Effect of drying temperature on berry press residue anthocyanin stability and profile

Abstract

Berry press residues represent a valuable source of bioactive compounds, particularly anthocyanins and polyphenols, which exhibit strong antioxidant properties. Berry press residues have wide application potential in food systems due to their health benefits as well as colouring capabilities. However, the effects of drying on anthocyanin stability are not fully understood across berry species and drying methods. This study evaluated the effects of conventional hot air and vacuum drying at temperatures ranging from 30 to 90 °C, as well as freeze drying, on the total polyphenolic content (TPC), total anthocyanin content (TAC), antioxidant activity (DPPH assay), and individual anthocyanin profiles in press residues from 10 berry species. Freeze drying preserved the highest levels of both TPC and TAC, while vacuum drying at moderate temperatures (30–60 °C) demonstrated comparable stability and outperformed conventional drying. All thermal methods showed accelerated degradation of anthocyanins above 75 °C, with notable compound losses at 90 °C. Species-specific responses were observed, with chokeberries and honeysuckle berries being particularly susceptible to high-temperature degradation. Chromatographic analysis revealed that rutinoside and glucoside anthocyanins were more thermally stable than sambubioside and diglucoside forms. Strong correlations were found between TPC and antioxidant activity (r = 0.89), whereas the contribution of anthocyanin was more variable (r = 0.66). This study provided a systematic cross-species comparison of 10 berry press residues dried under identical conditions, revealing species-specific degradation thresholds and demonstrating vacuum drying as a method for the substitution of freeze drying. Clear structure-stability relations across 24 individual anthocyanins were demonstrated, offering novel mechanistic insights for optimisation of industrial anthocyanin-rich by-product valorisation. Future research should explore the molecular mechanisms underlying anthocyanin degradation and assess process scalability for industrial applications. Optimising drying protocols may enable sustainable upcycling of berry by-products into high-value functional ingredients.

Introduction

Berry press residues are a by-product of juice processing, consisting of seeds, residual pulp, and berry skins. Despite their high nutritional value, which includes dietary fibre, vitamins, and antioxidants, they are an underutilised food industry by-product with potential innovative applications in nutrition and can also contribute to sustainable practice promotion within the food industry (Diez-Sánchez et al., 2023). Consumption of berry residues has been associated with the reduction of lifestyle-related diseases, for example, cardiovascular diseases and diabetes, while the high fibre contents can improve gut health by promoting beneficial gut microbiota (Calabuig-Jiménez et al., 2022; Huang et al., 2025). Utilisation of berry press residues also reduces amounts of produced waste; the overall environmental impacts associated with fruit and berry processing contribute to sustainability in food production (Agraso-Otero et al., 2025). Through valorisation of berry processing waste, we can create value-added products for incorporation into functional foods or nutraceuticals (Stanca et al., 2024; Zhang et al., 2022). An important aspect associated with berry press residue-derived products is the acceptance and consumer perception of products that are created from food industry side-streams – behavioural changes can be considered as the main drivers of food innovation uptake (Pérez-Marroquín et al., 2023; Hellali and Koraï, 2023). Berry press residue stability and shelf-life can be significantly increased when the biomass is dried, thus optimal drying methods can ensure that the health-beneficial compounds like anthocyanins and polyphenolics are retained in the product. Preserving these compound groups during drying can enhance the overall value of the ingredients by maintaining their antioxidant properties (Enaru et al., 2021; Cheng et al., 2023).

The impact of drying methods on the anthocyanin and polyphenolic contents is a relatively well researched area – previous studies have investigated the effects of freeze-drying (Donno et al., 2025), conventional drying (Grández-Yoplac et al., 2021), infrared drying (Barba et al., 2022), microwave-assisted drying (Zhang et al., 2023), and far infrared drying (Liu et al., 2022) on various materials (Pedisić et al., 2025). While many studies focus on individual drying methods, comprehensive, comparative studies that evaluate and compare several methods under the same conditions are lacking. There is limited data on specific degradation rates or characteristics of individual anthocyanins during the drying process, which is an important aspect when optimising the drying process to preserve foods or food by-products rich in these compounds. Also, most studies investigating the drying effect on berry press residues focus on a limited number of species, for example, chokeberry, bilberry, and blueberry (Zhang et al., 2023), which leaves a gap with respect to other types of biomass rich in anthocyanins and polyphenolics. Understanding anthocyanin degradation in berry press residues is an important aspect in the optimisation of their application in the food industry, food waste valorisation, and improvement of the nutritional value of this type of biomass (Chen et al., 2022). The comprehension of how different drying methods and temperatures affect anthocyanin stability can enhance the extraction efficiency and support the development of high added value ingredients for food production, for example, preservatives or food colourants (Tena and Asuero, 2022; Nemetz et al., 2021). Valorisation of berry by-products through anthocyanin recovery reduces the environmental impact and generates economic benefits for the food industry by offsetting the carbon footprint (Diaconeasa et al., 2022).

The aim of the study was to evaluate the influence of different drying techniques and temperatures, as well as their impact and the content of anthocyanins and total polyphenols in a variety of commonly produced berry press residues. The degradation of individual anthocyanins due to temperature, in the presence or absence of oxygen (vacuum drying), has been evaluated to determine specific degradation patterns across the studied berry species.

Materials and methods

Chemicals

The reagents used in the study were kuromanin chloride, ≥ 96.0% purity (Extrasynthese, France), formic acid, ≥ 98.0% purity (Sigma-Aldrich, United States), potassium chloride, 99.0–100.5% purity (Sigma-Aldrich, United States), sodium acetate, ≥ 99.0% purity (Sigma-Aldrich, United States), ethanol, ≥ 96.0% purity (Sigma-Aldrich, United States), methanol, ≥ 99.0% purity (Sigma-Aldrich, United States), 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), ≥ 97.0% purity (Sigma-Aldrich, United States), L-ascorbic acid, 99.7–100.5% purity (Merck KGaA, Germany), isopropyl alcohol, ≥ 99.5% purity (Honeywell, United States), Folin–Ciocalteu reagent, 2 M (Sigma-Aldrich, United States), sodium carbonate, ≥ 98% purity (Sigma-Aldrich, United States), gallic acid, ≥ 99.0% purity (Sigma-Aldrich, United States), 2,2-diphenyl-1-picrylhydrazyl (DPPH), (Sigma-Aldrich, United States), hydrochloric acid, ≥ 37.0% purity (Sigma-Aldrich, United States), phosphoric acid, ≥ 85.0% purity (Chem-Lab, Belgium).

Plant material

Black chokeberry (Aronia melanocarpa (Michx.) Elliott), highbush blueberry (Vaccinium corymbosum L.), bilberry (Vaccinium myrtillus L.), honeysuckle berry (Lonicera caerulea L.), elderberry (Sambucus nigra L.), black currant (Ribes nigrum L.), red currant (Ribes rubrum L.), bog cranberry (Vaccinium oxycoccos L.), strawberry (Fragaria × ananassa Duchesne ex Rozier), and raspberry (Rubus idaeus L.) pomaces were provided by the local juice producer LLC “Very Berry” (Dārzciems, Gaujiena parish, Smiltene county, Latvia) in February 2025. Berry press residues were stored in polyethylene bags at −20 °C.

Sample preparation

Conventional drying

Approximately 15 g of each berry press residue sample was spread in an even layer on a Petri dish and dried in a conventional dryer (Gallenkamp, United Kingdom) at five different temperatures: 30 °C, 45 °C, 60 °C, 75 °C, and 90 °C for 24 h.

Vacuum drying

Approximately 15 g of each berry press residue sample was spread in an even layer on a Petri dish and dried in a vacuum dryer (Memmert VO, Germany) at five different temperatures: 30 °C, 45 °C, 60 °C, 75 °C, and 90 °C, under 20 mBar pressure for 24 h.

Freeze drying

Approximately 50 g of each berry press residue sample was spread in an even layer on a Petri dish and dried in a freeze-dryer (BenchTop Pro SP Scientific, England) for 72 h, with the condenser temperature set at −55 °C under 50 mTorr pressure.

Homogenisation of berry press residue samples

After drying, each sample was cooled to room temperature. Samples were homogenised using a CM 102 grinder (GRAEF, Germany) by pulsing for 20 s, then transferred to a polyethylene bag and stored at −20 °C until further use.

Ultrasound-assisted extraction of berry press residue samples

Extraction of anthocyanins and polyphenolics was done according to Klavins et al. (2018) with modifications. Depending on the expected anthocyanin content in the sample, 2–5 g of each berry press residue powder was weighed into a 100 mL glass bottle and extracted for 15 min at room temperature with 50 mL of 70% acidified ethanol solution (0.5% formic acid) using a 60 W ultrasonic bath Sonorex DIGIPLUS DL (Bandelin, Germany). The solid to liquid ratio was set so that it did not exceed 1:10 to ensure complete extraction. The water in the ultrasound bath was exchanged between each set of samples to ensure that the water does not exceed 30 °C. After extraction, the samples were filtered through a Whatman 11 filter paper into 50 mL polypropylene tubes and stored at −20 °C until further analysis. All concentrations were calculated on a dry sample basis.

