Effect of Pretreatment with Low-Frequency Ultrasound on Quality Parameters in Gulupa (Passiflora edulis Sims) Pulp

Round 1

Reviewer 1 Report

The manuscript entitled: Effect of pretreatment with low-frequency ultrasound on quality parameters in Gulupa (Passiflora edulis Sims) pulp" presents rather low scientific novelty. Were determined only titratable acidity and pH, soluble solids, antioxidant capacity, ascorbic acid, color determination, and microbiological analysis. I am not sure that is important to investigate the pulp treatment since the Passiflora edulis is mainly consumed freshly. Do I am wrong? It is well known that ultrasounds treatment improving microbiological stability. In most cases, the standard deviation is remarkably low. Can you please provide an explanation? They are also present some speculations for e.g. Line 224-226: "In general, the increase in antioxidant capacity is possibly due to a more significant disruption of cell walls, facilitating the release of phenolic residues attached to traces of pectin, cellulose, hemicellulose, and lignin in the cell wall [34]." - to prove this the determination of phenolic compounds should be done. Such an experiment would remarkably increase the scientific quality of the present manuscript. Generally, the manuscript seems to be well written, unfortunately, the scientific quality/novelty is low and therefore I am against publication without increasing the scientific novelty of the present manuscript.

Author Response

Reviewer 1

The manuscript entitled: Effect of pretreatment with low-frequency ultrasound on quality parameters in Gulupa (Passiflora edulis Sims) pulp" presents rather low scientific novelty. Were determined only titratable acidity and pH, soluble solids, antioxidant capacity, ascorbic acid, color determination, and microbiological analysis. I am not sure that is important to investigate the pulp treatment since the Passiflora edulis is mainly consumed freshly. Do I am wrong? It is well known that ultrasounds treatment improving microbiological stability. In most cases, the standard deviation is remarkably low. Can you please provide an explanation? They are also present some speculations for e.g. Line 224-226: "In general, the increase in antioxidant capacity is possibly due to a more significant disruption of cell walls, facilitating the release of phenolic residues attached to traces of pectin, cellulose, hemicellulose, and lignin in the cell wall [34]." - to prove this the determination of phenolic compounds should be done. Such an experiment would remarkably increase the scientific quality of the present manuscript. Generally, the manuscript seems to be well written, unfortunately, the scientific quality/novelty is low and therefore I am against publication without increasing the scientific novelty of the present manuscript.

Reply,

Thanks for your comments

One of the biggest problems in the market for fresh fruits exported in the world is the rapid deterioration that they present before reaching the final consumer, which is reflected in losses of up to 15% of exports. An option that benefits producers are pulp export because it can have greater control of stability. However, the use of preservatives is not an option in the long term since there is a tendency to reject products that use them in their formulation. Gulupa is consumed fresh, and it is also a trendy ingredient in the preparation of desserts, for which the cold-preserved pulp can be used.

This problem requires effective solutions, where current technologies such as ultrasound can make a significant contribution. Very recent research has been done on the use of ultrasound in juice preservation. However, in some matrices, such as pulps, there is still much to explore.

As can be seen in the introduction section, some new references were incorporated. The new references highlight how ultrasound is still used as pretreatment of food matrices to facilitate the preservation of metabolites of interest, responsible for the antioxidant activity and the quality characteristics of the fruit. Likewise, it is possible to observe that this technology facilitates dehydration and drying by other technologies such as freeze-drying and osmotic dehydration.

“US technology is useful in enhancing the rate of food processing techniques such as cutting, filtration, freezing, extraction, pickling, and drying [12,13]. The US's mechanical effect facilitates dehydration through the drying process [14]. Moreover, the US improves the freezing process by increasing ice crystals formation before the freeze-drying (FD) [15,16]. Besides, the US is a food dehydration method that avoids food decomposition due to a low heat application [17] based on moderate temperatures generated [18].

The effect of ultrasound (US) pretreatment on freeze‐dried quince slices was reported by Yildiz, G., & Izli, G. [21]. They showed higher preservation of bioactive compounds and improved physical properties compared to the untreated fruit samples.

