16 Improving map through conceptual models M.L.A.T. M. Hertog, Katholieke Universiteit Leuven, belgium and N.H. Banks, Zespri Innovation Ltd, New Zealand 16.1 Introduction Conceptual models are descriptions of our understanding of a system that are used to shape the implementation of solutions to problems. The quality and quantum of innovation that will occur in development of modified atmosphere packaging (MAP) strongly depends upon the insights gained from robust of: ptual models of components of MAP. In this chapter, we outline a number of simple principles about modified atmosphere(MA)systems that we believe will assist industries that apply MA technology to move beyond the rather empirical pack-and-pray'approach that still predominates in commercial practice. This chapter will focus on the applications of MAP for the horticultural food industry, dealing with respiring plant produce, whole or minimally processed. However, most of the principles discussed will also hold for MAP of meat or processed food MA is generally used as a technique to prolong the keeping quality of fresh and minimally processed fruits and vegetables. In the widest sense of the term MA technology includes controlled atmosphere storage, ultra low oxygen storage, gas packaging, vacuum packaging, passive modified atmosphere packaging and active packaging 30, 38,39,7 Each of these techniques is based on the principle that manipulating or controlling the composition of the surrounding atmospheres affects the metabolism of the packaged product, such that the ability to retain quality of the product can be optimised. The different techniques come with different levels of control to realise and/or maintain the composition of the atmosphere around the product. While controlled atmosphere storage can rely on a whole arsenal of machinery for this purpose, active ackages rely on simple scavengers and/or emitters of gases such as oxygen
16.1 Introduction Conceptual models are descriptions of our understanding of a system that are used to shape the implementation of solutions to problems.58 The quality and quantum of innovation that will occur in development of modified atmosphere packaging (MAP) strongly depends upon the insights gained from robust conceptual models of components of MAP. In this chapter, we outline a number of simple principles about modified atmosphere (MA) systems that we believe will assist industries that apply MA technology to move beyond the rather empirical ‘pack-and-pray’ approach that still predominates in commercial practice. This chapter will focus on the applications of MAP for the horticultural food industry, dealing with respiring plant produce, whole or minimally processed. However, most of the principles discussed will also hold for MAP of meat or processed food. MA is generally used as a technique to prolong the keeping quality64 of fresh and minimally processed fruits and vegetables.75 In the widest sense of the term, MA technology includes controlled atmosphere storage, ultra low oxygen storage, gas packaging, vacuum packaging, passive modified atmosphere packaging and active packaging.30,38,39,71 Each of these techniques is based on the principle that manipulating or controlling the composition of the surrounding atmospheres affects the metabolism of the packaged product, such that the ability to retain quality of the product can be optimised. The different techniques come with different levels of control to realise and/or maintain the composition of the atmosphere around the product. While controlled atmosphere storage can rely on a whole arsenal of machinery for this purpose, active packages rely on simple scavengers and/or emitters of gases such as oxygen, 16 Improving MAP through conceptual models M.L.A.T.M. Hertog, Katholieke Universiteit Leuven, Belgium and N.H. Banks, Zespri Innovation Ltd, New Zealand
338 Novel food packaging techniques carbon dioxide, water or ethylene either integrated in the packing material or added in separate sachets. Passive MA packaging, as an extreme, relies solely the metabolic activity of the packaged product to modify and subsequently maintain the gas composition surrounding the product Although much research has been done to define optimum MA conditions for a wide range of fresh food products, the underlying mechanisms for the action of MA are still only superficially understood. The application of MA generally involves reducing oxygen levels(O2)and elevating levels of carbon dioxide(cO2)to reduce the respiratory metabolism. Parallel to the effect on the respiratory metabolism, the energy produced to support other metabolic processes, and consequently these processes themselves, will be affected accordingly. This still covers only part of the story of how MA can affect the metabolism of the packaged produce. The physiological effects of MA can be diverse and complex. In MAP, the success of the package strongly depends on the interactions between the physiology of the packaged product and the physical aspects of the package, MAP is a conceptually demanding technology. Much of the work in the area of MAP has been, and still is driven by practical needs of industry. This has enabled commercial development based upon pragmatic solutions but has not always contributed substantially to advancing the conceptual basis upon which future innovation in MA technologies depends. As a result, there is a substantial potential for models to contribute