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Theses and dissertations (Applied Sciences)

Permanent URI for this collectionhttp://ir-dev.dut.ac.za/handle/10321/6

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    Production process improvement and characterization of starch nanocrystals
    (2023-05) Nzama, Nkosingiphile Lucky; Amonsou, Eric Oscar
    Starch nanocrystals (SNCs) are promising biomaterials for novel applications in foods, cosmetics, and medicine. In general, acid hydrolysis below the gelatinization temperature of starch is the most common method used for nanocrystals production. Major drawbacks associated with this method are the extended hydrolysis time required (up to 5 days) and the low yield (4–15%) of SNCs. Different methods, including physical and enzymatic pretreatments of starch prior to acid hydrolysis, have been investigated. Among these methods, enzymatic hydrolysis can be regarded as a promising and green strategy for the creation of pores in starch to enhance acid diffusion into the inner regions during SNCs fabrication. Debranching enzymes such as pullulanase are gaining attention in the food industry due to their ability to modify the starch structure and properties through selective hydrolysis of the branched chain of α-1,6-glycosidic bonds. However, pullulanase has not yet been applied as a pretreatment method aiming at starch nanocrystal preparation. Therefore, the pretreatment of starch granules with pullulanase and β-amylase (i.e., to hydrolyze the linear α-1,4-linkages) concurrently could be a novel technique to modify starch surfaces for faster production of SNCs and improved yield. To improve the efficiency of starch nanocrystals production and properties, pullulanase (15 U/g starch) was used alone or together with β-amylase (50 and 100 U/g starch) to modify the starch before acid hydrolysis. The compound enzyme system of pullulanase:β-amylase (15 : 50 U/g starch) had the most pronounced effect on starch morphology compared to a single enzyme system by creating a dense and more porous structure on starch surfaces as evidenced by microscopy images, a high degree of oil absorption and extent of hydrolysis data. Nanocrystals were produced after 3 days with modified starches instead of 5 days. The yield of SNC was approx. 25 wt.%, which is 3 times greater than that of the conventional SNC preparation method. SNC derived from the modified starches were small in size (less than 50 nm) and appeared mostly as platelet and isolated round particle aggregates. Nanocrystals from modified starches showed the A-type crystalline structure similar to the native starch, but with a significant increase in the degree of crystallinity (from 32.85% to 45.28%.), and the short-range molecular order during the early stage of acid hydrolysis. Starch hydrolysis using compound enzymes consisting of pullulanase and βamylase hydrolysis seems to be the most effective and green to produce SNC in a shorter time and with increased yield and enhanced properties. SNCs were incorporated in different concentrations (0, 5, 10, 15, and 20 wt.% starch) together with stearic acid to improve cassava starch-based nanocomposite film properties using a solution casting method. The addition of SNCs from 5 to 15% in combined with stearic acid into starchbased nanocomposite films presented better water resistance, water vapor permeability, and tensile strength than native cassava starch film. Conversely, beyond 15% SNC content, nanocrystals seem to aggregate which impaired the tensile strength of the nanocomposite films. The surfaces of the nanocomposite films were relatively smooth and homogenous after the addition of nanocrystals at up to 15 wt.% concentration compared to native starch film as demonstrated by the atomic force microscopy (AFM). Furthermore, the opaqueness of the nanocomposite films progressively increased with the SNC content, which might be beneficial in the packaging of foods that are easily degraded when exposed to light and high moisture. XRD analysis revealed sharp peaks at approximately 2θ of 13.5° and 20.3°, which are characteristics of typical V-type crystalline pattern in starch films prepared with added steric acid. This further indicates the formation of amyloselipid complexes in films. The inclusion of SNC in films also enhanced their thermal stability. Therefore, the combined effect of SNC at different concentrations and stearic acid into cassava starch-based films was a successful approach to further improve the mechanical reinforcement and barrier properties of nanocomposite films.
