1.1Background
The horticultural industry is very important to the New Zealand economy. Over recent years (2003-2007), exports of fresh (e.g. kiwifruit, pipfruit, asparagus, squash, onion and tomato) and processed (e.g. processed sweet corn, pea, carrot, and tomato) horticultural products increased in value from approximately $1.97 to 2.53 billion (MAF 2007). To maintain international markets and prices to sustain growers, the key focus of horticultural producers is to maintain and improve already high product quality (MAF 2007). Packaging is considered one key aspect of postharvest technology to both maintain quality and extend shelf life of horticultural products. This can be achieved through improved protection from environment conditions (e.g. light, oxygen, moisture, and contaminations) or enhancing cooling rates to minimise heat accumulation (Merts 1996; Tanner 1998), providing information through labels, and providing improved convenience (e.g. easy opening, and reclosable package) (Ahvenainen 2003). In recent years, packaging technology offers interactive management (e.g. real-time traceability) throughout the supply chain using radio frequency identification (RFID) and global positioning (GPS) tracking (Järvi- Kääriäinen 2003; Han et al. 2005).
Horticultural products, either intact or minimally processed (fresh-cut), are living tissues that continue their metabolic activities after harvest and during storage (Gil et al. 2002; Kader 2002; Lanciotti et al. 2004). Once the optimum storage temperature and relative humidity to minimise the rate of ripening and senescence are established (Thompson & Mitchell 2002), modification of the atmosphere surrounding the products is a common practice to further reduce postharvest quality changes and extend shelf life (Kader et al. 1989). However, optimal packaging atmospheres can be adversely affected by dynamic changes in temperature and relative humidity through the cool chain (Day 1989). This in turn may pose a significant risk to preservation of the product’s quality because modified atmosphere packaging (MAP) relies on the interactions between the gas atmosphere, metabolic activity of the packaged produce and the properties of the packaging to achieve the desired outcomes (Zagory 1995).
For a given permeability of the packaging material, the packaged product adjusts its metabolic activity according to the concentrations of the main respiratory gases (CO2 and
O2). If the metabolic activity increases, this will increase CO2 production and lower O2
availability (Kader et al. 1989). The respiration rate of horticultural products is more strongly affected by temperature than the permeability of most existing packaging films utilised in MAP. Thus even a small temperature increase can cause rapid accumulation of CO2 and depletion of O2 in the package (Cameron et al. 1995). Continuous decreases in
the O2 level can initiate anaerobic respiration or fermentation, which generally results in
poor quality, such as off-flavour development and increased pathogenic susceptibility (Labuza & Breene 1989; Yam & Lee 1995).
To improve the efficacy of MAP, active packaging technologies have been incorporated to provide alternative interactive controls between the packaged food, package, packaging atmosphere and the environmental conditions to better achieve and retain optimal modified atmospheric conditions inside the package (Rooney 1995b; Brody 2002). In the context of horticultural products, “active” may refer to systems for control of environmental
parameters such as moisture content or gas atmosphere composition, to the control of microbial growth, or the control of the physiology of the product. According to Rooney (1995b), this is mainly achieved by scavenging mechanisms, which remove one or more components from the internal environment, or desorption mechanisms, which allow the controlled release of one or more active components.
Whilst the scavenging systems are commercially available, desorption systems are less well studied and yet offer interesting possibilities for extending shelf life and quality of perishable product (Utto et al. 2005). Although a number of the desorption-based active packaging systems have been commercialised (for example sachet releasing ethanol vapour commercially sold under the trade names of Antimold Mild and Negamold Freund Industrial Co., Japan), generic approaches to design and optimise packages are mainly empirical or rely on proprietary information from companies.
Fundamental knowledge needed to better implement their new technology relating to packaging design of desorption-based active packaging systems is valuable. To maximise application of this knowledge it should be incorporated into a framework that would permit engineers and/or technical users to predict outcomes of a new package design for ‘what-if’
scenarios for given products in typical horticultural postharvest handling systems. To simulate and understand ‘what-if’ scenarios relevant to specific new package designs, one may perform many experiments by “trial and error” testing. However this approach has the disadvantages of being costly in experimental development and time consuming, and it is difficult to transfer results obtained from one product-package-storage system to others. Alternatively, the development of predictive tools based on mathematical models, in which the key variables relevant to the new package as well as the handling system can be
specified or estimated, will allow new packaging systems to be designed and screened prior to prototype testing. Development of such predictive tools will formalise the understanding of the underlying phenomena occurring in these systems, and in so doing will contribute significantly to food packaging research.
At the start of this project, the fundamental knowledge needed to construct mathematical models for the design of the such active packaging systems, were limited in availability and scope, and this research to fill these knowledge and technical gaps was justified.
1.2Research aim
The aim of this research project was to formalise the fundamental knowledge of desorption controlled release active packaging in the form of mathematical models. These models will allow the design of systems that can dynamically deliver and sustain one or more active components at effective levels in a package atmosphere, incorporating the postharvest responses of the product and the storage conditions.
1.3Research objectives
To achieve the research aim, the following specific objectives were set as the research milestones:
(1) To develop a conceptual model which accommodates all relevant aspect(s) of the design of active packaging systems, where the active agents can be delivered and sustained at the required level to regulate quality changes through influencing
product physiology and inhibiting growth of spoilage or pathogenic microorganisms, or a combination of both.
(2) For a representative specific application (i.e. a given horticultural product and active control system), to:
(a) Develop a model active packaging system for experimental investigation of the postharvest behaviour of the selected product
(b) Determine the effective concentrations of the chosen active agent and their effects on postharvest attributes of the selected horticultural products, including microbial suppression, respiration rate and ethylene production under simulated storage condition.
(c) Identify the equilibrium and kinetic mechanisms which govern the sorption of the active agent by, and its release from, the chosen carrier material, and how key packaging and environmental factors affect the equilibrium concentration through storage.
(d) Mathematically model and experimentally validate (including assessing the limitations of the model) the distribution of the active agent within the model active packaging system, under a known storage condition.
(3) To use the developed modelling tools to investigate what-if scenarios to gain further insights into release patterns of the active agent for different active packaging systems.
(4) Summarise model methodology and associated input data requirements for design of packaging systems for other food and horticultural products
1.4Structure of thesis
In Chapter 2, a review of relevant literature is presented, covering the current trends and opportunities in the development of the active packaging systems with an emphasis on horticultural products, and the basic physical phenomena from which mathematical models of active packaging system will be developed.
In Chapter 3, the active MAP system of interest, a hexanal controlled release sachet for tomatoes was experimentally studied. Effects of hexanal vapour on postharvest qualities of tomatoes such as antimicrobial activity, rates of respiratory O2 consumption and ethylene
generation, and apparent rates of hexanal uptake by tomatoes were studied.
Chapter 4 presents studies of the effective hexanal permeability of films (used as sachet materials and outer bag) and sorption isotherms for hexanal on silica gel.
Chapter 5 presents the conceptual model developed for active MAP systems. A decision tree for facilitating the selection among modelling options was also developed and utilised for modelling the hexanal-based active MAP for tomatoes. The formulations of
mathematical model and its MATLAB language codes to predict dynamic hexanal concentration in the package headspace are presented.
Chapter 6 presents results of model validations against experimentally collected data of a range of active MAP systems and discusses the model performance through sensitivity analyses considering uncertainties associated with model inputs.
Chapter 7 subsequently presents applications of the mathematical model to predict ‘what- if’ scenarios of hexanal releases which were not experimentally measured and to be utilised as means to understand key mass transfer processes involving in the simulated scenarios.
Lastly, Chapter 8 presents the general discussion on the findings from the present work along with recommendations for future work to provide further insights for better understanding and optimisation of active packaging systems for horticultural products.