Abstract
The general objective of this PhD research was to investigate the impact of emerging non-thermal technologies, i.e., High Hydrostatic Pressure (HHP) and Pulsed Electric Fields (PEF), on starch properties and subsequently the flour functional properties of cassava flours. Six Philippine varieties of different parental lines and agronomical characteristics were acquired and characterized first for this study. Results revealed stronger influence of intervarietal differences than intravarietal variations, whereby non-starch components, thermal properties, and volatile profile strongly differentiated the six Philippine varieties. Starch was the major component of all cassava varieties ranging from 75.7–92.2% and served as focal interest for non-thermal modification.
Key HHP processing variables, i.e., pressure (0.10 or untreated, 300, 400, 500, and 600 MPa), holding time (HT; 10 and 30 min) and flour concentration (FC; 10, 20, and 30%), known to drive modification in starch was applied on flour at different combinations. In general, 600 MPa consistently transformed the crystalline starch into an amorphous one. HHP-induced gelatinisation led to enlarged starches with loss of birefringence, reduced relative crystallinity percentage, and changes in short-range order. The three-way interaction between the process variables was evident in the significant progressive rise in onset gelatinisation temperature and degree of gelatinisation, and the decline in gelatinisation enthalpy from 500 to 600 MPa with decreasing FC and increasing HT.
PEF treatment was applied at mild electric filed strengths (1, 2, and 4 kV/cm) combined with elevated levels of specific energy input (250 – 500 kJ/kg). At 2 and 4 kV/cm with specific energy of 450 – 500 kJ/kg, PEF was consistently able to alter cassava starch’s external structure with swelling and disintegration albeit some granules remained intact. This led to disruption of internal crystalline structure, supported by progressive loss of birefringence and significantly lower absorbance ratio at 1047/1022 cm-1, which is characteristic of gelatinisation. These physical and microstructural changes of the inherent starch promoted the shift of gelatinisation temperatures to higher temperature and reduced the gelatinisation enthalpy.
In brief, both treatments induced gelatinisation, albeit at different intensity. Granular disintegration and aggregation were more substantial in PEF treated flour samples but showed less uniform impact on starch granules than HHP. These significant changes on the inherent starch led to easier hydrolysis by digestive enzymes, causing increased percentage of rapidly digestible starch and decreased resistant starch fraction. For HHP-treated flours this was initiated at 500 MPa–10%FC–30 min HT combination until 600 MPa and only at 2–4 kV/cm with 450–500 kJ/kg for PEF-treated flours. Whereas the slowly digestible starch fraction can be increased only at 500 MPa for HHP but did not significantly change with the PEF treatments. Interestingly, the rate of starch digestion did not significantly change during the intestinal phase for both technologies.
Native or untreated flours were compared to two modified flours from both the HHP and PEF (representing a highly and less gelatinised flour) on their volatile retention performance of commercial mango flavor (100 ppm). The new amorphous starch structures significantly reduced the overall volatile retention capacity of the cassava flours compared to native counterparts. Nevertheless, monoterpenes, sesquiterpenes, and related hydrocarbons were progressively retained during storage and showed higher increase in retention after 48 hr than native flour for both technologies. While more alcohols were retained during storage by the highly gelatinised flour, compared to the native and the less gelatinised samples. Indeed, retention and stability during storage was influenced by the new properties of starch and the flour as a whole.
This research is first to describe the starch-related changes in cassava flour when subjected to HHP and PEF. The granular alterations and microstructural disruption associated with non-thermal gelatinisation of starches, influenced the functional properties of flour i.e., enhanced starch digestibility and lower but varied ability to bind volatiles. Moreover, through manipulation of the processing factors, cassava flours with tailored properties can be created.