Last month researchers from the University of Illinois, USA, reported that commercially available frozen broccoli almost entirely lacks the ability to form the potentially beneficial compound sulforaphane from the phytonutrient glucoraphanin because the processes involved in the production of frozen broccoli destroys an enzyme known as myrosinase that forms a vital part of the conversion reaction.
Now, writing in the Journal of Food Science, the same research team has demonstrated how the food industry can alter its processes to ensure frozen broccoli remains as beneficial as fresh broccoli.
"Whenever I've told people that frozen broccoli may not be as nutritious as fresh broccoli, they look so downcast," explained Professor Elizabeth Jeffery from the University of Illinois - who led both research studies.
"We discovered a technique that companies can use to make frozen broccoli as nutritious as fresh," she said. "That matters because many people choose frozen veggies for their convenience and because they're less expensive."
Freezing problems
Jeffery explained that the problem begins when soon-to-be-frozen broccoli is blanched (heated to high temperatures) in order to inactivate enzymes that can cause off-colours, tastes, and aromas during the product's 18-month shelf life.
But, the heating process not only wipes out enzymes that can cause off-tastes and colours but also destroys the myrosinase enzyme which is necessary to form sulforaphane - a potentially powerful anti-cancer compound, she said.
"We know this important enzyme is gone because in our first study we tested three commercially frozen broccoli samples before and after cooking. There was very little potential to form sulforaphane before the frozen broccoli was cooked and essentially none after it was cooked as recommended," explained Edward Dos - first author of the study.
As a result, the US-based team performed some further investigations which experimented with blanching broccoli at slightly lower temperatures than the 86ºC that is currently industry standard. They found that using a temperature of 76ºC resulted in the preservation of around 82% of myrosinase without compromising food safety and quality.
Dosz also noted that the team had some success in using other food sources of myrosinase to boost broccoli's health benefits by exposing the frozen broccoli to myrosinase from a related cruciferous vegetable - the daikon radish.
He revealed that when the team sprinkled 0.25% daikon radish —an amount invisible to the eye and undetectable to taste —on the frozen broccoli, the two compounds worked together to form sulforaphane.
"That means that companies can blanch and freeze broccoli, sprinkle it with a minute amount of radish, and sell a product that has the cancer-fighting component that it lacked before," Dosz commented.
Cooking issues?
After demonstrating that the beneficial properties of fresh broccoli can be restored to frozen broccoli, the team had one final question to tackle: Whether sulforaphane would survive the heat of microwave cooking?
"We were delighted to find that the radish enzyme was heat stable enough to preserve broccoli's health benefits even when it was cooked for 10 minutes at 120ºF," said Dosz. "So you can cook frozen broccoli in the microwave and it will retain its cancer-fighting capabilities."
Industry take up
Jeffery said that she hopes food processors will be eager to adopt the processes outlined in the research paper so that they can market frozen broccoli that has all of its original nutritional punch.
Until they do she said that consumers will need to spice up their frozen, cooked broccoli with another food that contains myrosinase to bring the cancer-fighting super-food up to nutritional speed.
Sakata Vegetables Europe supported the study.
Source: Journal of Food Science
Published online ahead of print, doi: 10.1111/1750-3841.12221
" Modifying the Processing and Handling of Frozen Broccoli for Increased Sulforaphane Formation"
Authors: Edward B. Dosz, Elizabeth H. Jeffery