Rheological Characteristics of Carob Pekmez

Abstract

This study investigated the physicochemical properties and rheological behavior of carob pekmez. Rheological measurements were undertaken at 30, 40, 50, 60, and 70°C using a rotational viscometer at the shear rate of 5, 10, 20, 50, and 100 1/s. The flow characteristics of carob pekmez were described by the power law and Herschel-Bulkley models. The Herschel-Bulkley model was found to be the best to describe the rheological property with the coefficient of determination higher than 0.998. Carob pekmez exhibited a time-independent shear thickening behavior. An Arrhenius equation was used to describe the effect of temperature on viscosity and E_a value of the carob pekmez was calculated as 103.86 kJ mol⁻¹.

Keywords:

• Carob
• Pekmez
• Chemical and rheological properties
• Concentrated fruit juices

INTRODUCTION

Pekmez is one of the traditional food products in Turkey and it is commonly produced from grape and mulberry by concentration of juices up to 70–80 soluble dry matter content.[1] Fruits containing high amounts of sugar like apple, carob, plum, watermelon, apricot, sugar beet, fig, and grape are used mainly in the pekmez production. Since pekmez contains high amounts of sugar, mineral, and organic acid, it is a very important food product for human nutrition.[2,3] Pekmez is a concentrated and shelf-life extended form of fruit juice produced by boiling without the addition of sugar or other food additives.[4] Carob pekmez is produced from Ceratonia Siligua fruits. Ceratonia Siligua, located in the Mediterranean region and Southwest Asia, Carob has high content of carbohydrates (45%, sucrose at more than 30%), appreciable amounts of protein (3%) and low levels of fat (0.6%).[5] It is a natural sweetener with a flavor and appearance similar to chocolate and is often used as a chocolate or cocoa substitute.[6] Carob also can be consumed as a fresh fruit, and it is one of the fruits commonly used in pekmez production.

Pekmez has been produced with different techniques considering species of fruits used in production. For carob pekmez production, the fruits (carob) are gathered from trees and then are smashed by a mallet into granulated form using a wooden mortar. After that, the granulated carobs and water are put into open vessels in equal proportion for 3 days so that the soluble materials can be extracted. The extract is filtered in order to separate solid materials. Then, it is boiled by stirring in order to concentrate up to 72°Brix in an open vessel. The concentrated product develops various colors, ranging from light brown to dark brown, depending on the concentration process. The final product is called carob pekmez.

In the food industry, the rheological characteristic of concentrated fruit juices is a significant property in addition to chemical and physical properties.[1] Furthermore, rheological characteristics of pekmez are of particular interest in the design of flow processes, standardization, product development, quality control, and unit operations as it provides information on its physical characteristics. Viscosity, an important quality factor for providing food products with quantifiable attributes,[7] is also considered an important physical characteristic related to the quality of liquid food products.[8] There is a number of research works published about rheological[1,9,10] and chemical properties of grape pekmez[2,3,11,12,13,14] and mulberry pekmez.[1] However, research on rheological behavior of carob (Ceratonia Siligua) pekmez has not been reported until now. Therefore, the aim of this study was to characterize the rheological behavior of carob pekmez.

MATERIALS AND METHODS

The commercial carob (Ceratonia Siligua) pekmez was supplied from a local supermarket (Erzurum, Turkey), and all the chemicals used in its chemical analysis were of reagent grade.

Chemical and Physical Analyses

Total and soluble dry matter, protein, ash, pH and titratable acidity were determined according to standard method AOAC;[15] pH was determined with a ATI ORION 420A model pH meter; titratable acidity, expressed as percentage of citric acid, was determined with 0.1 N NaOH up to pH 8.1; soluble dry matter was determined with an Abbe-Zeis refractometer; protein content was determined by the Kjeldal method (Nx6.25).

