Yeast powder

Citric acid, trisodium citrate dehydrate, sodium bicarbonate, sodium carbonate anhydrous and 30% hydrogen peroxide were supplied by Kermal, Tianjin, China. Ethylene diamine tetra-acetic acid disodium, sodium chloride, magnesium acetate and cobalt (II) chloride were provided by J&K Chemical, Beijing, China. Tris, isopropyl-b-thiogalactopyranoside (IPTG), peptone, yeast powder, and BCA protein assays kit and Bradford protein assays kit were supplied by Solarbio, Beijing, China. His Trap FF was supplied by GE Healthcare Life Sciences, Chiltern, Buckinghamshire, UK. All other chemicals were of analytical grade.

The fruits of Citrus Reticulate Blanco ‘Chachiennsis’ (Chachi) were harvested from an orchard in Xinhui, Guangdong Province, China, on 8 November 2020. The peels were collected and sun-dried for 5 days followed by vacuum sealing in plastic bags. The samples were stored in a desiccator at room temperature. Aspergillus niger CGMCC 3.6189 was purchased from China General Microbiological Culture Collection Center (Beijing, China). The Folin–Ciocalteu, potato dextrose agar (PDA), and yeast powder were provided by Solarbio^® Science and Technology Co., Ltd. (Beijing, China). The ABTS and 1, 1-DPPH were bought from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). The HPLC standards (chlorogenic acid, caffeic acid, p-coumaric acid, ferulic acid, narirutin, hesperidin, naringenin, hesperetin, nobiletin, and tangeretin) were purchased from Chengdu-Must Technology Co., Ltd. (Chengdu, China). The other chemicals were all of analytical grades and were acquired from Sinopharm Chemical Reagent Co., Ltd. (Zhenjiang, China).