Measurement of cooked rice stickiness with consideration of contact area in compression test.
The cooked rice stickiness is conventionally measured as the maximum detachment force required to separate the probe from the sample, after compression on the platform of a texture analyzer. A corrected stickiness (the measured stickiness force divided by the contact area) was newly created to avoid the deviation of stickiness influenced by the contact area varying with the rice sample's irregularity, regarding shape and size. The contact area could be estimated from the volume, density, and thickness of the samples. The compression force triggering the stickiness was also converted to the corrected compression force (the force divided by the contact area). Two varieties of rice (short-grain rice and glutinous rice) were cooked with different amount of water to prepare the samples with various stickiness levels. The contact area was mostly higher in the glutinous rice than short-grain rice. The contact area increased with the cooking water amount. Three parameters were compared such as the stickiness (the measured force), the primarily corrected stickiness (the corrected stickiness at a fixed value of compression force), and the secondarily corrected stickiness (the corrected stickiness at a fixed value of the corrected compression force). The difference between the rice samples was the most pronounced in the secondarily corrected stickiness. The correlation between the instrumental and sensorial stickiness was also the highest in the secondarily corrected stickiness. Conclusively, the new corrected parameters enhanced the sample discrimination capability and the agreement with the sensorial stickiness than the uncorrected stickiness. PRACTICAL APPLICATIONS: Stickiness is the important quality factor associated with cooked rice preferences. Instrumental stickiness is a detaching force of the preliminarily compressed rice grain by cylindrical probe of texture testing equipment. When it comes to a force measurement, the contact area is usually considered to convert the force to stress. But, for the samples like cooked rice grain, with smaller contact area than that of probe, the contact area is not easy to know because it is not simply the probe bottom area. So the contact area had not been employed to measure cooked rice stickiness. In this study, an established method was devised to figure out the contact area. The corrected stickiness by considering the contact area proved to be more accurate than the original stickiness through sensory evaluation.