Electrical Conductivities of Liquid Egg Products and Fruit Juices Exposed to High Pulsed Electric Fields

Abstract

Electrical conductivity can be used to monitor important changes in a food product during pulsed electric field (PEF) processing. Electrical conductivities of selected fruit juices (namely apple, orange, and pineapple juices) and liquid egg products (namely whole egg, yolk, and egg white) were determined online during a PEF treatment. The property was measured at broad processing temperatures ranging from 5 to 55°C. Electrical conductivity increased linearly with increasing temperatures for all the products. The liquid egg products have the highest overall electrical conductivity varying from 0.22 to 1.1 S/m whereas fruit juice products have the lowest electrical conductivity ranging from 0.13 to 0.63 S/m. Regression equations of electrical conductivity as functions of temperature were developed. This paper provides a database and equation correlations of food electrical conductivity that could be used to design and optimize PEF process.

Keywords:

• Pulsed electric fields
• Liquid egg products
• Fruit juices
• Electrical conductivity
• Temperature

INTRODUCTION

Pulsed electric field (PEF) treatment is considered a non-thermal method of pasteurizing liquid foods by inactivation of spoilage and pathogenic microorganisms at comparatively lower temperatures thus maintaining top product quality.[1–5] The technology involves the application of short pulses (microseconds pulse duration) of high voltage to food sample placed between two electrodes. The applied pulse energy destroys the bacterial cell membrane by mechanical effect with minimum heating of the food.[3]

PEF processing has been successfully used for variety of liquids and pumpable food products such as orange and cranberry juices,[6] and apple juice and cider[7] without any loss of their natural characteristics. It has also been successfully used to inactivate several kinds of food spoilage and pathogenic microorganisms including Escherichia coliO157:H7,[4,7–9] Salmonella species[10,11] with inactivation rates up to 6 log reduction for certain microorganisms.

The electrical conductivity of the medium to be treated is its ability to conduct electric current.[12,13] It is an important variable that determines the extent of biological changes such as electropermeabilization, electrofusion, motility, and microbial inactivation produced during PEF treatment.[3] In general, foods with high electrical conductivities are difficult to treat since they generate low peak electric fields across their treatment chambers due to the high current that is typically generated during PEF treatment of such products.[3] In that case, pasteurization of high electrical conductivity food product may be accomplished with low efficiency. On the other hand, low conductivity products are generally more amenable to an effective PEF treatment. Electrical conductivity is related to the efficiency of energy transferred during PEF treatment. Consequently, it is suitable to lower a food's conductivity in order to obtain greater microbial inactivation rate for the same applied electric field and with an application of equal input energy.[3]

Higher increases in temperature occur with increasing electrical conductivity during electrical treatment of products.[13–16] Appropriate treatment chamber that would match the characteristic impedance of a PEF unit (in the case of a square pulse) can be designed from product electrical conductivity. Matching impedances of the load and PEF unit is required in order to allow higher energy transfer and proper treatment of the food.[3] Determination of electrical conductivity of liquid foods over a wide range of temperature is critical for the design and optimization of PEF process.

Electrical conductivity can be measured using conductivity or LCR meter. Ruhlman et al.[17] used electrical conductivity meter to measure electrical conductivities of different products including apple and orange juices at temperatures between 4 and 60°C. Other authors have also used conductivity meters to determine electrical conductivities of different food products including orange, apple and pineapple juices at a constant temperature of 20°C.[18] However, most commonly available conductivity meters may not be suited for measurement of high conductivity products. Marcotte et al.[15] used a static ohmic heating cell to measure electrical conductivity of some hydrocolloid solutions. For PEF applications, it is more suitable to determine electrical conductivity online while the product is treated in a treatment chamber. This will allow measurement of conductivity and monitoring of changes in the product during a PEF treatment. This approach has not been reported in the literature. Data on the relationship between temperature and electrical conductivity of liquid foods is scarce and has not been investigated using the PEF technology as a measurement device.[6,19] Not much published data is currently available on electrical conductivities of a wide range of high conductive food products such as liquid egg over a wide range of processing temperatures. The objectives of this study were to determine the electrical conductivities of selected liquid foods at various temperatures, to determine the relationship between electrical conductivity and temperature during PEF treatment of different liquid products.

MATERIAL AND METHODS

Three acidic fruit juice products namely orange juice, pineapple juice, and apple juice; and four neutral liquid egg products namely 15.64% solid content whole egg A, 24.54% solid content whole egg B, egg yolk, and egg white were selected for the experiment (Table 1). The orange, pineapple, apple juice samples were purchased from a local grocery store. The whole egg A (15.64% solid content) sample was also purchased from a local grocery store. The egg white, the whole egg B (24.54% solid content), and the yolk samples were obtained from a local egg processing company.

