Prediction of Adulteration in Honey Using Rheological Parameters

Abstract

Honey is a sweet substance that can be adulterated easily with inexpensive sweeteners by making syrup. Water being the second major constituent of honey, makes its flow behavior one of the important parameter for quality determination. In the present study, the rheological characteristics of honey samples adulterated with different concentrations of jaggery syrup were studied using rotational rheometer with parallel plate geometry. The viscosity of honey and adulterated honey samples was determined by varying temperature and percentage of jaggery concentration. All the adulterated honey samples behaved as non-Newtonian fluid. The viscosity increased linearly with increase in concentration of adulteration and varied from 2.48–4.80 Pa s as adulteration increased from 5 to 30%. Oscillatory tests were performed to find the possible effect of storage time on different honey samples. It was found that adulteration decreased the shelf life of honey, thus rheology can be considered one of the important parameters to determine adulteration of honey.

Keywords:

• Honey
• Rheology
• Viscosity
• Activation Energy, Brix

INTRODUCTION

Honey is the natural sweet substance produced by honey bees from nectar of plants or from secretions of living parts of plants or excretions of plant sucking insects on the living parts of plants.^[1] Honey has been used as medicinal remedy from ancient times and even now it has been reported to exhibit antibacterial and antiviral activity^[2−4] and used for the cure of ulcers, bed sores, and other skin infections resulting from burns.^[5,6] Honey is treated as an important dietary food product worldwide. It is a premium and expensive food product. In order to increase its quantity to make more profit, pure honey is adulterated with inexpensive adulterants such as sugar syrup, beet invert syrup, corn syrup, and jaggery syrup, etc. These adulterants increase the mass of honey but they decrease nutritional properties, thus degrading its quality.

Several studies have been done to detect adulteration in honey using different techniques. Spectroscopic methods such as Fourier transform infrared (FTIR) spectroscopy was studied for the detection of adulteration in honey using inverted cane sugar, corn syrup, and inverted beet syrup.^[7−9] Addition of adulterants in honey such as jaggery was detected using near infrared spectroscopy and was able to predict the percentage adulteration in it.^[10]

Quality analysis of honey using isotopic ratio technique was done to detect adulteration of sucrose solution and high fructose corn syrup (HFCS) by determining ¹³C/¹²C ratio of carbohydrates and protein of the honey.^[11] Oligosaccharides profile was studied to detect adulteration of corn syrups and HFCS using high-performance anion-exchange chromatography (HPEAC).^[12]

Addition of these adulterants also affects the sugar and water content of honey. As honey is primarily composed of sugars such as fructose and glucose, the change in composition profile by adulteration effectuates its viscosity and thus, rheology. Many researchers have studied rheology of honey to determine the botanical origin, temperature effect, etc. Viscosity determination of sugar added honey samples with the help of a model was developed to predict the viscosity of honey based on sugar composition.^[13]

The rheological behaviour of pure honey mixed with propolis was studied and it was observed that viscosity of pure honey was reduced by the addition of propolis extract.^[14] Pseudo plastic behavior of Galician honey at 25°C was observed at low values of shear rate.^[15] Honey samples adulterated with starch, molasses, glucose, and distilled water has shown variation in their physico-chemical and rheological properties.^[16] Several studies have been done showing different honey samples following Newtonian law of Viscosity and Viscosity variation with temperature.^[17,18]

A lot of work has been done on the viscosity-temperature relationship of honey and different models were developed to explain its characteristics.^[19−21] Four models namely Williams-Landel-Ferry (WLF), Arrhenius, Vogel-Tammann-Fulcher (VTF), and power-law were investigated and it was observed that the WLF model was suitable for modeling the rheological behavior of different honey.^[22] An oscillatory test which is very much significant in rheology of honey has shown that viscous component predominates over elastic component.^[23] Also some of the modern oscillatory measurements showed that crystallization of honey is dependent on botanical origin, temperature, and storage time.^[24]

Among all the adulterants, jaggery is commonly used in North India mainly due to its physical appearance which is very much like multi-floral honey. The present study aims to study the rheological characteristics of honey samples adulterated with jaggery at different concentrations. Jaggery is commonly available in North India which is also known as Gur. It consists of 30–35 g/100 g of water, 40–60 g/100 g of sucrose, 15–25 g/100 g of invert sugars and trace elements.^[25] Samples under study were prepared by mixing honey with jaggery syrup. Brix value of syrup was kept same as honey sample.

