Role of millets in pre-diabetes and diabetes: effect of processing and product formulation

Abstract

The incidence of pre-diabetes and diabetes has been increasing recently worldwide and considered as a major growing non-communicable disease. Millets are eco-friendly crops which could sustain extensive climatic conditions. The productivity of millets had increased in recent years to meet the nutritional needs of the increasing global population. The factors which affect the starch digestibility pattern in millets are protein, fat, resistant starch, dietary fibre, and anti-nutrients. However, the interplay of these components also affects the starch digestibility pattern in millets during various processing methods such as thermal, non-thermal, chemical, and their combination. The incorporation of native and processed millet in food products varies the in-vitro and in-vivo glycaemic index. The current study further discusses the potential applications of millet in food formulations for pre-diabetic and diabetic population. Hence the appropriately processed millets could be a suggested as a suitable dietary option for pre-diabetic and diabetic population.

Introduction

Millets are ancient crops cultivated in sub-Saharan Africa and Asia due to their drought tolerant capacity (Meena et al. 2021). It requires lesser irrigation and maturation time when compared to other cereal grains. It belongs to C4 group of plants which utilizes more carbon dioxide to release more oxygen and hence called as ‘environmentally friendly crops’. The millets are considered to have great importance due to their agro-industrial demand and nutritive value (Kumar et al. 2018). The global area under cultivation for millets had declined by 25.71% from 1961 to 2018. But recently due to the increased consciousness in health and awareness of millet benefits, the global millet production increased by 30,089,625 tonnes in 2021 (FAOSTAT 2021). The productivity of millets showed an increasing trend in recent years in India followed by Nigeria, Sudan, and other countries. Also, the millets are the sixth highest yielding cereal crop in the world (FAOSTAT 2021).

Diabetes mellitus is an endocrine disorder mainly characterized by hyperglycaemia were depending on the action of insulin, it is classified as type-1 (deficient insulin production) and type-2 (combination of insulin resistance and action) (American Diabetes Association 2010). The pre-diabetes is an intermediate hyperglycaemia characterized by fasting blood glucose ranging between 100 and 125 mg/dl, oral glucose tolerance test ranging between 140 and 199 mg/dl and HbA1C ranging between 5.7 and 6.4%. In 2021, the global prevalence of diabetes was estimated to be 537 million and projected to increase up to 783 million by 2045. The global prevalence of impaired glucose tolerance in 2021 was around 10.6% and estimated to increase up to 11.4% by 2045 (Sun et al. 2022a). The greater number of deaths related to diabetes are seen in lower and middle income countries where the major amount of energy consumption are limited to cereal grains such as refined rice, wheat, and maize (Sun et al. 2021; Anitha et al. 2021). Hence the diversification of cereal grains can be expanded to millets in developing countries to prevent pre-diabetes and diabetes mellitus as the millet carbohydrates are especially known for their reduced starch digestibility profile which lowers the blood glucose levels in the body (Anitha et al. 2021; Saleh et al. 2013,2013). Hence, this review focuses on (1) studying the millet grain components and effect of processing on glycaemic control (2) The potential of millet incorporated food formulations in diabetic and pre-diabetic population.

Nutritional content of millets

The millets are round and small-seeded grains belonging to Poaceae family. The major millets cultivated in Africa and Asia are sorghum (Sorghum bicolor), pearl millet (Pennisetum glaucum), finger millet (Eleusine coracana), proso millet (Panicum miliaceum), foxtail millet (Setaria italica), kodo millet (Paspalum scrobiculatum), barnyard millet (Echinochloa frumentacea), and little millet (Panicum sumatrense) (Kumar et al. 2018; Saleh et al. 2013). Millets are a major source of macro and micro nutrients and nutritionally superior to other cereal grains such as rice and wheat (Table 1). They contain around 52.9–60.6% total starch, 6.2–14.1% protein, 1.49–5.43% fat, 1.39–8.0% ash, and 10.22–18.90% total dietary fibre (Bora et al. 2019a; Longvah et al. 2017). Finger millet (364 mg/ 100 g) contains higher amount of calcium when compared to other cereal grains. The iron content in millets ranges between 50.36 and 1.26 mg/100 g (Longvah et al. 2017; Verma et al. 2015; Sharma et al. 2017). The free polyphenolic contents of millets ranged between 87 and 2518 µg/g were the kodo millet had the highest. The bound phenolic content of the millets ranged between 1107 and 7570 µg/g. The polyphenol present in the millets are known to reduce oxidative stress and reduce degenerative diseases (Bora et al. 2019a). Millets contains more than 75% of unsaturated fatty acids were the linoleic acid (18:2) was highest in all the millets (Bora et al. 2019a).

Table 1.

