Introduction

Prunus persicaL. Batsch, a member of the Rosaceae family, is used as an initial species for many cultivars commonly cultivated worldwide. Prunus persica fruit is a stone fruit that is suitable for direct consumption and constitutes an interesting material for processing. At present, there are several thousand varieties within this species, with the majority adapted to warm climatic zones. The largest producers of highly developed creative cultures are Turkey, Spain, the USA, France, and Italy1,2. Due to the large diversity of Prunus persica species, peaches available in the market differ not only in taste values but also in chemical composition which has significance for human health. Therapeutic properties of Prunus persica fruit, stones, flowers, and leaves have been exploited for years, especially in folk medicine in China, the fruit’s country of origin. Prunus persicafruit has been associated with health-promoting properties in diseases of the cardiac muscle, kidneys, liver, and gallbladder. Prunus persica flowers aid in the treatment of stomach ailments and rheumatism, while Prunus persica stones exhibit antimycotic and bactericidal properties, and leaves have a positive effect on women’s health3,4,5,6.

Peach, a low-calorie, easily digestible fruit with little effect on blood glucose levels, is recommended for weight reduction diets due to its high pectin content, a soluble fiber thus limiting the supply of easily digestible carbohydrates. The basic chemical components of Prunus persica fruits include carbohydrates (mainly glucose and fructose), organic acids, vitamins, and minerals. In addition, this species contains pectins and carotenoids, which impart the characteristic orange color to the skin. Noteworthy are also their other biologically-active compounds like polyphenols or triterpenes4,7,8,9.

In recent years, peaches have garnered increasing attention from both consumers and food processors, not only for their attractive sensory values but also their chemical composition and health-promoting properties. Primary processing directions include the production of jams, compotes, canned products, and occasionally juices or smoothies. However, seasonal availability and conventional processing technologies often lead to significant degradation of bioactive compounds in peach fruit10,11. Therefore, less invasive solutions are sought, to maintain a similar nutritional value to that of the raw material. Drying, one of the oldest food processes emerges as an interesting and effective way to increase the shelf life of peaches. Dried fruits, considered an alternative fat-free snack, have gained much attention12,13. Most dehydrated fruits are produced by air drying. While this method has many advantages (mainly economic), conventional drying technology reduces the rehydration ratio, volume, and porosity of final products and influences the degradation of bioactive compounds14. Therefore, less invasive solutions that would ensure a similar nutritional value of processed peach to that of the raw material are required. To achieve this objective, it makes sense to use microwave-vacuum drying (VMD) or combine this technique with traditional convection drying (CD). Over the past two decades, there has been increasing interest in combination drying due to several advantages such as faster drying rates, shorter drying times, decreased energy consumption, and improved quality of final products. In this combination, hot air removes water in a free state from the product surface, whereas microwave energy removes water from the product interior, increasing drying rates and better retaining the quality of the dry products15,16,17.

However, the impact of using these two techniques (CD-VMD) on the quality of peach drought has not been studied so far. Therefore, this study aims to be innovative, assessing the possibility of using sublimation techniques (as control), CD at different temperatures (50 °C, 60 °C, 70 °C), microwave drying at different power levels (120 W, 240 W, 360 W, and 360/120 W), and combining these two techniques—convective predrying followed by vacuum-microwave finish drying (CD-VMD—50 °C/120 W, 60 °C/120 W, 70 °C/120 W) in the production of peach snacks. This study presents the drying kinetics, physicochemical parameters (water activity, color, final moisture content), and the profile of phenolic compounds and prohealth properties (antioxidant activities, and antiaging activities) of obtained dried peaches. In this way, it was possible to highlight the differences between the selected methods, the relationship between the process conditions and the quality of the final products, and also to select the best method of drying that allows for the maximum retention of bioactive compounds. Thus, the conducted research will enable the development of a method for obtaining functional snacks that can be used in the daily diet of adults, adolescents, and children.

Materials and methods

Reagents and standards

Acetonitrile and methanol used for LC/MS and UPLC analysis were purchased from Merck (Darmstadt, Germany). All reagents for the determination of health-promoting properties through an in vitro assay (sodium acetate, phloroglucinol, Trolox, TPTZ, AAPH, fluorescein disodium, potassium persulfate, disodium and dipotassium phosphate, starch from potato, α-amylase from porcine pancreas (type VI-8), p-nitrophenyl-α-D-glucopyranoside, α-glucosidase from Saccharomyces cerevisiae (type I), lipase (EC 3.1.1.3) from porcine pancreas (type II), p-nitrophenyl acetate, butyrylcholine chloride, dimethyl sulfoxide [DMSO], acetylthiocholine iodide, acetylcholinesterase, butyrylcholinesterase) were purchased from Sigma-Aldrich (Steinheim, Germany). ( −)-Epicatechin, ( +)-catechin, procyanidin B1, quercetin-3-O-glucoside, kaempferol-7-O-neohesperiodoside, hesperidin-7-rutinoside, isorhamnetin-3-O-rutinoside, luteolin-7-glucoside used for the identification of bioactive compounds in peach fruits were purchased from Extrasynthese (Lyon Nord, France). Chlorogenic and neochlorogenic acids were provided by TRANS MIT GmbH (Giessen, Germany).

