EFFECT OF PROCESSING CONDITIONS ON PHYSICAL, CHEMICAL, COOKING, AND SENSORPROPERTIES OF OFADA RICE(ORYZA SATIVA L) GRAIN AND FLAKES

Abstract

Rice is a staple food in Nigeria with consumer preference for imported varieties due to good quality. Ofada rice (OS6) is a local variety characterised with unpleasant smell and unappealing appearance. Previous researches to improve Ofada rice quality have not satisfactorily solved the problem. This study was designed to determine effect of processing conditions on physical, chemical, cooking and sensory properties of Ofada rice and flakes.

Trial experiments were done to determine variables and responses of experimental design. The variables (storage, soaking, parboiling and drying) were interacted using response surface methodology and responses (physical, chemical, cooking, and pasting properties) for each experimental combination were analyses. Physical (length, width, breadth, head rice yield, brokenness, chalkiness and colour) properties of the samples were determined using ASABE methods. Chemical (protein, fat, ash, carbohydrate, metabolisable energy, free fatty acids, and amylase, zinc, iron, magnesium, copper, potassium and calcium) properties were determined using AOAC methods. Methods of AACC were used for determination of cooking (cooking time, water uptake ratio, solid loss and elongation) and pasting properties. Ready-to-eat rice flakes were produced by pressing and roasting pressure-cooked rice dough. Proprietary software was used to model and optimise process parameters for both grain and flakes. Predicted optimum conditions for rice grain and flakes processing were validated using experimental values. Adsorption isotherm of flake was determined at 25, 35 and 45 ⁰C for 14 days using gravimetric method. Sensory characteristics of cooked rice and flake produced at optimised condition were determined using panelists.Data were analysed using regression and ANOVA at p=0.05.

The ranges of length, width, breadth, head rice yield, brokenness, chalkiness and colour lightness of the rice grain were 5.5-6.3 mm, 2.5-3.0 mm, 1.9-2.4 mm, 50.7-76.5 %, 0.8-83.1 %, 0-49.6 %, and 66.0-78.8 respectively. The rice contained protein (8.4- 10.4%), fat (0.7-3.8%), ash (0.1-1.0%), carbohydrate (78.6-84.4 %), metabolisable energy (390.3-395.2 kcal/100g), free fatty acid(1.2-4.8%) and amylase (18.5-26.5 %). Storage, soaking, parboiling and drying temperature significantly influenced cooking properties of rice grain. Iron (24.0-75.0 mg/kg), zinc (13.0-42.5 mg/kg) and magnesium (234.3-366.2 mg/kg) were the major minerals found in the rice grain. Drying temperature and storage duration significantly influenced pasting properties of the grain. Coefficient of determination (R2) of predicted models ranged from 0.1 to 0.8. The best rice grain was obtained at grain storage for eight months and 19 days, soaked for one day and 20 h, parboiled at 113.0 °C, and dried at 43.2 °C while most acceptable flakes were obtained at grain storage of nine months, soaked for four days and 17 h, parboiled at 106 °C and dried at 30 °C. Deviation between experimental and predicted values ranged between 0.9 and 0.93 % for rice grain and flakes. Adsorption isotherms of the flakes best fitted into Guggenheim Anderson De Boer model (R2=0.96). Flavour and appearance of the rice and flake produced at optimised condition were acceptable to panelists.

Appropriate storage duration and processing conditions have been established and found to successfully eliminate unpleasant odour and improved appearance of Ofada rice grain. Ofada rice was found suitable for production of ready-to-eat flakes which is an alternative means of harnessing potential of Ofada rice.