n′, n″ values represent the flow behavior index values.

2.10. Statistical Analysis

All the treatments and analyses were performed in triplicate. The results were compared by one-way analysis of variance (ANOVA) using JMP 6.0 Statistics Software (SAS Institute, Inc., Cary, NC, USA, ABD). The results were given as mean ± standard deviation (SD), and the significant differences (p < 0.05) were evaluated by using Tukey’s post hoc and independent t-tests. The figures were drawn using Origin 9.0 (OriginLab Co., Northampton, MA, USA).

3. Results and Discussion

3.1. Chemical Composition and Physicochemical Properties of Pectin Samples

The pectin yield from unfermented apple pectin (AP) was 23.2 ± 1.31% and significantly increased to 32.4 ± 1.95% and 30.2 ± 2.10% in FAP and FAPP, respectively (Table 1). When Xu et al. [7] extracted pectin from apple and grape wine pomaces that had been fermented by Saccharomyces cerevisiae, the fermentation decreased the pectin yield compared to the pectin yield of unfermented fruit pomaces. The decreased pectin yield could be attributed to pectinolytic enzymes secreted by the yeast during wine fermentation. In our study, however, compared to those of non-fermented raw materials, the pectin yield was increased by vinegar fermentation. In contrast to wine (yeast) fermentation, the enzymes secreted by the microorganisms that dominate vinegar fermentation may not possess pectin depolymerization activities. When Scharf et al. [20] analyzed the softening of pickled cucumbers through the pectolytic enzyme activities of the ingredients (dill, onion, mustard seed, and vinegar), almost no polygalacturonase, pectinesterase, or pectin lyase activity was reported for vinegar. The pectin extraction in acidic conditions consists of two overlapping reactions. The first reaction includes the dissolution of insoluble protopectin from the cell wall, while the second entails the hydrolysis of solubilized pectin into the degraded products. The first reaction dominates until all of the pectin is extracted from the plant cell wall; while, if the second reaction dominates, the pectin yield will decrease [21]. Previously, acetic acid was recommended as an effective extraction solvent for pectin from apple pomace rather than mineral acids (HCl, HNO3, and H2SO4) [22]. A higher concentration of acetic acid can provide a more stable and sufficient supply of hydrogen ions because acetic acid is a weak electrolyte with balanced dissociation properties. The increased supply of hydrogen ions may increase the disruptive effects during the extraction process, leading to the release of larger molecules such as pectin. During the vinegar fermentation (about 4 weeks) in the acetic acid-rich environment, the apple cell walls may have become more permeable, which could enhance the separation of pectin from the cellulose and hemicellulose structures and the solubilization of protopectin in the HCl-acid extraction step that followed [22].
Water-insoluble pectic material, proteopectin, is present in intact tissue and can be broken down by protopectinases (PPase) enzymes to yield polymerized soluble pectin [23]. Although no study has reported PPase activity in vinegar, the culture filtrates of yeast and yeast-like fungi, such as Trichosporon penicillatum, Bacillus subtilis, Kluyveromyces fragilis, Galactomyces reesei L., Trametes sp. [24], and Aspergillus oryzae [25], have been reported to have PPase activity. In the study by Sakai and Okushima [26], for example, citrus peels were inoculated with a variety of Trichosporon penicillatum that secretes the protopectin-solubilizing enzyme. Compared to pectin that was extracted using conventional methods, microbial pectin had an elevated yield and different properties (e.g., higher Mw, increased viscosity, and more neutral sugars). The PPases are a diverse group of enzymes. The A-type PPase interacts with the smooth regions of protopectin, which are composed of homogalacturonans. Conversely, the hairy sections of protopectin, which are made up of neutral sugar side chains and rhamnogalacturonan, interact with the B-type PPase. Therefore, it could be speculated that the different varieties of enzymes that solubilize pectin from the insoluble protopectin might have been secreted by the microorganisms dominant in vinegar flora.
The protein content of the pectins decreased from 1.25 (AP) to around 0.70 g/100 g (dw) in the fermented sources (Table 1). The DM (%) values of the samples determined by the titration method and FT-IR analysis were in a similar region and ranged from 50 to 58% (Table 1), indicating that all samples can be classified as high-methoxyl pectin. The DM values of FAPP and AP were not significantly different, whereas FAP yielded a lower value. FAP was extracted from the solid residue of vinegar-fermented apple slices, whereas FAPP was extracted from the solid residue of vinegar-fermented apple pomaces. The vinegar fermentation includes both initial anaerobic yeast fermentation and the conversion of alcohol to acid by acetobacter under aerobic conditions. During the short first wine-fermentation stage, the pectin methyl esterase (PME), which is secreted by yeasts, may cause the cleavage of the methyl groups [24]. Whereas, the raw material of FAPP had fewer soluble sugars, which probably resulted in lower yeast activity and prevented it from producing enough PME to act on the methyl groups of pectin. As expected, the total acidity (%) and Brix of the apple pomace vinegar (0.59%, 1.9) were lower than those of the apple slice vinegar (1.01%, 1.0). The higher acetic and lactic concentrations of apple slice vinegar (5013 ± 25.4 and 614 ± 6.61 mg/L) compared to apple pomace vinegar (3628 ± 49.4 and 534 ± 14.3 mg/L), as additionally determined by HPLC, indicated the lower concentration of fermentation products in the apple pomace vinegar.
The FAP pectin has the highest lightness (L*), as well as the lowest redness (a*) and yellowness (b*) values (Table 1). During the crushing and dejuicing processes, the raw materials of AP and FAPP were already subjected to oxidation by the endogenous enzymes. The surface area of the raw material of FAPP was higher than that of FAP; the raw material of FAP was probably less affected by polyphenol degradation during fermentation, leading to it yielding lighter colors and a higher TPC value.

