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
642 Novel food packaging starts to accumulate while the O, level starts to decrease(bottom). In response to the hanging gas conditions, gas exchange rates are inhibited(top). Driven by the increasing concentration gradients between package and surrounding atmosphere, O, and CO, start to diffuse through the packaging film. Combined, this slows down the change in gas conditions. Eventually, gas exchange by the product and diffusion through the film reach steady state levels at which the consumption and production of O, and cO2 equals the influx and efflux by diffusion When one considers gas exchange as a function of O2 and CO2 levels, one is generally inclined to look at the atmospheric composition surrounding the product as the driving force. However, the actual place of action of gas exchange inside the cells, in the mitochondria. Depending on the type of product, this means that an O2 molecule has to diffuse through the boundary layer surrounding the product, through a wax layer, cracks, pores or stomata, through intercellular spaces, has to dissolve in water, and has to pass the cell membrane to get into the cell. 3 The CO2 molecule produced by the gas exchange has to travel the same way in the opposite direction. The driving force for the diffusion comes from the partial pressure difference for O2 and CO2 between the fruit's internal and external atmospheres generated by the gas exchange. The intracellular. in-situ. O, and CO, concentrations are much more relevant for
Gas diffusion When one considers gas exchange as a function of O2 and CO2 levels, one is generally inclined to look at the atmospheric composition surrounding the product as the driving force. However, the actual place of action of gas exchange is inside the cells, in the mitochondria. Depending on the type of product, this means that an O2 molecule has to diffuse through the boundary layer surrounding the product, through a wax layer, cracks, pores or stomata, through intercellular spaces, has to dissolve in water, and has to pass the cell membrane to get into the cell.13 The CO2 molecule produced by the gas exchange has to travel the same way in the opposite direction. The driving force for the diffusion comes from the partial pressure difference for O2 and CO2 between the fruit’s internal and external atmospheres generated by the gas exchange. The intracellular, in-situ, O2 and CO2 concentrations are much more relevant for Fig. 16.3 A typical example of the dynamics of MA. Due to the gas exchange, CO2 starts to accumulate while the O2 level starts to decrease (bottom). In response to the changing gas conditions, gas exchange rates are inhibited (top). Driven by the increasing concentration gradients between package and surrounding atmosphere, O2 and CO2 start to diffuse through the packaging film. Combined, this slows down the change in gas conditions. Eventually, gas exchange by the product and diffusion through the film reach steady state levels at which the consumption and production of O2 and CO2 equals the influx and efflux by diffusion. 342 Novel food packaging techniques
mproving MAP through conceptual models 343 H, HO O Fig. 16. 4 A schematic outline at the micro level of MAP where the product is considered to generate its own MA conditions due to the resistance of the skin. The internal gas conditions are responsible for affecting large parts of the metabolism either directly or via the gas exchange. This will influence quality related product properti determining the quality(Q) as perceived by the consumer. Depending on the Ma conditions, microbes can interact with the products physiology influencing its final the gas exchange than the fruit external gas conditions. Generally, it is assumed however, that the largest resistance in the diffusion pathway from the surround lings into the fruit exists at the skin of the fruit. Therefore the largest gradient in concentration occurs at the skin while the concentration differences within a fruit are small Even at identical external atmospheres, different species of fruit will have completely different intermal gas compositions due to their different skin permeances. Fruit with a wax layer, like apples, have a much lower permeance than leafy vegetables like cabbages, which generally have a large amount of stomata present. The skin permeance of different apple varieties will be strongly affected by thickness of their natural wax layers. Due to such a wax layer, the skin of tomato and bell pepper is relatively impermeable, forcing all
the gas exchange than the fruit external gas conditions. Generally, it is assumed however, that the largest resistance in the diffusion pathway from the surroundings into the fruit exists at the skin of the fruit. 12,14 Therefore the largest gradient in concentration occurs at the skin while the concentration differences within a fruit are small. Even at identical external atmospheres, different species of fruit will have completely different internal gas compositions due to their different skin permeances. Fruit with a wax layer, like apples, have a much lower permeance than leafy vegetables like cabbages, which generally have a large amount of stomata present.18 The skin permeance of different apple varieties will be strongly affected by thickness of their natural wax layers. Due to such a wax layer, the skin of tomato and bell pepper is relatively impermeable, forcing all Fig. 16.4 A schematic outline at the micro level of MAP where the product is considered to generate its own MA conditions due to the resistance of the skin. The internal gas conditions are responsible for affecting large parts of the metabolism either directly or via the gas exchange. This will influence quality related product properties determining the quality (Q) as perceived by the consumer. Depending on the MA conditions, microbes can interact with the product’s physiology influencing its final quality. Improving MAP through conceptual models 343
