Performance of Single and Two-Stage Cross-Flow Ultrafiltration Membrane in Fractionation of Peptide from Microalgae Protein Hydrolysate (Nannochloropsis gaditana)

Round 1

Reviewer 1 Report

Review for: The performance of single and two-stage cross-flow ultrafiltration membrane in fractionation of peptide from microalgae protein hydrolysate (Nannochloropsis gaditana)

Authors: Nur Izzati Md Saleh, Wan Azlina Wan Ab Karim Ghani, Mazlina Mustapa Kamal and Razif Harun

The authors evaluate the performance of fractionation of peptides of a water feed containing microalgae protein hydrolysate. They use two multilayer stacked ultrafiltration (UF) membranes prepared from individual membranes of different pore sizes (Molecular weight cut-off 10 kDa and 5 kDa). The authors investigate the influence of factors such as pH and pressure on the fractionation performance of individual membranes and the combined membrane.

The work presented in the manuscript is an incremental advance over other reports in the field. The authors do not present any novel scientific information. Moreover, a lot of the arguments made in the paper are presented without any scientific investigation of the phenomena occurring in the system.

Major points:

1. Page 4, Measurement of peptide content: The authors mention that they followed a previous report by Church et al. In this report, a mixture of ortho-phthaldialdehyde and beta-mercaptoethanol was used, that resulted in the formation of indole derivatives with primary amines. The authors should mention these facts in a short paragraph. The authors do not provide sufficient information of the analytical test for proteins.

2. How is the two-stage filtration carried out? The authors should provide experimental details. Figure 1 only shows a single stage filtration. Was the membrane manually changed for the second stage?
3. On page 5, the authors say that 5kDa cut-off membrane had higher flux than 10kDa cut-off membrane. That seems counter-intuitive. The 5kDa membrane should filter more proteins, and hence, should be lower flux. Could the authors provide an explanation?
4. Could the authors please explain how the total permeate flux for a two-stage process be higher than the individual membrane flux?
5. On page 7, the authors mention “The highest permeate flux for single membrane was using 5 kDa membrane might be due to its high surface porosity compared 10 kDa membrane.” The authors should carefully analyze this argument. It seems very unscientific. Just difference in pore sizes does not give any indication of surface porosity.
6. There is no analysis done on the fractionated peptide permeate. The authors should provide analysis of the isolated fractions with other analytical techniques such as gel permeation chromatography.

Minor points:

1. Why were the two membranes with molecular cut-offs of 5k and 10k used? What is the reasoning behind choosing these molecular weights?
2. Page 2: “Filtration with filter paper was performed to remove the microalgae residual”. Please provide the specifications of the exact filter paper used.

In conclusion, the manuscript presents an incremental advance over the current state-of-the-art and needs significant effort to be attractive to the membrane filtration community.

Author Response

Author Response File: Author Response.pdf

Reviewer 2 Report

In this paper, the authors investigated the operation conditions of cross-flow ultrafiltration to efficiently separate low molecular weight peptides and reported the results. Overall, I think that this paper could be published in Processes only after many revisions have been made. My detailed comments are as follows.

1. Authors must correct many grammatical errors and careless mistakes. In particular, it is judged that English correction by native speakers is essential.
2. The surface charge (zeta potential) of the used hollow fiber membrane should be measured and reported.
3. The chemical structure of the peptide and the membrane surface state for each pH condition should be presented through a schematic drawing and the transport mechanism should be explained based on this. 

Author Response

Author Response File: Author Response.pdf

Reviewer 3 Report

Before the publication, the manuscript should be improved. Here are some remarks.

In the title „The performance of … ultrafiltration membrane in fractionation of peptide …” there is „fractionation” – nothing is said about the fractionation. E.g. in the Conclusions we find:

„In conclusion, low molecular weight of peptide from microalgae protein hydrolysate of N,gaditana was successfully fractionated using cross-flow  ultrafiltration membrane.”

- where are some quantitative measures of this „successfully” fractionation? How are the peptide fractions characterized?

            The volume permeate flux, J, is calculated from the formula (1). In the text, the values of J with standard deviations (?) are given. How these deviations were calculated? In p. 5, these deviations are also expressed in % - is that really so? the permeate flux is without units – it should be corrected.

            In p. 6, the peptide transmission values are given with the % deviation – is it really in % ? How the deviation was calculated? The repetition of experiments is not mentioned.

            The information like e.g. “The best operating parameters … were found at flow rate of 23 mL/min” is valid only for the used membrane cartridge which is not described sufficiently. It would be useful to provide information on the linear velocity of feed solution along the membrane surface and the width of channels.

            The “empty” sentences, i.e. bringing no information, like:

“Increasing TMP can increase the flux and hence raised the driving forces for ultrafiltration [25].” – it is obvious that TMP is the driving force; apart of that, the construction of sentence is wrong.

I’m not a native English speaker but I feel that apart of that example there are many more examples of bad English, e.g.:

p.1: “the peptide produced is normally have relatively low molecular weight”;

p.2: “UF membrane as separation method” – “membrane” is not a “method”;

p.2: “which could reduce permeated flux” - permeate;

p.5: “the total permeate flux was higher than single membrane”;

p.7: “This situation made the filtration at second stage much easier since less resistance in the membrane and improved the filtration process.”;

p.8: “In another study showed that the increase of pressure”;

p.8: “peptide transmission by using low molecular weight membrane size which was 5 kDa membrane was favored instead of 10 kDa membrane.”;

p.12: “If the charges were shielded …., the range of … was reduced.”.

Generally, the tenses are used incorrectly.

Author Response

Round 2

Reviewer 1 Report

Review for: Performance of single and two-stage cross-flow ultrafiltration membrane in fractionation of peptide from microalgae protein hydrolysate (Nannochloropsis gaditana)

Authors: Nur Izzati Md Saleh, Wan Azlina Wan Ab Karim Ghani, Mazlina Mustapa Kamal and Razif Harun

The authors have significantly revised the manuscript and in the revised form, the manuscript adds scientific value to the field of protein fractionation using UF membranes. There are still some minor points to address before publication of the manuscript.

1. On page 2, “Operating UF membrane in cross-flow filtration possesses more advantages over dead-end filtration such as excellent ability to handle wide variations in particle size and solids concentration, has an excellent system in minimizing waste and excellent in producing yield with high purity [15].” The authors should include more recent references here (the cited article is from 1996!). Please include 10.1016/j.chemosphere.2020.127179, 10.1016/j.memsci.2019.02.033, 10.1016/j.memsci.2019.117604, etc. as newer advancements to prevent fouling of membranes and improve UF performance).

2. In Fig 10, PH should be pH.

3. For Figure 11, please provide the wavelength used for the detection.

4. Please provide Fig 12 in higher resolution.

In conclusion, the manuscript is suitable for publication contingent on addressing the minor points.

Author Response

Reviewer 2 Report

The authors have carefully revised the manuscript according to the referees’ comments. In my opinion, this manuscript could be accepted for publication in processes.

Author Response

Reviewer 3 Report

After the introduced corrections/supplement with missing details the manuscript seems to be suitable for publication.

English should be checked once more, e.g.:

"the peptide is typically has relatively low
molecular weight";

"using UF membrane as a separation technique";

etc. ...

Author Response