Influence of seed coat colour on nutrient content of sorghum genotypes in storage

Introduction

Sorghum [Sorghum bicolour (L.) Moench], a cultivated diploid (2n = 20) tropical cereal C 4 grass plant is a fifth most important cereal crop grown in the world followed by rice, wheat maize and barley. It is a monocotyledon plant of tropical origin, belonging to Poaceae family. Sorghum is called as camel of crops due to its high tolerance to water and temperature stress and also high photosynthesis efficiency. It is a warm temperate and tropical cereal and one of the most widely grown dry land food grain in India. It is nutritionally superior to rice because of its high mineral and fibre content. Starch (60-75%) is the main component of sorghum grain, followed by proteins (7-15%), non-starch polysaccharides (2-7%) and fat (1.5-6%). The average energetic value of whole sorghum grain flour is 356 kcal/100gm (Dicko et al., 2006) [3] . Seed coat is vital input in seed production. Seed coat colour is simple and excellent indicator of seed quality parameters. One of the unique features of sorghum is its variation in grain colour. The colour of seed coat of grains is most obvious physical characteristic which varies widely with white, cream, red, black etc. Seed coat colour is assumed to relate to tannin content and, hence, the nutritional quality of the grain. Seed storage and retention of seed viability is always an important consideration in agricultural practice. Poor storage conditions greatly affect seed vigour. The deterioration rate depends on storage condition i.e., temperature, relative humidity, seed moisture and types of storage container etc. Types of containers also regulate temperature, relative humidity and seed moisture content. It has been reported in the literature that fluctuation in temperature, humidity and prolonged storage result in considerable nutrient losses (Kumar and Singh, 1984) [5] . Changes in physical characteristics have also been observed during storage of cereal grains by Kent (1974). The association between the physical changes and the changes in the chemical composition of food has made the biochemical and nutritional quality control of the stored products increasing essential. The objective of the study was to investigate the effects of storage on nutritient content of sorghum genotypes differing in seed coat colour.

Materials and Methods

Seed materials

The different seed coat colour sorghum genotypes were procured from Department of genetics and plant breeding, College of Agriculture, Raichur. The seeds were harvested in the month of February 2021 and kept for storage under ambient condition from August 2021 to May 2022 (ten months) in the cloth bag. The experiment was laid out in Completely Randomized Design with total 20 genotypes replicated four times.

The protein content of seeds was estimated as per micro kjeldhal procedure as described by Jackson (1973) [4] . One gram sample was kept in the digestion flask, with a little quantity of catalyst mixture (K 2 SO 4 + CuSO 4 ), 10 ml of 96 percent sulphuric acid was added and kept for complete digestion. Digested sample was distilled. The distilled amount of ammonia was titrated with 0.1 N H 2 SO 4. The nitrogen and protein content was worked out as per following formula and expressed in percentage.

Nitrogen (%) = Normality of H2SO4 X BR X 0.014 × 100 Weight of sample Protein (%) = percent of nitrogen x 5.95

Starch was extracted by anthrone reagent as described by Clegg et al. (1956). It was extracted with perchloric acid after removal of sugars by extracting the sample with hot ethanol In hot acidic medium starch is hydrolysed to glucose and dehydrated to hydroxymethyl furfural. This compound on reaction with anthrone formed a green colour.

The content of zinc and iron was estimated using atomic absorption spectrophotometer (AAS) with zinc and iron hollow candle lamps (Jackson, 1973) [4] . The digested material with suitable dilutions was aspirated in to an AAS flame and concentration of zinc and iron was recorded in mg kg -1 by referring to standard curve (Plate 4). The zinc and iron concentration was calculated using the formula.

Micronutrient (Zn and Fe) = Concentration x volume of extract Weight of the sample

The crude fibre content of flour sample was determined by sequential acid and alkali hydrolysis method. One gram of fresh and dried ground sample was taken in a crucible. The sample was boiled in 1.25 percent sulphuric acid and subsequently boiled in 1.25 percent sodium hydroxide solution. The sample was dried in hot air oven at 100 °C till all the moisture is evaporated (around 20 min). The weight of the crucible before ashing was noted down. The obtained dried sample was made into ash in a muffle furnace at 200 °C for 4 h. After ashing, the crucibles was cooled in a desiccator and reweighed. The residue obtained after subtraction of the ash was regarded as fibre. The crude fibre was obtained by using the following equation.

Crude fibre (%) = W 1 -W 2 × 100 W

Where, W1 = Weight of the sample before ashing (g) W2 = Weight of the sample after ashing (g) W = Weight of the sample (g).

Tannin was estimated as per vanillin hydrochloride method. Extract 1g of ground seed in 50 ml methanol mix occasionally by swirling. After 20-28 hours, centrifuge and collect supernatant. Pipette out 1 ml of supernatant. Quickly add 5 ml of vanillin hydrochloride reagent. Read in spectrophotometer at 500 nm after 20 minute. Prepare the blank with vanillin hydrochloride reagent alone. Prepare standard graph with 20-100 µg catechin using diluted stock solution. From the standard graph, calculate the amount of tannin as catechin equivalent (CE) as per the absorbance values.

Data analysis

The mean data of the laboratory experiments was statistically analyzed by adoting appropriate statistical methods as outlined by Panse and Sukhatme (1985) [9] . The critical differences were calculated at one percent level of probability wherever 'F' test was found significant.

