Usage of ohmic heating process in milk evaporation, electrical conductivity, performance analysis and determination of some quality properties

In this study, the moisture content of 7.85±0.02 kg water/kg DM milk sample was concentrated to a content of approximately 3±0.12 kg water/kg DM using ohmic heating assisted evaporation (OHAE) and traditional evaporation process. The temperature change, total processing time and electrical conductivity value of the concentrated milk sample using the OHAE process were investigated. In addition, the performance and some quality characteristics of concentrated milk by traditional evaporation process and ohmic heating assisted evaporation process were examined and the two methods were compared statistically. The heating time of milk from 20 °C to boiling point was between 10-30 minutes at 3 different voltage gradients (7,11 V/cm) using OHAE and increasing voltage gradient shortened the processing time (p<0.05). The evaporation time of the milk sample with an initial moisture content of 7.85±0.02 kg water/kg DM to approximately 3±0.12 kg water/kg DM value varies between 31-92.5 minutes and the increasing voltage gradient has a positive effect on the processing time. In addition, it was determined that the traditional evaporation process was completed in 41 minutes (p<0.05). During the evaporation process, it was determined that there was an increase approximately 1 °C in the boiling point, depending on the increasing concentration. In OHAE process, there was a linear relationship between time and moisture during the milk concentration to about 3±0.12 kg water/kg DM content, and the R2 value was greater than 0.99. With the developing technology, system and/or process efficiency is considered as an important parameter. In the present study, the performance values of OHAE process were examined and the energy efficiency value ranged between 67.1-56.3% and the energy efficiency of the traditional evaporation process was 37.1%. The exergy efficiency value varies between 59.3-49.7% and the exergy value obtained in the conventional evaporation process is 32.7% lower than the ohmic heating process. Hydroxymethylfurfural (HMF) and color properties of the concentrated milk obtained from OHAE evaporation process were investigated. The HMF value of the concentrated milk samples were between 10.2-18.0 ppm, and the HMF content of the milk sample obtained by the traditional evaporation process was 17.7 ppm. The HMF content of sample from OHAE process is lower, but the increase in the processing time due to the decreasing voltage gradient affects the HMF content negatively. As a result, the moisture content of 7.85±0.02 kg water/kg DM milk was successfully concentrated to a value of approximately 3±0.12 kg water/kg DM by OHAE. In the ohmic heating application, shorter processing time and higher efficiency in terms of energy and exergy efficiency were obtained. Concentrated milk sample with lower HMF content in terms of quality characteristics was obtained in 11 and 9 V/cm ohmic applications. Key Words: Ohmic heating, evaporation, electrical conductivity, energy, exergy, quality