Spectrophotometric analyses

Measurement of total anthocyanins by the pH differential method

Total anthocyanin content (TAC) in the berry press residues was determined using the pH differential method (Lee et al., 2005). Potassium chloride buffer solution (0.025 M, pH 1.0) and sodium acetate buffer solution (0.4 M, pH 4.5) were prepared, and the pH of each solution was adjusted using concentrated (11.65 M) hydrochloric acid.

Samples were filtered through 0.45 μm membrane filters before analysis. In a 96-well plate, 5 μL of the filtered dark berry press residue extracts (blackcurrant, honeysuckle berry, bilberry, elderberry, chokeberry, blueberry, and blank) or 50 μL of the filtered light berry press residue extracts (raspberry, strawberry, cranberry, red currant, and second blank) were diluted to a total volume of 250 μL with potassium chloride buffer (pH 1.0) in one plate and with sodium acetate buffer (pH 4.5) in the second plate. The microplates were placed in a Nano+ spectrophotometer (Tecan, Switzerland), shaken for 1 min at a 2 mm amplitude, incubated for 20 min in the dark, shaken again for 1 min, and absorbance was measured at 520 nm and 700 nm. Anthocyanin content was calculated according to Equation 1:

where

MW = cyanidin-3-O-glucoside molecular weight (449.2 g/mol);

DF = dilution factor;

ε = 26,900 M extinction coefficient, L/mol/cm for cyanidin-3-O-glucoside;

l = well path length in cm = (absorbance measured for distilled water).

The extraction yield, g anthocyanin/100 g dry berry press residue, was calculated using the following Equation 2:

Measurement of total polyphenolics by Folin–Ciocalteu method

Total polyphenolic content (TPC) in the berry press residues was measured using the Folin–Ciocalteu colorimetric method (Siriwoharn et al., 2004). Samples were filtered through 0.45 μm membrane filters and diluted before analysis. For honeysuckle berry and chokeberry extracts, 100 μL of sample was diluted with 900 μL extraction solvent.

In a 96-well plate, 95 μL of deionised water (100 μL for the blank) was pipetted, followed by 5 μL of diluted, filtered berry press residue extract. Then 85 μL of Folin–Ciocalteu reagent was added, and the plate was shaken for 5 min at 2 mm amplitude and incubated for 30 min. Then 65 μL of 7.5% sodium carbonate solution was added, shaken for 1 min at 2 mm amplitude and incubated for 9 min. Then the absorbance was measured at 765 nm.

A calibration curve made from gallic acid was used to calculate the total polyphenol concentration. Nine calibration points in the concentration range of 2.0–19.8 μg/mL were prepared from a 99.2 μg/mL gallic acid working solution by pipetting 5–50 μL gallic acid working solution into a 96-well plate and diluting to 100 μL with deionised water, then repeating the previously described procedure. A calibration curve was constructed using the obtained absorbance data, with R² = 0.993 (Supplementary Figure 1). The limit of detection (LOD) and limit of quantification (LOQ) were 1.31 μg/mL and 3.96 μg/mL, respectively.

Measurement of radical scavenging potential by DPPH method

Radical scavenging potential of berry press residues was determined using an adapted 2,2-diphenyl-1-picrylhydrazyl (DPPH) method (Klavins et al., 2022). Samples were filtered through 0.45 μm membrane filters and diluted before analysis, following the same procedure as in the total polyphenolics analysis method. In a 96-well plate, 45 μL of 96% ethanol (50 μL for the blank) was pipetted, 5 μL of diluted filtered berry press residue extract and 150 μL of DPPH reagent were added, stirred for 1 min at 2 mm amplitude and incubated for 9 min. The absorbance was measured at 517 nm.

A calibration curve made from Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) standard solution was used to calculate the antiradical activity. Six calibration points in the concentration range of 3.81–19.05 μg/mL were prepared from a 152.4 μg/mL Trolox standard solution by pipetting 5–25 μL of the Trolox standard solution into a 96-well plate and diluting to 50 μL with 96% ethanol, then repeating the previously described procedure. A calibration curve was constructed using the obtained absorbance data, with R² = 0.9992 (Supplementary Figure 2).

Qualitative and quantitative analysis of anthocyanins by UPLC-PDA-MS/MS

To determine the berry press residue anthocyanin profile, the European Pharmacopoeia 7.0 instrumental method “Fresh bilberry fruit dry extract, refined and standardised” (01/2016:2394) with some modifications was used. Before analysis, the berry press residue extracts were filtered through 0.45 μm membrane filters. Chromatography vials were filled with 1.00 mL sample and determination of anthocyanin profile was carried out using Waters Acquity UPLC H- class (Waters, United States) consisting of a ultra-high-pressure gradient unit (Quaternary Solvent Manager), an auto injector (FTN), a column oven (CH-A), a photo-diode array PDA eλ detector and MS detector (XEVO TQD with Zspray ESI/APSI/ESCi) with electrospray ionisation (ES-), capillary voltage 3.50 kV, cone voltage 40 V, ion source temperature 150 °C, spray temperature 400 °C, spray gas flow 300 L/h, a cone gas counterflow of 50 L/h, a collision energy of 15 V, argon at 3,5 ∙10⁻³ mBar as collision gas, an ion recording range of 200–1700 m/z (0.15 s), and an operating mode of SIR. Anthocyanin separation was carried out at 35 °C using an Acquity UPLC BEH C18 column (1.7 μm, 2.1 × 150 mm). The mobile phases were deionised water (A), methanol (B), formic acid (C) and acetonitrile (D) (Table 1). The flow rate was set to 0.250 mL/min, and the gradient elution was set according to Table 1. Data was collected using Waters Empower software. Identification of individual anthocyanins was based on the PDA detection at 520 nm and further UPLC-ESI-MS/MS analysis to identify the molecular ions ([M]+) and diagnostic fragment ions corresponding to the anthocyanin aglycones. MS/MS spectra were further compared with reference spectra available in the mzCloud mass spectral database (Thermo Fisher Scientific), data reported in the European Pharmacopoeia and relevant literature to support compound assignment.

Quantification of individual anthocyanins was done using external calibration prepared from Cyanidin-3-glucoside (kuromanin chloride) ≥ 96.0% (Extrasynthese, France). Calibration curve was prepare din the concentration range from 0.01 mg cyanidin-3-glucoside (C3G) eq./mL to 2.4 C3G eq./mL (Supplementary Figure 3).

Statistics and data analysis

Data visualisation and statistical analysis have been done using the programme “R” (version 2023.06.2 + 561), using ggplot2 (version 3.5.1), corrplot (version 0.95), dplyr (version 1.1.4), GGally (version 2.3.0) and readxl (version 1.4.3) data packages. Correlation analysis between TAC, TPC, and DPPH was performed using Spearman’s rank correlation coefficients to evaluate the associations between total anthocyanin content, total polyphenolics, and antioxidant activity. Individual correlations between drying methods in each berry press residue sample, depending on the temperature, were done using Pearson’s correlation analysis for TAC, TPC and DPPH. A pairwise correlation matrix was constructed using the ggpairs function with LOESS regression fits for identification of non-linear trends and kernel density estimates.

Results

Effect of temperature and drying method on total polyphenolic content

The effect of conventional and vacuum drying at different temperatures on total polyphenolic content (TPC) in 10 types of berry press residues was evaluated using the Folin–Ciocalteu method. Figure 1 shows the changes in total polyphenol concentration depending on drying temperature and method for each berry species (Figure 1).

Figure 1

Freeze drying, used as a baseline control to indicate possibly the lowest degradation within the samples, yielded the highest polyphenolic retention across all tested berry species, with mean TPC values as high as 8,377 mg/100 g DW of chokeberry press residues.

The concentration of polyphenols decreased in the samples with increasing temperature from 30 °C to 90 °C. Exceptions in this trend were observed for cranberries (using both drying methods) and strawberries (using vacuum drying). These findings confirm the widely reported efficiency of freeze drying in preserving thermolabile bioactive compounds due to the absence of oxidation and thermal stress (Ma et al., 2023). Freeze drying thus represents a method that can induce minimal polyphenolic degradation, providing polyphenolic retention similar to fresh material.

Conventional hot air drying showed relatively stable TPC values in the temperature range from 30 °C to 75 °C, indicating that moderate drying temperatures do not necessarily result in polyphenolic loss and, in some cases, are comparable to the used control freeze drying. However, at 90 °C, the TPC decreased sharply and was reduced significantly in most samples (Figure 1). It was observed that the drying temperature and TPC showed a negative relation, where increasing the temperature significantly reduced the TPC. The sharp reduction at 90 °C suggests a temperature threshold at which the degradation of polyphenolics is likely accelerated, possibly due to the combined effects of thermal breakdown and oxidation. The non-linear response of TPC to temperature implies that polyphenolic stability cannot be assumed to decline gradually, but rather that certain processing conditions can trigger increased degradation.

