Continuous Vacuum Drying of Whole Milk Foam

Abstract

The principles developed on a batch scale for preparing vacuum foam-dried whole milk have been successfully applied on a continuous basis. An integrated pilot plant and means for creating a foam in vacuum and distributing it in a thin layer onto a moving belt are described. In the continuous process, pasteurized homogenized market milk is concentrated in an agitated, falling-film vacuum evaporator from 12% solids to a solids content (43% average) corresponding to a viscosity at 100 F of 32–43 centistokes. The concentrate is homogenized at 3,000-500 and 135 F. Nitrogen is metered into the concentrate stream and a scraped-surface heat exchanger then simultaneously chills the concentrate to 35 F and disperses the gas. Finally, within a drying chamber maintained at reduced pressure, the gassed concentrate expands to a foam which is applied and dried on a continuous solid belt. Results of a study on the effects of process variables are reported and reproducible operating conditions are given for preparing products of excellent dispersibility and initial flavor. The study was restricted to dryer chamber pressures (9 mm Hg and below) found to preserve the foam integrity during drying (as in the case of batch drying). With this strict translation of the batch principles to a continuous operation, mass and heat transfer limitations were encountered, resulting in product rates which, when extrapolated to commercial scale operation, are considered too low to be of commercial significance. Thus, current work is directed toward improving product rate while maintaining product quality. © 1962, American Dairy Science Association. All rights reserved.