Materials

Peach fruit (“Kijowska Wczesna” cultivar) was manually harvested at the Research Station for Cultivar Testing in Zybiszów near Wrocław (51°3′51.11″N, 16°54′43.56″E) at the “ready-to-eat” ripening stage during the 2020 season and transported directly to the laboratory. Immediately after transportation, the fruits were processed.

Drying process

Just before drying the peaches were pitted and cut into 3 ± 1 mm slices. Four methods of dehydration were used: (i) freeze drying (FD) (Alpha 1-4LSC; Martin Christ GmbH, Osterode am Harz, Germany); (ii) convective drying (CD) (convective drier designed by the research team at the Agricultural Engineering Institute of Wroclaw); (iii) vacuum microwave drying (VMD) (VM-200; Plazmatronika S.A., Wroclaw, Poland), and (iv) combined convective predrying and microwave vacuum finished drying (CD-VMD).

The initial mass of dried samples was 100 g. In the case of FD, the process lasted 24 h, the pressure was reduced to 0.960 kPa, the temperature in the drying chamber was − 60 °C, while the temperature of shelves reached 22 °C. The CD process was conducted using three different hot air temperatures: 50 ± 1 °C, 60 ± 1 °C, and 70 ± 1 °C (air velocity was 1 m/s). During VMD, the initial microwave power was set to 120 W, 240 W, 360 W, and 360/120 W (microwave power was reduced to 120 W before the samples reached the moisture content of 1 kg/kg dm). The pressure in the VMD chamber varied between 4 and 6 kPa. The combined process CD-VMD was conducted using three different hot air temperatures (50 ± 1 °C, 60 ± 1 °C, and 70 ± 1 °C), and one microwave power which was 120 W.

The moisture ratio (MR) was determined using the following equation:

$$MR(1)=\frac{\text{M}\left(\text{t}\right)-\text{ Me}}{\text{M}0-\text{Me}}$$
(1)

where M(t), M0, and Me denote moisture content achieved after drying time, initial moisture content, and equilibrium moisture content, respectively. The moisture content of dried samples was determined by drying the previously ground samples in a vacuum dryer (SPT-200; ZEAMiL Horyzont, Krakow, Poland) for 24 h at a temperature of 60 °C and a pressure of 100 Pa.

Temperature measurement during VMD of peaches was checked with an infrared camera Flir i50 (Flir Systems Inc., Stockholm, Sweden) immediately after taking them out of the VM dryer.

Determination of polyphenolic compounds by the LC–MS-PDA-Q/TOF and UPLC-PDA methods

The dried peaches’ extract of polyphenols for analysis was prepared as described previously by Wojdyło et al.18. The contents of polyphenolic compounds were determined according to Nowicka et al.5. The analysis of polyphenolic compounds in peach fruits was carried out using an Acquity UPLC system (Waters Corp., Milford, MA, USA) equipped with a photodiode and a fluorescence detector with the mass detector G2 Qtof mass spectrometer (Waters, Manchester, UK). The retention time and absorbance values (280 nm for flavan-3-ols, 360 nm for flavonols, 320 nm for phenolic acid, and 520 nm for anthocyanins) of pure standards were compared to the obtained results and used to identify polyphenolic compounds in dried peach samples. The quantification was performed using calibration curves (injection of standard solutions of known concentrations ranging from 0.05 to 0.5 mg/ml) of the selected compounds. In addition, the content of polymeric procyanidins was analyzed by the phloroglucinol method19. All samples were measured in triplicate, and the results were expressed as mg per 100 g dry mass.

Water activity, and color parameters

The water activities of the dried peach samples were determined in triplicate using a Novasina water activity meter (LabMas-terav., Lachen, Switzerland) at 20.0 ± 0.5 °C.

The color of the 11 dried peache samples was determined using an A5 Chroma-Meter (Minolta CR300; Osaka, Japan), referring to color space CIE L*a*b*. The lightness (L*), redness-greenness (a*), and yellowness-blueness (b*) values were determined using Illuminant D65 and a 10° observer angle. Values were the mean of five replicates.

The total change in color of the dried samples (dE) was calculated using the following equation described by Nowicka et al.20.

$$dE= \sqrt{{(L0-L)}^{2}+(a0-a{)}^{2}+ (b0-a{)}^{2}}$$
(2)

where L0, a0, and b0 were the values of fresh peaches, and L, a, b were the values after the drying process.

Analysis of health-promoting properties of obtained dried peaches by in vitro methods

The antioxidant properties measured by ORAC and ABTS• + assays were performed according to the procedures of Re et al.21 and Ou et al.22, respectively. The obtained results of these two methods were expressed as millimoles of Trolox per 100 g dm. All the assays were measured in triplicate.

The α-amylase, α-glucosidase, and lipase inhibitory effects (antidiabetic activity) of the dried peaches were determined according to the procedure described by Nowicka et al.23. Acarbose was included as a positive control for α-amylase and α-glucosidase assays, while Orlistat was used as a positive control for pancreatic lipase assay. The results were expressed as IC50 values.

The antiaging activity, expressed as acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibitory effects, was determined according to Ellman’s method described previously by Wojdyło et al.18. The results of the enzyme inhibition assay were presented as IC50 values.

All tests were performed in triplicate using a microplate reader Synergy™ H1 (BioTek, Winooski, VT, USA).

Statistical analysis

All data presented in this study are expressed as the mean value (n = 3) ± standard deviation. All statistical analyses were performed with XLSTAT 2017 (Addinsoft, New York, NY, USA), and Statistica version 13.0 (StatSoft, Krakow, Poland). The significance of differences (p ≤ 0.05) between means was evaluated by a one-way ANOVA followed by Duncan’s multiple-range test.

Results and discussion

Drying kinetics

The drying kinetics of peaches for convective drying (CD), vacuum microwave drying (VMD), and combined convective predrying and microwave vacuum finished drying (CD-VMD) are presented in Fig. 1. The drying time was longest during CD at 50 °C (660 min) and shortest during VMD at microwave power 360 W (48 min). During CD, an increase in the drying temperature led to a shortened drying time, and an increase in microwave power during VMD resulted in an accelerated drying rate. Similar relationships were observed during the drying of sour cherry24, pomegranate fruit25, and beetroots26. During VMD, an increase in microwave power led to higher sample heating with the sample temperature reaching 99 °C at 360 W and 86 °C at 120 W. During VMD, in the initial stage of drying, when samples have high moisture content, high microwave power can be applied without significantly heating the drying material. Therefore, VMD with a reduction of microwave power from 360 to 120 W (360/120 W) has been proposed. In this approach, when the sample temperature during drying reaches approximately 80 °C, the microwave power is reduced. VMD application at 360/120 W resulted in a reduction of drying time by more than 30% compared to VMD at 120 W while ensuring that the maximum temperature of dried peaches did not exceed 80 °C. VMD with power reduction has been successfully used in drying beetroots27, and jujube fruits28. CD of peaches is efficient and relatively inexpensive, especially in the initial phase of drying when the material has high moisture content29. The drying rate during VMD is significantly higher than during CD30. Therefore, the combination of these methods during combined drying (CD-VMD) significantly reduced the drying time compared to CD alone. The predrying temperature of the CD had no significant effect on VMD drying time or the maximum temperature of peaches during VMD.

Fig. 1
figure 1

Drying kinetics of peaches. Valor dehydrated by (a) convective drying (CD), (b) microwave vacuum drying (VMD) and (c) combined convective pre-drying and microwave vacuum finished-drying (CD/VMD).

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The drying kinetics of peaches are described by the modified Page model (Eq. 3). A logarithmic model (Eq. 3) was fitted to the experimental points representing a decrease of MR with time of CD and VMD using Table Curve 2D Windows v2.03 (Jandel Scientific Software, USA). This drying model is commonly used due to its relatively simple form and high applicability.

$$MR(2)=A\cdot {e}^{-k\cdot {\tau }^{n}}$$
(3)

The modified Page model was successfully used to describe the drying kinetics of convective drying of peaches29,31. Table 1 shows the constants of the modified Page model. The root mean square error (RMSE) ranged from 0.0033 to 0.0173, while the values of the coefficient of determination (R2) ranged from 0.9869 to 0.9992, indicating a good agreement between the experimental data and the thin layer modeling equation. An increase in the temperature of the CD increased the value of constant drying k; similar results were obtained during VMD, where an increase in microwave power affected the increase of parameter k. Similar results were obtained during CD, VMD, and CD-VMD of pomegranate arils and rind25.