to the field of maP by making the complex and vast amount of, sometimes fragmental, expert knowledge available to packaging industries In this chapter, we bring together existing concepts, models and sub-models on MAP to build an overall conceptual model of the complex system of MAP Starting from this overall model, dedicated models can be extracted for specific tasks or situations. The benefits and drawbacks of the modelling approach are discussed, together with an identification of the future developments needed to create advantage to MAP commercial operations 16.2 Conceptual models The ideal model integrating all critical aspects of MAP would inevitably have a multidisciplinary nature and a complexity that, at least in its mathematical form is far beyond the scope of this chapter. Here we attempt to provide a sound conceptual model to assist understanding of the underlying mechanisms. Going in aggregation level from the macro(palletised packs) via the meso(individual packs)to the micro level(packaged product) the emphasis shifts from physics and engineering to include more and more biology; physiology and microbiology. In parallel to this shift, the level of complexity and uncertainty Increases
carbon dioxide, water or ethylene either integrated in the packing material or added in separate sachets. Passive MA packaging, as an extreme, relies solely on the metabolic activity of the packaged product to modify and subsequently maintain the gas composition surrounding the product. Although much research has been done to define optimum MA conditions for a wide range of fresh food products,37 the underlying mechanisms for the action of MA are still only superficially understood. The application of MA generally involves reducing oxygen levels (O2) and elevating levels of carbon dioxide (CO2) to reduce the respiratory metabolism.38 Parallel to the effect on the respiratory metabolism, the energy produced to support other metabolic processes, and consequently these processes themselves, will be affected accordingly.10 This still covers only part of the story of how MA can affect the metabolism of the packaged produce. The physiological effects of MA can be diverse and complex.13 In MAP, the success of the package strongly depends on the interactions between the physiology of the packaged product and the physical aspects of the package; MAP is a conceptually demanding technology. Much of the work in the area of MAP has been, and still is, driven by practical needs of industry. 29 This has enabled commercial development based upon pragmatic solutions but has not always contributed substantially to advancing the conceptual basis upon which future innovation in MA technologies depends. As a result, there is a substantial potential for models to contribute to the field of MAP by making the complex and vast amount of, sometimes fragmental, expert knowledge available to packaging industries. In this chapter, we bring together existing concepts, models and sub-models on MAP to build an overall conceptual model of the complex system of MAP. Starting from this overall model, dedicated models can be extracted for specific tasks or situations. The benefits and drawbacks of the modelling approach are discussed, together with an identification of the future developments needed to create advantage to MAP commercial operations. 16.2 Conceptual models The ideal model integrating all critical aspects of MAP would inevitably have a multidisciplinary nature and a complexity that, at least in its mathematical form, is far beyond the scope of this chapter. Here we attempt to provide a sound conceptual model to assist understanding of the underlying mechanisms. Going in aggregation level from the macro (palletised packs) via the meso (individual packs) to the micro level (packaged product) the emphasis shifts from physics and engineering to include more and more biology; physiology and microbiology. In parallel to this shift, the level of complexity and uncertainty increases. 338 Novel food packaging techniques
mproving MAP through conceptual models 339 Fig. 16.1 A schematic outline at the macro level of map where forced airflow and urbulent convection are responsible for heat and mass transfer to and from the individual 16.21 Macro level The macro level is schematically presented in Fig. 16. 1. Much research has been undertaken on heat and mass transfer, the effects of boundary layers and different flow patterns given different geometries, types of cooling and ventilation-,- The same techniques have been applied to the storage of livi and non-living food and non-food products all over the world. These techniques enable, in general, a good understanding of the storage environment of palletised or stacked packs, whether or not MA packs. Cooling is needed to remove heat from the packages and continuously to counteract the heat produced by the iving product. Both forced airflow and turbulent convection are at this level major contributors to the transport of heat, water, gases and volatiles, to and
16.2.1 Macro level The macro level is schematically presented in Fig. 16.1. Much research has been undertaken on heat and mass transfer, the effects of boundary layers and different flow patterns given different geometries, types of cooling and ventilation.20,28 The same techniques have been applied to the storage of living and non-living food and non-food products all over the world. These techniques enable, in general, a good understanding of the storage environment of palletised or stacked packs, whether or not MA packs. Cooling is needed to remove heat from the packages and continuously to counteract the heat produced by the living product. Both forced airflow and turbulent convection are at this level major contributors to the transport of heat, water, gases and volatiles, to and from the packs. Fig. 16.1 A schematic outline at the macro level of MAP where forced airflow and turbulent convection are responsible for heat and mass transfer to and from the individual MA packs. Improving MAP through conceptual models 339
340 Novel food packaging techniques 16.2.2 Meso level t the level of individual packs(Fig. 16.2) the emphasis moves towards natura convection and diffusion processes driven by concentration and thermal gradients. Heat produced by the product is conducted directly, or through the atmosphere in the package, to the packaging material and, eventually, is released to the air surrounding the pack. Water vapour, respiratory gases, ethylene and other volatiles are exchanged between the package atmosphere and surrounding atmosphere by diffusion through(semi-) permeable packaging materials. Those packaging films can be either selective semi-permeable films or perforated films. In the case of perforated films especially, the diffusion rate of a gas can be influenced by a concurrent diffusion of a second gas. A counter current generally hinders diffusion while a current in the same direction promotes diffusion of the first gas Inside the package, the metabolic gases are either consumed(o2)or produced (H,0, CO2, C2H4 and other volatiles)by the product. Each of these gases may promote or inhibit certain parts of the products metabolism. In the end, the overall metabolism of the packaged product is responsible for maintaining the products properties. As long as the product properties relevant for the quality as perceived by the consumer stay above satisfactory levels the product remains acceptable The steady state gas conditions realised inside an Ma pack are the result of both the influx and the efflux through diffusion and the consumption and T ChK H2o volatiles Fig. 16.2 A schematic outline at the meso level of map where heat and mass transfer from and to the packaged product are ruled by natural convection and diffusion processes. he packaging film acts like a selective semi-permeable barrier between the package and the surrounding atmosphere. Temperature has a marked effect on all processes going at the meso level
16.2.2 Meso level At the level of individual packs (Fig. 16.2) the emphasis moves towards natural convection and diffusion processes driven by concentration and thermal gradients. Heat produced by the product is conducted directly, or through the atmosphere in the package, to the packaging material and, eventually, is released to the air surrounding the pack. Water vapour, respiratory gases, ethylene and other volatiles are exchanged between the package atmosphere and the surrounding atmosphere by diffusion through (semi-) permeable packaging materials. Those packaging films can be either selective semi-permeable films or perforated films. In the case of perforated films especially, the diffusion rate of a gas can be influenced by a concurrent diffusion of a second gas.57 A counter current generally hinders diffusion while a current in the same direction promotes diffusion of the first gas. Inside the package, the metabolic gases are either consumed (O2) or produced (H2O, CO2, C2H4 and other volatiles) by the product. Each of these gases may promote or inhibit certain parts of the product’s metabolism. In the end, the overall metabolism of the packaged product is responsible for maintaining the product’s properties. As long as the product properties relevant for the quality as perceived by the consumer stay above satisfactory levels the product remains acceptable. The steady state gas conditions realised inside an MA pack are the result of both the influx and the efflux through diffusion and the consumption and Fig. 16.2 A schematic outline at the meso level of MAP where heat and mass transfer from and to the packaged product are ruled by natural convection and diffusion processes. The packaging film acts like a selective semi-permeable barrier between the package and the surrounding atmosphere. Temperature has a marked effect on all processes going on at the meso level. 340 Novel food packaging techniques
mproving MAP through conceptual models 341 production by the product which are themselves strongly dependent on the composition of the package atmosphere. For instance, water loss by the product is the main source for water accumulating in the pack atmosphere. The product elevates humidity levels within the pack to an extent that depends upon relative water vapour permeances of film and product. This elevated humidity inhibits further water loss to a progressively greater extent as relative humidity approaches saturation. This substantial benefit carries a risk of condensation that is exacer bated by temperature fluctuations. Condensation creates favourable conditions for microbial growth that will eventually spoil the product and, as water condenses on the film, will also depress the overall permeance of the package The time needed for a package to reach steady state is important as only from that moment on is the maximum benefit from ma being