The broad objective of this research was to study and characterize different sweetpotato cultivars from Papua New Guinea (PNG) and Australia for their physicochemical, functional, digestibility and processing properties. The study was conducted in two phases. In the first phase, twenty five sweetpotato cultivars were studied, and in the second phase, a selected number of cultivars were processed using extrusion. Both phases involved characterisation for physical, chemical, functional and digestibility properties.
The first phase of the study was guided by the wide range of sweetpotato cultivars in the Pacific region. The cultivars, white-, orange-, cream-, and purple-fleshed, and with dry matter from 15-28 g/100g, were obovate, oblong, elliptic, curved, irregular in shape, and essentially thin-cortexed (1-2 mm). Flour yield was <90 g/100g solids. Most cultivars had amylose content <30%, while the protein contents were <8%. The potassium content ranged from 605-1405 mg/100g solids, while the other major minerals measured in the sweetpotato cultivars included sodium, calcium and phosphorus. The sugar contents varied from 0-8 g/100g solids, and sucrose was the main sugar, especially in the orange-fleshed cultivars.
The cultivars differed in their pasting properties, with four classes of shear-sensitive behaviours; highly shear-thinning (HSS), moderately shear-thinning (MSS), slightly shear-thinning (SSS) and shear-thickening (STK). Extended rehydration for ~16 hr in water slightly changed the pasting behaviours for the STK cultivars (L135, L49, Magenta Gold) to SSS behaviours. Substituting water with 0.05M silver nitrate (AgNO3) solution as the dispersing medium marginally affected the shear sensitivity. The AgNO3 treatments generally increased the peak, trough and final viscosities possibly due to inactivation of the endogenous α-amylase, which could have partially hydrolysed starch to low molecular weight dextrins and/or glucose during heating in Rapid Visco-Analyser (RVA).
Differential scanning calorimetry (DSC) showed single-stage gelatinisation behaviours, with cultivar-dependent temperatures (61-84oC) and enthalpies (12-27 J/g dry starch). Scanning electron microscopy (SEM) showed heterogenous granule shapes (oval-, round- and angular), and X-ray diffraction (XRD) revealed A-type diffraction patterns in the cultivars, with four distinct peak intensities at 15.0° (5.9Å), 17.0° (5.2Å), 17.9° (4.9Å) and 22.8° (3.9Å) 2-theta angles. Studies on the sweetpotato flour and starches revealed no significant differences in the morphological and crystallinity properties between the state (starch or flour), time of scan and either conditioned (48 hr over saturated potassium carbonate) or non-conditioned samples. The conditioning did not change the d-spacing values and crystallinity (~30%) levels.
Digestibility properties were cultivar-dependent (p<0.05), and the cultivars all displayed monophasic digestograms. Modelling using the modified first-order kinetic (MFOK) model adequately described (r2 >0.99, p<0.05) the digestograms, and the rate and extent of starch digestion were obtained. Irrespective of the modelling approach (gastric-pancreatic, GP; pancreatic, P; and gastric-pancreatic enzyme blank, GPEB) used, the cultivars showed high glycemic index (GIavg) and glycemic load (GL) properties with values of >60% (60g/100g) and >40% (40g/100g solids) respectively. Digestibility properties were affected by non-starch components in the flour, particularly sugars (e.g. glucose and maltose). Some cultivars were not completely digested, and the level of undigested starch, possibly resistant starch (RS) of type-1, type-2 and/or both were calculated to be as high as 30%. Alpha-amylases from porcine pancreas (PPA), Aspergillus oryzae (AOAA) and MegazymeMT thermostable (MTAA) were not significantly (p>0.05) different in their starch digestion activities.
Four of the sweetpotato cultivars (Beerwah Gold II, Northern Star, Snow White, L49) were studied for their extrusion behaviours in a co-rotating twin-screw extruder at three moisture (30, 35, 40%) and screw speed (150, 220, 300 rpm) levels. Low moisture increased the die pressure (2-6 bar) and specific mechanical energy (280-600 kJ/kg) of the extruder. Physicochemical, functional and digestibility properties of the extrudates were dependent on extrusion conditions, and cultivar-specific. Extrusion moisture increased the longitudinal expansion (15-30 m/kg) of the extrudates, which were almost completely gelatinised (100% degree of gelatinisation). In-vitro starch digestion revealed that gastric digestion in the extrudates ranged from 8-18 g/100g dry starch, while the rate of starch digestion was 3.0-3.7 min-1. Gastric digestion in the non-extrudates was from 2-11 g/100 g dry starch, with the rate of starch digestion being 0.1-0.8 min-1. Extrusion also affected the extrudate colour, with the ΔL* and hue angle (HA) values decreasing, while the Δa*, Δb*, total colour change (ΔE), chroma (CR), and browning indices (BI) were increased, possibly due to browning reactions (e.g. carotenoids) or caramelisation of sugars and phenolics.
Carotenoid properties of the extrudates and non-extrudates, studied using standard spectrophotometry and high performance liquid chromatography (HPLC) methods, revealed the cultivars differed (p<0.05) in the total carotenoids and β-carotene contents, and the Original Beauregard cultivar had the highest carotenoids contents among the cultivars. The spectrophotometry (84-1720 μg/g solids) method generally over-estimated the total carotenoid content compared to the more specific HPLC (23-355 μg/g solids) method, possibly because of the relatively non-specificity of the former. With the extruder and screw configuration used, extrusion at 40% moisture and 300 rpm screw speed maximally retained carotenoids at more than 80%. Generally, processing such as extrusion stabilizes sweetpotato for increased utilization, but large losses of carotenoids, for example, can result. The critical extrusion factors (temperature, moisture content and screw speed) to optimize carotenoid retention in sweetpotato for maximum health benefits were identified.
Sweetpotato minerals are important in the nutrition and health properties of the root crop. Using the extruded and non-extruded sweetpotato, mineral bioaccessibility of the cultivars was investigated by comparing minerals released during in-vitro starch digestion with the total elemental contents. Inductively Coupled Plasma with Optical Emission Spectroscopy (ICP-OES) was used for the analysis. In both gastric and pancreatic stages, potassium, calcium and sodium were readily accessible and released. However, the data obtained were inconsistent, and there were no time-dependent behaviours. Recommendations are made for future studies on mineral bioaccessibility in sweetpotato.
This research has shown diverse properties of sweetpotato cultivars available in PNG and Australia. This is the first study on such a range and number of cultivars, and the information generated will contribute to strategies to maximize utilisation of sweetpotato in view of its health and nutrition benefits. In addition, the information from the present research will guide transfer of processing and analytical skills to develop sweetpotato food technology in the Pacific region and worldwide. Amongst others, the present study identifies seasonal and environmental effects, as areas for further studies, on sweetpotato in the Pacific region.