Table 1 Drying constans A, k and n of the model describing the drying kinetics of peaches by different methods, drying time and a final moisture content of obtained peaches.
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Physical parameters of dried peach fruit

Table 4presents the physical properties of the obtained dried products, i.e. water activity and color of the final formulations. Water activity is a key parameter characterizing the quality, physicochemical, and microbiological stability of dried products. Parameter values ranging from 0.900 to 1.000 indicate high product humidity, while values in the range of 0.000–0.550 are characteristic of foods with low water content. Low water activity inhibits the growth of microorganisms and slows down enzymatic and lipolytic reactions32. Water activity in the obtained products ranged from 0.154 (peaches dried using FD) to 0.432 (peaches dried using CD at 50 °C). The water activity varied significantly (p≤ 0.05) depending on the drying technique used. The highest water activity was observed in samples dried by CD, followed by peaches dried using a combination of CD-VMD methods, dried products obtained by using VMD, and products obtained by sublimation. This relationship is a result of the effectiveness of individual drying methods. CD, using hot air as the main water removal medium, is less effective compared to microwaves, which can excite bound water molecules that are difficult to remove33. Regardless of the drying process used, these products had water activity below 0.550, classifying them as dried products and ensuring microbiological stability during storage.

In addition to water activity, color parameters were determined in the CIELab system. The L* parameter, responsible for the brightness of the sample, ranged from 52.58 (peach dried using CD at 50 °C) to 77.95 (peach dried using FD). The darkest dried peaches were obtained through CD processes, both as a single process and in combination with VMD. This is a result of the disadvantages of the convection method. The CD method, widely used and considered a first-generation method, despite having relatively low operating costs and high flexibility, causes strong oxidation due to long exposure to oxygen and temperature, leading to changes in color and chemical properties34. Conversely, the highest brightness of the product after FD indicates effective protection of oxidation-sensitive ingredients by the vacuum used in this process.

In addition to the L* parameter, the values of parameters a* and b* were determined in dried peaches. All obtained peaches showed a greater share of red than green color, with values ranging from 3.29 to 6.47. It was observed that the factor causing a significant reduction in the share of red color was a temperature of 60 °C in the CD process (in combination or alone). However, parameter a* did not correlate positively with anthocyanin content. Hence, it can be concluded that this parameter was influenced to a greater extent by enzymatic oxidation products than by the anthocyanins responsible for the red color of the sample. The values of the b* parameter for dried peaches ranged from 18.93 (sample dried using FD) to 32.79 (sample dried using VMD 360/120 W), placing the obtained formulations in the yellow space, a direct result of the carotenoid content in the dried material.

Another color parameter calculated for the study was dE (Table 4). This parameter describes the human eye’s ability to differentiate between the colors of two products making it an extremely important parameter for the processing industry. It is assumed that a consumer can typically distinguish between the colors of two samples when dE ≥ 5 units35. In this study, all samples were compared to the freeze-dried product, i.e. the one that retains the color of fresh fruit to the greatest extent. The interpretation of the obtained results allows us to conclude that the use of all other drying techniques—CD, CD-VMD, and VMD under various temperature conditions and microwave power—results in a significant color change visible to the human eye. The value of the dE parameter ranged from 14.23 to 21.21, with the greatest differences observed for products dried using CD or CD-VMD. This observation, similar to the observations made for the brightness of the product, indicates extensive oxidation processes resulting from prolonged contact with hot air.

Determination of phenolic compounds in dried peach fruit

As presented in Table 2, the content of polyphenolic compounds in dried peaches was influenced by five groups of compounds, totaling 22 identified polyphenolic compounds. The dominant group was the polymers of procyanidins, accounting for an average of 82.5%, followed by phenolic acids (10.6%), flavanols (monomers and dimers 4.6%), and finally, flavonols (1.3%) and anthocyanins (1.0%) (Table 3). The contribution of individual fractions in shaping the total content of polyphenolic compounds is similar to that determined by the authors in fresh fruit23. However, it is essential to note that variations in the quality of identified compounds, apart from the drying process, may also result from differences in cultivars, cultivation methods, or climatic conditions36. In this case, drying methods and their parameters undoubtedly had a significant influence on bioactive components.

Table 2 Phenolic compounds identified by LC–MS-PDA-Q/TOF in dried peaches obtained by different methods.
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Table 3 The content of polyphenolic compounds present in dried peaches obtained by freeze drying (FD), convective drying (CD), vacuum drying (VMD), and combined convective pre-drying and microwave vacuum finished-drying (CD/VMD) [mg/100 g dm].
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The total polyphenolic compounds in dried peaches ranged from 1221 mg/100 g dm (peaches after being dried by the CD-VMD method at 60 °C/120 W) to 2082 mg/100 g dm (peaches after application of VMD at 240 W). Generally, it is possible to distinguish factors, within a given drying technique, that had a significant impact on the final concentration of polyphenolic compounds. In the case of CD, drying time and exposure to oxidation had a greater effect than drying temperature. Therefore, samples dried for a shorter time but at a higher temperature (at 70 °C) were characterized by a higher content of polyphenols (1521 mg/100 g dm) than those dried for a longer time at lower temperatures (1409 and 1340 mg/100 g dm at 50 °C and 60 °C, respectively).

