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From: Food Quality & Safety magazine, December/January 2005

Measuring Quality in Cultured Dairy Products

Test method comparisons can hone cultured dairy product characterizations.

by Len Thibodeau

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.

Determining Flowability

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.

Firmness Factor

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.

Both the flow-ability (viscosity) and spoon-ability (yield point) of cultured dairy products may also be compared by using a single test. A texture analyzer can be used to drive a cylindrical probe slowly into the product while measuring the force required during probe travel. A typical test with a texture analyzer consists of driving a probe of specific dimensions at a constant speed into a product at specified distances, then withdrawing it. The increase in positive force (or load) recorded just after the probe contacts the sample surface is an indicator of stiffness, or modulus, in the gel structure of the yogurt. In Figure 4 this slope is indicated by the dotted lines labeled “modulus”.

As the probe descends and displaces the sample, the combination of stiffness and flow-ability contributes to the force profile. The shallower slope and lower, flatter force plateau of the full fat yogurt are caused by its softer gel set and smoother flow. Its appearance is glossy compared to the graininess noticed in the low and non-fat variety. This characteristic is represented by the absence of peaks along the plateau of the full fat yogurt. Then, as the probe withdraws, a negative load is encountered because the yogurt adheres to the probe. These force profiles become the primary test result.

Samples can be compared, and quality control can be assured, by repeating the same test in successive products and comparing results. A manufacturer might use this information to adjust the formulation until the force profiles compare favorably to that of the full fat product. This result would correlate to a more similar “mouth feel” to the consumer.

Sample and test considerations which will affect results of a texture analysis test are:

  • Sample temperature;
  • Dimension of sample containers;
  • Flat and level surface of the sample;
  • Homogeneity (fruit bits will add variability for example);
  • Dimensions of test probe;
  • Probe travel speed and penetration distance.

Each of the test techniques described in this article can provide information about one or more of the physical properties of cultured dairy products. When used in combination these techniques will complement and reinforce each other.

Len Thibodeau is a senior sales engineer for Brookfield Engineering Laboratories (Middleboro, Mass.). He can be reached 508-946-6200, ext. 199 or l_thibodeau@brookfieldengineering.com.

Norovirus Grown in Lab

The norovirus, a highly contagious source of food poisoning, has been successfully grown in a laboratory for the first time by researchers at Washington University in St. Louis, Mo., a feat that could speed the development of a vaccine.

Noroviruses are highly contagious and cause diarrhea, vomiting and other problems. Two years ago, these viruses caused repeated outbreaks of illness on cruise ships that were widely reported in the media.

Scientists showed that the mouse norovirus MNV-1 could be grown inside cells from mice with defective immune systems. The university said those findings could ease the way for further research about the mouse virus and may help researchers seeking to duplicate the accomplishment with human forms.

“By looking at the mouse virus we’d grown in the lab, we were able to identify a part of the capsid, the virus’s protein shell, that is essential to its ability to cause disease,” senior author Dr. Skip Virgin, a professor of pathology and immunology and molecular biology, said in a prepared statement.

“If this part of the capsid has an equivalent in human noroviruses, altering or disabling it may give us a way to produce forms of the viruses that are weak enough to serve as vaccines,” he added.

According to the U.S. Centers for Disease Control and Prevention, noroviruses are involved in about half of all food poisoning cases and annually cause about 23 million cases of acute gastroenteritis in the United States.

Norovirus disease is characterized by frequent vomiting and diarrhea over the course of one to two days. The most infamous norovirus, the Norwalk virus, was first identified after a 1968 outbreak at a school in Norwalk, Ohio. The Norwalk virus also caused a series of repeated outbreaks on cruise ships in 2002 and in military personnel in Afghanistan.

All previous attempts to culture human noroviruses in tissues in the laboratory have been unsuccessful, and Virgin says that as a group, noroviruses have defied characterization for decades because there has not been a way to get the virus to “grow outside of a human host.”

In 2003, Christianne Wobus, Ph.D., and Stephanie Karst, Ph.D., two postdoctoral fellows in Virgin’s lab, identified MNV-1, the first known mouse norovirus. Virgin’s group showed that mice’s ability to fight MNV-1 relied heavily on the innate immune system, the branch of the immune system that attacks invaders soon after they enter the body.

In its report, Virgin’s group reveals that MNV-1 likes to infect cells of the innate immune system. In tests in mice, the researchers found the virus thrived in macrophages, immune system cells that normally engulf and destroy pathogens, and in dendritic cells, sentry-like cells that pick up and display proteins from pathogens.

“We think there may be dendritic cells just beneath the lining of the human gut that are providing the gateway the virus needs to cause disease,” Virgin says. To grow the virus in the lab, researchers took dendritic cells and macrophages from mice with defective innate immune systems and exposed them to the virus.

“The virus grew beautifully,” Virgin says. “It’s a very facile and robust system.”

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