Investigation of the Kinetic Dynamics in the Intermittent Microwave–Hot-Air Combined Drying of Peanut Pods

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript presents a study on the significance of drying peanut pods using intermittent microwave-assisted hot air drying. Overall, the presentation of methods and results is commendable. The experimental design is robust, and the findings are both interesting and insightful. However, several areas require improvement to enhance clarity and comprehensiveness, ensuring that readers can fully grasp the study's contributions.

 

Major Issues

 1. Incomplete Methodological Details:

-The manuscript lacks essential details such as the power rating of the microwave generator and the flow rate used during the experiments. These parameters are crucial for reproducibility and understanding the drying process's dynamics.

-While Figure 2.1 is referenced, it is not self-explanatory. It remains unclear whether the equipment operates in a batch or continuous mode. Additionally, the use of red arrows to indicate both material and air flow is confusing.

-The black object situated in the center of the drying chamber is not described. Clarification is needed on how peanut pods are loaded into the chamber and the mechanism by which microwaves are directed into the drying space.

2. The study sets the intermittent ratio at 0.3 without adequate explanation. If the intermittent ratio is intended to control microwave power, the authors should specify the duration for which the microwave is turned on and off. Furthermore, the exact power settings of the microwave generator during these intervals should be disclosed to provide a complete understanding of the experimental setup.

3. Lines 337-339 discussing these tables should be relocated to the beginning of Section 3.4 to provide context. Tables 3-8 are not adequately introduced within the text. It was not clear what the first column (No) of Table 3 meant. As a result, Lines 318-333 are challenging to comprehend. A clear description of each table's content and structure is necessary for reader comprehension.

4. The fitted parameters (e.g., k, n, a, b, g) are not presented in Tables 4-8. Including these parameters is essential for readers to understand the modeling aspects and validate the study's findings.

5. Section 3.6 need significant improvement. Figure 7 was not even introduced before going into Lines 370-377, which explains why average drying rate increases with microwave power and air temperature. The explanation is also redundant because similar information has been given in previous sections. Likewise, the manuscript failed to describe how Specific Energy Consumption was affected by different parameters. The authors rushed into explaining the phenomenon which was difficult to follow.

6. In Line 235, Equation 16 utilizes absolute humidity of inlet air, which typically has units of kg/m³. The equation appears incorrect if absolute humidity is used without appropriate units. Please provide the units for each parameter to ensure the equation's accuracy and clarity.

7. It is challenging to discern the impact of higher air temperature based on Figure 4. Enhanced explanation or clearer graphical representation is needed to effectively communicate these findings.

8. The statement that temperature elevation reduces vapor pressure is scientifically inaccurate. Temperature elevation reduces the air relative humidity but not the vapor pressure.

 

Minor Issues

1. While the overall English writing is satisfactory, certain words are uncommon or misused. For example:

     -Line 310: "collectively" – Consider using a more precise term.

     -Line 380: "exacerbate" – Ensure the term accurately reflects the intended meaning.

2. Lines 41-45: The: It is strange to claim that the economic feasibility of traditional methods was hindered by something.

3. Line 47 claims that microwave drying is uniform, yet Line 53 says that microwave drying along may lead to surface heating and non-uniform drying. The authors should not describe microwave drying as a uniform method in the beginning because that’s not what it is famous for.

4. Figure 1 includes an image of an oven and a scale for moisture content measurement, which appears unnecessary. Consider removing.

5. Clarification in Calculation Formula (Line 156): To enhance clarity, add "for drying rate (DR)" when describing the calculation formula.

6. Table 2 cites a review article for five models. It is preferable to cite the original sources for each equation to provide direct references and acknowledge the primary contributors.

 

7. Line 393, significantly shorter time compared to who?

Author Response

Reviewer #1: The manuscript presents a study on the significance of drying peanut pods using intermittent microwave-assisted hot air drying. Overall, the presentation of methods and results is commendable. The experimental design is robust, and the findings are both interesting and insightful. However, several areas require improvement to enhance clarity and comprehensiveness, ensuring that readers can fully grasp the study's contributions.

