Manufacture of virtually all milk and dairy products involves heat treatment, which is mainly aimed to kill microorganisms and inactivation of enzymes or to achieving some other, mainly chemical changes. This depends on the intensity of the treatment, i.e., the combination of temperature and duration of heating. Consequently heat treatment may also cause undesirable changes, although desirability may depend on the product involved or on its intended use, such on browning, development of a cooked flavor, loss of nutritional quality, inactivation of bacterial inhibitors, and impairment of rennet ability [1].

Pasteurization is a mild process, designed to inactivate the major pathogenic and spoilage bacteria found in raw milk. It should produce minimal chemical, physical and organoleptic changes in the product [2]. Range of thermal treatments were used to reduce the bacterial population of milk, which include thermisation, batch and HTST pasteurization, high temperature pasteurization (ESL), UHT treatment and in-container sterilization [3] as well as microwave pasteurization [4-6]. Virgiliu [7] stated that the next methods of pasteurization are used may be slow pasteurization or low pasteurization at the temperature of 62-65°C for 30 minutes, respective 20 minutes, where as midst pasteurization or shorten pasteurization is performed at the temperature of 72-78°C for 15 seconds, known as, High Temperature Short Time pasteurization (HTST) is performed between 85-90°C, for a few seconds, followed by sudden cooling at 10°C.

There are several non-thermal treatments can be used to destroy microorganisms in foods which including high-pressure treatment, pulsed electric field technology, ultra sonication, centrifugation and microfiltration; however, only the last two technologies are used commercially for milk [8,9]. There is other milk pasteurization method; it’s called solar milk pasteurization, in this method the solar energy was used to milk pasteurization [10]. Ohmic heating is a thermal process in which heat is internally generated by the passage of alternating electrical current (AC) through a body such as a food system that serves as an electrical resistance [11]. The main advantages of ohmic processing are the rapid and relatively uniform heating achieved [12]. Direct ohmic heating has problems with fouling and corrosion. Fouling creation of milk was studied by Ayadi [13].

In fact using ohmic heating for milk pasteurization at 72°C for 15 sec. with 220 volt AC produce low quality pasteurized milk because of the whey proteins are became denatured and occur reaction between poles and milk and constituted deposits on the poles and fouling. The aim of this study is to milk pasteurization by different electrical fields at 20, 27.5, 55 V/cm (80, 110, 220 V, AC) as a non-thermal treatment to improvement pasteurized milk quality.

Apparatus of milk pasteurization by electrical field

The apparatus was designed and manufactured in the Food Engineering Laboratory, Faculty of Agriculture, University of Basrah, Iraq. The device consists of a reservoir made of stainless steel 316 double wall. Its capacity 25 liters, pump its discharge 20 liters / min and a heat plastic tube with internal diameter 1 cm and a length of 3 m. Its fixed on it 20 electrodes and the distance between the electrodes of 4 cm It is made of stainless steel 316, electrode diameter 0.3 cm and 2 cm long figure 1 and 2.

Figure 1: Schematic diagram of Milk pasteurizer by electrical field.

Figure 2: Photograph of Milk pasteurizer by electrical field.

This design doesn’t need holding tube, thermal valve, heat exchanger and high pressure pump, but it depends on the electrical field (AC) generated between electrical poles. The apparatus is doesn't has complicated parts, and its parts are simple as tank, heat plastic pipe, stainless steel electrical poles, pump and valve. Electrical conductivity is calculated according to equation (1): [14,15].

Time of the holding milk in the apparatus is calculated from the equation (2): [16].

(1)

I: current (A), L: distance between poles (m), V: voltage (V), A: section area and: electrical conductivity (S/m).

(2)

t: staying time (sec.), A: section area (m2), Ld: pipe length (m) and m: milk mass flow (kg/m².sec.)

The apparatus productivity was calculated through the total milk output from apparatus (L/h).

While system Performance Coefficient (SPC) is calculated from equation (3): [17,18].

(3)

Where:

(4)

M: mass (kg), Tw: final temperature (°C), Tin: primary temperature (°C), Eg: amount of given energy (J) and Qt: the amount of taken heat (J).

In which the electric field strength, E (V/cm), is calculated by equation (5): [19].

(5)

Where d is the inter-electric distance (cm).

