Enzymatic Glucose and Xylose Production from Paper Mill Rejects

Round 1

Reviewer 1 Report

This is a worthy article for publication that only requires some modifications.  The results are quite dramatic and certainly of interest to the wide community of scientists and technologists interested in biorefineries and in the valorisation of (paper) waste.  Some of these results need further/better explanation:

• explain the better than 100% entry in Table 3 and why this doesn't show that the standard deviation of only 1% isn't inadequate
• the comments near the bottom of page 9 are inconsistent - one suggests lignin assists enzymic activity (and hence the excellent results with OCC) while another says "lignin inhibits access of the enzymes" (and hence the poor results with ONP).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors proposed to use cellulose-containing products by definition as sources of glucose and cellulose, described experiments on the pretreatment of these products and subsequent enzymatic hydrolysis, conducted a comparative analysis, choosing the most appropriate source and method of its pretreatment. The very idea of ​​"recycling" paper waste is always welcome, but such recycling is hardly better than the possibility of reusing cellulose fiber for paper of the lowest grades. The only merit of the article is the large number of potential sources considered by the authors: “three classes of recycled paper fines (old corrugated containers, old newspaper, and mixed office waste) and two industrial papermaking rejects streams from different recycling mills (one mill processes linerboard and the 12 other old corrugated cardboard". To improve the text presented, the following questions and comments are given:

Comments:

1. The relevance of the idea of ​​recycling "recycled paper fiber rejects" is obvious! In other words, it makes no sense for the authors to update the idea of ​​recycling "recycled paper fiber rejects" in the introduction. Especially it makes no sense to compare with the complexity of the enzymatic hydrolysis of native biomass. It is necessary to justify the impossibility of using this type of waste "recycled paper fiber rejects" for the manufacture of low-grade paper and to propose as a raw material for biomolecules of the green chemistry platform - glucose and xylose. Supplement your article with three articles or reviews from 2020-2022. on this topic. I am sure that the use of recycled paper in the cited works may not be found. Therefore, the proposal of the authors claims to be a certain novelty.
2. What was the purpose of defibrillation if you planned to carry out enzymatic hydrolysis? What useful information can be extracted from the given defibration data, in particular fiber length (Table 1)? As proof that they worked with paper waste? Or was it possible to remove impurity non-cellulose-containing substances in the process of defibration?
3. What is the reason for choosing the three types of pretreatment?
4. Please clarify that in the studied sources is really Klason lignin or substances that, like Klason lignin, are not soluble in sulfuric acid during the analysis? The data in Table 2 shows sources with a Klason lignin content of 69-72%, but what is the simple ash content in them?
5. Why is there no ash content in table 2, although the name does not exclude its presence? And further in the ash content are discussed.
6. In table 3, it would be appropriate to present the yields of total carbohydrates in terms of the weight of the substrates.
7. Analysis of Figure 2: positively, the yields of glucose and xylose obtained after treatment with water are not much inferior to the treatment of "recycled paper fiber rejects" with diluted reagents. In any case, the use of reagents will lead to new problematic waste. The authors should pay attention and still justify their desire to use solutions of sodium hydroxide and sulfuric acid for processing "recycled paper fiber rejects".
8. The article does not contain data on the yield of substrates from “recycled paper fiber rejects” after their pretreatment. Most likely, these outputs were not defined by the authors.
9. Despite the absence of ash content in the article, which the authors discuss in the “Discussion” section, the question arises: what new, unusual for those who understand enzymatic hydrolysis, did the authors discover? The reviewer does not find an answer to this question.
10. The authors state that the MOW samples contain crystalline cellulose. And this was the reason for the low efficiency of enzymatic hydrolysis. They are wrong! Just the lignin present in them (69-72%) deactivated the enzymes by 100%.
11. TOTAL: The reviewer highly recommends proving that no one has ever offered products similar to "recycled paper fiber rejects" for enzymatic hydrolysis. If there were such proposals, then how exactly do the authors bypass the already described enzymatic hydrolysis of recycled paper. It is necessary to cite reviews and experimental articles of 2020-2022 as “reference” ones on the topic of enzymatic hydrolysis of pre-treated secondary products or intermediates from woody biomass.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf