Evaluation of Chemical and Functional Properties of Pectin-like Polymers Extracted from Tomato Using Conventional Acid Extraction

Abstract

The present study focuses on the extraction, characterisation, and functional properties of pectin-like polymers from tomatoes. The results revealed that the highest pectin yield (35.5%) of the dry weight was extracted at pH 1, whilst the lowest yield (25.4%) was extracted at pH 3. Fourier Transform Infrared (FTIR) spectra displayed major peaks at 2900–3300 cm⁻¹ and 900–1100 cm⁻¹, which are typical of carbohydrate polymers. A compositional analysis revealed the presence of six monosaccharides (glucose, arabinose, fucose, galactose, mannose, and galacturonic acid) together with trace amounts of xylose, which are typical of pectin (or pectin-like) structures. This suggests that the pectin-like polymers have galactan and/or arabinan side chains. The emulsifying activities and stabilities were ≥50% and ≥96%, respectively. The pectin-like polymers also demonstrated notable antioxidant activities (70%) when determined using the 1-diphenyl-2-picrylhydrazyl (DPPH) assay.

1. Introduction

Tomato (Solanum lycopersicum), a member of the Solanaceae family, is rich in nutrients, proteins, phenolic compounds, dietary fibre, and vitamins A and C [1,2]. The food industry uses tomatoes to produce ketchup, tomato juice, and tomato puree [1]. Dietary fibre from tomatoes has been reported to lower the risk of cardiovascular disease, reduce blood sugar, and prevent colon cancer [2,3,4]. Dietary fibre is divided into two fractions: soluble dietary fibre (SDF) and insoluble dietary fibre (IDF). SDF consists of carbohydrates, mainly pectin and β-glucan [2]. During storage and processing of tomato products (soup, tomato juice, puree, etc.), variations in the nutritional content, quality, and bioactivity can occur due to modifications in the plant cell wall polysaccharides/pectin structures [1]. Moreover, factors such as extraction conditions, ripening stage, and seasonal variations influence pectin yield, chemical composition, and molecular weight [5,6,7]. This is therefore important to consider, as properties such as chain length, conformation, composition, molecular weight, degree of branching, and degree of methyl esterification can affect the rheological properties and subsequent performance of food products. For instance, previous studies have shown that modifications in the pectin content of tomato puree alters the viscosity [4,5,6,7,8].

Pectins are composed of regions with different chemical structures, which include the homogalacturonan (HG) region, rhamnogalacturonan I (RG-I) region, rhamnogalacturonan (RG-II), and, depending on the plant source, they may also contain xylogalacturonan (XGA) and apiogalacturonan regions [2]. Pectin consists mainly of galacturonic acid residues covalently linked via an α-(1–4) glycosidic linkage; to be classified as pectin, the galacturonic acid content should be >65%, and polysaccharides containing lower amounts are classified as pectin-like materials. Rhamnose residues attached to the pectin chain may also be covalently connected to D-galactose, L-arabinose, or D-xylose or, less commonly, with D-glucose, D-mannose, and L-fucose [3,4]. The 1,2 linked side chain, neutral sugars, and L-rhamnose residue along the pectin chain interrupt the linear structure of pectin, thereby leading to a more complex heteropolysaccharide structure [5]. The linear polymer (HG) region is also known as the smooth region, whilst the branched regions (RG-I and RG-II) are known as the hairy regions [4]. Pectin is extracted commercially from apple pomace and citrus fruits by acid extraction [6,7]; the extraction conditions (temperature, pH, and time) are usually monitored to obtain a high-quality yield of pectin. However, due to the numerous uses of pectin in the food and pharmaceutical industries, as well as their reported health benefits, different studies have been carried out recently to investigate the extraction and characterisation of pectin from novel sources. Examples of novel pectin sources include okra pods [8,9,10]; eggplants [11]; sugar beets [12,13,14]; terminalia pectin [15]; tomato waste [16]; and banana peels [17]. These studies have demonstrated the importance of a fundamental understanding of the effect extraction/processing conditions on the structure-function relationship of pectins or pectin-like polysaccharides. Therefore, the present study aims to investigate and characterise the composition of pectins or pectin-like polysaccharides extracted from tomatoes under different conditions, with the goal of understanding their structure–function relationship.

