Process optimisation and utilisation of resistant starch from sorghum (Sorghum bicolor (L.) Moench)

Abstract

Sorghum, commonly named as great millet are gluten free grains with good nutritional profile. Sorghum grains contain higher amount of resistant starch than other cereals and seems to be more slowly digestible. Resistant starch (RS) is the sum of starch and products of starch degradation which is not absorbed in the small intestine of healthy individuals. In this context, the present study, entitled "Process optimisation and utilisation of resistant starch from sorghum (Sorghum bicolor (L.) Moench), was undertaken with the objectives of optimising the conditions for resistant starch formation in sorghum, its quality evaluation and product development In the present study, the sorghum starch samples were autoclaved at 120°C and 140°C with 10 per cent moisture for 15, 30, 60 minutes consecutively (T2-T7). The above mentioned procedure was repeated by the replacing the moisture with 20 per cent (T8-T13) and 30 per cent (T14-T19). Sample autoclaved at 120⁰C for 15 minutes with 20 per cent moisture was found to have maximum amount of resistant starch (38.19%). The selected starch sample (T8) was subjected to repeated autoclaving and cooling cycles for 2, 3, 4 times. Treatment T8 after 3 repeated autoclaving and cooling cycles, showed maximum RS content (39.22%). As the number of cooling cycles increased the amount of resistant starch in the sample increased. The selected sorghum resistant starch along with sorghum starch were evaluated for its quality aspects. Sorghum resistant starch had a higher pH value of 6.02 whereas native starch had a pH of 5.70. The moisture content of sorghum resistant starch (9.93%) was observed to be lower than that of sorghum starch (12%). The water holding capacity of resistant starch was estimated and found to be 2.03 g water/g flour which was statistically different from the water holding capacity of sorghum starch which was estimated to be 1.09 g water /g flour. The bulk density of sorghum resistant starch was 0.83g/mm3 whereas that of sorghum starch was estimated to be 0.88 g/ mm3. The retrogradation property of resistant starch (60%) was lower than that of sorghum starch (65 %). The gelatinisation temperature of resistant starch (74.50ºC) was estimated to be slightly higher than that of sorghum starch (72ºC). The carbohydrate content of resistant starch was found to be 95.33g/100 g and that of sorghum starch was found to be 95.43g/100 g. The starch content of resistant starch (87g/100 g) and sorghum starch (87.32g/100 g) showed no significant difference. The amylose content of resistant starch was found to be 27.20 per cent and in the sorghum starch it was 23.10 per cent. The amylopectin content of resistant starch was 59.80 per cent whereas that of sorghum starch was 67.22 per cent. The total sugar content of resistant starch was 3.53 per cent whereas sorghum starch had a total sugar content of 3.61 per cent. The in vitro digestibility of resistant starch was 10.31 per cent which was significantly lower than that of sorghum starch (62%). The prepared starch was packed in laminated pouches and stored for a period of three months. The pH of the flour gradually increased from 6.02 to 6.99 after the third month of storage. The bulk density was initially 0.83 g/mm3 which increased to 0.97 g/mm3 after the third month of storage. The water holding capacity of the flour was found to be 2.03 g water/ g flour initially which decreased to 1.96 g water/g flour after third month of storage. The reterogradation property was found to be 60.67 per cent initially which increased to 62.33 per cent, 63.58 per cent and 64.21 per cent after the first, second and third months respectively. The effect of storage on nutrient composition of the sorghum resistant starch was also studied. The carbohydrate content was found to be 95.33 g/100 g initially, which decreased after third month of storage (92.41g/100g). The starch content of sorghum resistant starch had a statistically significant difference on storage. It was initially found to be 87g/100g which decreased to 84.72 g/100g after three month of storage. The total sugars also decreased from 3.523 per cent to 2.99 per cent on storage. The in vitro digestibility of the starch decreased with increasing storage. It was initially 10.31 per cent which decreased to 10.30, 10.06 and 10.02 per cent after first, second and third month respectively. The glycemic index of the resistant starch and sorghum