L serine

Tylosin, aivlosin tartrate, tilmicosin, and l-serine were purchased from Solarbio (Beijing, China). l-Serine was dissolved in double-distilled H₂O to a 500 mM stock solution. After filtering the stock solution with a sterile hydrophilic polyvinylidene difluoride membrane with a pore size of 0.22 μm (Biosharp, China), it was stored at −20°C. The solution was diluted according to the concentration required.

Soybean seeds were bought from Shanggong Food Co., Ltd., Zaozhuang, Shandong, China. Commercial lyophilized LAB culture (Angel Yeast Co., Ltd., Yichang, Hubei, China) was used to ferment soymilk. The most suitable temperature for growth is 42 °C, and the culture contains two strains (Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus). Glucono-δ-lactone (GDL) was also purchased from Angel Yeast Co., Ltd., Yichang, Hubei, China. Bromophenol blue, bovine serum albumin, Coomassie Brilliant Blue R250, and Coomassie Brilliant Blue G250 were obtained from Feijing Biotechnology Co., Ltd., Fuzhou, Fujian, China. Sodium dodecyl sulfate (SDS), tris base, glycine, sodium tetraborate decahydrate, and o-phthalaldehyde (OPA) were purchased from Yien Chemical Technology Co., Ltd., Shanghai, China. L-serine, casein tryptone, pancreatin (P7340), pepsin 1:3000 (P8390), pig bile salts (G8310), α-Amylase (G8290), and β-mercaptoethanol were obtained from Solarbio Science and Technology Co., Ltd., Beijing, China. Trypsin 1:200 (porcine pancreas, S10034), Chymotrypsin (S10001), and peptidase (S31740) were brought from Yuanye Biotechnology Co., Ltd., Shanghai, China. Pre-stained molecular mass standard protein markers (15–130 kDa) and the SDS-PAGE gel rapid preparation kit were purchased from Ranjeck Technology Co., Ltd., Beijing, China.

L-Proline (L-Pro), L-methionine (L-Met), L-valine (L-Val), L-histidine (L-His), L-isoleucine (L-Ile), L-leucine (L-Leu), L-threonine (L-Thr), L-tryptophan (L-Trp), L-tyrosine (L-Tyr), L-alanine (L-Ala), L-serine (L-Ser), L-phenylalanine (L-Phe), L-arginine (L-Arg), L-asparagine (L-Asn), L-aspartic acid (L-Asp), L-cysteine (L-Cys), L-glutamine (L-Gln), L-glutamic acid (L-Glu) and L-lysine (L-Lys) are from Beijing Solarbio Science & Technology Co., Ltd. D-Proline (D-Pro), D-methionine (D-Met),D-leucine (D-Leu) and D-tryptophan (D-Trp) are from Tokyo Chemical Industry Co., Ltd. D-Guanosine is from Beijing Solarbio Since & Technology Co., Ltd. Quinine is from Saen Chemical Technology (Shanghai) Co., Ltd.
Analytical reagents methanol (HPLC grade) was purchased from Spectrum Chemical, and formic acid was purchased from Sigma Aldrich. Ultrapure water from a Heal Force water purification system (Pudong, Shanghai, China) was used to prepare solutions and the mobile phase.
The aa measurements were performed on Jasco-J1700 electronic CD at ambient temperature. The HPLC was performed on Agilent 1260 Infinity system and fitted with an Agilent TC- C18, 5 µm, 4.6 mm × 150 mm column. The column temperature was maintained at room temperature. The HPLC flow rate was 1 ml min⁻¹ with TC-C18.