Filtration and drying characteristics of casein

Abstract

Casein precipitated by acidolysis of the skimmed milk was filtered under constant pressure conditions followed by the cake drying in a vacuum tray dryer. Filtration characteristics of the cake were expressed in terms of average specific cake resistance (αav) and cake solids fraction or porosity. An interesting trend was observed at lower feed concentrations of casein in the filtration slurry. In the initial period of filtration, αav, was found to be lower under the prevailing conditions as compared to the increased value of αav in the later part of filtration. This phenomenon accentuates the strong relationship between the differential pressure across the filtration unit and αav. To corroborate these findings, the pressure drop across the cake was found to influence its compressibility, n, that varied between 0.4 and 0.8. The differential pressure studied was in the range of 0.5–2.5bar. The influence of gas deliquoring on cake saturation (S) was studied to obtain the threshold pressure, an useful design parameter for pressure filtration.The drying of casein cake in a vacuum tray dryer showed the typical drying rate curve with constant rate period followed by the extended falling rate period. The drying curve was analyzed in terms of moisture diffusivity, which in turn was found to be a strong function of temperature. To overcome long drying time of few hours, the casein cake after partial dehydration was extruded to get the benefit of higher surface area to volume ratio. The extruded pellets were effectively and efficiently dried to the final desired moisture content. It was shown that the total drying time and drying technique influence to a large extent the protein denaturation.