Abstract:
Ofada rice(Oryza sativa, variety OS-6) is an indigenous rice variety highly sought mostly for its flavour. However, there are subjective noticeable variations in these flavour attributes which sometimes cast doubt on the rice quality. Profiling of rice flavour compounds and their formation during processing is sparsely documented. Hence, this study was to identify and characterise flavour compounds in Ofada rice as affected by paddy pre-treatment process.
The D-optimal mixture design of steeping time (1, 3 and 5 days), initial steeping temperature (30, 65 and 100oC), parboiling temperature (80, 100 and 120oC) and drying temperature (30, 50 and 70oC) as pre-treatment variables were adopted to condition Ofada rice paddy sourced from Ogun State Agricultural Development Program (OGADEP). The pre-treated 25 samples were processed into parboiled rice using standard procedures. The pH and total Titratable Acidity (TTA) of the fermenting steep water were determined, and bacterial type identified by standard methods. Proximate composition, amylose, Free Fatty Acid (FFA), flavonoid, Total Phenolic Content (TPC), Total Antioxidant Capacity (TAC), Ferric Reducing Antioxidant Power (FRAP) and 1,1-diphenyl-2-picrylhydrazyl radical scavenging assay (DPPH) of the processed rice were determined by standard procedures. Amino and organic acids in processed rice were analysed by liquid chromatography-mass spectrometry,while rice extracts from solvent extraction method were analysed using gas chromatography-mass spectrometry.Parboiled rice was cooked and evaluated using 25-member panelists. Data were analysed using descriptive statistics and ANOVA at α0.05.
The pH and TTAranged from 4.9 to 6.9, and 0.1 to 0.4 mg/mL, respectively. Predominant bacteria in the fermenting steep water were proteobacteria (Acinetobacter, Azotobacter, Pseudomonas, Enterobacter and Citrobacter) and firmicutes(Lactobacillus, Bacillus, Paenibacillus, Brevibacillus and Aneurinibacillus). Moisture, ash, fat, protein, crude fibre, carbohydrate, amylose contents and FFA of the rice were 4.2-10.2, 0.2-0.7, 0.6-1.7, 7.4-9.5, 0.6-1.7, 78.3-84.6, 17.4-22.3, and 0.7-3.8%, respectively. Flavonoid (0.6-1.5 mgQuercetin/g), TPC (25.4-62.6 mg GAE/g), TAC (0.5-3.9 mgGAE/g), FRAP (19.0-33.4 mgTrolox/g) and DPPH (277.8-1372.7 µg/mL) were obtained. Glutamic acid (8.5–26.1%), alanine (7.4-23.6%), proline (2.4-18.8%), asparagine (2.2-4.8%), aspartic acid (1.3–4.1%), threonine (1.2-3.6%), methionine(0.6-3.2%), serine (0.8-2.1%) and glycine (0.6-1.9%) were the major amino acids detected. Organic acids identified were 2-aminobutyric, nicotinic, pantothenic, pyruvic, lactic, citric, succinic and fumaric acids. The samples extracts contained 30 alcohols, 28 aldehydes, 50 hydrocarbons, 13 nitrogenous compounds, 36 ketones, 33 esters, 15heterocyclics, and 12 phenols. Aroma impacting compounds [pentanoic acid (green), 2-methyl-butanoic acid (cheese-like), butanoic acid (rancid), 1-octanol (metallic), decanal (citrusy), hexanal (fatty), 2-octenal (nutty), 4-methyl-2-heptanone (spicy) and 2-methoxy-4-vinylphenol (clove-like)] were detected mostly among samples steeped for five days.Only initial steeping temperature and duration significantly influenced composition of the compounds.Rice produced from paddy steeped for a day at 30oC initial steeping temperature, parboiled at 80oC and dried at 50oC was most acceptable to the panelists.
Flavourants identified in Ofada rice predominantly contained fatty, metallic, clove-like, citrusy, nutty and green flavour notes which varied with steeping treatments. Ofada paddy rice should not be steeped beyond a day for flavourants acceptability.
