Rheology of Commercial and Model Borojó Jam Formulations

Abstract

The rheological behavior of model and commercial borojó (Borojoa patinoi Cuatrec.) jams was analysed as a function of temperature (5–60°C). Model borojó jam formulations were prepared by modifying pectin concentration. Both small-amplitude oscillatory shear and steady flow measurements were performed. Rheological results were also compared with those obtained with a traditional commercial peach jam. The Herschel-Bulkley model was used to describe the steady flow behaviour. In general, all rheological parameters increased with pectin concentration and decreased with temperature. Arrhenius-type relationships were used to quantify the influence of temperature on several viscoelastic and viscous flow parameters. Model borojó jam formulations exhibited higher temperature dependence of linear viscoelastic functions than peach jam samples but lower than borojó commercial jam. The influence of temperature on the yield stress was more important in model borojó formulations than in both commercial jams.

Keywords:

• Borojó
• Jam
• Pectin
• Rheology
• Temperature
• Viscoelasticity

INTRODUCTION

Borojoa patinoi Cuatrec. is an understory tree, i.e., cultivated under a shelter wood system, member of the Rubiaceae family, and restricted to tropical climates, especially in humid areas, mainly growing in Panamá, Colombia, and Ecuador. The cultivation system involves low production costs and conserves much of the biodiversity of the natural forest. Borojó is an exotic and apple-sized fruit with some medicinal and aphrodisiacal properties, which command a high price in the local markets and is traditionally used in drinks and other handcrafted preparations.[1 ⁻ 3] The Borojó fruit weighs around 750 g, between 250 and 1000 g, and consists of pulp, seed, and peal. Often the pulp and seed represent around 83 and 10%, respectively. The pulp is acid, dense, and contains a significant level of carbohydrates, calcium, and soluble solids. Although borojó pulp is currently used to produce processed products like juices, jams, compotes, candies, ice creams, or energy drinks,[3] the commercialization of such products is very limited to the native countries and their physicochemical, rheological, or textural properties are scarcely investigated. In this respect, recent works[4,5] have explored the possibility of using the spray-drying and freeze-drying techniques to open up new alternatives for their commercialization in international markets, avoiding some difficulties in handling, transport, and packing.

The rheological, textural, and sensory properties of jam formulations manufactured from other much more universally popular and accepted tropical fruits like pineapple and mango have been reported.[6 ⁻ 9] The manufacture of borojó jam with suitable physicochemical, sensory, and rheological properties and dissemination of these properties is a way to increase the appraisal of this fruit in other markets. Jam is an intermediate moisture food product obtained by cooking the fruit ingredient mixed with sugar and other ingredients (preservative, colouring, or flavouring agents) and concentrating to get the right consistency, thick enough to hold the tissues.[10] Jam must contain no less than 65% soluble solids.[11] Pectin, sugar, acids, and water are present in approximately the following proportions: 0.5–1% added pectin, 50–75% sugar, 1% acid, and 33–38% water (on fruit pulp basis).[12] One of the factors affecting jam's quality is the pulp content. For instance, in Ecuador, the legislation requires that jam formulations should be made with at least 45% wt. of original fruit.[11]

Rheology of complex food products plays an important role not only in quality control, determining in many cases the consumer acceptance, but also in the design of unit operations like pumping, mixing, stirring, evaporation, heat exchange, etc.[13,14] In this sense, a full rheological characterization of such formulations must cover both the viscous flow behaviour and linear viscoelasticity.[15 ⁻ 17] Jam can be considered a gel-like solid-in-liquid system with a structure that immobilizes the liquid and confers viscoelastic properties.[17] However, fracture occurrence under large deformations is not clearly detected like in real gels (gelatin, jelly, surimi, etc.). The flow behaviour of most conventional fruit jams has been widely investigated in the past, especially regarding the modeling of the shear stress-shear rate relationship.[8,18,19] However, less attention has been paid on the time-dependent flow behaviour[7] and linear viscoelastic properties.[9] From this existing literature, it can be deduced that the most important factors affecting the rheology of jam formulations are some compositional parameters and interactions among components like pectin and sugar contents and temperature. In this work, the linear viscoelastic response and flow behaviour of commercial and model borojó jam formulations were explored by means of small-amplitude oscillatory shear (SAOS) and steady-state flow measurements, respectively, by analyzing the influences of pectin concentration and temperature. Results were also compared to those obtained with a traditional commercial peach jam.

MATERIALS AND METHODS

Materials and Preparation of Model Jam Formulations

Commercial borojó jam was supplied by Gamboinos factory (Francisco de Orellana, Ecuador). Commercial peach jam (Helios, Spain) was purchased in a local supermarket. Model borojó jam formulations were prepared using borojó pulp previously characterized,[20] commercial grade sugar purchased in a local supermarket, and high-methoxyl pectin, quick set, 153 USA SAG, Unipectine RS 150 (Cargill Texturizing Solutions, France). Borojó fruits were obtained from Sandrita farm (Los Ríos, Ecuador). Physicochemical characteristics of borojó pulp employed are included in Table 1.

Table 1  Physicochemical data of original borojó pulp and jam formulations studied

		Jam formulations
Parameter	Borojó pulp	Commercial peach jam	Commercial borojó jam	Borojó jam (0.25% pectin)	Borojó jam (0.50% pectin)	Borojó jam (0.75% pectin)	Borojó jam (1.0% pectin)
Moisture (%)	69.41	38.15	28.23	28.03	32.49	32.18	32.74
Ashes (%)	0.73	0.34	0.39	0.33	0.30	0.35	0.32
Soluble solids (°Brix at 20°C)	32.0	68	72	67	65	67	68
pH	2.96	3.16	3.16	3.10	3.11	3.10	3.14
Acidity (g of malic acid/100 g)	2.61	3.24	3.11	3.08	3.09	3.10	3.08

Model borojó jams were prepared using a conventional procedure. The amount of sugar to be added (45:55 pulp/sugar ratio) and the amount of water to be evaporated was calculated according to the Ecuadorian norm[11] to reach at least 65 °Brix and 50% pulp in the final product. Borojo pulp used in this work contains 2.6% of pectin (wet basis), and concentrations of high-methoxyl pectin added were 0.25, 0.50, 0.75, and 1% wt. Model jam formulations were prepared on the basis of 250 g of borojó pulp. Initially, 90% of the pulp was mixed at 80°C with only 10% of the total sugar calculated to facilitate evaporation. Pectin was then added before reaching 25 °Brix, in order to favour complete dissolution, previously blended with sucrose in a 1:5 weight ratio, while stirring vigorously. Evaporation was continued at a temperature not exceeding 90°C until 35 °Brix, during 6–10 min at atmospheric pressure. At this time, the rest of the sucrose was added rapidly reaching 65–67 °Brix and then the remaining 10% of borojó pulp was added. Borojó pulp acidity (Table 1) ensures a pH value lower than 3.3 during jam preparation and, therefore, further pH adjustment was not required. Afterwards, borojó jams were packed in 250-mL cylindrical jars at 88°C under vacuum, which were then cooled down to room temperature (25°C) and stored at 4°C.

Physicochemical Characterization

Physicochemical analyses of jam samples studied were performed in duplicate, using standard methods.[21] The physicochemical variables studied were soluble solids, moisture, ashes, pH, and acidity. Soluble solids content was determined by means of refractometric measurements, at 20°C, in a refractometer (model Abbe, Atago Co. Ltd., Japan), according to the AOAC method 932.12. Moisture was determined in a Thermo Scientific Heraeus vacuum oven, model VT6130 M (Germany), at 70°C and 100 mm Hg, for 24 h (AOAC method 920.151). pH was determined at 25°C in an Orion pH meter (Research Inc., Boston, MA, USA), according to the AOAC method 981.12. The acidity of jam samples was determined by titration and expressed in grams of malic acid per 100 g of jam (AOAC method 942.15). Ash content was determined in a muffle furnace, model Neyo M-525 (Division Barkmeyer, Yucaipa, CA, USA), at 525°C, according to the AOAC method 940.26.

Rheological Characterization

Small-amplitude oscillatory shear (SAOS) tests were conducted in a controlled-stress rheometer (model Rheoscope, ThermoHaake, Germany), within the linear viscoelastic region using a plate-plate geometry (35 mm diameter, 1 mm gap) in a frequency range of 5·10⁻³–16 Hz. Linear viscoelasticity range was previously determined by means of stress sweep tests at 1 Hz. Viscous flow tests were performed in a Physica MCR-501 rheometer (Anton Paar, Austria), which allows a better controlled-rate operation mode (0.1 μN m torque sensitivity), by applying a stepped-shear rate ramp in a range of shear rates of 10⁻²–10² s⁻¹, using parallel plates (diameter: 25 mm, 1 and 2 mm gaps, depending on sample consistency) with grooved surfaces to overcome wall slip phenomena usually observed in colloidal food systems.[22,23] Figure 1a shows the comparison of viscous flow curves obtained using both the controlled-rate and the controlled-stress modes for a selected sample. As can be observed, the controlled-rate mode allows the consecution of a more uniform distribution of shear stress (or apparent viscosity) data in the whole shear rate range studied, which cannot be obtained in the controlled-stress mode as a consequence of the marked yielding characteristics of the samples studied. Of course, the mechanical spectra obtained in SAOS measurements, inside the linear viscoelastic range, are not influenced by the type of rheometer or operation mode employed (Fig. 1b). All rheological measurements were carried out at temperatures comprised between 0 and 60°C. At least two replicates of each test were performed on fresh samples.