Table 1 Proximate composition of liquid food product.[20]

	Composition
Products	pH	Water(g)	Protein(g)	Carbohydrate(g)	Fat (g)	Organic acids(g)	Minerals(g)
Egg white	7.8	87.30	11.10	0.70	0.20	–	0.70
Whole egg	8.1–8.3	74.10	12.90	0.70	11.20	–	1.10
Yolk	6.4	50.00	16.10	0.30	31.90	–	1.70
Orange juice	3.8	88.10	0.65	9.40	0.18	1.18	0.37
Pineapple	3.5	86.10	0.40	11.36	0.10	0.72	0.20
Apple juice	3.6	88.10	0.07	11.06	–	0.77	0.41
The nutrient amount is given in gram (g) per 100 g of material.

Electrical Conductivity Measurement

Electrical conductivity was measured using a continuous PEF chamber. The 3.7 ml chamber consisted of 2 parallel stainless steel electrodes with 1 cm gap and surrounded with a polypropylene insulator. A schematic of the experimental set-up is shown in Fig. 1. The sample was introduced into the chamber and electrical conductivity was measured directly while the sample was exposed to pulsed electric field. The pulsed electric field system used was a 30 kV generator with internal impedance of 100 Ω and delivered a maximum energy of 18 J/pulse. The generator generated bi-polar, instant reversal square wave electric field pulses. During the experiments, the high voltage leads from the generator were connected to the treatment chamber (as shown in Fig. 1), and the liquid product was circulated through the treatment chamber. The product temperature was maintained at different set values between 5 and 55°C by passing the fluid product through a heat exchanger coil immersed in a water bath before it entered the treatment chamber. The product was circulated at the flow rate of 6 ml/s using a peristaltic pump. Thermocouples were installed at the inlet and outlet of the treatment chamber to continuously monitor product temperature. When a desired product temperature was attained, single shot electric field pulse (pulse width 2 μs, electric field intensity 15 kV/cm) was applied while voltage and current across the sample were captured simultaneously using a 2 channel digital oscilloscope (TDS3000, Tektronix, Wilsonville, OR). Electric resistance and conductivity were calculated according to Eqs. 1 and 2, respectively.

(1)

(2)

where R_l (Ω), V_l (V), l A, σ(S/m), A (m²), and d (m) are the resistance load, voltage, current cross the food, electrical conductivity of the food, electrodes surface and the gap between electrodes, respectively. Three replicate measurements were conducted at each set temperature. In order to evaluate the accuracy of the measurements by the PEF system, electrical conductivity was also measured for the whole egg products at 5 and 55°C using a desktop electrical conductivity meter (ORION, Model 120, MA, USA). Measured values of electrical conductivities using the PEF and the conductivity meter for the product were compared. Statistical and regression analyses of data were conducted using SAS version 8 (SAS Institute Inc., Cary, NC).

Figure 1 Experimental setup for the determination of electrical conductivity of liquid food products.

RESULTS AND DISCUSSION

Electrical conductivities of the selected liquid foods at the different temperatures were measured using a continuous flow PEF processing system. A statistical analysis of the data obtained showed that the effect of product type and temperature significantly (at the 5% level) influenced electrical conductivity. The variation of electrical conductivities of the various products with respect to temperature is presented in Figs. 2 and 3. Electrical conductivity increased linearly with increasing temperature. This result agrees with the reports of Palaniappan and Sastry,[13] Yongsawatdigul et al.,[14] Marcotte et al.,[15] Henningsson et al.,[16] and Ruhlman et al.[17] The values of electrical conductivity for the whole egg products measured using the PEF generator compared well (within 3%) with values measured using an electrical conductivity meter.

Figure 2 Electrical conductivity of liquid egg products versus temperature. Experiments were carried on four liquid egg samples at temperatures varying from 5 to 55°C. Whole egg, 24.54% solids (▴); Whole egg, 15.64% solids (▪); Egg white (♦); Yolk (•).

Figure 3 Electrical conductivity of fruit juices versus temperature. Experiments were carried on three fruit juice samples at temperatures varying from 5 to 55°C. Orange juice (♦); Apple juice (▴); Pineapple juice (•);

Whole egg A and B (15.64 and 24.54% solid contents, respectively) and egg white have the highest overall electrical conductivity compared to the other products, whereas the apple juices and egg yolk had lower electrical conductivities at each given temperature. The conductivities of egg white, whole egg (24.54% solid), apple juice and orange juice at 15°C were 0.57, 0.53, 0.17, and 0.31 S/m, respectively (Figs. 2 and 3). The obtained results at 15°C are consistent with the data reported by Zhang et al.[6] and Dunn and Pearlman[19] at about the same temperature. For apple and orange juice at 20°C, electrical conductivity values were obtained as 0.19 and 0.34 S/m, respectively, which were also close to the values reported by Ruhlman et al.[17] using an electrical conductivity meter as a measurement device. In addition, the electrical conductivity of pineapple juices, obtained as 0.3 S/m at 20°C is consistent with the value obtained by Raso et al.[11] using an electrical conductivity meter. The results show that the values of electrical conductivities obtained in this study using direct measurements during PEF treatment agree with published data at the comparable temperatures.

