MAP performance under dynamic temperature conditions 571 product evaporates more water than the cold air can contain, quickly oversaturating the air with an excess water condensating on the inside of the cold packaging material(Fig. 27. 1d). During the subsequent period,con densation slowly disappears again by evaporation and diffusion through the film with fluctuating temperatures the amount of condensate fluctuates as well. Once temperature is increased to 12C there is a fast drop in the amount of condensate These relative fast changes are due to changes in the air saturation levels for water vapour as a function of temperature. This example shows that condensation can be rapidly induced but once present is hard to remove without Increasing temperature again When the void volume in the package is eliminated(Fig. 27. 1b and c)steady state gas conditions are rapidly realised within half a day. Because of the warm lettuce, the CO2 level peaks to initially extremely high levels, rapidly disappearing when the product cools down. By reducing the void volume we have removed the buffering capacity of the system as a consequence of which the gas levels respond much more vigorously to the fluctuating temperature and also become more sensitive to fast fluctuations. When temperature is increased to 12%C. the increase in CO, is much faster than before When the film is replaced by a microperforated material, permeance of the ackaging film has become almost independent of temperature. The resulting gas conditions are now different(Fig. 27. 1b and c) with O2 going towards 3kPa and CO2 continuing to increase with time. The reason for not reaching steady state conditions is the relatively much lower permeance for CO2 as compared to the permeance for O2. Therefore the steady state conditions for CO2 are at much higher CO2 levels than before, which takes more time and the Ma package never reaches this situation. Because of the temperature independency of film permeance the fluctuations in O2 levels respond vigorously to changes in temperature. The final temperature increase to 12C results in a drop of O2 to IkPa and an increase of o2 towards 40-50kPa. This increase is clearly the result of fermentative CO2 production that, due to the low permeance for CO2 is trapped inside the package. As the accumulating CO2 has an inhibitive effect on the respiration of lettuce, O2 consumption is inhibited, resulting in a subsequent slight increase of the O level The outlined simulations were focused on a single average MA pack. When the dynamic temperature condition is applied to a batch of MA packages, each prepared package will differ slightly from another. Given that biological riance is the most variable parameter, we simulated a batch of 500 packages assuming 25% variation on product respiration rates, and 10% variation on packed product weight and film thickness(Fig. 27.2). The simulation result clearly shows the effect of variation in Ma design parameters on the resulting MA gas conditions. At the same time it shows that variation in MA gas conditions depends on time and temperature. As, depending on the respiration rates, some packages establish MA conditions faster than others, initially a large variation in MA gas conditions is observed. Some packages reached a level of 2kPa O2 within three hours after packing while others took two days to reachproduct evaporates more water than the cold air can contain, quickly oversaturating the air with an excess water condensating on the inside of the cold packaging material (Fig. 27.1d). During the subsequent period, con￾densation slowly disappears again by evaporation and diffusion through the film. With fluctuating temperatures the amount of condensate fluctuates as well. Once temperature is increased to 12ºC there is a fast drop in the amount of condensate. These relative fast changes are due to changes in the air saturation levels for water vapour as a function of temperature. This example shows that condensation can be rapidly induced but once present is hard to remove without increasing temperature again. When the void volume in the package is eliminated (Fig. 27.1b and c) steady state gas conditions are rapidly realised within half a day. Because of the warm lettuce, the CO2 level peaks to initially extremely high levels, rapidly disappearing when the product cools down. By reducing the void volume we have removed the buffering capacity of the system as a consequence of which the gas levels respond much more vigorously to the fluctuating temperature and also become more sensitive to fast fluctuations. When temperature is increased to 12ºC, the increase in CO2 is much faster than before. When the film is replaced by a microperforated material, permeance of the packaging film has become almost independent of temperature. The resulting gas conditions are now different (Fig. 27.1b and c) with O2 going towards 3kPa and CO2 continuing to increase with time. The reason for not reaching steady state conditions is the relatively much lower permeance for CO2 as compared to the permeance for O2. Therefore the steady state conditions for CO2 are at much higher CO2 levels than before, which takes more time and the MA package never reaches this situation. Because of the temperature independency of film permeance the fluctuations in O2 levels respond vigorously to changes in temperature. The final temperature increase to 12ºC results in a drop of O2 to 1kPa and an increase of CO2 towards 40–50kPa. This increase is clearly the result of fermentative CO2 production that, due to the low permeance for CO2 is trapped inside the package. As the accumulating CO2 has an inhibitive effect on the respiration of lettuce, O2 consumption is inhibited, resulting in a subsequent slight increase of the O2 level. The outlined simulations were focused on a single average MA pack. When the dynamic temperature condition is applied to a batch of MA packages, each prepared package will differ slightly from another. Given that biological variance is the most variable parameter, we simulated a batch of 500 packages assuming 25% variation on product respiration rates, and 10% variation on packed product weight and film thickness (Fig. 27.2). The simulation result clearly shows the effect of variation in MA design parameters on the resulting MA gas conditions. At the same time it shows that variation in MA gas conditions depends on time and temperature. As, depending on the respiration rates, some packages establish MA conditions faster than others, initially a large variation in MA gas conditions is observed. Some packages reached a level of 2kPa O2 within three hours after packing while others took two days to reach MAP performance under dynamic temperature conditions 571