A Portable Battery-Operated Sensor System for Simple and Rapid Assessment of Virgin Olive Oil Quality Grade^†

Round 1

Reviewer 1 Report

The manuscript "A Portable Battery-Operated Sensor System for Simple and Rapid Assessment of Virgin Olive Oil Quality Grade" by Grossi et. al reports a built device for the determination of free-acidity and the peroxide index of olive oil samples. Overall, the manuscript is well-written and the results are clearly described. I liked the idea of the manuscript and the presentation is suitable. However, prior to the recommendation for publication, I ask for minor revisions as follows: 

a) What's the novelty of the presented paper with previous studies from the same group? I am afraid that the study has no substantial differences from e.g. reference 35: Grossi, M.; Palagano, R.; Bendini, A.; Riccò, B.; Servili, M.; García-González, D. L.; Gallina Toschi, T. Design and in-house 580 validation of a portable system for the determination of free acidity in virgin olive oil. Food Control, 2019, 104, 208-216.

b) There are many self citations within the references and the authors did not reference recent works on the evaluation of olive oil quality, such as: 10.1016/j.talanta.2020.120920 and 10.1016/j.foodchem.2021.129243

Author Response

Please, find the response to the reviewer’s comments in the attached PDF file.

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper the authors present a portable sensors system for the quality evaluation of the olive oil. The developed system exploits the electrical impedance spectroscopy technique to measure variations in the electrical properties of a set of different olive oil samples, when are mixed with suitable reagent. The authors developed a battery-supplied device and presented measurements results concerning 40 different olive oil species (EVOO, VOO and LOO) having different value of peroxide index and free acidity.

The subject is of great interest and any mind of improvement in the state of the art are welcome, despite this the reviewer has some concerns related to the work, recommending a major revision before to be accepted for publication in this journal.

• In the reviewer opinion the introduction section needs to be rewritten. It seems to be a quite useless list concerning sensors systems or application that are out of scope. It would be useful to focus the attention on the proposed work application or techniques.
• It is not clear what is the novelty or improvements presented with the proposed work with respect to the state of the art. In literature there are several works exploiting EIS measurements for the olive oil quality monitoring, please clarify the position of your study.
• Did the authors consider the effect of the electrodes oxidation on the measurements?
• In view of the obtained results concerning measurements performed using DW or HAS as reagent, did the authors consider the effect of the dielectric properties of the medium on the measurements results?
• The reported results in Table 5 and 6 are not very clear. Please, clarify the table to make the results more readable.

Author Response

Please, find the response to the reviewer’s comments in the attached PDF file.

Author Response File: Author Response.pdf

Reviewer 3 Report

The development of quick and simple quality control systems for food products, in particular olive oil, is certainly an important task. The authors applied a fairly simple principle of the free acidity estimation from the electrical conductance measured by Electrical Impedance Spectroscopy (EIS) of the oil emulsion with a simple reagent.

However, publication of the manuscript in its current form is not possible. The presented study almost completely repeats the previously published work of the authors (ref 35). Current work uses exactly the same device (compare photographs and the electronic system in Figures); it uses the same algorithms for analyzing the sample and temperature compensation. The object of analysis has not changed (olive oil), as well as the estimated parameter - free acidity. The reagent is the same - a mixture of 60% ethanol and water. The number of samples has been increased, but there are no fundamentally different conclusions when comparing ref 35 and the submitted manuscript. Actually, the declared goals of the compared works are the same: "The aim of this work is to present a portable electronic system to measure the free acidity in olive oil samples: the instrument is batteryoperated and can be used for quick “in-situ” measurements in the oil production or bottling site" (ref 35); " In this paper, a battery-operated portable sensor system for supporting the quality analysis of olive oil is presented. The system working principle is based on the measurement of the electrical characteristics of an emulsion between a suitable reagent and the olive oil sample" (this work).

The authors should both in the title and in the text of the work clearly indicate the fundamental difference of the presented work from the previously completed study.

In addition, the authors do not explain in any way what determines the composition of the added reagent to obtain an emulsion, but simply declare its composition. Why exactly ethanol and at a concentration of 60%? Perhaps the results would be better with different concentrations or using a different solvent?

It's good that the measurements are compared at the same temperature (using compensation algorithm), but why 23.5 °C?

Since the authors use the electrical conductivity measurement as primary information, it is not surprising that for samples with a large peroxide index, their prediction algorithm does not work (Figure 5). Figure 7 illustrates a well-known principle in analytical chemistry - the relative error of determination increases as the measured value decreases in absolute value. The electrical conductivity is affected by any ions in the medium, i.e. such a measurement is not selective. A simple addition of salt (even without UV irradiation of the sample) can lead to incorrect results. Judging by the results, additional measurements in the emulsion of distilled water and oil do not help. Therefore, from a practical point of view, it would be interesting if the authors proposed an algorithm for eliminating such errors.

Author Response

Please, find the response to the reviewer’s comments in the attached PDF file.

Author Response File: Author Response.pdf

Reviewer 4 Report

This manuscript demonstrates a portable sensor system to support the quality grade assessment of virgin olive oil. The system is battery powered and the working principle is based on the measurement of the electrical conductance of an emulsion between a chemical reagent and the olive oil sample. Overall, this manuscript is informative, I have several concerns, comments, and suggestions for the authors’ consideration:

1. What’s the significance of rapid assessment of virgin olive oil quality grade?

2. What’s the gold standard to assess the olive oil quality? How’s the system compared to the gold standard?

3. How fast to complete one cycle test?

Author Response

Please, find the response to the reviewer’s comments in the attached PDF file.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thr reviewer appreciate the effort due by authors to follow the reviewer suggestion and consider the paper suitable for publication.

Reviewer 3 Report

The authors significantly improved the manuscript and responded to all comments. I can recommend it for publication, if other reviewers do not mind.

Reviewer 4 Report

I would like to recommend the current version to publish, thanks.