Acidic products generally exhibit higher electrical conductivities. However, considering the results obtained for egg yolk, it is apparent that pH alone cannot explain the variation in electrical conductivity values. Studies have shown that electrical conductivity of liquids is influenced by the nature of ions (chemical composition) and ionic movement in the liquid, which are all temperature dependent.[13] Consequently, the electrical conductivities of liquid egg products were higher than the values for fruit juices owing to the great amount of ionic species concentration such as salts and acids in the formulation that act as electrolytes, which generated electric current through the product.[12] Other authors have indicated that ionic mobility increases with rising temperature and decreases with the amount of solid content in a product.[13,15] Egg white had the highest electrical conductivity followed by whole egg B (24.56% solid), whole egg A (15.64% solid), and egg yolk. This phenomenon was attributed to the greater mineral (sodium) concentration in egg white and to the dielectric non-conductive compounds or solid content (fats) in egg yolk which act as insulators thereby preventing ions migration.[21–23] Egg white has higher moisture content than yolk. The moisture content of product plays an important role in the migration of positive and negative ions that is responsible for conduction of electrical current. It has been shown that electrical conductivity tends to increase with the increasing moisture content of food containing protein compounds. Ion solvation could increase when more water molecule are available, resulting in increasing ionic mobility.[14,24]

The values of electrical conductivity obtained for whole egg were between the values obtained for egg white and yolk values. These results are understandable since whole egg is made of yolk and egg white. The results also suggest that the non-ionic constituents such as fat in yolk suppress electrical conductivity.[13] Though, the whole egg conductivities were very close to those of egg white, this phenomenon might be due to the non-conductive compound in whole egg that had a small physical obstacle on ion migrations.[22,25] Electrical conductivities of whole egg A (15.64% solids) were lower than the values obtained for whole egg B (24.52% solids) at corresponding temperatures. This result appears to contradict the conclusion of Palaniappan and Sastry[13] that lower solid content tend to increase electrical conductivity of a medium. The discrepancy may be explained as due to higher concentration of ionic species (especially sodium) in the whole egg B (24.52% solids) samples than in the whole egg A (15.64% solids) samples.

The electrical conductivities of fruit juices increased while the temperature increased. Higher values were obtained for orange juice followed by pineapple and apple juices. The results were close to values reported in the literature for different temperature ranges.[17,18] Again the pattern of the result may be attributed to the ionic concentrations of the samples. Most electrical conductivity meters may have less accuracy in measuring higher electrical conductivities of liquid food products since they are typically designed to measure liquid products with lower electrical conductivities (in the order of micro siemens). Also conductivity meters may not replicate the condition of a product during PEF processing. Therefore, using the PEF system as a measurement device clearly has two main advantages especially in a PEF research. It can give accurate values that could be used to design an adequate test chamber and generator for further experiments and the electrical conductivity can be measured online.

As Reported by Ruhlman et al.[17] for different liquid food products, regression analysis of data results in a linear equation relating electrical conductivity to temperature for all the products. The regression parameters are shown in Table 2. The R² values of all the equations were greater than 0.996 thus indicating the suitability of the regression equation.

Table 2 Product equations correlation of electrical conductivity (σ) and temperature (T).

Product	Regression equation	R^2
Egg white	σ = 0.0133T + 0.3729	0.9983
Whole egg (II)	σ = 0.0119T + 0.3482	0.9976
Whole egg (I)	σ = 0.0107T + 0.296	0.9966
Yolk	σ = 0.008T + 0.1704	0.9978
Apple juice	σ = 0.0046T + 0.1067	0.9978
Pineapple juice	σ = 0.0068T + 0.1546	0.9978
Orange juice	σ = 0.008T + 0.1912	0.9960

CONCLUSION

Electrical conductivity of measured samples depends highly on temperatures. Increasing product temperature increased electrical conductivity. Regression equations predicated electrical conductivities of the selected liquid products based on their temperatures. This database should provide critical tool that can be used to design and adequately operate a pulsed electric field process. In particular, knowledge of the electrical conductivity-temperature relationships will permit determination of the change in temperature (ΔT) during a treatment, a better control of the process and will provide a tool for efficient treatment of the food.

ACKNOWLEDGMENT

The authors acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC).