Mathematical modelling of modified atmosphere packaging systems for apples : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process and Environmental Technology at Massey University
ENZA New Zealand (International) is considering the use of modified atmosphere packaging (MAP) as an adjunct to cool storage for cartons of apples. The objectives of this study were to measure modified atmosphere development in apple cartons and to develop a mathematical model that could be used as a tool for package design and optimization. Storage trials were carried out with film-lined cartons of 'Braeburn', 'Royal Gala', and 'Granny Smith' apples. Measured package O2 and CO2 concentrations showed excellent reproducibility for cartons with heat-sealed liners. Liners closed by folding produced less modified and less consistent package atmospheres, especially for thicker films (40 µm versus 25 µm). Macroscopic holes in the liners resulted in almost total loss of atmosphere modification, whereas microscopic holes resulted in smaller changes apparent for O2 concentrations only. A high incidence of film damage could quickly erode any potential fruit quality benefits imparted by the liners. Packing of warm rather than pre-cooled fruit resulted in much faster rates of atmosphere modification, without the development of unduly low O2 or high CO2 concentrations. The detrimental quality effects of slower cooling rates for film-lined cartons may outweigh any benefits of more rapid modified atmosphere development. Short-term exposures (less than 24 hours) to 20°C resulted in relatively short-lived and non-critical disturbances to package atmospheres. Periods of more than 3 days at 20°C led to a significant risk of anaerobic conditions or harmful CO2 levels forming within the fruit, especially within the 40 µm liners. Folding rather than heat-sealing of liners did not reduce this risk. The MAP model simulated fruit respiration as a function of temperature and fruit O2 and CO2 concentrations; O2, CO2, N2, and water vapour exchange between the fruit, package, and external atmospheres; condensation of moisture within the package; and moisture sorption by paper-based packaging materials. Gas concentrations and temperature throughout (i) the fruit and (ii) the package atmosphere were each assumed to be uniform with position. The model can be applied to a wide range of packages under variable-temperature storage regimes. The model closely predicted observed trends in experimental data collected during the MA storage trials, but tended to under-predict CO2 concentrations and performed less well under conditions of extremely modified atmospheres. Sensitivity analyses showed that this lack of fit was not greater than that which could be explained by uncertainties in respiration and permeability data. It is recommended that future work be aimed at resolving the worst of these uncertainties before a significant amount of effort is directed towards further model development. The MAP model was considered sufficiently accurate for it to be usefully applied to the design and optimization of MAP systems.