EXPERIMENTAL METHOD AND ANALYSIS

Sample Preparation

Multifloral raw honey was collected from Punjab region (India) and was stored at ambient temperature for a month before it was used for the study. Jaggery used for the experiments had pH 5.11, Brix 77.73 and a refractive index 1.48203. The jaggery syrup for adulteration was prepared by mixing jaggery and distilled water in the ratio of 80:20 at 50°C. The mixture was stirred and sonicated continuously for 5 h. A total of six samples were prepared by adding the jaggery syrup to the honey in different concentration (w/w) varying from 5 to 30% with an increment of 5%. Then the samples were placed in water bath at 50°C for 2 h and sonicated for 5 h to produce a homogeneous solution. The prepared samples were brought at room temperature before analysis.

Rheological Measurement

Viscosity of honey samples was measured using Bohlin rheometer CVO 100 (Malvern Instruments Ltd.). Parallel plate geometry was used with a plate having radius of 20 mm for all the measurements. The gap between the plates was set to 1000 μm. All the rheological measurements were performed at 25°C. For each test, approximately 2–3 g of the sample was placed on the bottom plate. Two types of tests were performed namely viscometry and oscillation. The flow behavior of honey was studied by measuring the viscosity at 25°C and with temperature gradient of 5 to 30°C at a constant stress of 10 Pa. The viscosity of the honey samples along with the adulterated samples was determined as a function of temperature using Arrhenius equation.

(1)

where, η_o is the pre exponential factor (i.e., value of viscosity as T approaches infinity); E_a the activation energy; R the universal gas constant; and T the absolute temperature in Kelvin. A linear plot of natural logarithm of viscosity (ln η) versus inverse of temperature (1/T) was used to obtain the activation energy of all the samples. Oscillatory measurements were performed to get the values of storage modulus (G’) and loss modulus (G”). For amplitude sweep, a controlled stress was applied from 0.1 to 80 Pa at a constant frequency of 1 Hz. The linear viscoelastic region was selected for performing frequency sweep test. Frequency was varied from 0.1 to 40 Hz by applying a constant stress of 10 Pa and strain of 0.409. Oscillation tests were carried out at a constant temperature of 25°C. The rheological measurements were repeated twice and Bohlin software was used to obtain the rheological parameters. Water content of honey was determined by measuring its refractive index using the Refractrometer (RFM 840, Bellingham and Stanley, UK) at a constant temperature of 20°C and was converted to 25°C by adding correction factor of 0.0023/°C.^[26]

RESULTS AND DISCUSSION

The variation in moisture content with respect to concentration of adulteration is shown in Fig. 1. As concentration of jaggery syrup increases there is a linear reduction in moisture content of the honey samples. The moisture content ranges from 19.7 to 21.6 g/100 g. The addition of jaggery syrup to honey increases its solid content because of the presence of sucrose and invert sugars that results in a decrease in the moisture content. Figure 2a illustrates the viscosity of different honey samples adulterated with jaggery syrup at a constant stress of 10 Pa at 298 K for initial 300 s. The viscosity was measured as a function of time. It was observed that there was a slight increase in viscosity as time increases showing time dependency. The rheopectic behavior of honey and jaggery adulterated honey samples were observed from the above graph indicating non-Newtonian behavior. It was found that viscosity increases with increase in percentage adulteration and the increase shows a linear behavior. As viscosity is related to the flow behaviour hence decrease in the moisture content due to adulteration results in the increase in viscosity as shown in Fig. 2b.

FIGURE 1 Moisture content of samples having different concentration of adulteration of jaggery.

FIGURE 2 (a) Variation of viscosity with respect to time for honey and adulterated samples at constant shear stress of 10 Pa; (b) Viscosity of samples having different concentration of adulteration.

Figure 3 shows steady shear rheograms of honey samples adulterated with different concentrations of jaggery syrup at 298 K. The shear rate was varied from 0–20 per second and corresponding stress values were obtained. It is seen from the graph that pure honey shows Newtonian behavior as listed in literature for Chinese honey, Greek Honey, light and dark colored honey, and Indian honey.^[27−30] It is clear from the figure that all the adulterated samples behave as Bingham plastics and its flow behavior can be explained by Bingham model:

(2)

where, σ is shear stress; σ₀ is yield stress; η is the viscosity and γ is the shear rate. At constant stress of 10 Pa and at 298 K, viscosity decreases exponentially with the varying shear rate (Fig. 4). It was observed that higher shear rate is required for the viscous flow in case of raw honey whereas lower shear rate for adulterated honey. Honey adulterated with 30% of jaggery syrup has more viscosity as compared to raw honey sample.

