Study of the Mechanical Properties of Gels Formulated with Pectin from Orange Peel ^†

Abstract

Pectin is a polysaccharide that is known for its gelling properties and its applications in the pharmaceutical industry. This can be divided into two structural groups, high methoxyl pectins (HMP) and low methoxyl pectins (LMP). Currently, there is little information on the properties of the orange pectin in which LPM predominates. The aim of this study was therefore to investigate the mechanical properties of gels produced with pectin isolated from orange peels. The results showed similar values to those found in the literature, except for hardness. The gels produced from the pectin could be used in the industry, the formulation varying depending on the application.

1. Introduction

Pectin is a heteropolysaccharide found in plant cell walls and is known for its gelling properties and applications in the pharmaceutical industry. The power of pectin lies in the fact that it may strongly modify the structure of a solution to generate a gelled network, as well as the fact that it is of natural origin and has numerous healthy properties, which has resulted in its increased use for the formulation of edible gels. Indeed, the combination of characteristics such as vegetable origin, functionality, safety at high concentrations, commercial availability for a wide variety of products, and ease of production and application, are some advantages of pectins over other gelling agents [1,2]. The gelling property of an edible product can be beneficial in many ways. Some typical examples can be as simple as the pleasure and relaxing texture of a smooth, gelled dessert [3]. On the other hand, gels have other technical applications, such as the intake of bitter drugs, or the in situ release of drugs for specific pharmaceutical applications [4]. In the food industry, pectins have been used in a wide variety of products including beverages, confectionery, bakery, dairy, and meat.

Recall that the central molecule of pectin is a linear chain of α(1,4)-D-galacturonic acid, occasionally interrupted by (1,2)-L-rhamnose residues. Pectins can be divided into two structural groups: high methoxyl pectins (HMPs) with a degree of esterification (DE) (or methoxylation/methylation (DM)) higher than 50%, and low methoxyl pectins (LMPs) with a DE lower than 50%. Some carboxyl groups of galacturonic acid can be substituted with amidated groups. This class of pectins are called amidated low methoxyl pectins (ALMPs) and are characterized by their degree of amidation (DA). Intrinsic factors such as DE and DA [5], the degree of polymerization (DP), and methoxylation patterns are key parameters that affect the behavior of pectins. In addition, extrinsic factors such as pectin and calcium concentration, pH, temperature, total soluble solids, different types of sugars, and metal ions, significantly impact the characteristics of a pectin-based gel. Many interactions can be anticipated when pectin molecules are used in product formulation with other molecules, such as carbohydrates and proteins. HMPs, LMPs, and ALMPs have different gelation mechanisms. According to Singhal [6], the orange has low methoxyl pectins when they are extracted at 100 °C.

The aim of this study was investigating the mechanical properties of gels produced with pectin isolated from orange peels.

2. Materials and Methods

2.1. Pectin Extract

To obtain the extract, the method of Canteri-Schemin et al. [7], with some modifications was used. The pectin was extracted with a solution of citric acid (Brand: Anedra, chemically pure) with a pH = 2.3 (measured using a Boeco pH meter, model BT500), and distilled water was used as a solvent. The process was carried out in flasks under reflux, with condensation at boiling temperature. The flasks were heated at 100 °C by electric heating mantles. The peel was added to the cold solvent at a peel/solvent ratio (Rs) of five. The initial time was considered when the solvent boiled, the total time for the process was 60 min. Agitation was achieved by the solvent moving on its own due to the boiling state. Once the processing time was completed, the extract from the exhausted peel was separated by means of a cloth filter. Filtration was carried out while hot.

Samples for analysis were taken immediately after filtering due to the low microbiological stability of the filter.

2.2. Physicochemical Determinations

2.2.1. Degree of Esterification

The degree of esterification of the pectin was determined according to the Dominiak technique [8]. Pectin samples are washed in a 60% 2-propan-ol solution containing 5% HCl, then washed with a 60% and 10% 2-propan-ol solution. Next, 0.2 g of the washed and dried material was dissolved in 100 mL deionized water and the sample is titrated with a 0.1 M NaOH solution using phenolphthalein as an indicator (the volume of the 0.1 M NaOH solution is referred to as V₁). The sample was then saponified by adding 10 mL of 1 M NaOH solution, followed by stirring for 15 min. Subsequently, 10 mL of 1 M HCl was added, and the sample was titrated again with 0.1 M NaOH until the color changes (volume $V_2$). The degree of esterification DE was calculated according to Equation (1).

$$DE=\frac{V_2}{V_1+V_2}·100$$

(1)

2.2.2. Alcohol Precipitation

The modified Ranganna [9] technique was used. The extract was mixed with 3 volumes of ethanol. It was stirred for 3 min and left to rest for 1 h. The precipitate was then separated using a cloth filter and dried to a constant weight in a vacuum oven (AHR 8601) at a temperature of 45 °C. Then, it was ground in a mill (Control Química S.A., Model MC-1). The sample was packaged in glass vials and stored in a desiccator.

2.2.3. Gel Preparation

The Ranganna [9] technique was used. Measure 425 mL of water into a previously tared beaker. Add 10 mL of the 6% sodium citrate solution and the 60% citric acid solution. The mixture was heated up to 80 °C with constant stirring. Low methoxyl pectin was mixed with 30 g of sugar and it was placed in the glass. When the mixture was warm, 25 mL of the calcium chloride solution was added. Then, the mixture was stored at 24–26 °C for 18–24 h in corresponding containers for texture measurements.

As observed in the technique, the amount of pectin and calcium chloride to be added were considered variables since they are the parameters being evaluated. The Brix degrees will always remain fixed at around 35% (those normally used in a low-calorie formulations).