Vacuum drying displayed a specific pattern where the characteristics of conventional and freeze drying were combined. At lower temperatures (30–60 °C), mean TPC values between vacuum and freeze drying were statistically indistinguishable (Figure 1). At 75 °C, vacuum drying shows higher TPC retention than conventional drying, suggesting that reduced oxygen availability delays degradation. However, at 90 °C, TPC were reduced significantly for most of the tested berries, closely resembling the values of conventional drying at the same temperature. These findings suggest that oxygen exclusion is effective at mitigating degradation under mild to moderate temperature conditions, but once the temperature exceeds certain thresholds, the exclusion of oxygen is insufficient to mitigate the degradation effects.

When comparing phenolic contents between samples dried at 30 °C and 90 °C, clear berry- and method-specific differences were observed (Figure 2). Generally, the TPC values decreased significantly with the temperature increase for all the berries and methods evaluated; however, the magnitude at which the losses happened varied. Chokeberry and honeysuckle berry TPC decreased by 50% or more under conventional and vacuum drying, indicating high susceptibility of these matrices to thermal degradation. In contrast, raspberries, strawberries, and cranberries exhibited a more moderate decline in TPC (2–19%), suggesting greater thermal stability (Figure 2).

Figure 2

Across most berry types, vacuum drying consistently showed less degradation than conventional drying, leading to smaller relative differences between the low and high drying temperatures. This protective effect was particularly evident in raspberry, strawberry, and cranberry samples, where vacuum drying resulted in only 2–5% TPC loss (Figure 2). A strong trend was observed: drying at 90 °C resulted in lower TPC than 30 °C, confirming that the temperature is the dominant driver of phenolic compound degradation, regardless of the drying method used.

All drying methods showed high TPC variability, depending on the berry type and matrix composition. Berries with more robust cell structure (e.g., chokeberry) or berries with naturally higher TPC may withstand processing better than more delicate species such as strawberry. As the results indicate, it is important to assess the possible matrix effects across different species since the drying conditions cannot be generalised without considering inherent compositional differences. Taken together, the obtained results highlight a significant interaction between drying method and temperature regarding TPC. Freeze drying provides a reliable retention of phenolic compounds followed by vacuum drying at moderate temperatures (30–60 °C), giving similar TPC retention with lower energy requirement and processing time than freeze drying. However, both conventional and vacuum drying exhibit a degradation threshold between 75 °C and 90 °C, beyond which phenolic compounds were rapidly degraded, emphasising the need for controlled conditions during the drying process as well as balancing the energy consumption with final product quality.

Effect of temperature and drying method on total anthocyanin content

The total anthocyanin content (TAC) of the different berries was strongly influenced by both drying methods and temperature. The highest TAC was observed in chokeberry dried under vacuum at 45 °C, reaching up to 3,826 mg/100 g DW, while the lowest value was recorded in raspberry subjected to conventional drying at 90 °C (26 mg/100 g DW). These radically different values represent and highlight the combined effects of berry type and drying conditions on anthocyanin stability.

In general, chokeberry exhibited the highest TAC across all drying temperatures and methods, confirming its status as one of the richest natural sources of anthocyanins. Also, blueberry, bilberry, and honeysuckle berry maintained relatively high TAC values. In contrast, cranberry and raspberry showed considerably lower anthocyanin levels and distinct degradation patterns across the drying temperatures using both drying methods (Figure 3). The control treatment – freeze drying – generally preserved anthocyanins more effectively than the thermal drying methods, with values close to or exceeding those obtained at the lowest (30 °C) conventional or vacuum drying temperatures. Vacuum drying tended to provide higher TAC values than conventional drying, particularly in the temperature range 30–60 °C, where anthocyanin retention remained relatively high compared to the decrease between 75 and 90 °C. At 90 °C, anthocyanin degradation was evident in all tested berry samples, although the extent of reduction was species dependent. Anthocyanin degradation for cranberry, blueberry, elderberry, honeysuckle berry, raspberry, and strawberry showed a clear linear relation (Figure 3).

Figure 3

Reduction in TAC was observed when comparing berry press residues dried at 30 °C and 90 °C (Figure 4). Significant reduction was observed across all berry types investigated, with the highest reduction observed in raspberries dried conventionally (up to 59.4%). Least reduction in TAC was observed for conventionally dried redcurrant press residues (9.7%). Regarding TAC content and the drying method used, it was observed that the vacuum drying method, in all types of berry press residues, retained more anthocyanins than the conventional drying (Figure 4). This indicates that the specific degradation of anthocyanins is dependent on the presence of oxygen – prolonged exposure in elevated temperatures with oxygen present (conventional drying) leads to more pronounced anthocyanin degradation. In berries like bilberry, blueberry, chokeberry, where the TAC contents are the highest, the degradation using vacuum drying was recorded to be from 20 to 26% as opposed to the >40–50% in conventional drying (Figure 4).

Figure 4

The results demonstrate the heat sensitivity of anthocyanins in berry press residue and highlight the importance of selecting the appropriate drying method for the preparation of dried products. Variation in TAC among berry species highlights the importance of matrix composition in stabilising or protecting anthocyanins: bilberry and blueberry showed good anthocyanin retention, while raspberry and honeysuckle berry anthocyanins were the most heat sensitive. Moreover, for retention of anthocyanins, it is advisable to choose drying methods that avoid the presence of oxygen and use temperatures in the range from 30 °C to 60 °C.

Effect of temperature and drying method on antioxidant capacity

DPPH radical scavenging activity was determined for all the berry press residue samples dried at different temperatures with each of the tested methods. Freeze drying, the mildest of the drying methods that avoids elevated temperatures and oxygen exposure, showed the highest DPPH radical scavenging activity for all of the tested berries, with the highest activity observed for freeze-dried chokeberry press residues (12,930 mg TE/100 g DW), followed by honeysuckle berry (8,014 mg TE/100 g DW). These results indicate that maintaining low processing temperatures is critical for preserving antiradical scavenging activity, as significant reductions were observed for both thermal drying methods even at 30 °C. Vacuum drying at moderate temperatures (30–60 °C) was proven to retain the antioxidants responsible for DPPH activity better than conventional drying. While most berry press residues followed this pattern, some berry-specific deviations were evident, for example, honeysuckle berry showed increasing DPPH radical scavenging activity over the tested range of temperatures (Figure 5). This suggests that, in addition to polyphenolics, other compounds (e.g., Maillard reaction products formed during thermal processing) may contribute to the antioxidant potential. These results are in line with the hypothesis that both temperature and the drying method can affect the antioxidant capacity of a product during processing. Although the trend of DPPH radical scavenging activity is non-linear with the TPC or TAC, it is worth noting that vacuum drying is the optimal method to be used for retention of valuable antioxidants in this type of waste biomass.

Figure 5

TAC, TPC, and DPPH correlation

The relationship between total polyphenolic content (TPC), total anthocyanin content (TAC), and antioxidant activity (DPPH) was assessed across all drying treatments, and the results are summarised in Figure 6. Strong positive associations were observed between TPC and DPPH (r = 0.891^***), while TAC correlated positively with both TPC (r = 0.765^***) and DPPH (r = 0.662^***). These results suggest that total polyphenolic levels were the dominant contributors to antioxidant potential, whereas anthocyanins, while important, were not the sole determinants of radical-scavenging capacity. Importantly, these correlations were consistent across different drying methods, highlighting that the strength of association was not simply an artefact of processing. Within conventional, freeze-, and vacuum-dried samples, TPC remained strongly correlated to DPPH (r = 0.895, 0.964, and 0.901^***), emphasising the robustness of this relationship. TAC–TPC correlations were also significant under all conditions (r = 0.718–0.891), with the closest coupling observed for freeze-dried material, suggesting that anthocyanins behave as a relatively stable sub-fraction of total phenolics in this treatment, especially under milder processing conditions. By contrast, TAC–DPPH associations were slightly weaker (r = 0.627–0.782), pointing toward a more indirect or variable contribution of anthocyanins to antioxidant capacity, however, statistically, they show a strong relation.

Figure 6

The smoothed regression fits (Figure 6) further emphasise these relationships - the TPC–DPPH relation trend showed a nearly linear fit across the observed range, with a tendency toward stronger activity at higher phenolic concentrations, whereas TAC displayed greater dispersion and wider confidence intervals, particularly at intermediate values. Density plots along the diagonal (Figure 6) highlight that freeze-dried samples exhibited a broader distribution of both TPC and DPPH values, consistent with their higher correlation strength, while vacuum- and conventionally dried samples clustered more tightly. These findings suggest that while anthocyanins contribute to antioxidant activity, the potential of total phenolics is a more important and more consistent contributor. Thus, phenolic content as a whole, rather than anthocyanin concentration alone, appears to drive the radical-scavenging properties of the studied material, with processing method influencing the magnitude of the effect but not the overall pattern of association.

Stability of individual anthocyanins in dried berry press residues

Analysis of individual anthocyanin profiles revealed that certain structures were markedly more sensitive to thermal degradation than others (Table 2). Across all berries and drying methods, 24 individual anthocyanins were found. Cyanidin 3-O-sambubioside showed the highest mean reduction (54.2%), indicating exceptional thermal lability. Similarly, cyanidin 3,5-O-diglucoside and cyanidin 3-O-sophoroside were highly degraded (41.9 and 35.1% on average, respectively), with only marginal differences between conventional (C) and vacuum (V) drying (Table 2). By contrast, rutinoside derivatives such as cyanidin 3-O-rutinoside displayed much lower reductions (19.8% under C, 16.0% under V), suggesting greater structural resilience to both heat and oxygen exposure. A compilation of chromatograms of the analysed blackcurrant shows the reduction in differently dried berry press residue anthocyanin profiles (Figure 7).

