Control of Food-Quality Factors in Spray Drying

Abstract

Research over the past 15 to 20 years has provided considerable fundamental insight into mechanisms of phenomena governing quality of spray-dried food products. Important advances have been made in understanding rates of drying, retention of volatile flavor and aroma substances, changes in particle size and shape (morphology), stickiness and agglomeration, and extents of thermal degradation reactions. Recent research results in each of these areas, obtained at Berkeley and elsewhere, are described and interpreted. From these results conclusions are drawn regarding effective means of improving and controlling product-quality factors.

Losses of volatile components are governed by a ternary diffusion process with the drops. The loss rates are greatest in the immediate vicinity of the atomizer, before sufficient drying of the drop surfaces occurs for a selective-diffusion mechanism to become dominant. Foaming and incorporation of an emulsified oil phase or surfactants all have substantial effects on volatiles loss. These effects can be interpreted through transport analyses.

Particle inflation during drying requires entrainment or nucleation of an air bubble as a first step, and is then governed by evaporation of water vapor into the bubble. Tendencies to form surface folds on particles are governed by the viscosity of the concentrated solution, and are described by a dimensionless group involving viscosity, surface tension, particle size, and exposure time.

Bulk mixing, convection patterns, and drop trajectories may be interpreted through the configuration of the dryer, as well as the ratio of the air supply rate to the rate of air entrainment into the spray by momentum exchange as the drops decelerate. These patterns determine the local temperature and humidity and thus directly influence rates of drying and loss of volatile components.