The hidden attributes of Brassica crops lie in their abilities to reduce the incidence of human cancer and coronary diseases when consumed over periods of years as part of a balanced diet. Medical evidence for these attributes has accumulated substantially in the past decade, and these properties are coming into prominence with the general public (Mazza, 2004) (see Table 8.1).
In this respect, the Brassica crops, such as broccoli (calabrese), have especial interest because of their sulphorathane content which is associated with the reduction of active oxygen in tissues and may provide protection from cancer and coronary diseases. Previously, Brassica crops lacked consumer appeal largely due to the presence of sulphur-containing amino acids that released unacceptable odours when overcooked and which can be mildly toxic. Now, broccoli (calabrese) is becoming a very popular convenience vegetable worldwide with significant benefits for long-term human health. It has been recommended as a dietary additive for cancer prevention since the early 1980s (Nestle, 1998).
The health benefits of Brassica vegetables are related to their content of glucosinolates. These are a group of sulphur-based secondary metabolites that are present in at least 16 families of the dicotyledonous plants (Fahey et al., 2001). Wild and domesticated brassicas contain >100 different forms of the glucosinolate molecule; they all have a common basic structure composed of three parts: a p-d-thioglucose group, a sulphonated oxime moiety and a variable side chain. The latter can be a straight-chained alkyl or alkenyl structure, a ring-shaped aromatic group or an indolyl formation. Many common vegetable brassicas such as Brussels sprout or broccoli contain all three moieties. They remain inactive in intact cells, but damage releases myrosinase enzymes which breaks down the glucosinolate into several products, notably nitriles and isothiocyanates. These are the source of hot and bitter flavours in brassicas, especially condiments. Their original function
Table 8.1. Examples of the constituents of brassicas that may enhance human health. Component Potential benefit
Lutein Contributes to healthy vision
Sulphoraphane Neutralizes free radicals; may reduce cancer risk
Lignans May protect against heart disease and some cancers;
lowers low-density lipoprotein (LDL) cholesterol, total cholesterol and triglycerides Allyl methyl trisulphide, dithiothiones Lowers LDL cholesterol, maintains a healthy immune system
Adapted from Mazza (2004).
may have been to provide natural pesticides active against some insect and vertebrate pests. Some glucosinolate breakdown products have been thought to produce toxic, or anti-nutritional effects in grazing animals. Consequently, plant breeders have aimed to reduce their content in crops such as forage rape and fodder kale. They have also been ascribed goitrogenic effects in humans, but recent evidence offers a contrary view of their value.
There is a growing body of epidemiological and experimental evidence showing that the consumption of Brassica vegetables specifically reduces the risk of cancer in the human lung and alimentary tract (Chu et al., 2002; Lester, 2006). Evidence suggests, for instance, that sulphorathane, an isothiocyanate present in broccoli (calabrese), aids in the detoxification of carcinogens, others such as those present in watercress help in the excretion of toxic agents from tobacco smoke while yet another group inhibits the proliferation of cancerous cells.
Since glucosinolates are highly labile, their health benefits depend on many variables relating to intake and metabolism following a range of factors which affect their concentrations in the crop and following its harvesting, storage and processing. Recent laboratory studies using rapid cycling B. rapa (see Chapter 2) suggest that zinc influences glucosinolate content and the resultant bitterness and medical properties of brassicas (Coolong et al., 2004). In some crops such as red and white cabbage, concentrations of glucosinolates remain stable for several months after harvest. Where postharvest processing causes physical damage, then this is likely to lead to the activation of myrosinase and the release of glucosinolate breakdown products. Thus as the use by consumers of brassicas in chopped ready-to-use salads containing cabbage or broccoli (calabrese) increases, the concentration of anti-carcinogenic agents is likely to increase. Where the crops are subjected to heat treatment as in the blanching of chopped broccoli (calabrese), there will be reductions in the concentrations of glucosinolates. There is evidence that fermentation as in the production of sauerkraut or kimchi causes a complete loss of glucosinolates. These putative health-promoting effects of Brassica vegetables have substantial implications for producers, retailers and consumers. Breeding strategies are being developed aiming to produce new 'healthier' cultivars utilizing the abundant genetic variation present in B. oleracea (Johnson, 2000).
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