Storability evaluation of Golab apple with acoustic and penetration methods

Apple fruit (Mauls domestica Borkh, Rosaceae) after citrus fruits, grape and banana, is the fourth important fruit in the world and is considered the most important fruit of temperate regions. In terms of trade volume, Iran is fourth producer and 17th exporter in the world. Among Iranian cultivars of apple fruit, known as “Golab apple”. Golab apple is one of the fragrant and tasty varieties and meanwhile is very sensitive and also its period of the postharvest shelf life is very short. In a study, the firmness of pear fruit during 4 weeks of storage was monitored using non-destructive impulse response (I-R) and destructive Magness-Taylor (M-T) puncture tests. The results of this study showed that the dominant frequency, stiffness coefficient and elasticity coefficient as a function of time could be expressed as a decreasing linear function (Gómez et al., 2005). Tiplica et al., (2010), showed that acoustic measurement can be a useful tool to discriminate different apple batches with a low error rate. Starting from the spectrum of the signal recorded by a microphone after the impact of a small hammer on the fruit, 18 key features were identified and used for the classification of apples belonging to 10 different varieties. The study aimed to evaluate apple firmness measured using both the penetrometer and acoustic methods. The methodologies were applied to Royal Gaya and Golden Smoothee apples harvested from 12 different orchards in Catalonia (Spain), on six different dates, and over three seasons. The results obtained showed a noticeable correlation between Magness Taylor firmness and acoustic measurements in Royal Gala, but no correlation was found for Golden Smoothee. In this study, also, acoustic measurements seemed to be a good tool for evaluating changes in tissue firmness during long-term storage (Molina-Delgado et al., 2009). In another study, it was presented a novel approach based on the simultaneous profiling of the mechanical and acoustic response of the flesh tissue to compression, using a texture analyzer coupled with an acoustic device. The methodology was applied to a 86 different apple cultivars, measured after two months postharvest cold storage and characterised by 16 acoustic and mechanical parameters. The results demonstrate the good performance of our combined acoustic-mechanical strategy in measuring apple crispness as it is perceived by human senses (costa et al, 2011). Hence, present study was about postharvest durability evaluation of this apple in cold storage and effect of methylcellulose coating on durability of this sensitive apple for both intact and damaged ones.
After obtaining Golab apples, from one of the gardens of Karaj (Alborz province, Iran), 240 of them were selected. Our aim in this study was to evaluate the firmness of apples with two penetration (destructive) and acoustic (non destructive). The tests were performed in Agricultural Engineering Research Institute in Karaj. Firmness is one of the fruit characteristics that changes during storage. In present study, this characteristic of the apple fruit was assessed by two mentioned methods. Half of the apples were damaged with identified and controlled impact. In the next stage, another half of apples in both groups (the intact apples and the bruised apples) were coated with methylcellulose. Effect storage on apple in four groups, including: Intact and uncoated apples, intact and coated (with methylcellulose) apples, bruised and uncoated apples and bruised and coated apples during about ten weeks of cold storage at 2˚C and 85% RH was studied by the acoustic and the penetration tests. Acoustic parameters including: natural frequency, firmness index, elasticity coefficient were measured by recording audio signals resulting from non destructive impacts of a pendulum using a sound analyzer microphone and then the conversion of those parameters were performed from the time domain to the frequency domain by the corresponding formulas and software. Penetration test measurements were performed using a texture analyzer and its software. The tests were carried out every week. Statistical analysis of the results was carried out using Excel 2007 and SPSS 16 software and the significance of the results was determined using Duncan's test at the 5% confidence level.
Analysis of variance showed effect of independent variables including: effects of coating, impact and time and also interaction effects on dependent variables including: natural frequency, acoustic index and modulus of elasticity and penetration index on the tested apples. Effects of coating and time were significant at the 5% confidence levelon all dependent variables. But the impact and interaction effect were not significant on dependent variables (Table 1). In general, bruise and lack of coating on the apples during the 10 weeks of storage, were reduced acoustic parameters. In the penetration test, changes were similar to acoustic test (Table 2). In this test, all curves have downward trend and combination of independent variables: coated and intact apples were reasons of more penetration resistance of apples in all of the groups. The condition was continued until the end of storage time, despite of the downwards slopped curves in all groups. In penetration test, coated apples keep more firmness than other groups (groups of apples without coating) and thus the apple's quality would stay better, too (Fig.7).
In general, the following results were obtained from this research: The results showed that the acoustic and penetration parameter were decreased during 10 weeks of storage. Reduction of these parameters continued until the end of storage period, but this reduction was significant only up to eighth week. Also at this time, the acoustic parameters (natural frequency, firmness index, elasticity coefficient) and penetration firmness in intact and coated apples were 14.26%, 4.11%, 14% and 40% respectively higher compared to other apples. Due to the more tangible acoustic parameters changes, especially acoustic index and modulus of elasticity (having the more slope than the penetration firmness). One could use acoustic tests for more accurate evaluation of apples firmness and quality changes. Finding correlation between acoustic parameters and penetration parameter showed that, correlation between acoustic parameters in each case is greater than correlation between these parameters with penetration parameter.