Response: Thank you for your thoughtful and encouraging comments. We appreciate your recognition of the experimental design and the significance of our findings. We will carefully address the areas for improvement you mentioned to enhance the clarity and comprehensiveness of the manuscript. Your feedback is invaluable, and we are grateful for your time and expertise in reviewing our work.

 

# Major Issues

1. Incomplete Methodological Details:

-The manuscript lacks essential details such as the power rating of the microwave generator and the flow rate used during the experiments. These parameters are crucial for reproducibility and understanding the drying process's dynamics.

Response: Thank you for your valuable comments. Regarding the power rating of the microwave generator and the flow rate used during the experiments, this study utilized 6 kg of peanut pods, with the microwave generator's power range set between 0-7.2 kW. Based on preliminary experiments, we found that higher microwave power could cause the peanuts to burn, so we selected moderate microwave power densities for the drying tests. Specifically, the power densities used were 0.4 W/g, 0.6 W/g, and 0.8 W/g, with the hot air temperature set at 40°C and 45°C, and the air velocity fixed at 0.7 m/s (This air velocity is the apparent air velocity calculated based on an air flow rate of 1110 m³/h and a cross-sectional area of 0.4354 m²). Detailed information is set out in lines 148-155 of the revised draft.

The rated power for each power density is calculated as follows:

• At 0.4 W/g, the rated power is 2.4 kW
• At 0.6 W/g, the rated power is 3.6 kW
• At 0.8 W/g, the rated power is 4.8 kW

When there is no specific power density set, the maximum power of the microwave generator is 7.2 kW.

These parameters have been added to the revised manuscript to ensure the reproducibility of the experiments and to aid in a better understanding of the drying process dynamics. Thank you again for your thorough review, and please feel free to let me know if you have any further questions.

 

-While Figure 2.1 is referenced, it is not self-explanatory. It remains unclear whether the equipment operates in a batch or continuous mode. Additionally, the use of red arrows to indicate both material and air flow is confusing.

Response: Thank you for your insightful feedback. The equipment operates in a batch mode for drying peanut pods. I understand that using red arrows in Figure 2.1 to represent both material and air flow could cause confusion. I have revised the figure to clarify this distinction, making it easier for readers to understand. Additionally, I have added further explanations in the text to improve clarity. Please refer to lines 131-138 of the revised manuscript for the specific details. Thank you again for your valuable suggestions.

 

-The black object situated in the center of the drying chamber is not described. Clarification is needed on how peanut pods are loaded into the chamber and the mechanism by which microwaves are directed into the drying space.

Response: Thank you for your valuable remarks. Permit me to offer a more detailed account based on the design and operation of the system.

The black object at the center of the drying chamber is the vibrating motor, and I have provided an explanation of this part in Figure 1. Its principal function is to evenly distribute peanut pods during the drying procedure. The vibrating motor contributes to forming a uniform and thin layer of peanut pods, ensuring consistency and efficiency in the drying process. By vibrating the pods, it prevents clustering or uneven drying, enabling more efficient moisture removal. Detailed changes can be found on line 1 of the revised draft.

Regarding the loading mechanism, the peanut pods are initially lifted by the hopper lifting system. The pods are conveyed to the top of the drying chamber, where they are fed into the distribution hopper. The hopper guarantees the uniform distribution of peanut pods to the drying chamber via the material valve. Once inside the drying chamber, a vibrating motor ensures that the pods are dispersed in a thin and even layer, promoting uniform microwave exposure.

Concerning microwave energy distribution, the system employs microwave emitters, which are represented by six black circular components at the center of the figure. These emitters are fixed above the microwave bed and arranged in a rectangular waveguide system. When the microwave switch is activated, microwave energy is emitted from above and spreads over the peanut pods in the drying chamber. Waveguides assist in directing and focusing microwave energy into the drying space, ensuring a uniform energy distribution. The microwave energy is then evenly distributed throughout the drying chamber, minimizing the formation of "hot spots" and guaranteeing that all parts of the peanut pod are heated uniformly and consistently.

