Science Snippets!

Broccoli (Brassica oleracea L.) is a rich source of vitamins, dietary fiber, and glucosinolates that contain anti-carcinogenic properties. Glucoraphanin is the most predominant glucosinolate in broccoli, and when it breaks down upon chewing, glucoraphanin turns into sulphoraphane which is believed inhibit cancerous cell growth.

Many studies have been performed on the effects of heating (methods) on glucosinolate concentrations in broccoli. Working from previous research, Hwang and Kim (2012) studied different cooking methods and times and their effects on glucosinolates, carotenoids and tocopherol (vitamin E). The authors boiled (1, 5 or 10 min), steamed (1, 5 or 10 min) or micro-waved (1, 5 or 10 min) broccoli. All samples were cooled and then freeze dried. The processed samples were analyzed for glucosinolates, carotenoids and tocopherols by HPLC.

Glucosinolates: Five individual glucosinolates were detected in fresh broccoli samples with glucoraphanin the primary one. The initial (e.g. before heat treatments) glucosinolate content was 27.51 micromole/gram. Glucosinolate content decreased (approximately 64 to 70%) as cooking time increased in all three cooking methods (Hwang and Kim, 2012). However, the glucosinolate concentration remained higher after micro-waving compared to boiling or steaming. The authors postulate that glucosinolates are higher in micro-waved samples since less water is used in this method and thus less leaching of glucosinolates in the “cooking” water.

Carotenoids: beta-carotene and lutein concentrations increased as heating time increased across all heating treatments. Lutein content increased from 3.27 mg/100 g to approximately 6.0 mg/100g after 10 minutes of heating. Beta-carotene showed a similar trend from approximately 1.5 mg/100g to 2.4-2.8 mg/100 g after 10 minutes of heating.

Tocopherols: Hwang and Kim (2012) analyzed alpha- and gamma tocopherol content during the heating treatments. Similar to the carotenoids, alpha- and gamma tocopherol concentration increased as boiling/steaming/micro-waving times increased. The authors hypothesize that the membrane bound carotenoids and tocopherols are released during the heat treatments.

Hwang and Kim (2012) conclude that micro-waving and steaming broccoli resulted in the least loss of glucosinolates, while boiling resulted in the greatest loss. However, all heating methods released the bound carotenoids and tocopherols---compounds that are also beneficial to human health.

Reference:

Hwang, E-S and G-H Kim. 2012. Effects of various heating methods on glucosinolate, carotenoid and tocopherol concentrations in broccoli. International Journal of Food Sciences and Nutrition. 1-9.