3.2. Monosaccharide Composition of Pectin Samples

Vinegar fermentation affected the abundance of individual monosaccharides in the pectin samples. As anticipated, the most predominant monosaccharide was GalA (Table 1). The mol ratio (%) of GalA was highest in FAP (80.1%) and lowest in FAPP (61.9%). Ara and Gal were the primary components of the pectin side chains. In AP and FAPP, GalA was followed by Gal > Ara, whereas in FAP, it was Ara > Gal. Compared to AP, vinegar fermentation increased the proportions of Ara when the raw material was either apple slices or pomace; however, this increase was greater in FAPP. In comparison to AP, FAPP showed a slight increase in the proportions of Gal, Rha, and Xyl, whereas they all decreased in FAP. This could suggest that the proportions of side chains in vinegar-fermented pectins would vary depending on the state of the raw material. According to the literature, the monosaccharide composition of extracted pectins was affected by the wine fermentation of apples and grapes, resulting in higher proportions of Ara, Gal, Rha, Xyl, and Man but lower proportions of GalA, Glc, and Fuc [7].
Homogalacturonans (HGs) consist of a linear chain of α-1,4-D-GalA units that are partially esterified with methanol at the C6 position, and the ratio of the esterified units to the total amount of GalA units is expressed as the degree of methylation. Rhamnogalacturonan I (RG-I) has several side chains made up of sugars and branched oligosaccharides, such as side chains of Rha, Gal, Ara, and arabinogalactans [27]. RG-II is a structurally complex pectin; its building blocks are GalA, Rha, Gal, and rare sugar residues, and the presence of Xyl was attributed to xylogalacturonans [7]. The remnants of non-pectic polysaccharides (such as hemicellulose) bonded to the side chains of pectin may account for the existence of Man and Xyl.
All the pectin samples were dominated by HG domains. HG levels (mol%) ranged from 59.6 to 77.4, and RG-I levels (mol%) were between 20.7 and 35.6. HG/RG-I ratios of 2.54, 3.78, and 1.67 were calculated for AP, FAP, and FAPP, respectively (Table 1). The Rha/GalA ratio, which was 0.03 for AP and FAP and 0.04 for FAPP, indicates the proportion of GalA residues to Rha residues. This ratio provides information about the RG-I backbone with respect to HG content. Therefore, a higher value indicates a compound that contains a greater abundance of RG-I chains. The changes in pectin structure induced by vinegar fermentation varied depending on the state of the raw material. In comparison to AP, the quantity of the HG region increased, while the RG-I region was reduced in FAP, indicating a decrease in the proportion of the hairy region and the abundance of the smooth region. However, using apple pomace as the raw material for vinegar resulted in a decrease in the quantity of the HG region, while the RG-I region became more prominent in the pectin that was extracted (FAPP). In the study of Xu et al. [7], the RG-I domain was more dominant in the pectin of apple and grape winery pomace. HG and RG-I regions are proposed to be covalently linked in the pectin structure and cannot be separated without chain-cleaving enzymes [28].
The average size of the neutral side chains joined to the RG-I backbone is represented by (Gal + Ara)/Rha; therefore, a higher value suggests higher branching degrees or a longer side chain. GalA/(Fuc+Rha + Ara + Gal + Xyl) indicates the linearity of the pectin. Compared to AP, FAP had higher linearity, and the length of the neutral side chains that are attached to the RG-I backbone was significantly lower. In contrast, the linearity of FAPP decreased as the length of its side chains increased. Xu et al. [7] reported an increase in the degree of branching and reduced linearity of both grape and apple pectin extracted from wine-fermented pomaces and a lower average size for the side chains in fermented grape pomace pectin but not in fermented apple pomace pectin, and those changes were associated with the actions of both the endogenous enzymes of the pomace and the yeast enzymes.
The substrates of the vinegar fermentation were different in FAP and FAPP, serving as soluble sugar-rich and deficient sources for microbiota, respectively. This difference may have led to the creation of different levels of enzymes that act on pectin, resulting in different structures. The acetic acid concentration of the vinegar to which FAP was subjected during the incubation period was higher than that of FAPP, and the glycosidic linkages of neutral monosaccharides to the RG-I region are known to be more susceptible to acid hydrolysis.

