Comparative effect of thermo sonication and conventional heat process on lipid oxidation, vitamins and microbial count of milk

Introduction: Milk is the most consumable dairy product in the world which is predominantly treated to heat pasteurization and sterilization processes to increase safety and shelf life (Meshref 2008). Heat treatment can cause undesirable protein denaturation, non-enzymatic browning, loss of vitamins and volatile flavor compounds, freezing point depression, and flavour changes. Due to using high temperature of heat in this process, the nutritional composition and sensory properties of milk are changed (Herceg et al., 2012). Today, there is interest in use of high-intensity ultrasound waves with a mild heat treatment process to deactivate microbes and enzymes leading to spoilage and degradation of safety in foods. The aim of this study was to evaluate the effect of ultrasound process as an alternative for high temperature heat process on microbial count, lipid oxidation as qualitative parameter and vitamins as nutritional characteristics of milk.
Materials and methods: Fresh milk was prepared from the Firoozkandeh cow farm (Sari-Mazandaran-Iran) and transferred to the laboratory in cold conditions then it was treated by ultrasonication techniques at 3 temperature levels of 45, 55 and 65 ° C and duration 5, 10 and 15 min using ultrasound probe type (350 W) in two intensities of 50 and 75% and ultrasound bath (280 W). The control sample was heated at 90 ° C for 10 min. Total viable count of bacteria, total count of mold and yeast, count of coliforms, psychrophilic bacteria and Escherichia coli in raw milk, control milk and treated milk with ultrasound were measured. Plates with 30 to 300 colonies were selected to count of colonies and microbial changes of milk samples were reported based on Log CFU/ml. The effect of thermo sonication on the lipid oxidation of milk was evaluated by measuring peroxide value and thiobarbituric acid value. Riboflavin and thiamine as water-soluble vitamins and retinol and tocopherol were measured as lipid-soluble vitamins by HPLC method.
Results and discussion: It was found that an increase in ultrasonication temperature from 45 to 65°C and duration from 5 to 15 min, increased lipid oxidation of milk and the samples treated with ultrasound bath had the least amount of peroxide value (0.019±0.01 meq O2/Kg oil) and thiobarbituric acid value (0.03±0.0 µg/Kg). Raw milk has the least oxidation because no heat process was applied to it. The presence of air bubbles during sonication and solid particles of milk leads to the formation of nuclei for free radicals and intensifies the oxidation process. The formation of primary and secondary compounds resulting from fat oxidation depends on the variables of ultrasound treatment, the time and temperature of treatment with ultrasound, as well as the type and intensity of ultrasound. When sound waves pass through a liquid; in each section of the fluid, a continuous compression occurs and an exponential cycle occurs. Because of the sound energy cannot be absorbed by molecules, the fluid is drawn to one side and the cavitation phenomenon occurs, which is associated with the formation of small gas bubbles. The collision of gas bubbles encounters the formation of other waves. Given that the effects of ultrasound depend on the number and severity of explosive bubbles inside the liquid over time and type of treatment environment. In same sonication condition the effect of increasing in temperature on vitamin degradation was higher than that of ultrasound type, and the highest levels of thiamin (10.80±0.06 µg/100g), riboflavin (97.55±0.68mg/100g) and retinol (14.9±0.09µg/100g) degradation occurred in the control sample. The effect of temperature on riboflavin and thiamine amount was significant (P<0.05) and with increasing in temperature riboflavin and thiamine amount decreased and there was a significant difference (P<0.05). The ultrasound process was effective in reducing riboflavin and thiamine amount, resulting in a decrease in their amount, and the probe type of ultrasound caused the most vitamin degradation. The separate and interaction effects of each factor were not significant on retinol content. The retinol content decreased with increasing in temperature but the difference was not statistically significant. The reason for the decrease in tocopherol during sonication treatment can be attributed to the anti-radical properties of vitamin E. Also, the effect of ultrasound on water-soluble vitamins and lipid-soluble soluble vitamins showed lower levels of degradation of these vitamins compared with conventional heat-treated specimens. Very few studies have investigated the direct and indirect effects of ultrasound on the nutritional value of food, and the use of ultrasound at low temperatures reduces the probability of nutrient degradation. Increasing in time of ultrasound was effective in reducing the count of microorganisms but it had a statistically significant effect (P<0.05) on reducing the number of Escherichia coli, mold and yeast. Ultrasound type and ultrasound temperature had a significant effect on the total viable count of bacteria, coliforms, mold and yeast and psychrophilic bacteria (P<0.05). Probe type ultrasound is very suitable for the 75% oscillation range in the destruction of a variety of microorganisms, but increases the fat oxidation. The effect of none of the studied variables on count of Escherichia coli was statistically significant (P<0.05). When probe type of ultrasound used, the antimicrobial effect of sonication increased with increasing in intensity of ultrasound due to the cavitation effect. Increasing the time of ultrasound process has reduced the number of bacteria and in this regard, probe ultrasound has been more effective than ultrasound bath, which is consistent with the results of the present study. In addition to the cavitation phenomenon, ultrasound application results in a change in permeability of the membrane. Entering high amounts of water into the microbial cells can easily damage the cells.
Conclusion: The results showed that ultrasound has been able to reduce the microbial load of milk and it made fewer changes in vitamins than milk treated with conventional method. In this regard, the ultrasound probe has been more effective at 75% intensity. The use of ultrasound probe type at 55 ° C and 75% intensity for 10 minutes is recommended as a non-destructive process for milk pasteurization.