Results and Discussion

Seed coat colour effect the nutrient content of sorghum genotypes significantly during storage ( (2011) [1] studied different seed coat coloured cowpea genotypes and stated that, the decrease in protein content in unpigmented genotype over storage might be due to increased protein content which might have activated the proteolytic enzymes as a result of ageing effect leading to depletion of food reserves. Similar results were also found by Mokashi (2017) [8] in cowpea and Thombre et al. (2017) [11] [2] in paddy, Mokashi et al, 2017 [8] in cowpea.

The black seed coat colour genotype G 20 showed significantly highest iron content at initial and at end of storage period (52.88 mg kg -1 and 50.19 mg kg -1 respectively) followed by dark brown colour genotype G 17 (50.49 mg kg -1 and 46.98 mg kg -1 ). Whereas, genotype G 3 exhibited lowest iron content at initial month and at end of storage period (33.25 mg kg -1 and 30.16 mg kg -1 respectively).

The decline in iron content may be attributed due to fluctuation in temperature and relative humidity. Similar results were explained by, Pontieri et al. (2021) [10] in sorghum and Mokashi et al, 2017 [8] in cowpea. They stated that decrease in iron may be due to fluctuation in temperature as well as relative humidity during the prolonged storage resulting in considerable nutrient losses. The black seed coat colour genotype G 20 showed significantly highest zinc content at initial and at end of storage period (32.24 mg kg -1 and 29.98 mg kg -1 respectively) which was on par with dark brown colour genotype G 17 (30.48 mg kg -1 ) at initial month of storage. Whereas, genotype G 3 exhibited lowest zinc content at initial month and at end of storage period (26.12 mg kg -1 and 20.79 mg kg -1 respectively). These results indicate that mineral content of sorghum was affected by both genetic and environmental factors as suggested by Dhanam, (2009) [2] in paddy, Mokashi, (2017) [8] in cowpea and Pontieri et al. (2021) [10] in sorghum.

During initial month of storage, G 20 black seed coat colour genotype (2.64%) showed significantly highest crude fiber content while G 3 (2.10%), white seed coat colour genotype noticed lowest crude fiber content. After the end of ten months of storage, significantly highest crude fiber was recorded by black seed coat genotype G 20 (2.58%) which was on par with G 17 (2.53%), G 18 (2.51%), G 16 (2.48%), G 15 (2.44%), G 12 (2.37%) and the lowest was noticed by white seed coat colour genotype G 3 (1.77%).

Pontieri et al, (2021) [10] suggested that the highest crude fiber content in black seed coat colour genotype might have show dependency and linear correlation with protein content of grain, hence protein content and crude Fiber are interdependent. Increased protein content in pigmented genotypes makes them highest in crude fiber content and increased health benefits in addition to those conferred from phenolic compounds. These findings are also similar to the readings of Dhaman (2009) [2] in paddy, Mokashi (2017) [8] [6] in cowpea, Mabhuaudi and Modi, (2013) [7] in bambara groundnut and Mokashi (2017) [8] in cowpea.

Conclusion

Nutritional quality of cereal grains was adversely affected as a result of storage. All the nutrient content in sorghum genotypes decreased and significant losses have been occurred to various extents upon storage. In the view of these facts, it is suggested that black seed coat colour sorghum genotype is rich in nutrient content even after prolonged ten months storage and it can be stored for longer period of time.

References

1. Anuradha C, Gaur PM, Pande S, Gali KK, Ganesh M, Kumar J, et al. Mapping QTL for resistance to botrytis grey mould in chickpea. Euphytica. 2011;182(1):1-9.
2. Dhanam S. Effect of dairy effluent on seed germination, seedling growth and biochemical parameter in paddy. Botany Research International. 2009;2(2):61-63.
3. Dicko MH, Gruppen H, Traoré AS, Voragen AG, Van Berkel WJ. Sorghum grain as human food in Africa: Relevance of content of starch and amylase activities. African Journal of Biotechnology. 2006;5(5):384-395.
4. Jackson ML. Soil chemical analysis. New Delhi: Prentice Hall (India) Pvt. Ltd.; 1973.
5. Kumar R, Singh R. Levels of free sugars, intermediate metabolites and enzymes of sucrose-starch conversion in developing corn grains. Journal of Agricultural and Food Chemistry. 1984;32:806-808.
6. Laurena AC, Van Den T, Mendoza ME. Effect of tannins on the in vitro digestibility of cowpea (Vigna unguiculata). Journal of Agricultural and Food Chemistry. 1984;32(11):1045-1048.
7. Mabhaudhi T, Modi AT. Growth, phenological and yield responses of a bambara groundnut (Vigna subterranea (L.) Verdc.) landrace to imposed water stress under field conditions. South African Journal of Plant and Soil. 2013;30(2):69-79.
8. Mokashi YS. Effect of seed coat color on seed quality, nutrition and seed longevity in cowpea [Vigna unguiculata (L.) Walp.] [MSc thesis]. Bengaluru (IN): University of Agricultural Sciences; 2017.
9. Panse VG, Sukhatme PV. Statistical methods for agricultural workers. New Delhi: Indian Council of Agricultural Research; 1985. p. 158-161.
10. Pontieri P, Troisi J, Calcagnile M, Bean SR, Tilley M, Aramouni F, et al. Chemical composition, fatty acid and mineral content of food-grade white, red and black sorghum varieties grown in the Mediterranean environment. Foods. 2022;11(3):436.
11. Thombre MT, Damame SV, Lokhande PK, Naik RM. Physio-biochemical evaluation of soybean (Glycine max L.) genotypes exhibiting variable seed coat colour. Journal of Oilseeds Research. 2017;34(1):32-37.