2. Materials and Methods

2.1. Materials

Tomatoes were purchased (September 2022) from a local supermarket (Sainsburys, Huddersfield, UK). They were then chopped into small pieces (approximately 1 cm³), weighed, and dried in an oven at 45 °C for 48 h prior to polysaccharide extraction. Ethanol (96% w/w), hydrochloric acid 37%, acetone ≥99.9%, and concentrated sulphuric acid were used as supplied. 1-diphenyl-2-picrylhydrazyl (DPPH), sulfamic acid 99.3%, sodium azide ≥99.5%, Bradford reagent, bovine serum albumin (BSA) 99%, sodium hydroxide, sodium tetraborate (borax) 99.0%, phenol, trifluoracetic acid, potassium sulfamate ≥99.0%, m-hydroxydiphenyl, glucose (Glc), galactose (Gal), arabinose (Ara), mannose (Man), xylose (Xyl), fucose (Fuc), galacturonic acid (GalA), glucuronic acid (GlcA), galactosamine (GalN), glucosamine (GlcN), and rhamnose (Rha) were purchased from Sigma-Aldrich (Gillingham, UK).

2.2. Tomato Polysaccharide (TP) Extraction

A conventional acid extraction method was used to extract pectin-like materials from tomatoes using various parameters (time, pH, and temperature) with modifications based on previous studies of pectin extraction [12,18] (

Table 1. Properties of the pectin-like polymers extracted from tomatoes using a full factorial design.

Sample	pH	Time (Hours)	Temperature (°C)	Yield (%AIR)	Protein (%)	GalA (wt. %)	DE (%)	DA (%)	Glc (wt. %)	Phenolics (mg GAE/g)
1	1	2	60	28.60 ± 4.81 a	12.90 ± 2.05 a	20.5 ± 0.7 a	80.55 ± 4.26 a	26.1 ± 1.1 a b	14.9 ± 2.3 a b	25.5 ± 0.2 a
2	3	2	60	32.80 ± 3.94 a	11.52 ± 3.01 a	18.1 ± 5.0 a	77.85 ± 1.53 a	36.4 ± 1.8 a	13.8 ± 1.0 b c	26.4 ± 0.2 a
3	1	4	60	30.80 ± 1.70 a	13.75 ± 3.63 a	22.6 ± 9.1 a	79.05 ± 2.11 a	24.8 ± 2.6 b	17.8 ± 0.4 a	23.9 ± 0.1 a
4	3	4	60	31.90 ± 4.70 a	12.60 ± 0.40 a	22.5 ± 2.1 a	83.45 ± 2.60 a	25.5 ± 2.8 a b	16.5 ± 0.4 a b c	26.4 ± 0.2 a
5	1	2	80	30.30 ± 3.36 a	15.50 ± 4.21 a	23.4 ± 3.7 a	82.85 ± 4.00 a	26.2 ± 5.0 a b	16.2 ± 0.4 a b c	26.0 ± 0.1 a
6	3	2	80	28.90 ± 0.10 a	11.95 ± 0.84 a	20.4 ± 3.0 a	80.90 ± 8.30 a	25.3 ± 4.5 a b	15.8 ± 1.0 a b c	28.0 ± 0.2 a
7	1	4	80	35.50 ± 3.52 a	12.25 ± 0.91 a	21.9 ± 6.4 a	74.45 ± 0.82 a	25.7 ± 5.0 a b	14.6 ± 0.4 a b c	25.3 ± 0.1 a
8	3	4	80	25.40 ± 0.61 a	11.50 ± 0.00 a	23.5 ± 2.2 a	74.50 ± 9.83 a	28.1 ± 0.5 a b	13.0 ± 4.8 c	25.2 ± 0.1 a

All extractions were performed in triplicate and presented in mean values ± SD. Means that share a letter are not significantly different. GalA: galacturonic acid; DE: degree of esterification, DA: degree of acetylation; and Glc: glucose.

