5-Methyltetrahydrofolate, the naturally abundant folate vitamer, has been proposed as an alternative to folic acid for fortification. However, it is less stable than folic acid. In a formate buffer (pH 3.5), folic acid was entirely preserved after heating the solution for 3 h at 80 °C. In contrast, 5-methyltetrahydrofolate was completely degraded in less than 15 min. As in the buffer, 5-methyltetrahydrofolate in apple or carrot purées degraded rapidly without the addition of ascorbic acid. By adding ascorbic acid, the stability could be increased, but the chosen amount was crucial. An excess of vitamin C compared to 5-methyltetrahydrofolate was not always sufficient for the complete protection of 5-methyltetrahydrofolate during 3 h at 80 °C. Only by adding 2840 μmol/kg of ascorbic acid (equivalent to 500 mg/kg), 5-methyltetrahydrofolate seemed to remain stable. Degradation started after approximately 60 min when 570 μmol/kg of ascorbic acid (equivalent to 100 mg/kg) were added; after 120 min with 1420 μmol/kg (equivalent to 250 mg/kg). In addition, a temperature decrease to 70 °C or 60 °C did not increase the stability of 5-methyltetrahydrofolate.