3.3. FTIR, NMR, XRD, Molecular Weight, and SEM Analysis

FTIR spectra (Figure 1) were used to identify functional group differences, and their binding configurations among AP, FAP, and FAPP revealed that the chemical bonds across the pectin samples were similar, with only minor variations. The wide and distinct peaks at 3358 cm−1 and 3314 cm−1 include the OH stretching absorption due to inter- and intramolecular hydroxyl groups [29]. A peak at 2928 cm−1, common to all samples, was assigned to the C-H bonds of CH, CH2, and CH3 groups. All pectin samples exhibited characteristic signals at 1738 and 1634 cm−1, corresponding to the esterified and free carboxyl groups of GalA units, respectively [30]. FAP exhibited a decreased ester carbonyl band at 1738 cm−1, indicating a lower degree of methylation (DM). However, the wide peak at 1634 cm−1, corresponding to carboxylate ion stretching, was consistent across all pectin samples. The bands in the region of 1350–1450 cm−1 are reported to be related to esterified CH3 groups, in all pectins, the peak appeared at 1371 cm−1 and 1440 cm−1 attributed to the symmetric and asymmetric stretching of CH3, respectively [31]. Furthermore, the strong peaks at 1014 cm−1 indicated high homogalacturonan (HG) content, with HG regions predominating in all pectin samples [32]. AP and FAP showed a lower signal at the shoulder peaks of 1045 cm−1 and 1076 cm−1, which is related to neutral sugars such as Ara, Xyl, and Gal, compared to the FAPP [33]. FAPP contained a higher ratio of neutral sugars, as could be seen in the monosaccharide composition (Table 1).
XRD analysis provides information on whether the structure of polymers is amorphous or crystal. The samples exhibited a predominantly amorphous nature but also had a crystal region between 10° and 25° (Figure 1). In common with previously published findings, the crystalline signals at about 2θ of ca. 13° and ca. 21° were observed in all samples. The order of the carboxylic groups in the solid state has a significant impact, resulting in a more crystalline polymer [34]. The diffractograms of FAP and FAPP differed from that of AP in terms of the width and intensity of the crystalline peak. The second peak (at around 21°) became more defined in FAP and narrower with decreased intensity in FAPP. The decrease in the intensity of the peaks is likely to have occurred due to the disruption of hydrogen bonds within the polysaccharide networks [7].
Figure 2 shows the 1H NMR and 13C NMR spectra of the samples. The solvent signal (D2O) was linked to the signal at 4.70 ppm. One large singlet at 1.0 ppm was attributed to ethanol (CH3-CH2OH), which was used in the extraction step to precipitate pectin. The methyl groups of Rha were assigned to the signals at 1.21 and 1.23 ppm. Two signals at around 1.98 and 1.89 ppm were due to the acetyl groups binding at 2-O and 3-O GalA, respectively. High proton signals around 5 ppm, like those of the anomeric H-1 (5.07 ppm) and H-5 (4.96 ppm), disclosed non-esterified GalA residues [35], and the anomeric proton of Ara was observed at 5.22 ppm [14]. In all three samples, the signals from the protons in the methoxy groups (-OCH3) of esterified pectin produced a large and sharp singlet at 3.61 ppm. Five major signals in Figure 2 were attributed to the esterified D-GalA as H-1 protons at 4.90 ppm, H-2 protons at 3.46 ppm, H-3 protons at 3.77 ppm, H-4 protons at 3.98 ppm, and H-5 protons at 4.46 ppm, which were consistent with previous studies [7,36,37]. The very-low-field