The combination of a vibrating motor for uniform material distribution and a waveguide system for uniform microwave energy distribution ensures an efficient and uniform drying process. This design helps to maximize drying performance while minimizing energy consumption and preventing peanut pods from over-drying or under-drying.

I hope this clarifies what you need. If you have any further inquiries or require additional information, please feel free to ask. Thank you once again for your insightful feedback.

 

2. The study sets the intermittent ratio at 0.3 without adequate explanation. If the intermittent ratio is intended to control microwave power, the authors should specify the duration for which the microwave is turned on and off. Furthermore, the exact power settings of the microwave generator during these intervals should be disclosed to provide a complete understanding of the experimental setup.

Response: Thank you for your valuable input. The interval ratio of 0.3 refers to the percentage of time the microwave transmitter is on during each cycle. Specifically, in each 10-minute cycle, the microwave oven is on 30% of the time (i.e., 3 minutes) and off 70% of the time (i.e., 7 minutes). For example, when the microwave power density is 0.4 W/g, the corresponding microwave power is 2.4 kW. Similarly, when the microwave power density is 0.6 W/g, the microwave power is 3.6 kW; when the microwave power density is 0.8 W/g, the microwave power is 4.8 kW. In each cycle, the hot air was uninterrupted and the microwave test stand was operated for 3 minutes to dry the peanut pods, after 3 minutes, the microwave transmitter was turned off and the hot air continued to dry the peanut pods uninterrupted for the remaining 7 minutes. We hope the above explanation clarifies the intermittent use of microwave power and its experimental setup. For more details, please refer to lines 148-155 of the revised draft.

Once again, we sincerely appreciate your careful review and constructive suggestions. Your feedback has greatly improved the clarity and quality of the manuscript.

 

3. Lines 337-339 discussing these tables should be relocated to the beginning of Section 3.4 to provide context. Tables 3-8 are not adequately introduced within the text. It was not clear what the first column (No) of Table 3 meant. As a result, Lines 318-333 are challenging to comprehend. A clear description of each table's content and structure is necessary for reader comprehension.

Response: Thank you for your thorough review and valuable suggestions regarding the paper. In response to your comments, I have made the following revisions:

1. Regarding the suggestion to move lines 337-339 to the beginning of Section 3.4: I understand your point that relocating this content might make the article seem redundant. The purpose of lines 337-339 in the original draft was to guide the reader's understanding of the tables presented below. I believe keeping them in the current section helps provide a smooth transition when introducing the tables. To address this, I have added further explanations in lines 215-222 of the revised manuscript to help readers better understand the context and structure of the tables.
2. Regarding the meaning of the first column (No) in Table 3: Thank you for pointing this out. I have now clarified the meaning of each column in Table 3 in the revised manuscript to ensure that the content is clearer and easier to understand.
3. Regarding the explanations for Tables 3-8: I have provided thorough explanations for the content of Tables 3 through 8 to ensure that their structure and content are better understood by the readers. The specific revisions can be found in lines 332-354 and 367-400 of the revised manuscript. Please refer to these sections for more detailed explanations.

Thank you again for your detailed review and constructive feedback. Your comments have been very helpful in improving the quality of the paper. If you have any further questions or suggestions, please feel free to let me know.

 

4. The fitted parameters (e.g., k, n, a, b, g) are not presented in Tables 4-8. Including these parameters is essential for readers to understand the modeling aspects and validate the study's findings.

Response: Thank you for your valuable feedback. Based on your suggestion, I have included all relevant fitting parameters (such as a, b, k, n, g, etc.) in Tables 4-8 and provided detailed explanations for these parameters in the revised manuscript. Please refer to the updated tables and corresponding textual explanations for further details. Thank you again for your helpful suggestions in improving the manuscript.

 

5. Section 3.6 need significant improvement. Figure 7 was not even introduced before going into Lines 370-377, which explains why average drying rate increases with microwave power and air temperature. The explanation is also redundant because similar information has been given in previous sections. Likewise, the manuscript failed to describe how Specific Energy Consumption was affected by different parameters. The authors rushed into explaining the phenomenon which was difficult to follow.

Response: Thank the reviewers for their valuable comments. Based on your feedback, I have improved Section 3.6 to ensure that it is fully introduced before introducing Figure 6, and clearly explains the relationship between specific energy consumption and drying rate, while avoiding duplication, and describes in detail the impact of different parameters on energy consumption in comparison. See Section 3.6 for details. This revised paragraph first introduces Figure 6 and gives a clear explanation, illustrating the influence of microwave power and hot air temperature on energy consumption and drying rate, and avoids repeated interpretation. Specifically, I combined the chart data provided by you to clarify how microwave power and hot air temperature work together to affect specific energy consumption, and made a reasonable explanation and summary. I hope this modification meets your requirements. If there are any other adjustments that need to be made, please feel free to let me know.

 

6. In Line 235, Equation 16 utilizes absolute humidity of inlet air, which typically has units of kg/m³. The equation appears incorrect if absolute humidity is used without appropriate units. Please provide the units for each parameter to ensure the equation's accuracy and clarity.

Response: Thank you for your valuable feedback. Based on your suggestion, I have revised Equation 16 to ensure that the units for absolute humidity are correctly included, and I have also added the units for each parameter in the equation. The specific revisions can be found in Equation 2-16 and Lines 246-253 of the revised manuscript. Thank you again for your thorough review, which has helped us improve the manuscript.

 

 

7. It is challenging to discern the impact of higher air temperature based on Figure 4. Enhanced explanation or clearer graphical representation is needed to effectively communicate these findings.

Response: Thank you for your valuable feedback. I apologize for the oversight in the original description. The intention was to convey that the hot air temperature has little effect on the drying rate, but due to my carelessness, the wording in the manuscript was misleading. I have re-analyzed the data for Figure 3, and I have made the necessary revisions. You can find the detailed modifications in lines 298-312 of the revised manuscript.

Thank you again for your helpful comments.

 

8. The statement that temperature elevation reduces vapor pressure is scientifically inaccurate. Temperature elevation reduces the air relative humidity but not the vapor pressure.

Response: Thank you for your comments and for pointing out the scientific inaccuracies. Based on your feedback and the available literature, I have revised the statement as follows:

With increases in microwave power density and temperature, the moisture content in pea-nut pods decreased significantly, mainly due to the change in moisture gradient and relative humidity. Microwave heating increases the energy of water molecules, causing violent vibrations that generate heat inside the peanut, thus accelerating the outward migration of free water. This process increases the water gradient, which increases the driving force of outward diffusion, resulting in a large amount of free water loss. In addition, with the in-crease in drying temperature, the removal rate of free water is accelerated, which is conducive to its diffusion. At the same time, the high temperatures reduce the relative humidity of the surrounding air and increase the water vapor pressure difference between the in-side and outside of the peanut pod.

This revision is in line with the scientific principle that temperature affects relative humidity, not vapor pressure. For detailed changes, see lines 317-326 of the revised draft. Thank you again for your insightful feedback.

 # Minor Issues

1. While the overall English writing is satisfactory, certain words are uncommon or misused. For example:

Response: Thank you for your valuable feedback regarding the language usage. I have carefully reviewed the manuscript and made necessary corrections. After making my own revisions, I also commissioned MDPI's language editing service for additional refinement to ensure the use of more appropriate and commonly used terms.

Once again, thank you for your helpful suggestions, which have greatly contributed to improving the clarity of the writing.

 

-Line 310: "collectively" – Consider using a more precise term.

Response: Thank you for your feedback. To make the sentence sound even more professional, I have replaced "collectively" with a more precise and technical expression. Here's the updated version: The synergistic effect of these factors promotes the migration of moisture from inside the pod to the outside, intensifying the moisture gradient and accelerating the drying process.

In this revision, I replaced "collectively" with "The synergistic effect of these factors", which is a more formal and scientifically precise phrase, emphasizing how the factors work together to drive the process. I hope this meets your expectations!

 

-Line 380: "exacerbate" – Ensure the term accurately reflects the intended meaning.

