The effect of spray drying on the physicochemical properties of products

The impact of spray drying on the physico-chemical properties of spray dried lactose and spray dried polyethylene glycol (PEG) 4000/lactose samples was investigated. Isothermal microcalorimetry was used to evaluate the amorphous content of the spray dried material. The parameters that influence the area under the micocalorimetric crystallisation curve were investigated in order to best understand the application of isothermal microcalorimetry to determine amorphous content of materials. It was shown that in order to obtain good quantitative data for crystallinity, it is necessary to use saturated salt solutions that give a slow supply of vapour at 25°C. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) provided qualitative data for the lactose and PEG 4000 polymorphs in the spray dried samples. Gas chromatography was used to measure the alpha/beta anomeric ratio in the lactose samples. Variations in feed concentration influenced the degree of disorder in the spray dried products. Surprisingly, the % disorder in materials was substantially greater than the amount of lactose dissolved. The results were attributed to a milling effect on the suspended lactose particles. The crystalline fraction in the spray dried materials was mainly the alpha-anhydrous form. Furthermore, only the sample with the highest feed concentration retained any amount of the monohydrate form. None of the lactose feed concentrations that were studied, resulted in any detectable amounts of crystalline beta-lactose. Products spray dried from solutions prepared at different feed temperatures were found to be completely amorphous. Increases in feed temperature are known to decrease the solution alpha/beta ratio. However, the alpha/beta ratio in the products increased with feed temperature which demonstrated that beta-alpha mutarotation occurred during the spray drying process. The highest feed temperature products retained a similar beta-content as the feed due to little opportunity for beta-alpha transition during the spray drying process. On exposure to relative humidity (RH), amorphous-crystalline transition occurred. The crystallisation lag times for the samples were longer for products made with a high feed temperature due to the greater amorphous beta-lactose content. The effect of spray drying lactose in the presence of PEG (12% by weight of lactose) produced interesting findings. PEG was expected to inhibit lactose crystallisation, but the results confirmed that the spray dried PEG/lactose samples were crystalline. During the spray drying process, PEG presumably slowed the evaporation rates such that crystallisation occurred. The 10 or 20g/100ml PEG/lactose solutions yielded the monohydrate, alpha-anhydrous, beta-lactose and the extended chain PEG crystals. Whereas, the 30 or 40g/100ml suspension produced only alpha-monohydrate, alpha-anhydrous and the extended chain PEG crystals. This implied that complete beta-alpha transition occurred during the spray drying process. Further work is still needed to obtain a complete understanding of how PEG enhances the crystallisation of lactose during the spray drying process. The overall studies have improved the understanding of the influence of process variables on the properties of the spray dried products, which can be useful in the design and optimisation of spray granulation processes.