signals at 174.8 ppm and 173.6 ppm correspond to the free C-6 of methyl-esterified and free carboxylic groups, respectively. In the anomeric carbon region, the signals at 99.6 and 99.4 ppm were linked to the C-1 of esterified and nonesterified units, respectively [38,39]. The C-6 O methyl ester group (O-Me) of the pyranosidic ring was identified as the source of the signal at 60.31 ppm [36], and the signals at 71.0, 71.4, 76.5, and 73.2 ppm were related to C 2-5 of GalA [40,41]. The spectrum of AP was more complex than those of the pectins derived from the fermented materials (FAPP and FAP), indicating that a wide range of natural compounds were also present in AP. Additionally, the peaks became more obvious and clear in the apple pectins obtained from vinegar-fermented sources. For instance, the peaks related to methyl groups of Rha (at around 1.2 ppm) could not be differentiated in AP, while in the fermented materials, the intensity of these peaks increased, with stronger peaks seen in the fermented pectins, especially in FAPP.
The Mw distribution of the samples is given in Figure 3. The chromatograms of the pectin derived from non-fermented apple pomace (AP) eluted two peaks, with the first corresponding to 1255 kDa with a mass fraction of 34.9%, and the second to a lower Mw (340 kDa) with a mass ratio of 65.2% (Figure 3). The first large peak could be indicative of pectin substances that were abundant in galacturonic acids, while the second peak, which was eluted later, may represent neutral sugars and other components present in the pectin extract. The Mw of pectins produced from different varieties of apple pomaces was reported to be >805 kDa when the extraction solvent was citric acid. When Zhou et al. [42] extracted pectin from thinned-young apples, the Mw of pectins ranged from 592 to 1787 kDa, depending on the extraction solvent. When the raw material for pectin extraction was the residue of fermented apple slices, the mass fraction of the first peak (Mw of 1294 kDa) was 89%, which could indicate that fermentation provided purity along with the hydrolysis of low-Mw components tailored to the main peak. In FAPP, the larger peak eluted earlier corresponded to a higher Mw (6851 kDa), and the mass ratio of low-Mw components (478 kDa) increased to 91.4%. According to Xu et al. [7], wine fermentation increased the Mw of apple pectin from 1859 to 1925 kDa. Similar to the Mw of their pectin obtained from apple wine residue, the highest Mw among our samples (FAPP) was coupled with the biggest average size of the neutral side chains attached to the RG-I backbone and the lowest fraction of HG regions (Table 1). A polydispersity index (Mw/Mn) value below 1.2 indicates a narrow weight distribution, while a value above this suggests a wide weight distribution [42]. AP and FAPP both had similar Mw/Mn values, but FAP had the smallest value, suggesting that among the samples, it had a narrower distribution of polymer fractions with a more uniform Mw distribution. The processing conditions, such as drying, vinegar fermentation (incubation temperature and duration), and pectin extraction, were all the same; therefore, the difference observed in the Mw distribution can be related to the properties of the raw material used for the pectin extraction.
The fermented pectins had a smoother structure across all images (Figure 4) compared to AP. FAP exhibited a less smooth surface, whereas FAPP displayed a more homogeneous and smoother surface with slight wrinkling.

