Developments in fluidised bed freeze drying

Developments in fluidised bed freeze dryin

Green synthesis of silver and copper nanoparticles using ascorbic acid and chitosan for antimicrobial applications

Silver and copper nanoparticles were produced by chemical reduction of their respective nitrates by ascorbic acid in the presence of chitosan using microwave heating. Particle size was shown to increase by increasing the concentration of nitrate and reducing the chitosan concentration. Surface zeta potentials were positive for all nanoparticles produced and these varied from 27.8 to 33.8 mV. Antibacterial activities of Ag, Cu, mixtures of Ag and Cu, and Ag/Cu bimetallic nanoparticles were tested using Bacillus subtilis and Escherichia coli. Of the two, B. subtilis proved more susceptible under all conditions investigated. Silver nanoparticles displayed higher activity than copper nanoparticles and mixtures of nanoparticles of the same mean particle size. However when compared on an equal concentration basis Cu nanoparticles proved more lethal to the bacteria due to a higher surface area. The highest antibacterial activity was obtained with bimetallic Ag/Cu nanoparticles with minimum inhibitory concentrations (MIC) of 0.054 and 0.076 mg/L against B. subtilis and E. coli, respectively

Formulation optimisation of mixed sugar/protein/maltodextrin encapsulants for spray drying L. acidophilus using the response surface method

Three sugars (maltose, fructose, and lactose) have been combined in different formulations with three protein based powders (whey protein, skim milk, and soy protein) to assess the survivability of L. acidophilus after spray drying at 80°C followed by optional further exposure to simulated gastric intestinal juice (SGI) or bile solution. The results showed that the highest survival rate was found in a recipe consisting of 87.5% skim milk and 12.5% maltose, while the lowest rates were found in formulations containing no protein. Maltose and lactose provide higher survival rate than fructose which may reflect the higher glass transition temperature of maltose/lactose mixtures. Similar trends were found with cells rehydrated in SGI and bile solutions

A freeze-drying microscopy study of the kinetics of sublimation in a model lactose system

Freeze drying microscopy has been used to probe the lyophilisation kinetics of lactose solutions of various concentrations, at temperatures ranging from -50. °C to -30. °C and under a constant pressure of 1. Pa. Sublimation front velocities were determined by recording a sequence of video images of the sublimation and analysing the frontal progression using MATLAB. Initial experiments showed poor reproducibility. To combat this, silver iodide (AgI) was added as an ice nucleator, which raised nucleation temperatures and improved reproducibility when compared to non-AgI experiments. The lower supercooling on nucleation when AgI was used also produced larger ice crystals, which enabled the crystal microstructure of the more dilute samples to be more clearly observed. This showed long thin crystals, and the orientation of these crystals with respect to the direction of the frontal movement strongly affected frontal progression rates, which explained the earlier reproducibility problems. A twin resistance mass transfer model, comprising a fixed edge resistance and a resistance which increased with frontal depth, was able to describe the sublimation kinetics. The edge resistance first increased and then decreased with solids content. The resistance per unit depth increased exponentially with solids content, so much so that there is an optimal solids content in relation to the rate of production of dried material. Resistances were also much higher when crystals were oriented with their major axis perpendicular to the direction of frontal movement. Freeze drying rates were approximately proportional to the saturation vapour pressure of water, however the long-held belief that water vapour pressure is the main driving force for mass transfer in freeze-drying systems may be an oversimplification as this only reflects driving forces in the vapour phase (pores) rather than within the solid

Spray-freeze-drying of whey proteins at sub-atmospheric pressures

Spray-freeze-drying (SFD) involves spraying a solution into a cold medium, and freeze-drying the resultant frozen particles, which can be performed by contacting the particles with a cold, dry gas stream in a fluidized bed, typically at atmospheric pressure. This enables much faster drying rates than are usually possible by conventional freeze-drying, due to the small particle sizes involved. However, the quantities of gas required for atmospheric fluidized bed freeze-drying are prohibitively expensive. This has led to a process modification whereby fluidization is performed at sub-atmospheric pressures, which still allows rapid freeze-drying, but using much less gas. This study demonstrates the fluidized bed spray-freeze-drying technique at sub-atmospheric pressures (0.1 bar) using whey protein isolate solution (20% w/w solids) at gas inlet drying temperatures ranging from -10°C to -30°C. The process yields a powder consisting of highly porous particles and shows little loss of solubility for β-lactoglobulin and a-lactalbumin, the principal proteins in the isolate. A wet basis moisture content of 8.1% was achieved after freeze drying at -10 °C for only 1 hour, whilst at 30 °C a longer drying time (100 minutes) produced a wetter product (14% w.b.)

