Exploring oxygen diffusion and respiration in pome fruit using non-destructive gas in scattering media absorption spectroscopy

Highlights

• GASMAS for pome fruit respiration measurement.

• Fruit mimicking model system for GASMAS oxygen sensor validation.

• Influence of skin and additional surface coating on gas exchange rate.

Abstract

Pome fruit stored under a controlled atmosphere (CA) often suffers hypoxia due to the mismatch of O₂ level in the storage rooms and the fruit’s O₂ consumption. Fruit response-based O₂ sensing and control could be an efficient approach to reduce the hypoxia-related physiological disorders and therefore increase the shelf life of stored fruit. This research aims to validate and evaluate the application of nondestructive gas in scattering media absorption spectroscopy (GASMAS) O₂ sensing in fruit post-harvest. In the first stage, the GASMAS O₂ sensor was validated on a fruit mimicking multilayer model system where a high correlation was observed between the measured and reference O₂ partial pressures (r² ≥ 0.9). Later, the GASMAS sensor was evaluated on two apple cultivars (Malus x domestica ‘Golden delicious’, Malus x domestica ‘Nicoter’) and one pear cultivar (Pyrus communis ‘Conference’). The observed GASMAS signal from the ‘Golden delicious’ apples were nearly 2 times higher than the signal from ‘Nicoter’ apples and 5 times higher than from the ‘Conference’ pears. In the next stage, GASMAS measurements were taken on water submerged ‘Golden delicious’ apple and ‘Conference’ pear to investigate the difference in O₂ consumption in those fruit. The calculated relative O₂ changes during respiration and evolution of the O₂ partial pressure after nitrogen treatment for both the fruit were found different. It was hypothesized that these findings may be attributed to variations in fruit porosity. And finally, the influence of skin and additional surface coating on the gas exchange was studied by immersing unpeeled, peeled and coated samples in the gaseous nitrogen for 2 h before measurement. The coating was found to reduce the gas exchange compared to the unpeeled samples which already exhibited a lower exchange rate than the peeled samples.

Introduction

Oxygen is a key element of many metabolic pathways in plants which includes respiration. Respiration results in physiological and biochemical changes in living tissues. In climacteric fruit like pome fruit, the respiration rate surges in concert with the ethylene production rate at the onset of ripening (Busatto et al., 2017). To prevent fast ripening and senescence of pome fruit, controlled atmosphere (CA) storage is commonly used to reduce the ethylene biosynthesis and respiration. This is achieved by reducing the O₂ and increasing the CO₂ partial pressures of the cool room (Thompson and Bishop, 2016). Because of the resistance of the skin and cortex tissue concerning diffusion, O₂ and CO₂ gradients develop and the local O₂ partial pressure may decrease to the levels beyond which the respiration metabolism stalls and fermentation takes over. The ATP yield of the latter pathway is much lower during which the fruit can no longer sustain essential maintenance processes such as membrane repair. This may lead to loss of cell integrity, the occurrence of browning reactions due to oxidation of phenolic substrates (Ho et al., 2013) and the development of internal browning symptoms. Novel storage methods such as dynamic controlled atmosphere (DCA) storage (Bessemans et al., 2016) impose even much lower O₂ levels compared to conventional CA storage. Therefore knowledge of local O₂ and CO₂ partial pressure inside the fruit during hypoxic storage is essential in understanding the changes that occur in fruit physiology during controlled atmosphere storage which helps to develop novel dynamic controlled atmosphere methods.

