“We discovered a technique that companies can use to make frozen broccoli as nutritious as fresh,” said Elizabeth Jeffery, Professor of nutritional sciences at the University of Illinois in the US.
“That matters because many people choose frozen veggies for their convenience and because they’re less expensive.”
Cancer-protective benefit
As little as three to five servings of broccoli a week helped protect against cancer, but that wasn’t true for bags of broccoli that had been frozen, she added.
The problem began when broccoli is blanched before freezing, or heated to high temperatures, to inactivate enzymes causing unpleasant colours, tastes and aromas during its 18-month shelf-life, she explained.
Extreme heat destroys the enzyme myrosinase, which is necessary to form sulphoraphane, the powerful cancer-preventive compound in broccoli.
Sulphoraphane
“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 sulphoraphane before the frozen broccoli was cooked and essentially none after it was cooked as recommended,” said Edward Dosz, one of the research team.
Sulphoraphane is formed when fresh broccoli is chopped or chewed, bringing its precursor glucoraphanin and the enzyme myrosinase into contact with each other. The researchers first thought that thawing frozen broccoli in the refrigerator might rupture the plant’s cells and kick-start the enzyme–substrate interaction. It didn’t work, Dosz said.
Boost broccoli’s health benefits
But they had previously had success using other food sources of myrosinase to boost broccoli’s health benefits. So the researchers decided to expose frozen broccoli to myrosinase from a related cruciferous vegetable.
When they sprinkled 0.25% of daikon radish – an amount that’s invisible to the eye and undetectable to taste buds – on the frozen broccoli, the two compounds worked together to form sulphoraphane, Dosz said.
“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,” he said.