Response: Thank you for your feedback. To address your concern, in this revision, I replaced "exacerbate" with "aggravate", which better reflects the intended meaning in the context of temperature differential and heat transfer efficiency. This term conveys the idea of an increase in the temperature difference, which aligns more precisely with the physical process being described.

 

2. Lines 41-45: The: It is strange to claim that the economic feasibility of traditional methods was hindered by something.

Response: Thank you for your insightful comment. I understand your concern regarding the phrasing. You are correct that the term "economic feasibility of traditional methods was hindered" may not be the most accurate in this context. I have revised the text to better reflect the challenges faced by traditional drying methods without implying an overly negative impact on their economic feasibility. The updated version now provides a clearer and more nuanced explanation. Please refer to the revised text in lines 45-47 for the changes.

Thank you once again for your valuable feedback!

 

3. Line 47 claims that microwave drying is uniform, yet Line 53 says that microwave drying along may lead to surface heating and non-uniform drying. The authors should not describe microwave drying as a uniform method in the beginning because that’s not what it is famous for.

Response: Thank you for your insightful comment. I apologize for the inconsistency in my description, which was due to my oversight. You are correct that microwave drying is not typically regarded as a uniform method, especially in the initial stages. I have now corrected this mistake, and the revised text reflects a more accurate description. Please refer to lines 49-58 in the revised manuscript for the necessary adjustments. Thank you again for your valuable feedback!

 

4. Figure 1 includes an image of an oven and a scale for moisture content measurement, which appears unnecessary. Consider removing.

Response: Thank you for your valuable suggestion. I agree with your point that the image of the oven and scale for moisture content measurement is not essential in this context. I have removed the figure as per your recommendation. Thank you again for your helpful feedback.

 

5. Clarification in Calculation Formula (Line 156): To enhance clarity, add "for drying rate (DR)" when describing the calculation formula.

Response: Thank you for your advice. I agree with you, and I apologize for my oversight. According to your suggestion, I have modified the calculation formula to improve the clarity. Detailed amendment suggestions are in line 168-169 of the revised draft.

Thank you again for your valuable feedback.

 

6. Table 2 cites a review article for five models. It is preferable to cite the original sources for each equation to provide direct references and acknowledge the primary contributors.

Response: Thank you for your insightful suggestion. I completely agree with your point, and I apologize for my oversight. As per your recommendation, I have now updated Table 2 to cite the original sources for each equation, ensuring direct references and proper acknowledgment of the primary contributors. The detailed information can be found in Table 2.

Thank you again for your valuable feedback.

 

7. Line 393, significantly shorter time compared to who?

Response: Thank you for your insightful comments. I did not find this issue in the manuscript for line 393 that you mentioned. I was unable to locate the exact information you refer to, so I sincerely apologize for any ambiguity in my manuscript.

Upon further review of the manuscript at a subsequent time, I found a similar unclear description in lines 309-312. What should be expressed here is that the drying time is significantly shorter when using higher microwave power compared to lower microwave power. See lines 295-299 of the revised manuscript for detailed changes.

Thank you again for pointing this out.

Reviewer 2 Report

Comments and Suggestions for Authors

1)     Page 1 Line 24-25: Keywords are sorted by the first letter of each word.

2)     Introduce：Please explain why this study was conducted and how it differs from existing studies. I think the present introduction is not clear enough.

3)     Line 138：There should be Spaces between numbers and units, please check the full text.

4)     Line 139：kg is used as the unit of material quality in the article, should it be unified into w/kg? Please check the full text to ensure unity of units.

5)     Line 216：Please explain the basis for selecting these models.

 

6)     Please point out the shortcomings of this study in the results and discussion.

Comments on the Quality of English Language

The language needs further polishing.

Author Response

Reviewer #2:

1)  Page 1 Line 24-25: Keywords are sorted by the first letter of each word.

Response: Thank you for your helpful suggestion. I have made the necessary revision and sorted the keywords by the first letter of each word, as recommended. The detailed changes can be found in Lines 25-26 of the revised manuscript. Thank you again for your valuable feedback.

