Dielectric Impedance Measurements of Chocolates

Swiss chocolates are famous worldwide thanks to their high quality of taste. Switzerland is also renowned as the country with the highest chocolate consumption per capita [1]. Today, many popular brands can be found even in the supermarket. Are you curious about what makes the chocolates special? And which non-destructive technique (NDT) can be employed to measure them?

In this blog post, we use dielectric (impedance) spectroscopy to confirm the difference and aging effect of chocolates. The measurements are carried out with a Zurich Instruments MFIA Impedance Analyzer, an instrument also made in Zurich, Switzerland. We examine 3 different chocolates: Caramel Salé, Noir Extra (85% cacao content), and Noir Satin (65% cacao content). These chocolate pieces are about 6-mm-thick, and larger than the electrode area (with a diameter of 38 mm) of our dielectric fixture. This ensures that the dielectric spectroscopy can be conducted properly, where the result shows no strong dependence on position.

As explained in [2], dielectric measurements often need a model with more than 2 equivalent circuit elements. For instance, contact resistance Rs often plays a significant role at low frequencies. Therefore, the capacitance (Cp) derived from a simplified Rp||Cp or Cp||D model in LabOne software turns out frequency-dependent (ideally, at MHz or lower, the dielectric constant should really remain a constant). Nevertheless, the result is still valid for a qualitative comparison. We can distinguish the 3 chocolates, and also determine their different aging effects in a non-destructive manner.

Figure 1 shows the result of Noir Extra. For better readability, we only present the zoom-in view between 1 kHz and 500 kHz, despite actual measurements covering a much wider range from 100 mHz to 5 MHz. The red trace is measured fresh, with the following measurements taken at a ~6-hour interval. The orange trace taken after 6 hours shows a slight increase. If we look at 100 kHz, the capacitance rises from 3.89 pF to 3.91 pF, hinting likely an aging effect due to oxidation. The capacitance continues to increase in the blue trace, measured after 12 hours. All the measurements later (green, after 18 hours, and purple, after 24 hours) overlap, indicating aging has already become stabilized. With a bit of correction and normalization to the chocolate's dimension, we calculate a (relative) dielectric constant of 2.44. This value is higher than chocolate powder (2.0) in literature [3], but do note that the Noir Extra chocolate is not a pure substance. Other ingredients make the taste special, and change the dielectric properties at the same time.

Figure 2 shows the result of Caramel Salé. The fresh measurement (red trace) shows a capacitance of 4.73 pF at 100 pF, with which we can calculate a dielectric constant of 2.99. What is more interesting is that Caramel Salé chocolate reveals a strikingly different aging trend from Noir Extra. The capacitance decreases at first in the initial 12 hours (orange and blue), but increases again at 18 hours (green), and becomes stable afterward (purple, 24 hours). Obviously, this difference comes likely from the added caramel content. Further study might be helpful to unveil the chemistry inside.

To rule out the possibility that aging may originate from the electrical current in our tests, thereby proving the method is non-destructive, we use a weaker test signal at 300 mV on Noir Satin. Figure 3 shows the outcome. Since the current is lower, the measured capacitance traces appear to be noisier at the low-frequency end (below 1 kHz, not shown). However, we are able to confirm the same aging trend as in Noir Extra, where the measured capacitance only increases with time until saturation. This further implies that the decreasing capacitance in Caramel Salé is related to caramel, which is not present in Noir Extra nor Noir Satin. Using the capacitance of 4.08 pF at 100 kHz, we can obtain a dielectric constant of 2.56 in Noir Satin, only slightly higher than in Noir Extra (2.44). This small difference may arise from the cacao content.

To summarize, we find that chocolates can be differentiated with dielectric impedance spectroscopy. These non-destructive measurements are sensitive enough to resolve the tiny changes in the chocolates' capacitance over time, therefore useful to study the aging behavior. More importantly, please remember to eat your delicious chocolates once opening the package, ideally within 12 hours.

If you are interested in discussing how impedance measurements can help in food science research, please get in touch.

Reference

[1] Wikipedia. https://en.wikipedia.org/wiki/Swiss_chocolate

[2] Zurich Instruments blog. https://www.zhinst.com/ch/en/blogs/taking-dielectric-measurements-to-liquid-state

[3] List of material dielectric constants. https://www.vega.com/-/media/pdf-files/list_of_dielectric_constants_en.pdf