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Liquid and gas chromatography
For the determination of monosaccharides, the most common analytical techniques are gas chromatography (GC) and high performance liquid chromatography (HPLC) (Cheng, Tsai and Chang, 2006). In GC, flame ionization detection or mass spectrometry (MS) may be used to quantify sugars (Medeiros and Simoneit, 2007). Concerning HPLC refractive index detection, pulsed amperometric detection, evaporative light-scattering detection and MS are common (Medeiros and Simoneit, 2007). Considering cassava cell wall hydrolysates, determination of constituent neutral sugars has been specifically by GC (Favaro et al., 2008; Adetunji et al., 2016; Ngea et al., 2016) and high performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) (Salvador et al., 2000; Salvador et al., 2002; Ngea et al., 2016). A pioneering study into cassava cell walls was conducted by Salvador et al. (2000) using HPAEC-PAD. The technique is reputed to provide superior separation and detection of neutral monosaccharides (Garleb, Bourquin and Fahey, 1989). It separates monosaccharides by using strongly alkaline solutions, in which the weak acidic properties of sugar molecules are exploited (Cataldi, Campa and Benedetto, 2000). HPAEC-PAD reportedly has an advantage over GC based methods in separation of multi-component mixtures containing monosaccharides, disaccharides and amino acids (Cataldi et al., 2000). However, the hydrolytes of purified residual cassava cell walls are most likely not complicated by the presence of disaccharides and amino acids.
Effects of storage temperature and dilute NaOH steeping on starch yield
As described in Chapter 4.1, frozen cassava chips (approx. 9 cm3) were thawed at 4oC for 24 h prior to utilisation. Cassava chips (2.4 kg) were vacuum packed into multi-layered vacuum bags (polyethylene 55 µm, ionomer resin 10 µm and polyamide 15 µm) (Plastikon Pretoria, South Africa) and stored for 14 days. Vacuum packaging was done to inhibit growth of aerobic spoilage microorganisms. Three storage temperatures, 24-25oC, 4oC and -20oC were investigated. Selection of storage temperatures was based on conditions favourable for the growth of moulds and yeasts capable of producing cell wall degrading extracellular enzymes (Amoa‐Awua, Frisvad, Sefa‐Dedeh and Jakobsen, 1997). After the 14 days storage, the cassava chips were ground into smaller pieces using an electric rotary meat mincer fitted with an 8 mm opening plate. Ground cassava pieces (400 g) were mixed with 400 ml 0.75% (w/v) NaOH solution, final pH 11.5. Control samples were mixed with only distilled water. All treatments were steeped for 2 h at 25oC. The steeped cassava pieces were pulverised in a Waring blender for 1 min. using low and the high speed each for 30 s. The pulp was suspended in 5x its volume distilled water and wet milled using Retsch EZ200 wet mill (Haan, Germany) with a 500 µm screen opening size. Figure 4.2.1 summarises the stages followed during cassava starch extraction.
1 INTRODUCTION
2 LITERATURE REVIEW
2.1 INTRODUCTION
2.2 PHYSICO-CHEMICAL PROPERTIES OF CASSAVA ROOTS
2.3 CASSAVA ROOT PARENCHYMA CELL WALLS
2.4 ALKALIPHILIC MICROORGANISMS AND CELL WALL HYDROLYSIS
2.5 DEVELOPMENTS IN CASSAVA WET MILLING STARCH EXTRACTION
2.6 PREPARATION AND ANALYTICAL TECHNIQUES FOR STUDY OF CASSAVA CELL WALLS
2.7 CONCLUSIONS
3 HYPOTHESES AND OBJECTIVES
3.1 HYPOTHESES
3.2 OBJECTIVES
4 RESEARCH
4.1 MECHANISM OF CASSAVA TUBER CELL WALL WEAKENING BY DILUTE SODIUM HYDROXIDE STEEPING
4.2 COMBINED EFFECTS OF ENDOGENOUS FERMENTATION FOLLOWED BY DILUTE SODIUM HYDROXIDE STEEPING ON CASSAVA ROOT STARCH EXTRACTION
4.3 SOLID STATE CASSAVA ROOT FERMENTATION USING ALKALIPHILIC BACILLUS AKIBAI, B. CELLULOSILYTICUS AND B. HEMICELLULOSILYTICUS CULTURES ALONE, AND IN COMBINATION TO IMPROVE CELL WALL DISAGGREGATION
5 GENERAL DISCUSSION
5.1 METHODOLOGICAL CONSIDERATIONS CONCERNING ANALYSES OF THE RESIDUAL CASSAVA CWM
5.2 SUMMARY OF THE MAIN RESEARCH FINDINGS
5.3 PROPOSED MECHANISMS FOR WEAKENING OF CASSAVA ROOT PARENCHYMA CELL WALLS
5.4 POTENTIAL APPLICATION OF DILUTE NAOH AND ALKALIPHILIC BACILLUS SPP. IN CASSAVA ROOT UTILISATION
6 CONCLUSIONS
7 REFERENCES
8 PUBLICATIONS, PRESENTATIONS AND POSTERS BASED ON THIS RESEARCH
