Insight into the physicochemical characteristics and biological features of dietary polysaccharides extracted from palm kernel cake

Palm kernel cake (PKC), a major by-product of the palm oil industry, is rich in non-starch polysaccharides. In this study, two polysaccharide fractions, precipitated with acetic acid (PPA) and ethanol (PPE), respectively, were extracted from PKC using a 2 mol/L NaOH solution. The molecular weight, sugar composition, structural characteristics, morphology, antioxidant activity, as well as in vitro stimulated digestion were investigated in detail. The results revealed that due to its poor solubility of PPA in water, the detected molecular weight of PPA was only 2040 g/mol, which was significantly lower than that of PPE (65,300 g/mol). Although PPA and PPE had a similar sugar composition with varying contents, mannose was the predominant monosaccharide in both, accounting for 87.71% and 60.40%, respectively. Both PPA and PPE were primarily composed of crystalline mannan, consisting of mannopyranosyl units linked by (1→4)-β-glycosidic bonds, along with a small amount of lignin. PPA possibly contained a higher proportion of crystalline mannan, whereas PPE had a larger amount of arabinoxylan and galactomannan. Atomic force microscope revealed a stacked morphology for both PPA and PPE. PPA exhibited a higher scavenging rate against DPPH• and ABTS+• but a weaker HO• scavenging activity and reducing power compared with PPE. Within the polysaccharide concentration range of 0.5–5.0 mg/mL, PPA and PPB demonstrated the strongest scavenging activity against ABTS+•, with the highest scavenging rates exceeding 91%. However, PPA and PPB exhibited the weakest scavenging activity against HO•, with their highest HO• scavenging rates reaching only 44.91% and 55.86%, respectively. The antioxidant activities of both PPA and PPE were weaker than that of ascorbic acid. PPA remained almost stable in the in vitro simulated saliva fluid, while PPE exhibited weaker resistance to it. Both PPA and PPE exhibited weak resistance to the in vitro simulated gastric digestion fluids, but remained relatively stable in the in vitro simulated small-intestinal digestion fluid. The differences in physicochemical properties between PPA and PPE likely played an important role in their distinct biological activities. These findings suggest potential utilization of PKC in exploring dietary polysaccharides with favorable antioxidant activity and unique digestive characteristics.