Rheological properties of strawberry fruit coating with methylcellulose

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.