Effect of infra-red heat treatment on the biochemical characteristics of little millet during storage

Introduction

Millets are a group of small-seeded cereal grains belonging to the family Poaceae and have been cultivated for centuries in Asia and Africa because of their ability to withstand drought, low soil fertility, and erratic rainfall. Their resilience and nutritional value have made them increasingly important for sustainable food systems under changing climatic conditions (Manik et al., 2025; Meti et al., 2024a and Meti et al., 2024b) [12,13,14] . Little millet commonly known as kutki, sawa, samai, and samalu, is one of the important minor millets cultivated in India and several Asian countries (Padulosi et al., 2009) [16] . It has gained considerable attention owing to its superior nutritional quality and potential health benefits. Little millet is a rich source of protein (10.0-15.0%), dietary fibre, essential minerals such as magnesium (120-133 mg/100 g) and phosphorus (251-260 mg/100 g), and bioactive compounds, making it a valuable ingredient for the development of functional foods (Veena et al., 2005) [19] . It also contains appreciable amounts of polyunsaturated fatty acids, flavonoids, and essential amino acids, including cysteine, methionine, and lysine, which contribute to its nutritional value (Indirani, 2021) [8] . The grain is characterized by a low glycaemic index, high resistant starch, and slowly digestible carbohydrates, making it suitable for diabetic individuals and health-conscious consumers (Deshpande et al., 2015; Chethan and Malleshi, 2007) [4,15] . In addition, its high dietary fibre content supports digestive health and contributes to improved metabolic wellbeing. These nutritional and functional attributes have increased the demand for little millet as a healthy alternative to conventional cereal grains. Despite its nutritional advantages, the presence of a tightly adhering husk poses challenges during milling, resulting in lower dehusking efficiency and reduced head grain yield. Infra-red (IR) heat treatment has emerged as an effective pre-milling treatment to overcome the challenges associated with husk removal. The rapid and uniform heating provided by infra-red heat weakens the adhesion between the husk and kernel, thereby improving dehusking efficiency and head grain yield (Kumar et al., 2022; Oomah et al., 2014 and Kumar et al., 2009) [9,10,15] . These findings highlight the potential of IR heat treatment as an efficient pre-processing technology for improving the milling performance of cereal and millet grains. Storage is an important stage that influences the quality and shelf life of processed millets. During storage, biochemical changes such as lipid oxidation, enzymatic activity, moisture redistribution, and degradation of bioactive compounds may affect their nutritional quality. Evaluating these changes in infra-red heat treated dehusked little millet provides insights into the effectiveness of the treatment that improve milling efficiency while preserving the nutritional quality of the grains throughout storage. Therefore, the present study was undertaken to investigate the effect of infra-red heat treatment on the biochemical characteristics of little millet during storage.

Materials and Methods

Sample preparation and storage conditions

Little millet (variety LM-8437-17) was procured from the Millet Processing Unit, University of Agricultural Sciences (UAS), Raichur, Karnataka, India. Infra-red (IR) heat treatment was carried out using the IR heat treatment unit developed at the Department of Processing and Food Engineering, UAS, Raichur. Little millet was cleaned and treated with IR heat at a feed rate of 500 kg/h and an IR power level of 5 kW, followed by dehusking using a commercial pneumatic rubber roll millet dehusker (Agrosaw, RB3, Haryana, India). For the storage study, untreated dehusked little millet (control) and little millet subjected to IR heat treatment followed by dehusking were packed in PET (400 µm) packaging material and stored under room temperature conditions for six months. The effect of IR heat treatment on the biochemical characteristics viz., proximate composition (moisture content, carbohydrate content, crude protein, crude fat, crude fibre and total ash) and anti-nutrients (phenols, tannins and phytic acid) were evaluated at 30 days intervals throughout the storage period by following the standard procedures as outlined in Table 1 (Meti et al., 2024b) [14] . All chemicals of analytical grade were purchased from M/s. Sigma-Aldrich, Bengaluru and M/s. Fisher Scientific, Bengaluru. [1] Carbohydrate (%) AOAC, 2016 (Method No.975.14) [1] Crude protein (%)

AOAC, 2016 (Method No.992.23) [1] Crude fat (%)

AOAC, 2016 (Method No.920.39) [1] Crude fibre (%)

AOAC, 2016 (Method No.962.09) [1] Total ash (%)

AOAC, 2016 (Method No.923.03) [1] Anti-nutrients Phenols (mg/g) Malick and Singh (1980) [11] Tannins (mg/g) Price et al. (1978) [17] Phytic acid (mg/g) Wheeler and Ferrel (1971) [18]

Results and discussion

Proximate composition Moisture content

The changes in moisture content of untreated dehusked (Lc) and IR heat treated dehusked little millet (Lt) during storage are presented in Fig. 1. It was observed that the initial moisture content for Lc and Lt was recorded at 11.24% (w.b.) and 9.12% (w.b.), respectively. The moisture content of Lc decreased to 11.01% (w.b.) at 120 days but increased to 11.21% (w.b.) at 180 days. Similarly, for Lt, the moisture content decreased to 8.90% (w.b.) at 120 days and increased to 9.10% (w.b.) at 180 days. Significant differences in moisture content at 180 days are observed among the control and IR heat treated samples, which might be attributed to their initial moisture content. The fluctuations in moisture content recorded during the storage studies may be ascribed to variations in the environmental conditions within the storage area, such as temperature and humidity. Huang et al. (2021) [7] also observed a reduction in moisture content for dehulled millets, from 13.27 to 12.63% and noted a similar trend in heat treated dehulled millets, where moisture content decreased from 12.97 to 11.77% over a 90 days period of storage.

