Fresh and frozen broccoli were analyzed for retention of ascorbic acid, riboflavin and thiamine by semi‐automated fluorometry before and after steaming, boiling or microwave cooking for 5 min. Retention of vitamins in general was higher for fresh than for frozen broccoli. Steamed fresh broccoli retained most ascorbic acid, riboflavin, and thiamine. Boiled fresh samples retained least riboflavin and ascorbic acid and microwaved samples, the least thiamine. In frozen broccoli, microwaved samples retained most ascorbic acid and riboflavin, and steamed samples retained most thiamine.
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Hudson et al. (1985) studied this question.