Firmness evaluation of irradiated tomato in combination with Polyethylene coat using Acoustic technique

Tomato is one of the most important vegetable crops in Iran and is grown commercially in every state in the country. Tomatoes are climacteric in nature and climacteric fruits submitted to gamma irradiation exhibit a delay of ripening. In the specific case of tomatoes, irradiation generally delays ripening when the treatment is applied at the pre-climacteric stage. Mechanical properties are very important quality property in tomato. Determination of the storage time effect on the evolution of this quality is a desirable objective for producers, distributors and marketing agencies, who need reliable firmness measuring instruments for commercial purpose. Previous studies carried out by different researchers show that impact techniques can be used to evaluate firmness of fruits successfully. Quick and nondestructive methods for measuring texture are critical for controlling postharvest quality of crops and fruits. Polymer films are ubiquitous in the food industry, fulfilling a range of functions including a significant role in reducing food waste. Also, it can be possible to extend shelf-life with some coatings, including polymer films.
In this study, tomatoes were irradiated in Atomic Energy Organization of Iran. For irradiation of samples, cobalt 60 (Gamma Cell 220) was used. The samples were irradiated with doses of 0 (control), 0.25, 0.5 and 1 kG, then transferred to the laboratory and kept in a refrigerator at 6 ± 1 ° C and 65 ± 4% relative humidity for 21 days. In order to pack, polyethylene film with medium density (thickness of 0.3 mm and depth and width, respectively, 6.5 and 19 cm) were obtained from the Iranian Polymer Research Institute. Experiments were performed on samples coated with polyethylene film in 7 days intervals. Acoustic tests were performed using Impact Test System. The system was covered with aluminum chamber and inside with wood and clouds. The system was internally equipped with the sample location on the floor, the impact mechanism and the receiver of the signal. In this audio excitation system, samples were taken with the help of a blow. By applying a blow to the sample surface, the audio signal was sent to the sample and then measured by a receiver placed at a distance of 7 mm from the sample. The experiments were performed every 7 days. During the experiment period, the physical and mechanical properties of the samples were measured. Data were analyzed using SPSS.20 software. Correlation between hit parameters and firmness of penetration test were checked out using regression in Excell .2013 software, and predicted a suitable model for determining the firmness of tomato.
The results showed that after 21 days of storage time, the amount of mass and density for irradiated sample with a 0.5 kGy dose in polyethylene film compared to control sample (uncoated) decreased to 22.53 and 12.34 %, respectively, and had more resonant frequency to 26.58%. Also, irradiated sample with a 0.5 kGy dose in polyethylene film compared to samples irradiated with doses of 1 kg and non-irradiated samples in polyethylene film had higher modulus of elasticity and firmness to 37.74, 26.48 % and 25% and 18.12%, respectively. The maximum penetration force in destructive testing had strong relationship with the parameters obtained from Acoustic test such as resonant frequency, Strength index and Modulus of elasticity with a correlation coefficient to 0.893, 0.913 and 0.886, respectively. Combination of polyethylene film with 0.5 kG irradiation on the changes in mass and volume, strength coefficient, resonance frequency, elasticity modulus and rigidity obtained from the impact test have the best results. Also, combination of polyethylene coating and 21-day storage time on mass and volume changes, strength coefficient, resonant frequency, elasticity modulus and stiffness from the impact test reduced the storage performance. According to the results of this study, by irradiation of a sample of tomatoes with a 0.5 kG and using a polyethylene film, it can be stored with a slight change in stiffness and texture for a period of 21 days, and possibly more, which causes increase the shelf-life in the warehouse, reduce storage costs in the warehouse by conventional methods and increase the possibility to export. As a summing up, irradiation in combination with Polyethylene films is a good way to preserve tomatoes tissue and Acoustic textures is a useful way to be replaced with Penetration test