Succinic acid

Crystal violet, N-acetyl-L-glutamic acid, succinic acid and anthrone were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Luria-Bertani (LB) agar, LB broth and MacConkey agar were purchased from Guangdong Huankai Microbial Sci. & Tech. Co., Ltd. (Guangdong, China). Glycerin, pure anhydrous glucose, NaCl, FeCl₂, and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were purchased from Shanghai Yuanye Bio-Technology Co., Ltd. (Shanghai, China). Methanol, formic acid, and ammonium acetate (HPLC grade) were purchased from Thermo Fisher Scientific (Waltham, MA, USA). Phosphate buffered saline and concentrated sulfuric acid were purchased from Shanghai Runjie Chemical Reagent Co., Ltd. (Shanghai, China).

Terbium chloride hexahydrate (TbCl₃·6H₂O), europium chloride hexahydrate (EuCl₃·6H₂O), glucose, fructose, vitamin B₆, vitamin B₂, vitamin C, kaempferol, quercetin, caffeic acid, p-coumaric acid, and vanillic acid were obtained from Aladdin (Shanghai, China). Dipicolinic acid (DPA), proline, gluconic acid, citric acid, succinic acid, MgCl₂, ZnCl₂, CuCl₂, and MnCl₂ were obtained from Macklin (Shanghai, China). Other reagents were all of analytical grade. Honey samples were directly collected from apiaries during the period of flowering. Litchi (Litchi chinensis) and longan (Dimocarpus longan) honey samples were collected in Fujian province, and astragalus (Astragalus membranaceus) honey samples were collected in Gansu province. Ultrapure water (18.2 MΩ·cm) was used in all the experiments.