Rodriguez et al. [22] demonstrated that ultrasound pretreatment significantly in-creased the dehydration rate and solid gain of Sanhua plum during osmotic dehydration without affecting the fruits. Žlabur et al. [23] found that the US pretreatment of honeyberry fruits increased the vitamin C content regardless of the drying technology used.

During atmospheric FD of apples, carrots, and eggplants, US application decreased the drying time and improved the FD process. Colucci et al. [19] demonstrated that the US caused no destructive effect on eggplants' antioxidant potential or quality aspects. Before FD, the US pretreatment of onions increased the flavonoids, quercetin, and phenolic content up to 20%. It improved the antioxidant activity of dried vegetables, only if the US treatment was for short periods but, prolonged sonication harmed the antioxidant activity and antioxidant content compounds [20].”

 The experiments were carried out carefully to keep the interference as little as possible. We believe that the most considerable deviations occurred in color because the gulupa pulp is not uniform in appearance and presents two color patterns, a dark color provided by the seed and the aril's yellow color.

We agree about the need to determine the phenol content as a support for the antioxidant capacity. However, the phenol content's determination was not possible for the pandemic situation that affected (and continuing affecting) our country. However, several research types have previously demonstrated that the antioxidant activity is mainly due to phenolic, anthocyanin, and organic acid compounds [1–3], and we believe that our suggestions and hypothesis are all well supported by all these previous studies.

1. Jiménez, A.M.; Sierra, C.A.; Rodríguez-Pulido, F.J.; González-Miret, M.L.; Heredia, F.J.; Osorio, C. Physicochemical characterisation of gulupa (Passiflora edulis Sims. fo edulis) fruit from Colombia during the ripening. Food Res. Int. 2011, 44, 1912–1918.
2. Yepes, A.; Ochoa-Bautista, D.; Murillo-Arango, W.; Quintero-Saumeth, J.; Bravo, K.; Osorio, E. Purple passion fruit seeds (Passiflora edulis f. edulis Sims) as a promising source of skin anti-aging agents: Enzymatic, antioxidant, and multi-level computational studies. Arab. J. Chem. 2021, 14, 102905.
3. Granados Conde, C.; Tinoco Guardo, K.P.; Granados Llamas, E.; Pájaro-Castro, N.P.; García Milano, Y. Chemical characterization and determination of the antioxidant activity of the pulp of Passiflora edulis Sims (gulupa). Rev. Cuba. Plantas Med. 2017, 22.

We have made the corrections proposed by reviewers. Thanks to the valuable contributions, the article highlights its scientific novelty and its relevance in this important section (Food Science and Technology) of the journal. We believe that it is relevant to promote industrial development and opens the doors for scaling the technique at an industrial level.

We wish to thank you all for your constructive comments in this review. In the corrected document, we try to address the issues raised as best as possible. Kindly see the corrections effected below. Corrections are also highlighted in the corrected manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The presented study "Effect of pretreatment with low-frequency ultrasound on quality parameters in Gulupa (Passiflora edulis Sims) pulp" deals with the influence of ultrasound on the quality of the export product (pulp) Gulupa. Perhaps it would be better to support (including references) the sections discussing the use of ultrasound and its implications (pros and cons) than to make general statements about transport and distribution problems. Three references to the biocidal effects of ultrasound are a relatively small reference point for the presented study. In this respect, it would be appropriate to discuss more the already known effects in terms of different frequencies and intensities (energy). Possible more complex procedures (variable parameters, e.g. oscillating intensity or sweep frequency) are also not mentioned. How much are these procedures used in practice? It seems they are used. Then it must be better pointet out, what is the scientific novelty of this study?

During the procedure, the pulp was in a plastic (polyethylene) bag. How was the homogeneous action of ultrasound ensured? Can't such a procedure contaminate the product with microplastics? Is a laboratory experiment comparable when upscaled to the dimensions and volumes used in Gulupa production?

The most fundamental comment is on Figure 1. In no case can the results be presented in the form of the line (curves) used! The results must be presented in the form of a bar chart (including mistake bars)!

