MAP performance under dynamic temperature conditions 567 LDPE film. ' One can argue whether a temperature-independent atmosphere inside the package is important in its own right. The aim of MAP is to retain quality. With constant gas conditions at increasing temperatures, respiration rate and the rate of quality decay will still increase due to the increased temperature The O2 levels in MA packages that make use of perforated films are even more sensitive to changes in temperature, as diffusion through the holes(i.e diffusion through a barrier of standing air) is almost independent of temperature An increase in temperature will induce increased O2 consumption by the product without inducing a substantial increased influx through the packaging material, ng in a fast drop of the steady state O2 ley Besides reducing O2 levels in MAP, CO2 levels are increased to further inhibit the product's metabolism- High CO2 levels also inhibit decay by suppressing the growth of microbes, although sometimes the COz levels needed to suppress icrobial growth exceed the tolerance levels of the vegetable produce packaged 4,6 This identifies another dilemma in controlling the gas conditions in MAP For most polymeric packaging films the permeance for CO2 is 2-to 10-fold higher than for O2, under aerobic conditions O2 depletes much faster than CO will accumulate. Assuming a respiratory quotient of l and a steady state O2 level of 2kPa, the maximum achievable steady state CO2 level between 2 and 9kPa depending on the film material. To achieve higher steady state CO2 levels without inducing fermentation, microperforated films should be used that have comparable permeances for O2 and CO2. When using microperforated films, O will deplete about as fast as CO2 accumulates, such that the sum of Oz and co partial pressure remains around 20kPa. A microperforated Ma package designed for 2kPa O2 can therefore generate CO2 levels of around 18kPa. For soft fruit like strawberries, these high CO2 levels are needed to prolong shelf- life , However, after prolonged storage at high CO2(>15 kPa)CO2 injury becomes visible from tissue defects and fermentation off-flavours 10, 14 When exposing MA packages to dynamic temperature conditions there is a direct risk of inducing fermentation and an added secondary risk of inducing CO2 damage due to the accumulating fermentative CO2. Especially for microperforated packs where the permeance does not increase with temperature the risk of inducing fermentation and consequently the accumulation of high CO2 levels is much larger Scavengers to constrain the accumulation of CO2 Chapter 3)might limit the secondary risk of CO2 damage but cannot prevent the direct risk of inducing fermentation 27.3.3 High humidity With horticultural products generally consisting of up to 90% water and with their economic value often determined by the saleable weight of the crop, moisture loss needs to be limited under all conditions. Depending on how andLDPE film.7 One can argue whether a temperature-independent atmosphere inside the package is important in its own right. The aim of MAP is to retain quality. With constant gas conditions at increasing temperatures, respiration rate and the rate of quality decay will still increase due to the increased temperature. The O2 levels in MA packages that make use of perforated films are even more sensitive to changes in temperature, as diffusion through the holes (i.e. diffusion through a barrier of standing air) is almost independent of temperature. An increase in temperature will induce increased O2 consumption by the product without inducing a substantial increased influx through the packaging material, resulting in a fast drop of the steady state O2 levels. 27.3.2 High carbon dioxide Besides reducing O2 levels in MAP, CO2 levels are increased to further inhibit the product’s metabolism.2 High CO2 levels also inhibit decay by suppressing the growth of microbes, although sometimes the CO2 levels needed to suppress microbial growth exceed the tolerance levels of the vegetable produce packaged.4, 6 This identifies another dilemma in controlling the gas conditions in MAP. For most polymeric packaging films the permeance for CO2 is 2- to 10-fold higher than for O2, 9 under aerobic conditions O2 depletes much faster than CO2 will accumulate. Assuming a respiratory quotient of 1 and a steady state O2 level of 2kPa, the maximum achievable steady state CO2 level varies between 2 and 9kPa depending on the film material. To achieve higher steady state CO2 levels without inducing fermentation, microperforated films should be used that have comparable permeances for O2 and CO2. When using microperforated films, O2 will deplete about as fast as CO2 accumulates, such that the sum of O2 and CO2 partial pressure remains around 20kPa. A microperforated MA package designed for 2kPa O2 can therefore generate CO2 levels of around 18kPa. For soft fruit like strawberries, these high CO2 levels are needed to prolong shelf￾life.8, 13 However, after prolonged storage at high CO2 (> 15 kPa) CO2 injury becomes visible from tissue defects and fermentation off-flavours.10, 14 When exposing MA packages to dynamic temperature conditions there is a direct risk of inducing fermentation and an added secondary risk of inducing CO2 damage due to the accumulating fermentative CO2. Especially for microperforated packs where the permeance does not increase with temperature, the risk of inducing fermentation and consequently the accumulation of high CO2 levels is much larger. Scavengers to constrain the accumulation of CO2 (Chapter 3) might limit the secondary risk of CO2 damage but cannot prevent the direct risk of inducing fermentation. 27.3.3 High humidity With horticultural products generally consisting of up to 90% water and with their economic value often determined by the saleable weight of the crop, moisture loss needs to be limited under all conditions. Depending on how and MAP performance under dynamic temperature conditions 567