In the combined CD-VMD process, higher degradation of bioactive compounds was observed when using a lower temperature during CD (50 °C and 60 °C), associated with running VMD for a longer time, leading to exceeding the sample’s limit temperature of 80 °C. This resulted in a significant decrease in the content of polyphenolic compounds in the combined technique, observed in both the 50 °C/120 W and 60 °C/120 W variants. The dried material in the case of the 70 °C/120 W variant reached a maximum of 65 °C (during VMD), significantly reducing the losses of bioactive compounds. Additionally, this variant allowed the processing time to be shortened by more than 15% compared to the 50 °C/120 W variant, playing a significant role in stabilizing the compounds in question. Hence, changes in the CD-VMD process are attributed to oxidation processes during drying and prolonged exposure to thermal degradation of phenolic compounds with increased heat intensity, as indicated by Wojdyło et al.24 also, during the drying of sour cherry fruit.

In the case of drying using only VMD, the polyphenolic compound content varied significantly based on the process parameters. The highest bioactive compound content was observed in peaches dried using 240 W (2082 mg/100 g dm) followed by peaches dried at 360 W/120 W (1855 mg/100 g dm), peaches dried at 120 W (1805 mg/100 g dm), and peaches dried at 360 W (1580 mg/100 g dm). For this process, the final polyphenol concentration was influenced by the total duration of the process and the temperature of the dried material. Exceeding 95 °C, the limit temperature, resulted in intensive degradation of the compounds, observed in the case of VMD at 360 W. However, it should be emphasized that VMD proved to be a more effective method for obtaining dried peaches with a high polyphenolic compound content compared to FD.

The qualitative and quantitative analysis of each phenolic compound showed that flavonols were the most numerous groups in terms of quality (Table 2). A total of 11 compounds from this group were identified in dried peaches, with all of them identified only in freeze-dried peaches. The most stable during the drying process were: quercetin-3-O-rutinoside ([M-H]− at m/z = 609.02; MS/MS fragment at m/z = 301.02, and Rt = 6.31); quercetin-3-O-galactoside ([M-H]− at m/z = 463.09; MS/MS fragment at m/z = 301.02, and Rt = 6.37); quercetin-3-O-glucoside ([M-H]− at m/z = 463.09; MS/MS fragment at m/z = 301.02, and Rt = 6.52); isorhamnetin-3-O-rutinoside ([M-H]− at m/z = 623.14; MS/MS fragment at m/z = 315.15, and Rt = 6.91); keampferol-7-neohesperiodise ([M-H]− at m/z = 593.01; MS/MS fragment at m/z = 285.08, and Rt = 7.50), and hesperidin-7-rutinoside ([M-H]− at m/z = 609.21; MS/MS fragment at m/z = 301.03, and Rt = 8.68). All six compounds mentioned above were determined after the drying process using all techniques, i.e. CD, VMD, combined techniques CD-VMD, and FD. In contrast, keampferol-4`-O-acetyl-β-D-glucopyranoside with [M-H]− at m/z = 489.10 degraded in most of the used drying processes. The presence of this compound was noted only after FD, CD at 50 °C, and CD-VMD (50 °C/120 W). VMD drying was also unfavorable for kaempferol-3-O-α-L-rhamnopyranosyl (1 → 2)-[3-O-acetyl]-β-D-glucopyranoside with [M-H] − at m/z = 635.17, while CD technique and combining this process with VMD resulted in the degradation of luteolin-7-glucoside with [M-H] − at m/z = 285.28. The highest content of flavonols was found in samples dried by FD (26.21 mg/100 dm) and CD-VMD (60 °C/120 W—26.03 mg/100 g dm), but other peaches dried by using a combined technique of CD-VMD also exhibited a high content of this fraction—20.14 mg/100 g dm (50 °C/120 W), and 18.24 mg/100 g dm (70 °C/120 W). The lowest content of flavonols was found in samples dried with VMD, especially at 360/120 W and 120 W with the total concentration of these compounds in the samples being 11.34 and 13.09 mg/10 g dm, respectively.