). After extraction the suspensions were centrifuged at 4200 rpm for 10 min, precipitated with absolute ethanol (four volumes), and left to stand at room temperature for 1 h. The precipitate was then washed in acetone and then filtered. The precipitated pectin-like materials were freeze-dried for 48 h (Christ Alpha 2-4 LSC basic). The yield of the extracted pectin-like materials was calculated using Equation (1):

$$\mathrm{Extraction} \mathrm{yield} \mathrm{of} \mathrm{polysaccharide} (\%w/w) = {\displaystyle \frac{\mathbf{D}\mathbf{r}\mathbf{i}\mathbf{e}\mathbf{d} \mathbf{p}\mathbf{o}\mathbf{l}\mathbf{y}\mathbf{s}\mathbf{a}\mathbf{c}\mathbf{c}\mathbf{h}\mathbf{a}\mathbf{r}\mathbf{i}\mathbf{d}\mathbf{e}}{\mathbf{D}\mathbf{r}\mathbf{i}\mathbf{e}\mathbf{d} \mathbf{p}\mathbf{o}\mathbf{w}\mathbf{d}\mathbf{e}\mathbf{r} \mathbf{w}\mathbf{e}\mathbf{i}\mathbf{g}\mathbf{h}\mathbf{t}}\times 100\%}$$

(1)

2.3. Chemical Properties

The chemical properties of the extracted polysaccharide were analysed using several analytical techniques. High-performance anion exchange chromatography coupled with pulsed amperometry detector (HPAEC-PAD) was used to evaluate the neutral sugar composition of the tomatoes polysaccharides (TP). Briefly, the extracted pectin was hydrolysed using 2 M TFA at 100 °C for 4 h in a water bath. The sample was left to cool at room temperature and then placed under a stream of nitrogen at 60 °C to remove the TFA. The dried sample was dissolved in deionised water, and then the solution was filtered using a syringe filter (0.45 µm). The solution was injected into an HPAEC-PAD system (Dionex ICS-3000 HPAEC-PAD, Dionex Corporation, Sunnyvale, CA, USA), where the mobile phase used consisted of ultrapure water, NaOH (200 mM), NaOH (10 mM), and sodium acetate (1 M) in NaOH (150 mM). The flow rate was 0.3 mL/min with an injection volume of 25 µL. After each injection, a regeneration step was included. The retention times and peak areas of the standard neutral sugars (glucose, galactose, arabinose, mannose, xylose, fucose, galacturonic acid, glucuronic acid, galactosamine, glucosamine, and rhamnose) were used to determine the monosaccharide compositions of tomato polysaccharide extracts. The sulfamic colorimetric assay (sulfamic m-hydroxyphenyl) [19] was used to determine the galacturonic content of the polysaccharide samples. The Bradford assay was used to determine the total protein content of the tomato polysaccharide samples using bovine serum albumin (BSA) as a standard, and the total phenolic content of the polysaccharide was evaluated using the Folin–Ciocalteu method, using gallic acid as a standard [20], and results are reported as gallic acid equivalents (GAEs). The degree of esterification was evaluated using the titrimetric method [18,21], while the degree of acetylation was determined using the McComb and McCready method [22].

2.4. Structural Analysis

To determine the structural features of the polysaccharides extracted from tomatoes, Fourier Transform Infrared (FTIR) spectra were obtained, scanning in the range of 4000 to 500 cm⁻¹ using a Nicolet 380 FT-IR spectrometer (Thermoelectron Corporation, Waltham, MA, USA). The spectra were then used to determine the presence of proteins, double bonds, and anomeric configuration for the different polysaccharide samples. For an NMR analysis, 2 mg of polysaccharide sample was dissolved in 2 mL of deuterium oxide (D₂O) and left for 24 h; the sample was freeze-dried and then redissolved in 600 µL of D₂O and added to an NMR tube. The sample was analysed using a Bruker Neo 600 MHz and Brunker Avance (AVI) 500 MHz spectrometer (Bruker, Coventry, UK). The NMR (¹H and ¹³C NMR) analyses were carried out at 70 °C, and acetone was used as the internal standard.

2.5. Functional Properties

The functional properties of the tomato polysaccharide samples were evaluated for two key parameters: antioxidant and emulsifying activities. The antioxidant activities of tomato polysaccharides were assessed using the DPPH radical-scavenging method, as described by [23], with slight modifications. Briefly, 5 mg of the polysaccharide sample was dissolved in 5 mL of deionised water. To this mixture, 2 mL of DPPH solution (0.1 mM) in ethanol (95%) was added, and the solution was mixed thoroughly and left to stand for approximately 30 min at room temperature. A blank was also prepared using 2 mL of DPPH solution without the addition of polysaccharide. UV–Vis spectroscopy was used to measure the absorbance of the mixture at 517 nm, and the scavenging activities were measured through the following equation (Equation (2)):

$$\mathrm{DPPH} \mathrm{scavenging} \mathrm{activities} \% = [{\displaystyle \frac{(\mathit{b}\mathit{l}\mathit{a}\mathit{n}\mathit{k} \mathit{a}\mathit{b}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{a}\mathit{n}\mathit{c}\mathit{e}-\mathit{s}\mathit{a}\mathit{m}\mathit{p}\mathit{l}\mathit{e} \mathit{a}\mathit{b}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{a}\mathit{n}\mathit{c}\mathit{e})}{\mathit{b}\mathit{l}\mathit{a}\mathit{n}\mathit{k} \mathit{a}\mathit{b}\mathit{s}\mathit{o}\mathit{r}\mathit{b}\mathit{a}\mathit{n}\mathit{c}\mathit{e}}\times 100.}$$

(2)

The tomato-polysaccharide-emulsifying properties were determined using the method adapted from [11,18]. The emulsification activity (EA) was assessed at room temperature (~20 °C), and the emulsion stability (ES) was determined by storing the emulsions at room temperature and 4 °C for up to 30 days.

2.6. Statistical Analysis

The experimental data were analysed using Minitab 19 to examine the relationship between the extraction conditions (pH, time, and temperature) and the measured polysaccharide properties (yield, galacturonic acid content, protein content, and degree of methylation). The main effect plots were applied to visualise the interaction between the extraction variables and responses. Pareto plots were used to identify which extraction parameters were statistically significant (α = 0.05). A one-way analysis of variance (ANOVA) was performed using the Tukey method at a 95% confidence level to show significant difference between groups.

3. Results and Discussion

3.1. Extraction Yield

Extractions were carried out at two different pHs (1 and 3) to study the interaction between the pH of extraction and the polysaccharide yield (Figure 1A). The extraction yields are dependent on the extraction conditions (Table 1). Although, besides pH/temperature (Figure 1B), none of these parameters were statistically significant at the 95% confidence limit. The highest yield of polysaccharide (35.5%) was extracted at pH 1, 4 h, and 80 °C (Figure 1A), while the lowest yield (25.4%) was extracted at pH 3, 4 h, and 80 °C. This increase in yield is expected to be due to the strength of the acid used in the extraction process, which hydrolyses the insoluble protopectin into soluble pectin (or pectin-like) polysaccharides, thereby increasing the yield [24]. This agrees with the findings from honeydew melon [25], eggplant [11], sugar beet [12], and sugar beet pulp [26]. Furthermore, as the acidity increases (pH decreases), the cell walls of plants are digested, and the high-molecular-weight protopectin is broken down into smaller fragments, which are subsequently released from the matrix, leading to an increase in yield [11]; in contrast, at a higher pH, the yield decreases, as fewer pectin-like polysaccharides are released. Temperature also plays a major role in the extraction process: as temperature increases from 60 to 80 °C, the extraction yield decreases (Figure 1A). This may be because an increase in temperature results in the degradation of pectin-like polysaccharides into smaller molecules, which are more difficult to precipitate resulting in lower yields [27]. Among all extraction conditions, time had the smallest effect on the yield (Figure 1A).

A Pareto chart (Figure 1B) describes the interaction between the extraction conditions and the yield; this clearly shows that the interaction between the pH and temperature (AC) is the only significant factor (α = 0.05).

3.2. Chemical Characterisation

Coextracted protein varied from 11.50 to 15.50% with extraction conditions, although none of these extraction conditions were statistically significant at the 95% confidence limit (Table 1 and Figure 2F). The protein contents measured in this work are much greater than 0.9–7.5%, 1.5–5.4%, and 3.0–3.3% reported for sugar beet pulp [26], green bell peppers [18], and citrus peels [28], respectively. These differences in protein contents can be attributed to the extraction methods and the different plant sources. The neutral sugar and uronic acid analysis of tomato polysaccharides revealed the presence of six monosaccharides (Glc, Ara, Fuc, Gal, Man, and GalA), together with trace amounts of xylose. These findings suggest that galactans and arabinans are the main side chains in the structure of pectin-like polysaccharides extracted from tomatoes [29]. Rhamnose was not reported directly because, under the experimental conditions, rhamnose and arabinose co-eluted, making them difficult to distinguish; however, the methyl groups of unbranched α-1,2 linked rhamnose and branched α-1,2,4 linked rhamnose were identified in the ¹HMNR spectra at the regions 1.1–1.2 and 1.3–1.4 ppm, respectively. Similar monosaccharide compositions have previously been reported for polysaccharides extracted from tomato peel [30,31]. The presence of GalA, Ara, and Gal suggests that the extracted polysaccharides consist of pectin-like materials. Large amounts of glucose (13.0–17.8%) with small amounts of fucose (3.6–5.0%) and mannose (4.2–6.8%) were present also in the tomato extracts. The high glucose content (Figure 2C) may be attributed to cellulose and/or hemicellulose hydrolysis during the extraction process [29,32]. The detection of neutral sugars, such as xylose, mannose, and fucose, varies depending on the source material [33]. The galacturonic acid contents were determined calorimetrically and are shown in Table 1. The GalA content varies from 18.1 to 23.5% (Figure 2A), slightly lower values compared to the galacturonic acid (30.9%) extracted from tomato peel extracts [31] or tomato waste (12.2–40%) [16]. This may be attributed to different tomato varieties and the extraction conditions used [34]. Moreover, from Table 1, the GalA content was the major sugar component of the tomato extract and can be used to determine the purity of extracted pectin. To be classified as pectin, the GalA content needs to be >65%, and polysaccharides containing lower amounts are classified as pectin-like materials [35]. Therefore, the extracts from tomato should be classified as pectin-like materials. The degree of esterification of pectin-like polysaccharides from tomato ranges from 74.45 to 83.45% (Table 1), which is greater than 50% and therefore are classified as high methoxyl (HM). The DE of pectins (Figure 2B) influences their functionality (emulsification properties, solubility, and gel formation), thus calculating the degree of esterification is very important [33,36]. Knowledge of the degree of esterification of pectin is a key step in terms of gel quality and the rate of gel formation [37]. The degree of acetylation of the tomato polysaccharides varies with the extraction conditions and ranges from 23 to 36.4% (Table 1); the values are similar to data reported for sugar beet pulp pectin (16–35%) [19]. In the main effect plot for DA (Figure 2D), pH, time, and temperature had major effects, and the highest value of DA was obtained at pH 3. This implies that the DA content increased as the pH increased (low acidic strength). Highly acidic media favour the de-esterification of galacturonic unit residues, which undergo acid hydrolysis, thereby releasing acetic acid. These findings agree with data published for sugar beet pulp pectin [26]. Furthermore, increasing the temperature and time appeared to decrease the DA; again, this has also been reported for sugar beet pulp polysaccharides [19]. Therefore, hasher conditions tend to reduce the DA. Significant amounts of phenolic compounds (24–28 mg GAE/g) were coextracted with pectin during the extraction process (Figure 2E). The values were towards the lower end of the values previously reported for tomato (16–61 mg/g) [38]. Characterisation of the phenolic contents in pectin is important because of the role they play in pectin’s antioxidant properties.

3.3. FTIR Spectroscopy of Tomato Polysaccharides (TP)

FTIR was performed on the extracted materials to confirm the presence of polysaccharides by comparing the sample spectra with known polysaccharide spectra from the literature. The FTIR spectra of tomato extracts displayed peaks between (2900–3300 cm⁻¹) and (900–1100 cm⁻¹), which are typical of polysaccharides. The peak at 3218.7 cm⁻¹ indicates the O-H stretching of the polysaccharide, which is caused by intra- and intermolecular hydrogen bonding of the galacturonic polymer, while a peak at 2980.0 cm⁻¹ indicates the C-H stretching of a CH group (CH, CH₂, and CH₃) [15]. The peaks at 1028.2 cm⁻¹ and 1252.1 cm⁻¹ were assigned to the glycosidic bond (C-O) and pyranose ring (C-C) stretches, respectively [8,11,15,39,40]. Samples containing uronic acids tend to have strong peaks in the 900–1100 cm⁻¹ region of the spectrum. In this present work, intense peaks were observed in tomato polysaccharide spectra, which suggests that the tomato extract contains pectin-like materials. Moreover, a peak at approximately 1730 cm⁻¹ observed at various pectin sources indicates stretching vibration of ester carbonyl group (COOR) and, under certain circumstances, can be used to calculate the degree of esterification [41]. It is worth noting that in this present work there was no peak observed at 1730 cm⁻¹ in the tomato polysaccharide spectra, which is most likely due to the relatively low amounts of galacturonic acid. Overall, these observations indicate that these polysaccharides contain uronic acids and are most probably pectin-like polysaccharides, which agrees with the sugar composition (Section 3.2).

3.4. Antioxidant Activity (DPPH Assay)

Upon addition to the sample, the DPPH free radical accepts electrons from the samples and is reduced to form a stable complex; as a result, the purple colour of the solution changes to yellow, and the concentration of DPPH radicals is reduced. The antioxidant activities are presented in

Table 2. Values of free radicals’ scavenging effect under different extraction parameters.

Sample	Free Radicals’ Scavenging Effect of DPPH (%)
1	82.81 ± 5.32
2	73.35 ± 7.41
3	78.56 ± 5.36
4	85.03 ± 3.60
5	85.08 ± 3.72
6	71.40 ± 6.70
7	82.56 ± 1.70
8	82.76 ± 1.61

All experiments were performed in duplicate and presented as mean values ± SD. There is no significant difference between the mean values at the 95% confidence limit.

, with values ranging from 71.4 to 85.0%, which was greater than the values observed for husk tomatoes (51.87–71.84%) [33]. The free-radical-scavenging activity increases with higher polysaccharide concentrations, indicating a direct relationship between the two. The high scavenging activities can also be attributed to the polysaccharide composition and important structural features that include galacturonic content, phenolic content, degree of esterification, and protein content [11,42,43].

3.5. Emulsifying Properties

According to

Table 3. Values of emulsifying activities and emulsion stability of TP under different extraction parameters.

Sample	EA (%)	ES at 22 °C After 1 Day (%)	ES at 22 °C After 30 Days (%)	ES at 4 °C After 1 Day (%)	ES at 4 °C After 30 Days (%)
1	48.7 ± 1.8	99.0 ± 1.4	98.0 ± 2.8	98.0 ± 3.1	98.0 ± 3.5
2	48.5 ± 1.5	97.0 ± 1.3	100.0 ± 0.0	91.0 ± 9.4	100.0 ± 0.0
3	50.0 ± 0.0	97.0 ± 1.4	100.0 ± 0.0	100.0 ± 0.0	93.0 ± 4.2
4	50.0 ± 0.0	98.0 ± 0.1	100.0 ± 0.0	97.0 ± 4.2	96.0 ± 5.7
5	50.0 ± 0.0	98.0 ± 2.8	100.0 ± 0.0	96.0 ± 5.7	98.0 ± 2.8
6	48.7 ± 1.8	97.0 ± 4.2	99.0 ± 1.4	95.0 ± 1.4	98.0 ± 3.5
7	50.0 ± 0.0	97.0± 4.2	99.0 ± 1.4	100.0 ± 0.0	94.0 ± 5.7
8	50.0 ± 0.0	99.0 ± 1.6	99.0 ± 1.4	100.0 ± 0.0	97.0 ± 1.4

All experiments were performed in duplicate and presented as mean values ± SD. There is no significant difference between the mean values at the 95% confidence limit.

, the highest emulsifying activity of tomato polysaccharides was 50%. These values were higher than results published for sour orange peel (40.7%) [44] and citrus peel (46.5%) [45]. The emulsion activities observed in this present work clearly show that tomato polysaccharides can reduce the surface tension between oil-in-water or water-in-oil systems and subsequently can be relied upon in the production of emulsion. Emulsion stability was also observed at different temperatures over several days. The emulsion stability at 4 °C and at room temperature after one day were between (91–100%) and (92–99%), respectively. However, after 30 days, the emulsion stability at 4 °C and room temperature were (93–100%) and (98–100%), respectively. These values show the high degree of stability of the emulsion stored at different temperatures, and it also appears that the emulsion stability is not significantly different regardless of temperature. This differs with findings from previous publications [26,45], who reported that emulsions were more stable at 4 °C compared to those stored at a higher temperature of 22 °C. Pectin-like polysaccharides from tomatoes could therefore be used to adequately stabilise oil/water emulsion systems. The main effects plot for emulsifying activities (Figure 3) shows that the pH, extraction time, and extraction temperature only had a minor effect on the emulsifying activities. Protein coextracted with polysaccharides have been linked with emulsion activities and stability [40]. Therefore, it is worthwhile noting that, in this present work, protein (11.5–15.5%) coextracted with pectin-like polysaccharides from tomatoes could be responsible for its high emulsion stability.

4. Conclusions

Pectin-like polysaccharides were successfully extracted from tomatoes using the conventional acid extraction method with various extraction conditions (pH, time, and temperature), and the results showed that the different extraction parameters affected the extraction yield, composition, and chemical properties of the polysaccharides. The highest extraction yield (35.5%) was achieved at low pH (highly acidic medium), which agreed with previous studies [18,19,26,32,46,47,48] on pectins and pectin-like polysaccharides from other plant sources. Galactose, galacturonic acid, arabinose, fucose, mannose, and glucose were the major constituents found in tomato extracts using HPAEC, which confirmed the presence of pectin-like polysaccharides. Rhamnose was not reported as rhamnose co-eluted with arabinose, making it difficult to separate using HPAEC; however, rhamnose peaks were present at ~1.4 and 1.5 ppm [8] in the ¹H NMR spectra, and an acetyl peak at ~2.1 ppm [8] was also clear. Furthermore, the signals in the anomeric region between 5.0 and 5.4 ppm were assigned to galacturonic acid residues and were in good agreement with previously published results [8,40,41,49,50]. However, it should be noted that the spectra were only partially analysed due to the complex nature of the ¹H NMR spectra and the difficulty in dissolving these samples at the high concentrations required to obtain a good signal-to-noise ratio (Figure 4).

The FTIR confirmed major peaks at 2900–3300 cm⁻¹ and 900–1100 cm⁻¹, which are typical of polysaccharides. The GalA content of the tomato pectin ranged from 18.1 to 23.5%; therefore, these polysaccharides cannot be classified as pectin for pharmaceutical or food use since GalA was less than the 65% benchmark set by the Food and Agricultural Organisation (FAO) [11]. The DE of the extracted pectin was > 50%, and they were therefore classified as high methoxyl. In addition, the extracted pectin was found to contain substantial amounts of protein (11.5–15.5%) and phenolic compounds (23.9–28.0 mg/g). The extracted polysaccharides demonstrated promising antioxidant properties, emulsifying activities, and notable emulsifying stability. The antioxidant and emulsion activities could be attributed to the rhamnogalacturonan-I region structure [51,52,53,54], phenolic [51,53], protein [55], and acetyl contents [56], as well as the polysaccharide content. There is also the possibility that the presence of pectin-like polymers and phenolic compounds may have a synergistic effect on the antioxidant activity [53]. No relationship between arabinose and the phenolic content was observed, probably due to there only being small amounts of arabinose present. Although there were only limited differences between samples for most of the measured parameters, which traditionally could be considered as a drawback, it also suggests that polysaccharides extracted from tomatoes, like those from green bell peppers [18], are unlikely to be greatly affected by unintended changes in the processing conditions during isolation. However, this is not the case for other polysaccharide extracts, for example, polysaccharides from citrus [14,57], honeydew melon [25,48], or sugar beet [19].