**1. Introduction**

Chinese cabbage (*Brassica rapa* L. ssp. *pekinensis*) is the most widely cultivated and consumed vegetable in East Asia with the characteristics of high yield, good cold resistance, long supply period, and rich nutrition. The inner leaf color of Chinese cabbage is mainly white and yellow. Purple leaf Chinese cabbage (PCC) is mainly generated by the cross of common green Chinese cabbage with red leaf mustard (*Brassica juncea* Coss.), purple flowering Chinese cabbage, or red bok choy (*Brassica rapa* L. ssp. *chinensis*) [1,2]. It has become increasingly popular due to its beautiful color, special flavor, and high level of anthocyanidins [2]. Anthocyanidins, a class of flavonoid substances, exist in different colors in fruits, flowers, and vegetables, such as purple, blue, and red. They contain a C6-C3-C6 carbon skeleton and -OH or -OCH3 groups and specific sugar or acylated sugar residues located at C3, C5, and C7 positions [3,4]. Based on the type and location of the substituents, anthocyanidins are generally classified into six major groups: cyanidin, delphinidin, petunidin, malvidin, peonidin and pelargonidin, and the main anthocyanidin in PCC is cyanidin accumulated in the vacuoles [2,3,5]. Anthocyanidins have a wide range of anti-inflammatory, cardioprotective, chemotherapy, and hepatoprotective effect for human disease prevention [6]. Studies have proved that anthocyanidins have a good role in the chemoprevention and treatment of breast cancer [7]. Blueberry anthocyanidins can effectively improve the solubility of lipids [8], and extracted anthocyanidins from apples

**Citation:** Wang, Y.-Q.; Liu, G.-M.; Hu, L.-P.; Zhao, X.-Z.; Zhang, D.-S.; He, H.-J. Prediction of Anthocyanidins Content in Purple Chinese Cabbage Based on Visible/Near Infrared Spectroscopy. *Foods* **2023**, *12*, 1922. https://doi.org/10.3390/ foods12091922

Academic Editors: Zhiming Guo, Zhao Zhang and Dong Hu

Received: 22 March 2023 Revised: 4 May 2023 Accepted: 5 May 2023 Published: 8 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

have an inhibitory effect on gastric cancer cells [9]. Due to their human health benefits, anthocyanidins have received more and more attention from public in recent years [9,10]. Creating colorful leaf vegetables, such as PCC, which contain abundant anthocyanidins, is of significant commercial interest and the new trend of breeding.

Visible/near-infrared spectroscopy (NIR) is a widely used technique in the agriculture and food industry with the advantages of fast, non-destructive, environmentally friendly, and accurate analysis. NIR is a molecular vibrational spectrum with wavelengths ranging from 400–750 (visible) and 750–2500 nm (near-infrared), in which the absorption signals of the reflected chemical components are assigned mainly to overtone and octave vibrations of hydrogen-containing groups, including C-H, N-H, O-H, and S-H [11,12]. Theoretically, no two compounds produce the same visible/near-infrared spectra since their unique composition of atoms [12]. It has been widely used in the field of bioactive compound detection in vegetables and fruits, and its applicability has been proven. Prodromidis et al. have successfully used FT-IR and UV-Vis spectroscopy to measure the onion anthocyanidins during heating [13]. Johnson et al. used attenuated total reflection Fourier transform infrared spectroscopy to predict the total anthocyanidin content in ethanolic extracts of plum with an R<sup>2</sup> of 0.93 [14]. Additionally, using NIR spectroscopy in the prediction of anthocyanidins content and antioxidant activity in grape juice is feasible [15]. Tian et al. established a prediction model for the detection of water content and anthocyanidins content in purple potatoes by visible near-infrared hyperspectroscopy [16]. With the development of algorithms, chemometrics, and artificial intelligence, the application of NIR spectroscopy will be extended for fast screening and quantitative analysis of anthocyanidins.

Purple leaf Chinese cabbage has become a popular breeding interest; meanwhile fast and accurate determination of the anthocyanidins contents is an important task for improving its nutritional quality. The commonly used determination methods for anthocyanidins are based on ultrasonic or microwave-assisted liquid extractions and high-performance liquid chromatography (HPLC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) detection [17,18]. However, the extraction is complicated and time-consuming while the reagents used may be harmful to the environment and human health [19], and the equipment are more expensive and require experts for analysis. By comparing with chemical analysis techniques, spectroscopic techniques are relatively simple and do not require further expansion of sample preparation [20]. NIR spectroscopy could be a powerful tool to fulfill this task. To date, no studies have focused on the quantitative prediction of anthocyanidins in PCC by NIR spectroscopy. It is an urgent need to build a suitable NIR method for simple and fast prediction of anthocyanidins to help the breeders and producers since the prediction models established by different food matrices cannot be simply applied to PCC. Therefore, this study aims to develop an accurate quantitative prediction method for anthocyanidin content in PCC using NIR spectroscopy, which laid a foundation for the fast and convenient detection of the nutritional quality of agri-food and the rapid screening of purple vegetable breeding materials.

#### **2. Materials and Methods**