Nutritional content of whole grain cereals (Longvah et al. 2017; Bora et al. 2019a; Patil et al. 2015; Verma et al. 2015; Sharma et al. 2017; Sharma and Gujral 2019a)

Cereal grains	Total starch (%)	Protein (%)	Lipid (%)	Ash (%)	Total Dietary fibre (%)	Iron (mg/100 g)	Calcium (mg/100 g)
Wheat	57.53 ± 1.86	10.59 ± 0.60	1.47 ± 0.05	1.42 ± 0.19	11.23 ± 0.77	3.97 ± 0.78	39.36 ± 5.65
Rice	71.31 ± 1.91	9.16 ± 0.75	1.24 ± 0.08	1.04 ± 0.18	4.43 ± 0.54	1.02 ± 0.35	10.93 ± 1.79
Sorghum	59.70 ± 1.70	9.97 ± 0.43	1.73 ± 0.31	1.39 ± 0.34	10.22 ± 0.49	3.95 ± 0.94	27.60 ± 3.71
Pearl millet	60.6 ± 0.7	14.1 ± 0.0	5.43 ± 0.64	2.1 ± 0.1	11.49 ± 0.62	6.42 ± 1.04	27.35 ± 2.16
Finger millet	63.1 ± 1.4	6.2 ± 0.1	1.92 ± 0.14	8.0 ± 0.0	18.90 ± 0.35	4.62 ± 0.36	364 ± 58.0
Little millet	55.6 ± 1.3	7.9 ± 0.0	1.49 ± 0.01	4.2 ± 0.0	14.90 ± 0.54	8.18 ± 0.01	22.02 ± 0.01
Foxtail millet	55.1 ± 0.2	10.5 ± 0.1	2.55 ± 0.13	3.5 ± 0.0	15.20 ± 0.40	2.34 ± 0.46	15.27 ± 1.28
Banyard millet	55.8 ± 2.9	10.0 ± 0.1	2.02 ± 0.06	4.3 ± 0.1	14.70 ± 0.44	6.91 ± 0.11	23.16 ± 1.50
Kodo millet	52.9 ± 0.3	6.2 ± 0.0	3.6 ± 0.03	3.6 ± 0.0	17.20 ± 0.23	50.36 ± 0.44	24.83 ± 0.17
Proso millet	64.5 ± 0.7	10.9 ± 0.1	3.89 ± 0.35	8.0 ± 0.0	13.60 ± 0.59	1.26 ± 0.44	16.06 ± 1.54

Millet grain components affecting the starch digestibility

Resistant starch

RS is a fraction of starch which resists hydrolysis in the stomach and small intestine leading to microbial fermentation in the large intestine (Tian and Sun 2020). The RS is known to reduce the starch digestibility by restricting the enzyme activity (Kaimal et al. 2021). They are mainly classified into five types RS 1, RS 2, RS 3, RS 4, and RS 5. The RS 1 is enclosed within a food matrix which is present intact within cereal grains and legumes when ingested prevents the starch from being hydrolysed by digestive enzymes due to cell wall barrier effect and making it resistant (Kraithong et al. 2022). The RS 2 is present in high amylose maize starch, raw potato, and green bananas. The compact and crystalline nature of RS 2 makes it inaccessible to digestive amylases (Jiang 2013). The RS 3 is present in retrograded starches. The retrogradation of starch happens when the coiled amylose recrystallizes after gelatinization. The coiled amylose is leached out during the gelatinization process and it re-aligns into a compact double helical crystalline structure which is resistant to enzymatic digestion (Raigond et al. 2015; Kaimal et al. 2021). RS 4 relates to chemically modified starch. The chemically modifications such as cross-linking, substitution, and esterification are performed to native starches to improve the physicochemical and functional properties. Thus, these structurally modified starches are resistant to enzymes and delays the digestion of starch (Raigond et al. 2015). The RS 5 refers to amylose lipid complex or amylose lipid protein complex either inherently present or formed during processing. The interplay between lipid components such as long chain fatty acids with amylose propagates the hydrophobicity to starch which is resistant to digestive enzymes (Bojarczuk et al. 2022). The RS rich diet is known to reduce the post prandial blood glucose levels by increasing the insulin sensitivity. Hence the RS can be used as an aid to control diabetes (Tian and Sun 2020). The FAO had projected a shortage of major cereal grains such as maize, rice, and wheat by 2028 which might impact the starch-based industry. In this regard, millets could be a major substitute to manufacture RS from starch as it inherently possesses a higher amount (Kaimal et al. 2021). The resistant starch content in millets varies between 21.99 and 30.87%. The RS content is higher in millets when compared to wheat which could be attributed to the lower starch digestibility of millets (Sharma and Gujral 2019a).

Protein

The protein present in the millets tends to play an important role in reducing the starch digestibility rate. The hydrophilic interactions between protein and starch leads to formation of complexes that may act as a barrier and restricts the action of digestive enzymes. (Annor et al. 2017). The study conducted by Annor et al. (2013) revealed that the deproteinization of kodo millet flour showed a significant increase in RDS and eGI confirming the role of protein as a barrier around the starch granules to reduce its digestibility. Similar observations were also recorded by Jin et al. (2019) where the removal of protein from foxtail millet flour increased the starch digestibility. The study conducted by Zheng et al. (2020) revealed that the starch protein mixtures developed from proso millet starch with zein, soya protein, and whey protein isolate by heat treatment had decreased RDS with concomitant increase in RS and SDS. The authors suggested that the starch may be entrapped by hydrogen bonds with hydrophilic groups of protein resulting in the reduced activity of digestive enzymes. The study conducted by Fu et al. (2020) revealed that prolamin extracted from foxtail millet when fed to diabetic mice normalized blood glucose homoeostasis with reduced triglyceride accumulation in serum and liver; also improved the pancreatic beta cell functioning of diabetic mice.

Lipid

The lipids also form complex with starch and reduces the rate of digestibility. Annor et al., (2013) observed that defatted kodo millet flour had increased RDS with a reduction in SDS and RS leading to an increase in eGI which was related to the formation of complexes between the amylose and lipid. The study conducted by Annor et al. (2015) on foxtail, pearl, proso, and finger millet starches with addition of lipids such as palmitic acid, oleic acid, and linoleic acid had reduced the starch digestibility. This phenomenon was attributed to formation of RS which depended on type of fatty acid to resist the amylase hydrolysis. Similar study was conducted by Jin et al. (2019) wherein observed that the addition of fatty acid such as palmitic, oleic, linoleic, and elaidic acid to foxtail millet starches had influenced the starch digestibility thus proving that the lipids present in the millet have a hypoglycaemic effect.

Polyphenols

Polyphenols, abundant in millets, are known for their health promoting properties such as anti-oxidative, anti-diabetic, anti-cancerous, anti-hypertensive, and anti-inflammatory properties (Shahidi and Chandrasekaran 2013). They consist of both free and conjugated forms of phenolic acids, that include derivatives of hydroxybenzoic and hydroxycinnamic acid. Polyphenols are found mainly in the seed coat layer of the millet grain (Shahidi and Chandrasekaran 2013). The study conducted by Shobana et al. (2009) on finger millet seed coat phenolics revealed that the inhibition of key enzymes like alpha amylase and glucosidase are necessary for targeting the postprandial hyperglycaemia. The finger millet seed coat matter containing 11.2% of polyphenols, when fed to diabetic rat reduced the fasting blood glucose, serum urea, serum creatinine, serum cholesterol levels, and HbA1C levels which indicated that millet polyphenols ameliorated the abnormalities associated with diabetes (Shobana et al. 2010). Pradeep and Sreerama (2015) observed that polyphenols extracted from barnyard, foxtail, and proso millet inhibited alpha amylase and glucosidase enzymes. The germination of grains with higher phenolic index promoted higher inhibitory activities on these enzymes. Polyphenols isolated from barnyard millet showed significant inhibition of alpha glucosidase in various models such as S. cerevisiae, rat intestinal a-glucosidase, Caco-2 cells, and in-vivo oral glucose tolerance in rats. Hence barnyard millet could be used as a potential grain to normalize post prandial blood glucose levels in diabetics (Seo et al. 2015).

Dietary fibre

The millets are usually rich in dietary fibre contents when compared to other cereal grains. The dietary fibre is classified as soluble fibre content ranging between 3.34 and 5.90% and insoluble fibre content ranges between 8.06 and 13.00% in millets. Hence, the dietary fibre present in the millets contributes to reducing the activity of digestive enzymes and lowering the starch digestibility (Sharma and Gujral 2019a). Annor et al. (2017) reviewed that processing of millet altered the dietary fibre contents. The synergistic effect of dietary fibre and polyphenols leads to inhibition of the alpha amylase enzyme which has a potential role in management of diabetes and the viscous property of soluble fibre present in the millets tends to decrease the postprandial blood glucose concentrations.

Anti-nutritional factors

The millets consist of various anti-nutritional factors such as phytic acid, tannins, lectins, alkaloids, and protease inhibitors. These anti-nutritional factors are known to reduce the absorption of nutrients and hinder their bioavailability (Sharma et al. 2021). Sharma and Gujral (2019a) observed that the anti-nutritional factors such as phytic acid and tannins present in millets played an important role in reducing the starch digestibility by restricting the enzyme activity.

Processing of millets and its effect on starch digestibility

Millets are processed by physical, chemical, enzymatic, and genetic processes mainly to enhance the physicochemical, functional, and digestibility properties. Annor et al. (2017) have reviewed the effects of milling, heat moisture, fermentation, and germination on starch and protein digestibility in millets which varies depending on the processing conditions.

Physical treatments

The physical treatments given to millet grains/starches are thermal, shear, moisture, and radiation which is gaining more popularity than other treatments because of the absence of any toxic substances during the modification process. These physical treatments are given to modify the starches/ grains to enhance the functional properties and can be easily adopted in food industry. They may be categorised into thermal and non-thermal processing of millets given in Table 2 (Annor et al. 2017; Zia-ud-Din et al. 2017).

Table 2.

Thermal, chemical, and combination processing of millets relating to the starch digestibility and its action

SI no	Millet	Processing	RDS (%)	SDS (%)	RS (%)	PGI	Action	References
Thermal treatments
1	Foxtail millet flour	Heat moisture	12.46	8.65	19.25	–	Increase in SDS and RS, reduced glycaemic response	Amadou et al. (2014)
2	Barnyard millet grain	Microwave treatment			15.56	–	High RS content due to microwave drying	Kanagaraj et al. (2019)
3	Foxtail millet Starch	Annealing	41.53	16.15	42.32	–	The annealing promoted the interaction of amylose with amylose and amylopectin which increased the RS	Babu et al. (2019)
4	Pearl millet starch	Heat moisture	36.6–42.2	39.1–40.2	18.1–23.8	–	SDS and RS increased restricted the activity of alpha amylases	Sandhu et al. (2020)
5	Sorghum grain	Infra-red treatment with 30% moisture		12.64–30.87	40.07–51.85	48	Starch kafrin complex might have increased the RS and reduced GI	Semwal and Meera (2021)
6	Sorghum starch	Infra-red treatment with 30% moisture		4.57–14.27	37.55–47.92	61	Starch protein interaction with infra-red treatment increased the RS and reduced GI	Semwal and Meera (2021)
7	Pearl millet flour	Heat moisture	52.61	7.68	1.75	–	Crystalline disruption by heat moisture treatment and increased accessibility of enzymes	Vinutha et al. (2022)
8	Pearl millet flour	Infra-red	51.13	9.46	1.77	–	Crystalline disruption by infra-red treatment and increased accessibility of enzymes	Vinutha et al. (2022)
Chemical treatments
9	Pearl millet starch	Cross-linking	46.1–50.6	34.5–36.1	13.6–19.4	–	Increase in RS reduced the activity of alpha amylase	Siroha and Sandhu (2018)
10	Pearl millet starch	Acetylation (5.0%)	45.4	27.3	27.3	–	Increase in RS restricted the activity of digestive enzyme	Siroha et al. (2019)
11	Foxtail millet starch	Succinilayation	76.31	15.39	8.29	–	Reduced in RDS with increase in SDS and RS	Babu and Mohan (2019)
Combination of treatments
12	Foxtail millet flour	Heat moisture and fermentation	15.92	18.42	22.68	–	Combination has promoted increase RDS and RS due to proteolysis of starch	Amadou et al. (2014)
13	Foxtail millet starch	Acid treated and succinilayted	69.58	12.22	18.19	–	Acid hydrolysis increased amylopectin regions which increased RS	Babu and Mohan (2019)
14	Foxtail millet	Ultrasonication and annealing	40.57	13.84	45.59	–	The combination treatment promoted the interactions of amylose with amylose and lipid which lead to increase in crystallinity and increase in RS	Babu et al. (2019)
15	Pearl millet flour	Heat moisture and infra-red	49.23	12.34	1.75	–	SDS increased	Vinutha et al. 2022)

RDS Rapidly digestible starch; SDS Slowly digestible starch; RS Resistant starch; PGI Predicted glycaemic index

Thermal treatments

The thermal treatments include dry heat, infra-red, microwave, annealing, and heat moisture (Babu et al. 2019; Kanagaraj et al. 2019; Amadou et al. 2014; Sandhu et al. 2020; Vinutha et al. 2022). Kanagaraj et al. (2019) studied the effect of dry heat on barnyard millet using different techniques such as pan roasting, fluidized bed drying, tray drying, and microwave. Among these treatments, the microwave drying for 20 min yielded a higher RS content (15.56 g/100 g). Amadou et al. (2014) revealed that heat moisture treated foxtail millet flour had increased RDS (12.46%), SDS (8.65%), and RS (19.25%). The heat moisture treatment given to pearl millet (18.1–23.8%) and foxtail millet (42.32) starches had doubled the RS content with a reduction in RDS (Babu et al. 2019; Sandhu et al. 2020). The increase in RS content was attributed to the interactions between amylose with amylose and amylopectin causing a compact crystalline structure restricting the action of enzymes (Babu et al. 2019). The infra-red treated pearl millet flour had an increase in RDS with reduction in SDS (Vinutha et al. 2022). The treatments caused the disruption of crystalline structure of the starch granules thus enhancing the accessibility of digestive enzymes leading to an increase in RDS contents. However, Semwal and Meera (2021) showed that the sorghum corneous grain treated with infra-red increased the RS content upto 40.07–51.85% due to the interactions between starch-protein complex formed and might have restricted the enzyme attack with a corresponding decrease in GI. The starch isolated from the modified grain also had an increased RS content (37.55–47.92%) and reduced GI (Semwal and Meera 2021). The differences in starch digestibility properties of pearl millet flour and sorghum corneous endosperm might be attributed to the variations in particle size, IR wavelength, and temperature. The starch isolated from microwave treated foxtail millet had increased RDS (27.25–52.10%) and decreased RS (23.63–53.88%) with an increase in duration of treatment. The microwave treatment damaged the crystalline and amorphous regions of starch which converted the RS into SDS and RDS. Whereas, the increase in moisture content caused an increase in SDS content due to formation of intermediate dense chains in amorphous and ordered organizations of starch (Zhi et al.2022).

Non-thermal treatments

The non-thermal modification is considered as an environmentally friendly technology and known to preserve the nutrients, colour, texture, and flavour while simultaneously destroying the pathogens. Some of the non-thermal modifications include ultrasound, ozone, high pressure homogenization, and pulse electric field (Zia-ud-Din et al. 2017). The studies on non-thermal treatments for millets are very limited when compared to thermal treatments.

Ultrasonication treatment involves generation of sound waves beyond 20 Hz. The application of ultrasound waves to starch leads to formation of gas bubbles, cavitation effect, and generation of free radicals (Raghunathan et al. 2021). The effect of ultrasonication has also been studied by Babu et al. (2019) for the modification of foxtail millet starch and showed that there was a slight increase in RS content from 18.20 to 20.14%, whereas SDS reduced.

The high pressure homogenization treatment involves the application of pressure ranging between 100 and 1000 MPa (Wu et al. 2022). Applying 200 MPa of pressure to starch along with heat led to the gelatinization of starch granules along with disruption of amorphous and crystalline regions of starch (Raghunathan et al. 2021). The study conducted by Sun et al. (2022b) on proso millet starch revealed that the ultrahigh pressure homogenization treatment decreased the RS content upto 41.31–50.29% when compared to the native starch due to the disruption of structural integrity and enhancing the degree of hydrolysis.

The cold plasma is generated by either dielectric barrier discharge or jet plasma. These treatments generate plasma gas and decomposes water molecules leading to OH radical generation and crosslinking of two polymeric chains (Raghunathan et al. 2021). It further causes the collision of ions which depolymerizes the starch granules leading to breakage of amylose and amylopectin chains (Raghunathan et al. 2021). The etching of surface of starch granules were also observed which leads to the formation of pores that facilitates the entry of water into the starch (Raghunathan et al. 2021). The cold plasma treated proso millet starch for 1 min had higher RS content (52.89%) because of the interactions which increased the crystallinity and thus restricted the access of digestive enzymes (Sun et al. 2022b).

The ozonisation of cereal starches deals with discharge of ozone gas from ozone generator which leads to oxidation of starch by conversion of hydroxyl groups to carbonyl groups in C-2, C-3, and C-6. The depolymerization leads to breaking of alpha 1, 4 glycosidic bonds (Raghunathan et al. 2021). The study conducted by Wang et al. (2022) revealed that the ozonation of finger millet starch had increased in RS content (29.60%) leading to lower digestibility as the carbonyl and carboxyl groups produced during modification inhibited the active sites of the substrate rendering the starch molecule resistant to digestive enzymes.

Chemical treatments

The chemical modification is mainly applied to starch by introducing a new functional group to bring desired physico-chemical properties (Zia-ud-Din et al. 2017). The millet starch could be modified using acetylation, crosslinking, and acid hydrolysis (Siroha and Sandhu 2018; Siroha et al. 2019; Babu et al. 2019). Siroha and Sandhu (2018) studied the effect of crosslinking pearl millet starches with sodium trimetaphosphate and sodium tripolyphosphate at varying levels of 9.2, 26.2, and 29.2%. It reduced RDS and SDS with a subsequent increase in RS content. The modification of starch depended on the degree of cross linking. Higher degree of crosslinking inhibited the swelling of granules thus reducing the starch digestibility. Siroha et al. (2019) observed that acetylated pearl millet starches had 27.3% increase in RS content with 5.0% acetic anhydride concentration. Further, the authors suggested that digestive enzymes fail to act on the chemically modified starch and hence reduced the starch digestibility with an increase in RS content. Similar results were observed by Sharma et al. (2016) in octenyl succinylated pearl millet starches were the RS content increased with introduction of octenyl succinate groups. The esterification of the pearl millet starch had protected the starch granules from being digested by alpha amylase. Babu et al. (2019) observed that succinylation treatment of foxtail millet starch showed no significant changes in the RS content while combining acid hydrolysis with succinylation increased the RS content up to 18.51%, with a reduction in RDS up to 69.58%. They further concluded that acid hydrolysis breaks the amylopectin regions of the starch granules and combines with succinyl groups restricting the active sites of substrates to alpha amylase and amyloglucosidase, resulting in reduced starch digestibility.

Combination of treatments

The studies relating to combination of treatments were studied by Amadou et al. 2014 werein, foxtail millet flour was treated with a combination of heat moisture and fermentation. However, the starch digestibility increased due to the breaking down of the protein matrix which enabled easy accessibility of the starch granules to digestive alpha amylase. On the other hand, Babu et al. (2019) observed that ultrasonication, annealing, and combination of treatments given to foxtail millet starches had increased the RS content by 45.59% when compared to untreated starch. The ultrasonication resulted in breaking of the long starch chains into small chains which rearranged into small starch chains upon annealing leading to increase in crystallinity. Babu and Mohan (2019) studied the combined effect of ultrasonication and succinylation treatments on foxtail millet starch which increased the RS content as ultrasonication created pores on the surface of the starch granules which allowed the succinyl groups to fit into these groves thus blocking the activity of digestive enzymes on the starch granules. Similarly, when heat moisture was combined with succinylation, an increase in the RS content was found in foxtail millet starch (Babu and Mohan 2019). These modified starches can be used to develop low GI food formulations.

Other processing technologies

The millets can be also processed through other technologies such as germination/malting, enzymatic, and fermentation to improve the nutritional, sensory, and technological characteristics of food products (Saleh et al. 2013). The germination or malting induce biochemical modification within the millet grains to improve the protein and starch digestibility (Saleh et al. 2013). Sharma and Sharma (2022) observed that germination of foxtail millet grains increased the starch digestibility due to the activation of alpha amylase, beta amylase, and beta glucosidase resulting in breakdown of starch granules. Similar observation was seen in proso millet were the bioprocessing of millets results in disruption of protein and fibre associated with starch led to an increase in in-vitro starch digestibility (Sharma et al. 2022). The fermentation improves the protein digestibility and nutrient quality of millets with reduction in anti-nutrients (Saleh et al. 2013). Sharma and Sharma (2022) and Sharma et al. (2022) observed that reduction of anti-nutrient content such as phytic acid, tannins, and saponins by germination and fermentation in foxtail and proso millet had increased the in-vitro starch digestibility. Sharma and Gujral (2020) observed that soaking followed by sprouting of finger, barnyard, kodo, foxtail, proso and little millet caused a major reduction in RS (12.22–26.22%) and SDS (8.77–25.05%) with an increase in RDS (15.16–26.22%) content when compared to native. The sprouting might have improved the amylase activity leading to break down of starch chains and lead to an increase in the starch digestibility. It also causes the reduction of anti-nutrients such as phytic acid and tannins which might have created a larger area within the matrix and susceptible to enzymatic attack leading to an increase in starch hydrolysis rate. The authors further observed that sprouting of millets increased the PGI (44.77–63.77) when compared to native (Sharma and Gujral 2020). In enzymatic modification, isoamylase or pullulanase can be used to debranch millet amylopectin leading to an increase in amylose content (Kaimal et al. 2021). The finger millet resistant starch prepared by enzymatic method had a greater hydrolysis rate when compared to other treatments (Wang et al. 2022). These processing technologies using appropriate enzymes can be used to alter the starch digestibility pattern of millets.

