T01028: Glucosinolate stability in culinary processing and storage

Study Duration: October 2002 to September 2005

Contractor: University of Essex

Research to date suggests that Brassica vegetables (green cabbage, cauliflower, broccoli and Brussel sprouts) in the diet contribute significantly to a decreased risk of cancer, particularly for tumours of the lung, stomach, colon and rectum. This effect is mediated by glucosinolates, (a group of natural products) which are broken down in the gut to produce chemopreventative products called isothiocyanates. As isothiocyanates are highly unstable during cooking, it is not known how best to store and cook glucosinolate-rich vegetables in order to produce a palatable yet highly chemopreventative meal component.

Initially the project is isolating and purifying seven common glucosinolates found in green cabbage, broccoli, cauliflower and Brussels sprouts, and their isothiocyanate and amine derivatives. Three of these will be compared with stable isotope substituted standard supplied from project T01027 which is running in parallel with this contract. Analytical methods will be developed and validated before being used to measure levels of the glucosinolates in green cabbage, cauliflower, broccoli and Brussels sprouts.

This project complements project no T01027
T01027

Seven glucosinolates and related isothiocyanate and amine derivatives were purified or obtained through organic synthesis. Analytical methods were developed to quantify dietary glucosinolates, isothiocyanates and related amine derivatives using LC-MS/MS. The glucosinolate content of four Brassica vegetables (broccoli, Brussels sprouts, cauliflower and green cabbage) were analysed to determine the effects of different cooking, preparation and storage regimes.

Results showed that in the short-term most of the glucosinolates were preserved in vegetables kept at ambient temperature or in a refrigerator at 4-8&#176C, the loss being slightly greater when the vegetables were stored in a refrigerator. However, after 7 days storage there was a considerable reduction in glucosinolate content (10-20% at ambient temperature and 20-30% in a refrigerator) and those stored at ambient temperature showed visible signs of decay. Vegetables stored in a -85°C freezer showed significant loss of glucosinolates (30-40%) due to their degradation with the formation of isothiocyanates. This was attributed to release of myrosinase (an enzyme that breaks down glucosinolates to isothiocyanates and other metabolites) following freeze-thaw fracture of plant cells.

Shredding of vegetables resulted in up to 75% loss of glucosinolates after 6 hours. The decrease in glucosinolate concentration was affected by the size of the pieces, the time after shedding that samples were taken for measurement and the type of vegetable, the loss being greatest in cauliflower. Only 30-50% of the lost glucosinolates were detected as isothiocyanates. This was assumed to be due to further breakdown of the isothiocyanates; however, amines were not detected.

Boiling vegetables led to a significant loss of glucosinolates with levels falling to 60 -70% of the original value after 10 minutes and 30-40% of original levels after 30 minutes boiling. This loss was due to leaching of glucosinolates into the boiling water.

Apart from boiling, most of the other cooking methods tested (stir-fry, steaming and microwaving) had little influence on glucosinolate levels in Brassica vegetables. Data from this project supported the results from T01027 in showing that boiling in water is the only cooking method that led to a large loss of glucosinolates. The contractors concluded that as isothiocyanates are reported to be chemoprotective, cooking procedures that do not inactivate all the plant myrosinase may be desirable.

The final report is available from the Agency’s Information Centre.
To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk)

Song LJ, Morrison JJ, Botting NP and Thornalley PJ, (2005). Analysis of glucosinolates, isothiocyanates and amine degradation products in vegetable extracts and blood plasma by LC-MS/MS. Anal. Biochem., 347, 234-43.

Song L, Iori R and Thornalley PJ, (2006). Purification of major glucosinolates from Brassicaceae seeds and preparation of isothiocyanate and amine metabolites. J. Sci. Food and Agric., 86, 1271-80.

Song L and Thornalley PJ, (2007). Effect of storage, processing and cooking on glucosinolates in Brassica vegetables. Food Chem.Toxicol., 45, 216-24.

For any enquiries concerning this research project, please contact the relevant Programme contact or email: science@foodstandards.gsi.gov.uk