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Myrosinase may greatly improve the health impact of frozen broccoli.

Frozen broccoli is gaining popularity due in part to its improved shelf life. Brassica vegetables contain myrosinase, a thiohydrolase that releases sulforaphane from glucoraphanin. Since myrosinase is not heat-stable, blanching may destroy its activity, depleting frozen broccoli of its major health-promoting component--sulforaphane, which has exhibited antimicrobial and anti-cancer properties.

Broccoli is usually blanched to inactivate any degradative enzymes in the product. Peroxidase, a relatively thermally stable enzyme in broccoli, is currently used for measuring whether the blanching process is complete. Lipoxygenase, responsible for most off-flavors and colors produced during the frozen storage of broccoli, is less thermally stable than peroxidase.

Scientists at the University of Illinois ran experiments to determine if heating broccoli to destroy lipoxygenase, but not peroxidase, would protect against the inactivation of myrosinase. They found that an external source of myrosinase may greatly improve the availability of sulforaphane, and optimize the health impact of frozen broccoli.

The researchers grew broccoli--Brassica oleracea var. Green magic--at the university. Market-ready heads were blanched in a temperature range of 66 C to 96 C. Then they were frozen at -80 C, thawed at 4 C for up to nine hours, and microwaved.

The investigators assayed peroxidase and lipoxygenase using color reactions. Myrosinase-dependent sulforaphane formation was measured using gas chromatography with a flame ionization detector against a benzyl isothiocyanate internal standard. The scientists also locally purchased Daikon radish--Raphanus sativus var. Longipinnatus. They freeze-dried and powdered the radish. The powder, an external source of myrosinase, was added to the frozen broccoli immediately before it was thawed.

When the broccoli was blanched at 76 C for 145 seconds, its lipoxygenase content was inactivated by more than 90%. However, 86% peroxidase activity and 83% myrosinase-dependent sulforaphane production remained.

But when the frozen broccoli was either microwaved directly--for 50 seconds with a 20-second rest period--or thawed for nine hours and then microwaved for 30 seconds, no sulforaphane formed. Yet, when freeze-dried radish powder was added during the nine-hour thaw, 74% of the sulforaphane content of the raw broccoli remained in the frozen, thawed, microwaved product.

Further information. Elizabeth H. Jeffery, Department of Food Science and Human Nutrition, University of Illinois, 499 Bevier Hall, Urbana, IL 61801; phone: 217-333-3820; email:
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Publication:Emerging Food R&D Report
Date:Jul 1, 2013
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