AVOCADO FIRMNESS MONITORING WITH VALUES OBTAINED BY MEANS OF LASER DOPPLER VIBROMETRY

Avocado (Persea americana Mill.) ripeness is usually evaluated by destructive firmness assessment, yet quality is notorious for being heterogeneous within a consignment. This problem, which is especially true for imported avocado fruit, lends itself to searching for non-destructive methods for firmness evaluation. Firmness of objects can be analysed by impulse-response. This technique utilizes recording of a vibration signature and interpretation of the resonant frequency. In this study a laser Doppler vibrometer (LDV) was used for non-contact recording. The aim of this study was to show the feasibility of using a LDV to monitor avocado firmness. In order to create avocado batches of different homogeneity, three groups were treated with and without ethylene, inside or outside a box. After day 0 a third of the fruit were transferred into boxes (325 L) to be treated with ethylene for 24 h. A third was kept in boxes untreated and another third was kept on open trays. Avocado fruit cultivar ‘Hass’ were ripened at 18°C and measured on days 0, 2, 3, 4 and 6. Individual fruit were impacted once and two LDV measurements were taken simultaneously at the stem-end and seed-end. This was repeated twice around the fruit. Force-deformation measurements in compression mode were performed by means of uniaxial testing (Instron model 5542) on two opposite sides (day 0 n=8, then n=24 i.e., 8 per treatment/d). Significant differences were found between firmness of avocados over time and across treatments. Firmness decreased exponentially as expected (304.1 to 2.1 N) over six days. The LDV results showed significant differences between days, treatments and laser-location. The resonant frequency of the fruit decreased linearly until day 4 and then decreased more slowly (1671 to 476 Hz). On average, the frequency found at the seed-end of the fruit was higher than the resonant frequency at the stem-end. This is thought to be due to the seed itself, which could influence the vibration pattern. Resonant frequencies showed good correlation to the logarithm of firmness (r=0.87) and therefore were shown capable of monitoring avocado firmness. INTRODUCTION Avocado (Persea americana Mill.) fruit are an attractive addition to the diet due to their unique flavour and texture. In addition, avocados have reported health benefits (Ashton et al., 2006; Ding et al., 2007; Henrotin, 2008), as research has shown that eating avocado fruit flesh may contribute to lowering cholesterol levels (Ledesma et al., 1996). Avocado mesocarp tissue has high concentrations of desired nutrients such as unsaturated fatty acids and seven carbon carbohydrates. The OECD (2004) recommends that the quality of avocado fruit can be rated according to size, estimated oil content (dry matter), absence of defects and fruit firmness. The degree of ripeness of many climacteric fruits like avocado can be correlated with fruit firmness. It has been shown that fruit often do not soften uniformly due to physiological gradients (Landahl et al., 2009). Fruit and vegetables are qualitatively evaluated by organoleptic properties. Firmness is an important textural characteristic. Standard firmness evaluation is Proc. 4 International Conference Postharvest Unlimited 2011 Eds.: P.M.A. Toivonen et al. Acta Hort. 945, ISHS 2012 240 destructive and often subjective. In the past many studies have reported on attempts to predict firmness by non-destructive techniques (Ruiz Altisent et al., 2010). From as far back as the 1960s (Abbott et al., 1968) researchers have investigated the possibility of predicting firmness by impulse response (vibration) measurements. The recording of vibration signatures can be done by accelerometer or more recently by piezoelectric film (Lu et al., 2008), or non-contact by either microphone (Roth et al., 2007, 2008; Lu et al., 2008; Molina-Delgado et al., 2009) or by laser Doppler vibrometer (Blahovec et al., 2008; Tanawaki et al., 2010; Landahl et al., unpublished). The impulse response techniques are greatly influenced by angle and location of impact (Sun et al., 2010). A laser-Doppler vibrometer (LDV) is a precision optical instrument used for determining vibration at a defined point (Polytec UK Ltd., 2011). The technology is based on the Doppler-effect; sensing the frequency shift of back scattered light from a moving surface. The Laser-Doppler vibrometer works on the basis of optical interference, requiring two coherent light beams, with their respective light intensities I1 and I2, to overlap; the resulting intensity is not just the sum of the single intensities, but is modulated. According to interference theory the overall intensity of two beams is zero if they have a path length difference of half of one wavelength. Changing the optical path length per unit of time manifests itself as the Doppler frequency shift, so that the modulation frequency of the interferometer pattern is directly proportional to the velocity of the object. However, the velocity of the object cannot indicate the direction it is moving in; for this purpose, an acousto-optic modulator (Bragg cell) is placed in the reference beam; This means that it is now possible not only to detect the amplitude of movement but also to clearly define the direction of movement. In principle the LDV can directly measure displacement as well as velocity. In this case, the frequency is not analysed, instead the LDV counts the bright-dark fringes on the detector. Displacement measurement is better suited for low frequency measurements. The present study aimed to minimise the heterogeneity of a batch, which often occurs due to different agricultural practise, climate, season and duration of avocado transit by measuring all fruit non-destructively and sorting them according to the results. A feasibility study was conducted to test non-destructive laser Doppler vibrometry as an assessment tool for firmness in avocado fruit. MATERIALS AND METHODS Plant Material All imported avocado fruit (Malaga, Spain) were supplied by M.W. Mack Ltd. (Kent, UK) and were transferred to a 6°C cool room immediately after delivery (8 boxes of size code 18 avocado fruit cultivar ‘Hass’). Fruit from Spain arrived in the UK within a week after picking and a cool chain was applied of 5-6°C. On day 0 (baseline) fruit were randomised and labelled and allowed to warm up to room temperature. Then tests were performed on 8 fruit. Forty fruit were transferred into gas tight boxes (ca. 325 L) to be treated with ethylene for 24 h in a separate 18°C room. Fruit were not treated with 1-methylcyclopropene. Controlled ripening was initiated using exogenous ethylene (100 μl L; BOC Gases Ltd., Surrey, UK; Terry et al., 2007b). Forty fruit were kept in boxes untreated and another 40 were kept on open trays in a temperature controlled 18°C room. All fruit were ripened at 18°C for 6 days. Tests were performed in a sequence. First non-destructive laser Doppler vibrometry (LDV) was performed, then firmness was measured destructively on two opposite sides of the fruit and last mesocarp tissue was snap frozen in liquid nitrogen for sugar analyses (thin slice cut across longitudinal axis of fruit). During ripening 8 fruit were measured per treatment each on days 2, 3, 4 and 6 (n=8+4x24=104). Frozen samples of fruit tested on days 0, 2, 3 and 6 were analysed for sugar content (n=80). Textural Evaluation Avocado fruit firmness was measured on opposite sides of peeled fruit. All texture