The study, published in the Journal of Functional Foods, tested the ability of the enzyme myrosinase to catalyse the formation of potentially health beneficial sulforaphane in commercially-available frozen broccoli after suggestions that the processes involved in the production of frozen broccoli products may denature the myrosinase enzyme – leaving the broccoli unable to produce the potential anti-cancer compound.
“Current processing and cooking techniques used for the production of frozen broccoli yield a product which is deficient in the formation of sulforaphane,” said the research team – led by Professor Elizabeth Jeffery from the University of Illinois, USA.
“Our work attributes this to the inactivation of the enzyme myrosinase, which occurs in part during thermal processing and in part by the consumer during cooking,” they added.
Jeffery and her colleagues noted that blanching - where heat in the form of water or steam, is used to inactivate degradative enzymes to prolong the shelf life of the product - is a common practice used in frozen vegetable processing.
“Coupled with the cooking procedures that frozen broccoli undergoes, this process far exceeds the reported temperature stability of the broccoli enzyme myrosinase.”
Broccoli is the only commonly eaten vegetable that contains meaningful quantities of glucoraphanin – a naturally occurring compound that is converted into the bioactive compound sulforaphane.
The research team noted that sulforaphane has been suggested to have many health benefits, including acting as a potent cancer-preventative compound and possibly assisting in the management of inflammation linked to type 2 diabetes..
Previous studies have backed such an idea, finding that people who consume a few portions of cruciferous vegetables such as broccoli every week have a lower risk of chronic diseases, including cancer and heart disease.
However, the team noted that the conversion of glucoraphanin to sulforaphane to requires the presence of the enzyme myrosinase.
Jeffery and her team investigated the ability of myrosinase to catalyze the formation of sulforaphane within commercially-available frozen broccoli.
"Before the frozen broccoli was cooked, results showed that only two of the three samples had the potential to form sulforaphane," said the authors.
"Furthermore, only one of these (sample three) formed measurable sulforaphane during the 20 min hydrolysis," they added - noting that these increased slightly for a 24hr hydrolysis.
However, Jeffery added that a comparison with fresh broccoli that was picked and freeze dried within 2 h of harvest without undergoing blanching, showed that the quicker freeze drying process resulted in 46% and 27% more sulforaphane at 20 min and 24 h, respectively, than the highest commercial broccolis.
All samples then underwent each of two microwave heating methods; the samples were either cooked in a loosely covered bowl or in a microwave steamer bag.
"Following either protocol, we found that no sulforaphane was formed after microwave heat treatment," said the team.
"The results show that in these products, there was very little potential to form sulforaphane prior to cooking and essentially none after the recommended cooking method was performed," they concluded
Source: Journal of Functional Foods
Volume 5, Issue 2, Pages 987–990, doi: 10.1016/j.jff.2013.01.033
“Commercially produced frozen broccoli lacks the ability to form sulforaphane”
Authors: Edward B. Dosz, Elizabeth H. Jeffery