Investigation of viscoelastic behavior of persimmon fruit during storage using stress relaxation models

Persimmon fruits are subjected to different environmental conditions from harvest to consumption. Foods, those are exhibiting characteristics of both liquid and solid, are described as viscoelastic where stress relaxation and strength properties are time dependant. Knowledge of viscoelastic properties of foods and agricultural materials are important when considering harvesting, handling, transportation, processing, and storage. Also the data on viscoelastic properties are required as an input for mathematical models, which describe and predict internal stress and cracking during different handling and processing procedures. The mechanical properties are considered one of the most important four parameters, which reflect the quality of food material. Those parameters include texture, firmness and chewability. Viscoelastic material exhibit stress relaxation phenomena, which is one of the most important factors in characterizing agricultural materials. The objectives of the current study were: (1) to obtain data to describe the stress relaxation characteristic of Persimmon fruits during Storage, (2) to investigate the effect of storage time on stress relaxation properties, and (3) to find the best model to describe the obtained stress relaxation data.

The persimmon fruits were manually picked up at maturity stage from persimmon trees in a garden, in Fars province, Iran. They were handled to Biophysics laboratory in Agricultural Engineering department, at Shiraz University, on the foam sheets in one layer array for minimizing any likely compaction. Initial moisture content was determined by gravimetric method in an oven at 70C till obtaining constant successive weight loss of 0.001 g. The initial moisture content was 44.26% (d.b). Since the rheological characteristics of fruits are important for any processing operation, viscoelastic behavior of persimmon fruits during storage in environmental conditions was investigated using estimating its relaxation parameters from experimental stress relaxation data. The rheological behavior of persimmon fruits was evaluated to uniaxial compression test for relaxing the stress during 480 s. Texture Profile Analyser (TPA) (TA.XT2 plus model, Stable Microsystems, England) with Exponent Lite (Version 4,0,8,0, UK) software was used to compress whole date samples and export data into Excel worksheets. TPA was equipped with a 30 kgf load cell. The cross head speed was adjusted on 1 mm/s, and samples compressed with 30% strain. The cross head then kept at 30% of strain for relaxing the stress in 480 s duration. A chamber with temperature control circuit was fabricated around the TPA cross head to prepare constant temperature identical to sample temperature with an accuracy of ±0.1C during the stress relaxation tests. All tests were performed with ten replications. Three popular stress relaxation models, namely the generalized Maxwell with three elements, Nussinovitch, and Peleg were fitted to experimental data and was used from R-square, standard error of estimation (SEE) and percent of average relative error (ARE%) indexes for evalution of models.

The force–time curves were converted to stress–time curves by dividing the true compression force values to the corresponding true values of contact surface area. Analysis of variance showed that storage time had significant effect on initial stress relaxation. The results showed that the magnitude of initial stress (stress at time zero) required to create a fixed strain on whole fruit was significantly decreased up to 80% during storage in environmental conditions, but the relaxation time was increased at the range of 312.10 to 353.90 s. Based on the results, elastic behavior of persimmon fruit was enhanced and its firmness decreased during fruit storage. Persimmon is sugar rich fruit that its behavior is principally partly controlled by physico-chemical properties of sugar. Changes in sugar type as a result of enzymatic action are taken place during the storage process. However, in the present study all samples were in the time of maturation, and therefore the physical state of sugar was influenced by free water which is resulted in increasing in fruit moisture content. Stress relaxation curves were plotted at different storage time. Some variations were apparently observed among the curves. Sucrose decreased initial stress decay rate and increased proportion of un-relaxed force. Variations in stress decay rate might refer to the amount of initial stress which was affected by physic-chemical properties of the sugar. Recall that nonlinear regression method with non-negative constraint was used to derive the models constant.

All three models were valid for quantifying the viscoelastic behavior of persimmons; however the generalized Maxwell with three elements (with R2=0.999, SEE=0.017 and ARE=0.19%) and Nussinovitch (with R2=0.996, SEE=0.057, ARE=0.36%) were the best models in predicting experimental data.