Creaminess is the term used for the velvety coating sensation normally assessed in the tongue and palate. It is one of the most relevant textural properties of risotto. Understanding creaminess requieres a multidisciplinary effort: from physical and chemical techniques to sensory perception studies and human-food interaction knowledge. After a review of the literature, we can conclude that it is undoubtedly related to thickness and smoothness. Other factors such as flavour or surface properties may also contribute to the perceived creaminess. An attempt to quantify creaminess was proposed by Kokini and Cussler1:
Creaminess = (Thickness)a(Smoothness)b
where a = 0.54 ± 0.1, b = 0.84±0.1. The expression above has been experimentally verified for a variety of commercial foods: liquids, cream like products, semi-solid high fat food and frozen products showing a correlation coefficient of 0.9. But, do we know how to quantify these two sensory properties?
Smoothness. It has been suggested that it is related to the size and shape of the particles, as well as to the distance between particles. The evaluation of smoothness is related to the frictional forces on the tongue. Kokini and Cussler1 suggested that smoothness could be inversely proportional to the friction force (Ff), Ff = μ Ft where μ is the friction coefficient and Ft is the normal force done by the tongue on the food.
Thickness. It is related to shear forces exerted on the tongue. These forces are related to the viscosity of the product. The precision of our mechanoreceptors feels differences of 1mPa·s in viscosity. The viscosity of certain Newtonian liquids, such as 1-2% rice starch solution is of the same order of magnitude. Different models have been proposed to find a correlation between instrumental and sensory viscosity (what we have above referred as thickness). Cutler et al.2 found for Newtonian fluids that the logarithm of the perceived thickness was linearly correlated to the logarithm of the measured viscosity with a correlation coefficient r=0.995.
How to measure viscosity (η)?
The various types of viscometers are classified according to the principle they use. For practical reasons we will only consider two types before we decide which one use in the experiments.
- Capillary viscometer.The time (t) for a given volume of fluid (v) to pass through a capillary is measured. The flow follows the Hagen-Poiseuille equation:
η = πP R4 t / 8vl ,
where P is the pressure, l the length of the tube and R its radius. The pressure is usually created by the gravity force. Nowadays, we can fine a huge variety of capillary viscometers. The Lamb-Lewis viscometer is a low cost and easy to use instrument that provides a clear end point to decide when stop the timer. The detailed procedure to made that viscometer in the lab is given in the Official Methods and Analysis (2000): Method 967.16.
- Falling Ball viscometerIt is based on the principle of the limiting velocity of a falling sphere that is reached when the friction force of the fluid on the ball is compensated by the acceleration due to the force of gravity. The time for the ball to fall through the fluid is measured. The Stokes equation can be used when the diameter of the ball is much more smaller than the diameter of the tube that contains the fluid.
η = 2[ (ρs – ρL )gr2 ]/9V,
where ρs is the sphere density and ρL the fluid density, r is the sphere radius, V the limiting velocity and g is the gravity acceleration.
This is another low cost method that can be easily implemented in the lab. It is advisable to allow sufficient distance as the sphere has to reach the limiting velocity. Also, the larger the ball the faster will fall. Although commonly steel balls are used, glass marble spheres have a lower density and therefore will fall slower. One of the greatest advantages of this method is that uncertainties can be better determined than in capillary viscometers. Lommatzsch et al.3 deduced an expression following theoretical considerations regarding the effect of the walls and the inertial effects governed by the Reynolds number. By knowing the uncertainty in: the mass of the ball, the diameter of the ball, the diameter of the tube and the density of the fluid we can calculate the precision needed in the limiting velocity for a desired uncertainty in viscosity. The average limiting velocity can be calculated by recording the position (zi) of the ball at constant time intervals (Δt), then:
V =( zi+1 – zi )/ Δt
1 Kokini, Jozef L. and Cussler, E.L. 1983. Predicting the Texture of Liquid and Melting Semi-Solid Foods in Journal of Food Science. 48(4):1221-1225
2Cutler, A. Norman and Morris, Edwin R. and Taylor,L. 1983.Oral perception of viscosity in fluid foods and model systems in Journal of texture studies. 14(4): 377-395
3Lommatzsch, T. and Megharfi, M. and Mahe, E. and Devin, E. 2001. Conceptual study of an absolute falling-ball viscometer in Metrologia. 38(6)