Figure 7

A consistent pattern emerged in which rutinosylated and glucosylated forms (e.g., cyanidin 3-O-rutinoside, delphinidin 3-O-glucoside, malvidin 3-O-glucoside) were comparatively stable, typically showing 18–30% reduction, whereas diglucosides and sambubiosides were markedly less stable, with 40–60% losses (Table 2). This structural trend suggests that acylation or rutinosylation may confer protection against heat-driven cleavage and oxidative breakdown, whereas more labile linkages (e.g., sophoroside and sambubioside groups) are especially vulnerable.

When comparing drying methods, vacuum drying systematically was able to avoid anthocyanin loss across almost all analysed anthocyanins and samples, although the magnitude varied. For instance, cyanidin 3-O-arabinoside decreased by 38.5% under conventional drying but only 25.1% under vacuum, while delphinidin 3-O-galactoside dropped from 37.0% (C) to 23.9% (V) (Table 2). However, for compounds already highly unstable (e.g., cyanidin 3-O-sambubioside), the protective effect of vacuum was limited, with degradation remaining high under both drying methods (59.1% vs. 49.3%) (Table 2).

Together, these findings highlight that anthocyanin degradation is not uniform across berry matrices but strongly influenced by the molecular structure of individual anthocyanins. Glycosylation pattern appears to be a major determinant of thermal stability, with rutinosides and simple glucosides showing the greatest resilience, while sambubiosides and diglucosides are disproportionately degraded. These trends provide novel insight into structure–stability relationships of anthocyanins during drying, underscoring the importance of considering individual anthocyanin chemistry, in addition to total anthocyanin content, when evaluating the nutritional and functional quality of processed berry products.

Discussion

This study demonstrated that both assessed drying methods and the used temperature critically influence the stability of polyphenolic compounds, anthocyanins, and the related antioxidant activity of the berry press residues. Among the methods tested, freeze drying consistently retained the highest levels of bioactive compounds across all berry species, aligning with the findings that this drying method is the mildest in terms of degradation of bioactive compounds due to the absence of both heat and oxygen during the treatment (Wojdyło et al., 2009; Sadilova et al., 2006). Findings in this study demonstrated that vacuum drying at moderate temperatures (30–60 °C) can yield comparable retention of total polyphenolics (TPC) and total anthocyanins (TAC), suggesting it as a viable, more energy-efficient alternative. The protective effect of vacuum drying appears to be due to the reduced oxidative environment, which slows degradation processes otherwise accelerated in conventional drying. This observation is consistent with the conclusions of Kechinski et al. (2021), who reported that oxygen exclusion substantially improved anthocyanin stability in freeze-dried and encapsulated berry products. In this work, berries such as raspberries, strawberries, and cranberries retained up to 98% of their TPC under vacuum drying at low temperatures, in contrast to the 40–60% losses observed under conventional drying at 90 °C. These differences show the role of oxidative stress in polyphenolic compound degradation and highlight how even moderate thermal stress becomes an even more important parameter when oxygen is present. Degradation of the investigated compound groups is not simply a linear function of temperature. A critical finding from this study was the non-linear nature of polyphenol and anthocyanin degradation, where it became especially evident between 75 °C and 90 °C. This threshold effect is consistent with kinetic models of anthocyanin degradation, which describe rapid compound breakdown beyond a certain activation energy (Patras et al., 2010). At 90 °C, the integrity of both TPC and TAC was markedly compromised across nearly all berry types and drying methods, suggesting that once this temperature threshold is surpassed, degradation is accelerated by both heat and oxygen. This is further supported by Saikia and Mahnot (2020), who showed that fruit and vegetable antioxidants undergo abrupt losses at elevated temperatures due to thermal decomposition (Saikia and Mahnot, 2020). Interestingly, the magnitude of degradation varied considerably between the tested berry species, which suggests that matrix effects might play a significant role. Chokeberry and honeysuckle, for instance, displayed the highest initial levels of TPC and TAC but also showed the most intensive degradation losses under high-temperature treatments. This might be attributed to the denser matrix and higher initial antioxidant loads, which could in turn make them more vulnerable to thermal stress, as previously suggested by Zhang et al. (2023). Conversely, raspberries, strawberries, and cranberries, with their more delicate cellular structures, retained greater TPC stability, particularly under vacuum drying. This aligns with findings from Reis et al. (2018), who noted that porosity and moisture migration rates influence drying efficiency and compound preservation (Reis et al., 2018).

The anthocyanin profile analysis provided further insights into the degradation behaviour of individual anthocyanin compounds. Our data show a clear relation between the structural characteristics and anthocyanin stability. Cyanidin derivatives with sambubioside or diglucoside groups exhibited degradation rates exceeding 50%, whereas rutinoside and glucoside forms were substantially more stable. Similar trends were observed in a review article by Khoo et al. (2017), who emphasised the role of glycosylation in enhancing anthocyanin colour characteristics and the stabilising of the flavylium cation structure or forming protective copigmentation complexes. The degradation resistance of rutinoside derivatives under both vacuum and conventional drying supports the view that anthocyanin structure is a major determinant of stability, regardless of drying method used (Li et al., 2021).

Beyond compound stability, this study also addressed how the changes in TPC and TAC affect antioxidant capacity. DPPH radical scavenging activity was positively correlated with both TPC (r = 0.891) and TAC (r = 0.662) (Figure 7), though the association was significantly stronger for polyphenols. This suggests that while anthocyanins contribute to antioxidant potential, they are not the main contributing factor. These findings are consistent with earlier studies by Kähkönen et al. (1999) and Jakobek (2015), which emphasised that phenolic acids, flavonols, and flavan-3-ols often play a more central role in antioxidant responses than anthocyanins alone. The strong and consistent TPC–DPPH relationship across all drying methods (r > 0.89) further supports this conclusion (Figure 7), indicating that total phenolic content is a more reliable predictor of antioxidant function in thermally processed berry matrices than anthocyanin contents.

Taken together, the results of this study not only reinforce known principles of thermal degradation but also provide novel insights into the interplay between berry matrix, anthocyanin structure, and processing technologies. One particularly valuable implication is that vacuum drying at 45–60 °C can serve as a practical and scalable compromise between the nutrient and phytochemical preserving ability of freeze drying and the energy demands of conventional heat-based drying. Additionally, the data suggests a potential for optimisation of drying strategies based on berry species and target compounds. For instance, when maximising anthocyanin retention in chokeberry or blueberry, vacuum drying below 60 °C appears most effective. For polyphenol-rich matrices intended for antioxidant extracts, a broader temperature tolerance may be acceptable, particularly if vacuum drying is employed, however, the balancing between product final costs must be considered. In terms of practical applications, these findings are particularly relevant for the valorisation strategies employed for berry press residues, which is a growing field in food sustainability, as suggested by recent research (Huang et al., 2025; Jagelavičiūtė et al., 2025; Pedisić et al., 2025). As food and beverage industries generate increasing amounts of polyphenol-rich by-products, understanding how to preserve these bioactive compounds during stabilisation processes for storage becomes crucial. The findings of this study offer guidance for designing drying protocols that retain the nutritional and functional properties of berry waste streams, potentially enabling their use in nutraceuticals, functional foods, or natural colourants.

Conclusion

The influence of drying methods and different drying temperatures on 10 berry press residues was evaluated. Generally, the effects and trends on TPC and TAC degradation were consistent regardless of the method or temperature used. Freeze drying, used as the control method, consistently provided the highest retention of both TAC and TPC, confirming its role as the reference standard for preserving bioactive compounds. Vacuum drying outperformed conventional drying at moderate temperatures (30–60 °C), producing anthocyanin and phenolic levels that were statistically comparable to those achieved by freeze drying. However, at 90 °C, both vacuum and conventional drying led to pronounced degradation, demonstrating that while oxygen exclusion can slow oxidative losses, it cannot mitigate the thermal stress that occurs at temperatures above 75 °C. The obtained results show a relation between the structural characteristics and anthocyanin stability – cyanidin derivatives with sambubioside or diglucoside groups were identified as the most vulnerable to degradation while rutinoside and glucoside forms were substantially more stable. Overall, these results highlight a dual perspective: while TPC demonstrates moderate resilience to heat, anthocyanins represent the most vulnerable fraction of berry phenolics. The combined analysis therefore reveals that preserving berry quality requires not only careful control of drying temperature, but also an awareness of compound-specific stability profiles. Chokeberry, bilberry, and blueberry emerge as potential candidates for higher-value processing, while raspberry and cranberry demand stricter technological control to retain their anthocyanin-rich character. Berry press residues hold potential for incorporation in various food products as functional ingredients. Drying is the preferred method to increase the shelf life of berry press residues for further use in the food industry – it is economically viable, and it produces a stable product that can be introduced in a variety of food articles. Our findings emphasise the importance of drying method choice and demonstrate how drying optimisation can provide higher added-value ingredients for the food industry.

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