Total sugars (TS), reducing sugars and sucrose (SU) were analized by the Lane-Eynon volumetric method.[16] Reducing sugar concentration was measured before inversion. Total invert sugar was determined after inversion at 67–70°C for 5 minutes by adding HCl in a ratio of 1–10 (v/v) and with the pH brought to around 7 with 4N NaOH. Sucrose was calculated by subtracting the reducing sugar concentration from the total invert sugar and multiplying the result by 0.95. Then the reducing sugar and non-reducing sugar (sucrose) concentrations were taken together as TS concentration.[16] Hydroxymethylfurfural was determined by the IFFJP[17] with a spectrophotometer (Schimadzu, UV-120–01 Model) at 550 nm. Color of carob pekmez was analyzed by measuring Hunter L (brightness; 100: white, 0: black), a (+: red; −: green) and b (+: yellow; −: blue) parameters with a colorimeter (Model CR 200, Chromometer, Minolta, Japan). For color analysis, the instrument was calibrated with a white reference tile before measurements.

Rheological Behavior

The viscosity (Pa s) of carob pekmez was determined at 30, 40, 50, 60, and 70°C using a rotational viscometer (Model Poulten RV-8, Selfe Elee Ltd., Wickford Essex SSII 8BJ, England) equipped with spindle 5 at the shear rate of 5, 10, 20, 50, and 100 1/s. Three readings were taken per sample at 30 seconds intervals. A 300 mL beaker was used for viscosity measurements and a thermostatic water bath was used to control the processing temperature within the 30–70°C ± 1°C range.

Rheological behavior of fruit juices has been described by several models. In this article, the flow curves of carob pekmez were described by power law relationship (Eq. 1)[18] and the Herschel-Bulkley model (Eq. 2):[19]

(1)

(2)

where, K is the consistency index (Pa sⁿ); γ is the shear rate (s⁻¹), n is the flow behavior index (dimensionless); σ = shear stress (Pa); and, σ₀ = yield stress (Pa).

Statistical Analysis

In this article, 5 different shear rate levels (5, 10, 20, 50, and 100 1/s) and temperatures (30, 40, 50, 60, and 70°C) were selected as experimental factors. The experiment was set up as 5 × 5 the factorial experimental design, and the analysis was carried out according to completely randomized blocks design with three replications (SPSS for Windows Release 10.01, SPSS Inc.[20]

RESULTS AND DISCUSSION

Chemical and Physical Characteristics

The chemical and physical characteristics of carob pekmez are given in Table 1. The total dry matter, soluble dry matter, titratable acidity, and pH values were, 75.90 g/100 g pekmez, 72 g/100 g pekmez, 0.60%, 5.09, respectively. Acidity in pekmez is calculated as citric acid. These results agreed with the data reported by Şimşek and Artık[21] and Yogurtçu and Kamışlı.[22] Şimşek and Artık[21] reported that carob pekmez has 75.01% total dry matter, 71.7% soluble dry matter, 0.61% titratable acidity, and 5.35 pH values. There are a number of other researches about physical and chemical characteristics of different pekmez,[1,11,14,23,24] but no information is available on the physical and chemical characteristics of carob pekmez.

Table 1 The chemical and physical characteristics of carob pekmez

Parameters	Values
Total dry matter	75.90% w/w
Soluble dry matter	72% w/w
Moisture	24.10%
Total sugar	71.20 g 100 g^−1
Invert sugar	49.10 g 100 g^−1
Sucrose	22.11 g 100 g^−1
Ash	2.48% w/w
Protein	0.33% w/w
Hydroxymethylfurfural (HMF)	21.32 mg L^−1
pH	5.09
Titratable acidity (citric acid)	0.60% w/w
Color	L: 19.35 a: +4.35 b: −6.01

As shown in Table 1, the amounts of total sugars, reducing sugars and sucrose are 71.20 g 100 g⁻¹, 49.10 g 100 g⁻¹ and 22.11 g 100 g⁻¹, respectively. Şimşek and Artık[21] also reported that carob pekmez contained 62.16%–68.79% total sugars and 40.36%–44.38% sucrose. Carob pekmez contained high amounts of total sugar which composes of approximately 70% invert.

