Models of the fate of glucosinolates in Brassicaceae from processing to digestion

Abstract.

Glucosinolates are secondary metabolites of Brassica vegetables. Glucosinolates are not bioactive themselves, but their hydrolysis products isothiocyanates have been associated with health benefits. The concentrations of glucosinolates and their break down products are strongly affected by processing of the vegetables, but are also affected by digestion conditions.

During thermal treatment of Brassicaceae, such as domestic cooking, different mechanisms affecting the content of glucosinolates can take place and were modelled in the present study: Lysis of plant cells and compartments, leaching of glucosinolates into the cooking water and thermal degradation of glucosinolates in both the intact vegetable tissue and in the cooking water. These mechanisms were described mathematically and the model parameters for broccoli, Brussels sprouts, red cabbage and white cabbage were estimated based on experimental results. Differences between the thermostability of the same glucosinolates originating from different Brassicaceae could be detected, as well as differences between the thermostability of the same glucosinolates in the vegetable matrix compared to that in cooking water. This mathematical model and the estimated parameters can be used to simulate the different glucosinolate contents in prepared foods considering the processing method. This should be a useful tool in food research and industry to make predictions about the nutritional quality of foods and to optimize their health related quality attributes.

In broccoli, the glucosinolate glucoraphanin and its breakdown products were further studied in an in vitro digestion study and an in vivo chewing study with five subjects. Upon cell damage, e.g. during chewing, the glucosinolate glucoraphanin is hydrolyzed by the endogenous enzyme myrosinase and, depending on the environmental conditions, sulforaphane or sulforaphane nitrile are produced.

The effect of steaming time (raw or steamed for 1, 2 or 3 min) and meal composition (with and without addition of protein (bovine serum albumin or lipid (olive oil)) on the conversion of glucoraphanin were studied in an in vitro digestion model and the bioaccessibility of released breakdown products investigated. The main formation of sulforaphane and sulforaphane nitrile from glucoraphanin occurred during the in vitro oral phase. The content of glucoraphanin, sulforaphane and sulforaphane nitrile did not degrade after digestion. Sulforaphane concentrations were up to 10-times higher in raw and 1-min steamed broccoli samples after the digestion compared to broccoli that was steamed 2 or 3 min. The addition of bovine serum albumin and olive oil had no influence on the formation and bioaccessibility of sulforaphane or sulforaphane nitrile. Meal preparation seems to have a much more pronounced effect on SF formation and bioaccessibility compared to meal composition.

In an in vitro study the effect of chewing time (11 s, 22 s, 30 s and 40 s) on differently steamed broccoli ( raw or steamed for 0.5-min, 1-min 2-min and 3-min) was studied. Chewing time influenced the amount of hydrolysis of glucoraphanin in raw and short steamed broccoli that contains active myrosinase (raw, 0.5-min and 1-min steamed), but not in broccoli that had been steamed longer. Steaming time showed to influence the oral hydrolysis of glucoraphanin. Both chewing time and steaming time influence the enzymatic breakdown of glucoraphanin in the mouth. Longer chewing times of raw and short steamed broccoli (0.5-min and 1-min), which contains active myrosinase, lead to more hydrolysis.