Another group of compounds identified in dried peach fruit were phenolic acids. In total, 6 compounds of this fraction were determined. Qualitative analysis showed that these compounds were relatively stable. The exception was p-coumaric acid ([M-H]− at m/z = 163.11; MS/MS fragment at m/z = 119.01, and Rt = 4.40), which was completely degraded during CD and CD-VMD. However, since it remained in the dried peaches after VMD drying, it can be concluded that the exposure to the long-term oxidation process caused the degradation of this ingredient, rather than the temperature during the process which had a much lesser impact. On the other hand, the content of neochlorogenic acid ([M-H]− at m/z = 353.09; MS/MS fragment at m/z = 191.05, and Rt = 2.81), and p-hydroxybenzoic acid ([M-H]− at m/z= 137.01; and Rt = 3.52) increased after the drying process using CD, CD-VMD, and VMD techniques, compared to FD. For neochlorogenic acid, there was an average increase of 131% (compared to FD), and for hydroxybenzoic acid, about 126%. Fruit peels are known to contain significantly more neochlorogenic acid37,38,39 and p-hydroxybenzoic acid40, than their flesh. Therefore, it is likely that the impact of high temperatures in the CD process and/or high-power during CD-VMD or VMD processes may result in greater cell disintegration in the fruit peel, and thus enable greater extraction of compounds that are finally more available. This phenomenon is beneficial from a nutritional point of view in obtaining formulations. A similar relationship was also presented by Hooshmand et al.41. So finally, the richest in total phenolic acids were peaches after FD (185.34 mg/100 g dm) and peaches after being dried through processes using high temperatures and/or high microwave power (CD at 60 °C (164.21 mg/100 g dm), CD at 70 °C (162.85 mg/100 g dm), VMD at 360 W (156.77 mg/100 g dm), VMD at 360/120 W (163.93 mg/100 g dm), combined CD-VMD at 50 °C/120 W (162.43 mg/100 g dm), and CD-VMD at 60 °C/120 W(156.41 mg/100 g)).

A similar relationship, as observed in the case of neochlorogenic acid and p-hydroxybenzoic acid, was also noted for anthocyanins identified in dried peach fruit (cyanidin-3-glucoside—[M + H]+ at m/z = 449.11; and cyanidin-3-O-rutinoside—[M + H]+ at m/z= 595.11). This confirms the validity of the assumption that higher temperatures and microwave power lead to a more robust extraction of polyphenolic compounds from the peel, potentially resulting in their increased availability in dried fruit. This observation is confirmed by the fact that the dried variety of peach (“Kijowska wczesna”) accumulates anthocyanins only in the peel, with no detection in the pulp23. However, it should be noted that, for anthocyanins, exceeding a sample temperature of 90 °C, during drying, results in a significant reduction in their content. Therefore, to maintain their concentration, the process should be conducted in a manner that ensures that this temperature is not exceeded. Finally, the highest anthocyanin content was determined in samples dried by VM at 120 W (21.42 mg/100 g) followed by FD (19.25 mg/100 g dm), while the lowest content was found in dried peaches after VMD at 360 W (10.67 mg/100 g dm) and 360/120 W (5.79 mg/100 g dm).

Significant variations in the content of polyphenolic compounds were also observed in flavan-3-ols, encompassing both monomers and dimers, and polymeric procyanidins. Three compounds from the group of monomeric and dimeric flavan-3-ols were identified: procyanidin B1, ( +)-catechin, and ( −)-epicatechin. It was demonstrated that during the drying process ( −)-epicatechin was a more stable form of the monomer (in all variants), while the ( +)-catechin content was significantly reduced during the drying process (from 34.80 mg/100 g dm in peaches after FD to 12.00 mg/100 g dm in peaches after VMD at 240 W). Conversely, the content of procyanidin dimer and polymers changed depending on the drying technique used. And so, after the CD and CD-VMD processes, there was a complete reduction of procyanidin B1, and a significant decrease in the content of procyanidin polymers (by about 20% compared to peaches after FD). This trend may be attributed to prolonged exposure to oxygen and high temperature, which in turn led to the process of depolymerization, and their conversion into elementary units. This type of transformation was also observed by other authors in their works42,43. In contrast, peach fruits dried by VM exhibited a completely different pattern, with an increase in the content of procyanidin B1 and polymeric proanthocyanidins (on average by 62% and 11% compared to dried peaches after FD, respectively). This increase can be explained by the impact of high microwave power, causing the association of monomer compounds to polymerized forms. Additionally, due to the high content of pectins, an encapsulation process occurred, preventing depolymerization during drying and exposure to high temperatures. This is a favorable phenomenon, as polymerized forms exhibit high antioxidant activity, as confirmed by numerous authors20,28,42.

Prohealth potency and antioxidant capacity by in vitro methods

Biological activity in the obtained peaches was assessed in terms of antioxidant capacity using the ABTS and ORAC methods as well as in vitro enzymatic methods to determine the α-amylase, α-glucosidase, pancreatic lipase, AChE, and BuChE inhibition effect. The results are presented in Table 4.