The glass Ostwald (Size A) is used in estimating the viscosity of raw milk and pasteurized milk in the electric field by the method of [20] to estimate the viscosity. By using of tables [21] for the extraction of specific gravity and viscosity of water at different temperatures are calculated throughout the equation:

(6)

The milk density was determined by the method that mentioned by Pearson [22] by using Pyknometer.

Chemical tests included determination of moisture where estimated percentage of moisture was recorded according to the method described in the AOAC [23]. Each of protein, fat, lactose and ash are estimated by the method of Egan [24]. Acidity and pH of the raw milk and pasteurized milk are measured and then after the storage period amounting 21 days,according to the method applied by Egan [24]. Phosphatase enzyme in the raw and pasteurized milk is detected according to the enzyme method that included commercial treaty (kit) that described in the Ryan [25].

Microbiological tests are conducted on raw milk and pasteurized and after a period of storage for 21 days casting method are used to calculate the number of dishes microbiology in milk samples [26]. Estimation included total bacteria count and estimate yeasts and molds, staphylococcus.

Statistical analysis: One way ANOVA and post–hoc comparison statistical analyses which performed by using SPSS 11.0 statistical packages are applied [27].

Electrical conductivity is the measurement of a substance transmits electric charge, expressed in Siemens per meter, which is a ratio of the substance density to electric field strength and is affected by the chemical composition of a substance [28]. Results showed that the electrical conductivity was increased with increasing milk temperature for all electrical fields (55, 27.5 and 20 V/cm) respectively. As evident from figure 3, which show the relationship between electrical conductivity and temperature of milk at different electrical fields. When temperature were 10°C, 40°C the electrical conductivity were 0.798, 0.858 S/m respectively at 55 V/cm. The increasing in the electrical conductivity with temperature is caused by to the increasing of current with increasing temperature, as Kong [29] whom stated that the current increase with increasing temperature. Data in figure 3 showed the relationship between electrical conductivity and temperature of milk was linear equation for all electrical fields such as σ=0.002T+0.7765 at 55 V/cm and determination coefficient is R²;=0.997. The dependence of the electric conductivity of milk on temperature is very important and the relationship between them is linear equation as stated [30,31]. Consequently, the results showed that electrical conductivity was increased with increasing electrical field.

Figure 3: Electrical conductivity of milk as a function of temperature.

On the other hand the results showed in figure 4 that the viscosity of milk decreased with increasing temperature and this because the increase in temperature leads to lower milk fatty blocs responsible for the high viscosity of milk. The relationships between viscosity and temperature is first-order equations for all electrical fields such as in the 55 V/cm the relationship between temperature and viscosity is υ=-3E- 05T + 0.002, where 10 ≤ T ≤ 40. The coefficient of determination R= 0.9963. Results also showed that the differences in viscosity between electrical fields are not significant at all temperatures.

Figure 4: Viscosity vs. Tempeture of milk at electrical field pasteurization.

Milk density was reduced with increasing milk temperature as shown in figure 5. This reducing due to the rising of milk temperature up to 40°C as Muhsin, [32] stated that milk density reduced with increasing temperature and electrical field. The results also showed that the differences in the density between electrical fields were not significant at all temperatures.

Figure 5: Density vs. Temperature of pasteurized milk by electrical field.

Final Output milk temperature was 40 ± 1.49°C at 55 V/cm and 35 ± 1.39 at 20 V/cm significantly reduced by reducing electrical field, this reducing is due to reducing electrical conductivity. That temperature (40°C) is very necessary in milk filling and packaging processing (Table 1). The results showed that the holding time doesn’t significantly affected by electrical field. The holding time is 0.708 ± 0.0042 (sec.) at 55 V.cm, wish to prevent formation of fouling and deposits on the poles because the high voltage (220 volt) cause whey protein denaturation and milk overheating closed to poles and poles corrosion. This result was agreed with Ayadi [33]. So there is a strong effect of electrode material and significant effect of the current density, numerical simulation showed that even a relative thin layer of deposits can be over heated, and could accelerate change in whey proteins leading to the final growing phase of fouling [34].

Table 1: Final Output milk temperature, Staying time, Productivity and System Performance Coefficient of apparatus.

