68 ACTA BOT. CROAT. 79 (1), 2020 Acta Bot. Croat. 79 (1), 68–77, 2020 CODEN: ABCRA 25 DOI: 10.37427/botcro-2020-007 ISSN 0365-0588 eISSN 1847-8476 The impact of drying on bioactive compounds of blue honeysuckle berries (Lonicera caerulea var. edulis Turcz. ex Herder) Mateja Senica1*, Franci Stampar1, Sezai Ercisli2, Barbara Sladonja3, Danijela Poljuha3, Maja Mikulic-Petkovsek1 1 University of Ljubljana, Biotechnical Faculty, Department of Agronomy, Chair for Fruit Growing, Viticulture and Vegetable Growing, Jamnikarjeva 101, SI-1000, Ljubljana, Slovenia 2 Ataturk University, Agricultural Faculty, Department of Horticulture, 25240 Erzurum, Turkey 3 Institute of Agriculture and Tourism, Karla Huguesa 8, HR-52440 Poreč, Croatia Abstract – Drying fruit is one of the simplest ways to extend the shelf-life of fruit, especially berries. Both higher temperature and time of heating significantly change the contents of some primary and secondary metabolites in honeysuckle fruit. Differences in their contents arising from different heat treatments were determined with the aid of high-performance liquid chromatography (HPLC) coupled with mass spectrophotometry (MS). The con- tent of sugars showed a small change with drying, while organic acid contents decreased with a longer drying time. Ascorbic acid was totally degraded, regardless of the time or heating temperature. Different phenolic groups responded differently to heat intensity and time of drying. Flavanols were more sensitive to higher temperature than to duration of heating and they decreased by more than 70% at 75 °C. In contrast, the content of hydroxy- cinnamic acids, increased with drying by more than 75%, regardless of the time and temperature. Keywords: heating, Lonicera, phenolics, time of drying *Corresponding author e-mail: mateja.senica@bf.uni-lj.si Introduction Small fruit berries, such as blueberries, strawberries, blackberries and raspberries, are widely consumed all over the world. They are a rich source of polyphenolics, especial- ly flavonols and anthocyanins, which have health benefits (Sablani et al. 2010, Mikulic-Petkovsek et al. 2012). Minor fruits, such as quince, rose hip, hawthorn, saskatoon, choke- berries and honeysuckles, are easier to grow and hardy in na- ture, producing a crop even under adverse soil and climat- ic conditions (Gündüz and Özbay 2018). Their fruits have a unique aroma and taste and play a vital role in nutrition and as a source of livelihood, in particular providing em- ployment and income generation for rural and tribal groups (Vijayan et al. 2008, Mikulic-Petkovsek et al. 2012, Ercisli et al. 2012, Cuce and Sokmen 2017). The blue honeysuckle berry (Lonicera caerulea var. edulis Turcz. ex Herder) (WFO 2019) from the Lonicera genus and the plants can be organi- cally grown. Their natural growth areas are wetland spaces along rivers, marshes or forest clearings in northeastern Asia and America (Thompson 2008, Miyashita et al. 2009). The berries are similar in color to blueberries, dark purple with a waxy coating, but with an obvious difference in the ber- ry shape. Blue honeysuckle berries have a more elongated or cylindrical shape than the blueberry (Thompson 2008, Hummer et al. 2012). The taste is bitter to tart-sweet, a mix- ture of known berry flavors (Hummer et al. 2012). The ber- ries from blue honeysuckle have become popular because of their health-promoting properties. They contain nutraceuti- cal compounds, such as vitamins, minerals, polyphenolics, iridoids and saponins (Jurikova et al. 2009, 2012, Becker et al. 2017, Oszmiański and Kucharska 2018). Additionally, they contain a low sugar content compared to some other fruits, which could make them a good source of nutrition for peo- ple with diabetic troubles (Palíková et al. 2009). The berries have a short shelf-life and it is important to keep the product fresh to maintain its nutritional value as far as possible. Most storage techniques require low temperatures, which are diffi- cult to maintain throughout a distribution chain (Sagar and Kumar 2010). In addition to freezing, drying is the simplest DRYING OF BLUE HONEYSUCKLE BERRIES ACTA BOT. CROAT. 79 (1), 2020 69 procedure for preserving fruit (Senica et al. 2016). Not only does it extend the shelf-life of fruit, but it retains the char- acteristics of natural products, reduces the costs of packing, storage and transportation, due to the reduced weight and volume of the product, and additionally inhibits the growth of micro-organisms (Wang and Xu 2007, Chauhan and Sriv- astava 2009, Sagar and Kumar 2010, Mundada et al. 2010). Another advantage is the higher prices of commodities and accessibility of food in the season when fresh berries are not available. Disadvantages of drying are an alternation of col- or, in terms of browning, lipid oxidation, degradation of en- zymes and change of aroma, flavour and taste. The texture of dried products is influenced by their moisture content, composition, pH, and product maturity. During drying, the collapse of cell structures causes, reduction in size through shrinkage of the berries (Sagar and Kumar 2010). Freshly harvested blue honeysuckle berries have a short shelf-life. In Europe, fresh blue honeysuckle berries can be purchased from mid-May to the end of June. There are ma- ny food preservation methods for prolonging the presence of fruit in the market and to ensure product nutritional and health quality. The most suitable fruits for drying are apples, pears, plums, grapes, apricots, figs, persimmons and peach- es. They can be purchased dried on the market or prepared at home. They are principally dried on average for less than 30 hours (depending on the fruit) at 60 degrees (Garden- Robinson 2012, FAO 2019). There have been a several stud- ies investigating anthocyanin contents during different pres- ervation processes in blue honeysuckle berries (Khattab et al. 2016, Oszmiański et al. 2016) and some other fruit species, such as blueberries (Brownmiller et al. 2008, Sablani et al. 2010), persimmon (Karaman et al. 2014, Senica et al. 2016), gooseberry (Kucner et al. 2014) and strawberry (Wojdyło et al. 2009). Mineral content changes were observed in straw- berries stored at different temperatures (Çavuşoğlu 2018). The aim of this study was to identify appropriate drying con- ditions for preparing dried blue honeysuckle berries while achieving a high quality. We evaluated the influence of a combination of different temperatures (40, 50, 60, 65 and 75 °C) and times of heating (240, 200, 66, 30 and 20 hours) on the thermal stability of ascorbic acid, sugars, organic ac- ids and phenolic content of blue honeysuckle berries. Materials and methods Plant material The blue honeysuckle berries were from the cultivar 'Au- rora', grown organically in Slovenia. The berries were hand- harvested at the fully ripe stage on 11 June 2017, at the lo- cation Šmartno pri Litiji (46°2'38.7ʺ N 14°50'47ʺ E, 250 m a.s.l.). Fresh berries were immediately used for control treat- ment. Other berries were dried in a drying oven at different durations and temperatures. In addition to the control, we ap- plied five treatments: at 40 °C for 240 hours (10 days), at 50 °C for 200 hours, at 60 °C at 66 hours, at 65 °C for 30 hours and at 75 °C drying for 20 hours. Fifty grams of blue honeysuckle berries were used per repetition (10 repetitions). Drying process The drying process was carried out using a hot-air dry- ing oven (Suša 6, wood dryer, Splošno mizarstvo, Slovenia) at 40, 50, 60, 65 and 75 °C. Drying was continued for up to 30 h or the time needed to reach 15% of moisture or lower. Determination of ascorbic acid in dried honeysuckle berries For control 5 g of berries mashed and extracted with 10 ml of 2% meta-phosphoric acid was used. Half of one gram of dried berries from each drying treatment was ex- tracted with 5 ml of 2% meta-phosphoric acid. The control and all dried mixtures were then left at room temperature for 1 hour on a shaker (Grant-Bio POS-300, Grant Instru- ments, Shepreth, England) for ascorbic acid extraction. The samples were afterward centrifuged at 4 °C at 10 000 rpm for 7 min and filtered through a Chromafil A-20/25 mixed es- ter filter (Macherey-Nagel, Düren, Germany) into vials and left to wait until HPLC analysis. Determination of ascorbic acid was carried out with the Thermo Finnigan Surveyor HPLC system (Thermo Scientific, San Jose, CA). Conditions were previously described in Mikulic-Petkovsek et al. (2016) study. Contents were expressed in mg of ascorbic acid per 100g of dry weight. Determination of sugars and organic acids in dried honeysuckle berries Sugar and organic acid contents among various dried honeysuckle berries were estimated according to the method of Mikulic-Petkovsek et al. (2016). For control 5 g of fresh honeysuckle berries was mixed with an Ultra-Turrax T-25 macerator and extracted with 25 ml double-distilled water. Half of one gram of dried honeysuckle berries was extracted with 10 ml of double-distilled water. Each sample was then left at room temperature for 1 hour on a shaker. Samples were afterwards centrifuged at 4 °C at 10 000 rpm for 7 min and filtered through a Chromafil A-20/25 mixed ester filter into vials and allowed to wait until HPLC analysis. Determi- nation of individual sugars and organic acids was carried out with the Thermo Finnigan Surveyor HPLC system, as pre- viously described in Mikulic-Petkovsek et al. (2016) study. Contents were expressed in mg per g of dry weight. Determination of individual phenolics The extraction of phenolic compounds for 5 different honeysuckle berry products was carried out as described by Senica et al. (2016) with some modifications. For control honeysuckle berries were homogenized with an Ultra-Tur- rax T-25 and 5 g of fruit paste was extracted in 30 ml-centri- fuge tubes with 15 ml methanol containing 3% formic acid. For other treatments, one gram of dried honeysuckle berries was extracted with 10 ml of methanol containing 3% formic acid. All samples were then placed in a cool ultrasonic bath for 1 hour. The mixtures were then centrifuged for 10 min at 12 000 rpm. Each supernatant was filtered through a Chro- mafil AO-20/25 polyamide filter (Macherey-Nagel, Düren, Germany) and transferred into vials until HPLC and MS SENICA M, STAMPAR F, ERCISLI S, SLADONJA B, POLJUHA D, MIKULIC-PETKOVSEK M 70 ACTA BOT. CROAT. 