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Electrical Impedance Spectroscopy for Commercial Milk Characterization

In an article published in Applied Sciences, researchers have proposed a novel method utilizing electrical impedance spectroscopy (EIS) to categorize the various kinds of commercial ultra-high temperature milk frequently found in grocery shops and supermarkets worldwide.

Study: UHT Milk Characterization by Electrical Impedance Spectroscopy. Image Credit: Parilov/Shutterstock.com

The most widely consumed type of milk in Europe is the ultra-high temperature (UHT) pasteurized milk. The measured admittance curves derived from the electrical impedance spectroscopy and the values of the equivalent circuit model enabled the classification of the milk type (whole, semi-skimmed, fat-free) and the differentiation of lactose-free milk.

The various circuit parameter values derived from an equivalent circuit model provided intriguing data regarding the grading of the samples. The categorization enabled the distinction between milk stored in the fridge and at ambient temperature and the recognition of expired milk by detecting its degradation before it was humanly visible.

Electrical Impedance Spectroscopy and Food Safety

Milk is a crucial component of human nutrition for growth and maintaining health. Today's market offers a wide range of milk that is categorized based on various characteristics, including:

  • Thermal processing method (pasteurized, raw, ultra-pasteurized)
  • Fat content (fat-free, skimmed, semi-skimmed, whole)
  • Nutrient concentration (fortified with vitamins)
  • The presence or absence of lactose

Thorough and cautious testing methods have been developed to ensure milk quality as milk is susceptible to deterioration and adulteration. For example, methods to evaluate the presence of heavy metals in milk were developed in light of the rising environmental pollutants. A method for determining lactose in commercial UHT milk samples was designed and validated using liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). However, the creation and optimization of alternative procedures is the main focus of the study due to the expense and complexity of the typical studies.

Electrical impedance spectroscopy (EIS) is a reliable research technique in food safety. It can help study milk quality, detect aflatoxins, quantify the bacteria content in raw milk, study milk characterization and composition, and detect milk adulteration.

The study uses the electrical impedance spectroscopy technique and other characterization techniques. It necessitates special sample preparation and utilizes hardware and sensors specially designed for this purpose. The experiments in the present work focus on studying electrical properties such as conductivity and dielectric permittivity instead of impedance.

Analytical Methods of Electrical Impedance Spectroscopy

For the characterization, samples of semi-skimmed (SSM) and whole (WM) types of UHT milk from four distinct brands were considered.

Lactose-free (LFM) UHT milk samples were also considered for three of the four brands due to easy availability. Fat-free (FF) UHT milk samples, one of which was lactose-free, were considered for two brands.

The Agilent 4284A benchtop LCR meter was used to record the impedance measurements. The tests were conducted between a 20 and 100 kHz frequency range. A dedicated piece of software was used to connect the LCR Meter to a computer, making it easier to visualize the measured admittances and store the data for later processing. Before measurements were taken, the calibration corrections were made to remove any error due to the impedance of the wires.

The samples were measured at 23 °C, and six distinct packets of these samples were classified for each type of UHT milk. Except for the expired samples, all the samples under comparison had the exact expiry date.

The characteristics of all UHT milk types were compared by category (whole, semi-skimmed), and the varieties of UHT milk from the same brand were contrasted, emphasizing the differences between whole, skimmed, fat-free, and semi-skimmed. The study also focused on the variation in characteristics of UHT milk stored in or outside the refrigerator. Characteristics of expired milk were also analyzed.

Electrical Impedance Spectroscopy Ushers in a New Era of Commercial Milk Characterization

Although admittance measurement is an intriguing method for characterizing milk with known composition, it might not work as a blind detection tool. The equivalent circuit model forming the electrical description of the milk's response was obtained from the curves of the measured admittances to perform a more quantitative study that can dispel any uncertainties.

A parallel circuit between a capacitor, a resistance, and a series of capacitance forms the base of the equivalent circuit model of milk. In the equivalent circuit model, the resistance R denotes the milk conductance while the capacitance C denotes the capacitance of the two electrodes separated by the characterized milk.

Concerning three of the four brands examined, it is evident from the results of the equivalent circuit model that whole milk has a lesser value of capacitance than semi-skimmed milk. Such an observation might be because fats in the milk have a lower dielectric constant than its other constituents.

The results of the equivalent circuit model also indicate that, although semi-skimmed milk and lactose-free milk contain the same amount of fat, the lactose-free milk samples have a higher capacity.

New Method to Examine Commercial UHT

The researchers proposed a novel method to analyze commercial UHT milk samples bought directly off the shelf without prior preparation, using a traditional impedance meter and interpreting the data from the equivalent circuit model.

The current report presents the results of an electrical impedance spectroscopy study on the UHT milk sold in the Italian markets. It shows that it is possible to distinguish the various types of UHT milk within each brand, both by the values of the equivalent circuit model components and by comparing the admittance measurement curves.

Electrical impedance spectroscopy (EIS) is a valid technology for laboratory application, but more work is needed before it can be used as a transportable control method. If efforts were undertaken in this regard, it might become a legitimate way of control at refreshment points, where milk consumption is typical, and quality control is consequently crucial for the protection of consumers.

Reference

G. Scandurra, E. Cardillo, C. Ciofi, L. Ferro. (2022) UHT Milk Characterization by Electrical Impedance Spectroscopy. Applied Sciences. https://www.mdpi.com/2076-3417/12/15/7559/htm

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