Functional and nutritional properties of infrared and microwave heat moisture-treated sorghum meals

Proximate analysis: Sorghum meal samples were analyzed for moisture, ash, and crude fat using procedures 925.10, 923.03, and 920.39, respectively, from the Association of Official Analytical Chemists (AOAC, 2019). Protein (N × 6.25) was determined using the Dumas combustion method.

Total starch content: Total starch content of the sorghum types was determined using the megazyme method. Total phenolic content: The total phenolic content was determined using the Folin-Ciocalteu method by Waterman and Mole (1994) and reported as catechin equivalent.

Tannin content: Tannin content was determined according to Price et al., (1978). In brief, sorghum meal (1g) was weighed into a 25 ml beaker. Concentrated HCl in methanol (10 ml of 1% v/v) was used for the tannin extraction process by stirring on a magnetic stirrer for 2 h. Vanillin-HCl reagent was added to the extracted tannin and incubated in the dark for 20 min.

Nitrogen solubility: The nitrogen solubility index was determined according to Ogundele et al., (2017). Meals (1 g) were mixed for 1 hr at 30°C in 20 ml of 0.1 M NaCl solution. After centrifuging (9500 x g, 15 min), the supernatant was filtered using Whatman No.1 filter paper. Two more times, the residue from the suspension was washed in 10 ml of 0.1 M NaCl solution at pH 7. After being frozen at -10°C for the entire night, the filtrate was freeze-dried using a 13KL Instruvac Lyophilizer at Midrand, South Africa. 

Water absorption capacity and water solubility index: Sorghum meal solubility was determined using the method described by Mapengo & Emmambux, (2020). Samples of (1 g, dry basis) meal were cooked in 10 ml of distilled water at two different temperatures, 50 °C and 91 °C in a shaking water bath at 150 rpm for 30 min, with the mixture being vortexed every 5 min. The sample solution was centrifuged at 9154.3 x g for 15 min. 

Pasting properties: Sorghum meal samples were pasted according to Wokadala et al., (2012) with modification using the rheometer with a starch cell (Physica MCR 101, Anton Paar, Germany). The samples were then held at 91°C for 15 min, followed by cooling from 91°C to 50°C at a rate of 5.5 °C/min and allowed to stand for 2 min.

X-ray diffractions crystallinity: XRD was conducted using PANalytical X’pert PRO (Eindhoven, Netherlands) on native, HMT-treated, and pasted samples as described by Mapengo et al., (2019) with slight modification. 

In vitro starch digestibility: In vitro starch digestibility of the pasted sorghum meals was done using the method described by Goñi et al., (1997).