79 (1), 2020 analysis. Separation of phenolic compounds was performed on a mass spectrometer (LCQ Deca XP MAX, Thermo Sci- entific) with electrospray ionization (ESI) operated in nega- tive and positive ion modes. The ESI parameters were de- scribed by Senica et al. (2016). Analyses were carried out using the Accela HPLC system (Thermo Scientific, San Jo- se, CA), equipped with a diode array detector (DAD), con- trolled by CromQuest 4.0 chromatography workstation soft- ware, with technical characteristics as described by Senica et al. (2016) with the mobile phase gradient according to Wang et al. (2002). Individual phenolic compounds were identi- fied by fragmentation with HPLC-MS, comparison of reten- tion times with standards and monitoring UV-VIS spectra from 200–550 nm. Calibration curves were prepared from all standards and the individual compounds were identified and quantified by comparison with pure standards. Chemicals Quinic acid, shikimic acid, 5-caffeoylquinic acid, neo- chlorogenic acid 3-caffeoylqinic acid, cyanidin-3-gluco- side, ellagic acid, naringenin and luteolin-3-rutinoside and ascorbic acid standards as well as meta-phosphoric acid and methanol were obtained from Sigma Aldrich Chemie (Stein- heim, Germany). We obtained fructose, glucose, sucrose, cit- ric, malic, fumaric and tartatic acid, standards for sugars and organic acids; additionally epicatechin, quercetin-3-galacto- side, quercetin-3-glucoside, quercetin-3-rutinoside, p-cou- maric acid, procyanidin B2, luteolin-3-glucoside, genistein and kaempferol-3-glucoside for standards of phenolics from Fluka Chemie (Buchs, Switzerland). Phenolic standards cat- echin, p-coumaric acid and caffeic acid were obtained from Roth (Karlsruhe, Germany); quercetin-3-xyloside, quer- cetin-3-arabinofuranoside from Apin Chemicals (Abing- don, UK) and isorhamnetin-3-rutinoside, loganin, petuni- din- and peonidin-3-glucoside from Extrasynthese (Genay, Frence). Ultrapure water used to prepare all water extrac- tions and the mobile phases was obtained from the Milli-Q system (Millipore, Bedford, MA, USA). For phenolics where standards were lacking, they were tentatively identified based on their fragmentation pattern obtained from MS2/MS3 anal- ysis and by comparison with data from the literature. Their contents were calculated using chemically similar phenolic compounds. Thus quercetin glycosides were quantified in equivalents of quercetin-3-galactoside, isorhamnetin glyco- sides in equivalents of isorhamnetin-3-rutinoside, luteolin derivatives in equivalents of luteolin-3-glucoside, genistein derivate in equivalents of genistein, kaempferol glycosides in equivalents of kaempferol-3-glucoside, procyanidins in equivalents of procyanidin B2, pelargonidin- and peonidin derivatives on pelargonidin or peonidin-3-glucoside and el- lagic acid derivatives in equivalents of ellagic acid. Statistical analysis The results were analyzed statistically using a one way analysis of variance (ANOVA) with the statistical program R commander. Duncan’s mean separation tests were done for comparisons of the contents of the primary and second- ary metabolites studied. Statistically significant differences were accepted at P < 0.05. Results All compounds were expressed on a dry weight basis to ensure reliable comparison among different thermal treat- ments. At lower temperatures, they needed a longer time to dry and for water to be removed from the fruit. The contents of ascorbic and organic acids as well as of sugars in dried honeysuckle berries are shown in Tab. 1. In Tab. 2, the iden- tification of individual phenolic compounds is given, while their contents are presented in Tab. 3. Sugars, ascorbic and organic acid levels Contents of sugars, glucose, fructose and sucrose in blue honeysuckle berry fruit were determined. Fructose content ranged from 51 to 58%, glucose from 41 to 47% and sucrose from 1–5% of total sugar content (Tab. 1). Determined fruc- tose contents were from 15.5–22.6 g 100 g–1 DW, glucose 11.5 to 17.7 g 100 g–1 DW and sucrose 0.47 to 1.8 g 100 g–1 DW of honeysuckle berries. In general, sugar contents in dried berries dropped one third less than non-treated berries. The highest total sugar content was measured in honeysuckle Tab. 1. The content of sugars and organic acids (g 100 g–1 DW) in fresh (control) and blue honeysuckle berries dried at different tempera- tures and times. Means ± standard deviation are presented. Different letters (a-d) in rows denote statistically significant differences in some primary metabolites among fresh and dried blue honeysuckle berries by Duncan multiple range test (P < 0.05); n = 10. Parameters Control 40 °C (240 h) 50 °C (200 h) 60 °C (66 h) 65 °C (30 h) 75 °C (20 h) Fructose 22.57±0.51a 15.58±1.37c 17.5±0.87b 17.21±1.13b 17.83±0.41b 15.96±0.63c Glucose 17.75±0.28a 14.23±0.98c 15.29±0.72b 12.82±0.99d 13.24± 0.45d 11.50±0.54e Sucrose 0.86±0.21c 0.65±0.22c 0.47±0.31cd 1.40 ±0.52b 1.78±0.22a 0.18±0.04d Citric acid 18.10±1.30a 6.28±0.54f 9.74±1.16d 8.44±1.07e 15.14±0.50b 11.67±0.30c Fumaric acid 0.002±0.00d 0.009±0.002c 0.010±0.001bc 0.003±0.001d 0.013±0.001a 0.012±0.001b Malic acid 5.91±0.28b 3.71±0.40c 5.37±0.66b 2.63±0.46d 7.79±0.53a 7.82±0.28a Shikimic acid 0.002±0.001c 0.022±0.007b 0.038±0.008a 0.004±0.001c 0.019±0.002b 0.023±0.001b Tartaric acid 1.23±0.12d 2.49±0.52c 3.54±0.61b 1.38±0.05d 4.61±0.38a 4.37±0.10a Quinic acid 6.62±0.26a 4.76±0.42b 6.10±0.78ab 3.16±0.33c 6.56±0.46a 7.44±0.92a DRYING OF BLUE HONEYSUCKLE BERRIES ACTA BOT. CROAT. 79 (1), 2020 71 berries dried at 50 °C for 200 hours and at 65 °C for 30 hours, after which sugar contents were 20% lower than before dry- ing (Fig. 1). The lowest total sugar content was measured in honeysuckle berries dried at 75 °C for 20 hours, with their contents 33% lower than in the control. The drying process caused a decrease of ascorbic acid as well. Fresh berries contained 154.89 mg 100 g–1 DW of ascorbic acid. After the heat treatment there was no detect- ed vitamin C content in any dried blue honeysuckle berries. Five organic acids were identified in the blue honeysuck- le berries. The most abundant was citric acid (36 to 57% of total organic acids), followed by malic and quinic acids (19 to 28%), tartaric acid (4 to 14%), while fumaric and shikimic acids contributed under 1% of total organic acids (Tab. 1). In general, the contents of organic acids significantly varied according to the different heat treatments (Fig. 1), but all organic acids slightly increased (Tab. 1) with heating above 65 °C (Fig. 2). Phenolic compounds composition Forty different individual phenolics were quantified by HPLC-MS in the honeysuckle berries (Tab. 2). We detected and identified some less known phenolics, such as loganin- pentoside, an iridoid determined according to its molecu- lar ion at m/z 521 [M-H]– and its corresponding fragment ions m/z 389, 227. An isoflavone genistein hydroxyhexoside (Tab. 2) was confirmed according to the fragmentation pat- tern; from molecular ion at m/z 449 we got fragment ion Tab. 2. Identification of phenolic compounds in blue honeysuckle fruits in positive and negative ions with HPLC-MS, MS2 and MS3. Phenolic group [M]+ or [M-H]–(m/z) MS2 (m/z) MS3 (m/z) Phenolic compound Hydroxycinnamic acid 353 191, 179, 135 Neochlorogenic acid (3-caffeoylquinic acid) 353 173, 179, 191 Cryptochlorogenic acid (4-caffeoylquinic acid) 353 191, 179, 173, 135 Chlorogenic acid (5-caffeoylquinic acid) 337 191, 173, 163 5-Coumaroylquinic acid 325 163, 119 p-coumaric acid hexoside 515 353 191, 179, 173 Dicaffeoylquinic acid Hydroxybenzoic acids 463 301 257, 229 Ellagic acid hexoside Flavanols 289 245 Catechin 289 245 Epicatechin 577 425, 407, 289 Procyanidin dimer 865 577, 451, 425, 407, 289 Procyanidin trimer Flavones 447 285 Luteolin hexoside 593 447 285 Luteolin-3-rutinoside Isoflavones 449 269 Genistein hydroxyhexoside Flavonols 519 315 Isorhamnetin acetyhexoside 665 315 Isorhamnetin acetyl rhamnosylhexoside 609 315 Isorhamnetin hexosylpentoside 623 315 Isorhamnetin-3-rutinoside 489 285 Kaempferol acetylhexoside 579 285 Kaempferol hexosylpentoside 593 285 Kaempferol-3-rutinoside 447 285 Kaempferol-3-glucoside 505 301 Quercetin-3-acetylhexoside 433 301 Quercetin-3-arabinofuranoside 463 301 Quercetin-3-galactoside 463 301 Quercetin-3-glucoside 463 301 Quercetin hexoside 595 301 Quercetin hexoside pentoside 609 301 Quercetin-3-rutinoside 595 301 Quercetin-3-vicianoside 433 301 Quercetin-3-xyloside Flavanones 433 271 Naringenin hexoside Iridoid 521 389, 227 Loganin-7-pentoside Anthocyanins 611 449/287 Cyanidin-3,5-diglucoside 449 287 Cyanidin-3-glucoside 595 449/287 Cyanidin-3-rutinoside 595 433/271 Pelargonidin dihexoside 433 271 Pelargonidin-3-glucoside 625 463/301 Peonidin dihexoside 463 301 Peonidin-3-glucoside SENICA M, STAMPAR F, ERCISLI S, SLADONJA B, POLJUHA D, MIKULIC-PETKOVSEK M 72 ACTA BOT. CROAT. 79 (1), 2020 m/z 269. Ellagic acid hexoside was identified according to its molecular ion at m/z 463 [M-H]– and its correspond- ing fragment ions m/z 301, 229 and 257. All phenolic com- pounds were divided into 9 groups (Tab. 2). The groups in our study had different tolerances to temperature and time of heating (Fig. 2). The group of hydroxycinnamic acids (HCA) represent- ed only 3% of total phenolics in fresh blue honeysuckle ber- ries. The main contributors to HCA in fresh berries were neochlorogenic (3-caffeoylquinic acid) and dicaffeoylquin- ic acids, while in dried blue honeysuckle berries there were neochlorogenic and p-coumaric acids (Tab. 3). In our study their content was higher after the drying than in fresh ber- ries. The highest content of total HCA derivatives was mea- sured in honeysuckle berries dried at 65 °C for 30 h (355.94 mg 100 g–1) (Fig. 2). Flavanols (catechin, epicatechin and procyanidins) con- tribute approximately 50% of total analyzed phenolics in fresh berries. Their contents decreased with heating. The highest