Effect of Storage and Cooking on β-Carotene Isomers in Carrots (Daucus carota L. cv. ‘Stefano’)

Carrots are one of the highest dietary sources of β-carotene and are naturally high in the (all-E)-β-carotene isomer, which has higher bioavailability, provitamin A activity, and antioxidant capacity compared to Z (cis) isomers. The objectives of the present study were to investigate the effects of storage temperature, time, and cooking (boiling for 15 min) on the levels of carotene isomers in ‘Stefano’ carrots. Storing carrots at either 4 °C to simulate long-term storage or 20 °C to simulate marketing practices resulted in increases in (all-E)-β-carotene of 20.3% after 3 days at 4 °C and 34.4% after 14 days at 20 °C, respectively. The levels of Z isomers in raw carrots were low with (13Z)-β-carotene and (9Z)-β-carotene accounting for less that 1.8% of the total β-carotene present. Levels of (9Z)-β-carotene decreased during storage at either temperature, whereas storage at 4 °C resulted in a 109% increase in (13Z)-β-carotene after 56 days. Cooking significantly increased the levels of (13Z)-β-carotene and (9Z)-β-carotene and resulted in the production of (15Z)-β-carotene, which was absent in raw carrots. Storage at 4 °C for 15 days or more prior to cooking reduced the susceptibility of (all-E)-β-carotene to thermal isomerization during cooking, resulting in lower levels of all three Z-β-carotene isomers being generated, while storage at 20 °C for up to 21 days resulted in significantly higher levels of (all-E)-β-carotene before and after cooking but had no effect on Z-isomer production during cooking. Consequently, we conclude that, for the greatest health benefit, fresh carrots can be stored for up to 21 days at 20 °C or at 4 °C for up to 56 days without significant reduction in (all-E)-β-carotene and should be consumed raw or boiled for less than 15 min to limit Z-β-carotene isomer formation.