Assessment of apple (Malus × domestica Borkh.) fruit texture by a combined acoustic-mechanical profiling strategy

https://doi.org/10.1016/j.postharvbio.2011.02.006Get rights and content

Abstract

Texture of apple fruit originates from anatomic traits related to cell wall architecture and is one of its most important quality characteristics, thus there is the desire to better understand the different factors which contribute to apple texture. Here we present 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. Statistical treatment of the data with principal component analysis (PCA) allowed for the identification of three groups of variables, the mechanical ones being clearly distinguished from the acoustic ones. Moreover, the distribution of the apple cultivars in the multivariate PCA plot allowed characterisation of the cultivars according to their textural performance. Each cultivar was analyzed also with non-destructive vis/NIR spectroscopy in order to determine impartially the ripening stage. Sensory evaluation by panellists was performed on a selected group of cultivars and sensory data correlated with the acoustic-mechanical data. The results demonstrate the good performance of our combined acoustic-mechanical strategy in measuring apple crispness as it is perceived by human senses.

Research highlights

► Texture is mainly composed of acoustic and mechanical signatures. ► Apple texture was dissected by PCA in three groups of variables. ► Multivariate analysis separated acoustic from mechanical parameters. ► Two PCs characterized apple cultivars based on their textural performance.

Introduction

Texture represents one of the four principal factors defining food/fruit quality, together with appearance, flavour and nutritional properties (Bourne, 2002), and plays a key role in consumer acceptability and recognition of apples. In particular, textural characteristics of apples defined by “crispness”, “juiciness”, “hardness”, “firmness” and “mealiness” are often key drivers of consumer preference (Harker et al., 2003). Texture originates from several different physical properties rather than from a single trait, and depends on cellular structure and how this responds to applied forces (Szczesniak, 2002). In fruit and vegetables, crispness and crunchiness are mechanically expressed as a rapid decrease in force accompanied by a rapid textural fracture propagation. They normally represent the major traits contributing to general “food enjoyment” since both are considered by consumers as an indication of the freshness and healthy state of fruit (Szczesniak, 1988, Fillion and Kilcast, 2000, Fillion and Kilcast, 2002).

In apple it has been shown that among the textural traits, crispness accounts for 90% of texture appreciation, and it has been largely recognised as the key attribute affecting consumer acceptability (Hampson et al., 2000). Microscopically, when the wall of a turgid cell is broken under mechanical pressure, a sound pressure wave is generated, resulting in the typical “sound” perceived as the crispy phenotype. Crispness events and sound pressure waves are strictly dependent on the breaking propagation toward adjacent cells, where the pressure exerted on the outer cell wall causes a catastrophic rupture (Kilcast, 2004). In crispy apples, the cell breakage generates a sound wave which causes a vibration between molecules around their equilibrium, consequently propagating the pressure wave and thus producing the sound (Duizer, 2001).

In contrast, low cell wall turgidity, due to a higher elastic tension of the wall, or by pectic polysaccharide solubilisation in the middle lamella, determine cell separation instead of cell wall fracturing (De Belie et al., 2002), with a consequent rubbery texture typical of mealy apples (Reeve, 1970, Niklas, 1992, Andani et al., 2001).

Cell rupture (the event generating crispness) or separation (responsible for mealiness) have also a direct impact in the release or encapsulation of juice and aroma (Kilcast, 2004, Echeverria et al., 2008). Therefore, a crispy apple is generally preferred not only for texture characteristics, but also for an enhanced release of volatile compounds perceived by the receptors in the mouth space.

To date, the most complete description of crispness is provided by sensory testing carried out by expert or trained panellists. However, there are fundamental limitations to this approach due to the difficulties in employing such a strategy for assessment of large cultivar collections and breeding material. To overcome these constraints, instrumental approaches dedicated to fruit/food textural quality analysis have been developed (Roudaut et al., 2002), and documented in several studies reporting textural analysis in apple performed with different techniques, such as compression (Mehinagic et al., 2004), single-edged notched test (Harker et al., 2006), sound recording during mastication (Roudaut et al., 2002) and chewing sound measurements (De Belie et al., 2000, Ioannides et al., 2007). However, the comparison between the sound amplitude recorded during the fruit biting and the corresponding crispness sensorially evaluated resulted hardly reproducible and panellist dependent (De Belie et al., 2002).

Other methodologies based on mechanical approaches focus on the physical response of the samples, such as deformation, fracturing and compression, the latter being probably the most widely used for its simplicity (Roudaut et al., 2002). The use of the puncture test to predict consumer preference has been also reviewed and questioned (Harker et al., 2002).

Novel technological improvements in the direct measurement of fruit texture have recently been presented by Taniwaki et al. (2006), based on the concept that sensorially perceived crispness could be derived by the vibration produced during fracturing (proposed by Christensen and Vickers, 1981). This device, represented by a piezoelectric sensor able to detect the vibration caused by the sample's fracture, has been used to quantify a texture index (TI) in several species, including apple (Taniwaki et al., 2006, Taniwaki and Sakurai, 2008). This equipment was also further coupled with non-destructive vibration methods (Laser Doppler Vibrometer-LDV) to measure the change of both elasticity and texture index during the ripening of persimmons (Taniwaki et al., 2009a) and pears (Taniwaki et al., 2009b).

It is likely that the combination of different methodologies may represent a further improvement for more efficient texture investigation, as already demonstrated in almonds, where a similar strategy was successfully applied and correlated with sensory evaluation, (Varela et al., 2006). Recently, Zdunek et al. (2010) reported crispness and crunchiness evaluation of three apple cultivars by measuring the contact acoustic emission together with the puncture test, and showed that the sensory attributes related to crispness and crunchiness correlated better with acoustic emission events rather than firmness.

The purpose of our work was the improvement of the texture variability dissection, increasing the number of parameters acquired during the compression phase. We investigated whether mechanical and acoustic characterisation of a set of apple cultivars could provide a valuable methodology able to encompass all aspects related to apple texture, providing also a better link with sensory attributes.

Section snippets

Plant materials

In this experiment we selected 86 apple cultivars (Supplementary data Table S1) harvested in 2009 in the experimental fields of two institutions: the Research and Innovation Centre of Foundation Edmund Mach in San Michele all’Adige (Trento), and the Laimburg Research Centre for Agriculture and Forestry (Bolzano), both in the North of Italy (Trentino Alto Adige region) and with similar climatic conditions.

All fruit were collected at commercial harvest, following the main parameters used to

Acoustic-mechanical combined profile analysis

The TA-XTplus texture analyzer equipped with the AED system allowed the simultaneous acquisition of two different types of source data, acoustic and mechanical, that are efficiently collected by the same instrument, as well as plotted and analyzed by the same software.

The mechanical profile obtained as a response to compression is mainly composed by two parts (Fig. 1). In the first step a compression slope is observed until the yield point (F1), which marks the transition from the elastic

Conclusion

In this study we developed a novel strategy based on a combined mechanical and acoustic profiling to better categorise textural properties of apple fruit. We applied it to the largest apple collection investigated to date for this purpose (86 cultivars). Furthermore, the instrumental data were compared with sensory evaluation.

Multivariate data analysis allowed the identification of a clear separation of the mechanical set of parameters from the acoustic ones. Moreover, a good separation of the

Acknowledgements

This research was supported by the post-Doc project CANDI-HAP and by the Autonomous Province of Trento (APR 2009/2010). The authors are grateful to Marco Bulgarelli (Stable MicroSystem) for technical assistance and Marco Fontanari for phenotyping support.

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