Postharvest shelf-life discrimination of nectarines produced under different irrigation strategies using NIR-spectroscopy

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Abstract

Quick assessment of storage time in fruits is important for both growers and consumers due to the fresh fruit market is becoming increasingly demanding with regard to product quality. This study sought to evaluate the ability of near-infrared reflectance spectroscopy (NIRS) to classify intact nectarines in post-harvest storage, as a function of pre-harvest irrigation strategies applied and post-harvest cold storage duration. A total of 220 nectarine fruits (Prunus persica (L.) Batsch cv. ‘Sweet Lady’) were sampled after 7, 14, 21 and 28 days of refrigerated storage (0 °C, 95% RH) and at commercial harvest time. Two commercially-available spectrophotometers were evaluated for this purpose: a handheld MEMS spectrophotometer of 1600–2400 nm and a diode-array Vis/NIR spectrophotometer of 400–1700 nm. Models developed using partial least squares 2-discriminant analysis (PLS2-DA) correctly classified between 86 and 96% of samples by post-harvest storage time using the handheld instrument, and between 66 and 89% in the case of the diode-array spectrophotometer. Classification models based on pre-harvest irrigation treatment classified 57–84% of the samples correctly, due to the similarity in physical–chemical properties of fruits in both irrigation strategies. These results showed that NIRS could be used to monitor changes in nectarine quality parameters during pre- and post-harvest as an essential tool for decision-making both in-field and on-line.

Introduction

Quality in nectarine fruit can be evaluated in terms of several major physical and chemical parameters: size, shape, color, firmness, soluble solid content and acidity. Although maximum fruit quality is already determined by harvest time, and cannot therefore be improved but only maintained during the post-harvest ripening period (Crisosto et al., 1997, Hewett, 2006), it is essential for agricultural researchers, fruit growers and commercial packing houses to understand the influence of crop practices such as irrigation, fertilization, thinning, pruning, and application of plant-growth regulators, on harvest fruit quality parameters and the rate of fruit quality loss during post-harvest storage; this information provides a solid basis for the implementation of best practices to ensure high quality fruit and to prolong product shelf-life.

The adoption of regulated deficit irrigation (RDI) strategies, based on reduced irrigation during phenological stages when fruit growth is less sensitive to water stress (Chalmers, Mitchell, & Van Heek, 1981), is becoming a common practice in areas with low water availability; it is an efficient strategy for water saving in some fruit trees, and has no negative impact on yield (Chalmers et al., 1981, Fereres and Soriano, 2007, Girona et al., 2003, Girona et al., 2005, Li et al., 1989). However, although deficit irrigation has been identified as a factor modifying fruit quality (Naor, 2006), some studies which addressed to analyze the effect of deficit irrigation on quality parameters in stone fruits have shown the beneficial effect of this irrigation technique during post-harvest shelf-life (Crisosto et al., 1994, Gelly et al., 2004, Pérez-Pastor et al., 2007).

At present, measurement of the most relevant quality parameters in stone fruits during both on-tree ripening and post-harvest storage is performed by traditional methods, which are usually destructive, as well as time-consuming, costly and contaminant. Moreover, they enable quality control of an only few samples per batch, rather than of each individual fruit. The development of a non-destructive, multi-component, and instantaneous measuring technique is therefore very attractive, in order to monitor in-field crop trials and ensure fruit quality both at harvest and during the post-harvest storage process, with a view to meeting consumer requirements.

Near-infrared Reflectance Spectroscopy (NIRS) is currently one of the approaches best suited to these requirements; this is of particular interest, since fruit quality cannot be assessed on the basis of a single parameter (Nicolaï et al., 2007, Saranwong and Kawano, 2007, Sánchez et al., 2009, Slaughter and Abbott, 2004). In the near-infrared (NIR) spectral region, overtone frequencies of molecular vibrations absorb light quite readily. Because the overtone absorption bands are typically wide and overlapping, the spectroscopist cannot merely measure peak heights to perform quantitative analysis. Instead, multivariate regression analyses are used to correlate spectral features with concentrations or physical properties of interest (Bertrand, 2000, Workman and Shenk, 2004).

The application of NIRS for the analysis of intact fruits and vegetables, has become considerably more widespread in recent years, thanks to improvements in instrumental design and the development of low-cost, ergonomically-designed portable and handheld instruments, which are more robust and highly insensitive to external conditions (temperature, noise, humidity), thus allowing on-site measurement. As a result, the instrument can be taken to the product, rather than the reverse, as was previously the case with traditional NIR equipment. There have also been marked improvements in accessories, and especially in the development of transmission or interactance–reflectance fiber optic probes better suited to the analysis of intact products (Geyer et al., 2007, Pérez-Marín et al., 2009, Reita et al., 2008, Saranwong et al., 2003, Zude et al., 2006).

This study aimed to evaluate the ability of NIRS to classify intact nectarines by internal quality in post-harvest storage, as a function of pre-harvest irrigation strategies applied and post-harvest cold storage duration using PLS2-Discriminant analysis (DA). The performance of discrimination models developed with different NIR instruments, one particularly well suited to in-field measurements (handheld MEMS-based spectrophotometer) and the other appropriate for on-line use in the packing house (diode-array spectrophotometer) was also compared.

Section snippets

Experimental orchard

The study was carried out in 2007 in a 0.72-ha plot of nectarine trees in a commercial orchard located near Cordoba, Southern Spain. The nectarine trees (Prunus persica (L.) Batsch cv. ‘Sweet Lady’) were planted in 1990 at 6 m × 3.3 m (500 tree ha−1) on a deep alluvial soil of loam to clay-loam texture, with rows oriented N–S. The climate in the area is typically Mediterranean, with an annual average rainfall of around 600 mm, concentrated between autumn and spring, and an average annual ETo of

Irrigation, yield and fruit quality

Table 1 shows the irrigation applied in the FI and RDI treatments, together with values for fruit yield, fruit number and fruit size (measured as fruit weight and equatorial diameter) at each harvest date. RDI strategy reduced irrigation by 35% (108 mm of water saved) compared to the FI schedule, although no difference was found in yield between RDI and FI, with a mean value about 45 kg fruit tree−1 (Table 1). There were no differences between irrigation treatments with regards to fruit yield,

Conclusions

The results showed that NIRS can be used to estimate post-harvest cold storage time taking into account the influence of pre-harvest factors such as the irrigation strategy applied, without the need for the costly and laborious analysis involved in traditional methods; it can also be used for measuring physical–chemical parameters in nectarines as a means of assessing post-harvest quality and product shelf-life. It must be stressed that this technology provides, in a matter of seconds,

Acknowledgements

This research was funded by the Andalusian Regional Government under the Research Excellence Program (Project No. 3713 ‘Safety and Traceability in the Food Chain using NIRS’), and the European Commission IRRIQUAL Project (FP6-FOOD-CT-2006-023120). The authors thank E. Fereres for editorial comments on the manuscript.

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