2.2.4. Mechanical Properties

Texture profile analysis of pectin gels were carried out using a technique proposed by Rascón-Chú et al. [10]. The gels were formed in 6 mL glass beakers, and the TPA was obtained using a TA.XT2i texture analyzer (RHEO Stable Micro Systems, Surrey, UK). Gels were compressed at a constant speed of 1 mm/s up to a distance of 3 mm from the gel surface using a cylindrical tip of 20 mm diameter and a trigger force of 5 g.

The measurements were carried out at room temperature (25–28 °C).

2.3. Statistical Analysis

The effect of calcium and pectin concentrations was explained by Hoefler [11], which was taken into account to observe the effects in terms of the rheological behavior of the gel formed, evidencing that the maximum peaks of gel strength have been between approximately 20 and 40 mg of calcium for each gram of low methoxyl pectin.

Statistical process optimizations using RSM have been widely employed by a number of researchers [12,13]. The Box–Behnken design of RSM was used to investigate the effects of two different independent variables: pectin yield and calcium concentration. The levels of these variables were selected based on the work of Hoefler [11]. The experiments were performed in random order. ANOVA was also performed to assess whether there are significant differences between the different formulations.

The statistical design used is detailed in

Table 1. Statistical design of gelling experiences.

Calcium Concentration (mg/L)	Pectin Concentration (% w/w)
	2	2.5	3	3.5	4
20	1.1	1.2	1.3	1.4	1.5
25	2.1	2.2	2.3	2.4	2.5
30	3.1	3.2	3.3	3.4	3.5
35	4.1	4.2	4.3	4.4	4.5
40	5.1	5.2	5.3	5.4	5.5
45	6.1	6.2	6.3	6.4	6.5

.

Which yields a total of 30 different formulations. All samples were tested in triplicate with a coefficient of variation of less than 10%.

3. Results

After carrying out the analytical technique by quintuplicate to determine the degree of methoxylation, it was obtained that the extracted pectin has a degree of 32.5%, therefore it is considered to be low methoxyl (LMP), probably due to the intensity of the extraction treatment, consequently, gels must be prepared with added calcium.

Once the gels were prepared according to the technique described above, they were left to rest for 24 h at room temperature (24–28 °C). After that time, no appreciable syneresis (liquid loss) was observed.

Table 2. TPA results of pectin gels.

Cod	Hardness (g)	Adhesiveness	Cohesiveness	Elasticity	Gumminess	Chewability
1.1	38.00	−294	0.30	0.80	8.00	1.2
1.2	38.27	−295	0.32	0.82	8.20	1.1
1.3	52.97	−293	0.39	0.79	8.00	1.0
1.4	56.91	−264	0.35	0.80	8.30	1.4
1.5	57.82	−270	0.42	0.85	8.25	1.3
2.1	58.20	−268	0.38	0.15	8.20	1.3
2.2	59.04	−270	0.37	0.81	8.10	1.4
2.3	79.09	−285	0.37	0.85	8.15	1.3
2.4	84.54	−292	0.38	0.84	8.05	1.2
2.5	89.35	−300	0.41	0.85	8.08	1.5
3.1	80.05	−270	0.38	0.85	8.20	1.5
3.2	87.40	−265	0.42	0.88	8.40	1.3
3.3	93.04	−274	0.37	0.83	8.10	1.4
3.4	102.50	−286	0.40	0.89	8.35	1.5
3.5	123.20	−295	0.41	0.86	8.45	1.4
4.1	78.40	−280	0.39	0.84	8.25	1.3
4.2	84.23	−282	0.41	0.84	8.30	1.4
4.3	90.10	−289	0.39	0.82	8.31	1.5
4.4	100.30	−265	0.42	0.87	8.40	1.5
4.5	119.32	−270	0.40	0.89	8.50	1.4
5.1	49.20	−265	0.43	0.75	8.60	1.4
5.2	54.45	−246	0.35	0.81	8.90	1.2
5.3	60.15	−285	0.41	0.76	8.40	1.3
5.4	63.50	−289	0.38	0.74	8.65	1.2
5.5	70.00	−295	0.40	0.83	8.60	1.1
6.1	43.60	−290	0.35	0.69	8.80	1.2
6.2	50.80	−295	0.40	0.72	8.90	1.3
6.3	53.50	−301	0.39	0.76	8.53	1.2
6.4	56.00	−273	0.43	0.74	8.64	1.2
6.5	58.14	−270	0.45	0.76	8.70	1.1

shows the results of the TPAs produced by the texturometer for the different gel formulations.

Hardness is the only parameter that shows significant differences (p < 0.05) between the various gel formulations; the other parameters showed non-significant differences (p > 0.05), and the p values were calculated using Minitab 17 software. The values are within the order found by Pancerz et al. [14], who obtained a TPA of apple pectin gels at concentrations of 1.5% and 3%. The hardness values found here are a little higher than those of Rascón-Chu [10], however, the rest of the parameters are quite close to those obtained by these authors. Also, Urias-Orona et al. [15] determined hardness values of 3% apple pectin gels, which was very similar to those obtained in the present study. For a better understanding of the influence of the pectin and calcium concentration parameters on the hardness of the gels, a response surface, Figure 1, and a contoured surface, Figure 2, were plotted using Minitab 17 software.

4. Conclusions

The responses obtained coincide with what was anticipated in the literature. There was a region where the maximum hardness was obtained that corresponds to the interval of 30–35 mg/L of calcium. Naturally, as the pectin concentration increased, the calcium concentration was kept constant, and the hardness of the gel increased. The optimal formulation depends directly on the final use of the gel in the industry.