Studies of the encapsulation and release of carbon dioxide from amorphous and crystalline alpha-cyclodextrin powders and its application in food systems

Abstract

Carbon dioxide gas has been widely used in food production. Nevertheless, the conventional ways to utilize CO2 gas have limitations about safety, convenience, handling and storage. To offer a safe and convenient approach to use CO2 gas, the production of food-grade CO2 powder in which CO2 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 alpha-cyclodextrin (a-CD) or crystalline a-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 CO2-a-CD complex powder with high yield and stability was investigated using amorphous spray-dried a-CD powder followed by crystallization of the complex. Due to a lack of understanding of amorphous a-CD powder properties and the complexities of conventional methods to quantify CO2 in solid systems, the project commenced with the characterization of a-CD powders and the development of a simple system to determine the amount of CO2 encapsulated. The study of the structure of a-CD powders revealed that spray drying of a-CD solution resulted in a completely amorphous powder (Tg a 83oC). The differences in molecular structure between crystalline and amorphous a-CDs were illustrated by the analytical results of SEM, X-ray, FTIR, DSC, TGA and 13C NMR. The study of moisture sorption showed that an amorphous a-CD powder adsorbed more water than its crystalline counterpart at the same aw but it crystallized as it was equilibrated at higher than 65% RH (g13.70g moisture/100 g of dry solids). A simple system to quantify the CO2 in the complex through measuring the amount of CO2 released from the complex into an air-tight chamber headspace by using an infra-red CO2 probe was designed and tested. The concentrations measured using this new system and conventional acid-base titration were insignificantly different (p g 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.) a-CD powders at 0.4-1.6 MPa for 0-96 h showed that amorphous a-CD encapsulated a much larger quantity CO2 than the crystalline form at low pressure and short time (p l 0.05). An increase in pressure and prolongation of the time increased encapsulation capacity (EC) of a-CD, especially for the crystalline form. The highest EC of crystalline a-CD was 1.45 mol CO2/mol a-CD, which was markedly higher than that of amorphous a-CD (0.98 mol CO2/mol a-CD). Solid encapsulation did not affect the structure of amorphous a-CD, but slightly altered the structure of crystalline a-CD. Peak representing the encapsulated CO2 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 a-CD. To improve the stability of CO2 gas, crystallization of CO2-amorphous a-CD complex was developed. To achieve this, initially water was added to the amorphous a-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 a-CD was significantly increased up to 1.1-1.2 mol CO2/mol a-CD. Under the same conditions, the EC of crystalline a-CD showed a considerable decline with an increase of initial MC. The phase transformation of amorphous a-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 CO2 molecules into isolated cavities. However, a large amount of water on the complex surface (aw g 0.95) due to crystallization made it still low in stability. Dehydration of the crystallised complex produced from amorphous a-CD powder to improve its stability by desiccant adsorption using silica gel and CaCl2 desiccants, and release properties of the desiccated complex in air, water and oil mediums, were investigated. CaCl2 reduced the complex aw faster, with less CO2 loss during dehydration, than using silica gel. Dehydration dramatically improved the complex stability. The release rate of CO2 markedly increased with an increase in RH, and was much faster in water than in oil. However, almost none of the CO2 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 CO2 powder (0.5-0.6 mol CO2/mol a-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 25oC. This demonstrated the ease of use of CO2 powder in food products if CO2 gas is needed to extend the shelf-life of these products.