Quality testing of tomatoes is critical in many aspects of their growth, storage, and transportation. Sugar content (Brix) is a necessary criterion for evaluating whether a fruit is tasty. The timely and accurate mastery of quality parameters, such as the Brix distribution during the ripening process, is crucial for the scientific and efficient cultivation of tomatoes. Among the widely used detection methods, the Brix meter detection, which is based on the principle of light refraction, is a mean value measurement method based on the fruit juice, which cannot satisfactorily assess the fine distribution of different parts of a sample. The hyperspectral imaging technology leads to a significant amount of redundancy in the amount of data and is easily affected by the depth of light penetration and the water content of the sample, which is insufficient to detect and characterize the full range of details of the sample. Mass spectrometry is complex and cumbersome for sample preparation and does not support rapid detection. This study proposes a method for detecting and characterizing the sugar content of tomato fruits from the perspective of cellular phase information based on phase imaging technology.

The physiological properties of the samples were analyzed and characterized in this study from a cellular perspective, starting from the microscopic material basis of plant growth. Based on the characteristics of the quantitative phase microscopy, which uses the inherent contrast source of different refractive indices between different components in the cell, we performed phase imaging experiments on tomato pulp cells and extracted the cellular phase parameters, called "phase envelope volume" and "phase peak." The correlation between the phase parameters and Brix was analyzed by comparing them with the measurement results of the Brix meter. Based on the experimental data, a data cube of the two-phase parameters and Brix was constituted, and positive correlations between the phase parameters and Brix were obtained. This provides a basis for characterizing local Brix using phase parameters.

The proposed detection and characterization method can be applied to any part of a tomato plant and requires only a single frame of the phase map at any incidence angle. The extraction of phase parameters eliminates the tedious operation of decoupling the physical thickness and refractive index of the cells, and the entire analysis process only takes approximately 0.5 s. Based on the sensitivity of the phase information to the internal chemical composition of the cell, the phase parameter characterization method can also be applied to detect the physiological state of other fruits and vegetables in addition to sugar (Fig. 10). This study provides a reference for the refinement and precision in detecting the quality of agricultural products.

This paper focuses on the demand for rapid and accurate quality inspection of fruits and vegetables in modern agriculture. A detection method based on phase imaging is proposed. The inspection of tomato fruit sugar is used as an example to explain the related principles and procedures. In this method, only a single frame of the phase map from any angle of incidence is required. Two phase parameters, that is, "phase envelope volume" and "phase peak," are extracted from the phase map to quantitatively investigate the sugar content characteristic of a cell. The experimental results of the comparison with the detection based on the Brix meter indicate that the sugar content and the above phase parameters show a significant positive correlation. This provides a basis for characterizing the sugar content using the phase parameters. The local sugar content distribution of a tomato fruit was detected experimentally, and the results show good consistency with those of hyperspectral detection. The feasibility and effectiveness of this method have been demonstrated to a certain extent. This phase detection and characterization method requires only one frame of the phase image, and the related analysis process eliminates the cumbersome operation of decoupling the physical thickness and refractive index of the cell. This means that the hardware and time costs can be reduced. Based on the sensitivity of the phase information to the internal chemical composition of the cell, the phase parameter characterization method can also be applied to detect the physiological state of other fruits and vegetables, which may be used as a workable solution for the rapid and accurate detection of agricultural product quality.