Theses and Dissertations (Centre for Nutrition)

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    Antioxidant content and activity of selected African leafy green vegetables in the gastrointestinal tract (GIT) : determination using simulated GIT digestion
    (University of Pretoria, 2018) Bester, Marthan Nieuwoudt; skhumalo@uniswa.sz; Khumalo, Sanele Maxine
    Introduction The incidence of non-communicable disease such as cancer, diabetes, and cardio vascular disease (CVD) is increasing. Major contributing factors are diets that are poor in minerals, vitamins and antioxidants. Leafy green vegetables (LGV) are an important source of these nutrients. In addition, indigenous LGV are drought resistant, robust and easy to cultivate than more exotic LGV such as spinach and cabbage. Little is known about the effects of cooking and digestion on the polyphenol, flavonoid and ß-carotene content of African LGV as well as the associated antioxidant activity. The aims of this study are firstly to evaluate the effect of pH, the digestive enzymes and the phase of digestion on the polyphenols, flavonoids and β-carotene content and associated antioxidant activity of LGVs, spinach, pumpkin, cowpea, amaranth and jute mallow. Then to determine whether following digestion the antioxidant properties of the African LGVs is better than spinach which is commercially and widely cultivated. Spinach, pumpkin, cowpea, amaranth and jute mallow leaves were cooked for 30 minutes, the water was discarded and the remaining solid cooked LGV which is usually consumed was subjected to two methods of digestion. The first referred to as gastrointestinal model of digestion (GI) of digestion consisted only of the stomach and intestinal phase of digestion, while the second model which was more complex included the oral, stomach and intestinal (OGI) phases of digestion. Raw, cooked, pH adjusted and digested samples at each phase of digestion were collected. For each sample the total polyphenol content (TPC), the total flavonoid content (TFC) was determined with the Folin-Ciocalteu method and aluminum chloride methods respectively. The ß-carotene content was determined with HPLC. Antioxidant activity was determined with the Trolox equivalent antioxidant capacity (TEAC), the 1, 1-diphenyl–2-picrylhydrazyl (DPPH) and the oxygen radical absorbency capacity (ORAC) assays. Cooking decreased significantly the TPC, TFC, TEAC and DPPH of spinach, pumpkin and cowpea, while the ß-carotene content of these LGV remained unchanged. For amaranth and jute mallow, cooking increased the TFC, ß-carotene and DPPH values, while their TPC and TEAC remained unchanged. All the LGVs decreased their antioxidant activity when measured by ORAC assay, except for jute mallow which was unchanged. During digestion, the pH controls had varying effects with different assays, as it did not change the TPC of spinach, pumpkin and cowpeas, but decreased that of amaranth while the TPC of jute mallow was higher. Overall, the gastric and intestinal digestion increased the antioxidant activity of all the LGVs as measured by the ORAC assay. TPC and TEAC increased significantly for spinach, pumpkin and cowpea, while their TFC remained unchanged and antioxidant activity measured with the DPPH assay was lower. For the amaranth and jute mallow the TFC was decreased, DPPH increased, while TEAC was unchanged. Except for pumpkin the ß-carotene content was reduced following digestion. In general, the effect of pH was minimal while digestive breakdown of the LGV matrix resulted in a significant release of molecules with antioxidant activity. In the OGI model of digestion, the oral phase of digestion contributed significantly to the levels of polyphenols and flavonoids released from the matrix. The effect on ß-carotene content was variable. Antioxidant activity for all LGV was increased (TEAC assay), variable (DPPH assay) and reduced (ORAC) assay. When considering the effect determined by the ORAC assay which is considered to be the physiologically the most relevant assay, using models that only simulate gastric and intestinal digestion may over estimate antioxidant activity. Compared to spinach the antioxidant properties of the LGVs, although in some instances statistically different, were similar to spinach. In conclusion, LGVs evaluated in this study retained to various degrees the antioxidant activity following digestion. The biggest contributing factor to these antioxidant properties was not pH but the ability of the digestive enzymes to break down the food matrix resulting the release of antioxidant molecules. Based on the findings of this study are that African LGV are an ideal source of bioactive molecules that can prevent diseases of the GIT and if bio- available can effectively reduce the risk of diabetes, cancer and CVD.