344 Novel food packaging techniques oxidative r fermentative rco. A typical example ge as a function of Oz partial pressures(Po (ro is related to the oxidati fermentative CO2 production can take place esulting in increased CO2 production as compared to the decreasing O2 consumpti the gas exchange through the stem end of the fruit. Consequently, some fruits become internally anaerobic in conditions where others are still aerobic Water diffusion and water loss The diffusion of water vapour is limited by skin permeance in the same way as the diffusion of O2 and CO2, the slight difference being that diffusion of O2 and CO2 takes place mainly through pores connected to intercellular spaces while water vapour is more easily released through the whole skin surface. ,Water loss is driven by the partial pressure difference of water vapour between the fruits internal (close to saturation) and external atmospheres. Water loss is an important issue in relation to the overall mass loss, firmness loss and shrivelling or wilting of the product Inside an MA pack, water loss can also be responsible Ethylene effects Being a plant hormone, ethylene takes a special place among the gases and volatiles produced by the product because of its potential impact on the product's own metabolism. The pathways of biosynthesis and bio-action of ethy lene are still subject to extensive study. Most of the climacteric fruits show a peak of ethylene production at the onset of ripening. In most of these fruits, ripening can be triggered by exogenously supplied ethylene. This creates the situation that one ripening fruit in an Ma pack will trigger the other fruit to ripen
the gas exchange through the stem end of the fruit.22 Consequently, some fruits become internally anaerobic in conditions where others are still aerobic. Water diffusion and water loss The diffusion of water vapour is limited by skin permeance in the same way as the diffusion of O2 and CO2, the slight difference being that diffusion of O2 and CO2 takes place mainly through pores connected to intercellular spaces while water vapour is more easily released through the whole skin surface. 3,44 Water loss is driven by the partial pressure difference of water vapour between the fruit’s internal (close to saturation) and external atmospheres. Water loss is an important issue in relation to the overall mass loss, firmness loss and shrivelling or wilting of the product. Inside an MA pack, water loss can also be responsible for generating conditions favourable for microbial growth (high RH). Ethylene effects Being a plant hormone, ethylene takes a special place among the gases and volatiles produced by the product because of its potential impact on the product’s own metabolism. The pathways of biosynthesis and bio-action of ethylene are still subject to extensive study.40 Most of the climacteric fruits show a peak of ethylene production at the onset of ripening. In most of these fruits, ripening can be triggered by exogenously supplied ethylene. This creates the situation that one ripening fruit in an MA pack will trigger the other fruit to ripen Fig. 16.5 A typical example of gas exchange as a function of O2 partial pressures (PO2 in kPa). (rO2 ) is related to the oxidative part of CO2 production (rCO2 ) via the respiration quotient. Additionally, at low O2 levels fermentative CO2 production can take place resulting in increased CO2 production as compared to the decreasing O2 consumption. 344 Novel food packaging techniques
mproving MAP through conceptual models 345 simultaneously, due to the ethylene accumulating in the pack, MA can inhibit the normal development and ripening of products postponing climacteric ethylene production thus extending the keeping quality of the product. With kiwifruit, however, advanced softening of the fruit occurs before ethylene produced. Although the fruit is not yet producing any ethylene, the softening process is extremely susceptible to exogenously applied ethylene Product quality The quality of the packaged product is based on some subjective consumer evaluation of a complex of quality attributes (like taste, texture, colour, appearance)which are based on specific product properties(like sugar content volatile production, cell wall structure). These product properties generally change over time as part of the normal metabolism of the product. Those developmental changes that are directly influenced by O2 or CO2 or driven by the energy supplied by respiration or fermentation will all be affected by applying MA conditions, potentially extending the keeping quality of the product. Some processes are more affected than others due to the way they depend on atmospheric conditions. To understand the mode of action of MAP for a specific product, a good understanding of how the relevant product properties depend on gas conditions and temperature is required Spoilage and pathogenics MA conditions can also provide conditions favourable to the growth of microbe potentially limiting the keeping quality of the packaged product due to rot. This is especially the case for soft fruits or minimally processed fruit and vegetable salads when high humidity levels are combined with a tasty substrate. Some microbes are known to be opportunistic, waiting for their chance to invade the tissue when ripe, damaged or cut. In this case, MA conditions inhibiting the ripening of fruit in combination with proper handling and disinfection can prevent some of the problems. Other microbes more actively invade the tissue, causing soft patches on the fruit. More insight is needed on how MA can inhibit not only the metabolism of the product but also that of the microbes present on the products. High CO2 levels are generally believed to suppress the growth of microbes, although sometimes the COz levels needed to suppress microbial growth exceed the tolerance levels of the vegetable produce packaged. 17 Variation Although the general concept of MAP is now almost complete, there is one thing left that affects all other issues outlined so far: the effect of variation. variation can occur on different levels, like time and spatial variation in temperature control in storage. irregularities in the stacking of cartons influencing ideal flow patterns, irregularities in the thickness or perforation of films or differences between batches of film used. However, the most important non-verifiable factor is biological variation. Besides the more obvious differences between cultivars distinct differences exist between produce from different harvests, years, soils or
simultaneously, due to the ethylene accumulating in the pack. MA can inhibit the normal development and ripening of products postponing climacteric ethylene production thus extending the keeping quality of the product. With kiwifruit, however, advanced softening of the fruit occurs before ethylene is produced.6 Although the fruit is not yet producing any ethylene, the softening process is extremely susceptible to exogenously applied ethylene. Product quality The quality of the packaged product is based on some subjective consumer evaluation of a complex of quality attributes (like taste, texture, colour, appearance) which are based on specific product properties (like sugar content, volatile production, cell wall structure).61 These product properties generally change over time as part of the normal metabolism of the product. Those developmental changes that are directly influenced by O2 or CO2 or driven by the energy supplied by respiration or fermentation will all be affected by applying MA conditions, potentially extending the keeping quality of the product. Some processes are more affected than others due to the way they depend on atmospheric conditions. To understand the mode of action of MAP for a specific product, a good understanding of how the relevant product properties depend on gas conditions and temperature is required. Spoilage and pathogenics MA conditions can also provide conditions favourable to the growth of microbes potentially limiting the keeping quality of the packaged product due to rot. This is especially the case for soft fruits or minimally processed fruit and vegetable salads when high humidity levels are combined with a tasty substrate.7 Some microbes are known to be opportunistic, waiting for their chance to invade the tissue when ripe, damaged or cut. In this case, MA conditions inhibiting the ripening of fruit in combination with proper handling and disinfection can prevent some of the problems. Other microbes more actively invade the tissue, causing soft patches on the fruit. More insight is needed on how MA can inhibit not only the metabolism of the product but also that of the microbes present on the products. High CO2 levels are generally believed to suppress the growth of microbes, although sometimes the CO2 levels needed to suppress microbial growth exceed the tolerance levels of the vegetable produce packaged.7,17 Variation Although the general concept of MAP is now almost complete, there is one thing left that affects all other issues outlined so far; the effect of variation. Variation can occur on different levels, like time and spatial variation in temperature control in storage, irregularities in the stacking of cartons influencing ideal flow patterns, irregularities in the thickness or perforation of films or differences between batches of film used. However, the most important non-verifiable factor is biological variation. Besides the more obvious differences between cultivars, distinct differences exist between produce from different harvests, years, soils or Improving MAP through conceptual models 345