The results in table 2 has showed a slight reduction in moisture content (%), fat, lactose and ash when milk pasteurized by electrical field. This decline due to the slight rise in temperature which caused of the low percentage of fat in pasteurized milk may be due to some fatty globule adhesion with device in the pipeline because of the non-homogenized milk. All of the protein, acidity and pH are not significantly affected by the electrical field pasteurization process, as well.

Table 2: Chemical composition (%), acidity (%) and pH for raw and pasteurized milk by electrical field.

Results in table 3 showed the total count bacteria in raw milk was 154,000 CFU/ml The number of colonies after a pasteurization process by electrical field at 6400, 8000 and 9600 CFU/ml at 55, 27.5 and 20 V/cm respectively. This result was less than the minimum Iraqi standard specification, which stated that the total count bacteria in pasteurized milk of good quality are 10000 CFU/ml and 50000 CFU/ ml with acceptable quality milk. Moreover, these results are less than the minimum of the Australian standard specification for the year 2007, which showed that the total count of pasteurized milk be between 50000–100000 CFU/ml. Coliform bacteria Enterobacter task in milk as the presence in pasteurized milk evidence of the inefficiency of the pasteurization process. The results also showed that total count bacteria Enterobacter in raw milk was 600 CFU/ml and the number of colonies are reduced to zero after a pasteurization process at electrical field of 55 V/cm, this results are considered less than Iraqi and Australian standard specification which showed that the total number of coliform Enterobacter in pasteurized milk must be at least 10 CFU/ml. While the numbers of colonies were reduced to 20 and 90 CFU/ml at electrical fields 27.5 and 20 V/cm respectively. The presence of fecal bacteria in the milk denotes contaminated animal feces and therefore their presence in pasteurized milk evidence of the inefficiency of the pasteurization process. Also, total count for E. coli bacteria in raw milk is 86000 and number of colonies is reduced to zero after a pasteurization process by electrical field and these results are in accordance with Iraqi and Australian standard specification. Staphylococcus is reduced to zero at electrical field 55 V/cm. as well. Results indicated that and presence of alkaline phosphatase in raw milk and its absence in the electrical field pasteurized milk, and all living cells contain cell membrane. These membranes are comprised of lipids (fatty molecules) and proteins [35]. Electroporation occurs because the cell membrane has a specific dielectric strength, which can be exceed by the electric field. The dielectric strength of a cell membrane is related to the amount of lipids (acting as an insulator) present in the membrane itself. The formed pores can vary in size depending on the strength of the electric field, and can reseal after a short period of time. Excessive exposure causes cell death due to the leakage of intracellular components through the pores [36]. Therefore electroporation is highly damaging to a cell. Results showed the absence of any growth of yeasts and molds before and after the process of electrical field pasteurization, as well as for the impact of the storage period for pasteurized milk. Sterile taken samples and under hygienic conditions has been archived sample at room temperature for two days as observed undesirable changes in milk as a result of the pollution. The second sample was preserved in the refrigerator at 4°C for a period of 21 days has been conducted tests the microbiological every seven days. The results of the impact of storage periods for pasteurized milk indicated the minimum standard specification Iraqi and Australian for counting bacteria total since reached after 7, 14, and 21 days of storage 6925, 7534 and 8435 CFU/ml at 55 V/cm. As for the two types of coliform bacteria staphylococcus, yeasts and molds did not show any growth which shows the efficiency of the pasteurization process (Figure 6). The total microbial count at 55 V/cm was less than at 27.5 and 20 V/cm, and located within the minimum standard specification Iraqi and Australian specification.

Figure 6: Total microbial count vs. storage time at different electrical field.

Table 3: Microbiological data for raw and pasteurized milk by electrical field.

Throughout the results of the study, can pasteurization of milk by electrical field at 55 V/cm (220 V, AC) and milk temperature are not more than 40°C. The device proved highly efficient in eliminating microorganisms and pasteurized milk storage good results. The chemical composition and acidity are not affected by electrical field. Pasteurized milk quality was improved. Alkaline phosphatase was absent in the pasteurized milk at 55 V/cm but its present in the pasteurized milk at 27.5 and 20 V/cm. Stopping use the electrical fields 27.5 and 20 V/cm for milk pasteurization in this study.