Figure 1 Comparison of measurements carried out using both the controlled-rate (physica rheometer) and the controlled-stress (rheoscope rheometer) modes for a selected model borojó jam containing 1.00% added pectin sample. (a) Viscous flow curves; (b) SAOS tests.

RESULTS AND DISCUSSION

Physicochemical Parameters

Table 1 collects the values of parameters deduced from the physicochemical analysis of the different jam formulations studied. Physicochemical properties of borojó jams were very similar to those of both peach and borojó commercial jams. All model jams were processed to reach a soluble solid content close to 65 °Brix, which is basically the same obtained for the commercial peach jam but slightly lower than that measured in the commercial borojó jam. On the contrary, peach jam showed the highest moisture content.

SAOS Tests

Figure 2 shows the evolution of the storage (G′) and loss (G″) moduli with frequency within the linear viscoelastic region for model borojó jam samples formulated with different added pectin concentration at three selected temperatures. These formulations are compared with commercial borojó and peach jams. In all cases, the typical gel-like behaviour with G′ values higher than G″ in all the frequency range studied is apparent.[15] A power-law evolution (with approximate constant slope in log-log plots) with frequency was found for G′, whereas a tendency to reach constant values at low frequencies, i.e., a plateau region, was detected for G″. The same behaviour was reported for mango jams containing sucrose or sorbitol.[9] Therefore, neither these viscoelastic functions nor G* are susceptible to be described through power-law models as previously proposed[9,24,25] in the whole frequency range. Besides, SAOS functions increased with added pectin concentration. Differences of more than one decade were found by modifying the added pectin content from 0.25 to 1.0%, especially at low temperatures. Moreover, the frequency dependence of both moduli for model formulations is approximately the same to that found in the commercial borojó jam, with very similar values of SAOS functions in the case of jam formulation containing 0.5% pectin at 5°C. However, jam containing 0.25% pectin is more similar to the commercial sample above 25°C. This means that the temperature effect is greater for the commercial borojó jam. A rather different behaviour was shown by the commercial peach jam, which exhibited a much lower frequency dependence of both SAOS functions. Thus, for instance, G′ values for this jam are similar to those obtained with model borojó jam formulation containing 0.5% pectin in the low frequency range but are similar to those shown by the borojó jam with 0.25% pectin in the high frequency range, independently of the temperature (Fig. 2). Moreover, the relative elasticity is slightly affected by the pectin content. As can be observed in Fig. 3, the loss tangent (tan δ = G″/G′) generally decreased with increasing the pectin content, especially at high frequencies. However, all model formulations exhibited tan δ values comprised between those found in both commercial formulations analysed.

Figure 2 Frequency dependence of SAOS functions for the different jam formulations studied at selected temperatures.

Figure 3 Evolution of the loss tangent with frequency for the different jam formulations studied at 25°C.

The effect of temperature on some rheological parameters of different jams has been previously reported,[7,8,19] although mainly focusing on the viscous flow behaviour. From Fig. 2, a decrease in the SAOS functions by increasing temperature can be detected. As previously mentioned, the temperature effect seems to be different for model and commercial borojó jams. Temperature influence is much better illustrated in Figs. 4 and 5 for a model (0.75% added pectin) and commercial borojó jams, respectively. As can be seen, the linear viscoelastic functions of commercial borojó jam are more affected with temperature than observed for the model formulation. On the contrary, a very low thermal dependence was found for the commercial peach jam. The G′ value at 1 rad/s was selected to quantify the influence of temperature by using an Arrhenius-type equation:

(1)

where E_a,G′ is a parameter that evaluates the thermal dependence, similar to the activation energy (J/mol), R is the gas constant (8.314 J/mol K), T is the absolute temperature (K), and A_G′ is the pre-exponential factor (Pa). Equation (1) describes well the experimental G′ values, in the whole temperature range studied for most of the samples (R ² > 0.989, Fig. 6). However, model formulations with the lower pectin contents (0.25 and 0.5%) deviate from the Arrhenius evolution above 45°C, exhibiting a higher thermal dependence. Table 2 collects the activation energy values for all the samples studied. As previously discussed, the commercial borojó jam shows significantly higher E_a,G values than the rest of the samples analysed. The lowest E_a,G value was obtained for the model formulation containing the lowest amount of added pectin, followed by the commercial peach jam. In general, the thermal dependence of both G′ and G″ functions seems to slightly increase with pectin content. However, the loss tangent of model borojó jam samples slightly increased with temperature when added pectin concentration was higher than 0.75%, but, on the contrary, significantly decreased by increasing temperature, especially at low frequencies, for lower pectin concentrations (Fig. 7). This result can be explained attending to the effect of pectin concentration on biopolymer thermal-induced gelling behaviour. As Tsoga et al.[26] discussed, during heating pectin gels a balance between the formation of hydrophobic association and dissociation of hydrophilic junctions occurs. This balance is highly dependent on pectin concentration and pH, as described by Agoub et al.[27] Thus, for low pectin concentrations, or pH above pK_o , the least favourable conditions to intermolecular associations, an increase in temperature yields an initial decrease in G″ down to around 80°C whereas G′ remains almost constant.[27,28] The same effect on tan δ was observed at low frequencies for commercial borojó jam, although, in this case, this tendency was reversed at high frequencies (Fig. 7c), which indicates a change in G′ and G″ frequency dependence with temperature. In this sense, in this commercial formulation, a tendency to a crossover between both SAOS functions can be clearly observed at high temperature (Fig. 2c). On the other hand, the commercial peach jam presented much lower values of the loss tangent and almost unaffected with temperature, similarly to that found in borojó jams with high pectin contents.

Table 2  Activation energy values obtained from the fittings to Eqs. (1), (3), and (4) for the different jam formulations studied

Sample	Ea,G′ (kJ/mol)	E a,τ (kJ/mol)	Ea,k (kJ/mol)
Borojó jam (0.25% pectin)	9.04	23.37	29.08
Borojó jam (0.50% pectin)	16.45	32.26	27.88
Borojó jam (0.75% pectin)	22.94	32.73	26.87
Borojó jam (1.00% pectin)	17.14	27.86	30.27
Commercial borojó jam	35.92	18.65	24.07
Commercial peach jam	12.19	7.84	31.78

Figure 4 Frequency dependence of SAOS functions as a function of temperature for a model borojó jam containing 0.75% added pectin.

Figure 5 Frequency dependence of SAOS functions as a function of temperature for the commercial borojó jam formulation.

Figure 6 Variation of G′ values at 1 rad/s with temperature and Arrhenius’ fittings (solid lines) for the different jam formulations studied.

Figure 7 Evolution of the loss tangent with frequency as a function of temperature for selected borojó jam formulations.

Viscous Flow Behaviour

As previously reported,[8,18,19] jams typically display a shear-thinning behaviour associated to a tendency to reach a yield stress value. This flow behaviour has been frequently expressed in terms of the Herschel-Bulkley model:[29]

(2)

where τ is the stress, is the shear rate, τ _o is the yield stress, and k and n are consistency and flow indexes, respectively. Figure 8 shows the stress versus shear rate plots for the different borojó jam samples studied at 25°C, and fittings to the Herschel-Bulkley model (R ² > 0.994). The Herschel-Bulkley parameters, as a function of temperature, are collected in Table 3. As can be observed in Fig. 8, the flow behaviour of model borojó jams is qualitatively similar to that found with the commercial borojó jam and stress values increased with pectin concentration, as expected. Moreover, stress values of commercial borojó jam were more similar to those obtained using low pectin concentrations (0.25 and 0.5% added pectin). The reference peach jam also displayed similar stress values at low and moderate shear rates but deviated from them at high shear rates as a consequence of a lower flow index (Table 3). Thus, as can be seen in Table 3, τ _o values of both commercial jam formulations are generally comprised between the values obtained for model borojó jams containing 0.25 and 0.5% added pectin. Likewise, k values are also comparable to these model jams. On the contrary, much higher τ _o values, and viscosity values in the entire shear rate range studied, were found for model borojó jams including 0.75 or 1.0% added pectin concentration.