FIGURE 3 Rheograms of different honey samples adulterated with jaggery syrup.

FIGURE 4 Viscosity versus shear rate plot showing the effect of adulteration on viscosity.

Figure 5a shows that apparent viscosity decreases with temperature and the temperature dependence of viscosity follows the Arrhenius model. Viscosity of all the samples were measured by varying the temperature of the samples from 273–303 K. As the temperature of honey is raised, there is a gradual decrease in viscosity.^[31–^33] Beyond 295 K, the change in viscosity is very slow. As the temperature rises, viscosity decreases because particles in honey gain enough energy to overcome the intermolecular forces of attraction, thus decreasing the cohesive forces. Decrease in these cohesive forces reduces the viscosity in the honey. All the samples can be easily differentiated at lower temperatures from 273–295 K and showed similar behavior at higher temperature. It was also observed that viscosity increases with increase in adulterant concentration at a particular temperature, but this variation is not seen at high temperature.

FIGURE 5 (a) Viscosity versus temperature relationship in the range 273–303 K (0–30°C); (b) Arrhenius plot for various adulterated samples.

The activation energy of honey samples was calculated from Eq. (1) by plotting the graph between ln η and 1/T as shown in Fig. 5b. The calculated values of activation energy at different concentration of adulteration are given in Table 1. There is an increase in activation energy with increase in adulteration, because more energy is required by the particles of the honey samples to overcome the viscous forces. This confirms that increase in the concentration of jaggery syrup increases the viscosity.

TABLE 1 The activation energies of the jaggery adulterated honey samples

S.No.	Sample % of jaggery adulteration	Activation energy (KJ/mol)
1.	0	35.48
2.	5	36.07
3.	10	36.66
4.	20	37.00
5.	25	37.50
6.	30	38.48

Oscillatory tests were done to determine the possible effect of storage time on raw honey and adulterated honey with 30% jaggery syrup at 298 K (Fig. 6). It is observed that the viscous modulus (G”) dominates elastic modulus (G’) in both the samples until a particular frequency is reached and after that G’ increases as reported in literature.^[34−37] The elastic modulus (G’) of both samples remains the same. This shows that adulteration only affects the viscous properties of the honey. The amplitude of viscous modulus of 30% adulterated sample is more as compared to that of raw honey. So, more adulteration means a more viscous behavior of the honey sample which is also depicted by the viscometry tests. It is shown in Fig. 7 that the phase angle linearly decreases with the increase in the frequency of operation. The phase angle of the system is above 45°, which indicates the elastic nature of the sample under test. This phenomenon is more evident for the samples having adulteration above 20%.

FIGURE 6 Elastic modulus (G’) and viscous modulus (G’’) versus frequency of raw honey and 30% jaggery adulterated sample.

FIGURE 7 Phase angle versus frequency response of the various samples.

Variation of crossover frequency with different percentage of adulterated samples is shown in Fig. 8. It is clear from the graph that less time (higher frequency) is required by the sample to solidify as the concentration of adulteration increases and if time is delayed, the elastic behavior will dominate, leading to the complete crystallization of honey at lower frequency. This is also indicated in Fig. 7. So adulteration causes the degradation of honey and crossover frequency can be considered as one of the parameter to know the percentage of adulteration in honey.

FIGURE 8 Crossover frequency variation with different percentage adulterated samples.

CONCLUSION

Rheology plays an important role to study the effect of jaggery adulteration in honey. The study indicates that adulteration in honey affects both its viscometric and dynamic rheological behavior. Moisture content decreases with the rise in adulteration concentration. All the samples showed non-Newtonian behavior except pure honey. It was found that raw honey had minimum viscosity of 2.48 Pa s among the six samples whereas 4.83 Pa s was the maximum viscosity obtained by the 30% adulterated sample. Arrhenius model was able to justify the relationship between viscosity and temperature. There was an exponential decay in the viscosity with respect to temperature. The highest activation energy was achieved by the 30% adulterated sample which is 38.48 KJ/mol. The effect of storage time on the adulteration was depicted by oscillatory tests and it was found that lesser time will be taken by the highly adulterated sample to solidify and adulteration can be predicted at lower temperatures.

ACKNOWLEDGMENTS

The authors are thankful to Sr. TO (2) Mrs. Virmita Malhotra, C.S.I.O. Chandigarh for her help and discussions during the work. The research work was carried out at Central Scientific Instruments Organization (CSIR), Chandigarh (Punjab).