Document Type : Research Paper

Authors

• H Zaki Dizaji
• S Minaei
• T Tavakoli Hashtjin
• M Mokhtari Dizaji

Abstract

Introduction: It is known for a long time that ultrasound offers unique features in food industry and also agricultural industry for characterizing products in their intact state, with no sample preparation and no sample destruction. However, it is used still mostly in research environment and there is little available research about fruit quality assessment by ultrasonic technique in IRAN.
Knowing the quality of agricultural products not only from the perspective of export and domestic consumers is important interests, but it also helps to control and reduce its postharvest losses. Determination of the quality of agricultural products such as fruits and vegetables is important in commercially competitive modern agriculture. Physiological degradation of pomegranate results in reduced quality exhibited as peel softening and loss of freshness. Native land of the pomegranate (Punica granantum L) is IRAN and it is an important tree of the tropical and subtropical regions of the world which is valued for its delicious edible fruit.
 Among the native fruits grown for export, pomegranate has a special significance. According to the FAO statistical report, Iran is the first producer and exporter of pomegranate in the world. Despite its importance, its basic tissue attributes and whole fruit maturity has not been studied. On the other hand, pomegranate fruits are not maturity indicators obviously such as tomato. For this reason pomegranate was selected for the current research. In this study, ultrasonic technique is utilized as a suitable method for quality determination of pomegranate fruit.
Materials and Methods: Ultrasonic technique is one of the earliest nondestructive testing (NDT) methods, which is still under development for quality determination of agricultural products. In this research, pomegranate quality was evaluated using Ultrasonic technique and punch test (Magness-Taylor). In line with previous research work, a novel ultrasonic system dubbed “Ultrasonic Qualimeter System” (UQS) and its control programs, “Ultrasonic Qualimeter System software“(UQSS) with central frequency 40 kHz were  utilized to evaluate ultrasonic indices of pomegranate fruit in four quality classes of unripe(hard), ripe(medium), overripe(soft) and decayed(so soft). This ultrasonic system works based on processing the signal passing through the materials. The ordinary indices of the through-transmission ultrasonic test are wave velocity and attenuation coefficient. The other ultrasonic index is root mean square that is calculated in time zone of the digital signals. Firmness as a mechanical property, and ultrasonic wave velocity as an ultrasonic parameter, was selected to assess pomegranate quality. Evaluation of pomegranate quality was carried out through testing of its tissue and peel. The firmness index of pomegranate peels was metered by the punch test using universal material test machine (Hounsfield, H50 K-S, England).
Results and Discussion: UQS were successful in transmitting ultrasound wave through pomegranate tissue (1-2 cm thickness) and peel, but results of excited and received signal processing showed that due to its non-homogenous tissue pomegranate vigorously diminished the intensity of transmitted waves. By comparison, the attenuation coefficient of pomegranate peel and its tissue is higher than that of the other agricultural products such as potato and avocado.
Statistical analysis demonstrated that the quality of pomegranate fruit can be assessed using ultrasonic technique, so that decreasing freshness of pomegranate peel samples leads to decrease of wave velocity from 290 (unripe fruit) to 63 m/s (decayed fruit). In other words, depending on samples quality levels, transmitted wave velocity is varied about 230 m/s for pomegranate peel samples.
One of the mechanical properties that are most useful to demonstrate fruit quality conditions is stiffness. Initially, analyses showed that Chart trend of stiffness in four quality levels is similar to wave velocity. So non-linear regression models were developed with good correlation (R2=0.83) between the firmness and ultrasonic velocity. Results of regression analysis demonstrated that ultrasonic indices of pomegranate peels can be used for inspection of pomegranate quality conditions.
Conclusion: The first step in nondestructive assessment of any medium is introducing fitness index or indexes in which it can show the medium conditions. In this research, statistical analysis demonstrated that the quality of pomegranate fruit can be assessed by ultrasonic technique. However, it is necessary to carry out more research to improve this technique for widespread applications. To use this method, the ultrasonic system should be modified so that the transmitted and received transducers test the whole pomegranate by its peels.

Keywords

• Quality determination
• Signal conditioning
• ultrasonic index
• nondestructive testing (NDT)