3.4. TG/DTG and DSC Analysis

The thermal behavior of the three pectin samples, AP, FAP, and FAPP, was studied using DSC, DTG, and TG. The TG/DTG curves shown in Figure 5 exhibited a similar shape with three distinct sections at 50–190 °C, 190–400 °C, and 400–600 °C, consistent with previous studies [43,44]. During heating, both free and bound water evaporated within the 25–200 °C temperature range. In this initial stage, the weight loss followed the pattern FAPP > AP > FAP. In the second stage, pectin decomposed, involving the breakdown of hydroxyl and methoxyl groups as well as disruption of functional groups and chemical bonds. Significant degradation of the GalA chains occurred, while decarboxylation of side groups and carbon ring structures led to gas release and solid char formation [45]. The AP, FAP, and FAPP samples lost 67%, 73%, and 70% of their weight, respectively at this stage. In the third temperature range (400–600 °C), which corresponded to a slow mass loss, the remaining pectin content was 24.9%, 20.6%, and 23.5% for AP, FAP, and FAPP, respectively. The solid char, which included polyaromatic structures grafted with aliphatic and ketonic groups, underwent partial disintegration and compact stacking as the pyrolytic temperature increased [43].
The DTG curve (Figure 5), derived from the TG signal, indicates both thermal stability and weight loss rates [46]. All samples showed three mass-loss events. The first endothermic peak (around 75–80 °C) corresponded to water evaporation. The second and third peaks, observed at 243 °C for AP and FAPP and 237 °C for FAP, with a total mass loss of 9.82%, 5.98%, and 11.4%, respectively, signified pectin chain degradation [36]. Compared to the other samples, FAP had a lower peak temperature but a smaller peak height. A low onset temperature on the DTG graph indicates material instability, and the peak height represents the maximum degradation velocity. In the third region, peaks were observed in the same region, but with higher peak heights in AP and FAPP, suggesting higher maximum degradation velocities. The different patterns observed for the different pectin samples in DTG curves may be attributed to variations in DM. FAP, with a lower DM, exhibited a unique DTG curve pattern. Einhorn Stoll et al. and Liang et al. [46,47] stated that thermal stability appeared to have a correlation with DM. The formation of hydrogen bonds by –COOH in pectin was thought to enhance thermal degradation.
The DSC thermogram showed that AP, FAP, and FAPP had endothermic peaks at 110.4 °C, 108.9 °C, and 91.1 °C, respectively (Figure 6). The slight shift toward a lower temperature for FAPP may be due to higher water content and a conformational change. The endothermic peaks might be a result of hydrogen bonding between GalA units, water presence, or a conformational shift causing the galacturonan to change from its compact conformation to a more stretched conformation [43,48]. Moreover, each sample showed two exothermic peaks at similar temperatures. The first exothermic peaks for AP, FAP, and FAPP occurred at 251 °C, 246 °C, and 250 °C, respectively. The second exothermic peak appeared at 313 °C for AP and 311 °C for FAP and FAPP. Those peaks could be attributed to the fracture of certain bonds or functional groups, structural depolymerization, and chain-breaking within the polysaccharide [45].

3.5. Rheological Analysis

Figure 7 shows the flow behaviors of AP, FAP, and FAPP at different concentrations (5%, 7%, and 10%, w/v). All of the pectin solution viscosities declined at all concentrations when the shear rate increased, indicating shear-thinning fluid behaviors in aqueous medium due to disentanglement in the polymer chain. The Ostwald de Waele model was employed to compute the results (Equation (1)), yielding an r2 of 0.99 (Table 2). As expected, the consistency coefficient increased with increasing concentration. The presence of higher polymer concentrations led to an increase in hydrogen bonding, causing pectin molecules to bind together more tightly. This restricted movement leads to higher viscosity and shear-thinning behavior [49]. As seen in Figure 7, FAPP provided a higher viscosity and more pronounced shear-thinning flow behavior than AP and FAP. The differences in viscosity between the pectin samples could be primarily caused by variations in their GalA content and Mw [50]. FAPP had a larger Mw but lower GalA content than other samples, causing noticeably higher viscosity behavior even at lower concentrations. Wang et al. [51] also reported that pectin with a lower GalA content but a larger Mw yielded a higher viscosity. As Mw decreases, the number of junction zones per molecule decreases, resulting in a weaker gel due to reduced cross-linking [52]. In the current study, we noted that the pectin obtained from different residues subjected to vinegar fermentation (FAP and FAPP) exhibited distinct behaviors compared to the pectin extracted from the unfermented raw material (AP). FAP had the lowest viscosity at all concentrations, while FAPP had a significantly higher viscosity. This hierarchy could be due to the difference in their enzyme secretion during fermentation. It is possible that during initial fermentation, the yeasts proliferated more effectively on FAP (soluble sugar-rich source), leading to increased enzyme secretion. The PME enzyme could potentially have disrupted the methyl ester bonds in FAP, leading to changes in rheological characteristics. Moreover, the sugar might have led to increased alcohol production by yeasts, potentially inactivating enzymes further in fermentation [8]. The decreased length of the side chains of FAP could limit its ability to form hydrogen bonds and create a three-dimensional framework. Similarly, Schmelter et al. [53] discovered that decreasing side chains and breaking up the pectin’s main chain caused the viscosity to decrease.

3.5.1. Temperature Sweep Test

The η70 value of all samples at different concentrations decreased with an increase in temperature (Figure 7), which can be attributed to thermal expansion. Viscosity decreases as molecules move farther apart, and less energy is needed to break down the structure at higher temperatures due to the reduced intermolecular forces [54,55]. High r2 indicates that the η70 values in this investigation were correctly predicted by Equation (2) (Table 3). The activation energy (Ea) was used to describe how viscosity responded to temperature changes. At 5% (w/v) concentration, Ea was similar in AP and FAPP and lower in FAPP (Table 3). However, as the concentration increased, the Ea of AP and FAP were similar, while that of FAPP remained at lower levels. When the concentration increased from 5% to 10% (w/v), the Ea of FAPP decreased from 194 to 157.
The decrease in pectin concentration resulted in an increase in Ea, indicating that temperature has a more profound effect at lower concentrations. For FAP, the Ea remained constant as the concentration increased. However, for AP, the Ea was consistent for the 5% and 7% (w/v) concentrations but decreased when the concentration reached 10% (w/v).