Millet food formulation for pre-diabetic and diabetic in-vitro evaluation

The millets are being promoted for their low digestible properties when compared to other cereals, the status of the GI in millet products needs to be validated using both in-vitro and in-vivo methods (Table 3). The porridge and couscous prepared from parboiled pearl and proso millet grains had reduced in RDS and SDS contents with an increase in the RS content. The eGI also had reduced slightly when compared to the native grains (Bora et al. 2019b). Liu et al. (2020) observed that 50% incorporation of the heat moisture treated sorghum flour in noodles showed higher RS content (50%) with a lower eGI (53.41). The heat moisture treatment induced the formation of starch-lipid complex leading to the reduced GI by restricting the accessibility of digestive enzymes. Ren et al. (2016) evaluated the effect of incorporation of extruded foxtail millet flour in steamed bread and pancake which revealed that steamed bread had relatively higher GI when compared to the pancake. The extrusion results in destruction of starch granular crystallinity which lead to increase in starch digestibility. They also studied the in-vitro starch digestibility of cooked foxtail millet and porridge and noticed that the cooked millet had a lower eGI (54.3) when glucose was used as a reference. The pancake incorporated with 100% native millet flour had reduced eGI with high RS content (Ren et al. 2016). The waffles incorporated with 40% finger millet and 20% pearl millet flours increased the RS content (4.64%) which may result in lower glycaemic load (Chaitra et al. 2020). The products such as muffin, couscous, extruded snacks, and porridge were prepared using different forms of proso millet at different levels and it was observed that 100% proso millet containing products had a lower RDS with a higher SDS and RS contents thus reducing the eGI. The lower eGI was attributed to formation of starch-protein, starch-polyphenol and starch-fibre complexes (McSweeney et al. 2017). Li et al. (2020) observed that upon addition of millet dietary fibre in steamed bread decreased RDS and SDS with an increase in RS contents leading to reduction in the GI. The dietary fibre present in the millet bread contributed to the formation of RS and reduced the accessibility of digestive enzymes. The millet dietary fibre fortified steamed bread could be administered to diabetic patients as a substitute to wheat steamed bread. The flat breads prepared from various millet such as finger, foxtail, kodo, little, proso, and barnyard had reduced in RDS with an increase in RS contents when compared to flat bread made from 100% wheat. The increase in RS content was attributed to increase in the dietary fibre content which resists hydrolysis in the small intestine to ferment in the large intestine (Sharma and Gujral 2019b). The addition of finger millet in rice noodles reduced the RDS with an increase in RS contents leading to reduction in the post prandial blood glucose levels (Chen et al. 2021). Kamble et al. (2021) observed that pasta incorporated with 31.9% sorghum flour and 13.4% finger millet flour reduced GI in both uncooked (24.45) and cooked (24.99) form when compared to control pasta. This might be attributed to an increase in dietary fibre and anti-nutrient content of pasta causing restriction of enzymatic attack causing a lower GI.

Table 3.

Incorporation of millet grains into food products and the related in-vitro starch digestibility

SI No	Millet	Food products	RDS (%)	SDS (%)	RS (%)	PGI	Mode of Action	References
1	Foxtail millet	Steamed Bread (75% millet flour + 25% extruded millet flour)	46.3	44.9	8.8	86.3	The extrusion processing of foxtail millet leads to decrease in crystallinity which will enhance the gelatinization extent and accessibility of enzymes	Ren et al. (2016)
2	Foxtail millet	Pancake (100% millet flour)	39.1	45.0	15.9	57.2	Increase in RS reduced the glycaemic index	Ren et al. (2016)
3	Foxtail millet	Cooked millet	36.9	38.3	24.9	54.3	Increase in RS reduced glycaemic index	Ren et al. (2016)
4	Proso millet	Couscous (100%)	27.6	25.6	46.8	50.2	It contains significant amount of protein, polyphenols and fibre which lead to reduced glycaemic index	McSweeney et al. (2017)
5	Pearl millet grain	Parboiled millet porridge and Couscous	17.6–18.3	36.6	45.0–45.8	42.7–43.7	Reduction in RDS and increase in RS related to formation of amylose lipid complexes during parboiling reduced the GI	Bora et al. (2019b)
6	Proso millet grain	Parboiled millet porridge and Couscous	17.4–17.9	32.8–33.8	48.9–49.1	41.7–42.2	Reduction in RDS and increase in RS related to formation of amylose lipid complexes during parboiling reduced the GI	Bora et al. (2019b)
7	Finger, kodo, banyard,little, proso, and foxtail millet	Flat bread (100%)	19.45–31.06	32.28–47.21	0.36–1.42	67.06–75.85	Increase in RS content reduced the GI	Sharma and Gujral (2019a, b)
8	Sorghum	Heat moisture treated flour incorporated into noodles	20	25	50	53.41	Increase in RS due to the formation of amylose lipid complexes lead to reduction in GI	Liu et al. (2020)
9	Finger millet and pearl millet	Waffle (40% wheat flour + 40% finger millet flour + 20% pearl millet flour	24.78	22.05	4.64	–	RS increased due to increase in fibre content	Chaitra et al. (2020)
10	Finger millet	Rice noodles (30% finger millet)	50.00	42.0	9.0	–	Increase in RS reduced the starch digestibility	Chen et al. (2021)

