A dynamic model has been developed that describes the decrease in firmness of ‘Elstar’ apples during different types of conditions, based on the general knowledge how firmness is affected by chemical and biochemical reactions, and on the strict and consistent application of fundamental kinetics. The variable part of the firmness of apples is deduced to depend mainly on the pectic compounds in the middle lamellae. These pectic compounds are supposed to decay during storage in two distinct routes, one consuming/needing oxygen and one occurring with or without oxygen. Depending on the gas conditions (oxygen and carbon dioxide) the respiration rate of apples change. This respiration affects the rate of the oxygen consuming pectin degradation. The moment of harvest defines among other, the initial climacteric stage of the apples. Depending on the relative respiration rate, the rate of development of the climacteric stage changes. The climacteric stage in itself affects the rate of pectin decay: preclimacteric apples can be kept longer than climacteric ones. During CA or MA storage, an enzyme, probably polygalacturonase, accumulates depending on the length of the CA/MA period: the longer kept at CA/MA conditions, the faster firmness decays at the end of the storage period. On top of all those interactions, a temperature dependence is applied on all individual reaction rates. This temperature dependence is described by the well know Arrhenius law. The model can be applied to predict the firmness of ‘Elstar’ apples during all kinds of scenarios encountered in practice, to get an impression of the expected final firmness when reaching the consumer.
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