Pekmez is rich in sugar and a good source of energy. Because of high ash and sugar content, pekmez is very important in human nutrition especially for babies, children, sportsmen, and pregnant women.[14,21] However, this pekmez contains very small contents of protein (0.33%).

In the Turkish Pekmez Standard,[25] maximum HMF content in first quality product is reported as 25 mg HMF per kg of pekmez. In this study, a low HMF content of carob pekmez was found here (viz. 21.32 mg L⁻¹). Şimşek and Artık[21] found that carob pekmez contained 4.1–7.0 mg/kg HMF: HMF is not present in fresh, untreated foods, but rapidly accumulates during the heat treatment or long-term storage in carbohydrate-rich products such as processed fruits.[26] The higher pH stimulates the Maillard reaction in high sugar and protein containing medium.[14] As can be seen in Table 1, pH value was found to be 5.09 and carob pekmez contained high amounts of sugar.

As shown in Table 1, L, a, and b values measured were 19.35, +4.35, and −6.01, respectively. Şimşek and Artık[21] also reported that carob pekmez had L, a, b values, 18.28, 0.61, 0.51, respectively. A high redness (a) value is not desired because it occurs as a result of caramelization of sugars at high temperatures or Maillard reaction which takes place between amino acids and reducing sugars present. Therefore, a low redness (a) and a high brightness (L) values indicate a good quality pekmez.[22] Browning is the most common quality problem in pekmez.[13]

Rheological Behavior

Among the rheological models applied, the suitability of Herschel-Bulkley model was superior to power law. In order to obtain the flow behavior index (n) and consistency index (k) values in power law model, Eq. (1) was analyzed by linear regression, while Eq. (2) was analyzed by non linear regression to obtain yield stress (σ ₀ ) associated with other parameters in Herschel-Bulkley model. All the rheological parameters and coefficient of determination (r ²) at different temperatures were summarized in Table 2. As shown in Table 2, Herschel-Bulkley model was the most suitable model to describe the rheological behavior of carob pekmez with high coefficient of determination at all temperatures. The r ² for the Herschel-Bulkley model ranged from 0.998 to 0.999 showing best fit of flow curves. Therefore, it can be concluded that the Carob Pekmez was a Herschel-Bulkley fluid, non-newtonian type, since the values of flow behavior index (n), a measure of the departure from Newtonian flow,[27] ranged between 0 and ∞, and k and σ ₀ values were higher than zero.[28] Comparing flow behavior indices it was found that the results were lower using Herschel-Bulkley model than power law. However, the consistency index (k) decreased as temperature increased for both models. This tendency was also obtained in other studies.[1,9,10,29,30] It should be noted that the study of viscosity and flow behavior is extremely complex. Some liquids can display a flow behavior index greater than 1, but they are rarely encountered. Penna et al.[31] found that the flow behavior index of lactic beverages ranged from 0.940 to 1.245 at downward curve for Herschel-Bulkley model.

Table 2 Rheological parameters (± SE) of carob pekmez obtained by Power law (σ = K γⁿ) and Herschel-Bulkley models (σ = σ₀ + K γⁿ) at different temperatures

	Power law			Herschel Bulkley
Temperature (°C)	Flow behavior index (n)	Consistency index [K (Pa s^n)]	r ^2	Yield value [σ0 (Pa)]	Flow behavior index (n)	Consistency index [K (Pa s^n)]	r ^2
30	1.017 ± 1.1 × 10^−5	0.786 ± 3.5 × 10^−5	1.000	0.277 ± 0.161	0.992 ± 5.8 × 10^−6	0.870 ± 0.024	0.999
40	1.056 ± 1.7 × 10^−5	0.363 ± 5.5 × 10^−5	0.999	0.519 ± 0.171	0.989 ± 1.13 × 10^−5	0.487 ± 0.026	0.999
50	1.107 ± 2.8 × 10^−5	0.217 ± 9.1 × 10^−5	0.998	0.380 ± 0.135	1.018 ± 1.27 × 10^−5	0.315 ± 0.019	0.999
60	1.391 ± 1.27 × 10^−4	0.029 ± 4.2 × 10^−4	0.975	0.449 ± 0.078	1.042 ± 1.69 × 10^−4	0.127 ± 0.010	0.999
70	1.726 ± 2.86 × 10^−4	0.006 ± 9.3 × 10^−4	0.924	0.374 ± 0.340	1.080 ± 9.47 × 10^−5	0.088 ± 0.040	0.998