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    Synthesis of lactic acid using hydrogen cyanide extracted from cassava (Manihot esculenta) leaves
    (2022-05) Ilunga, Monga; Paul, Vimla; Zinyemba, Orpah; Muniyasamy, Sudhakar
    Racemic lactic acid (2-hydroxypropanoic acid) has gained interest in the food and non-food industries and in producing biodegradable and biocompatible lactic acid polymers. Although racemic lactic acid is conveniently synthesised by chemical synthesis via the DL-lactonitrile route, it can also be produced by the fermentation process provided that suitable microorganisms and substrates are used. However, regardless of the sustainability issues associated with the fermentation process, it is the preferred production method since the chemical process relies on fossil fuel resources. In this context, this study aims to extract hydrogen cyanide (HCN) from cassava (Manihot esculenta Crantz) leaves and then use it to chemically produce racemic lactic acid. Cassava leaves were chosen as a natural source of HCN since they release 20 times more HCN than the tubers. HCN is produced by endogenous enzymes (linamarase and hydroxynitrile lyase) hydrolysing the cyanogenic glucosides (linamarin and lotaustralin). Following 120 minutes of maceration at 30 °C, the released HCN was extracted for 45 minutes under vacuum at 35 °C – 45 °C and collected in 400 mL of 5.104 mol/L sodium hydroxide (NaOH) solution (absorbing solution) to give sodium cyanide (NaCN) solution. The extraction process was repeated until saturation of the absorbing solution was achieved. The final concentration of NaCN solution determined by the alkaline picrate method was found to be 4.0421 mol/L. Furthermore, the sodium carbonate (Na2CO3) and residual NaOH content in control and sample sodium cyanide solutions were also determined. The Na2CO3 content was 0.72 % in the control NaCN solution and 2.49 % in the sample NaCN solution. The residual sodium hydroxide content was 2.61 % in the control sodium cyanide solution and 4.20 % in the prepared sodium cyanide solution. 79.241 g of NaCN crystals (0.19 % yield, green NaCN) were obtained from 42.750 kg of fresh cassava leaves. The suggested approach was successful in preparing NaCN, as evidenced by X-Ray Diffraction (XRD), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS) results. Control and green NaCN both contained sodium carbonate impurities, as shown by these spectral techniques. Titration tests revealed that the latter was 0.61 % and 2.29 % in control and green NaCN, respectively. In addition, titration studies indicated that the residual NaOH content in control NaCN was 1.63 % and 4.68 % in green NaCN. The high carbonate content can be explained by the reaction between residual sodium hydroxide and atmospheric CO2. Reproducibility and repeatability tests were done to evaluate the reliability of the hydrogen cyanide extraction method. Racemic lactic acid was synthesised using a four-step process. 73 mL of DL-lactonitrile (2- hydroxypropanenitrile) (81.1 % yield, 59.7 % pure) was prepared by reacting 75 mL of acetaldehyde with hydrogen cyanide generated in-situ from green sodium cyanide (62.190 g in 150 mL of Milli-Q water) in the presence of 37 % hydrochloric acid (100 mL). 35 mL of crude racemic lactic acid (84.1 % yield, 14.9 % pure) was prepared by hydrolysing 40 mL of DLlactonitrile with 8 mol/L hydrochloric acid (40 mL). Crude racemic lactic acid underwent a two-step purification process in the presence of concentrated sulphuric acid (5 mL), used as the catalyst. 35 mL of crude lactic acid was first esterified with excess methanol (50 mL) to produce 32 mL of methyl DL-lactate (methyl 2-hydroxypropanoate) (71.6 % yield, 46.1 % pure). The ester was then hydrolysed with excess water (20 mL) to give 22 mL of purified racemic lactic acid (88.0 % yield, 56.0 % pure). The identity of the synthesised products was confirmed by comparing them against control samples using 1H Quantitative Nuclear Magnetic Resonance (1H QNMR) and ATR-FTIR. Their purity was determined by 1H QNMR, using dimethylformamide as the internal standard. The overall yield of synthesised racemic lactic acid was 43.0 %.