level, which was still only half that of fresh berries, was determined in berries dried at 60 °C for 66 h (374.13 mg 100 g–1). Berries dried at 50 °C for 200 h (190.45 mg 100 g–1 DW) had the lowest flavanol content (Tab. 3). The group of flavonols (quercetin, kaempferol and isor- hamnetin glycosides), with up to 18% of total analyzed phe- nolics, did not show a clear trend of increase or decrease with heating. At temperatures of 60 °C and 75 °C, the content of flavonols was higher than in the control, while their contents in other treatments slightly decreased (Fig. 2). Honeysuckle berries dried at 75 °C for 20 h had the highest flavonol con- tent (326.60 mg 100 g–1), while berries dried at 65 °C for 30 h had the lowest flavonol content (248.61 mg 100 g–1) (Tab. 3). Flavanones and flavones contributed less than 1% of to- tal phenolics. Naringenin hexoside of the flavanone group showed a decrease in content with longer heating time and an increase at higher temperatures. Flavones in our study de- creased with all heat treatments, but a higher decrease oc- curred after longer drying than at higher temperature (Tab. 3). In our study, anthocyanins comprising 26% of the total phenolics, had the lowest content among phenolics in all heat treatments (Fig. 2). The most abundant of total antho- cyanins was cyanidin-3-glucoside and the least was pelargo- nidin-dihexoside. Cyanidin-3-glucoside represented from 45 to 65% of total anthocyanins in the various drying treat- ments. Their level decreased by 64% with drying at 40 °C for 240 hours, by 80% at 50 °C for 200 hours, by 57% at 60 °C Fig. 1. The content of total sugars and organic acids during the different heating treatments expressed per 100 g DW. Different letters (a-d) mean significant differences among different heat treatments (P < 0.05) by Duncan’s multiple range test. Fig. 2. The content of different phenolic groups during the different heating treatments expressed per 100 g DW. Different letters (a-d) mean significant differences among different heat treatments (P < 0.05) by Duncan’s multiple range test. DRYING OF BLUE HONEYSUCKLE BERRIES ACTA BOT. CROAT. 79 (1), 2020 73 Tab. 3. Individual phenolics content (mg 100 g–1 DW) in six different blue honeysuckle berry products. Means ± standard deviation are presented. Different letters (a-f ) in rows denote statistically significant differences in individual phenolic levels among blue honeysuckle berry products by Duncan’s multiple range test (P < 0.05); n = 10. Control 40 °C (240 h) 50 °C (200 h) 60 °C (66 h) 65 °C (30 h) 75 °C (20 h) Hydroxycinamic acids Neochlorogenic acid (3-CQA) 19.69±0.34 e 154.56±8.97b 86.70±3.56d 161.58±20.94b 198.99±15.14a 119.10±23.42c Cryptochlorogenic acid (4-CQA) 0.57±0.01 c 56.22±5.72a 32.78±3.54b 57.30±8.80a 54.35±3.61a 40.34±8.92b Chlorogenic acid (5-CQA) 0.26±0.01a 0.04±0.00d 0.02±0.00e 0.05±0.00c 0.07±0.00b 0.01±0.00f Coumaroylquinic acid 1.84±0.01e 5.87±0.55d 9.21±0.46c 34.56±0.70a 19.75±1.10b 6.72±0.75d p-Coumaric acid hexoside 2.99±0.27d 68.05±5.65a 30.26±3.78bc 48.90±4.65ab 69.99±2.81a 14.96±1.84cd Dicaffeoylquinic acid 14.13±0.39a 10.51±0.73c 7.48±0.40d 14.38±0.56a 12.79±1.34b 11.77±1.71bc Hhydroxybenzoic acids Ellagic acid hexoside 16.09±1.75b 1.6±0.27e 7.12±0.41d 9.40±1.03c 10.73±0.72c 23.44±1.28a Flavanols (+)catechin 20.04±2.76a 16.03±1.31bc 7.93±0.63e 17.21±1.36b 14.59±1.11c 10.05±0.79d (-)epicatechin 232.83±23.50a 89.40±8.11c 81.62±14.64c 142.13±18.31b 91.77±6.70c 75.25±2.55c Procyanidin dimer 539.20±40.48 a 184.93±18.17c 100.73±14.99d 214.37±11.93b 169.72±6.68c 122.70±10.15d Procyanidin trimer 0.55±0.05b 0.49±0.03bc 0.16±0.02d 0.42±0.03c 0.21±0.03d 1.13±0.15a Flavones Luteolin hexoside 3.01±0.15a 1.55±0.81cd 1.13±0.09d 1.83±0.11bc 1.49±0.12cd 2.09±0.31b Luteolin-3-rutinoside 4.85±0.21a 3.84±0.65b 4.56±0.47a 4.81±0.48a 2.05±0.15c 4.56±0.51a Isoflavone Genistein hydroxyhexoside 1.91±0.07a 1.05±0.01d 1.05±0.01d 1.30±0.10c 2.07±0.19a 1.53±0.07b Flavonols Isorhamnetin acetylhexoside 1.47±0.16a 0.03±0.00b 0.01±0.00b 0.02±0.00b 0.01±0.00b 0.02±0.00b Isorhamnetin acetyl rhamnosyl hexoside 10.91±0.61 ab 10.47±1.31ab 6.31±0.38c 11.78±0.56a 10.26±1.06b 9.69±1.40b Isorhamnetin hexosylpentoside 1.95±0.11 d 4.49±0.86c 5.55±0.53b 6.20±0.29a 5.28±0.44b 6.35±0.10a Isorhamnetin-3-rutinoside 14.08±0.92a 9.21±0.97c 9.90±1.64c 12.99±0.44ab 9.32±1.17c 11.98±1.27b Kaempferol acetylhexoside 0.51±0.00d 1.30±0.16a 0.47±0.03d 0.97±0.03b 0.74±0.05c 0.89±0.09b Kaempferol hexosylpentoside 3.55±0.08 a 1.23±1.72b 0.58±0.05b 0.70±0.03b 0.75±0.05b 1.01±0.16b Kaempferol-3-rutinoside 2.20±0.25e 2.53±0.29de 3.03±0.37cd 3.56±0.34bc 3.88±0.58b 5.47±0.86a Kaempferol-3-glucoside 0.06±0.00a 0.10±0.01a 0.15±0.02a 0.12±0.01a 0.09±0.01a 0.09±0.01a Quercetin-3-acetylhexoside 1.95±0.12e 3.09±0.06b 2.54±0.51d 3.53±0.22a 3.04±0.28bc 2.64±0.44cd Quercetin-3- arabinofuranoside 18.90±0.96 a 7.68±0.99c 4.88±0.28d 9.46±0.38b 7.82±0.63c 7.15±0.70c Quercetin-3-galactoside 30.73±1.90 d 30.43±2.66d 23.90±3.05e 43.93±2.68b 35.98±2.97c 54.52±2.97a Quercetin-3-glucoside 4.29±0.12d 37.74±8.85a 30.38±4.40bc 34.06±3.37ab 24.20±1.92c 37.95±4.90a Quercetin-hexoside 2.33±0.19a 0.57±0.08d 0.15±0.03e 0.76±0.09c 2.37±0.14a 1.92±0.24b Q hexoside-pentoside 6.60±0.05a 3.11±0.82d 3.89±0.65c 3.88±0.31c 3.49±0.32cd 4.85±0.18b Quercetin-3-rutinoside 173.44±15.19a 163.47±16.10a 157.39±17.29a 165.13±11.90a 131.02±12.15b 167.08±14.52a Quercetin-3-vicianoside 16.24±0.55a 11.89±0.81cd 12.45±2.42bc 14.11±1.72ab 10.02±0.91d 14.56±2.09ab Quercetin-3-xyloside 0.40±0.03ab 0.32±0.03c 0.36±0.06bc 0.45±0.05a 0.34±0.04c 0.44±0.05a Flavanones Naringenin hexoside 1.36±0.04c 0.76±0.13e 0.98±0.09de 1.72±0.11b 1.19±0.14cd 2.27±0.37a Iridoid Loganin-7-pentoside 3.28±0.38b 4.23±0.75a 1.81±0.16c 2.06±0.10c 1.80±0.31c 0.99±0.03d Anthocyanins Cyanidin-3,5-diglucoside 29.53±0.36a 0.51±0.09d 0.16±0.01e 2.82±0.17b 2.11±0.10c 0.44±0.08d Cyanidin-3-glucoside 268.46±17.30a 83.41±8.49c 42.90±9.15e 105.21±5.19b 107.60±11.54b 67.53±1.81d Cyanidin-3-rutinoside 44.19±2.52b 30.61±2.49d 18.61±4.91e 30.27±1.28d 93.03±3.20a 37.31±5.17c Pelargonidin dihexoside 7.21±0.45a 2.89±0.33d 1.46±0.11e 5.74±0.52b 3.85±0.21c 2.87±0.22d Pelargonidin-3-glucoside 10.76±0.43a 3.50±0.16e 3.94±0.15d 3.63±0.46de 6.03±0.21c 6.42±0.11b Peonidin dihexoside 12.39±0.90bc 14.75±0.81a 6.79±0.60d 11.46±1.14c 13.38±0.38b 11.79±0.55c Peonidin-3-glucoside 40.61±1.52a 13.07±0.74c 6.58±0.59d 16.70±1.79b 15.72±0.35b 12.17±0.21c Total 1565.37±42.52a 1035.56±61.54c 715.94±57.15e 1199.52±74.99b 1141.90±59.62b 902.06±49.25d SENICA M, STAMPAR F, ERCISLI S, SLADONJA B, POLJUHA D, MIKULIC-PETKOVSEK M 74 ACTA BOT. CROAT. 79 (1), 2020 for 66 hours, by only 40% with berries dried at 65 °C for 30 hours and by 66% at 75 °C for 20 hours (Fig. 2). Discussion Oszmiański et al. (2016) reported that honeysuckle ber- ries mainly consist of water and soluble solids. Selected bio- active compounds start to alter soon after harvest. Particu- larly, after harvesting, different fruit ingredients are subject to various enzymatic and non-enzymatic reactions. Enzy- matic reactions involve some enzymes, which react in the presence of individual compounds and oxygen, resulting in a change in their contents and composition. Other, non-en- zymatic processes do not require enzymatic catalysis, but they include three main reaction pathways: Maillard reac- tion, caramelization and ascorbic acid oxidation (Sanz et al. 2001). The drying process additionally changes the contents of selective primary and secondary metabolites. Water solu- bility and heat sensitivity are the main two factors that al- ter the content of selected compounds in dried fruits (Kara- man et al. 2014). Honeysuckle berries have extremely firm skins, which impeded water evaporation in our study. We thus needed much more time to dry the berries to an ac- ceptable dryness (85%) than is needed for other often-dried fruits. FAO (2019) reported the desirable final moisture content in dried fruits to be 15%. In general, heat treatment means water conversion into vapor, which passes in a gas- eous state across a disrupted cell and consequently changes the concentration of solid components and the evaporation of some volatile compounds (Karathanos 1999, Yadav and Singh 2014, Karaman et al. 2014). At lower temperatures, the transformation process lasts longer than at higher temper- atures, in which water passes from the fruit faster. Enough moisture (85%) must be removed for the product to be con- sidered dried, otherwise mold starts grow on the berries in a few days (FAO 2019). Singh et al. (2006) found that a low temperature of drying caused minimum damage to dried material, retaining more nutrients in the fruits than other drying methods. Sugars in blue honeysuckle berries in general decrease with drying. Our sugar contents were in accordance with previous published studies (Oszmiański et al. 2016, Auzan- neau et al. 2018). One of the reasons for the decrease in the sugar content was the Maillard reaction. This is a complex series of reactions between amines, amino acids and proteins with sugars and it is the major cause of fruit browning dur- ing heating processes. The result of that reaction is reduced sugar content and the formation of brown pigments (Yilmaz and Toledo 2005). The Maillard reaction was not the only reason for sugar reduction. Caramelization also occurs, tak- ing place above the melting point of sugar, which darkens to a brown color and decreases the sugar content (Sanz et al. 2001). Furthermore, heating of the berries caused water evaporation and partial decomposition of sugars i.e. trans- formation to volatiles, such as water vapor and carbon diox- ide, or other types of carbon-containing volatiles (Karatha- nos 1999). The other elements of sugars were modified into crystallized structures (Senica et al. 2016), which only dif- fuse from the berry with difficulty. All heat treatment caused some extent of water evaporation and the formation of solu- ble sugars, but a longer duration at higher temperatures can destroy sugar molecules, which are prone to chemical trans- formation at elevated temperatures (Karaman et al. 2014). High ascorbic acid content