RDS Rapidly digestible starch; SDS Slowly digestible starch; RS Resistant starch; PGI Predicted glycaemic index

In-vivo validation

The in-vivo validation of millet-based products are required to ascertain their performance in human subjects when incorporated as food formulation in daily diet for pre-diabetic and diabetic population (Ugare et al. 2014; Geetha et al. 2020) (Table 4). The dehulled and heat treated dehulled barnyard millet prepared as upma, a traditional south Indian breakfast exhibited a lower GI (50 and 41.7 respectively) in healthy volunteers (Ugare et al. 2014). Further, it was evaluated in diabetic subjects for 28 days and the findings revealed that fasting blood glucose, triglycerides, cholesterol, LDL levels, and LDL:HDL ratio reduced with an increase in HDL levels (Ugare et al. 2014). Shukla et al. (2014) observed that 30% incorporation of finger millet in noodles reduced the GI to 45.13 in normoglycemic subjects, when compared to wheat noodles (62.59). The high dietary fibre content of finger millet noodles was reported to be responsible in controlling the blood glucose levels. Geetha et al. (2020) observed that roti, dumpling, and dosa made from finger millet and little millet-based food mixes containing additional ingredients such as pulse flour, bitter gourd, fenugreek, and flaxseeds had reduced the GI and GL in healthy subjects. The authors also conducted a long-term intervention study of these products in pre-diabetic subjects and noticed reduction in the blood glucose and HbA1C levels. Further, flakes from little millet also showed a low GI and GL in normoglycemic subject due to the formation of RS during processing (Patil et al. 2015). The food formulations developed from foxtail millet were also evaluated in healthy subjects, the GI had reduced depending on the product which followed the pattern like millet porridge > steamed bread > millet pancake with 25% extruded millet flour > millet pancake with 100% millet flour > cooked millet (Ren et al. 2016). An increase in GI and GL of decorticated finger millet in comparison to wheat was noticed in healthy subjects which was attributed to reduced fibre content in the millet (Shobana et al. 2007). Almaski et al. (2022) observed that 50% incorporation of finger millet flour in muffin reduced the blood glucose and insulinemic response in pre-diabetic and healthy subjects. The reduction in the above responses was attributed to the presence of polyphenol and fibre leading to competitive inhibition of the alpha amylase and reduction in the glucose absorption. The incorporation of 15% foxtail millet along with arrowroot flour and kidney beans in cookie preparation led to low GI in normoglycemic subjects. The low GI was attributed to the (1) fibre content, (2) resistant starch formation by amylose lipid complexes and (3) amylose, protein, and fat contents. This un-digested starch ferments in colon leading to reduced glycaemic response (Lestari et al. 2017). Kumari et al. (2020) observed that parboiled finger millet porridge caused a lower GI and GL rather than the porridge prepared from raw finger millet flour in normal subjects. The bread produced from steamed foxtail millet reduced the fasting blood glucose, post-prandial blood glucose, and lipid levels in subjects with impaired glucose tolerance for a period of twelve weeks. The authors related the above reduction was due to the presence of bioactive components. They also observed an improvement in HOMA-IR which signifies an improvement in insulin sensitivity and resistance. The increase in leptin concentration with subsequent reduction in IL-6 and TNF-α was also reported to be responsible for the improvement in blood glucose metabolism and insulin action (Ren et al. 2018). Among the traditional Maharashtrian millet-based recipes like pearl millet cheela, bhakri, sorghum bhakri, bajra khichadi, and mixed millet thalipeeth evaluated for the GI and GL, pearl millet cheela was found to have a low GI and GL. This suggests the beneficial effect of incorporation of millet based traditional recipes in the daily diet of diabetic population (Nambiar and Patwardhan 2015).

Table 4.

Incorporation of millet grains into products and the related in-vivo starch digestibility

