Modelling mealiness

To date, qualitative information is available with respect to the development of mealiness in apples as a function of the storage conditions (Harker and Hallett, 1992; De Smedt et al., 1998; Andani et al., 1999). De Smedt et al. (2001) constructed a comprehensive mechanistic model for quantitative prediction purposes. This model describes the changes in the middle lamella, the water transfer through the tissue and their interaction at the cellular level as affected by the relative humidity for both air and low oxygen storage. The model explains the time dependency of the hardness, tensile strength and juiciness of apple tissue. These mechanical parameters have been shown to be directly related to mealiness as perceived by sensory panels (Barreiro et al., 1998b; De Smedt, 2000).

Texture properties of apple, such as mealiness, are affected by the mechanical and chemical properties of the cell walls and middle lamellae, by the water status and, in particular, by turgor pressure of the cell. These properties change considerably during post-harvest storage and affect each other. For example, a key transformation in apple is the hydrolysis of pectin which requires water as a substrate. Water is available from inside the cells and is also produced through respiration. De Smedt et al. (2001), therefore, decided to include the following general features in the model:

• respiration

• changes of the middle lamella

• transfer of water in the apple

• relationships between fruit texture attributes and the middle lamella and cell turgor.

They assumed that the apple can be considered to be a homogeneous object. The only independent variable left is the time and, therefore, ordinary differential equations are sufficient to define the model structure. They also noted that this model should, hence, be considered as a crude approximation of the reality.

The model is based on a simplification of the histological structure of the apple (Fig. 9.4). The authors assumed that the apple consists of two compartments, the symplast, consisting of the entire network of cytoplasm interconnected by plas-modesmata, and the apoplast, consisting of the cell walls system and the intercellular space (Taiz and Zeiger, 1998). The symplast is separated from the apoplast by a semi-permeable membrane, the plasmalemma. Passive (diffusive) transport of water between both compartments is possible through the plasmalemma. The apoplast can exchange water with the environment via epidermal transfer. The apple skin, with its protective wax layer, is the major barrier to this transfer. The water loss of Cox's apples during a commercial storage period of 6 months at 3°C and 90% RH is typically 5% or greater.

Fig. 9.4 Schematic representation of an apple (reproduced from De Smedt et al., 2001 with kind permission of Elsevier Science).

Relative humidity can be considered tobeapropertyoftheenvironmentwhich affects the behaviour of the apple. It is aninputvariableof themodelandisavail-able to the post-harvest technologist to optimisethestorageprocess.Inthecell compartment, the respiration process was modelled by two chemical reactions: the hydrolysis of starch into hexose units and water and the oxidation of the hexose units into water and carbon dioxide.Intheintercellularspacethedisso-lution of pectins was modelled by a simple hydrolysis reaction.

By specifying mass balances and assuming simple chemical kinetics, De Smedt et al. (2001) derived a set of six differential equations that describes the changes in the water concentration in thecellsandintheintercellularspace,as well as the changes in hexose and starchconcentrationinsidethecellsandpectin in the middle lamellae. These state variableswererelatedbysimplealgebraic relations to measurable quantities such asjuiciness,crispinessandcompressive hardness together with experimentally obtained values of apples stored under normal air and controlled atmospherestorageconditions.

In Fig. 9.5 the experimental data of the five output variables measured by De Smedt et al. (2001) are shown as a function of storage time together with the simulated model values. The symbols represent the averages of 20 measurements. The 95% confidence intervals ofthemeanaregivenbyverticalbars.By examining Fig. 9.5 it can be seen that the model fits the data very well, although the model slightly underestimates the tensile strength (crispiness) in the case of apples stored in air (Fig. 9.5e). Juiciness and hardness were estimated more adequately (Fig. 9.5c and a). According to the model, the soluble solids for the apples stored under normal air composition kept on increasing after 100 days, while apples stored in CA conditions reached a more or less constant value (Fig. 9.5a). This could not be verified by experimental measurements because the measuring technique did not allow any more juice to be taken once the apples became rather mealy for the normal air storage condition. However, this prediction was plausible because of the concentration effect that can be expected because of the considerable weight loss (Fig. 9.5b). The model fitted the weight loss well.

Fig. 9.5 Change in the measured output variables during storage. Error bars denote 95% confidence intervals of the mean of 20 measurements (note, the value of soluble solids after 84 days of air storage is the mean of only three measurements) (reproduced from De Smedt et al, 2001 with kind permission of Elsevier Science).

Fig. 9.5 Change in the measured output variables during storage. Error bars denote 95% confidence intervals of the mean of 20 measurements (note, the value of soluble solids after 84 days of air storage is the mean of only three measurements) (reproduced from De Smedt et al, 2001 with kind permission of Elsevier Science).

Sensory experiments showed that the apples stored under normal air composition were more mealy than those stored in CA (Andani, 2000). According to this model this can be explained through an accelerated degradation of starch and sugar and a more pronounced dissolution of the middle lamellae.

The model of De Smedt et al. (2001) can be used advantageously to evaluate the effect of changes of storage conditions - unintentional or on purpose - and fruit characteristics such as size and maturity on the development of mealiness for cool store management purposes.

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