Some Physical, Chemical, and Rheological Properties of Sweet Sorghum (Sorghum Bicolor (L) Moench) Pekmez (Molasses)

Abstract

Some physical, chemical properties and the rheological behaviour of the sweet sorghum (Sorghum bicolor L Moench) pekmez (molasses) were determined. The rheological behaviour of the sweet sorghum pekmez (concentrated sorghum juice) with different soluble solid contents (75.1, 72.4, 66.5, and 59.4 °Brix) was determined in the temperature range of 10, 20, 30, 40, and 50°C using a rotational viscometer equipped with spindle 5 at the speed (share rate) of 5, 10, 20, 50, and 100 rpm. An empirical power law model was used to describe the rheological behavior of the sweet sorghum pekmez with correlation coefficients (R²) between 0.922 and 0.986. The sweet sorghum pekmez exhibited a pseudoplastic behavior. An Arrhenius equation was used to describe the effect of temperature on viscosity and E_a value of the sweet sorghum pekmez was calculated as 31 350 J/mol. Depending on the soluble solids contents, the activation energies for flow of diluted samples vary from 52.27 to 24.50 kJmol⁻¹. The effect of °Brix on viscosity can be described by the power-law equation. Experimental data were fitted to power law and exponential model in order to describe the effect of temperature and soluble dry matter content. Density and Electrical conductivity were measured 1.3915 g/cm³ and 13.53 mS/cm, respectively. The color as L, a and b value were measured 19.07, + 4.0, and + 2.18, respectively.

Keywords:

• Sweet sorghum
• Pekmez
• Viscosity
• Power-low model
• Exponential model
• Arrhenius equation

INTRODUCTION

Pekmez is one of the popular and traditional food products, and it is consumed generally for breakfast in Turkey. Pekmez is commonly produced from sugar-rich fruits such as grape and mulberry by concentration of juices up to 70–80 soluble dry matter content. On the other hand, it is also produced from sugar-rich sugarbeet, sugarcane, sweet sorghum and carob. Pekmez can also be produced from sugar-rich fruits like apple, plum, watermelon, apricot and fig.[1] Pekmez contains high amounts of sugar, mineral and organic acid; therefore, it is a very important food product in human nutrition.[2,3,4] Pekmez easily passes into the blood without digestion because most of its carbohydrate is in the form of monosaccarides like glucose and fructose. This is nutritionally important, especially for babies, children, sportsmen and in situations demanding urgent energy.

Sweet sorghum (Sorghum bicolor (L) Moench) is an important plant for animal feeding and human nutrition. It is the only crop that provides grain and stem that can be used for sugar, alcohol, syrup, jaggery, fodder, fuel and chewing. The juice of a good variety of sorgo grown under suitable conditions contains 13 to 17% sugar, of which 10 to 14% is sucrose.[5]

Sorgo for syrup is cut when the seed is nearly ripe, or at least in the stiff-dough stage. Before cutting, the leaves usually are stripped from the stalks by hand by a worker using a special two-pronged tool or a lath, paddle, or pitchfork.[6] After cutting, the stalks are topped with a knife to remove the heads, peduncles, and, often, two or three of the upper internodes. For large syrup-factory operations, the whole stalks can be topped, after which the stalks are chopped into short sections mechanically. A fair quality of syrup can be made from whole stalks that have been stored as long as about 6 days before crushing.[7] The juice is expressed from the stripped cane by the crushing of the stalks between revolving fluted iron rolls in a cane mill equipped with two rolls. The strained juice that sometimes is allowed to settle for a time is piped to an boiling vessels where it is boiled down to a syrup containing about 65–80°Brix. It has various colors from light Brown to dark Brown depending on the process. During the boiling, the juice is skimmed constantly to remove floating impurities such as chlorophyll, soil, plant fragments, proteins, gums, fats, and waxes.[5] After then, sorghum syrup concentrated in open vessels is cooled up to 40°C; this product is called sweet sorghum pekmez. Finally, sorghum pekmez is packaged and stored at room temperature.

In the food industry, rheological characteristics of concentrated fruit juices are a significant property in addition to chemical and physical properties. Moreover, rheological characteristics depend on both the chemical composition of fruits and processing conditions. However, a knowledge of the flow behaviour of concentrated juices will be useful in quality control, calculating energy usage, process control and equipment selection.[8] In the food industry, viscosity is one of the most important parameters required in the design of a technological process. On the other hand, viscosity is also important factor that determines the overall quality and stability of a food system.

There is a number of research about rheological[1,4,8] and chemical properties of grape pekmez.[9,10,2,11,3] However, there is little information about rheological and chemical properties of sorghum pekmez. Therefore, the purpose of this study was to characterize the rheological behaviour of sorghum pekmez and to determine chemical and physical properties.

MATERIALS AND METHODS

Materials

Sorghum pekmez was manufactured from sweet sorghum in the pilot plant of the Department of Food Engineering, Selçuk University in Konya in Turkey. This plant was grown in the experimental fields of the Agriculture Faculty, Selçuk University. Sorghum was harvested maturating of seeds. After harvesting, the leaves of plant were removed from the stalks by hand using a paddle. After the stalks were topped with a knife to remove the heads, peduncle, whole stalks were stored as long as 6 days before crushing. The sweet sorghum juice was expressed from the stripped cane by the crushing of the stalks between revolving fluted iron plays role in a cane mill equipped with two rolls. After the juice was strained in a steel vessel, it was filtered. The filtered sorghum juice was poured into a steel-boiling vessel where it was boiled down to syrup containing about 65–76°Brix. During boiling, the juice was skimmed constantly to remove floating impurities. After then, concentrated sweet sorghum juice was cooled up to 40°C; this product is called pekmez in Turkey. Finally, sweet sorghum pekmez was packed into 200 ml glass jars. Pekmez, obtained from the sweet sorghum juice, was stored at 20°C as long as one week until analyzed.

Initially, soluble solid of sweet sorghum pekmez was 76%. It is diluted to 75.1, 72.4, 66.5, and 59.4°Brix with distilled water for measuring of viscosity and samples were uniformly homogenized in an ultrasonic bath at room temperature.

Methods

Chemical and physical analysis

Total dry matter, crude protein, ash, total oil, and crude cellulose were determined according to standard method AOAC;[12] pH, titratable acidity, total soluble dry matter were determined according to Cemeroğlu.[13] pH was determined with a WTW InoLab 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 ATAGO model refractometer; protein content was determined by the Kjeldal method (N × 6.25).

Total sugar, invert sugar and sucrose were quantitated by the Lane-Eynon method.[13] Hydroxymethylfurfural (HMF) was determined quantitatively following the procedure described by the IFJJP,[14] based on the colorimetric reaction between barbutiric acid, p-toluidin and HMF, forming a red colored complex. The intensity red color was measured at 550 nm with Shimadzu, UV-Visible 160 A model spectrophotometer

For color analysis, the instrument was calibrated with a white reference tile (No: 14533046) before measurements. Color of sorghum 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 400, Chromometer, Minolta, Japan).

Electrical conductivity of a pekmez solution at 20% (dry matter basis) in CO₂-free deionized distilled water was measured at 20°C by using a WTW InoLab (Weilheim, Germany) conductimeter, and the result was expressed as mS/cm.[15] The density of the sorghum pekmez was determined with a standardized 10 ml pycnometer.[16] The mass of the solution was calculated from the weight difference between the empty pycnometer (ILDAM, Ankara, Turkey) and the filled vessel. The pycnometer filled with pekmez was incubated at 20°C for 1 h (Venticall 222, MMM Medcenter GmbH, Müncher, Germany) to equilibrate the sample before determination.[17]

Mineral composition

For analyzing the minerals of the fruit and pekmez, about 0.5 g dried and ground mulberry fruit and pekmez sample was put into a burning cup and 15 ml pure HNO₃ were added. The sample was incinerated in a MARS 5 Microwave Oven at 200°C and dissolved ash diluted to a certain volume with water. Concentrations were determined with an Concentrations were determined by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES).[18]

Rheological Measurements

The rheological behavior of the diluted sweet sorghum pekmez samples, having total soluble dry matters of 75.1%, 72.4%, 66.5%, and 59.4%, and temperatures of 10, 20, 30, 40 and 50°C, was studied using a Brookfield rotational viscometer (Model LAB-LINE Instruments, Inc, Melrose Park, IL.) equipped with spindle 5 at the speed of 5, 10, 20, 50, and 100 rpm. In addition, rehological behaviour of diluted sample to 72.4°Brix was determined at 30, 40, 50, 60, and 70°C. Enough samples in a 500 ml beaker were used for viscosity measurements and a thermostatic water bath was used to control the processing temperature at each temperature level (± 0.5°C).

Statistical Analysis

In this study, five different speed level and temperatures were selected as experimental factors. The first factor, temperature, has 5 levels (10, 20, 30, 40, and 50°C) and second factor, speed, has 5 levels (5, 10, 20, 50, and 100 rpm). The experimental design was established in a 5 × 5 the factorial design with 3 replicates, and the analysis was carried out according to completely randomized blocks design. Temperature by speed interaction has 25 combinations (5 temperature levels × 5 speed levels). All determinations were made in triplicate, unless otherwise specified.[19]

RESULTS AND DISCUSSION

Chemical and Physical Characteristics

Some chemical and physical properties of sorghum pekmez are given in Table 1. As shown in Table 1, sorghum pekmez contained high amounts of total sugar, which is composed of approximately 91% invert sugar as glucose and fructose. This is very important in human nutrition because of its easy digestibility and these sugars provide e readily available energy source since they easily pass to blood.[20] In addition, glucose, energy of the brain, enhances the transport of tryptophan through the blood-brain barrier and it is useful in seratonin synthesis that has a function in brain working.[21] However, this pekmez contains very much contents of sucrose and protein according to other mulberry and grape pekmez.[4]

Table 1 The physico-chemical characteristics of sorghum pekmez (n = 3)

Parameters	Mean ± SD	Parameters	Mean ± SD
Total dry matter (%)	78 ± 2	HMF (mg/l)	15.3 ± 1.0
°Brix	72.4 ± 0.4	PH	5.9 ± 0.0
Total sugar (%)	70.6 ± 1.2	Titratable acidity (%)	0.68 ± 0.01
Reducing sugar (%)	64.3 ± 2.2	Color	L: 19.07 ± 0.15
Sucrose (%)	6.0 ± 0.5		a: + 4.02 ± 0.05
Crude cellulose (%)	1.3 ± 0.1		b: + 2.18 ± 0.06
Crude protein∗ (%)	2.1 ± 0.1	Density (g/cm^3)	1.3915 ± 0.0024
Total Oil (%)	0.95 ± 0.09	Electrical Conductivity (mS/cm)	13.53 ± 0.12
Ash (%)	4.6 ± 0.1
∗ N × 6.25.

HMF, an indicator of quality deterioration, occurs as a result of excessive heating in foods containing carbohydrates.[22] Therefore, high amounts of HMF are not desired in processed concentrated syrup. In the Turkish Pekmez Standard,[23] maximum HMF for first quality product is declared as 75 mg/l. In this study, HMF content of sorghum pekmez was found low (15.3 mg/l).

As shown in Table 1, L, a and b values were measured 19.07, + 4.0, and + 2.18, respectively. A high redness (a) value is not desired because it occurs as a result of excessive caramelization of sugars. Therefore, a low redness (a) and a high brightness (L) values indicate a good quality pekmez.[24]

The electrical conductivity of the sweet sorghum pekmez is closely related to the concentration of mineral salts, organic acids and proteins; it is a parameter that shows great variability according to the origin and is considered one of the best parameters for differentiating between pekmez with different origins.[25,26] This parameter is shown in Table 1. The electrical conductivity of sweet sorghum pekmez (13.53 mS/cm) was found to be higher than honey (0,395 mS/cm).[27]

Mineral Composition

The mineral contents of pekmez were given in Table 2. All materials contained high amounts of Ca, K, Mg, Na, P, S, and Zn. These values were found as 682.9 ppm, 13,932 ppm, 402.5 ppm, 69.5 ppm, 30.9 ppm, 140.3 ppm, and 70.6 ppm respectively. Batu,[9] established that K, Mg, P, S, and Ca reported as major elements in grape pekmez samples. These mineral contents of sweet sorghum pekmez were found higher than that of Batu.[9] Minerals contribute to biological process, but which have not been established as essential, are bromine and lithium.[28] Lithium is another element with beneficial pharmacological properties; it has been used effectively in the treatment of manic depressive disorders. There is evidence to suggest that lithium is also an essential element.[29] However, knowledge of their mineral contents of vegetable materials is very important for human nutrition.