One aspect of the experiment is the frequency used, but what is the specific energy absorbed? What is the effect of heating the samples during their ultrasound exposure? Here, too, there is an important aspect - the comparability of the laboratory experiment with the upscale.

The numbers of valid digits should be constant. E.g. in Table 5. for a control at time t = 0, the value is given to one decimal place, but most numbers in table are given with integer precision.

The work has a high typographic level, but one can still have a few minor criticisms. It is recommended to add a space between the unit and degrees Celsius or between the number and frequency (kHz). The negative sign (including exponents) should be spelled correctly with "Minus Sign" and not "-". The sign for multiplication is "Multiplication Sign", not the letter "x" (e.g. Line 144). Variables should be written in italics.

Author Response

Reviewer 2

Comments and Suggestions for Authors

The presented study "Effect of pretreatment with low-frequency ultrasound on quality parameters in Gulupa (Passiflora edulis Sims) pulp" deals with the influence of ultrasound on the quality of the export product (pulp) Gulupa. Perhaps it would be better to support (including references) the sections discussing the use of ultrasound and its implications (pros and cons) than to make general statements about transport and distribution problems. Three references to the biocidal effects of ultrasound are a relatively small reference point for the presented study. In this respect, it would be appropriate to discuss more the already known effects in terms of different frequencies and intensities (energy). Possible more complex procedures (variable parameters, e.g. oscillating intensity or sweep frequency) are also not mentioned. How much are these procedures used in practice? It seems they are used. Then it must be better pointet out, what is the scientific novelty of this study?

Reply:

Several paragraphs are attached in the introduction text to explain this.

Generally, the US is used and referred to as high and low energy applications. When the frequency is between 20 kHz and 100 kHz, it is known as high energy ultra-sound (greater than 1 Wcm−2) or low frequency. When the frequency is more significant than 100 kHz, the energy supplied is less than 1 Wcm−2. High doses of energy provided to the sample can have the desired effects with the application of ultrasound, but at the same time, it can generate adverse effects such as the degradation of some compounds of interest. The previous statement is why the best effects must be found with the least amount of energy applied.

US technology is useful in enhancing the rate of food processing techniques such as cutting, filtration, freezing, extraction, pickling, and drying [12,13]. The US's mechanical effect facilitates dehydration through the drying process [14]. Moreover, the US improves the freezing process by increasing ice crystals formation before the freeze-drying (FD) [15,16]. Besides, the US is a food dehydration method that avoids food decomposition due to a low heat application [17] based on moderate temperatures generated [18].

The effect of ultrasound (US) pretreatment on freeze‐dried quince slices was reported by Yildiz, G., & Izli, G. [21]. They showed higher preservation of bioactive compounds and improved physical properties compared to the untreated fruit samples.

Rodriguez et al. [22] demonstrated that ultrasound pretreatment significantly in-creased the dehydration rate and solid gain of Sanhua plum during osmotic dehydration without affecting the fruits. Žlabur et al. [23] found that the US pretreatment of honeyberry fruits increased the vitamin C content regardless of the drying technology used.

During atmospheric FD of apples, carrots, and eggplants, US application decreased the drying time and improved the FD process. Colucci et al. [19] demonstrated that the US caused no destructive effect on eggplants' antioxidant potential or quality aspects. Before FD, the US pretreatment of onions increased the flavonoids, quercetin, and phenolic content up to 20%. It improved the antioxidant activity of dried vegetables, only if the US treatment was for short periods but, prolonged sonication harmed the antioxidant activity and antioxidant content compounds [20].

• During the procedure, the pulp was in a plastic (polyethylene) bag. How was the homogeneous action of ultrasound ensured?

Reply,

Homogenization was not required. A paragraph in the methodology was attached to explain this.

During the ultrasound treatment, agitation within the bag was not required due to the dispersion effect caused by the technique; it induces a longitudinal displacement of particles [8], favored by the low viscosity of the pulp. During the ultrasound treatments, an ice bath was applied to maintain gulupa pulp samples' temperature at 5 °C.

• Can't such a procedure contaminate the product with microplastics?

Reply,

A paragraph was attached indicating the possible migration effect of the packaging in the additional remarks section.

