Sorghum, a hardy grain known for its drought resistance, has long been a dietary staple in many parts of the world. Recent research from the University of Pretoria and Wroclaw University of Economics and Business, published in Food Chemistry in 2025, reveals groundbreaking methods to enhance sorghum’s nutritional value.
By combining microwave heat moisture treatment (MWHMT) with phenolic extracts from sorghum bran, scientists have successfully reduced starch digestibility, increased resistant starch content, and lowered the glycaemic index (GI) of sorghum-based foods.
Sorghum Nutritional Challenges and Health Benefits Explained
Sorghum is rich in phenolic compounds like tannins and flavonoids, which are concentrated in its bran layer. Phenolic compounds are naturally occurring chemicals in plants known for their antioxidant properties.
They play a vital role in protecting plants from environmental stress and have been linked to human health benefits, such as reducing inflammation and slowing digestion. In sorghum, these compounds interact with proteins and starch, forming complexes that hinder enzyme activity.
However, traditional processing methods, such as milling, often remove the bran to improve texture and shelf life. This strips away beneficial phenolics, leaving the starch in the endosperm highly digestible. Rapid starch digestion leads to high GI values, which can spike blood sugar levels.
Glycaemic index (GI) measures how quickly a carbohydrate-containing food raises blood glucose levels. High-GI foods (like white bread) cause rapid spikes, while low-GI foods (like legumes) provide slower, sustained energy release.
Additionally, sorghum’s protein matrix, dominated by kafirin proteins, forms tight structures that resist breakdown by digestive enzymes, further limiting its nutritional appeal. Kafirin is the primary storage protein in sorghum, similar to gluten in wheat.
Unlike gluten, however, kafirin forms disulfide bonds (strong chemical links between sulfur atoms) during cooking, creating a rigid network that shields starch from digestion. While this slows starch breakdown, it also reduces protein availability, making sorghum less nutritious as a protein source.
The 2025 study addresses these challenges by reintroducing phenolic extracts into decorticated (bran-removed) sorghum meal and applying microwave heat moisture treatment. This approach aims to create a food product that retains the health benefits of phenolics while improving starch and protein digestibility profiles.
How Microwave Treatment Enhances Sorghum Nutrition
The research team used white sorghum grains (Macia variety), decorticated to remove 30% of the bran. Decortication refers to the mechanical removal of the outer bran layer, which is rich in fiber and phenolics but often discarded to improve texture. Phenolic extracts were obtained from two red sorghum varieties: a non-tannin type (GM) and a tannin-rich type (GH).
The bran from these varieties was soaked in 80% ethanol, centrifuged (spun at high speed to separate solids), and freeze-dried to create concentrated extracts.
These extracts were then mixed with decorticated sorghum meal at 6% of its weight—a ratio reflecting natural phenolic levels in whole sorghum.
Next, the team applied microwave heat moisture treatment (MWHMT) to the samples. MWHMT is a thermal processing technique that uses microwave energy to heat materials with controlled moisture levels.
Unlike conventional heating, microwaves penetrate the food evenly, modifying starch and protein structures without destroying their physical form.
The sorghum meal was moistened to 25% water content and treated in a microwave reactor at 250 watts for 15 minutes, reaching temperatures of 110°C. After treatment, the meal was dried to 7% moisture to simulate real-world storage conditions. To evaluate the effects of these treatments, the researchers conducted several tests:
- Pasting Properties: Measured using a Rapid Visco Analyzer (RVA) to track changes in starch gelatinization. Gelatinization is the process where starch granules absorb water and swell when heated, contributing to food texture.
- X-Ray Diffraction (XRD): A technique that analyzes the crystalline structure of starch. Crystalline regions in starch are more resistant to digestion than amorphous (non-crystalline) areas.
- Fourier-Transform Infrared Spectroscopy (FTIR): Examined protein secondary structures (α-helix and β-sheet), which influence digestibility. α-Helix and β-sheet are common protein folding patterns; β-sheets form rigid, enzyme-resistant aggregates.
- In Vitro Digestibility: Simulated human digestion using pepsin (stomach enzyme) and α-amylase (salivary/pancreatic enzyme) to measure starch hydrolysis and protein breakdown.
Sorghum Starch Digestibility Reduced by Microwave Methods
There is a several microwave methods to reduced starch digestibility which are given below:
1. Starch Digestibility Dropped by Nearly 50%: The study revealed dramatic reductions in starch hydrolysis—the process by which starch breaks down into glucose.
Untreated sorghum meal had a hydrolysis index (HI) of 69.89%, meaning its starch was quickly converted into sugar.
However, adding non-tannin phenolic extracts lowered the HI to 59.27%, while tannin extracts reduced it further to 54.74%. Microwave treatment amplified these effects: HI values plummeted to 35.99% for non-tannin samples and 36.18% for tannin samples.
For context, foods with an HI below 55 are classified as low-GI, making treated sorghum ideal for managing blood sugar levels.
2. Resistant Starch Content Surged to Over 55%: Resistant starch (RS) refers to starch that escapes digestion in the small intestine and reaches the colon, where it feeds beneficial gut bacteria. It is associated with improved gut health, reduced inflammation, and sustained energy release.
Untreated sorghum contained 28.21% RS. With non-tannin phenolics, RS rose to 31.97%, and tannin phenolics pushed it to 34.65%. After microwave treatment, RS levels skyrocketed to 54.57% (non-tannin) and 55.87% (tannin)—values comparable to legumes and undercooked pasta.
This increase is attributed to phenolics forming physical barriers around starch granules, blocking enzyme access. Tannins, with their larger molecular size, were particularly effective at stabilizing these structures.
3. Protein Digestibility Presented a Trade-Off: While starch digestibility improved, protein digestibility faced challenges. Untreated sorghum had a protein digestibility of 54.77%.
Adding non-tannin phenolics left this value unchanged, but tannin phenolics caused a drastic drop to 18.14%. Microwave treatment further reduced digestibility, with non-tannin samples falling to 37.71% and tannin samples to 16.31%.
This decline is linked to tannins binding irreversibly to proteins, creating rigid aggregates that resist enzymatic breakdown.
4. Structural Changes Explained the Results: Light microscopy revealed that microwave treatment caused starch, proteins, and phenolics to form dense aggregates. These aggregates physically blocked digestive enzymes, reducing starch hydrolysis.
X-ray diffraction showed that untreated sorghum had 30.69% crystallinity (A-type starch), characterized by tight, orderly molecular arrangements. After microwave treatment, crystallinity increased to 34.94% in untreated samples and 35.29% in tannin samples, stabilizing starch against digestion. FTIR analysis highlighted shifts in protein structures:
- α-helix content decreased from 83.10% to 69.43% in tannin-treated samples
- β-sheet structures rose from 16.90% to 30.57%. β-sheets create rigid, enzyme-resistant formations, explaining the drop in protein digestibility.
Health Benefits of Low-GI Sorghum for Diabetes Management
The interactions between phenolics, starch, and proteins are central to these findings. Phenolics like tannins form hydrogen bonds with amylose, a linear component of starch. Amylose typically forms helical structures that trap lipids or phenolics, reducing starch swelling and enzyme access.
This aligns with earlier studies where compounds like ferulic acid lowered starch viscosity in maize. Meanwhile, tannins crosslink kafirin proteins through covalent bonds, forming indigestible complexes. While this reduces protein availability, it also stabilizes starch structures, enhancing resistant starch content. Microwave treatment plays a dual role.
- First, it reorganizes starch molecules into V-type crystalline structures (detected at 19.9° in XRD scans), which resemble helical coils that resist enzymatic breakdown.
- Second, it denatures proteins, disrupting disulfide bonds in kafirin and promoting β-sheet formation. These structural changes explain the synergy between phenolics and microwave processing.
For consumers, this means sorghum-based foods could help manage diabetes and obesity. Low-GI diets stabilize blood sugar, while resistant starch promotes satiety and feeds beneficial gut bacteria. However, the trade-off in protein digestibility suggests that sorghum should be paired with protein-rich foods like legumes for balanced nutrition.
Practical Applications for the Food Industry
The study’s findings open doors for innovative food products. For instance, treated sorghum flour could replace refined maize flour in gluten-free bread, noodles, and snacks, offering a healthier alternative for diabetics.
Phenolic-rich bran extracts might also serve as natural additives in cereals and energy bars, boosting antioxidant content without artificial ingredients.
Farmers and policymakers could focus on breeding sorghum varieties with higher phenolic content, such as the tannin-rich GH type used in the study. In regions like Sub-Saharan Africa and South Asia, where diabetes rates are rising, promoting sorghum-based diets could improve public health outcomes.
Despite these opportunities, challenges remain. Industrial-scale microwave processing requires optimization to ensure consistency and cost-effectiveness. Additionally, blending sorghum with protein sources like cowpea could mitigate digestibility limitations.
Future Research Directions
While the study provides robust in vitro data, human trials are needed to confirm blood glucose responses. Researchers might also explore enzyme engineering to break down tannin-protein complexes without affecting resistant starch. Further analysis of interactions between phenolics, fiber, and lipids could uncover additional health benefits.
Conclusion
This pioneering research demonstrates that microwave heat moisture treatment and phenolic extracts can transform sorghum into a low-GI, high-resistant-starch superfood. By harnessing natural compounds and modern technology, the study offers a blueprint for healthier, sustainable food systems.
Though protein digestibility remains a hurdle, the findings pave the way for innovative solutions to global nutrition challenges. As demand for functional foods grows, sorghum could play a vital role in combating diet-related diseases and improving health outcomes worldwide.
Power Terms
Decorticated: The process of removing the outer layer (bran) of a grain, such as sorghum, to leave the inner endosperm. This is done to reduce fiber content or focus on the starch-rich part. Decortication improves processing but may lower phenolic content. Example: White sorghum meal is decorticated to study starch-phenolic interactions.