Table 2 Elemental composition of sweet sorghum pekmez

Elements	ppm	Elements	ppm
Al	4.2 ± 0.7	Li	0.60 ± 0.08
B	97.0 ± 6.6	Mg	402.5 ± 79.5
Ba	1.2 ± 0.0	Mn	6.7 ± 0.3
Ca	682.9 ± 124.3	Na	69.5 ± 7.7
Co	0.35 ± 0.04	Ni	0.15 ± 0.08
Cr	0.11 ± 0.01	P	30.9 ± 1.8
Cu	0.82 ± 0.15	S	140.3 ± 34.5
Fe	19.1 ± 2.5	Sr	2.02 ± 0.37
K	13933 ± 716	Zn	70.6 ± 18.5

Rheological behavior of pekmez samples

Several rheological models have been employed to fit data on fruit juices. The rheological behavior of sorghum pekmez was described by the power law model:[30,31]

(1)

where η_a is the viscosity (Pa s), γ is share rate (s⁻¹), K the consistency index (pa s ⁿ ) and n is the flow behaviour index (dimensionless). Linear regression analysis was performed on the data to find K, n and correlation coefficient r ², the results of which are summarized in Table 3 and Table 5. The power-law model appears to be suitable for describing the flow behavior of sweet sorghum pekmez as indicated by high r ² values. Values of n and K ranged from 0.945 to 0.969, 0.3159 to 1.3143, respectively. The sweet sorghum pekmez (°Brix = 72.4) exhibited a pseudoplastic behavior in the range of 30–70°C and 10–50°C because the values of flow behaviour index (n), a measure of the departure from Newtonian flow,[30] were less than 1.[32,33] However, the consistency index (K) decreased as temperature increased. Results were also found similar when compared with other studies.[1,8,4]

Table 3 The parameters of power law model for the sorghum pekmez at different temperatures (°Brix = 72.4)

Temperature (°C)	n	K (Pa s^n)	r ^2
30	0.95	1.3143	0.97
40	0.95	0.7765	0.97
50	0.95	0.5399	0.99
60	0.97	0.3656	0.92
70	0.96	0.3159	0.92

Table 5 Soluble dry matters, consistency index, flow behavior index and activation energy for diluted samples

Soluble dry matter (°Brix)	T(°C)	K (mPa s^n)	n	r ^2	E a (kJmol^−1)	k o	r ^2
75.1	10	11736	0.80	1.00	52.27	2.44 × 10^−6	0.99
	20	4567	0.87	0.99
	30	2541	0.92	0.99
	40	1172	0.98	0.96
	50	754	0.95	0.92
72.4	10	4581	0.93	0.99	47.97	6.77 × 10^−6	0.99
	20	2376	0.95	0.99
	30	1314	0.95	0.97
	40	777	0.95	0.97
	50	340	0.95	0.99
66.5	10	1143	0.97	0.99	42.43	1.58 × 10^−5	0.99
	20	572	0.96	0.92
	30	304	0.94	0.97
	40	175	0.94	0.89
	50	146	0.94	0.89
59.4	10	349	0.95	0.99	24.50	1.09 × 10^−2	0.99
	20	268	0.96	0.99
	30	191	0.94	0.98
	40	124	0.95	0.98
	50	86	0.96	0.97

Data over the temperature range 30–70°C showed that the flow behavior index was affected by temperature. ANOVA results show that viscosity changes in temperature by speed combinations show significant differences (P < 0.001), and the plot of the viscosity versus speed for sweet sorghum pekmez was at different temperatures (10–70°C) is shown in Fig. 1. The viscosity of liquids generally decreases as temperature increase. As shown in Fig. 1, as temperature and speed increased, the viscosity of sorghum pekmez decreased. Similar observations had been reported by several researchers.[34,31,33,8,4] An activation energy is necessary for moving of a molecule, and as the temperature increases, the liquid flows more easily due to higher an activation energy in high temperatures.[35] In addition, Rha[36] noted that the decrease in viscosity with increasing speed (share rate) is related to the increasing alignment of constituent molecules. The consistency index, an indication of the viscous nature, can be used to describe the variation in viscosity with temperature using the Arrhenius equation.[32,37,38]

(2)

Figure 1 Effect of speed on the viscosity of sorghum pekmez at different temperatures.

where K is the consistency index (Pa s ⁿ ); k _o is the Arrhenius constant (Pa s ⁿ ); E _a is the activation energy (J/mol); R _g is the universal gas constant (J/mol); and T _a is the absolute temperature (K). The mean absolute percentage error was calculated to analyze the deviance of observed value from the calculated value.[39]

(3)

where Y_o represent observed values; Y_c the calculated values; and n the number of pairs. Applying the linear regression analysis in Eq. (2)(2), values of E _a and k _o were calculated; the Arrhenius equation parameters with correlation coefficients (r ²), and mean absolute percentage error are given Table 3. According to data in Tables 4 and 5, Arrhenius model is suitable for sweet sorghum pekmez. The value of activation energy 31.35 kJmol⁻¹ obtained at 72.4°Brix and temperatures the range 30–70°C for sweet sorghum pekmez in our study was higher compared with results reported by other authors for mulberry pekmez at 72°Brix (17.97 kJmol⁻¹).[4]

Table 4 The parameters of Arrhenius equation

Soluble dry matter (%)	k 0(Pa s^n)	E a(kJmol^−1)	r ^2
72.4	4.8 × 10^−3	31.35	0.98	1.6

The value of flow behavior index at temperatures the range 10–50°C was less than 1 in each case, implying to pseudoplastic nature of sweet sorghum pekmez and are consistent with earlier findings for other pekmez samples.[4,8,40] The flow behavior index (n) showed a increasing trend with increase in temperature at soluble solid concentration of 75.1, 72.4, 66.5, and 59.4, but it decreased at concentration of 66.5%. However, increase in the concentration resulted into decrease in the values of flow behavior index, signifying increase in the pseudoplasticity of pekmez.[40]

A decrease in the consistency index was observed with increase in temperature at all levels of soluble solid concentration, which meant a decrease in the apparent viscosity of sweet sorghum pekmez, with increase in temperature. The results are given in Table 5 and have correlation coefficients higher than 0.89. As can be shown in Table 5, the consistency index of the sweet sorghum pekmez varied considerably with temperature and concentration. Table 6 shows the effect of concentration on viscosity for different temperatures. On the other hand, the correlation coefficients were obtained to be around the unity, which means the agreement between experimental data and the proposed mathematical model is good.

Table 6 Effect of soluble dry matters on the viscosity of different concentration of samples at different temperatures

T (°C)	Model: K = k 1 (C^a ^1)			Model: K = k 2 exp (a 2 C)
	k 1 (mPa s)	a 1	r ^2	k 2 (mPa s)	a 2 (Brix^−1)	r ^2
10	2.86 × 10^−27	14.66	0.97	1.0018	1.98	0.99
20	5.77 × 10^−23	12.16	0.95	1.0039	1.77	0.97
30	2.49 × 10^−21	11.14	0.91	1.0041	1.74	0.92
40	1.53 × 10^−19	10.03	0.89	1.0041	1.72	0.91
50	1.37 × 10^−17	8.86	0.92	1.0053	1.64	0.94

At higher temperatures the viscosity decreases and at higher concentration the viscosity increases. The effect of temperature on the flow behavior of samples was described by the Arrhenius relationships. The parameters of this equation are given in Table 5. The activation energy values increase with the soluble solid concentration. It can be observed that the effect of temperature in decreasing the viscosity of samples is more pronounced at higher concentration. This tendency is similar to that other clarified juices.[8,38,41,42]

The influence of concentration

The variation of viscosity with soluble dry matter contents can be described by various models.[38,42] In general, these models are power-law and exponential-type models:

(4)

(5)

where k ₁, k ₂, a ₁ and a ₂ are constants and C is the concentration in °Brix. The values of viscosity in Table 5 were made suitable for linear forms of the above equations by using the least-squares method in order to calculate different parametres of the equations. Table 6 presents the values of the parameters of the power-law and exponential relationships. Statistical analysis explains that the exponential model seems to describe better the effect of the soluble dry matters on the viscosity of diluted samples.

For a fixed temperature, activation energy for flow depends on the soluble dry matter contents. The variation of activation energy with concentration can be described by several models.[8] In this study two models were used: the power-law and the exponential models.

(6)

(7)

where A ₁, A ₂, B ₁ and B ₂ are constants. The values of E _a and their respective concentrations were fitted to Eqs. (6)(6) and (7)(7) by the least-squares methods to obtain the estimates of the parameters of the model. The calculated parameters for these models are given in Table 7. The dependency of E _a on the soluble solid content was better described by the power-law model.

Table 7 Effect of the soluble dry matters on the activation energy

Model	A	B	r ^2
Power-low
E a = A 1 C^B ^1	6.11 × 10^−5	3.17	0.93
Exponential
E a = A 2 exp(B2C)	1.09	0.89	0.92

Combined effect of temperature and concentration

It is very useful to acquire a simple equation describing the combined effect of temperature and concentration on pekmez viscosity for engineering applications. From the results obtained in the preceding section, the following models have been used:

(8)

(9)

where k ₃, k ₄, D ₁ and D ₂ are constants of the models. The equations were made linear to obtain their parameters. The values of these constants are given in Table 8. It can be observed that E _a values are the same in both of the models. Although it is suitable for two of the models, model 1 describes the influence of temperature with soluble solid materials better than model 2. So, a single equation Eq. (10)(10) is proposed to describe the viscosity of different concentrations of sweet sorghum pekmez:

(10)

(11)

Table 8 Combined effect of concentration and temperature on viscosity

Model	Equation	i	ki	J	Di	E a (kJmol^−1)	r ^2
1	8	3	3.27 × 10^−10	1	0.17	41.96	0.95	4.05
2	9	4	6.63 × 10^−26	2	11.33	41.96	0.94	4.39

CONCLUSIONS

Sweet sorghum pekmez contained high amounts of total sugar, which is composed of approximately 91% invert sugar as glucose and fructose. This is especially important in human nutrition. Sweet sorghum pekmez (75.1°Brix) was found to exhibit non-Newtonian behavior. The power-law model had an excellent fit to describe the type of the sweet sorghum pekmez. Viscosity of sweet sorghum pekmez decreased with increasing temperature as expected. The activation energy values increased with increasing soluble dry matter concentration. The effect of temperature on pekmez viscosity was described by the Arrhenius equation.

NOMENCLATURE

a 1	=	Constant of Eq. (4)(4) (dimensionless)
a 2	=	Constant of Eq. (5)(5) (°Brix−1)
A 1	=	Constant of Eq. (6)(6) (°Brix−1)
A 2	=	Constant of Eq. (7)(7) (kJ/mol)
B 1	=	Constant of Eq. (6)(6) (dimensionless)
B 2	=	Constant of Eq. (7)(7) (°Brix−1)
C	=	Soluble solid contents (°Brix−1)
D 1	=	Constant of Eq. (8)(8) (°Brix−1)
D 2	=	Constant of Eq. (9)(9) (dimensionless)
E a	=	Activation energy (kJ/mol)
K	=	Consistency index (Pa s n )
R g	=	Gas constant (J/mol)
T a	=	Temperature (K)
ηa	=	Viscosity (Pa s or mPa s)
k 0	=	Arrhenius constant (Pa s n )
k 1	=	Constant of Eq. (4)(4) (mPa s n )
k 2	=	Constant of Eq. (5)(5) (mPa s n )
k 3	=	Constant of Eq. (8)(8) (mPa s n )
k 4	=	Constant of Eq. (9)(9) (mPa s n )
n	=	Flow behavior index (dimensionless)
γ	=	Shear rate (s−1)
	=	Mean absolute percentage error