Abstract
This manuscript presents results of a research aimed at conducting a statistical analysis of correlation between the percentage addition of water to milk and its electrical parameters, which are used to determine the degree of milk dilution and the level of milk adulteration with water. The results of milk conductivity parameters analysed in the study showed admittance and conductance to decrease systematically with water addition from ca. 10.8 to 7.8 mS. In contrast, impedance and resistance values were observed to increase from ca. 91 to 128 Ω. Results of the measurements of capacitance parameters showed that with an increasing content of distilled water in milk the value of equivalent parallel capacitance was decreasing from 1.4 to 0.65 μF. In turn, along with water content increasing to 15%, the value of equivalent series capacitance was increased from 212 to 237 μF. Still, at water content of milk exceeding 15%, the value of that parameter remained unchanged. The statistical analysis demonstrated a close linear correlation (0.989 ≤ r ≤ 0.998) between the percentage of water added to the milk and electrical parameters of impedance, resistance, admittance, conductance, as well as equivalent parallel capacitance of milk.
INTRODUCTION
The quality of raw milk is determined by such its traits as: chemical composition, physical properties, hygienic purity, including microbiological and cytological ones, as well as sensory attributes and nutritive value. Milk quality is additionally affected by genetic and extra-genetic factors (nutritional, physiological, health-related, environmental), as well as by conditions the milk is kept in until being processed in a dairy plant. The major effect on milk composition is also ascribed to mastitis conditions that result, among other things, in reduced contents of casein and lactose, as well as increased levels of whey proteins and chlorides in milk. Changes in contents of milk constituents may also be affected by the season of year and climatic conditions.Citation[1,2] Citation
The use of fresh raw material characterised by desirable hygienic purity and not adulterated with extrinsic substances is a prerequisite for producing high quality dairy products. The key elements of technical-technological development and automation of production are fast instrumental methods of milk quality evaluation based on its physical characteristics. In terms of electrical properties, milk is an electrolyte characterised by good ionic conductivity that may be determined by applying such parameters: impedance and resistance, as well as admittance and conductance. In dairy technology, measurements of electrical parameters are applied for detection of milk originating from mastitic cowsCitation[3] and observation of milk components transformation during processing–for example, homogenization and milk storage.Citation[4 – Citation7] Measurements of electrical parameters have also been used in the technology of fermented dairy products for testing milk acidification dynamics–determination of lactose content, soluble mineral salts content, and pH.Citation[8,Citation9] On the other hand, studies by CitationTherdthai and Zhou (2001 Citation[10] were aimed at defining the recombined milk electrical model. The measurement of electric conductivity has additionally enabled determining the level of whey demineralisation in the technology of cheddar cheese production.Citation[11]
Currently, addition of water is one of the most common adulterations of milk. Unintended dilution of milk may also occur at a dairy plant with process water applied for washing the process lines. The addition of water to milk is assessed with methods used to determine changes in the milk freezing point (cryoscopy method) or changes in light refraction of components after fat extraction from milk. The majority of those methods are, however, costly and time consuming.Citation[5] Milk dilution with water contributes to a decrease in the concentrations of mineral salts and lactose in its serum, which results in changes of milk electrical properties, such as a decrease in electrical conductance or an increase in impedance, in particular.Citation[12,Citation13] For that reason, measurements of electrical parameters of milk can be used to develop methods for the fast assessment of milk dilution with water. Milk originating from mastitic cows or adulterated upon the addition of alkaline substances will exhibit higher electrical conductivity as a result of a considerably higher level of ions. In turn, milk diluted with water or originating from mastitic cows is characterized by a higher content of micellar casein accumulating electric loads, which is likely to result in a diminished electrical capacity of milk.
Considering the above, it has been assumed that water addition to milk affects a change in the concentration of components conducting an electric current. Based on that assumption, it is possible to develop an electrical parameter, the changes of which would be closely correlated with an increasing level of milk dilution with water. In order to verify that thesis, studies were undertaken that were aimed at conducting a statistical analysis of correlation between the percentage addition of water to milk and its electrical parameters, which are used to determine the degree of milk dilution and the level of milk adulteration with water.
MATERIAL AND METHODS
Milk obtained in the autumn season from the morning milking of cows was the object of the study. The selection of raw milk for the study was based on the somatic cells count (SCC ≥ 400 K/cm3) using Fossomatic 5000 and total bacteria count (TBC ≥ 100 K/cm3) using a Bactoscan 8000S apparatus. Raw milk was centrifuged at 40°C using a laboratory centrifuge LWG 24E (“Spomasz,” Gniezno, Poland) and standardised to fat content of 3.5% (experimental milk). The standardisation was aimed at eliminating the influence of diversified fat content in raw milk on its electrical properties. The experimental milk obtained following the standardisation process was cooled down to 20°C and preserved by the addition of 2% sodium azide at 1 cm3/1 dm3 of milk. To milk samples, distilled water was added in the quantities of: 2, 4, 6, 8, 10, 15, 20, 25, 30, 35, and 40%. Prior to assays, the samples were warmed up in a water bath to 20°C and kept at that temperature for 2 h to stabilise the system of components and physicochemical properties. Control samples were those of experimental milk without the addition of distilled water (0%).
Tests of electrical characteristics were conducted for raw milk, experimental milk, and distilled water used to dilute the milk samples. To this end, 200 cm3 of milk (or milk and water mix) sample were poured into 75 × 55 × 94 mm glass containers equipped with two plate electrodes of acid resistant steel installed at two opposite (smaller area) walls of the container. Next, the samples were placed in an air-conditioned chamber to obtain the temperature required. Once the temperature has stabilised (after 2 h), measurements were conducted for the following electrical parameters: impedance (Z), resistance (R), admittance (Y), conductance (G), equivalent parallel capacitance (Cp ), and equivalent series capacitance (Cs ).
Measurements, in three repetitions for each sample, were carried out using an HP 4263B type measuring device (Hewlett Packard, Palo Alto, CA, USA) at the voltage of 200 mV and the frequency of 100 Hz. The optimal current frequency was determined in the earlier studies.Citation[12,Citation14] The tests of electrical properties were conducted according to the electrical diagram developed by the authors ()Citation[15]. The experiment was conducted in six repetitions (six tests). The results of the studies obtained from the experiments were subjected to a statistical analysis aiming at determining the correlation and regression curve, for which the values of electrical parameters of milk (Z, Y, R, G, Cp, Cs ) were independent variables, whilst the percentage quantity of water added to milk (w) was a dependent variable. The statistical analysis of data obtained was conducted using the Excel and Statistica software on the basis of a one-way analysis of variance (ANOVA).
Figure 1 Scheme of a measuring system of electric properties of food products: Z–impedance, R–resistance, Cs–equivalent serial capacitance, Cp–equivalent parallel capacitance, M–measuring device.Citation[15]
![Figure 1 Scheme of a measuring system of electric properties of food products: Z–impedance, R–resistance, Cs–equivalent serial capacitance, Cp–equivalent parallel capacitance, M–measuring device.Citation[15]](/cms/asset/4d51f57a-7cd1-4ef4-b85f-5591f003f79c/ljfp_a_483614_o_f0001g.gif)
RESULTS AND DISCUSSION
The first stage of the experiment involved determinations of electrical parameters of raw milk and distilled water used for milk dilution. The average milk Z and R were at the level of around 92 Ω, Y and G at ca. 11 mS, whereas the Cp accounted for ca. 1.3 μF and Cs for ca. 231 μF. The values of Z and R parameters determined for water were higher than those measured for raw milk, while the values of parameters Y and G, as well as Cp and Cs were lower ().
Table 1 The values of electrical parameters of raw milk and distilled water applied for milk dilution
The next stage of the study was aimed at assaying electrical properties of experimental milk obtained from raw milk standardised to 3.5% fat content diluted with distilled water at 0–40% of milk volume. The results obtained showed changes in the values of both conductivity (Z, R, Y, G) and capacitance (Cs, Cp ) parameters as affected by water addition to milk (). Results of the measurements of the conductivity parameters of milk showed Y and G parameter to decrease successively from ca. 10.8 to 7.8 mS with water addition to milk. On the other hand, the value of Z and R parameters were observed to increase from ca. 91 to 128 Ω. Results of the measurements of capacitance parameters showed that with an increasing content of distilled water in milk the value of Cp was decreasing from 1.4 to 0.65 μF. In turn, along with water content increasing to 15%, the Cs value was increasing from 212 to 237μF. Still, the value of that parameter remained unchanged at water content of milk exceeding 15%.
Table 2 Changes of electrical parameters of experimental milk dependently percentage addition of distilled water
The statistical analysis conducted in the study () demonstrated a close statistical correlation (α = 0.01) between the percent of water addition to milk and electrical parameters tested (Z, R, Y, G, Cp ). The correlation coefficient (r) calculated for those correlations was high and ranged from 0.989 to 0.998. The value of a correlation coefficient computed for the correlation between the percentage addition of water and the Cs accounted for 0.764 (at a significance level of α = 0.003). The correlations studied were described by regression equations y = a + bx (), which allow computing the percentage of water added to the milk. The computations indicate that the actual percentage of water added to the full milk (0–40%) can be determined most accurately on the basis of admittance tests (; ).
Figure 2 Mathematical dependence between percentage water addition (W) to experimental milk in function of admittance, at 100 Hz frequency measured.
Table 3 Regression analysis of percentage water addition (W) to experimental milk in function of electrical parameters (Z, Y, R, G, C p , C s )
Table 4 The results of calculation of water content in full fat milk on the basis of determined regression equations for selected electrical parameters
The results obtained in this study confirm findings of CitationMabrook and Petty (2003b)Citation[13] who showed that milk conductance was decreasing with an increasing water addition to milk and an increasing fat content of milk. Those studies indicate also that the admittance values of skimmed and full milk decrease under the influence of water addition. Electrical conductivity of full fat milk measured at 100 kHz showed abnormal increase after the addition of small quantities of water (up to 2%).Citation[13] That effect is probably linked to fat hydrolysis which causes release of free fatty acids forming calcium and sodium salts. The increase in fatty acids' salts quantity can influence an increase in the quantity of sodium and calcium ions conducting electric current in milk during the initial phase of its dilution. Further milk dilution contributed, on the other hand, to decreasing the concentration of dissociating mineral salts in the water phase of milk and, as a result, the electrical conductivity of its decreased. In the case of skim milk, that phenomenon was not observed. According to those authors, if the fat content in milk is known, the measurement of electrical conductivity can be used for estimating the level of water addition to milk on the condition that the measurement is conducted under strict temperature control and using specific frequency of the electric current.Citation[13]
CONCLUSIONS
The addition of water to experimental milk resulted in decreasing admittance and conductance, as well as increasing impedance and resistance, which indicated deterioration of electrical conductivity of milk with the increase of its dilution with water. Statistical analysis showed a close linear mathematical correlation (0.989 ≤ r ≤ 0.998) between the percentage of water added to the milk and electrical parameters of impedance, resistance, admittance, conductance, as well as equivalent parallel capacitance. The greatest dependence and the closest to really counted amount of water added to the milk (0–40%) can be determined on the basis of admittance values. In the future, the results could be the basis for further studies aiming at the development of a rapid method for determining both the degree of investigated milk dilution with water and for its adulteration level on the basis of its electrical parameter measurements.
Related Research Data
REFERENCES
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