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Firming Up Your Yogurt

Quality control in manufacturing has never been more important than it is today. Food products can be especially challenging because of the subjective nature of the many varied parameters leading to consumer acceptance. Beyond the safety issues are the characteristics of appearance, aroma, flavor and texture. This last parameter, texture, includes such sensory characteristics as thickness, firmness, chewiness and stickiness, among others.

This article will examine three different techniques for measuring some important parameters in cultured dairy products. The data shown are for yogurt, but the techniques would apply also to sour cream, pudding, gelatin preparations and the like.

Viscosity: For seventy years Brookfield Engineering Labs., Inc. has been providing a range of instrumentation to measure thickness, more analytically known as viscosity. Measuring viscosity with a rotational viscometer involves rotating a spindle in the sample. This technique, although accurate, can provide a misleading and confusing result when applied to cultured dairy products. This is due to the nature of the product itself rather than the method or the equipment used.

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Dairy products, such as yogurt, develop an easily broken gel structure during culturing. This structure is what makes a newly opened container of yogurt appear solid until it is stirred. Once the gel structure is broken by stirring, one then sees the familiar smooth, thick, creamy yogurt to which we are accustomed. Rotating the spindle during viscosity measurement also disturbs the gel structure. This results in a large, rapid drop in apparent viscosity at the beginning of the measurement period. The Yogurt Viscosity graph shows a drop in viscosity from 375,000cP to nearly 50,000 cP in only three minutes. 50,000 cP is a good measure of the viscosity of the yogurt after it has been stirred and is an indication of its "mouth feel" to the consumer.

Note that, it is difficult to determine from this one test the initial strength of the gel structure before the yogurt was disturbed. It is desirable for the manufacturer to produce a consistent gel structure in the yogurt to meet consumer expectations. The gel structure of yogurt is analogous to its yield stress and another way to measure this property is necessary.

Yield Stress: Measurement of yield stress can determine the strength of the delicate gel structure of a cultured dairy product without disturbing it. The YR-1 Yield Stress Rheometer slowly increases the torque force applied to a vane geometry spindle until rotation begins in the yogurt. Since the dimensions of the vane spindle and the torque constant of the rheometer are known, the test result of yield stress can be shown in Pascals versus strain in radians, as seen in the Yogurt Yield Stress graph. A yogurt with a firmer gel structure would have a steeper slope line and a higher yield stress value than would one with a softer gel structure.

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Texture Analysis: A novel way to gain comparative information about both viscosity and yield stress in a single test requires the use of a texture analyzer. A texture analyzer is essentially a load monitoring device to which a probe is attached. A test consists of driving the probe into the sample a predetermined distance at a constant speed. While the probe is descending, the force with which the sample is resisting is monitored.

The graph Yogurt Texture shows such a load profile. The slope of load force at the beginning of the test indicates the modulus, or rigidity, of the gel structure. The load force becomes steady toward the end of the end of the test once the probe is totally immersed and is descending further into the sample. This plateau levels at 395 g in this case and is an indication of both the viscosity of the yogurt as it flows past the descending probe and the strength of the new gel being broken beneath the probe.

Sample temperature, the container holding the sample, as well as every parameter of each test method will affect the test results. Consistency of all test conditions is imperative if repeatable test results are to be obtained.

Brookfield Engineering Laboratories, Inc.
11 Commerce Blvd.
Middleboro, MA, 02346