Table 4 The water activity, colour parameters, antioxidant activity, acetylcholinesterase, butyrylcholinesterase inhibitory activities of dried peaches obtained by using different drying methods.
Full size table

Antioxidant capacity

Peaches dried using various methods exhibited significantly different antioxidant capacities, ranging from 0.53 mmol Trolox/100 g dm (sample obtained using CD-VMD 60 °C/120W) to 5.25 mmol Trolox/100 g dm (sample dried using VMD 360 W) in the ORAC method and from 0.55 mmol Trolox/100 g dm (peach dried with CD-VMD 60 °C/120W) to 3.35 mmol Trolox/100 g dm (sample dried with VMD 360 W) in the ABTS method. It is worth emphasizing that there was a very strong positive correlation (R = 0.970) between the two methods. It was observed that the samples with the highest antioxidant capacity were peaches dried by using VMD 360 W followed by VMD 240 W, and CD at 70 °C. Conversely, the lowest antioxidant effect was recorded in samples obtained by using the combined CD-VMD methods. Based on the obtained results, it can be concluded that the use of different drying techniques leads to diverse changes in chemical compounds, resulting in a wide variety of dried products in terms of antioxidant capacity. The strongest correlation coefficient between the content of polyphenolic compounds and antioxidant capacity was found for samples obtained by the combined CD-VMD method (R = 0.946); these samples were also characterized by the weakest antioxidant capacity. In contrast, a negative correlation (R= − 0.100) between antioxidant capacity and polyphenolic compounds was observed for samples dried by using VMD methods, which exhibited the highest antioxidant potential. The use of high temperature during CD (70 °C) or high microwave power (240 or 360 W) significantly reduces the overall process time, leading to shorter exposure of the dried material to oxygen and high temperature. This, in turn, contributes to the preservation of bioactive compounds and the development of high antioxidant capacity. In the case of peach fruit, the final antioxidant capacity is influenced not only by polyphenol compounds but mainly by tetraterpenoids, including carotenoids, which are sensitive to oxidation. This observation aligns with findings from studies on fresh peaches of various varieties23and was confirmed by other authors, who pointed out that the antioxidant activity of the studied raw materials depends on the presence of functional groups in the terminal ring and the number of conjugated double bonds (carotenes with 11 coupled bonds being more active than xanthophylls)44,45. Therefore, maintaining a high concentration of carotenes in peaches by minimizing the drying process time may result in a better antioxidant effect, even with significant degradation of polyphenolic compounds or xanthophylls under such conditions.

A-amylase, α-glucosidase, and pancreatic lipase inhibition effect

The antidiabetic properties of the dried peaches were evaluated by analyzing their ability to inhibit α-amylase, α-glucosidase, and pancreatic lipase, digestive enzymes found in the pancreas and intestines responsible for the hydrolytic breakdown of carbohydrates (oligosaccharides and disaccharides) and fat digestion in the case of lipase. Preventing the hydrolysis of complex sugars can be particularly beneficial for diabetics, who, due to their impaired production or secretion of insulin, must specifically monitor their glycemia levels and maintain appropriate blood sugar levels. Another strategy to counteract hyperglycemia involves reducing lipid absorption by suppressing pancreatic lipase activity23,46.

As shown in Table 4, the obtained dried peaches turned out to be a strong inhibitor of pancreatic lipase, while their effectiveness against α-amylase and α-glucosidase was comparatively lower. The IC50 values for pancreatic lipase ranged from 26.04 mg/ml (dried peaches obtained by CD-VMD 70 °C/120 W) to < 3.00 mg/ml in samples obtained by CD at 50 °C, CD-VMD at 50 °C/120 W, VMD at 360 W, and VMD at 360/120 W. This suggests that to develop inhibitory properties against pancreatic lipase, the temperature limit used during CD or the combined CD-VMD technique should be 60 °C. This relationship indicates that the process should be carried out at lower temperatures, even if it means prolonging the drying time. This is probably because the lipase inhibitory effect is strongly positively correlated with the content of flavonols (R = 0.942 for peaches dried by CD, and R = 0.479 in the case of peaches dried by CD-VMD method), and in the case of products dried using CD-VMD is also strongly positively correlated with the content of phenolic acids (R= 0.994). These compounds are thermolabile and could undergo degradation at higher temperatures. A similar effect was observed in our previous research on fresh peach fruit. These studies demonstrated a strong inhibitory effect of peach fruit against lipase and this inhibition potential was strongly correlated with the content of phenolic acids23.

