Studies of the encapsulation and release of carbon dioxide from amorphous and crystalline alpha-cyclodextrin powders and its application in food systems
Carbon dioxide gas has been widely used in food production. Nevertheless, the conventional ways to utilize CO 2 gas have limitations in terms of safety, convenience, handling and storage. To offer a safe and convenient approach to use CO 2 gas, the production of food-grade CO 2 powder in which CO 2 release can be controlled was investigated. Conventionally, such gas powder has been produced via molecular encapsulation, accomplished by compression of the gas into either a solution of
... extrin (-CD) or crystalline -CD in a solid state. However, shortcomings (low yield or stability of the complex) of these techniques have prevented their actual application. In this project, an innovative method to produce CO 2 --CD complex powder with high yield and stability was investigated using amorphous spray-dried α-CD powder followed by crystallization of the complex. Due to a lack of understanding of amorphous α-CD powder properties and the complexities of conventional methods to quantify CO 2 in solid systems, the project commenced with the characterization of α-CD powders and the development of a simple system to determine the amount of encapsulated CO 2 . The study of the structure of α-CD powders revealed that spray drying of α-CD solution resulted in a completely amorphous powder (T g 83 o C). The differences in molecular structure between crystalline and amorphous α-CDs were illustrated by the analytical results of SEM, X-ray, FTIR, DSC, TGA and 13 C-NMR. The study of moisture sorption showed that an amorphous α-CD powder adsorbed more water than its crystalline counterpart at the same a w but it crystallized as it was equilibrated at higher than 65% RH (>13.70g moisture/100 g of dry solids). A simple system to quantify the CO 2 in the complex through measuring the amount of CO 2 released from the complex into an air-tight chamber headspace by using an infra-red CO 2 probe was designed and tested. The concentrations measured using this new system and conventional acidbase titration were insignificantly different (p > 0.05). This was also validated by the gas chromatography method. A study of solid encapsulation of crystalline (9.84% MC, w.b.) and amorphous (5.58% MC, w.b.) α-CD powders at 0.4-1.6 MPa for 0-96 h showed that amorphous α-CD encapsulated a much larger quantity of CO 2 than the crystalline form at low pressure and short time (p < 0.05). An increase in pressure and prolongation of the time increased encapsulation capacity (EC) of α-CD, especially for the crystalline form. The highest EC of crystalline α-CD was 1.45 mol CO 2 /mol α-CD, which was markedly higher than that of amorphous α-CD (0.98 mol CO 2 /mol α-CD). Solid encapsulation did iii not affect the structure of amorphous α-CD, but slightly altered the structure of crystalline α-CD. Peak representing the encapsulated CO 2 in the complex was clearly observed on the FTIR (2334 cm -1 ) and NMR (125.3 ppm) spectra. However, the complexes were not stable enough for actual application, especially those produced from amorphous α-CD. To improve the stability of CO 2 gas, crystallization of CO 2 -amorphous α-CD complex was developed. To achieve this, initially water was added to the amorphous α-CD powder to increase its MC to around its crystallization induced level (13, 15 and 17% MC, w.b.), and complexation was undertaken under 0.4-1.6 MPa and compared with crystalline CD complexation. The results showed that the EC of amorphous α-CD significantly increased up to 1.1-1.2 mol CO 2 /mol α-CD. Under the same conditions, the EC of crystalline α-CD showed a considerable decline with an increase of initial MC. The phase transformation of amorphous α-CD powder during complexation was clearly observed in the analytical results of SEM, FTIR, X-ray, NMR and DSC. The crystals of the complex have a cage-type structure entrapping the CO 2 molecules into isolated cavities. However, a large amount of water on the complex surface (a w > 0.95) due to crystallization made it still low in stability. Dehydration of the crystallised complex produced from amorphous -CD powder to improve its stability by desiccant adsorption using silica gel and CaCl 2 desiccants, and release properties of the desiccated complex in air, water and oil media, were investigated. CaCl 2 reduced the complex a w faster, with less CO 2 loss during dehydration, than using silica gel. Dehydration dramatically improved the complex stability. The release rate of CO 2 markedly increased with an increase in RH, and was much faster in water than in oil. However, almost none of the CO 2 was released from the complex kept in airtight packaging during storage. One potential application for controlling the mould and yeast growth in cottage cheese was investigated by direct mixing of the dehydrated CO 2 powder (0.5-0.6 mol CO 2 /mol α-CD) into the product before packing. The results showed a significant inhibition of the mould and yeast growth during storage of cottage cheese at temperatures of 7 and 25 o C. This demonstrated the ease of use of CO 2 powder in food products if CO 2 gas is needed to extend the shelf-life of these products. iv Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis.