Title: Modeling of 1-MCP transport in apple cool stores by means of computational fluid dynamics
Other Titles: Modelleren van 1-MCP transport in bewaarcellen voor appel met behulp van CFD-technieken.
Authors: Tsige, Alemayehu
Issue Date: 24-Jun-2013
Abstract: Control of ripening is one of the many challenges in keeping quality of apples during postharvest storage. In practice, fruit are stored in boxes or bins (made of plastic, cardboard or wood and of different design) that are stacked in a specific pattern in temperature controlled ultra-low oxygen storage rooms to maintain product quality. Additionally, 1-methylcyclopropene (1-MCP), a synthetic plant growth regulator, is used commercially to delay the ripening of fruits. It is applied in gas form (as a fumigant) in cool storage rooms. Air circulation is important for the uniform distribution of the active substance in the storage room. Wood, cardboard and other plant based porous materials used in bins have 1-MCP sorption capacity and may influence the treatment. Understanding the effect of these factors on the 1-MCP treatment is important to optimize the application process. The majority of research has focused on assessing the dose-response ofvarious commodities to 1-MCP. However, there is a lack of information on how the actual cool storage room environment affects the treatment. Experimental investigation of the spatio-temporal distribution of 1-MCP is expensive, time consuming, and some of the important variables are difficult to measure. To make progress in this area, models are developed in this study.The objective of the study was to develop a CFD model that calculates the cool store airflow and the convection, diffusion and adsorption of 1-MCP in the cool store air, apple fruit and other materials in the cool store. To achieve this, the research started by calculating the diffusion-adsorption kinetics of 1-MCP in fruits and in selected non-target solid materials. A diffusion-adsorption model was developed for the 1-MCP distribution in materials of which the parameters were identified by nonlinear data-fitting the model to jar-test experimental data. Apple fruit (Golden Delicious and Jonagold) and the following bin construction materials were investigated: high density polyethylene (HDPE), oak, poplar wood and card lining. The range in the magnitude of the diffusion coefficient, adsorption coefficient, and concentration of active sites in the various solids was 10,000-, 8-, and 30-fold, respectively. Subsequently, a direct CFD model of 1-MCP applications in a 500 L cool storage container was developed that combined the diffusion-adsorption model with a model of airflow inside the container. The direct CFD model incorporated the boxes and fruits explicitly. This approach was validated with distributed time-response measurements of the 1-MCP concentration in a container. The model allowed investigating the effect of air circulation, bin material of construction, plastic lining and dose reduction on uniformity of the 1-MCP distribution in the air, box material and fruit and the adsorption therein. The 1-MCP distribution in the container atmosphere was uniform in the presence of air circulation. Treatment without air circulationresulted in considerable non-uniformity. The concentration of adsorbed 1-MCPvaried inside the fruit during the treatment and certain minimum treatment duration is required to reach equilibrium depending on the initial dose. The direct CFD models were used to assess the time to reach equilibrium, degree of saturation of the binding sites and to estimate equilibrium distribution of the 1-MCP gas which, at an initial dose of 1µL L-1, equals 11, 34 and 55 % as unbound in fruit, bond in fruit and remaining free in air, respectively.In order to appraise the condition in large cool storage rooms of commercial magnitude, porous medium CFD models were developed and used. Porous medium models are needed to approximate the complex geometry of the product stack in large bins and cool stores. The performance of the porous medium modeling approach was first investigated by comparing it with a direct CFD model and through validation using pilot cool room experimental data. A strategy to perform a comprehensive model parameter sensitivity study was developed using a design-of-experiment (DOE) approach. The velocity field in and around the stack were well reproduced by the porous medium model. Concentration predictions were in average 4% and maximally 7% over the average measured values. The verified porous medium approach was subsequently implemented for the 1-MCP application in commercial cool storage rooms. Usingthe large scale models, the effect of room shape, air circulation rate, material of construction of the boxes and dose reductions were investigated. This study confirmed that within the range of air circulation rates of commercial cool stores a uniform distribution of 1-MCP is achieved. Plastic lining used to seal bins and protect fruit from moisture loss had no perceptible effect on theuniformity as well as on the rate of adsorption of the gas. Wooden boxes immobilized a significant amount of 1-MCP from the treatment atmosphere but did not affect the uniformity. At reduced dosage or in a leaking room, wooden boxes can seriously reduce the available dose for the fruit to a degree that efficacy be compromised. It was also found that the dose required for complete treatment depends on amount of fruit stored in a given cold storage room (filling density). Hence, filling-density based dose prescription is proposed in this study than the currently practiced solitary prescription used in the 1-MCP technology. Lower doses can be prescribed for rooms with plastic bins and for increased treatment duration from the usual 24h to 48h, when treating apples.This work demonstrated well the applicability of a generic CFD model to simulate the airflow and gas distribution in cold storage rooms as a basis for process optimization.<w:latentstyles deflockedstate="false" defunhidewhenused="true"  <w:lsdexception="" locked="false" priority="0" semihidden="false"  
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Division of Mechatronics, Biostatistics and Sensors (MeBioS)

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