3.5.2. Dynamic Viscoelastic Analysis

The viscoelastic characteristics of the pectin samples at various concentrations were examined as a function of frequency using the frequency sweep test, which revealed a relationship between the angular frequency and the modulus (G′ and G″) of the pectin samples. The G′ and G″ findings for each concentration in Table 4 were compiled using the power low model (Equation (3)). The findings revealed that an increase in all pectin concentrations resulted in higher K′ and K″ values, indicating a more pronounced viscoelastic structure.
Furthermore, at all concentrations, the loss modulus (G″) of each pectin sample exceeded the storage modulus (G′) within the angular velocity range of 0.1–100 rad/s, indicating a dominant liquid-like viscous character over the elastic behavior. In agreement with the steady flow properties, G′ and G″ values were higher in FAPP than in AP and FAP at the same concentration (Figure 8). At a 5% concentration, the K′ values were 0.20, 0.02, and 0.31, while the K″ values were 0.94, 0.43, and 2.32 for AP, FAP, and FAPP, respectively. This trend was similarly observed at other concentrations. The observation may be clarified by pointing out that the application of vinegar fermentation to apple pomace resulted in an increase in Mw and neutral sugar side chains, which, in turn, led to a rise in pectin chain mutual entanglement and an increase in G′ and G″ [51]. A frequency sweep test allows for the observation of a material’s rheological properties at different shear rates: high frequencies corresponding to short times and low frequencies to long times [56,57]. As a result, the pectin extracted from vinegar-fermented apple pomace appeared to possess greater elasticity as the angular frequency increased (specifically, in high-shear-rate situations), which could be attributed to the stronger network formation in FAPP.

4. Conclusions

Our findings showed that vinegar fermentation and the state of the raw material play a significant role in modifying the physicochemical and structural properties of the apple pectin that was extracted. Vinegar fermentation increased the pectin extraction yield while reducing the protein content. When apple slices were used as the raw material for vinegar fermentation, the pectin (FAP) that was extracted from the residues had a lower DM value and less thermal stability, yielding a slightly higher proportion of the HG region. Whereas compared to the pectin from unfermented apple pomace (AP), vinegar fermentation decreased the linearity of the pectin (FAPP) that was extracted from the residues of vinegar-fermented apple pomace, resulting in a smaller proportion of the HG region. The molecular weight distribution of the samples revealed that the elution of two peaks and their mass fractions were changed in pectins extracted from fermented sources. In FAPP, low-molecular-weight components became dominant, and the average molecular weight of the sample increased. FAPP showed a greater viscosity and a more noticeable shear-thinning behavior with higher G′ and G″ values than those of other samples. At all concentrations, FAP displayed the lowest viscosity characteristic. Thus, FAP and FAPP could be employed for food applications that require different viscoelastic behaviors. Considering that the vinegar fermentation in this study was carried out by spontaneous fermentation, future studies could explore how pectin characteristics would be affected under controlled conditions, particularly utilizing different starter cultures. Furthermore, the attributes of pectin could be investigated through the process of vinegar fermentation, utilizing alternative sources and by-products.

Author Contributions

A.M.C.: Investigation, Methodology, Writing-original draft preparation; R.M.Y.: Methodology, Writing—review and editing; A.K.: Conceptualization, Validation, Resources, Supervision, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflicts of Interest

The authors confirm that they have no conflicts of interest to declare for this publication.

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Fermentation 10 00556 g001
Figure 1. FTIR spectra (A), XRD patterns (B) of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 1. FTIR spectra (A), XRD patterns (B) of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g002
Figure 2. 1H NMR and 13C NMR spectra of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 2. 1H NMR and 13C NMR spectra of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g003
Figure 3. Molecular weight distribution of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 3. Molecular weight distribution of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g004
Figure 4. Photographs and SEM images of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 4. Photographs and SEM images of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g005
Figure 5. TG/DTG curves of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 5. TG/DTG curves of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g006
Figure 6. DSC thermograms of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 6. DSC thermograms of samples (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g007
Figure 7. The viscosity curves and temperature dependency of η70 values of samples at different concentrations (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 7. The viscosity curves and temperature dependency of η70 values of samples at different concentrations (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Fermentation 10 00556 g008
Figure 8. Dynamic rheological properties of samples at different concentrations (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
Figure 8. Dynamic rheological properties of samples at different concentrations (AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace).
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Table 1. The composition and physicochemical properties of pectin samples.
Table 1. The composition and physicochemical properties of pectin samples.
ParametersUnitAPFAPFAPP
Yield%23.2 ± 1.31 b32.4 ± 1.95 a30.2 ± 2.10 a
DM 1 57.2 ± 0.56 a50 ± 0.11 b57.9 ± 0.6 a
DM 258.5 ± 0.10 a55.2 ± 0.20 b58.5 ± 0.10 a
Protein contentg/100 g dw1.25 ± 0.21 a0.76 ± 0.08 b0.70 ± 0.02 b
Total phenolic contentmg GAE
/100 g dw		1.10 ± 0.02 b1.36 ± 0.07 a0.97 ± 0.17 b
Monosaccharide composition
Mannose (Man)mol%0.25 ± 0.02 c0.89 ± 0.00 a0.83 ± 0.01 b
Rhamnose (Rha)2.00 ± 0.01 b2.69 ± 0.13 c2.34 ± 0.03 a
Glucuronic acid (GlcA)0.03 ± 0.01 c0.39 ± 0.02 a0.12 ± 0.01 b
Galacturonic acid (GalA)72.2 ± 0.03 b80.1 ± 0.57 a61.9 ± 0.05 c
Galactose (Gal)15.9 ± 0.02 b6.43 ± 0.07 c16.8 ± 0.08 a
Xylose (Xyl)0.69 ± 0.01 b0.37 ± 0.02 c1.70 ± 0.14 a
Arabinose (Ara)7.89 ± 0.01 c8.84 ± 0.44 b14.2 ± 0.12 a
Fucose (Fuc)1.06 ± 0.08 b0.29 ± 0.02 c2.10 ± 0.04 a
Homogalacturonan (HG)70.4 ± 0.19 b77.4 ± 0.71 a59.6 ± 0.08 c
Rhamnogalacturonan I (RG-I)27.9 ± 0.09 b20.7 ± 0.64 c35.6 ± 0.26 a
GalA/(Fuc + Rha + Ara + Gal + Xyl)2.62 ± 0.01 b4.30 ± 0.16 a1.67 ± 0.01 c
(Gal + Ara)/Rha11.9 ± 0.03 a5.68 ± 0.14 c13.2 ± 0.10 b
Rha/GalA 0.03 ± 0.00 b0.03 ± 00 b0.04 ± 0.00 a
Color
L* 72.2 ± 1.39 b88.1 ± 0.63 a73 ± 0.28 b
a* 6.63 ± 0.22 a2.86 ± 0.31 b6.59 ± 0.08 a
b* 17.7 ± 0.31 b12.2 ± 0.41 c21.1 ± 0.51 a
Data are expressed as mean ± S.D. The results were given as dry weight (dw). Means with different letters in the same row are significantly different (p < 0.05). DM (degree of methylation): Degree of methylation measured by titration 1, and by FTIR 2. GAE: gallic acid equivalent, HG (%) = GalA (mol%) − Rha (mol%); RG-I = 2Rha (mol%) + Ara (mol%) + Gal (mol%). AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace.
Table 2. Coefficient of determination (r2), consistency coefficient (K), and flow behavior index (n) values of pectin solutions at different concentrations determined by the Ostwald de Waele model.
Table 2. Coefficient of determination (r2), consistency coefficient (K), and flow behavior index (n) values of pectin solutions at different concentrations determined by the Ostwald de Waele model.
Concentration (w/v)Sampler2K (Pa.sn)n
5%AP0.991.05 ± 0.01 Bc0.84 ± 0.006 Aa
FAP0.990.51 ± 0.01 Cc0.85 ± 0.010 Aa
FAPP0.992.50 ± 0.06 Ac0.78 ± 0.004 Ba
7%AP0.994.21 ± 0.10 Bb0.76 ± 0.007 Ab
FAP0.992.31 ± 0.05 Cb0.74 ± 0.01 Bb
FAPP0.9911.9 ± 0.27 Ab0.66 ± 0.005 Cb
10%AP0.9935.8 ± 0.31 Ba0.53 ± 0.003 Bc
FAP0.9920.1 ± 0.10 Ca0.54 ± 0.004 Ac
FAPP0.9978.4 ± 1.24 Aa0.45 ± 0.006 Cc
Data are expressed as mean ± S.D. Different superscript uppercase letters show significant differences between the samples in the same concentration. Different superscript lowercase letters show significant differences of the samples according to the concentration (p < 0.05). AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace.
Table 3. Coefficient of determination (r2), Arrhenius equation constant (Ao), and activation energy values (Ea) of pectin solutions at different concentrations determined by the Arrhenius model.
Table 3. Coefficient of determination (r2), Arrhenius equation constant (Ao), and activation energy values (Ea) of pectin solutions at different concentrations determined by the Arrhenius model.
Concentration (w/v)Sampler2AoEa (kJ/mol)
5%AP0.990.000019 Ab195 ± 0.49 Aa
FAP0.940.000232 Ab177 ± 12.50 Ab
FAPP0.990.000353 Ac194 ± 4.04 Aa
7%AP0.990.000527 Bb196 ± 1.33 Aa
FAP0.990.000220 Bb200 ± 3.98 Aa
FAPP0.990.001764 Ab182 ± 6.31 Bb
10%AP0.990.004714 Ba166 ± 5.18 Bb
FAP0.990.000839 Ca195 ± 1.99 Aab
FAPP0.990.010370 Aa157 ± 0.42 Cc
Data are expressed as mean ± S.D. Different superscript uppercase letters show significant differences between the samples in the same concentration. Different superscript lowercase letters show significant differences of the samples according to the concentration (p < 0.05). AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace.
Table 4. Elastic viscous modulus (K′) and consistency coefficient values (K″) of pectin solutions at different concentrations determined by the Power-law model.
Table 4. Elastic viscous modulus (K′) and consistency coefficient values (K″) of pectin solutions at different concentrations determined by the Power-law model.
Concentration (w/v)SamplesR′K′n′r″K″n″
5%AP0.970.20 ± 0.01 Bb0.73 ± 0.01 Cc0.990.94 ± 0.03 Bc0.87 ± 0.01 Aa
FAP0.990.02 ± 0.00 Cc1.49 ± 0.09 Aa0.990.43 ± 0.01 Cc0.88 ± 0.01 Aa
FAPP0.990.31 ± 0.01 Ac0.99 ± 0.01 Ba0.992.32 ± 0.08 Ac0.77 ± 0.01 Ba
7%AP0.990.36 ± 0.04 Cb1.17 ± 0.04 Aa0.994.35 ± 0.16 Bb0.74 ± 0.01 Ab
FAP0.990.62 ± 0.08 Bb0.84 ± 0.08 Bb0.992.33 ± 0.18 Cb0.72 ± 0.02 Ab
FAPP0.992.05 ± 0.15 Ab0.95 ± 0.02 Bb0.9910.9 ± 0.18 Ab0.65 ± 0.01 Bb
10%AP0.996.70 ± 0.26 Ba0.84 ± 0.02 Ab0.9924.7 ± 0.03 Ba0.58 ± 0.02 Bc
FAP0.995.38 ± 0.32 Ca0.76 ± 0.01 Bb0.9913.8 ± 0.08 Ca0.59 ± 0.01 Ac
FAPP0.9922.8 ± 0.54 Aa0.70 ± 0.01 Cc0.9953.2 ± 0.45 Aa0.50 ± 0.01 Cc
Data are expressed as mean ± S.D. Different superscript uppercase letters show significant differences between the samples in the same concentration. Different superscript lowercase letters show significant differences of the samples according to the concentration (p < 0.05). r′, r″: coefficient of determination values, n′, n″: flow behavior index values. AP: pectin extracted from unfermented apple pomace; FAP: pectin extracted from the residue of vinegar-fermented apple slices; FAPP: pectin extracted from the residue of vinegar-fermented apple pomace.
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Muslu Can, A.; Metin Yildirim, R.; Karadag, A. The Properties of Pectin Extracted from the Residues of Vinegar-Fermented Apple and Apple Pomace. Fermentation 2024, 10, 556. https://doi.org/10.3390/fermentation10110556

AMA Style

Muslu Can A, Metin Yildirim R, Karadag A. The Properties of Pectin Extracted from the Residues of Vinegar-Fermented Apple and Apple Pomace. Fermentation. 2024; 10(11):556. https://doi.org/10.3390/fermentation10110556

Chicago/Turabian Style

Muslu Can, Asli, Rusen Metin Yildirim, and Ayse Karadag. 2024. "The Properties of Pectin Extracted from the Residues of Vinegar-Fermented Apple and Apple Pomace" Fermentation 10, no. 11: 556. https://doi.org/10.3390/fermentation10110556

APA Style

Muslu Can, A., Metin Yildirim, R., & Karadag, A. (2024). The Properties of Pectin Extracted from the Residues of Vinegar-Fermented Apple and Apple Pomace. Fermentation, 10(11), 556. https://doi.org/10.3390/fermentation10110556

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