C. Judson King

Freeze‐drying of foodstuffs

Recent research and development concerning the freeze‐drying of foodstuffs is reviewed, with emphasis on factors relating to the engineering of freeze‐drying processes. Major areas considered are the physical description and analysis of drying rates, the influence of processing and storage conditions on product quality, improved processing approaches, and quality control.

FACTORS GOVERNING SURFACE MORPHOLOGY OF SPRAY-DRIED AMORPHOUS SUBSTANCES

ABSTRACT Changes in particle morphology (size, shape, and appearance) have been monitored during drying of drops of foods and food-related materials. The apparatus produces a single stream of drops of uniform size, using a vibrating-orifice device for drop production. The drop size and the time-temperature history of the drops as they fall can be varied and controlled. Qualitative observations are reported for drying of aqueous solutions of lactose, maltodextrin, skim milk, and coffee extract, with different feed concentrations. Particular emphasis is placed upon the tendency for development of folds upon the particle surface. A mechanistic model is developed, relating the tendency for folding to the extent of viscous flow of surface material in response to a surface-energy driving force. This model gives semi-quantitative agreement with observations for solutes of different molecular weight (and hence different viscosity) and for different feed concentrations.

Spray Drying: Retention of Volatile Compounds Revisited

ABSTRACT Retention or loss of trace volatile compounds during spray drying can be vital for product quality. Examples of cases where loss or retentions of volatile substances are important include retention of balanced flavor and aroma in food products, removal of odiferous substances, and control of the release of volatile organic compounds to the atmosphere. Factors and mechanisms determining losses of these substances are reviewed. Losses can occur during atomization, from undisturbed drops and as a result of morphological development. On the basis of this insight, several avenues for controlling the retention of volatile substances are identified and analyzed. These include control of atomizer pressure or rotation speed, choice of spray angle, configuration of air input, lteration of the air temperature profile, feed concentration, the presence of an oil phase and/or suspended solids, foaming of the feed, feed composition, and steam blanketing of the atomizer. The quality of a spray-dried product can ...

The onset of natural convection from time-dependent profiles

Abstract Experiments were conducted on the initiation of natural convection in deep pools (penetration depth is small compared with the fluid depth). Fluids covering a wide range of Prandtl number were heated from below. A constant value of Rayleigh number based upon density boundary layer thickness ( Ra 1 ) correlates the effects of surface boundary condition and fluid depth. These effects and those of Prandtl number and wall spacing can be interpreted in terms of growth rate-amplification theory; however a constant value of the Peclet number at the onset of convection was found to correlate the data better than did a constant value of the amplification factor. Theoretical reasons for this result are presented.

FACTORS INFLUENCING FLOW PATTRENS, TEMPERATURE FIELDS AND CONSEQUENT DRYING RATES IN SPRAY DRYING

ABSTRACT The PSI-Cell model is used to predict flow patterns and drying rates for laboratory-scale spray dryers. The liquid feed is water, with atomization producing a spectrum of droplet sizes. An analysis of the cause-and-effect relationships determining droplet trajectories, temperature fields, the location and magnitude of the backflow recirculation effect, and drying rates provides insight into the factors governing spray-air contacting and dryer performance. Independent effects are assessed for spray angle, dryer column diameter, initial droplet size distribution and droplet velocity (separately and as linked through atomizer pressure), air/water flow ratio, and liquid volatility. Inferences are made for effective dryer design, selection of operating conditions, and logic for scale-up.

Let engineers go to college

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THE MECHANISMS OF TRANSPORT OF WATER AND N-PROPANOL THROUGH PULP AND PAPER

Measurements have been made of transport rates of water, n-propanol, and mixtures of the two through samples of kraft pulp and filter paper. Comparison of the rates for the two components and the measured effects of temperature and relative humidity provide insight into the rate-limiting mechanisms of transport within these materials. Results show that gas-phase transport within pores and interstices dominates the mass-transfer process. Condensed-phase transport also occurs for water at high moisture contents, more so for paper than for pulp. The lesser contribution of condensed-phase transport for pulp may reflect the substantially lower bulk density of pulp. The dominance of gas-phase transport for pulp is also borne out by results of a second experiment, wherein pressure drops for gas permeation through pulp were measured following step changes in the humidity of the permeating gas.

Wetting and adsorption properties of hydrophobic macroreticular polymeric adsorbents

Abstract Wetting and adsorption properties have been measured for Amberlites XAD-2 and XAD-4, and Ambersorb XE-340 (Rohm & Haas Corp.). Substantial adsorption of polar organic solutes from water occurs for these resins in both prewet and nonwet states. Adsorption onto prewet resins is equivalent to that at aqueous-paraffin and aqueous-vapor interfaces, suggesting that only dispersion forces act at the interface. Adsorption onto nonwet adsorbents is attributable to the solid-vapor interface, and can be linked to sorption at the solid-liquid interface through the contact angle of water, using the Young-Dupre equation. Adsorbents which have higher nonwet adsorption capacities are also more easily wet by aqueous solutions. If a contact angle of 50° for wetting is assumed, based upon published results for the wetting of fiber membranes, the predicted wetting and adsorption properties of XAD-2 and XE-340 agree with the experimental results.

Energy conservation in regenerated chemical absorption processes

Abstract Energy savings achievable through the use of split flow and absorber intercoolers are explored for absorption of CO 2 and H 2 S by aqueous solutio

HEASUREHENT OF INSTANTANEOUS RATES OF LOSS OF VOIATILE COMPOUNDS DURING DRYING OF DROPS

ABSTRACT A novel technique serves to monitor instantaneous rates of loss of a volatile solute from a suspended drop during drying. A highly sensitive electron capture detector is used to monitor concentrations of SF6 released into a flowing gas stream from a suspended, drying drop. Simultaneously, the appearance and morphological development of the drop are monitored with a video camera. This provides the wherewithal of relating instantaneous rates of loss of the volatile solute to particular events during the development of particle morphology. Initial experiments have been carried out with drops of aqueous solutions of glucose, sucrose, maltodextrin and coffee extract. The results clearly display the onset of the volatiles-retentive selective diffusion phenomenon. There is also substantial loss of the volatile component later in the drying process, when the drops undergo repeated ex ansion, bursting and cratering due to the formation of internaf bubbles. These experiments appear to be the first quantita...