• The use of polypropylene bags for packing the pulp and subsequent ultrasound treatment has advantages because it avoids the subsequent contamination in the packing process. However, ultrasound likely affects the package causing the release of incorporated or unbound substances; It has been demonstrated that ultrasound, through different mechanisms, including cavitation, can degrade polymers by reducing molecular weight by breaking the most susceptible chemical bond without changing the chemical nature of the polymer [67,68]. Although it occurs in extreme conditions and has been tested only using solvents that facilitate the components' mobility, this degradation can generate a migration to the food [69].

• Is a laboratory experiment comparable when upscaled to the dimensions and volumes used in Gulupa production?

Reply,

The technique can be used at an industrial level; however, the possibility of upscaled the process was indicated in a paragraph in the additional remarks section.

The extension of the technique to more technical industrial processes should evaluate the available ultrasound equipment. Currently, ultrasound applications are based on three methods: direct application to the product, coupling with the device, and submergence in an ultrasonic bath [8]. The first two techniques offer greater control of the energy delivered to the sample, and they can be used as long as the microbiological safety conditions at the packaging stage allow it. The use of the ultrasound bath is conditioned to migration processes of the packaging towards the food; although there are few studies in this regard, it must be ensured that the migration is below the limits established for implementation. A general aspect in the process is that it is necessary to avoid heating the sample due to the degradation that biomolecules can undergo, which we want to conserve and potentiate; in this sense, refrigeration systems' coupling must be essential in scaling of process.

• The most fundamental comment is on Figure 1. In no case can the results be presented in the form of the line (curves) used! The results must be presented in the form of a bar chart (including mistake bars)!

Reply,

The change of the figure was made.

• One aspect of the experiment is the frequency used, but what is the specific energy absorbed? What is the effect of heating the samples during their ultrasound exposure? Here, too, there is an important aspect - the comparability of the laboratory experiment with the upscale.

Reply,

The specific energy absorbed is a parameter that was not calculated because the sample was not allowed to heat up. The temperature of the ultrasound bath was kept constant to avoid the degradation of pigments and other compounds. By removing the water's heat in the bath, we ensured that the microsites' temperature increases had a minimum duration and conserved the thermolabile compounds.

We know that this parameter is essential; a new work may be carried out in the future to determine this parameter in this matrix and its relationship with some biomolecules' degradation.

A paragraph was attached indicating the effect of heating the samples during their ultrasound exposure and the laboratory experiment's comparability with the upscale.

The extension of the technique to more technical industrial processes should evaluate the available ultrasound equipment. Currently, ultrasound applications are based on three methods: direct application to the product, coupling with the device, and submergence in an ultrasonic bath [8]. The first two techniques offer greater control of the energy delivered to the sample, and they can be used as long as the microbiological safety conditions at the packaging stage allow it. The use of the ultrasound bath is conditioned to migration processes of the packaging towards the food; although there are few studies in this regard, it must be ensured that the migration is below the limits established for implementation. A general aspect in the process is that it is necessary to avoid heating the sample due to the degradation that biomolecules can undergo, which we want to conserve and potentiate; in this sense, refrigeration systems' coupling must be essential in scaling of process.

• The numbers of valid digits should be constant. E.g. in Table 5. for a control at time t = 0, the value is given to one decimal place, but most numbers in table are given with integer precision.

Reply,

The change was made, and the decimal figures were unified in Tables 4 and 5.

• The work has a high typographic level, but one can still have a few minor criticisms. It is recommended to add a space between the unit and degrees Celsius or between the number and frequency (kHz). The negative sign (including exponents) should be spelled correctly with "Minus Sign" and not "-". The sign for multiplication is "Multiplication Sign", not the letter "x" (e.g. Line 144). Variables should be written in italics.

Reply,

Suggested adjustments were made.

Author Response File: Author Response.pdf

Reviewer 3 Report

Development of method for microbial decontamination and quality improvement of friut juices is one of the key issues of food science and technology. Ultrasound pre-treatments can widely use for food and food industry raw materials, as well. Authors of manuscript applsci-1106779 deals with the investigation of the effects of ultrasound treatments on the change of soluble solids, pH, acidity, colour, chemical compounds and antioxidanty capacity during the shelf life of gulupa pulp. Manuscript applsci-1106779 is generally well structured. Research motivations are clearly defined. Materials and methods are given clearly. Manuscript contains interesting and valuable results. Experimental results are discussed with relevant references.

