Abstract
Underutilized crops such as taro are known to contain valuable bioactive components. The water-soluble non-starch polysaccharide (WS-NSP) component of taro corm (mucilage) has been reported to have promising uses in food (e.g., for the enhancement of product texture and to stabilize food emulsion systems) and in health (e.g., anti-hyperglycaemic, anti-inflammatory, anti-oxidation, and anti-metastatic activities) applications. Despite its promising potential, the WS-NSP of taro (T WS-NSP) remains an understudied and underutilized by-product of the taro industry. The extraction of WS-NSP from a starch-rich system is challenging since it relies on the use of a conventional extraction (CE) method, which is time consuming. Published studies that utilized the CE method have often reported low yield and poor quality WS-NSP due to co-extraction of unwanted components such as starch, unconjugated protein, and other insoluble materials. Implementing purification processes may result in material degradation and loss of the material’s functionality and bioactivity. Therefore, due to these drawbacks, the present study investigated a new extraction method, termed improved conventional extraction (ICE) method that utilized a freeze-thaw technique to overcome the drawbacks and limitations of CE. The yield, composition, structure, physicochemical properties, and bioactivities (i.e., prebiotic potential and ability to regulate the production of the pro-inflammatory cytokine IL-8) of T WS-NSP extracted using the CE and ICE methods were evaluated and compared. The overall aim of this study was to determine whether the ICE method is able to provide better characteristics, properties, and bioactivities of the extracted biomaterial compared to CE.
The extraction methods influenced the yield, composition, structure, and properties of the T WS-NSP extract where T WS-NSP (ICE) had significantly (p ≤ 0.05) higher yield, neutral and acidic sugar fractions, lightness (L*), particle size and ζ-potential than the T WS-NSP (CE). Conversely, the T WS-NSP (CE) had significantly (p ≤ 0.05) higher amounts of protein, ash, and starch than the T WS-NSP (ICE). Both of the T WS-NSP extracts (CE and ICE) contained the monosaccharides arabinose, mannose, glucose, and galactose, with galactose being the main sugar unit. Rhamnose was only detected in the T WS-NSP (CE). The FTIR spectrums of the T WS-NSP (CE and ICE) were comparable but varied in their molecular orientation as amorphous (T WS-NSP CE) and semi-crystalline (T WS-NSP ICE). The complex and branched structural configuration of the T WS-NSP (CE and ICE) was evident in both of the 1H NMR and 13C NMR spectra. The T WS-NSP (ICE) had higher thermal stability (̴ 240 °C) than the T WS-NSP (CE) (̴ 200 °C). The dispersions (0.10 to 2.50 %, w/v) of the T WS-NSP (CE and ICE) exhibited pseudo-plasticity or shear-thinning behaviour (n < 1) characteristic of non-Newtonian fluid. The swelling capacity, water and oil holding capacities, emulsifying and foaming capacities, glucose dialysis retardation index, and the ability to inhibit the activity of the enzyme α-amylase were significantly (p ≤ 0.05) higher in the T WS-NSP (ICE) than in T WS-NSP (CE).
The T WS-NSP (CE and ICE) were highly indigestible as demonstrated by the high (88.53 % and 95.93 %) material recovery following the in vitro simulated digestion. The digested T WS-NSP contained the monosaccharides arabinose, galactose, glucose, and mannose with FTIR spectra that was comparable with the undigested T WS-NSP extract. The digestion process caused the semi-crystalline structure of the T WS-NSP (ICE) to be highly amorphous, similar to the digested T WS-NSP (CE). Both the undigested and digested T WS-NSP extracts showed a lack of or minimal cytotoxicity in HT-29 cells (viable cells ≥ 85 %) at concentrations in the range of 0.5 mg/ mL to 7.5 mg/ mL.
The T WS-NSP demonstrated a prebiotic effect by enhancing the growth of the probiotic bacteria Lactobacillus acidophilus, Bifidobacterium breve, and Bifidobacterium infantis. It also demonstrated immunomodulatory activity through its ability to regulate the production of the pro-inflammatory cytokine IL-8 by the unstimulated and TNF-α stimulated HT-29 cells without and in the presence of a necrotising enterocolitis (NEC)-positive associated pathogen (Klebsiella oxytoca) isolated from a baby. Both the prebiotic effect and immunomodulatory activity of the T WS-NSP were influenced by the extraction method and the digestion process. In the absence of the pathogenic bacterium, the digested T WS-NSP (25.02 %), the probiotic B. breve (45.61 %), and their synbiotic mixture (55.83 %) exhibited the highest IL-8 down-regulation capacities. In the presence of NEC-positive associated pathogen K. oxytoca, which up-regulated the IL-8 production, the digested T WS-NSP (24.45 %), the probiotic B. breve (37.83 %) and their synbiotic mixture (45.02 %) demonstrated the highest IL-8 down-regulation abilities.
Overall, the findings of the study suggest that the ICE method can be used as an alternative extraction method to the CE method for the extraction of T WS-NSP with enhanced qualities, properties, and biological activities. The properties and biological activities demonstrated by the T WS-NSP (ICE) suggest its potential use as a prophylactic agent for NEC and, in general, as a functional ingredient in the development of food products for gut health and immunity.