346 Novel food packaging techniques locations. Even within one batch, considerable variation between individual items can occur. 7 The amount of biological variation that can be expected generally depends on the organisation level looked at. Within packages, the product generally comes from one grower resulting in a relatively homogenous batch with limited fruit-to-fruit variation. Comparing different pallets involves product potentially originating from different growers and different harvest dates result in a much larger variation When developing small consumer MA packages, variation in the rate of gas exchange is almost impossible to take into account. The larger the package, the more these differences tend to average out. However. in the case of fruit nteractions,individual outliers can affect the other fruit in a pack, as with the spreading of rots, the onset of ripening through C2H4 production or with off- flavour development 16.3 Mathematical models Over the years, different elements of what has been discussed above have been subject to mathematical modelling. Other subjects are still to be explored Models describing the physics of MAP are usually more fundamental than the ones describing the physiology of MAP. This is due to the increased complexity and the lack of knowledge of the underlying mechanisms. For this reason, empirical models'(arbitrary mathematical equations fitted to experimental data)still prevail in post-harvest physiology. This section gives an overview of he type of MAP-related models available in the literature with the emphasis on lological aspects of MAP 16.3.1 Macro level With the strong development of computers, rapidly increasing computational power becomes available to food and packaging engineers. Associated with this, engineers can add numerical tools to their standard toolkit such as Computational Fluid Dynamics, infinite elements and finite differences. In general, when modelling heat and mass transfer, conservation laws are applied to formulate energy and mass balances. The space under study is subdivided a number of defined elements. Each of them is represented by one point within the three-dimensional space and is assumed to exchange mass and heat with its neighbouring elements according the heat and mass balances defined. The accuracy of such a model strongly depends on the number and size of elements defined and the knowledge of system input parameters. To improve both accuracy and computational time, smaller elements can be defined in areas with steep gradients and larger elements in the more homogeneous areas Theoretically, this approach is applicable at both the macro level to describe airflow in a cold room, at the meso level to describe diffusion within a pack, and at the micro level to describe gradients within the product. The main application
locations.42 Even within one batch, considerable variation between individual items can occur.67 The amount of biological variation that can be expected generally depends on the organisation level looked at. Within packages, the product generally comes from one grower resulting in a relatively homogenous batch with limited fruit-to-fruit variation. Comparing different pallets involves product potentially originating from different growers and different harvest dates result in a much larger variation. When developing small consumer MA packages, variation in the rate of gas exchange is almost impossible to take into account. The larger the package, the more these differences tend to average out. However, in the case of fruit interactions, individual outliers can affect the other fruit in a pack, as with the spreading of rots, the onset of ripening through C2H4 production or with offflavour development. 16.3 Mathematical models Over the years, different elements of what has been discussed above have been subject to mathematical modelling. Other subjects are still to be explored. Models describing the physics of MAP are usually more fundamental than the ones describing the physiology of MAP. This is due to the increased complexity and the lack of knowledge of the underlying mechanisms. For this reason, empirical ‘models’ (arbitrary mathematical equations fitted to experimental data) still prevail in post-harvest physiology. This section gives an overview of the type of MAP-related models available in the literature with the emphasis on the physiological aspects of MAP. 16.3.1 Macro level With the strong development of computers, rapidly increasing computational power becomes available to food and packaging engineers. Associated with this, engineers can add new numerical tools to their standard toolkit such as Computational Fluid Dynamics, infinite elements and finite differences. In general, when modelling heat and mass transfer, conservation laws are applied to formulate energy and mass balances.20 The space under study is subdivided in a number of defined elements. Each of them is represented by one point within the three-dimensional space and is assumed to exchange mass and heat with its neighbouring elements according the heat and mass balances defined. The accuracy of such a model strongly depends on the number and size of elements defined and the knowledge of system input parameters. To improve both accuracy and computational time, smaller elements can be defined in areas with steep gradients and larger elements in the more homogeneous areas. Theoretically, this approach is applicable at both the macro level to describe airflow in a cold room, at the meso level to describe diffusion within a pack, and at the micro level to describe gradients within the product. The main application is 346 Novel food packaging techniques