Several sensor principles to measure the local O₂ partial pressure in a variety of plant organs have been evaluated in the literature (van Dongen and Licausi, 2014). Traditional polarographic sensor-based O₂ sensing and profiling was common in earlier days. Macro or microelectrodes are inserted into the plant organ and the resulting electric current which is proportional to the amount of oxygen in the tissue is measured (Armstrong et al., 1993). In another method, gas is sampled from an interior surface of the plant tissue, and then analyzing it using gas chromatography. (van Dongen and Licausi, 2014). More recently, an optical O₂ sensing technique was developed based on an oxygen-sensitive fluorophore which is placed on a foil or fiber tip. By placing the sensor foil on a biological sample or inserting a fluorescent optical needle into the tissue, the intercellular oxygen concentration can be obtained (Ho et al., 2010; Rolletschek and Liebsch, 2017). However, all these methods are destructive and less applicable for internal oxygen measurements of fruit. Non-invasive measurement of internal gas concentrations in pome fruit tissue is very difficult though as inserting, for example, a needle oxygen probe may cause leakage of oxygen along the shaft of the needle and invalidate the measurements. Alternatively, the focus has been on gas transport models at multiple scales (Ho et al., 2008) that have been validated to some extent. However, there is a need for a non-invasive method to measure the local O₂ partial pressure of pome fruit in response to different storage atmospheres to validate those models further.

Gas in scattering media absorption spectroscopy (GASMAS) is a technique based on tunable diode laser absorption spectroscopy (TDLAS) which was proposed for non-destructive detection of gas from a highly scattering media (Sjöholm et al., 2001). The gas absorption features are 10,000 times sharper than those of the surrounding bulk media and each gas has its own wavelength-dependent absorption imprint. This promotes selective detection of a gas of interest from a turbid medium. A wavelength tuned and frequency modulated diode laser paired with a highly sensitive detector helps to perform selective and sensitive detection of gases from the noisy scattering media in GASMAS. Although the technique is predominantly used to measure oxygen and water vapor (Svanberg, 2013), it has also been validated for carbon dioxide (Svensson and Shen, 2010) and methane (Ding et al., 2014). GASMAS has already been applied in various areas like food packaging (Boonruang et al., 2012; Lewander et al., 2008), pharmaceutical preparations (Svensson et al., 2008), diagnostics of lungs and body cavities (Persson et al., 2007; Lewander et al., 2011) and to study the gas diffusion through the porous materials (Zhang and Svanberg, 2016). In the postharvest domain, it also been applied to study the role of fruit skin on respiration gas exchange (Persson et al., 2006), oxygen signal intensity variations during tropical fruit ripening (Zhang et al., 2014) and also to investigate the pressure changes on apple after vacuum impregnation (Tylewicz et al., 2012). Most of the GASMAS experiments were performed on high porosity fruit like apple where the signal to noise ratio is comparatively high. However, it would be even more valuable to quantify the intercellular oxygen levels of low porosity fruit like pears as it is more susceptibe to hypoxia.

One of the main limitations of the GASMAS technique is that the GASMAS signal is not only influenced by the gas concentration, but also by the path length traveled through the medium (Mei et al., 2014). Light propagation through fruit tissues is the result of a complex interplay of light scattering and absorption. The amount of scattering depends on the structure and density of cellular components, whereas the absorption is mainly caused by the presence of different chemical compounds. Light propagation simulations can provide insight into the light-tissue interaction in scattering media. The Monte Carlo (MC) method is suitable to simulate tissue layers with arbitrary optical parameters. The potential of MC simulations to predict light transport through fruit tissues has already been demonstrated on apples (Vaudelle and L’Huillier, 2015) and peaches (Ding et al., 2015). In the context of GASMAS sensing, MC simulations could help to understand the light-tissue interaction and model the relation between the oxygen concentration in a diffuse medium and the GASMAS signal.

Given the importance of non-destructive intercellular O₂ measurements for CA storage and the lack of reports on GASMAS application in low porosity fruit like pear, this study aims to validate and evaluate a GASMAS O₂ sensor for monitoring oxygen concentration in pome fruits. This study builds on the sentinel discovery work of non-destructive measurement of oxygen diffusion in apples reported by Persson et al. (2006) and extends it to respiration studies in intact pome fruit with different porosities with and without edible coating.

In the first part, the performance of the GASMAS sensor was validated using an Intralipid® liquid phantom model with a range of O₂ levels and scattering values covering typical values for pome fruit skin and flesh. MC simulation was used to obtain insight into the light propagation through the diffuse media and optimize the design of the model systems. In the second part, the applicability of the GASMAS sensor was evaluated on pome fruit with high and low porosity to study the relation between fruit porosity and O₂ depletion due to respiration. As edible coatings are often used in post-harvest storage, their effect on diffusion resistance was also studied.