The effect of the interaction between shear rate and temperature on apparent viscosity of carob pekmez was found to be statistically significant (P < 0.01), and the plot of apparent viscosity versus shear rate for carob pekmez at different temperatures is shown in Fig. 1. The apparent viscosity of carob pekmez increases with increasing shear rate (Fig. 1). As expected, the apparent viscosity of carob pekmez decreased as the temperature increased, see Fig. 1. This agrees with other research.[1,[32,33,34,35,36] This can be explained by activation energy which is necessary for moving of a molecule, and the liquid flows more easily due to higher activation energy at high temperatures.37] As the temperature increases, the degree of molecular movement increases, lowering the viscosity of the carob pekmez.

Figure 1 The flow behavior of carob pekmez at different temperatures and shear rates.

The consistency index, an indication of the viscous nature of food, can be used to describe the variation in viscosity with temperature using the Arrhenius equation.[38,39,40]

(3)

where, K is the consistency index (Pa sⁿ); K _t is the consistency index at a reference temperature (Pa sⁿ); E_a is the activation energy (J mol⁻¹); R_g is the universal gas constant (J mol⁻¹); and, T_a is the absolute temperature (K). When the logarithm of the viscosity was plotted versus the absolute temperature (1/T, where T is Kelvin), a straight line was obtained and the Arrhenius model fitted well with the experimental data and gave high r ² (0.933) value.

The value of activation energy (103.86 kJ mol⁻¹) obtained at 72° Brix for carob pekmez in our study was found higher than those reported by other authors for Gaziantep pekmez (concentrated grape juice) at 72.9°Brix (43.85 kJ mol⁻¹) Kaya and Belibagli,[10] for Mulberry pekmez at 72°Brix (17.97 kJ mol⁻¹) Sengül et al.[1] The highest Ea value indicates that carob pekmez has more sensitivity to the temperature. This value also indicates that carob pekmez has more heterogeneous texture and non-uniform particle distribution than grape and mulberry pekmez.

CONCLUSION

Carob pekmez contained high amounts of total sugar which composes of approximately 70% invert sugar. The rheological behavior of carob pekmez was found to be most adequately described by Herschel-Bulkley model. It can be concluded that the Carob Pekmez was a Herschel-Bulkley fluid, a non-newtonian shear thickening type. The effect of temperature on viscosity was described by the Arrhenius equation, with activation energy for carob pekmez at 72°Brix higher than those of grape and mulberry pekmez. This higher value indicates that carob pekmez has more heterogeneous texture and non-uniform particle distribution than grape and mulberry pekmez.

Notes

4. Gökçen, J.; Ömeroglu, S.; Ceritoglu, A. Üzümlerden elde edilen pekmez bulama, jöle, cevizli sucuk gibi tipik Türk gıda maddelerinin yapim yöntemlerinin geliştirilmesi olanaklarının araştırılması. TÜBİTAK. 1982, Marmara Bilimsel ve Endüstriyel Araştırma Ens. Gebze Yay. No. 65. Tükiye.

25. TS. Üzüm Pekmezi Standardi. TS 3792. Türk Standartlari Enstitüsü. Necatibey Caddesi, 112, Bakanliklar, Ankara, 1989.

37. Gürses, A. Bayrakçeken, S. Deneysel Fizikokimya. Atatürk Üniversitesi Yayın No: 807. K. K. Egitim Fak. Yayın No: 62. Ders Kitapları Serisi No: 48. Erzurum, Turkey, 1996.