in fresh blue honeysuckle berries is also found in the study of Jurikova et al. (2009). Ascorbic acid content can be affected by many factors like heat intensity, drying time, final moisture content and air ve- locity (Santos and Silva 2008). Ascorbic acid is a water-sol- uble compound (Khattab et al. 2017) and it is lost from the berries with vapor. Khattab et al. (2017) reported the reduc- tion of vitamin C by 90% at a temperature of 60 °C following 24 h drying. Our results are in agreement with Khattab et al. (2017) as ascorbic acid was lost from berries after less than 24 hours of drying at 75 °C. Degradation of ascorbic acid in dried fruit starts with the first-order reaction, followed by the effect of the reduction of the moisture content which, as the process of drying proceeds with the temperature effect, becomes predominant (Santos and Silva 2008, Goula and Adamopoulos 2006, Qiu et al. 2018; Di Scala and Crapiste 2008). The first reactions may depend on water activity, pH and the presence of degraded enzymes. With an increase of the water content, the aqueous phase becomes less viscous, which enhances diffusion in the media. This facilitates the reaction of oxidation and, consequently, the degradation of certain compounds (Santos and Silva 2008). The same au- thors also reported that both heat intensity and time of dry- ing have a major effect on ascorbic acid degradation. A lon- ger drying time results in a lower retention of ascorbic acid. In addition, higher temperatures (> 65 °C) and the conse- quent increase in relative humidity resulted in a lower reten- tion of ascorbic acid. During the drying process, moisture and temperature altered with drying time and their combination caused deg- radation, and the formation of organic acids. Tartaric, fu- maric and shikimic acids increased with both higher tem- perature and duration of heating. Quinic, citric and malic acids, the most abundant of the organic acids in blue honey- suckle berries seem to be sensitive to drying time (more than 30 hours), but their contents increased with temperatures above 60 °C. It seems that organic acids tolerate higher tem- peratures better than a long time of heating. Regardless of the temperature, dry matter was similar among treatments. Higher temperatures mean faster water transfer from the berries, which is linked to a higher loss of low-molecular weight components (Kucner et al. 2014, Zorenc et al. 2017), the same as with sugars. On the other hand, organic acid content seems to be more sensitive to a long duration of heating. Lower temperatures (40 and 50 °C) imply low po- rosity of the epidermal layer of various berries, which is re- flected in slower mass transfer, and the destruction of mol- ecules (Kucner et al. 2014). Chen et al. (2012) reported that organic acids, including citric acid, degraded via the gam- DRYING OF BLUE HONEYSUCKLE BERRIES ACTA BOT. CROAT. 79 (1), 2020 75 ma-aminobutyrate shunt pathway following a longer time of heating at lower temperatures. Blue honeysuckle berries are rich in phenolic compounds and their presence as established here is in accordance with some other studies (Senica et al. 2018a, b; Oszmiański et al. 2016). The last mentioned study put special emphasis on iri- doids with high health properties. In our study, significant changes occurred in their contents during the thermal treat- ment at different heating times and heat intensities. Total phenolic content has been reported to diminish as a result of food processing (Može Bornšek et al. 2015). The main reason for phenolic content decrease is the transfer of phe- nolics to the hypertonic solution. Heat treatment causes cel- lular disruption and the exposure of phenolics to oxidative and hydrolytic enzymes (Wojdyło et al. 2009, Karaman et al. 2014). The most important enzymes are polyphenol oxi- dases (PPO), which catalyze the oxidation of colorless phe- nolic compounds into o-quinones, which are red to brown in color. Heating and poor handling of fruits or vegetables cause greater PPO activation and, accordingly, a decrease in the content of some phenolic compounds (Sanz et al. 2001, Yilmaz and Toledo 2005). Additionally, with the drying pro- cess, a longer time or a higher heat intensity increases the presence of PPO (Kucner et al. 2014). Additionally, Wojdyło et al. (2009) reported that drying temperatures between 55 and 85 °C reduced the content of phenolics. They suggested that an irreversible oxidative process and prolonged expo- sure to thermal degradation may be the cause of altered lev- els of phenolic compounds. Unlike other groups of phenolics, the content of hy- droxycinnamic acids (HCA) increased with both heat- ing time and temperature. That is in agreement with sev- eral reports (Brownmiller et al. 2008, Wojdyło et al. 2009, Oszmiański et al. 2016). Zorić et al. (2014) also noted the high heat stability of HCA. Kaneko et al. (2016) reported that selected HCA have extremely high thermomechanical per- formance. The thermal degradation temperature for HCA was around 300 °C (Kaneko et al. 2016), which was