Encapsulation using cyclodextrin (CD) is widely researched in the food and pharmaceutical fields. Ethylene gas, a fruit ripening and plant growth regulation phytohormone, can be encapsulated into the α-CD to form the ethylene- α-CD inclusion complex (IC). The present research investigated the encapsulation methodology, controlled release of the gas from the IC
powder and the use of this powder in fruit ripening.
Liquid encapsulation of ethylene using the α-CD was performed at 0.2-1.5 MPa for 12-120 h and characteristics of the ethylene-α-CD IC powder were analysed using physiochemical methods. The ethylene concentration in the IC powder was found to be 0.98 to 1.03 mole ethylene/mole CD. The complex powder yield increased as a function of pressure and time of
treatment. The IC powder was characterised by the X-ray diffractometry (XRD) sharp peaks and scanning electron microscopy (SEM) images. It also exhibited chemical shifts for carbon atoms and the ethylene characteristic double bond (123.47±2 ppm) in nuclear magnetic resonance spectroscopy (CP-MAS 13C NMR) spectra and stretching and bending of carbon covalent bonds (C=C and C-H) in Fourier transform infrared spectroscopy (FTIR) spectra. The IC powder was also
characterised by the changes in thermal properties and water loss in the differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA).
Ethylene gas release from the ethylene-α-CD IC powder was studied at RHs of 52.9%- 93.6% and temperatures of 45 oC-105 oC. Increases in either RH or temperature accelerated the release of ethylene gas. Avrami’s equations gave a better fit for ethylene release kinetics than did the Power Law. The release parameter n corresponded with a diffusive mechanism at 52.9% and 75.5% RH and to a first-order mode at 93.6% RH. Temperature treatment of the IC powder exhibited diffusive release mechanisms. Ethylene was found to be stable within the IC powder at 52.9% RH. In contrast, complete release was recorded in 2 weeks at 93.6% RH. For quicker release at lower humidity and temperature conditions, deliquescent CaCl2 and MgCl2 were used. The deliquescent salts in an admixture ratio of 1:5 (salts / IC powder) were exposed to RH of 11.2%-93.6% at 18 oC. The salts were found to deliquesce at 32.7% RH and the saturated salt could dissolve the IC powder to give ethylene release. The release rate increased with higher RH and the highest release ratios were found at 75.5% and 93.6% RH after 24h.
Ethylene released from the IC powder was tested for the ripening of mango fruit. The trials were carried out in a laboratory and on a commercial in-transit pilot scale. In the laboratory experiments, fruit treated by the IC powder ripened in a shorter time of 9-10 days after harvest compared to untreated fruit of 15 days. The IC powder ripened mango showed more uniform quality than the untreated control fruit. In the in-transit ripening from Katherine (Northern Territory) to Adelaide (South Australia) over 2-4 days, the ethylene released from the IC powder with CaCl2 was at concentrations of 5.0-10.5 μL/L in the truck containers. In the shipment trials, shorter ripening time (by 3-6 days) also found versus untreated control fruit. The IC powder provided a simple and convenient option for in-transit fruit ripening, representing an innovation for the commercial supply chain of mango fruit.
Another novel encapsulation technique investigated in this work was the direct encapsulation of ethylene gas in an amorphous α-CD powder. The assumption made was that the amorphous structure of CD molecules in the powder can directly interact with ethylene gas. In this way, during encapsulation, the cavity of individual CD molecules can be occupied by ethylene molecules, thereby potentially improving the production yield. The amorphous α-CD powder was produced by spray drying of an aqueous α-CD solution. Accordingly, the novel solid encapsulation of ethylene using amorphous α-CD was tested. Ethylene gas was pressurised into amorphous α-CD at low (LM; 4.10% ± 0.20) and high (HM; 7.46% ± 0.60) moisture content levels under 1.0-1.5 MPa for 24-120 h. The ethylene concentration in the encapsulated powder was 0.45-0.87 mole ethylene/mole CD for the LM α-CD and 0.42 - 0.54 mole ethylene/mole CD for the HM α-CD. The release of ethylene from the ethylene encapsulated amorphous powder was studied at 11.2-52.9% RH over 1-168 h. Ethylene was released fast from the encapsulated powder as a function of RH increases. The α-CD powder before and after the encapsulation remained an amorphous structure, as shown in X-ray diffractometry (XRD) and scanning electron microscopy (SEM). Nuclear magnetic resonance spectroscopy (CP-MAS 13C NMR) spectra showed the ethylene gas to be present in the encapsulated α-CD product.
Overall, this research has confirmed the inclusion complexation between ethylene gas and the α-CD for the first time by using a range of analytical methods and has characterised its release kinetics from the IC powder. The application of releasing ethylene from the IC powder for the ripening of mango fruit offers a novel and convenient method for the fruit ripening industry, especially for in-transit ripening. This first time result of using amorphous α-CD for the encapsulation of ethylene gas will potentially contribute to the innovation of encapsulation methodology for the food, agricultural and pharmaceutical industries.