MAP performance under dynamic temperature conditions 569 The quality of shredded lettuce is often limited by browning of the cut edges pheres. Shredded lettuce is a product with a relative high respiration rate and a high responsiveness to temperature as expressed by the energy of activation of respiration(see Chapter 16). As a reference we simulated MAP of pre-cooled lettuce stored at a constant 4C and packed in a polymeric bag wit an energy of activation of about one-third of the lettuce itself (Fig. 27. 1b and c) Steady state gas conditions(10kPa COz and IkPa O2)are reached after about two and a half days of storage with O2 levels reaching 2 kPa after one-day storage. The realised steady state gas conditions correspond to the targeted optimum values for shredded lettuce. When one realises that minimally processed products generally have a limited shelf-life, the two and a half days needed to establish steady state conditions is relatively long For subsequent simulations an artificial dynamic temperature profile was created ( Fig. 27. 1a) consisting of one day at a constant 4C followed by a two- day period of slow fluctuating temperature around 4C and a subsequent one-day increased to a constant 12C. Instead of assuming the lettuce to be pre-cooled, lettuce was assumed to be at room temperature when packed. As a result of packing warm lettuce, depletion of O2 and accumulation of CO2 was accelerated in comparison to the reference situation(Fig. 27. 1b and c), the O2 level of 2 kPa was reached only half a day after packing. Both O2 and CO2 show fluctuating levels in response to the fluctuating temperature of the environment The fluctuations in O, and co2 follow the fluctuations in temperature after a short delay, as the product needs time to warm up and cool down. The larger the thermal mass and heat capacity of the product, the slower the product will respond to fluctuations in temperature. This explains why gas levels follow slow temperature fluctuations more clearly than they follow the fast temperature changes. Another reason why gas levels do not follow fast temperature changes is because of the void volume in the package, which buffers the change in gas conditions The direction of the fluctuation in CO2 level is the same as for temperature while the direction of the fluctuation in O2 level is the opposite. As temperature increases, film permeance increases. However, the rate of O2 consumption increases faster than the increase in film permeance resulting in dropping O2 levels. With dropping O2 levels fermentative CO2 production increases resulting in increasing levels of CO2. During the period of fluctuating temperature the same average gas levels are reached as seen before. When temperature is increased to 12C, the o2 level drops to 0.5kPa while CO2 accumulates up to I 8kPa due to the fermentation induced. It will be clear that such an increase in temperature to 12C when a package is designed to operate around 4C is fatal for the packed product. Depending on the product such temperature increase might irreversibly affect product quality Packing warm product has the advantage of rapidly establishing the targeted gas conditions. The downside is the induction of condensation as the warmThe quality of shredded lettuce is often limited by browning of the cut edges. This can be controlled by packaging in < 1% O2 and 10% CO2 atmos￾pheres.15, 17 Shredded lettuce is a product with a relative high respiration rate and a high responsiveness to temperature as expressed by the energy of activation of respiration (see Chapter 16). As a reference we simulated MAP of pre-cooled lettuce stored at a constant 4ºC and packed in a polymeric bag with an energy of activation of about one-third of the lettuce itself (Fig. 27.1b and c). Steady state gas conditions (10kPa CO2 and 1kPa O2) are reached after about two and a half days of storage with O2 levels reaching 2 kPa after one-day storage. The realised steady state gas conditions correspond to the targeted optimum values for shredded lettuce. When one realises that minimally processed products generally have a limited shelf-life, the two and a half days needed to establish steady state conditions is relatively long. For subsequent simulations an artificial dynamic temperature profile was created (Fig. 27.1a) consisting of one day at a constant 4ºC followed by a two￾day period of slow fluctuating temperature around 4ºC and a subsequent one-day period of fast fluctuating temperature. After this, temperature was rapidly increased to a constant 12ºC. Instead of assuming the lettuce to be pre-cooled, lettuce was assumed to be at room temperature when packed. As a result of packing warm lettuce, depletion of O2 and accumulation of CO2 was accelerated in comparison to the reference situation (Fig. 27.1b and c), the O2 level of 2 kPa was reached only half a day after packing. Both O2 and CO2 show fluctuating levels in response to the fluctuating temperature of the environment. The fluctuations in O2 and CO2 follow the fluctuations in temperature after a short delay, as the product needs time to warm up and cool down. The larger the thermal mass and heat capacity of the product, the slower the product will respond to fluctuations in temperature. This explains why gas levels follow slow temperature fluctuations more clearly than they follow the fast temperature changes. Another reason why gas levels do not follow fast temperature changes is because of the void volume in the package, which buffers the change in gas conditions. The direction of the fluctuation in CO2 level is the same as for temperature while the direction of the fluctuation in O2 level is the opposite. As temperature increases, film permeance increases. However, the rate of O2 consumption increases faster than the increase in film permeance resulting in dropping O2 levels. With dropping O2 levels fermentative CO2 production increases resulting in increasing levels of CO2. During the period of fluctuating temperature the same average gas levels are reached as seen before. When temperature is increased to 12ºC, the O2 level drops to 0.5kPa while CO2 accumulates up to 18kPa due to the fermentation induced. It will be clear that such an increase in temperature to 12ºC when a package is designed to operate around 4ºC is fatal for the packed product. Depending on the product such temperature increase might irreversibly affect product quality. Packing warm product has the advantage of rapidly establishing the targeted gas conditions. The downside is the induction of condensation as the warm MAP performance under dynamic temperature conditions 569