Z. Nadim; E. Ahmadi
Abstract
Introduction: The maintenance of the quality of fresh products is still a major challenge for the consumers. The most important quality attributes contributing to the marketability of fresh fruit include appearance, color, texture, flavor, nutritional value and microbial safety. Strawberry fruits should ...
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Introduction: The maintenance of the quality of fresh products is still a major challenge for the consumers. The most important quality attributes contributing to the marketability of fresh fruit include appearance, color, texture, flavor, nutritional value and microbial safety. Strawberry fruits should be firm but not crunchy. Decreased quality during postharvest handling is most often associated with water loss and decay. The postharvest life of strawberries can be extended by coating technique combined with refrigeration. Application of edible coatings is a conventional method to increase shelf life and maintain fruit quality. Edible coatings can provide an alternative to enlarge fresh fruits’ postharvest life. In this study, the effects of application of methyl cellulose edible coatings and storage time on some mechanical properties, including: the yield stress, yield strain, energy of rupture and modulus of elasticity and also, the viscoelastic behavior of the strawberry fruit was investigated.
Materials and Methods: MC (Methocel, Dow Chemical Company, Midland, MI) coating was prepared by solubilizing MC powder (3.0 g per 100 mL) in a water–ethyl alcohol mixture (2:1) at 75ºC under the high speedmixer (900 rpm) for 15 min. Coatings were used directly on the fruit surface. The physical and mechanical characteristics of fruits were analyzed on 2, 5, 8 and 11 days of storage. The puncture test and relaxation test were done using a texture analyzer (Zwick/Roell Model BT1_FR0.5TH.D14, Zwick GmbH Co., Ulm, Germany; using Xforce HP model of loadcell with capacity of 500 N, by 2 mv/v characteristic). General Maxwell model is widely used to analyze experimental results of the stress tests applied for relaxation. The obtained model coefficients were determined and evaluated from relaxation stress curves. Residues were determined using the sequential model. Usually, multicomponent models can properly describe the actual behavior of agricultural products. Results of factorial experiment in a completely randomized design were analyzed. In this study, the stress versus time graph was plotted and three-component Maxwell model coefficients were obtained.
Results and Discussion: In this current study, application of MC significantly reduced the fresh strawberries decay. Fruit decay in strawberries increased with storage time, but the coating reduced rate of decay with the length of storage. According to the results, the application of these coatings has a positive impact on yield stress and energy of rupture product texture during the storage. Average yield stress and rupture energy for the coated samples and control were 0.11, 5.71 and 0.09, 4.12 MPa respectively. The effect of treatment and storage time on the yield strain and elastic modulus were not statistically significant. The results show that provided Maxwell model satisfactorily (RMSE0.96) fits the experimental data. Also, with a retention time, the elastic component of the model is relatively reduced and the application of the coating prevents the decrease in relaxation time and improves the rheological properties of fruit. Relaxation time is different based on the characteristics of the viscoelastic or viscous substances, but this time is wider in elastic material. The relaxation time depends on the moisture content of the product, so the increase of humidity and soft, reduced relaxation time. Coatings are effective physical barrier to moisture loss and slower rates of weight loss in coated fruits because of the cover features for gas diffusion of stomata, the organelles that regulate the transpiration process and gas exchange between the fruit and the surroundings.
Conclusions: Edible films and coatings may reduce the moisture transfer, the rate of oxidation and respiration which are considered important to prolong the shelf-life of these products. This investigation showed that the MC coatings are effective for strawberries shelf life extension and retarded the senescence process in compared with control. The coat has been as a physical barrier for the gas exchange between the fruit and the environment. It was demonstrated that the coating reduced loss of firmness and delayed the softening of fruit and texture change. Fruit decay in strawberries increased with storage time, but the coating reduced rate of decay with the length of storage. Finally the results showed that coating, may increase overall acceptability, and increase the quality and shelf life of fruits.
Modeling
B. Ghasemi; A. Hemmat; A. Ghasemi; A. Habibi Rad
Abstract
Introduction: Apple is one of the most important horticultural crops of Iran. Its production in the country stands in the second place after citrus. Iran holds the fourth place in the world production of apples and gains a major share in the export of this product. Therefore, it is necessary to enhance ...