Carbohydrate content

Crude protein

From the Fig. 3, it was noted that as the storage period progressed, the crude protein of Lc and Lt decreased from 10.64 to 9.87% and from 10.16 to 9.42%, respectively, over 180 days. No significant differences (p > 0.05) in crude protein were observed at 180 days between the control and IR heat treated dehusked samples. Huang et al. (2021) [7] also reported a reduction in crude protein in dehulled millets, decreasing from 6.72 to 6.29% over a 90 days period.

Crude fat

The crude fat content of Lc decreased from 3.78 to 3.69%, while that of Lt decreased from 3.75 to 3.68%. During storage period, crude fat content did not differ significantly (p > 0.05) between the untreated and IR heat treated dehusked little millet samples (Fig. 4). The decrease in crude fat content during storage may be attributed to the hydrolytic activity of lipolytic enzymes produced by microorganisms associated with the grains, resulting in the breakdown of lipids. The findings align with those of Huang et al. (2021) [7] , who reported a reduction in crude fat in dehulled millets over a 90 days period, with levels decreasing from 2.93 to 2.72%.

Crude fibre

The changes in crude fibre content of untreated dehusked little millet (Lc) and IR heat treated dehusked little millet (Lt) during storage are presented in Fig. 5. The initial crude fibre content was 5.68% in Lc and 5.66% in Lt. During the storage period, these values gradually decreased to 5.37% and 5.42%, respectively. However, no statistically significant difference (p > 0.05) was observed between the untreated and IR heat treated samples. The reduction in crude fibre content may be attributed to the enzymatic degradation of structural polysaccharides by microorganisms associated with the grains, particularly through the activity of cell wall-degrading enzymes such as cellulases. A similar reduction in crude fibre during storage was reported by Fagbohun and Oluwaniyi (2015) [6] , who observed a decrease from 10.27% to 8.52% in sun-dried rice after five months of storage.

Total ash

From the Fig. 6, it was noted that as the storage period progressed, the total ash of Lc and Lt decreased from 1.50 to 1.47% and from 1.49 to 1.47%, respectively, over 180 days. The total ash content did not differ significantly (p > 0.05) between the untreated and IR heat treated samples. A reduction in total ash content during storage was reported by Huang et al. (2021) [7] , who observed a reduction from 1.40 to 1.30% in dehulled millets after 90 days of storage.

Anti-nutrients Phenols

The changes in total phenolic content of untreated dehusked little millet (Lc) and IR heat treated dehusked little millet (Lt) during storage are presented in Fig. 7. At the beginning of storage, the phenolic content was 4.71 mg/g in Lc and 4.62 mg/g in Lt. A gradual reduction was observed in both samples over the storage period, reaching 4.03 mg/g and 4.18 mg/g, respectively, after 180 days. However, the phenolic content remained statistically comparable (p > 0.05) between the untreated and IR heat treated samples. A similar reduction in total phenolic content during storage was reported by Babiker et al. (2018) [2] , who observed a decrease from 194.10 to 142.80 mg/100 g in the white pearl millet cultivar after two months of storage.

Tannins

The changes in tannin content of untreated dehusked little millet (Lc) and IR heat treated dehusked little millet (Lt) during storage are presented in Fig. 8. Over the storage period, tannin content reduced from 0.39 to 0.22 mg/g in Lc and from 0.33 to 0.21 mg/g in Lt. Despite this reduction, no statistically significant difference (p > 0.05) was observed between the untreated and IR heat treated samples. A similar decreasing trend in tannin content during storage was reported by Babiker et al. (2018) [2] , who observed a reduction from 174.43 to 105.62 mg/100 g in the white pearl millet cultivar over 60 days of storage. The reduction in tannin levels may be attributed to enzymatic degradation and other biochemical reactions occurring during storage.

Phytic acid

The variation in phytic acid content of untreated dehusked little millet (Lc) and IR heat treated dehusked little millet (Lt) during storage is illustrated in Fig. 9. Throughout the storage period of 180 days, phytic acid content decreased from 7.37 to 6.28 mg/g in Lc and from 7.32 to 6.21 mg/g in Lt with no significant difference (p > 0.05) between the untreated and IR heat treated samples. A comparable reduction in phytic acid during storage was also reported by Babiker et al. (2018) [2] , who observed a decrease from 77.59 to 66.54 mg/100 g in white pearl millet after 60 days of storage.

Conclusion

No significant differences were observed in the proximate composition and anti-nutritional factors between the untreated dehusked little millet and IR heat treated dehusked little millet over the storage period of 180 days. Therefore, Infra-red heat treatment effectively maintained the biochemical characteristics of dehusked little millet throughout storage, demonstrating its suitability as a premilling treatment for improving storage stability without adversely affecting its nutritional quality.

Acknowledgement

AcknowledgEmentsThe authors express their gratitude to the All India Coordinated Research Project on Post-Harvest Engineering and Technology, UAS Raichur Centre, for funding this research project. We also thank UAS, CAE, Dept. of Processing and Food Engineering, Raichur for providing assistantship and also Karnataka Science and Technology Promotion Society (KSTePS), Government of Karnataka for awarding Karnataka DST-Ph.D. Fellowship-2024 for Science and Engineering is thankfully acknowledged.

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