Automatic Ice-Cream Characterization by Electrical Impedance Spectroscopy

Marco GrossiDepartment of Electrical Energy and Information Engineering

University of Bologna, Italyhttp://www.researchgate.net/profile/Marco_Grossi

Food products are routinely screened for important organoleptic characteristics.

• Off-line screening: samples shipped to a laboratory for analysis

• On-line screening: sensors are used to investigate some properties of the sample

Automatic product recognition is very important in the case of ice-cream mixes

Different types of mixes require to set different parameters in the machines where the mixes are stored

Ice cream optimal machine setting requires the products to be clustered in three different groups: milk based creamy mixes, frozen yogurt and fruit based mixes

A sine-wave signal Vin(t)=VMin∙sin(ωt) is applied to the sample

Electrical Impedance Spectroscopy

The current through the sample Iin(t) = IMin∙sin(ωt+φ) is measured

The impedance of the sample Z(jω) = Vin(jω)/Iin(jω) is calculated

Z(jω) is measured over a range of frequencies (ω=2πf) of the test signal and fitted with a suitable equivalent electrical circuit

The estimated electrical parameters are used to evaluate characteristics of the sample under test

The sample electrical response in non-linear : the test signal amplitude VMin is low to guarantee the response to be confined in a pseudo-linear region

The measure of Z(jω) outside the pseudo-linear region can give useful additional informations about the sample properties

21 ice-cream samples have been tested using three different techniques:

Experimental Approach

1. Electrical Impedance Spectroscopy (EIS) in the linear region with sine-wave signal of amplitude 100mV and frequency in

the range 20Hz – 10kHz

2. Electrical response in non-linear region with sine-wave signal of frequency 20Hz and amplitude in the range 10mV – 2V

3. Measure of the sample pH with a Crison micropH 2000

Incubation temperature : 35 °C

Measurement Setup

#1 to #10milk based creamy

Tested Samples

#11 to #14frozen yogurt

#15 to #21fruit based

EIS in the Pseudo-Linear RegionThe impedance Z(jω) of the sample under test in direct contact

with the sensor electrodes can be modeled with the circuit

Qj

QR

jQRjZRjZ

m

mCPEm

2sin

2cos

1

The proposed electrical model features a very good correlation (R2 = 0.998) with the measured data

Rm and Q can not reliably discriminate between milk based and fruit based ice-cream mixes

Electrical Response in the Non-Linear RegionThe modulus of the impedance |Z(jω)| deviates from its pseudo-

linear region value |Z(jω)|10mV for VMin > 200mV

The deviation is stronger at lower frequencies, thus the measurements have been carried out at 20Hz

The deviation of |Z(jω)| from its pseudo-linear region value |Z(jω)|10mV can be modeled as function of VMin as

2

,10110

1

TMin

MinmV V

VLogjZjZ

The slope of |Z(jω)| vs VMin in the non-linear region is

21

,10 /

TMinMin VVLog

jZ

The slope of |Z(jω)| in the non-linear region can reliably discriminate between milk based and fruit based products, in

particular in the case of sensor B

Conclusions

The feasibility to discriminate different groups of ice-cream mixes has been shown

• The basic discrimination between milk based and fruit based mixes is possible with the electrical characterization of the

sample in the non-linear region

• A second level discrimination of the first group between creamy mixes and frozen yogurt is possible measuring the sample pH

If you want to know more about this, please read:

Grossi M., Lanzoni M., Lazzarini R., Riccò B. (2012). Automatic ice-cream characterization by impedance measurements for optimal machine setting. Measurement, 45, 1747-1754.