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