Sorghum is a key dietary staple, with starch making up 68%–75% of its composition. Despite its status as a slowly digestible starch compared to other grains, sorghum foods often have intermediate or high glycemic indices (GI) after processing. The GI measures how quickly a carbohydrate-containing food raises blood glucose levels, with lower values linked to reduced risks of metabolic syndromes such as diabetes and obesity. The modification of sorghum starch through heat-moisture treatment (HMT), a process which involves low moisture levels (10%–30%) and heating above the starch’s glass transition temperature, has been shown to reduce starch digestibility and the GI of food products.

A study funded by the DSI-NRF Centre of Excellence in Food Security examined the effects of infrared (IR) and microwave (MW) HMT on the functional and nutritional properties of different types of sorghum: white non-tannin, red non-tannin, and red tannin sorghum. The aim was to determine whether these treatments could improve the starch’s nutritional profile by reducing its digestibility and GI.

The researchers utilised three sorghum types produced commercially in South Africa: white non-tannin, red non-tannin, and red tannin sorghum. Sorghum meals were equilibrated to 25% moisture content before undergoing HMT using IR and MW energy at 250W for 15 minutes. The samples were then analysed for their thermal properties, protein and starch digestibility, pasting properties, and structural changes. The glycemic response of the starch was measured through an in vitro digestibility test, with key parameters such as rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) quantified. Protein digestibility was determined using pepsin-based assays.

Research key findings show that:

1. HMT increased the onset temperatures of starch gelatinisation, indicating improved thermal stability.
2. X-ray diffraction showed a partial shift from an A-type to a V-type crystalline pattern, with IR-treated samples exhibiting stronger crystallinity than MW-treated ones.
3. Estimated glycemic index (eGI) decreased in all sorghum types. For white non-tannin sorghum, eGI dropped from 86.50 (control) to 79.57 (IR-treated) and 82.49 (MW-treated).
4. Resistant starch (RS) content increased significantly, particularly in red tannin sorghum, which showed the highest RS after HMT.
5. Protein digestibility declined slightly after HMT. Native white non-tannin sorghum had the highest digestibility (61.39%), while red tannin sorghum had the lowest (23.78%).
6. Peak viscosity decreased across all sorghum types post-HMT, with IR-treated samples retaining slightly higher viscosity than MW-treated ones.
7. HMT reduced the water absorption index at high temperatures, with red tannin sorghum showing the lowest solubility due to tannin-protein interactions.

The research findings demonstrate that IR and MW HMT significantly improve the nutritional profile of sorghum by lowering starch digestibility and the GI. The IR-treated samples consistently showed better results compared to MW-treated ones in terms of RS content and thermal stability. Red tannin sorghum outperformed the other types in terms of RS content and GI reduction, attributed to its tannin’s enzymatic inhibitory effects.

The study highlights the potential of HMT-treated sorghum meals as healthier options for individuals at risk of metabolic disorders. These modifications make sorghum a promising candidate for functional foods targeting glycemic control and metabolic health.

The study concludes that HMT using IR or MW energy is an effective method for improving the functional and nutritional properties of sorghum. By reducing GI and increasing RS content, these treatments can enhance sorghum’s suitability as a dietary option for people with diabetes or obesity. Further research into consumer acceptability and scalability of these treatments is recommended.