Text

theses

ETD doctoral

African indigenous vegetables (AIVs) play important role in providing the needed food security, nutrition and economic opportunities in sub-Saharan Africa. African nightshades, including Solanum scabrum, S. nigrum, S. americanum and S. villosum, and others are among the most popular leafy green vegetables. Yet, seed companies and the vegetable industry have largely ignored and undervalued these indigenous leafy greens in favor of the more traditional European introduced vegetables. As a consequence, the nutritional factors of the African nightshades are not well understood and have been studied little compared to European centric vegetables. In addition, many Solanum species are known to contain toxic glycoalkaloids, and such concern is also associated with Solanum African nightshades. This dissertation is devoted to the study of the nutritional and anti-nutritive factors in African nightshades to identify the most nutritious edible nightshades, provide guidance for safe and nutritious consumption, and to provide the needed scientific knowledge to facilitate the proper promotion of these vegetables in sub-Saharan Africa and extended regions in developing countries to reduce hunger and improve nutrition. Such outcomes could also foster the creation of new agricultural and economic opportunities in these areas.

To accomplish these objectives, we first collected a wide array of genetic materials of the edible nightshades and then analyzed and chemically profiled them using various liquid chromatography coupled with mass spectrometry (LC-MS) methods and associated techniques that were developed over the course of this research. The methods and techniques developed were efficiently applied for investigation and quality control of the phytochemistry in the leaves and berries of African nightshades from different genetic sources and cultivation environments.

Chapters II to V are devoted to the chemistry of African nightshade leaves. In chapter II, the major phytochemicals were identified in the leaves, including phenolic acids such as chlorogenic acid; 23 glycosides of quercetin, kaempferol and rhamnetin; eight saponins of diosgenin, tigogenin and other analogues; and two glycoalkaloids of solasodine. In Chapter III, a phytochemical quantification method was developed and validated, which applied optimized acid-assisted hydrolysis to release the aglycones which were then quantified in tandem mass spectrometry (MS/MS). The impact of genetic sources and cultivation environment on phytochemical profile was also investigated and discussed. The results from all samples investigated showed that the leaves were safe for consumption due to the absence or very low content of glycoalkaloids and other anti-nutritives. Chapter IV focuses on the free amino acids in the edible leaves. A hydrophilic interaction (HILIC)-LC-MS/MS method was developed and validated for determination of free amino acids in African nightshades as well as other AIVs including Ethiopian mustard, spider plant and amaranth. Different machine learning methods were employed for AIV classification prediction based on the profile of free amino acids. An online dashboard was also constructed for interactive application. Chapter V examines the vitamin-A precursor compound beta-carotene as well as other micronutrients to gain further insight into the overall nutritional contribution of edible nightshades.

Chapters VI to VIII shifted the research focus from leaves to berries of the African nightshades. While Africans normally consume only the leaves in edible nightshades, other groups such as those in South American value and consume the berries and not the leaves, despite the recognition of the presence of glycoalkaloids. Chapter VI was dedicated to phytochemical identification. Here, a total of 54 phytochemicals were identified, including phenolic acids of chlorogenic acid and neochlorogenic acid; flavonol glycosides of quercetin and isorhamnetin; anthocyanins of petunidin, malvidin and delphinidin; and saponins of diosgenin and tigogenin. In particular, a range of glycoalkaloids of solasodine and its uncommon and potentially novel hydroxylated and methylated derivatives were discovered, with the structure putatively identified based on the structural scaffold-fragmentation pattern. Chapter VII focused on quantity determination or estimation of the identified compounds in differently sourced berries, and discussed the profile change across different berry maturation stages. While many genetic lines were found to possess toxic levels of glycoalkaloids, a few genetic lines were found to be lacking in such toxins and were found instead to be rich in polyphenols. Such lines may be promising as new foods in sub-Saharan Africa. As additional effort to enhance berry inspection and quality control, in Chapter VIII, a specialized novel in-source fragmentation MS/MS method was developed. This new high-throughput and sensitive method could be readily applied to rapidly distinguish safe from toxic berries.

ETD_11217

Ph.D.

Includes bibliographical references

doi:10.7282/t3-hxgk-sm48

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