Bioaccessibility of phenolic compounds and related antioxidant and health promoting properties of soured sorghum porridges

Abstract

The percentage of deaths due to non-communicable diseases (NCDs) such as diabetes, cancer, obesity and heart disease has been growing steadily in South Africa. Rapid urbanization and increased demand for convenience foods have driven changes in dietary choices towards those based on highly refined grains which has led to an increase in susceptibility to oxidative stress and NCDs among consumers. This brings into focus the importance of sorghum which is a cereal crop consumed in many parts of Africa and is well known as an important source of bioactive phenolic compounds with the potential to protect against diet-related NCDs. Phenolic compounds in sorghum occur mainly in bound forms which limits their extractability, bioaccessibility and subsequently their ability to exert health-promoting effects. Processing methods such as souring could release phenolics in sorghum from their bound forms and therefore enhance their bioaccessibility. This research studied the effect of souring using spontaneous fermentation and citric acid acidification on the bioaccessibility of phenolic compounds and related antioxidant and health promoting properties of sorghum porridges. Two type I non-tannin sorghums (red and white phenotype) were used. Soured sorghum porridges were prepared using spontaneous fermentation and exogenous acidification with citric acid. For determination of bioaccessibility of phenolic compounds, supernatants were obtained from sorghum porridges digested using a static simulated in vitro gastrointestinal digestion procedure. Total phenolic content (TPC) using the Folin-Ciocalteu assay, phenolic composition using Ultra Performance Liquid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry (UPLC-QToF-MS), esterase enzyme activity, ABTS and DPPH radical scavenging activity, Oxygen Radical Absorbance Capacity (ORAC), Nitric Oxide (NO) inhibition, Caco-2 cellular antioxidant activity as well as the capacity to protect against plasmid DNA oxidative damage were evaluated for the sorghum flours, porridges and supernatants of digested porridges. Phenolic compounds identified in the two sorghums were phenolic acids and flavonoids. The major phenolic acids class present were the hydroxycinammic acids. Flavonoids identified were flavanones (the major identified flavonoid class), flavonols, flavan-3-ols and flavones. Red sorghum flour, porridges and supernatants contained higher levels of total phenolic compounds compared to that for white sorghum.

Souring through spontaneus fermentation led to an increase in TPC and radical scavenging activity against ABTS, DPPH and ORAC values for the sorghum flours and resultant porridges. This increase was attributed to the acidic conditions imposed by the fermentation process combined with the action of esterase enzymes which facilitate ester bond hydrolysis and the release of bound phenolic compounds. Fermented red sorghum slurry exhibited higher esterase enzyme activity compared to that for white sorghum. On the other hand, acidification with citric acid had no effect on the TPC but produced more bioaccessible phenolic compounds compared to souring through fermentation. This souring method also enhanced the antioxidant activity in all the samples except for ABTS radical scavenging. Although souring had no significant effect on NO inhibition, red sorghum flours, porridges and supernatants of digested porridges had significantly higher NO inhibition capacity than those from white sorghum.

Simulated in vitro gastrointestinal digestion also enhanced the TPC and the radical scavenging properties of the porridges. This was attributed to the action of the enzymes used during the in vitro digestion process such as amylase, pepsin, pancreatin leading to breakdown of the food structure and enhanced extractability of phenolic compounds.

The flours, porridges and supernatants from both sorghum types exerted Caco-2 cellular antioxidant activity and protected the DNA from AAPH-induced oxidative damage. This is an indicator of the potential health promoting properties of the soured sorghum porridges by offering protection against diet-related NCDs.

This research has shown that souring, either through spontaneous fermentation or through exogenous acidification with citric acid can enhance the bioaccessibility of phenolic compounds and antioxidant properties of sorghum porridges. These soured sorghum porridges can therefore play a vital role in contributing towards efforts to combat and reduce the current rising trends of the occurrence and mortality from diet- and oxidative stress-related NCDs in Africa.