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Introduction: Apple is one of the most important horticultural crops of Iran. Its production in the country stands in the second place after citrus. Iran holds the fourth place in the world production of apples and gains a major share in the export of this product. Therefore, it is necessary to enhance the quantity and quality of the fruit in order to maintain and promote its position among the countries importing this product from Iran. Most of the mechanical damages to fruits and vegetables occur due to contact stresses under static, quasi-static and impact loading. To obtain stress distribution inside the fruit we can use finite element analysis. The aim of this study was to simulate the behavior of the apple as a viscoelastic body subjected to quasi-static loading and also to determine the failure criteria (maximum normal stress or shear stress) of apple flesh to estimate its susceptibility to mechanical bruising.Materials and methods: In this study, Golab kohanz apple was used. Two samples were removed from each apple using a core sampler, one was used for uniaxial compression and the other was used for confined compression test using Instron universal tension and compression machine. Spherical indenter and parallel plate tests were performed in order to study apple susceptibility to bruising at four deformation levels (1, 2, 3 and 4 mm) and the bruise volume was then measured after 24 hours. Stress-strain curves were plotted and then, the elastic and viscoelastic properties were obtained. Then, by using the data obtained from apple properties, the apple was modeled in Abaqus software as spherical and cylindrical shapes with viscoelastic behavior subjected to quasi-static loadings.Results and Discussion: The normal stress distribution of the modeled apple in the shape of a cylindrical sample is shown in Fig. 4. The value of maximum normal stress was obtained (0.51 MPa) at the contact point of the loading plate with the sample. Experimental and modeled stress-strain curves are shown in Fig. 5. Up to the bio-yield point, the two curves are nearly matched; and beyond that point, there are some overestimations in the predicted stress values. The location and pattern of failure have often been used to explain the cause of failure in fruits. When specimens of fruit are subjected to a uniaxial compression, the failure often occurs the maximum shear stress plane. Failure patterns in the tested samples indicate that the failure occurs due to shear stresses. Another explanation that has been used by researchers for shear failure is the bruising position inside the fruit after loading. The position of bruising in most of the tested apples was a distance away from the apple surface (Fig. 7).According to the experiments results at the three deformation levels of 2, 3, and 4 mm, the maximum generated normal stress inside the apple was above the point of failure of the cylindrical samples. Based on the empirical results, the bruising was almost zero for the apples subjected to one or two mm deformation (Fig. 9a). The experimental value of the shear strength of the Golab kahanz apple was obtained to be 0.23 MPa. The maximum shear stress inside the modeled apple due to the two mm deformation was 0.195 MPa, which was lower than the shear strength of the apple. On the other hand, by applying three and four mm of deformation, the maximum shear stresses were obtained to be 0.24 and 0.26 MPa, respectively, indicating that the induced stress exceeded the shear strength of apple flesh; therefore, the bruising was observed in the flesh of these apples. The location of the maximum shear stress corresponds to the location of bruising in the tested samples as shown in Fig. 9b.According to the obtained results from the modeling in the finite element software, we can use this software in order to recognize and investigate the damages in agricultural products during different loading conditions (Harvesting, transportation, packaging and storage).Conclusions: In this work, Golab apple was considered as a viscoelastic material and its behavior under quasistatic loading was modeled using finite element method. Elastic, viscoelastic properties and shear strength of apple flesh were obtained and used in the simulation. Comparison of modeling and experimental results shows that the model simulates the behavior of apples during quasistatic loading well. The location of bruise occurrence in the flesh of tested apple and the location of maximum shear stress in the simulated apple was the same. Therefore, the maximum shear stress criterion can be used to estimate the susceptibility of apple varieties to internal bruising under quasistatic loading. Modeling of apple as a viscoelastic sphere in Abaqus software assuming constant bulk modulus could properly simulate apple behavior under quasistatic loading.
A. Ghasemi; A. Ghodarzi; A. Hemmat
Abstract
The changes in viscoelastic properties of potatoes (Tubuls Agria) stored at 4 ˚C for 4 months were modeled and evaluated by using the creep-recovery test. Cylindrical specimens with 15 mm in diameter and 35 mm long were used. The samples were allowed to deform gradually under the constant stress (110 ...
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The changes in viscoelastic properties of potatoes (Tubuls Agria) stored at 4 ˚C for 4 months were modeled and evaluated by using the creep-recovery test. Cylindrical specimens with 15 mm in diameter and 35 mm long were used. The samples were allowed to deform gradually under the constant stress (110 kPa) for about 30 min. After removing the load, the sample recovery was registered. Deformation of the specimens with time were measured and recorded. Strain-time curve was plotted for loading and recovery process. Four-element (Burgers) mechanical model adequately described the creep response of the potato tissue. The coefficients of instantaneous elasticity, elastic and viscous coefficients of the retarded elastic part, Newtonian viscous flow and retardation time were determined, as they can be used to study the effect of storage conditions on the quality of the stored potatoes. These values were decreased significantly (P