four times higher than our highest studied drying temperature. Flavanols, flavonols, flavanones, flavones and isofla- vones made up the group of flavonoids. Their heat sensi- tivity caused by drying depends on the structural stability of the selected compound. In particular, compounds with a double bond in the structure need more energy in order to be degraded (Chaaban et al. 2017). Wang et al. (2000) re- ported that compounds from the flavanol group, especially epicatechin, are highly sensitive to oxidation processes. In our study, their contents greatly decreased regardless of the temperature and time of heating. Flavonols have two more double bonds in their basic molecular structures than the previously described flavanols. They are therefore more sta- ble during heat treatment. The structure of flavonones re- sulted in greater stability with respect to the intensity but not with respect to the duration of drying. Degradation of these contents occurred, but more slowly than with some more thermolabile compounds, such as anthocyanins and flavanols (Zorić et al. 2014). Anthocyanins, with high beneficial properties for human health, are mostly concentrated in the skin of the blue hon- eysuckle berry (Oszmiański et al. 2016). Accordingly they are more exposed to degradation, including from heating. Total anthocyanins showed significant differences among the various heat treatments (Figure 2). Berries dried at 65 °C for 30 h (241.72 mg 100 g–1) had the highest anthocy- anin content, but still only approximately half that of fresh berries (413.15 mg 100 g–1). Berries dried at 50 °C for 200 h had the lowest anthocyanin content. Khattab et al. (2016) also found a positive correlation between drying tempera- ture and anthocyanin degradation. Those results are also in accordance with Zorić et al. (2014) and Zorenc et al. (2017), who reported that degradation of anthocyanins is signifi- cantly greater at higher temperatures. A higher stability of anthocyanins during heating was achieved by using a low- er temperature and shorter duration of heating during pro- cessing (Wang and Xu 2007, Zorić et al. 2014). Our study showed that dried berries had the same losses at 40 °C as at 75 °C, which confirms that heat treatment in itself negatively affects the cyanidin content. Additionally, loss of anthocya- nins can be attributed to various factors, such as residual en- zyme activity or condensation reactions with other pheno- lics (Brownmiller et al. 2008). Some anthocyanins are more vulnerable than other phenols from that group, because of their different chemical structure (Srivastava et al. 2007), mainly due to different sugar and hydroxyl moieties. Cy- anidin glycosides are considered to be less stable in relation to heating, which is in accordance with our study, in which cyanidin-3-glucoside and cyanidin 3.5-diglucoside was 70 to 98% lower than in fresh berries. The results showed that peonidin and pelargonidin are more stable in relation to heat, which is in agreement with previous studies (Srivas- tava et al. 2007, Khattab et al. 2016). Dried blue honeysuckle berries are a durable and con- venient product available throughout the year. It is impor- tant to recognize that drying has many advantages; apart from cost reduction, it also accelerates water loss and con- sequently prevents the growth of bacteria, fungi, and other microorganisms. The chief disadvantage is the loss of some important nutritional compounds. Sagar and Kumar (2010) reported that an optimal drying system for the preservation of fruits should be cost effective, with a short drying time and minimum damage to the food product. The studied contents of some primary and secondary metabolites in our study responded differently to drying conditions. Sugars, fla- vonols and HCA appeared to be more thermostable sub- stances, especially HCA, which increased by more than 75% with drying, regardless of the drying time and temperature. On the other hand, anthocyanins and flavanols were highly thermolabile substances, their contents decreasing with an increase of both drying time and temperature. Additionally, ascorbic acid totally degraded with heat treatment. What is more, organic acids seem to be more sensitive to long expo- sure to drying, than to higher temperature of heating, while iridoids are more sensitive to higher heating temperatures. SENICA M, STAMPAR F, ERCISLI S, SLADONJA B, POLJUHA D, MIKULIC-PETKOVSEK M 76 ACTA BOT. CROAT. 79 (1), 2020 In conclusion, we found that the optimal treatment was drying at 60 °C for 33 h, which is in agreement with Gar- ba and Kaur (2014). Understanding the structural stability of selected active compounds in blue honeysuckle berries will help their processors to provide high quality dried berry products with rich nutritional properties. Acknowledgement The research is part of the program Horticulture No. P4-0013-0481, which is funded by the Slovenian Research Agency (ARRS). The authors would like to thank Haskap d.o.o. for contributting the plant material. 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