Measuring the Texture of Biscuits with Rheometers

No matter what they are called, biscuits or cookies must have the right bite and the right taste. While the first is quite subjective and can result in lengthy discussions, the latter can be tested with a rheometer in a very objective way.

Other than the classic measuring parameters deformation, shear rate and stress, a modern rheometer comes equipped with an additional sensor to detect normal force. So, a rheometer can do texture analysis when combined with its precise lift.

Measuring the Texture of Biscuits with Rheometers

Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Materials and Methods

A Thermo Scientific™ HAAKE™ MARS™ iQ Rheometer (Figure 1) was equipped with the 3-point bending-accessory [1] for testing biscuits by utilizing the 8 mm plate as the probe as seen in Figure 2. Other probes can be adapted easily to the measuring head by employing the universal adapters U1 with a 6 mm bore or U2 with a 4 mm bore.

The Thermo Scientific HAAKE MARS iQ Rheometer.

Figure 1. The Thermo Scientific HAAKE MARS iQ Rheometer. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Setup for breaking biscuits in start position.

Figure 2. Setup for breaking biscuits in start position. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

The measuring protocol begins with zeroing the normal force and lowering the probe to a position that is still high enough to permit the quick placement of a fresh biscuit onto the lower supports of the 3-point-bending accessory. A position that is around 2 mm above the biscuits upper surface has been utilized for the tests discussed here.

To support the biscuit, the lower supports of the 3-point-bending-tool were positioned 5 cm apart from each other and around 0.8 cm from its edges. As seen in Figure 2, the biscuit was placed onto the lower supports with its ce ntre under the probe.

The probe moved towards the biscuit at a speed of 0.1 mm/s from its starting position. The measurement began when the surface of the biscuit was detected with a trigger force of 0.1 N. The corresponding lift position was utilized as zero position h = 0, from where the amount of bending was measured.

The probe went downwards with 1 mm/s from this point on, while 200 data points per second were recorded. The probe went up again to the starting position automatically when its downwards movement had finished.

A message was then displayed by the Thermo ScientificTM HAAKETM RheoWinTM Software to place the next biscuit and begin the next measurement with a mouse click or from the touchscreen. The number of required cycles was programmed in the RheoWin Job.

As seen in Figure 3, the data from the last measurement was saved using an automatically generated filename at the end of every cycle. The user just needed to place the biscuits and confirm to begin the next measurement.

RheoWin Job using the GOTO-element to create a loop to perform the same breaking test 5 times and saving the results separately during each cycle.

Figure 3. RheoWin Job using the GOTO-element to create a loop to perform the same breaking test 5 times and saving the results separately during each cycle. Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

The measurements were performed in two sessions. Biscuits were tested immediately after opening their sealed package during the first session and afterwards, the remaining biscuits were left on the lab bench in their opened packaging.

The absorption of moisture from the ambient air is the main change that can be expected and this typically leads to a slight softening of the biscuits. The remaining biscuits were tested with the same test technique after two weeks.

Results

The axial force FN is plotted as a function of the probe position h and the maximum of the curve is determined to assess the data (Figure 4, Figure 5).

Breaking tests on fresh biscuits (5 repetitions).

Figure 4. Breaking tests on fresh biscuits (5 repetitions). Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

Breaking tests on biscuits aged for 14 days in the opened package (5 repetitions).

Figure 5. Breaking tests on biscuits aged for 14 days in the opened package (5 repetitions). Image Credit: Thermo Fisher Scientific – Materials & Structural Analysis

The results scatter around an average value and this is totally normal as no two biscuits have exactly the same pore structure and density distribution. So, to get a statistically sound result, it is recommended to always carry out a set number of breaking tests on the same product.

The mean average and the standard deviation were calculated as a basis for the comparison from the individual results and are shown in Table 1. The aged biscuits broke on average at a 22% higher force compared to the fresh biscuits.

Table 1. Amount of bending and force at curve maximum of fresh and aged biscuits. Source: Thermo Fisher Scientific – Materials & Structural Analysis

No. Fresh biscuits Biscuits aged for 14 days
Δh / mm Fn / N Δh / mm Fn / N
1 0.633 5.384 0.633 6.395
2 0.563 6.395 0.596 7.927
3 0.596 5.943 0.584 7.545
4 0.501 5.332 0.530 7.289
5 0.490 6.684 0.562 7.065
Mean value 0.537 5.948 0.581 7.244
StdDev 0.039 0.536 0.034 0.512
rel. StdDev 7.31% 9.02% 5.90% 7.07%

 

The absorption of moisture has led to a more flexible structure. This assumption is confirmed by the bending of the biscuits at maximum force, which is increased, on average, by 8%.

Conclusions

The HAAKE MARS iQ Rheometer can measure the effects of the storage of biscuits in the open pack on their texture when supplemented by a simple accessory. Over time the absorption of humidity softens the cookies and so will alter their crunchiness and their perception by the consumer.

In addition to the rheological characterization of the ingredients and the dough prepared from them, the HAAKE MARS iQ Rheometer also enables the quantification of the texture of the final product, providing a full analysis of every step from the raw materials to final product.

References

  1. Thermo Fisher Scientific Product information P014 “Sample fixture for bending and breaking tests for HAAKE MARS Rheometer“ Cornelia Küchenmeister-Lehrheuer, Klaus Oldörp

Acknowledgments

Produced from materials originally authored by Klaus Oldörp from Thermo Fisher Scientific.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers.

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