Low-temperature Vacuum Drying Of Natural Pigment And Its Mathematical Model

Natural pigment, which is mainly extracted from plants, animals and microbes, is safer than artificial pigment. It has the physiological activity, nutritional and pharmacological function. It is widely used in food, medicine and cosmetics industries. However, due to its vulnerability to light, heat and pH, it is prone to oxidation and decomposition. Its unit color value and shelf life are seriously affected by quality degradation during the processing. Drying, which is an important part in natural pigment processing, is a useful method to deal with the oxidation and decomposition. In order to improve the products'quality, this subject make a deep research on the influence of the different drying methods on the properties of natural pigment. The properties of the product, i.e., color value, color difference and moisture content etc., were investigated. The investigations of the low-temperature vacuum drying of natural pigment was made in details. The effects of operational parameters such as heating temperature, solution layer thicknesses and the level of vacuum on the product quality as well as its drying kinetics were studied, too.In order to investigate the effects of different drying methods on natural pigment quality, morphology and energy consumption, spray drying, freeze drying and low-temperature vacuum drying were selected for the dehydration of Gardenia yellow pigment solutions. For spray drying, the inlet air temperature was set from 180℃to 190℃, the outlet air temperature was set from 90℃to 100℃, 20% solutions (w/w) were selected as the feed concentrations and air flow rate was 40 m³/h. For freeze-drying, the sample was frozen at the temperature of - 40℃for two hours at first, then the solution was dehydrated under the absolute pressure of 400 Pa, The drying process lasted for 49 h. For the low-temperature vacuum drying process, the heating temperature was selected at 35℃, and vacuum 0.07 MPa, the sample thickness was 7 mm and the concentration was 15 %, the drying time was about 26 h. Through the comparison of the Gardenia yellow pigment powder's qualities, i.e. moisture content, color value, color difference and microcosmic morphology, it was found that spray drying had the most negative effect on the powder's qualities, but its dried product had uniform microscopic powder form. Meanwhile, the products'properties by low-temperature vacuum drying have no difference with that by freeze-drying. Finally, the energy consumption of three different drying methods were computed. It showed that spray drying got the lowest energy consumption and followed by low-temperature vacuum drying and freeze drying. In consideration of product quality and economic efficiency, low-temperature vacuum drying is a suitable for drying of natural pigment.The optimal technologies of low-temperature vacuum drying of the natural Gardenia yellow pigment was studied, as well. Parameters, i.e., heating temperature, the level of vacuum, sample concentration and thickness, were taken as affecting factors. L₉(3⁴) orthogonal experimental design was used. After the dried products reached the same moisture content, properties of Gardenia yellow pigment, i.e., color value (CV), color difference (CD) and drying time (t) were analyzed. From analysis of data, it was found that color value, color difference and drying time were significantly affected by the heating temperature. Color value were affected greatly by the sample thickness. With the comparison experiment of single-factor, drying time for heating temperature at 55℃was 21.05 % shorter than that at 35℃. It was also found that the other two parameters, i.e., sample concentration and the vacuum had relatively less impact on product quality and the drying time. Finally, the optimal conditions were obtained. They were heating temperature of 45℃, sample concentration of 20 %, thickness of 5 mm and the vacuum of 0.09 MPa. The powder properties of Gardenia yellow pigment at this conditions were moisture content of 4.35 %, color value of 87.8, and color difference of 74.12. The whole drying time was 1360 min.Natural gardenia yellow pigment solution was dried by low-temperature vacuum drying. In order to investigate the characteristics of low-temperature vacuum drying, eight mathematical models, i.e., Newton model, Henderson and pabis model and Page model etc., were examined using Matlab. The test was performed with the heating temperature of 45℃, the vacuum level of 0.08 MPa, solution thickness of 7 mm and solution concentration of 20 %. It was shown that Page model gave the best prediction of drying performance. Validation experiment showed that the predicted moisture ratio by Page model had a good agreement with the experimental results.In application, low-temperature vacuum drying was used to dry the natural Mulberry red pigment. L₁₆(4⁴) orthogonal experimental design was used. Parameters of heating temperature, the vacuum, sample concentration and thickness were also taken as affecting factors. After the dried products reached the same moisture content, the color value (CV) of the products and drying time were analyzed. From analysis of data, it was found that color value and drying time were significantly affected by the heating temperature, and the other three parameters, i.e., the vacuum, sample concentration and thickness, had an impact on the color value. Finally, the optimal conditions were obtained. They were heating temperature of 45℃, sample concentration of 15 %, thickness of 5 mm and the vacuum of 0.08 MPa. The powder properties of Mulberry red pigment at this conditions were moisture content of 4.26 %, color value of 76.2, and the drying time of 1360 min.