Are folates, carotenoids and vitamin C affected by cooking? Four domestic procedures are compared on a large diversity of frozen vegetables

Four home procedures such as boiling in water, steaming, pressure steaming and microwave cooking were tested on 13 frozen vegetables. Folates, carotenoids and vitamin C were characterized on uncooked and cooked vegetables and illustrated a very large variability among the studied vegetables. The effect of cooking was significant but it varied according to vegetables and phytochemicals. The best method to preserve the nutritional quality could be alternatively steaming, microwaving or pressure cooking, whereas boiling was generally the less suitable method. On the fresh weight basis, boiling involved a high loss of total vitamin C (average of -51% on fresh matter) and folates (-68%) and a slight loss of lutein (-15%) and b-carotene (-9%). On the dry weight basis, it remained the less suitable for vitamin C (-44%) and folates (-65%) but not for carotenoids, as it allowed a better extractability of lutein (+9%) and β-carotene (+20%)