Microwaving and streaming optimize levels of bioactive sulforaphane in broccoli

Emerging Food R&D Report, May, 2006

Sulforaphane (SF) is believed to be responsible for the anti-cancer properties found in broccoli. SF's mechanism of action involves the upregulation of detoxification enzymes, or the induction of apoptosis and the inhibition of cancer cell growth. Sulforaphane forms after plant tissue is disrupted, as when it is chopped. Its parent glucosinolate--glucoraphanin--is hydrolyzed by the myrosinase enzyme.

When the co-factor epithiospecifier protein (ESP) is present, an inactive hydrolysis product--sulforaphane nitrile (SFN)--forms at the expense of SF. Because ESP is more heat-sensitive than myrosinase, there is an opportunity to alter the SF-to-SFN ratio using heat. Hydrolysis product levels also depend on the glucosinolate levels in the plant, which differ according to plant genotype.

Researchers at the University of Illinois wanted to determine the effects of heating and genotype on SF and SFN levels in broccoli. The investigators microwaved, boiled or steamed fresh broccoli for a variety of time periods. They used gas chromatography to quantify hydrolysis products.

The scientists found that a broccoli's genotype can affect hydrolysis, with certain types of broccoli having significantly higher maximum SF levels than other types of product. The heating method used and the extent of heating also affected the formation of hydrolysis product. SF formation increased at early time points, then decreased as heating progressed. SFN levels continuously decreased.

The time at which maximum SF levels were reached varied with the method employed. For instance, about 30 seconds was required for microwaving, and from 1 to 3 minutes for steaming. Boiling caused a rapid loss of SF.

These results suggest that we can increase SF levels of broccoli by choosing specific preparation methods. Microwaving and steaming provide similar higher levels of SF, while boiling causes rapid losses of SF. It may be easier to obtain higher levels of SF by steaming the product because the window for microwave-induced SF is very short.

Further information. Elizabeth Jeffery, Department of Food Science and Human Nutrition, University of Illinois, Urbana-Champaign, 905 S. Goodwin Ave., 499 Bevier Hall, MC 182, Urbana, IL 61801; phone: 217-333-3820; email: ejeffery@uiuc.edu.