Microwave Heat Moisture Treatment (MWHMT): A method where grains or flour are treated with controlled microwave energy and moisture to alter starch structure. It enhances resistant starch content and reduces digestibility. Importance: Improves nutritional properties like lower glycaemic index. Example: Sorghum meal treated with MWHMT showed reduced starch hydrolysis.
Non-Tannin Phenolics: Plant compounds like phenolic acids or flavonoids that lack tannins’ protein-binding ability. Found in sorghum bran, they act as antioxidants. Importance: They slow starch digestion by forming barriers around starch. Example: Added to sorghum meal to study their effect on digestion.
Tannin Phenolics (Condensed Tannins): High-molecular-weight compounds in plants that bind proteins, reducing digestibility. Found in sorghum with pigmented testa. Importance: Lower starch and protein digestion by forming complexes. Example: Tannin extracts from red sorghum reduced protein digestibility in the study.
Resistant Starch (RS): Starch that resists digestion in the small intestine, reaching the colon for fermentation. Types include physically trapped starch (RS1) or retrograded starch (RS3). Importance: Lowers glycaemic response. Formula: RS (%) = (100 − glucose released at 180 min) × 0.9.
Glycaemic Index (GI): A scale (0–100) ranking how quickly a food raises blood glucose. Low-GI foods digest slowly. Importance: Diets with low GI reduce diabetes risk. Example: MWHMT sorghum with tannin phenolics had a lower GI.
Starch Hydrolysis Index (HI): Measures starch breakdown into glucose over time, compared to a reference (e.g., white bread). Formula: HI = (Area under sample’s glucose curve / Area under reference’s curve) × 100. Importance: Predicts GI.
Alpha-Amylase: An enzyme in saliva and pancreas that breaks starch into sugars. Inhibited by phenolics, slowing digestion. Importance: Sorghum’s phenolics reduce its activity, lowering glucose release.
Protein Matrix: A network of proteins (e.g., kafirin in sorghum) surrounding starch granules. Importance: Limits enzyme access, slowing starch digestion. Example: Sorghum’s protein matrix contributes to low starch digestibility.
Phytochemicals: Bioactive compounds in plants, like phenolics, with health benefits. Example: Sorghum’s 3-deoxyanthocyanidins. Importance: Antioxidants that combat oxidative stress.
3-Deoxyanthocyanidins: Unique pigments in sorghum with antioxidant properties. Unlike common anthocyanins, they lack a hydroxyl group. Importance: Contribute to sorghum’s health benefits and color.
Pericarp: The outer bran layer of a grain, rich in phenolics and fiber. Example: Sorghum’s pericarp contains up to six times more phenolics than the whole grain.
Testa: A seed layer between pericarp and endosperm. In tannin sorghum, it’s pigmented and contains condensed tannins. Importance: Determines sorghum’s tannin content.
Endosperm: The starchy core of a grain, providing calories. Sorghum’s endosperm has slow-digesting starch due to its protein matrix.
Germ: The nutrient-rich embryo of a grain, containing fats, vitamins, and minerals. Often removed during milling to extend shelf life.
Antioxidant Activity: The ability to neutralize harmful free radicals. Sorghum phenolics, like tannins, exhibit high antioxidant activity, protecting against diseases.
Proximate Analysis: A method to determine macronutrients (protein, fat, moisture, ash) in food. Example: AOAC methods used to analyze sorghum meal composition.
Dumas Combustion Method: A technique to measure protein content by burning samples and analyzing nitrogen. Formula: Protein = Nitrogen × 6.25. Importance: Faster than traditional Kjeldahl method.
Folin-Ciocalteu Method: A chemical assay to quantify total phenolics using a color reaction. Results are expressed as catechin equivalents (mg CE/g).
X-Ray Diffraction (XRD): Analyzes crystal structures in starch. A-type (common in cereals) and V-type (amylose-lipid complexes) patterns indicate starch digestibility.
Fourier-Transform Infrared Spectroscopy (FTIR): Detects molecular bonds (e.g., protein secondary structures). Example: Measured α-helix and β-sheet changes in sorghum proteins.
In Vitro Digestibility: Simulates human digestion in lab settings. Example: Starch/protein digestibility tests predict how nutrients are absorbed.
Rapidly Digestible Starch (RDS): Starch digested within 20 minutes, causing quick glucose spikes. Formula: RDS = Glucose at 20 min × 0.9.
Slowly Digestible Starch (SDS): Starch digested between 20–120 minutes, providing sustained energy. Formula: SDS = (Glucose at 120 min − Glucose at 20 min) × 0.9.
Aggregate Formation: Clusters of denatured proteins and phenolics that limit enzyme access to starch. Example: MWHMT sorghum with tannins formed aggregates, increasing resistant starch.
Reference:
Baah, R. O., Duodu, K. G., Harasym, J., & Emmambux, M. N. (2025). Nutritional and functional properties of decorticated and microwave heat moisture treated white sorghum meal with added non-tannin and tannin phenolic extract. Food Chemistry, 143261.