The tested dried peaches exhibited lower activity in inhibiting α-glucosidase and α-amylase, and this activity was significantly dependent on both the drying technique and process parameters. The inhibitory potential against α-glucosidase ranged from 42.52 mg/ml (peaches drying by FD) to 435.14 mg/ml (samples obtained by using VMD at 120 W). For α-amylase, the IC50value ranged from 66.57 mg/ml (samples drying by FD) to 380.99 mg/ml (samples drying by VMD at 230/120 W). The inhibitory effect on these enzymes was positively correlated with the content of anthocyanins, phenolic acids, and flavonols, with correlation coefficients of 0.460, 0.490, and 0.464, respectively. Other authors have also highlighted anthocyanins and flavonols as strong inhibitors of digestive enzyme23,47. The low concentration of these compounds in peach fruit may explain the observed slight inhibitory effect on both enzymes.

Cholinesterase inhibition effect

One of the main enzymes involved in the functioning of nerves and the transmission of information in the brain is AChE. Excessive activity of AChE can lead to rapid depletion of the neurotransmitter acetylcholine causing its deficiency may contribute to the development of dementia or Alzheimer’s disease (AD). Compounds inhibiting AChE activity to obtain stable neurotransmitter levels may have properties that slow down this degenerative process. Although BuChE plays a minor role in regulating acetylcholine levels, its activity increases progressively in patients with AD. In recent years, the scientific community has started to explore the possibility of using various plant materials and their compounds, either individually or in combination, against AD48. The study aimed to determine the antiaging activity, expressed as the ability to inhibit both AChE and BuChE (Table 4). It was observed that dried peaches had a high ability to inhibit both AChE and BuChE enzymes. The correlation coefficient for both methods was R= 0.828. The concentrations inhibiting enzyme activity by 50% ranged from 3.94 mg/ml (peaches dried by CD-VMD at 60 °C/120 W) to 8.21 mg/ml (peaches dried by VMD—360/120 W) for AChE and from 4.95 mg/ml (peaches dried by CD-VMD at 60 °C/120 W) to 24.15 mg/ml (peaches dried by VMD—360/120 W) for BuChE. Dried peach fruit proved to be a more effective AChE inhibitor than BuChE, and dried forms were more potent than fresh raw material in this regard23. This may result from the higher density of bioactive forms in the dried material and structural changes in these compounds. During the drying process, an increase in monomeric and dimeric forms of flavan-3-ols was observed at the expense of polymerized compounds. Moreover, as demonstrated by other authors, the drying process results in isomerization from E to Z forms of carotenoids, which are more active and have better bioaccessibility49,50. Moreover, the drying process may result in the extraction of bioactive compounds from deeper structures of plant matrices51. Although the authors highlight plant raw materials as acetylcholinesterase inhibitors due to the potential effect of polyphenolic compounds and carotenoids, the specific mechanism of their action has not yet been fully explored and it is difficult to clearly identify the compounds having the greatest antiaging potential. As suggested by Jabir et al.52 an appropriate combination of compounds may contribute to the development of cholinesterase inhibitory properties, but this requires further research in this area.

Conclusion

The research indicates that employing various drying techniques for peaches allows the production of products with distinct polyphenol compound profiles and health-promoting properties. Generally, it is possible to distinguish factors within each drying technique that have a significant influence on the final concentration of polyphenolic compounds. In the case of CD, drying time and exposure to oxidation had a more substantial impact than drying temperature. The combined CD-VMD process exhibited higher degradation of bioactive compounds when lower CD temperatures (50 °C and 60 °C) were used, linked to prolonged VMD duration and surpassing the limit temperature of 80 °C. Conversely, in the VMD process, the final polyphenol concentration was influenced by the total duration of the process and the temperature of the dried material. Exceeding the limit temperature of 95 °C resulted in intensive compound degradation. It should be emphasized that VMD was found to be the most effective method for shaping the final concentration of bioactive compounds.

Moreover, based on the results it can be concluded that dried peaches demonstrate stronger antioxidant properties and act as more potent inhibitors of pancreatic lipase and cholinesterases (both AChE and BuChE) compared to fresh fruits. In turn, the potential of the dried peaches to inhibit α-amylase, α-glucosidase has not been demonstrated. This is probably due to many heat-induced factors including improved compound extraction from internal plant matrix structures, an increase in the number of monomeric and dimeric forms of flavan-3-ols, and isomerization of carotenoids. However, a comprehensive understanding of the exact mechanism of action of these compounds requires further research in the future. They should be aimed both at understanding the mutual relations that occur between peach compounds, especially in the hydrophilic-hydrophobic relationship, and the possibility of forming highly active complexes from them, but also at in vivo studies that will allow for the assessment of the activity and bioavailability of dried peach compounds in a human model.