Cultured dairy products such as yogurt, sour cream and the like exhibit several physical properties which give them their unique character. Consumers may subjectively describe these as firmness, creaminess, thickness or even heaviness or lightness. Manufacturers can find subjective, sensory terms like these difficult to measure in the lab, and impossible to use for setting upper and lower quality control limits.
This article compares three analytical test methods that could be used by quality control to characterize the physical properties of cultured dairy products, with results that could be correlated to the more subjective, sensory properties.
In order to determine viscosity, or flow-ability, we must make the product flow. This is accomplished by rotating a vane spindle in the product (Figure 1). However, doing so quickly destroys the “set” gel structure first observed when these products are opened.
Figure 2 shows an apparent large drop in viscosity as soon as the measurement begins. These data are from non-fat yogurt, low-fat yogurt and full-fat yogurt of the same brand. It appears that within two minutes, nearly 80 percent of the initial viscosity of the samples has been lost. What is actually happening is that the initial gel structure is being broken up, just as the consumer stirs the product.
A test such as this can show two pieces of information which may be helpful to the manufacturer.
By using an established test method of specifying rotational speed, spindle and product temperature, initial firmness can be indicated and the break up of the gel structure can be monitored.
The viscosity after stirring can be determined by running the test until the viscosity levels off. In the case shown in Figure 2, such a test would take about 2 1/ 2 minutes. Viscosity of the low-fat sample is about 10 percent higher and the non-fat is 33 percent higher than that of the full fat variety.
However, viscosity alone does not tell the whole story. An important parameter that cannot be accurately determined from the above test is the strength or firmness of the gel before it is disturbed. While the curve for the non-fat yogurt in Figure 2 does appear to start out at a much higher level than that of the full fat yogurt, it is difficult from this test to pin-point exactly how much higher.
Measuring the undisturbed stiffness, or strength of the set gel structure of cultured dairy products can indicate its “spoon-ability,” which is the characteristic allowing the product to be scooped without dripping. An analytical measurement of this property can easily be done with a separate test using a vane spindle. The spindle may be lowered into the product with minimal disturbance of the product, and therefore minimal destruction of the gel. The test is conducted by applying a steadily increasing torque force on the vane spindle, until it begins rotating. The force required to begin spindle rotation is determined by the stiffness of the gel, and is termed the yield stress point of the gel.
Figure 3 shows the results of this test for all three types of yogurt. The full-fat variety has the softest gel structure as indicated by the lowest yield stress curve. The gel structure of the low-fat variety is 60 percent stronger, and the non-fat variety is the stiffest with a gel 125 percent stronger that of the full fat yogurt.
To the consumer, all of these products have very different firmness upon opening the containers. This result may seem surprising since the viscosity profiles appeared to be so similar. However, if one looks at the difference in viscosity at the beginning of the test, a similar result is indicated.