SI No	Millet	Processing	Subjects	GI	GL	Mode of Action	References
1	Finger millet	Decorticated hydrothermally treated flour incorporated into diabetic food formulations	Normoglycemic subjects (n = 8)	93.4	47	High GI due to the decortication of the grain and reduced dietary fibre content	Shobana et al. (2007)
2	Barnyard millet	Dehulled and heat treated at 60 °C with 4 cycles of intermittent cooling for 1 h	Normoglycemic subjects (n = 6)	41.7	–	The intermittent heating and cooling to retrogradation of amylose leading to formation of RS. The increase in RS lead to reduction in GI	Ugare et al. (2014)
3	Finger millet	Noodles (30% finger millet flour + 70% wheat flour)	Normoglycemic subjects (n = 10)	45.13	–	Incorporation of finger millet flour decreased the GI	Shukla et al. (2014)
4	Little millet	Flakes	Normoglycemic subjects (n = 10)	52.11	9.24	Low GI due the formation of RS during the partial gelatinization and melting of starch during the processing of grains	Patil et al. (2015)
5	Pearl millet	Cheela (traditional Maharashtrian recipe)	Normoglycemic subjects (n = 6)	44.94	18.06	Incorporation millet decreased the GI	Nambiar and Patwardhan (2015)
6	Foxtail millet	Steamed bread (75% millet flour + 25% extruded millet flour)	Normoglycemic subjects (n = 10)	89.6	–	High GI due to the incorporation of pre- gelatinized flour	Ren et al. (2016)
7	Foxtail millet	15% incorporated into cookie	Normoglycemic subjects (n = 12)	37.6	–	Incorporation of foxtail millet improved the dietary fibre content and reduced the GI	Lestari et al. (2017)
8	Foxtail millet	Steamed bread	Subjects with impaired glucose tolerance (n = 64)	–	–	Reduced the pre and post prandial blood glucose levels	Ren et al. (2018)
9	Little millet and finger millet	Millet based foods (dosa, dumpling and roti)	Normoglycemic subjects (n = 10)	37–53	11.05–18.43	Heat processing of millets contributed to low GI and GL	Geetha et al. (2020)
10	Finger millet	Parboiled and roasted made to a porridge	Normoglycemic subjects (n = 10)	38	9.5	Parboiling had increased the RS content and resulted in low GI	Kumari et al. (2020)
11	Finger millet	50% incorporated into muffins	Normoglycemic subjects (n = 15) Prediabetic subjects (n = 14)	–	–	Reduced blood glucose levels and IAUC due to presence of fibre and polyphenols in finger millet	Almaski et al. (2022)

GI Glycaemic index; GL Glycaemic load

Mechanism of starch digestibility of millets in-vitro and in-vivo

The possible mechanisms of reducing the starch digestibility by thermal, non-thermal, and chemical treatments are detailed in Fig. 1. The thermal treatment without moisture did not cause damage to the starch which remained intact and induced the formation of starch-lipid /starch-protein complexes (Kanagaraj et al. 2019). The undamaged cells and complexes restricted the activity of digestive enzymes and reduced the starch digestibility. However, the thermal treatments with moisture resulted in gelatinization of starch granules which was prone to amylose leaching. This resulted in reduced formation of starch-lipid /starch-protein complexes and permitting the entry of digestive enzymes thus increasing the RDS (Babu et al. 2016). The non-thermal treatments such as ultrasonication, cold plasma, high pressure homogenization, and ozonisation caused the scraping of cell surfaces with treatment and reduced the complex formation which gave easy access to enzymes resulting in increased starch digestibility (Babu et al. 2019; Sun et al. 2022b; Wang et al. 2022). The chemical treatments of millets involving attachment of functional group to starch acted as a physical barrier which restricted the enzyme access and reduced starch digestibility by increasing the RS (Siroha and Sandhu 2018; Siroha et al. 2019; Babu et al. 2019). The in-vivo mechanism of millet-rich/incorporated products when ingested are first hydrolysed by the salivary amylases in the mouth. The acid hydrolysis and pepsin in the stomach digests the protein associated with starch. Further, starch and its moieties get digested in the small intestine by pancreatic amylase leading to formation of glucose. The millet products rich in RS releases lesser amount of glucose which is absorbed in the small intestine, and the remaining undigested starch and its moieties ferments in the large intestine by colonic bacterial amylase finally contributing to the reduction in GI of the RS rich product (Kaimal et al. 2021). However, the rate of fermentation of RS from millet in colon are not extensively studied to understand the exact mechanism of in-vivo digestibility of millets.

Fig. 1.

Mechanism of starch digestibility in millets subjected to Thermal Treatments (TT), Non-Thermal Treatments (NTT) and Chemical Treatments (CT) (SL- Starch lipid complex, SP- Starch protein complex)

Conclusion

The millets are nutritionally rich when compared to other cereal grains and possess health beneficial components and hence referred to as nutri-cereal. The processing of millets promoted the formation of binary, ternary, and quaternary complexes leading to a reduced starch digestibility. The studies conducted revealed that among the treatments, the thermal and chemical treatments of millets showed doubling of the resistant starch content. Appropriate processing helps in the preservation of the anti-diabetic properties of millets. The incorporation of processed millets into food products varied the glycaemic index in-vitro as well as in-vivo and depended on the factors such as dietary fibre, complex formation, and polyphenols. These appropriately processed millets can act as a functional food which could help the pre-diabetic and diabetic population in dietary care.

Abbreviations

ADA

American diabetes association

eGI

Expected glycaemic index

FAO

Food and agriculture organisation

GI

Glycaemic index

GL

Glycaemic load

HbA1C

Glycosylated haemoglobin

HDL

High density lipoprotein

HOMA-IR

Homeostatic model assessment for insulin resistance

LDL

Low density lipoprotein

pGI

Predicted glycaemic index

RDS

Rapidly digestible starch

RS

Resistant starch

SDS

Slowly digestible starch

Author contributions

RVL Conceptualization, Writing-first draft and data curation. MSM Conceptualization, Supervision, Validation, and Writing—review & editing.

Funding

This research was funded by Department of Biotechnology, New Delhi, India (GAP-0462).

Data Availability

Not applicable.

Code availability

Not applicable.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

Not applicable.

Consent to participate

R.Vidhya Lakshmi and M.S.Meera consents to participate on request.

Consent for publication

The manuscript submission to Journal of Food Science and Technology (JFST) publication has been approved by all authors.