Diode laser absorption spectroscopy for studies of gas exchange in fruits
Introduction
Molecular oxygen is a biologically active molecule, and its concentration in fruits is of crucial importance for the ripening process and the quality of the fruits. In particular, the oxygen availability affects the metabolic process of respiration, which releases energy necessary to maintain the life processes by breaking down the organic compounds of the fruit. Thus, a decreased respiration rate causes a prolonged lifetime. The rate of the normal aerobic respiration process in which oxygen is consumed and carbon dioxide is produced, decreases with decreasing oxygen concentration and increasing carbon dioxide concentration. However, an oxygen concentration below a certain critical level initiates an anaerobic metabolic pathway for the respiration. This process, known as fermentation, produces off-flavours and causes a loss of quality and a rapid decay of the fruit. Also, a too high carbon dioxide concentration can cause damage to the fruit tissue [1], [2]. Thus, there is great interest in optimising gas concentrations during the postharvest time of fruits.
Methods developed to prolong the postharvest lifetime and inhibit physiological deterioration in fruits include controlled atmosphere (CA) storage and modified atmosphere packaging (MAP). The optimum partial pressures of oxygen and carbon dioxide in CA storage and MAP depend on the metabolic behaviours, especially the physiological tolerance to oxygen and carbon dioxide of the particular fruit [3]. In MAP, films with suitable permeability to oxygen and carbon dioxide are used to optimise the concentrations of the gases which the fruits are exposed to. An ideal type of package would be the one resulting in an oxygen concentration low enough to slow down respiration and maturation but higher than the critical concentration for initiation of anaerobic respiration [2], [4]. The respiration rate is also highly dependent on the temperature [5], [6]. The oxygen uptake increases with temperature while the permeability of the polymeric film frequently used in MAP does not increase to the same extent. Thus, it is very important to design the MAP according to the highest oxygen uptake that will occur due to temperature changes during shipping and handling [7]. To be able to design an optimal package, there is a need for methods to measure and model the internal gas concentrations and the gas exchange in fruits and package systems.
Techniques for assessing the gas content in the air around horticultural produces are readily available, although the implementation might be complicated. A common technique is to use a flush-through system in which the gas exchange can be estimated from the concentration difference between the inlet and outlet gas flow. In order not to make systematic errors, the flow has to be relatively high [8], [9]. However, this requires sensitive detection techniques to be able to monitor the small concentration differences, which is often accomplished by a gas chromatographic system combined with unselective detectors, or more selective spectroscopic devices like laser-based photoacoustic trace gas detectors [10]. When it comes to measurements of the internal gas contents and its dynamics, intrusive electrode-based probes or extraction of gas from the fruit interior by syringes are normally used [10], [11], [12]. These methods destroy the tissue, which might affect the ongoing physiological processes.
A non-intrusive, compact and easily implemented technique for measuring gas exchange inside fruits would thus be of interest for assessments of internal gas dynamics relevant to, for instance, the optimisation of CA storage and MAP of fruits. The aim of the present work is to introduce a new non-destructive in-situ technique to study gas inside fruits, based on diode laser absorption spectroscopy on highly scattering media, rather than to reveal new knowledge about fruit physiology in itself. In the present study, we focus on oxygen content and gas exchange measurements in fruits, although other gases such as carbon dioxide and water can be considered. We show the possibility of studying gas exchange in apples through their natural skins. The technique also makes it possible to determine the effect on the gas exchange due to the protective action of cling film, and to study the permeability of different films used in MAP. Although the present study is focused on measurements on fruits, in particular apples, the methodology is general and can be applied to a large number of problems concerning gas exchange in foods and in food packaging.
Section snippets
Gas absorption measurements
Gas concentrations can in principle be determined by measuring the gas attenuation of narrow-band light, over a certain distance and employing the well-known Beer–Lambertian law. The level of light unaffected by the gas can readily be assessed by also measuring the intensity at wavelengths close to, but outside the absorption profile.
This simple procedure requires a negligible variation of the absorption properties between the different wavelengths, inside and outside the absorption line
Introductory studies
In order to illustrate the general dynamics of gas in fruit samples a study of the equivalent mean path length, Leq, of oxygen for fruit slabs of different thickness was performed. Five varieties of apples, Jonagold, Granny Smith, Red Delicious, Golden Delicious, and Royal Gala, bought in a local supermarket, were studied. The different apples were sliced in slabs with well-defined thicknesses (±1 mm) and the equivalent mean path length was immediately measured, in order not to be affected by
Discussion
Non-intrusive measurements of oxygen gas inside intact and sliced apple fruits were demonstrated using the newly introduced GASMAS technique. In contrast to other techniques the gas is measured non-intrusively inside the tissue rather than as gas extracted from the fruits. Gas exchange through the intact skin and through packaging films was demonstrated by observing the response of the oxygen equivalent mean path length to a transient change in the ambient gas concentration. The measurements
Acknowledgements
This work was supported by the Swedish Research Council, the Asian-Swedish partnership programme of the Swedish International Development Cooperation Agency (SIDA), and the Knut and Alice Wallenberg Foundation.
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Permanent address: School of Science, Harbin Institute of Technology, Harbin, PR China.