Comments, suggestions

#1: It is not clear how was selected the frequency, power and exposure time for ultrasound treatment (preliminary experiments, references)?

#2: Have the authors experienced temperature increment during the ultrasound treatment? How was it controlled? (temperature increment can affect the colour, concentration of chemical components and antioxidant capacity)

#3: The quantity (mass, volume) of ultrasound processed pulp is not given clearly in materials and methods section.

#4: Have the authors data for absorbed/dissipated power of ultrasound treatments at different frequencies/exposure time?

Author Response

Reviewer 3

We wish to thank you all for your constructive comments in this review. In the corrected document, we try to address the issues raised as best as possible. Kindly see the corrections effected below. Corrections are also highlighted in the corrected manuscript.

Comments and Suggestions for Authors

Development of method for microbial decontamination and quality improvement of fruit juices is one of the key issues of food science and technology. Ultrasound pre-treatments can widely use for food and food industry raw materials, as well. Authors of manuscript applsci-1106779 deals with the investigation of the effects of ultrasound treatments on the change of soluble solids, pH, acidity, colour, chemical compounds and antioxidanty capacity during the shelf life of gulupa pulp. Manuscript applsci-1106779 is generally well structured. Research motivations are clearly defined. Materials and methods are given clearly. Manuscript contains interesting and valuable results. Experimental results are discussed with relevant references.

Comments, suggestions

• #1: It is not clear how was selected the frequency, power and exposure time for ultrasound treatment (preliminary experiments, references)?

Reply,

The approach to the treatments chosen for this work was carried out according to an analysis of the literature. Some impressive results were found, in which frequencies of 20 kHz were used (Bhavya & Hebbar, 2019; Fonteles et al., 2012; Soltani Firouz, Farahmandi, & Hosseinpour, 2019; Wang, Vanga, & Raghavan, 2019). These studies concluded that an increase was required of the exposure time to obtain better results. We proposed increasing the frequency to observe to what extent it was possible to continue improving the characteristics.

A paragraph in the methodology was attached to explain this.

• The treatments chosen in this work were based on the results from [10,11,24,26], in which frequencies of 20 kHz were used. During the ultrasound treatment, agitation within the bag was not required due to the dispersion effect caused by the technique; it induces a longitudinal displacement of particles [8], favored by the low viscosity of the pulp. During the ultrasound treatments, an ice bath was applied to maintain gulupa pulp samples' temperature at 5 °C.

• #2: Have the authors experienced temperature increment during the ultrasound treatment? How was it controlled? (temperature increment can affect the colour, concentration of chemical components and antioxidant capacity)

Reply,

A paragraph in the methodology was attached to explain this.

During the ultrasound treatments, an ice bath was applied to maintain gulupa pulp samples' temperature at 5 °C.

• #3: The quantity (mass, volume) of ultrasound processed pulp is not given clearly in materials and methods section.

Reply,

The methodology clarified the amount of sample.

"and 100 g of pulp were placed in sterile zip-lock polyethylene bags size 4 × 5.6 Inch (Whirl Pak, Missouri, United States), each of which was considered an experimental unit.

• #4: Have the authors data for absorbed/dissipated power of ultrasound treatments at different frequencies/exposure time?

Reply,

The specific energy absorbed is a parameter that was not calculated because the sample was not allowed to heat up. The temperature of the ultrasound bath was kept constant to avoid the degradation of pigments and other compounds. By removing the water's heat in the bath, we ensured that the microsites' temperature increases had a minimum duration and conserved the thermolabile compounds. We know that this parameter is essential; a new work may be carried out in the future to determine this parameter in this matrix and its relationship with some biomolecules' degradation.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript has been improved, however from my point of view not so much has been changed from the scientific point of view. The lack of determination of phenolic compounds, to support antioxidant capacity, was explained by the pandemic situation. This is the most often used today excuse.