Functional characteristics of egusi seed (Citrullus lanatus) hydrocolloid and oil in instant egusi soup

Abstract

The use of egusi melon in soup has been domesticated and egusi has predominantly been regarded as a secondary crop. The aim of this study was to evaluate the functional characteristics of egusi seed (Citrullus lanatus subsp mucosospermus) hydrocolloid and oil in instant egusi soup. An instant production of egusi soup by adding boiling water to an instant soup mix will promote the availability of this nutritious seed as a healthy meal option. Egusi oil was successfully extracted from egusi seed using supercritical carbon dioxide method. The percentage oil yield which measures the amount of oil derivable from egusi seed ranged from 46-53% w/w. There was a significant difference in the oil yield as pressure and temperature are increased at a constant CO2 flow rate of 30 g/h. The proximate composition of egusi oil was determined using standard AOAC method. The moisture contents were 1.3, 2.0 and 1.9% w/w, respectively for EO1 (60oC and 450 bar), EO2 (55oC and 600 bar) and EO3 (75oC and 600 bar). EO1 was significantly (p ≤ 0.05) lower in moisture content compared to EO2 and EO3. The fat content was 99.1% w/w (EO1), 98.3% w/w (EO2) and 98.9% w/w (EO3), with no significant difference in the three oil samples. The fatty acid composition was analyzed using gas chromatography. The fatty acid content of egusi oil was high in polyunsaturated and monounsaturated fatty acids, which was identified as linoleic (62%) and oleic (15%) acids. The saturated fatty acid (undecylic, myristic, palmitic and stearic) composition of egusi oil differed significantly (p ≤ 0.05) with EO1, having the lowest compared to EO2 and EO3. The index of atherogenicity (IA %) were significantly low 0.35, 0.38 and 0.38% w/w for EO1, EO2, and EO3, respectively. The thrombogenicity index (IT %) were 0.08, 0.09 and 0.09% w/w for EO1, EO2, and EO3, respectively with no significant difference. Peroxide Value (PV) measured using auto titrate Titrino plus, ranging from 11.60 for EO1 milliequivalents peroxide/kg to 12.60 for EO2 and 11.89 milliequivalents peroxide/kg for EO3. The oxidative stability index (OSI) was measured using Methrohm Rancimat at 120°C, expressed as the induction time of oxidation was 10.2, 11.5 and 5.3 h for EO1, EO2, and EO3, respectively, with E03 significantly higher than EO1 and EO2 (p ≤ 0.05). The iodine number, determined by AOAC direct titration method was high and ranged from 95 g/100 g for EO1 to 129 g/100 g for EO3, with EO3 being significantly high (p ≤ 0.05). The nutritional and functional properties of defatted egusi flour and hydrocolloid extracted using supercritical carbon dioxide extraction was also successfully achieved. Proximate analysis of defatted egusi (DEF) flour after supercritical extraction was carried out according to standard AOAC procedures. The moisture content of DEF ranged from 5.3 to 10.1% w/w, crude protein 48.3 to 60.4% w/w, crude fibre 3.4 to 4.5% w/w) and ash 5.3 to 6.8 % w/w). The protein content of defatted egusi flour differed significantly (p ≤ 0.05) between samples. The amino acid compositions of DEF showed glutamic acid had the highest concentration of 12.9, 11.8 and 9.8 mg/100 g for DEF1, DEF2, and DEF3, respectively with a significant difference (p ≤ 0.05) across the samples. In functionality, the water absorption and solubility index at a low temperature of 50oC ranged between 52.5 to 57.6% w/w and 68.0 to 73.3% w/w respectively for DEF1 to DEF3, which significantly differed between samples. The final viscosity of defatted egusi flour ranged from 126.7 to 126.3 cP, which differed significantly (p ≤ 0.05) between samples. Egusi flour is high in protein 60% w/w and carbohydrate 25% w/w was treated with hot water to extract its hydrocolloid. Functional properties of egusi hydrocolloid for the three defatted flour shows a stable emulsifier as the breakdown viscosity remained constant at (8.00 cP). Breakdown viscosity of egusi hydrocolloid confirmed its stability.