 

2)     Introduce：Please explain why this study was conducted and how it differs from existing studies. I think the present introduction is not clear enough.

Response: Thank you for your valuable feedback. I appreciate your suggestion regarding the clarity of the introduction. In response, I have made appropriate revisions to better explain the motivation for conducting this study and how it differs from existing research. The detailed changes can be found in lines 76-88 of the revised manuscript.

Thank you again for your helpful comments.

 

3)     Line 138：There should be Spaces between numbers and units, please check the full text.

Response: Thank you for your valuable suggestion. I have made the necessary revisions to ensure there are spaces between numbers and units, as recommended. The detailed changes can be found in Line 138 and throughout the manuscript. Thank you again for your helpful feedback.

 

4)     Line 139：kg is used as the unit of material quality in the article, should it be unified into w/kg? Please check the full text to ensure unity of units.

Response: Thank you for your valuable suggestion. In Line 139, the unit "kg" refers to the unit for material quality, and the value you mentioned (e.g., 0.4 W/g) represents the power density after conversion, based on a total microwave power of 2.4 kW used for drying. The conversion to W/g is intentional and does not impact the readability or consistency of the text. Nonetheless, I appreciate your careful review, and I believe the current unit usage is appropriate for the context. Thank you again for your feedback.

 

5)     Line 216：Please explain the basis for selecting these models.

Response: Thank you for your insightful question. The selection of these models is based on their widespread use and applicability in the literature for describing drying processes, particularly in relation to food materials such as peanut pods. Below is a brief explanation for the choice of each model:

Page Model: This model is an empirical modification of the Lewis model, where a dimensionless empirical constant (n) is introduced in the time term to eliminate the shortcomings of the original model. This parameter serves to adjust the time factor, allowing the model to better predict moisture loss.

Lewis Model: The Lewis model is commonly used to describe moisture loss during the drying process, especially in thin-layer drying. It is based on the assumption that the drying rate is proportional to the difference between the moisture content at any given time and the equilibrium moisture content, making it a simple yet widely applicable model.

Henderson Model: The Henderson model is frequently used in drying research as it effectively represents the drying kinetics of food materials under various drying conditions. It is particularly useful for fitting experimental data, as it accounts for the variation in drying rate over time and is suitable for both low and high moisture content ranges.

Diffusion Approximation Model: This model is based on the assumption that the drying process is primarily governed by the diffusion of water from the interior to the surface of the material. It is commonly applied when moisture diffusion dominates the drying behavior, making it suitable for materials with high moisture content, such as peanut pods.

Verma et al. Model: The model proposed by Verma et al. is often selected for its ability to predict drying behavior over a range of drying temperatures and air velocities. It provides a more comprehensive simulation of the drying process, taking into account the effects of varying environmental conditions. This model is particularly useful for studying drying kinetics under non-steady drying conditions, which is typical in many practical drying systems.

The selection of these models is based on their applicability to a range of drying conditions and their ability to accurately describe the drying kinetics of peanut pods, thereby providing a reliable framework for understanding and predicting drying behavior under different parameters.

I hope this provides a clear rationale for choosing these models, and I welcome your criticism if I'm wrong. Thanks again for your question!

 

6)     Please point out the shortcomings of this study in the results and discussion.

Response: Thank you for your valuable suggestion. I have followed your advice and included the shortcomings of this study in the Results and Discussion sections, with appropriate modifications. The details of these changes can be found in Lines 324-326, 352-354, 433-437, 452-456, and 483-505 of the revised manuscript. Thank you again for your constructive feedback!

 

The language needs further polishing.

Response: Thank you for your valuable comments. The manuscript has been submitted for professional English language editing services by MDPI. We hope this will provide you with a better reading experience. Thank you for your patience and careful review.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

please see the attached file.

Comments for author File: Comments.pdf

Author Response

 I have made the necessary revisions to the manuscript based on the reviewers' comments and provided detailed responses to the issues raised. 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The paper has been well revised according to the comments and is recommended for acceptance.