Study on the characterization of 6S-5-methyltetrahydrofolate calcium salt crystal form C and its stability improvement by ascorbic acid and cysteine

6S-5-methyltetrahydrofolate calcium salt crystal form C (MTHF-CAC) is a stable form of active folate. Compared with folic acid, MTHF-CAC is unaffected by the polymorphism of genes for folate-metabolizing enzymes and can be directly absorbed by the human body. In this study, MTHF-CAC was characterized by XRD, FTIR, DSC, and TGA. The effects of temperature, light, pH, and freeze–thaw cycles on the stability of MTHF-CAC solution were studied. The protective effects of ascorbic acid (AA) and cysteine (Cys) on MTHF-CAC were investigated by HPLC. The retention rate of MTHF-CAC solution was 82.6% after 6 days of storage at 4 °C but only 0.8% after heating at 75 °C for 330 min. Under UHT conditions, AA and Cys exerted protective effects on MTHF-CAC. 120 μg/mL of AA and Cys increased the retention of MTHF-CAC by 89.9% and 85.3%, respectively. The protective effect of AA was better than that of Cys. The research on the characterization and stability of MTHF-CAC provided the basic information for its application in the food industry.