Relationship between optical properties and soluble sugar contents of apple flesh during storage

https://doi.org/10.1016/j.postharvbio.2019.111021Get rights and content

Highlights

  • Automatic integrating sphere system was built for measurement of optical properties.

  • Optical properties at 400–1050 nm of apple flesh during storage were measured.

  • μa and μ's in different wavelengths were related to soluble solid and soluble sugar.

  • Prediction models were used to verify the relationship of μa, μ's and soluble sugar.

Abstract

Soluble solids (SS) in fruit are mainly composed of soluble sugars. This research aims to further the understanding of the detection mechanism of soluble solid content (SSC) based on optical technology by exploring the relationship between optical properties and soluble sugar contents. The total reflectance and total transmittance at 400–1050 nm of Fuji apple flesh stored at 25 °C for 50 d and 0 °C for 150 d were collected by an automatic integrating sphere system. The absorption coefficient (μa) and reduced scattering coefficient (μ's) were obtained by iteratively solving the radiative transfer equation using the inverse adding doubling algorithm. The relationship of μa and μ's with the contents of SS, total soluble sugars, fructose, glucose and sucrose were quantitatively analyzed at different wavelengths, and prediction models were established by partial least squares regression (PLSR). The results showed that the changes in μa, μ's, SSC and soluble sugar content presented similar trends during storage at the two test temperatures. As the storage time increased, the decreases in μa and μ's were accompanied by declines in SSC and soluble sugar content. In addition, μa and μ's at 550–1050 nm were both positively correlated with SSC and soluble sugar content, with correlation coefficients (r) of 0.834-0.992 and 0.737-0.981, respectively. Compared with the correlations at 550–780 nm, the correlations at 780–1050 nm between μa and SSC and soluble sugar content were enhanced, while the corresponding correlations with μ's were gradually weakened. In addition, SS was most strongly correlated with sucrose among the three types of soluble sugars. SS and sucrose had closer relationship with μa and μ's than fructose and glucose with μa and μ's. Moreover, their prediction models also performed better than the models for fructose and glucose, with Rp2 values of 0.731-0.804. Thus, the prediction of SSC based on Vis-NIR optical technology may be related to the high correlations between the absorption and scattering properties and the sucrose content.

Introduction

Sweetness is an important edible quality of fruit, which directly affects consumer preferences and purchasing behavior (Awad and de Jager, 2002). The sweetness of apple fruit is mainly determined by soluble sugar contents, including mainly fructose, glucose and sucrose (Li et al., 2012). The traditional analytical method of soluble sugar content determination, e.g., high-performance liquid chromatography (HPLC), requires expensive and complex preprocessing instruments, and its results can be affected by other factors, such as the composition of fruit, as well as the flow rate of the mobile phase and temperature (Ma et al., 2014; Sevcik et al., 2011). Soluble solids (SS) of apple fruit mainly consist of soluble sugars, and are positively correlated with soluble sugar contents (Zhao et al., 2017; Nie et al., 2012). The measurement of soluble solid content (SSC) is simpler and more feasible than measuring soluble sugar content. Therefore, SSC has been an important index for evaluating the sweetness of apple fruit for many years.

The traditional method for determining SSC in apple fruit is to peel the fruit, squeeze the pulp into juice and measure it with a portable Brix meter. However, this method requires destructive sampling and long sampling and analysis times. To meet the demand for nondestructive, rapid and economical SSC detection, research on spectroscopy technology (Oliveira-Folador et al., 2018; Pan et al., 2015; Mo et al., 2017), such as visible and near infrared spectroscopy (Vis-NIRS) and hyperspectral imaging spectroscopy (HIS), has become a hot topic. The principle of spectroscopy technology is as follows: when light comes into contact with tissue, it can either be absorbed or scattered by the chemical composition or physical structure of the tissue, resulting in the attenuation of light. Finally, the signals (reflection spectral or transmission spectral, etc.) collected using spectroscopy technology are changed and represent the chemical or physical information of the tissue. Currently, spectroscopy technology relies on chemometrics to build a prediction model for SSC. However, this technology has the following limitations: (1) the modeling process is invisible, and the stability and adaptability of the model need to be improved, as they are very vulnerable to the variety and maturity of samples (Xiao et al., 2018; Wang et al., 2017; Ma et al., 2015), instrument performance (Xiao et al., 2017), and the environment (Dong et al., 2016). (2) Absorption and scattering occur simultaneously in the propagation of light in turbid tissue. Spectroscopy techniques based on the Lambert-Beer law only consider the absorption of light by tissues and neglect the scattering effect, thus causing high detection error rates (Zaccanti et al., 2003). (3) Spectral signals collected by spectroscopy technology are the comprehensive effects of absorption and scattering in tissues and do not separate these two properties. It remains unclear how SS affects absorption and scattering at different wavelengths and then causes changes to the spectra and models. Thus, although spectroscopy technology has developed rapidly in the field of nondestructive determination of the quality of agricultural products (Liu et al., 2019; Sun et al., 2018; Magwaza et al., 2014; Xie et al., 2009), it lags behind in terms of the optical detection mechanism.

