PHOTONICS FOR AGRICULTURE: Studies of tropical fruit ripening using three different spectroscopic techniques

 By: Hao Zhang ; Jing Huang ; Tianqi Li ; Xiuxiang Wu ; Sune Svanberg ; Katarina Svanberg

The ability to discriminate between mature and immature fruits is very important both for fruit customers as well as for the fruit dealers. If the fruits are immature or overmature, the structure and taste are compromised and the value is reduced. Maturity signifies a quality stage where the fruits are acceptable for purchase but not necessarily ripe and in an optimal status for consumption [1,2]. Clearly, it is important to find the ripening stage when the fruits are considered to be of the desired quality. Many kinds of fruits can be harvested when considered mature but not ripe—ripeness will follow after some storage time, and is then followed by decay processes. Generally speaking, skin color, firmness, and size are the most used maturity indices for fruits [1,3]  and are often used by customers when purchasing the fruits. However, such assessments lack a fully reliable identification of the fruit ripening stage. Therefore, other biochemical and physiological parameters should be considered to determine the optimal fruit harvesting/consumption time. Sugar concentration, acidity, and starch content are the customary indices used to determine fruit ripening [1,4]. However, they cannot provide all the information needed to accurately identify the fruit ripening stage. Moreover, invasive methods are generally used to analyze these parameters to evaluate the fruit ripening.

In recent years, visible-near-infrared (vis-NIR) spectroscopy has been shown to be a promising, nondestructive method to evaluate fruit ripening, since it provides reliable information on internal characteristics of various fruit species [5–7]. However, this approach requires a very complex processing of data to build up calibration and prediction models [8]. Based on the vis-NIR spectroscopy, a simple and easy to perform measurement method, the “index of absorbance difference (IAD)” technique that strongly correlates with the chlorophyll content and the ethylene production of fruits, was introduced for nectarine and peach fruits [9–11]. However, this method cannot provide the precise days for the fruit ripening stage. Furthermore, it cannot differentiate the ripeness stage from the maturity of the fruits. In the present article, we demonstrate a noninvasive combination optical method based on reflectance and fluorescence spectroscopies together with the gas in scattering media absorption spectroscopy (GASMAS) technique, to study the ripening period of tropical fruits. The study focuses on measuring the changes of chlorophyll content using reflectance and fluorescence spectroscopies, and the changes of oxygen content using the GASMAS technique.

Chlorophyll is an essential pigment of fruits and is involved in fruit coloration, influencing the variations of color from green to yellow or red, when additional pigments are formed. For most fruits, the color will change during the ripening process. Therefore, the chlorophyll concentration is a major indicator of the physiology in fruit maturation/ripening. Usually, chlorophyll analysis is performed with spectrometry, including reflectance and fluorescence spectroscopies [12–14]. Thus, such techniques could be attractive tools to qualitatively estimate the changes of chlorophyll content in the fruit ripening process.

Molecular oxygen is a biologically active gas, and the oxygen concentration in fruits is of crucial importance for the ripening process and the quality of the fruits. Oxygen availability influences metabolic respiration, which leads to the synthesis of organic matter and the energy generation needed for the life processes. A common way to analyze gases in situ is to use absorption spectroscopy, which employs a narrow-band light source interrogating a fixed-length gas cell in combination with the use of the Beer-Lambert law. However, in porous materials such as fruits, the light is heavily scattered, which results in an undefined absorption path length. To handle this problem, the nonintrusive and easily implemented technique GASMAS was introduced, which is based on tunable diode laser absorption spectroscopy combined with wavelength modulation spectroscopy (WMS) techniques [15]. GASMAS has already been applied to the study of gas exchange in fruits [16,17]. We have now extended the use of the GASMAS technique to study the changes of oxygen content during the fruit ripening process.

Sketch of the system:

Experimental arrangement for the measurements of reflectance spectra as well as the fluorescence spectra 

(a) and for the monitoring of the second-harmonic wavelength modulation spectroscopy (2f WMS) signal of oxygen using GASMAS (b), TIA is a transimpedance amplifier.

Complete Article can be read at:
J. Biomed. Opt. 19(6), 067001 (Jun 02, 2014). doi:10.1117/1.JBO.19.6.067001
Open Access, available at: http://bit.ly/1ifMNtN