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    Improved iron and zinc availability in sorghum by phytate reduction through genetic modification, fermentation and phytase addition
    (University of Pretoria, 2012-05-15) johanita.kruger@up.ac.za; Kruger, Johanita; Prof A Oelofse; Prof J R N Taylor
    Iron and zinc deficiencies are highly prevalent in the developing world. In worst cases, the cause of iron and zinc deficiency may be actual insufficient dietary intake, but inhibitors of absorption, mostly found in plant foods, contribute substantially to these deficiencies. Sorghum is an important staple crop in Africa, often in populations with severe iron and zinc deficiencies. Sorghum is commonly consumed as whole grain. While the bran of sorghum contains the most iron, it also contains phytate and sometimes also tannins, depending on the cultivar, which further reduce the already low bioavailability of non-haem iron and zinc. The aim of this research was to evaluate the effect of reducing sorghum and maize phytate content through genetic modification (GM) on in vitro iron and zinc availability in porridges and raw cereal lager brewer’s wort. The effect of phytate reduction through GM on iron and zinc availability was also compared with the effect of phytate reduction through natural lactic acid fermentation and the addition of exogenous phytase. Iron and zinc availabilities were assessed using a dialysability assay (bioaccessibility), Caco-2 cell (uptake) study and an in vivo suckling rat pup model (absorption). A mineral solubility assay was used to analyse the soluble minerals in the raw cereal wort. GM low phytate (approx. 30-40% reduction) non-tannin and tannin sorghums and their respective null controls (NC) were processed into thick unfermented and fermented porridges. The inhibitory effect of the tannins on mineral availability seemed to prevent any increase in in vitro iron and zinc bioaccessibilities regardless of the level of phytate reduction. However, the additive effect of GM in combination with fermentation in reducing the porridge phytate content, caused a substantial increase in in vitro iron bioaccessibility in the non-tannin line. The percentage bioaccessible iron in the GM fermented porridge was approximately 30%, compared to the GM unfermented porridge (approx. 10%) or the NC fermented porridge (approx. 15%). At this level of phytate reduction, the dialysability assay could not detect any effect on zinc bioaccessibility. A larger phytate reduction through GM (approx. 80-90%) in non-tannin sorghum significantly (p<0.05) increased zinc uptake and absorption and iron bioaccessibility. Principal component analysis (PCA) showed an indirect correlation between phytate content and zinc uptake and absorption and iron bioaccessibility and absorption. The dialysability assay used in this research proved ineffective in estimating zinc absorption in GM sorghums. However, the dialysability assay can be used to estimate in vivo iron absorption from sorghum. The Caco-2 cell uptake study used in this research proved ineffective in estimating iron absorption in GM sorghums, as the iron uptake was possibly affected by the varying mineral (Ca, Fe, Zn, P) contents of the sorghums. More research is needed to determine the effect of naturally occurring variations in mineral contents of sorghum on the iron uptake by Caco-2 cells. With regard to raw cereal brewing a phytate reduction through GM (approx. 30- 40%) reduced the spent grain mineral (Fe, Zn, Mg, P, Ca) contents of sorghum by approximately 11-38%. While phytase addition during brewing reduced sorghum spent grain phytate content by 88% and mineral content by 17-59%, it did not, however, affect the maize phytate and mineral contents significantly (p≥0.05). This may be due to the fact that the phytate in maize is more soluble than in sorghum. The reduction in mineral content in the spent grain is an indication of the amount of minerals which would be solubilised during brewing. Compared to addition of exogenous phytase, GM has greater potential for increasing the overall nutritive quality of sorghum wort, as it also increased hot water extract and wort free amino nitrogen substantially. This research indicated that reducing sorghum phytate content through GM, fermentation and phytase addition increases iron and zinc availability from sorghum. However, none of the assays applied gave an unequivocal indication of the magnitude of increase in iron and zinc bioavailability. Also, factors like human health, food processing and other food components have a major influence on iron and zinc bioavailability and absorption in humans. For low phytate sorghum to increase iron and zinc status in subsistence growers of sorghum in semi-arid regions of sub-Saharan Africa where the prevalence of poverty and iron and zinc deficiency are high, it needs to be implemented together with nutrition education and dietary diversification, while simultaneously taking measures to address poverty and morbidity.
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    Antioxidant properties and cellular protective effects of selected African green leafy vegetables
    (University of Pretoria, 2012-06-07) Oelofse, Andre; Duodu, Kwaku Gyebi; Bester, Megan J.; s26556881@tuks.co.za; Mavhungu, Nangula Paulina
    Phenolic compounds in African green leafy vegetables (GLVs) may have a significant impact on human health. However, there is little information on the phenolic composition, antioxidant properties, as well as biological and cellular protective effects of these vegetables. The effects of boiling and extraction solvent on these compounds and on their antioxidant properties are also unknown. Phenolic content, antioxidant activity and cellular protective effects of four African GLVs in comparison with spinach, an exotic GLV, was determined. African GLVs had appreciable levels of total phenolics and antioxidant activity and in higher quantities compared to spinach. Boiling decreased the antioxidant content and activity of these vegetables and 75% acetone was more effective in extracting antioxidants from the GLVs compared to water. GLVs with high levels of phenolics also contained higher levels of antioxidant activity, suggesting that phenolics are likely to have contributed to radical scavenging activity of these vegetable extracts, even though the degree of scavenging varied in each extract of the vegetable species. The flavonoid compositions of raw and boiled African GLVs and spinach were determined using high-performance liquid chromatography. Epicatechin and rutin were the most dominant flavonoids found in both water and 75% acetone extracts. Among water extracts, pumpkin contained higher concentrations of detected flavonoids, while among the acetone extracts, cowpea exhibited higher concentrations. The effect of boiling was dependent on the type of vegetable and the specific flavonoids. There were no major differences observed between the type of flavonoids detected in extracts of African GLVs and those in spinach. However, similar to the results of total phenolics and antioxidant activity, the 75% acetone extracts of African GLVs also exhibited higher amounts of flavonoids than spinach. The protective effects of GLVs against oxidative haemolysis were dependent on the type of vegetable species. Boiling had variable effects depending on the species. The highest level of protection of erythrocytes against oxidative damage was offered by amaranth extracts, while extracts of raw jute mallow contributed to the damage of erythrocytes. The highest antioxidant protection activity against oxidative damage in plasmid DNA was offered by extracts of jute mallow and lowest by spinach.