Optical parameter measurement realizes the separation and quantification of the absorption (absorption coefficient, μa) and scattering properties (reduced scattering coefficient, μ's) in tissues by solving the radiative transfer equation (Tuchin, 2007). These optical parameters can be used to analyze the characteristics of light propagation in tissues (Zhang et al., 2019; Fang et al., 2016) and to build classification or prediction models for external and internal quality (Huang et al., 2018; Zhu et al., 2015; Nguyen Do Trong et al., 2014). Meanwhile, the optical parameter measurement provides an opportunity to study how the optical properties of a fruit are linked to its chemical composition and structure (Zhang et al., 2017; Rowe et al., 2014). The current optical parameter measurement technologies mainly include time-resolved (TR), spatially resolved (SR), frequency domain (FD) and integrating sphere (IS) technologies. Among them, IS has more advantages in terms of simplicity and high accuracy (Prahl et al., 1993). Fruit skin had higher contents of pigments and more compact cell structure than fruit flesh, and it had stronger absorption and scattering in the visible region (Wang and Li, 2013; Saeys et al., 2008). According to the findings of Tijskens et al. (2007) and Cen et al. (2013), the absorption levels around 525 nm and 675 nm were related to anthocyanin and chlorophyll, respectively, which could be used to evaluate fruit maturity. With fruit maturation, the green color of fruit skin gradually faded, and μa at 525 nm increased, while μa at 675 nm decreased. In addition, the relationships of optical properties with pectin substances and cell structure were calculated by Vanoli et al. (2009) and Cen et al. (2013), respectively. They found that μa at 630 nm correlated with polysaccharide aldehyde acid (GA) in residue insoluble pectin (RIP), with a correlation coefficient (r) of 0.64. μa and μ's at 675 nm positively correlated with the cell’s area and equivalent diameter of Golden Delicious and Granny Smith, with r values ranging from 0.581 to 0.941, respectively. Ma et al. (2018) found that the higher the SSC in apples, the stronger the absorption at 1198 nm, but the relationship between them could not be quantified. SSC prediction models based on μa and μs for different kinds of fruits, including apples (Nguyen Do Trong et al., 2014), kiwifruit (Valero et al., 2004), peaches (Cen et al., 2012), and pears (He et al., 2016; Nicolaï et al., 2008), have been established and the strong relationship between optical properties and SSC has been proved. However, to the best of our knowledge, a quantitative study on the relationship between optical properties and SSC and soluble sugar contents at different wavelengths has not been reported. Such a fundamental report may be helpful to explain the mechanism of optical technology in evaluating SSC and provide a reference for improving the accuracy and adaptability of the prediction model.

The overall goal of this study is to understand the quantitative relationships between optical absorption and scattering properties and soluble sugar contents in apple flesh during storage and to explain the reasons for detecting SSC by optical technology. Therefore, the research was carried out by (1) measuring μa and μ's of apple flesh at 400–1050 nm by our own automatic integrating sphere system; (2) quantifying the relationship between μa, μ's and the contents of SS, total soluble sugars, fructose, glucose and sucrose at different wavelengths; and (3) constructing prediction models for SSC and soluble sugar contents.

Section snippets

Samples and preparation

A total of 120 Fuji apple fruit with a size range of 80 to 85 mm was harvested from an orchard in Xuzhou, Jiangsu Province, China. Then, apples were equally divided into two groups. In one group, the samples were stored in a constant temperature and humidity chamber (25 °C, RH > 95%) for 50 d, and 10 samples were randomly measured every 10 d. In the other group, the samples were precooled at 0 °C for 24 h and then stored in the cold temperature (0 ± 1 °C, RH > 95%) for 150 d, during which 10 of

Validation of the optical property measurement system

Fig. 2a shows μa of water measured with our integrating sphere system and μa reported by Deng et al. (2012a, b). The measured spectral profile was consistent with that reported in the literature: a large absorption peak at 980 nm and smaller absorption peaks at 840 nm and 740 nm were observed. Additionally, the two sets of data at 800–1050 nm were highly similar, with an average error of only 5.63%. A considerable baseline offset was observed in the 400–800 nm region, which was also reported by

Conclusion

In this research, the relationships between optical properties at 550–1050 nm and SSC and soluble sugar content of apple flesh during storage were explored. The results showed that μa and μ's, along the entire wavelength, were both positively related to the contents of SS, total soluble sugars, fructose, glucose and sucrose, with r values of 0.736-0.992. In addition, μa was more correlated with these soluble components than μ's. In the NIR region, there was a stronger relationship between μa

Funding

This work was supported by the National Natural Science Foundation of China (NSFC), 31671926, 31671925 and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Declaration of Competing Interest

The authors declare that they have no conflict of interest.

Acknowledgements

The authors thank Xueming He of Zhejiang University and Aichen Wang of Jiangsu University for their valuable advice and help.

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