Characterization of starch accumulation in storage root of Marama (Tylosema Esculentum)
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Date
2018
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University of Namibia
Abstract
Starch is of substantial industrial significance for food and non-food uses. Plant roots and tubers such as potato and cassava are rich in starch and they are among the sources of starch for food consumption and industrial use. With climatic changes it is important to find crops that can be used as food and will still be able to survive the arid conditions. Marama is a plant that grows in the arid Namibian conditions, that bears a storage root that it is underutilized and has a potential to serve as an alternative source of starch. The main aim of this study was to determine the most suitable time for harvesting by evaluating the quality of marama root and its starch at the different harvest times. Effects of harvesting time on the proximate analysis of the marama roots as well as the thermal properties, size and physicochemical properties of the starch were investigated. Total starch of the marama roots (dry basis) increased with harvesting time, it ranged from 25.9 to 60.1% while the amylose content on starch basis decreased with harvesting time, ranging from 21.4-50.7%. Starch content was determined by enzymatic hydrolysation of starch to glucose and quantified colorimetrically by the Glucose oxidase-peroxidase reaction. Whereas amylose content was determined by the precipitation of amylopectin with lecin Concanavalin-A protein, amylose was then enzymatically hydrolysed to glucose and quantified using Glucose oxidase-peroxidase. Marama root starch granules were spherical, oval and lenticular in shape, the size of the granules increased with harvesting time and the mean granule (diameter) size ranged from 8.6 – 15.1μm. The youngest (2 month old) marama root had the highest crude protein content (33.6%). Crude protein content decreased from 33.6% down to 2.7% at the 12 months harvest time. The thermal properties of the freeze dried marama roots (2-12 month root samples) showed that it has an endothermic peak of 73.4-93.0 ˚C. This was higher than the gelatinization temperature of marama starch reported in literature suggesting that the other components of the marama root (proteins, ash, sugars and fibre) can affect the thermal properties. Literature reports that proteins, sugars and fibre delay the gelatinization of the starch. The Tp decreased with harvesting time, while the enthalpy change increased with harvesting time. The thermal properties of marama starch were affected by two factors : 1) the decrease in amylose content of the starch and therefore the increase in amylopectin content; hence an increase proportion of crystalline components (more ordered) the starch and therefore the increase in crystallinity of the starch crystallinity of the starch, which was evident in the increase in the ΔH as crops matured 2) the decrease in other components present in the root flour samples, hence a decrease in the interactions with the starch, the interactions may possibly have been responsible for the delay in endothermic peak, which was evident in the decrease in the To, Tp and Tc temperatures as the crop matured. It is therefore safe to conclude that time has an effect on the agronomic and physicochemical properties of the marama storage roots and its starch. The 2 months after planting is the optimal harvesting time for a good nutritional content of marama root, 2 month roots are rich in protein, fibre and ash and are less fibrous. Whereas the optimal harvest time for a good starch is dependent on the intended use of the starch. If a high amylose starch is preferred then the optimal harvesting time would be 2 months, while if normal starch is preferred then the 8 and 12 months present the optimal harvest time. However, the optimal harvesting time for better extractability is at 12 months after planting because of the higher total starch content. It is then recommended that future research focuses on the isolation and application of the marama root starch from roots harvested at different times in the food or non-food industries. This study also recommends that marama should be planted early in summer and harvested after 2 months when it is domesticated for a root vegetable.
Description
A thesis submitted in fulfillment of the requirements of the requirements for the Degree of Master of Science in Biology
Keywords
Starch accumulation, Marama