Table 3  Herschel-Bulkley parameters for the different jam formulations studied as a function of temperature

Sample	Temperature (°C)	τ0 (Pa)	k (Pas^n)	n
Borojó jam (0.25% pectin)	5	114.42	61.07	0.64
	15	51.52	54.54	0.60
	25	38.20	42.10	0.58
	35	28.70	25.98	0.63
	45	25.84	14.26	0.68
	60	18.79	8.67	0.72
Borojó jam (0.50% pectin)	5	275.24	68.53	0.84
	15	176.82	47.56	0.82
	25	105.06	32.89	0.83
	35	69.11	20.04	0.83
	45	46.65	14.58	0.79
	60	28.34	10.01	0.76
Borojó jam (0.75% pectin)	5	1012.19	96.24	0.71
	15	647.86	55.64	0.67
	25	449.15	34.31	0.72
	35	299.16	32.26	0.71
	45	173.62	19.74	0.78
	60	98.43	13.16	0.75
Borojó jam (1.00% pectin)	5	991.90	155.58	0.61
	15	666.74	123.41	0.61
	25	459.68	116.27	0.56
	35	332.66	95.41	0.55
	45	180.11	38.75	0.65
	60	128.26	16.89	0.70
Commercial borojó jam	5	93.75	80.45	0.66
	15	56.54	64.06	0.65
	25	44.77	57.43	0.63
	35	37.79	50.90	0.62
	45	29.39	22.10	0.59
	60	23.23	14.74	0.63
Commercial peach jam	5	90.00	49.72	0.40
	15	58.16	35.03	0.39
	25	57.69	28.52	0.39
	35	53.53	11.37	0.60
	45	48.69	8.68	0.59
	60	47.30	6.07	0.56

Figure 8 Viscous flow curves and Herschel-Bulkley's fittings (solid lines) for the different jam formulations studied at 25°C.

Besides this, as illustrated in Fig. 9, for a selected model jam formulation apparent viscosity (or stress) values decreased with increasing temperature but, however, the shear rate dependence was not significantly modified. Hence, as can be observed in Table 3, τ _o and k values decreased with increasing temperature but, however, n did not shown any systematic variation with temperature. Similarly, to that proposed above for G′, two Arrhenius-type relationships (R ² > 0.895) were used to quantify the influence of temperature on both τ _o and k parameters (Fig. 10):

(3)

(4)

where, in this case, E_{a ,τ} and E_a,k are the activation energies (J/mol) for the yield stress and consistency index, respectively, and A _τ and A_k are the pre-exponential parameters (Pa). The activation energy values obtained from these fittings are listed in Table 2. Similarly, to that previously discussed for linear viscoelasticity functions, the lower E_{a ,τ} value for model formulations was obtained for the sample containing the lowest amount of added pectin, comparable in this case to that found for the commercial borojó jam. However, not very different E_{a ,τ} values were obtained for formulations with higher pectin concentration. The lowest E_{a ,τ} value was displayed by the commercial peach jam, indicating a low thermal susceptibility in this parameter. Nevertheless, no important differences were obtained in E_{a , k} values obtained from the fitting to Eq. (4) for all the samples studied (Table 2).

Figure 9 Viscous flow curves and Herschel-Bulkley's fittings (solid lines) as a function of temperature for a model borojó jam containing 0.50% added pectin.

Figure 10 Variation of (a) the yield stress and (b) the consistency index with temperature, and Arrhenius’ fittings (solid lines) for the different jam formulations studied.

CONCLUSIONS

The rheological behaviour of borojó (Borojoa patinoi Cuatrec.) jams was studied as a function of temperature (5–60°C) and added pectin content (0.25–1.0% wt.). SAOS response reveals a typical gel-like behaviour, whereas the viscous flow can be fairly well described by the Herschel-Bulkley model. In general, linear viscoelastic functions and viscosity values increased with pectin concentration and decreased with increasing temperature. Model jam formulations containing low pectin concentrations (0.25–0.5% wt.) are more similar to both borojó and peach commercial samples used as references. Thus, the yield stress values for both commercial jam formulations are generally comprised between the values obtained for model borojó jams containing 0.25 and 0.5% added pectin. However, commercial peach jam exhibited a much lower frequency dependence of both SAOS functions than borojó jams. Arrhenius-type relationships were used to quantify the influence of temperature on the storage modulus (G′), yield stress (τ _o ), and consistency index (k). Model borojó jam formulations show higher temperature dependence of linear viscoelastic functions than peach jam sample but lower than borojó commercial jam. On the contrary, the influence of temperature on the yield stress is lower for both commercial jams, whereas the effect on the consistency index is very similar for all jam samples studied.