realised. In the extreme situation, the time to reach steady state could outlast the shelf-life of the packaged product. A typical example of how the atmospheric composition in an MA pack and gas exchange of the packaged product can change during time is illustrated in Fig. 163. The dynamics of reaching steady state depend upon the rates of gas exchange and diffusion and upon the dimensions of the package in relation to the amount of product contained. Packages with large void volumes take longer to reach steady state levels. Temperature has a major effect on the rates of all processes involved in establishing these steady state levelsand hence on the levels of the steady state gas conditions themselves 16.2.3 Micro level Gas exchange The complexity of the biological system inherent in each fruit(Fig. 16.4) contributes significantly to the uncertainties in current knowledge on issues critical to the outcome of Ma treatments. One of the central issues is the impact of Ma upon the products gas exchange, its consumption of O2 and production of CO2(Fig. 16.5). Total CO2 production consists of two parts, one part coming from the oxidative respiration in parallel to the 2 consumption and the other part originating from the fermentative metabolism. At high O2 levels, aerobic respiration prevails In this situation, the respiration quotient(RQ; ratio of Co production to O2 consumption), influenced by the type of substrate being consumed, remains close to unity. At lower oxygen levels, fermentation can develop, generally causing a substantial increase in RQ. This is due to an increased fermentative CO2 production relative to an O2 consumption declining towards zero. Besides the effect of O2 on respiration and fermentation, CO2 is known to inhibit gas exchange in some produce as well Although it would be convenient to consider gas exchange to be constant with time, there can be considerable ontogenetic drift in rates of gas exchange In so-called climacteric fruits especially, a respiration burst can be observed when the fruit starts to ripen. In addition, freshly harvested, mildly processed or handled fruit generally shows a temporary increased gas exchange rate Microbial infections can also stimulate gas exchange. o
production by the product which are themselves strongly dependent on the composition of the package atmosphere.35 For instance, water loss by the product is the main source for water accumulating in the pack atmosphere. The product elevates humidity levels within the pack to an extent that depends upon relative water vapour permeances of film and product. This elevated humidity inhibits further water loss to a progressively greater extent as relative humidity approaches saturation. This substantial benefit carries a risk of condensation that is exacerbated by temperature fluctuations. Condensation creates favourable conditions for microbial growth that will eventually spoil the product and, as water condenses on the film, will also depress the overall permeance of the package. The time needed for a package to reach steady state is important as only from that moment on is the maximum benefit from MA being realised. In the extreme situation, the time to reach steady state could outlast the shelf-life of the packaged product. A typical example of how the atmospheric composition in an MA pack and gas exchange of the packaged product can change during time is illustrated in Fig. 16.3. The dynamics of reaching steady state depend upon the rates of gas exchange and diffusion and upon the dimensions of the package in relation to the amount of product contained. Packages with large void volumes take longer to reach steady state levels. Temperature has a major effect on the rates of all processes involved in establishing these steady state levels4 and hence on the levels of the steady state gas conditions themselves. 16.2.3 Micro level Gas exchange The complexity of the biological system inherent in each fruit (Fig. 16.4) contributes significantly to the uncertainties in current knowledge on issues critical to the outcome of MA treatments. One of the central issues is the impact of MA upon the product’s gas exchange, its consumption of O2 and production of CO2 (Fig. 16.5). Total CO2 production consists of two parts, one part coming from the oxidative respiration in parallel to the O2 consumption and the other part originating from the fermentative metabolism.51 At high O2 levels, aerobic respiration prevails. In this situation, the respiration quotient (RQ; ratio of CO2 production to O2 consumption), influenced by the type of substrate being consumed, remains close to unity. At lower oxygen levels, fermentation can develop, generally causing a substantial increase in RQ. This is due to an increased fermentative CO2 production relative to an O2 consumption declining towards zero. Besides the effect of O2 on respiration and fermentation, CO2 is known to inhibit gas exchange in some produce as well. Although it would be convenient to consider gas exchange to be constant with time, there can be considerable ontogenetic drift in rates of gas exchange.8 In so-called climacteric fruits especially, a respiration burst can be observed when the fruit starts to ripen. In addition, freshly harvested, mildly processed or handled fruit generally shows a temporary increased gas exchange rate.10 Microbial infections can also stimulate gas exchange.70 Improving MAP through conceptual models 341