Antioxidant Properties of Selected African Vegetables, Fruits and Mushrooms: A Review, Mycotoxin and Food Safety in Developing Countries,

Abstract

Africa is blessed with vast amount of vegetables, fruits and mushrooms which are consumed for their nutrients or for their medicinal purposes. In recent years these vegetables, fruits and mushrooms have been shown to possess valuable antioxidants of great nutritional and therapeutic values. Antioxidants are substances which when present at low concentration compared to those of an oxidizable substrate [1] significantly delay or prevent the oxidation of that substrate. They are capable of preventing or attenuating damages such as lipid peroxidation, oxidative damage to membranes, glycation of proteins and inactivation of enzymes caused by free radicals. There are several evidences that show that oxidative stress resulting from reactive oxygen species including free radicals such as hydroxyl (OH.), superoxide (O2.-), nitric oxide (NO.), nitrogen dioxide (NO2.-), peroxyl (ROO.) and non free radical like hydrogen peroxide and singlet oxygen play an important role in the development of several pathological conditions such as lipid peroxidation, protein oxidation, DNA damage and cellular degeneration. These have been implicated in the aetiology of these pathological conditions related to cardiovascular diseases, diabetes, inflammatory diseases, cancer, Alzheimer and Parkinson disease, monogolism, ageing process and perhaps dementia [2,3-4, 5] . Free radicals and other reactive oxygen species are constantly formed in the human body during normal cellular metabolism e. g during energy production in the mitochondria electron transport chain, phagocytosis, arachidonic acid metabolism, ovulation, fertilization and in xenobiotic metabolism [6]. They can also be produced from external sources such as food, drugs, smokes and other pollution from the environment [7]. Organisms are endowed with endogenous (catalase, superoxide dismutase, glutathione peroxidase/reductase) and exogenous (vitamin C, E, β-carorene) antioxidant defense system against reactions of free radicals. However the generation of free radicals in the body beyond its antioxidant capacity leads to oxidative stress which has been implicated in the aetiology of several pathological 204 Mycotoxin and Food Safety in Developing Countries conditions such as lipid peroxidation, protein oxidation, DNA damage and cellular degeneration related to cardiovascular disease, diabetes, inflammatory disease, cancer and parkinson disease [8]. As a result of this much attention is been focused on the use of antioxidants especially natural antioxidant to inhibit and protect damage due to free radicals and reactive oxygen species. Synthetic antioxidant such as butylated hydroxyanisole(BHA), tert-butylated hydroxyquinone and butylated hydroxytoluene have been of utmost concern to many researcher because of their possible activity as promoters of carcinogenesis[9] Plant based antioxidant are now preferred to the synthetic ones because of their safety. Epidemiological studies have shown that the consumption of vegetables and fruits can protect humans against oxidative damage by inhibiting or quenching free radicals and reactive oxygen species [8].Many plants including fruits and vegetables are recognized as sources of natural antioxidants that can protect against oxidative stress and thus play an important role in the chemoprevention of diseases that have their aetiology and pathophysiology in reactive oxygen species (10, 11-12]. These positive effects are believed to be attributable to the antioxidants; particularly the carotenoids, flavonoids, lycopene, phenolics and β-carotene [13] Mushrooms which have long been appreciated for their flavour and texture are now recognized as a nutritious food as well as an important source of biologically active compounds of medicinal value [14]. Mushrooms accumulate a variety of secondary metabolites, including phenolic compounds, polyketides, terpenes and steroids. Also, a mushroom phenolic compound has been found to be an excellent antioxidant and synergist that is not mutagenic [15]. Studies have shown that tropical mushrooms are highly rich in proteins, minerals, vitamins, crude fiber and carbohydrate with low fat and oil content. The protein content of mushrooms has been reported to be twice that of vegetables and four times that of oranges and significantly higher than that of wheat [16, 17]. The high level of vitamins in mushrooms particurlary vitamin C and D has been reported as responsible for its antioxidative activity [17, 18]. Mushrooms contains also an appreciable quantities of crude fibres, although, little information exist on Total Dietary Fibre (TDF) content of mushrooms. The crude fibre content values reported from many studies suggest that mushrooms are potential sources of dietary fibre [16]. Mushrooms generally contain low fat and oil content [16]. Because of the low fat and oil content, they are recommended as good source of food supplement for patients with cardiac problems or at risk with lipid induced disorders. Also a lot had been reported on the nutrient; antinutrient and mineral composition of some edible mushrooms in Nigeria [19, 20] however there are few reported data on the antioxidant properties of commonly consumed mushrooms. This Chapter is therefore intended to discuss the antioxidant properties of selected African vegetables fruits and mushrooms