A comparison of the survival rates of E. coli K12 and L. acidophilus in spray drying

The survival of mid-exponential and the early-stationary E. coli K12 and L. acidophilus were investigated when spray drying and outlet air temperatures of 60, 70, 80, 90 and 100°C. The results showed that the early-stationary cell of both cultures had a greater heat resistance than the mid-log cell in every drying temperature. The best survival rate was found when spray drying at temperature lower 80°C and it is showed that L. acidophilus is stronger than E. coli K12 (irrespective of the growth phase)

A kinetic model for whey protein denaturation at different moisture contents and temperatures

The denaturation of whey protein samples that had previously undergone heat-treatment for different times at different temperatures and moisture contents was analysed by differential scanning calorimetry (DSC), using the DSC enthalpy as a measure of residual undenatured protein. Data were fitted to first order irreversible or reversible kinetic expressions, and the resulting rate constants were found to increase with both temperature and moisture content. The whole data set was then fitted as a function of time, temperature and moisture content, with rate constants varying according to either Arrhenius or Williams-Landel-Ferry (WLF) kinetics and with selected fit parameters made empirical functions of moisture content. The best fits were obtained using reversible WLF kinetics, which could be further slightly simplified without loss of accuracy. The model provides a platform for single- and multi-objective drying trajectory optimisation with respect to protein denaturation in dairy products

Spatial variation of starch retrogradation in Arabic flat bread during storage

Samples from different radial positions of flat Arabic bread were analysed by differential scanning calorimetry (DSC) immediately after baking and after up to 3 days storage. This showed that whilst almost complete gelatinisation initially occurs, higher levels of subsequent retrogradation (typically 1.5 to 3 times) occurred in an area intermediate between the centre and outside of the pita bread (viewed from above). This coincided with the region with the highest moisture content (30% w.b.) immediately after processing, and which is likely to have heated at the slowest rate. A parallel study using DSC which subjected dough samples to a temperature profile similar to that found in baking also found that relatively low heating rates of 20 °C min−1 produced slightly higher amounts of retrogradation (typically 5–25%) than higher heating rates of 200 °C min−1. In each case moisture contents during storage were comparable between samples, thus suggesting that the local heating rate experienced during baking is a key parameter that can explain differences in subsequent retrogradation in different regions of the pita bread

A study of particle histories during spray drying using computational fluid dynamic simulations

Computational fluid dynamics (CFD) models for short-form and tall-form spray dryers have been developed, assuming constant rate drying and including particle tracking using the source-in-cell method. The predictions from these models have been validated against published experimental data and other simulations. This study differs from previous work in that particle time histories for velocity, temperature, and residence time and their impact positions on walls during spray drying have been extracted from the simulations. Due to wet-bulb protection effects, particle temperatures are often substantially different from gas temperatures, which is important, because the particle temperature–time history has the most direct impact on product quality. The CFD simulation of an existing tall-form spray dryer indicated that more than 60% of the particles impacted on the cylindrical wall and this may adversely affect product quality, because solids may adhere to the wall for appreciable times, dry out, and lose their wet-bulb protection. The model also predicts differences between the particle primary residence time distributions (RTD) and the gas phase RTD. This study indicates that a short-form dryer with a bottom outlet is more suitable for drying of heat-sensitive products, such as proteins, due to the low amounts of recirculated gas and hence shorter residence time of the particles

Application of computational fluid dynamic (CFD) simulations to spray-freezing operations

A 3-D computational fluid dynamics (CFD) simulation for spray-freezing in a cold gas has been developed and used to identify design improvements. This model includes an approximate method to model the latent heat of fusion, and is able to track particle trajectories. The simulation predictions agreed reasonably well with experimentally measured gas temperatures and droplet velocities. The results suggest that a hollow cone spray is more effective in cooling the particles uniformly. The CFD simulation suggested that build up of an icy layer on the cone walls observed experimentally was due to incomplete freezing of larger particles (> 100 µm). Collection efficiencies could be raised (from 20% to 57%) by increasing the diameter of the chamber outlet