Drying kinetics,colour, total phenolic content and antioxidant capacity properties of kiwi dried by different methods

This study analysed the convective (60, 70 and 80° C), microwave (120 and 350 W) and freeze drying methods in terms of their effects on the drying characteristics, colour, total phenolic content (TPC) and antioxidant capacity of kiwi slices. Nine different mathematical models were applied to experimental data to achieve the most accurate calculation for drying curves. The Midilli et al. and Wang and Singh models proved to be the most suitable at explaining the drying kinetics of kiwi samples as compared to other models according to the statistical tests. Each drying method was significantly affected by colour parameters (L*, a*, b*, C, α and ΔE). The dried samples exhibited respectively 5–49 % and 10–47 % less TPC and antioxidant capacity compared to the fresh sample. According to the correlation analysis conducted between TPC and antioxidant capacity for kiwi slices, there is a positive correlation (R ²  = 0.7796). Microwave dried samples at 120 W particularly had the lowest TPC and antioxidant capacity. Freeze drying method yielded the closest values with respect to colour values, total phenol content and antioxidant capacity to those of fresh samples when compared to the other methods.     

Effects of drying methods on colour,amino acids,phenolic profile,microstructure and volatile aroma components of Boletus aereus slices

This study is to investigate the effects of hot air drying (HD) at 60–80 °C (HD-60, HD-70 and HD-80), microwave pretreatment combined with HD (MHD), ultrasound pretreatment combined with HD (UHD) and vacuum freeze drying (FD) on the quality of Boletus aereus slices. Key characteristics, including colour, free amino acids, microstructure, volatile aroma components, phenolic profile and antioxidant activity, were studied. Amongst these drying methods, FD was the best way to preserve colour and volatiles of Boletus aereus slices. Fifteen amino acids were detected in Boletus aereus slices, and HD-60 showed a higher content of total amino acids than other drying regimes. Two phenolic acids were identified in Boletus aereus slices, namely, protocatechuic acid and p-hydroxybenzoic acid. MHD presented the highest content of phenolic compounds, followed by FD. Antioxidant capacity was positively correlated with phenolic compounds of Boletus aereus slices. Moreover, the cell structure of Boletus aereus slices was damaged after drying, and FD had a more porous structure than other drying methods. These results will be beneficial to the further industrial production of Boletus aereus slices.     

Influence of drying treatments on the physical and chemical properties of cucumber

This work aimed at determining the properties of cucumber exposed to convective air drying and freeze-drying. The samples were analysed in terms of colour, texture, chemical properties (moisture content, fibre, ash, vitamin C and sugars), phenolic compounds and antioxidant activity in fresh and after drying. The trials in the convective chamber were done at 40 and 60 °C, in the drying tunnel at 60 °C and in the freeze dryer at ?50 °C. The results showed that the antioxidant activity and the phenolic compounds were not affected by any of the drying treatments tested, since the values were quite similar in the fresh product as compared to the dried cucumbers. With respect to colour, the freeze drying treatment was identified as the one originating less colour change, when compared with the colour of the fresh product. Finally, texture was less affected by drying in the chamber at 40 °C and freeze drying.     

Effect of drying methods on volatile compounds,fatty acids and antioxidant property of Thai kaffir lime (Citrus hystrix D.C.)

This study investigated the changes in colour, volatile compounds, fatty acids, total phenolic content (TPC), total flavonoid content (TFC) and antioxidant activity of kaffir lime leaves (makrud in Thai) Citrus hystrix D.C. after three drying treatments, namely hot‐air (HA) drying, low relative humidity (LRH) air drying and far‐infrared radiation (FIR) drying. Overall, when compared with fresh leaves, a slight decrease in L* values of the three dried samples was observed. TPC, TFC and ferric‐reducing/antioxidant power (FRAP) values were increased by LRH and FIR drying (LRH: 39%, 43% and 28%; FIR: 62%, 39% and 39%), while HA drying decreased TPC, TFC and FRAP values (28%, 21% and 1%), compared with fresh leaves. The main volatile compounds in fresh leaves were citronellal, followed by linalool and viridiflorol. Drying brought about an increase in the concentrations of volatile components, such as citronellal with HA, LRH and FIR drying (79%, 83% and 64%, respectively), compared with the results for fresh leaves (56%). LRH and HA drying increased essential fatty acid alpha‐linolenic acid (ALA, 18:3n‐3), while linoleic acid (LA, 18:2n‐6) was increased by FIR drying. The present study has provided useful information for the industrial use of dried kaffir lime production.     

Effect of temperature on the drying characteristics,colour, antioxidant and beta‐carotene contents of two apricot varieties

In this study, the drying characteristics, colour, oxygen reactive antioxidant capacity (ORAC) and beta‐carotene contents of two apricot varieties dried at different temperatures were compared. The hot air drying of apricot slices for both varieties consisted of a constant rate period (CRP) and two of falling rate periods (FRP). The CRP drying rate and the first and second FRP drying coefficients increased with drying temperature for both apricot varieties. The first and second FRP of both apricot varieties gave activation energies of 23.5–28.7 and 25.6–29.3 kJ mole^?1, respectively. The colour values (L*, a* and b*) of both dried apricot varieties decreased with increasing temperature, while the total colour change of both dried apricot varieties increased with temperature. The chroma, hue angle and browning index values of both dried apricot varieties decreased with increasing temperature, and the hydrophilic ORAC and beta‐carotene contents increased with drying temperature.     

Drying of Garlic Slices Using Convective Pre-drying and Vacuum-Microwave Finishing Drying: Kinetics, Energy Consumption, and Quality Studies

Garlic slices were dehydrated by convective drying (CD) and by a combined method consisting of convective pre-drying followed by vacuum-microwave finishing drying (CPD-VMFD). Drying of garlic using VMFD led to samples with high values of lightness and slightly lower values of the coordinates a* and b*. The treatment CPD-VMFD significantly decreased the drying time from 630 min for CD to 49 min and contributed to an important energy saving compared with CD (54.4–86.3 %). The total drying time was shorter when vacuum microwaves were introduced earlier and microwave power was higher. The highest temperature (114 °C) was found for samples pre-dried to a moisture content of 60 % and then dried at 360 W while showing the lowest energy consumption (98?±?1 kJ?g^?1). In general, the total antioxidant capacity increased after drying of garlic slices; however, the content of total phenolic compounds showed the opposite trend, and thus other compounds, such as allicin, should be involved in the antioxidant capacity of dried garlic. Garlic slices were CP dried until 30 % of moisture content followed by VMFD at 240 or 360 W showed the best results in terms of energy consumption and antioxidant capacity.     

Phenolic compounds and antioxidant activity are differentially affected by drying processes in celery,coriander and parsley leaves

Convective drying (CO) is the most common technique for drying herbs, although it may diminish phenolic compounds content. Microwave drying (MW) has been suggested as an alternative, although its effects on phenolic compounds remain to be explored. The aim of this study is to compare the effects of different convective temperatures (40, 80 and 120 °C) and microwave powers (100, 500 and 1000 W) on drying time, polyphenols classes (HPLC-DAD) and antioxidant capacity in celery, coriander and parsley leaves. Microwave procedure reduced drying time (MW: 3 h–7 min; CO: 25–1.5 h) resulting more effective than CO. Increasing temperatures led to an initial decrease in total phenolic content (TPC) and antioxidant activity followed by an increase, potentially by the release of bound phenolic acids. Regarding MW, the highest polyphenol content was generally observed at 500 W, probably due to the hydroxycinnamic acid's rise. Antioxidant capacity was similar at 500 and 1000 W. There was a good agreement between TPC and antioxidant capacity. Consequently, MW seems a good alternative to reduce drying time. However, the maintenance or improvement of phenolic activity depends on the food matrix. Indeed, the best drying system and conditions to preserve phenolic compounds were variable depending on the herb type: celery (CO at 40 °C), coriander (both CO at 40 °C and MW at 500 W) and parsley (MW at 100 W).     

Microwave assisted fluidized bed drying of nutmeg mace for essential oil enriched extracts: An assessment of drying kinetics,process optimization and quality

The drying mechanism of the myristicin enriched nutmeg mace has been optimized in a microwave assisted fluidization bed dryer (MWFBD) through multiple linear regression (MLR) and artificial neural network (ANN) modeling. The developed drying technique overcomes the non-uniformity heating problems in microwave heating and prolonged drying in fluidized bed drying. During the novel method of drying selected input variables were drying air temperature (DT) (40–50 °C) and microwave power (MP) (480–800 W) and output variables involving colour, oil yield, and myristicin quantity have been investigated by a continuous air velocity of 5.1 m/s. Six mathematical models about one to four conditions have fitted with an experimental design. Suitable enforcement of such models was evaluated through statistical measures. The coefficient of determination (R²) of MLR varied from 0.89 to 0.98, and the sum of squared error (SSE) varied within 6 × 10⁻³ to 158.18, while R² of ANN fluctuated from 0.82 to 0.95, and the mean squared error (MSE) varied between 0.006 and 0.1450, which shows MLR design performance superior than ANN design. The processing conditions of 48.24 °C DT and 637.431 W MP with a drying time of 1.3 h were identified as optimum conditions with a desirability value of 0.98 to obtain maximum oil yield (13.38%) and good colour (L* (20.83), a* (17.34), b* (8.62)) of nutmug mace. Moreover, no myristicin (5.92%) degradation was observed compared with the sun and convective drying. Among the tested models, page and logarithmic models gave a better prediction of moisture ratio.     

真空冷冻干燥对菊花多酚含量及其抗氧化活性的影响

以4种菊花(金丝皇菊、贡菊、雏菊、杭白菊)为原料,研究真空冷冻干燥对其多酚含量及抗氧化活性的影响。结果表明,真空冷冻干燥后,菊花总酚含量显著增加,其中雏菊的总酚含量增加率最高(增加了2.525倍);真空冷冻干燥后,除清除ABTS~+·能力差异不显著外,其清除DPPH·能力、铁还原能力以及ORAC变化显著,说明真空冷冻干燥对菊花多酚的抗氧化活性有较大影响。     

Fruit snacks from raspberries: influence of drying parameters on colour degradation and bioactive potential

Raspberries were dehydrated using air and freeze‐drying with wet and dry sugar infusion pretreatments. Product quality factors such as colour, bioactive compounds, antioxidant capacity and sensorial characteristics were analysed. Special emphasis was placed on the analysis of anthocyanin degradation and its relationship with colour deterioration and with polymeric compounds development and browning. Freeze‐dried raspberries presented a higher retention of bioactive compounds and a lower content of polymeric compounds than air‐dried ones. Dried samples without pretreatment (control) showed the highest retention of total phenolic content (freeze‐dried ≈82% and air‐dried ≈37% retention), but the lowest sensory acceptability. Although sugar infusion pretreatments caused an important loss of bioactive compounds (9–18% of TPC retention), a higher sensorial acceptability was obtained. Pretreatments with bisulphite and acid allowed obtaining the best quality attributes in terms of anthocyanin and polyphenol content, antiradical activity and colour retention. Polyphenol intake through pretreated dried raspberries (115–299 mg gallic ac./100 g intake) would be higher in some cases than that of usually consumed foods as vegetables, cereals and several fresh fruits.     

Effects of different drying methods on phenolic contents,antioxidant, and tyrosinase inhibitory activity of peach blossoms

Fresh peach blossoms of two varieties were dried by using different methods, which include shade drying, freeze drying, microwave drying and hot air drying (at 30, 60, 90, 120 °C). The effects of different drying methods on phenolic contents, antioxidant and tyrosinase inhibitory activity were evaluated. Among the seven drying treatments, microwave drying yielded the highest contents of total phenolics, flavonoids, anthocyanins, proanthocyanidins and individual phenolic compounds in dried peach blossoms, while hot air drying at 30 °C gave the lowest retention of total phenolics, flavonoids, anthocyanins. As compared with shade drying and freeze drying, hot air drying at 60, 90, 120 °C resulted in higher retention of phenolic content. The highest antioxidant activity in dried peach blossoms was obtained by microwave drying, while hot air drying at 30 °C gave the lowest. The effect of different drying methods on antioxidant activity of dried peach blossoms was consistent with that of phenolic content. The highest tyrosinase inhibitory activity was achieved by hot air drying at 120 °C for variety ‘yingchun’ and 90 °C for variety ‘huangjinmeili’, but there was no significant difference among hot air drying at 60, 90, 120 °C for both varieties. The tyrosinase inhibitory activities of peach blossoms by microwave drying and freeze drying were comparable and much lower than that of shade drying. The results indicated that microwave drying was a desirable method for the preservation of phenolic compounds and antioxidant activity in peach blossoms, while shade drying and hot air drying at high temperature were favorable for tyrosinase inhibitory activity.     

Degradation kinetics of total phenolic compounds,capsaicinoids and antioxidant activity in red pepper during hot air and infrared drying process

Degradation kinetics of total phenolic compounds, capsaicinoids and antioxidant activity in red pepper during both hot air drying and infrared drying were investigated, as well as the correlation between antioxidant compounds and antioxidant capacity was discussed in current study. The fractional conversion kinetic and first order kinetic models were proved to describe the changes of total phenolic compounds vs. drying time and moisture content, respectively. Degradation of capsaicinoids and antioxidant capacity vs. drying time and moisture content both followed fractional conversion kinetic model. Infrared drying accelerated the degradation rate of total phenolic compounds and capsaicinoids vs. drying time as compared with hot air drying at the same temperature; however, a contrary result was found that infrared drying decreased the degradation rate of both total phenolic compounds and capsaicinoids vs. moisture content. Total phenolic compounds exhibited a stronger relationship with antioxidant capacity of red pepper analysed by multiple linear regression analysis.     

Effects of microwaves,hot air and freeze-drying on the phenolic compounds,antioxidant capacity,enzyme activity and microstructure of cacao pod husks (Theobroma cacao L.)

The cacao pod husk (CPH) is the primary waste byproduct of the chocolate industry. One ton of cacao beans generate approximately ten times that amount of fresh CPH. The husk is rich in phenolic compounds. The aim of this study is to determine the effects of microwaves, hot air and freeze-drying on the phenolic compound content, antioxidant capacity, enzyme activity and microstructure of CPH. The results showed that fresh CPH contain 323.7 ± 26.5 mg gallic acid equivalents (GAE)/100 g dry mass (d.m.) of total phenolic compounds, and dehydration had a positive effect on the CPH phenolic content and antioxidant capacity. Catechin, quercetin, and (−)-epicatechin and gallic, coumaric and protocatechuic acids were identified in fresh and dried CPH. Drying the material using a microwave and freeze-drying preserved the husk microstructure. The results showed that microwave drying was a better drying method than hot air and as good as freeze-drying.Industrial relevanceThe food industry is currently searching for new sources of natural antioxidants. The cacao pod husk (CPH) is a good source of phenolic compounds that can function as antioxidants and could be used as ingredients in functional foods. However, due to their high water content, fresh CPH are perishable products, and for this reason, the first step would be dry the pod husk to improve its shelf life. The findings of this study showed that the use of microwaves to dry CPH may release polyphenols, thereby enhancing the antioxidant capacity and decreasing the polyphenol oxidase (PPO) activity while maintaining the microstructure. Thus, microwave drying could be of interest as a method for enhancing the extractability of phenolic compounds from CPH.     

Changes of quality characteristics of pepino fruit (Solanum muricatum Ait) during convective drying

Quality assessment of pepino fruit preserved through convective dehydration was investigated in this work. The effect of process temperature (50, 60, 70, 80 and 90 °C) on physicochemical properties, colour, non‐enzymatic browning, vitamin C, total phenolic content, antioxidant activity and firmness of the fruit were considered. When comparing the fresh with the corresponding dehydrated pepino samples, it was shown that the drying conditions resulted in important reductions of proteins and crude fibres. Discoloration of fruit was noticeable for all treatments due to effects of non‐enzymatic browning and changes in chromatic coordinates leading to a modification of the original colour. Regarding vitamin C and total phenolic content, an increase of drying temperature resulted in a considerable reduction of both initial compounds contents. Antioxidant activity showed an important decrease especially at low temperatures (e.g. 50 °C). Softening of the dried product was observed for all the treatments indicating structural modifications of the fruit due to thermal process.     

Changes of hydrogen peroxide and radical-scavenging activity of raspberry during osmotic, convective, and freeze-drying

This study was conducted to investigate the influence of different drying treatments on antioxidant (AO) activity and phenolic content of raspberry (Rubus idaeus), cultivar Willamette. Whole raspberry fruits were dried convectively (air-drying), osmotically, and freeze-dried. Acetone-water extracts of fresh and dried raspberries were assessed for total phenolic content by standard Folin-Ciocalteau method. Two AO assays were applied, a recently developed direct current (DC) polarographic assay based on decrease of anodic oxidation current of hydrogen peroxide and widely used radical scavenge against the 1,1-diphenyl-2-picrylhydrazyl (DPPH). Strong correlation has been obtained between both AO assays and total phenolic content. In addition, some individual phenolic compounds present in raspberry have been assessed using DPPH and DC polarographic assay. Comparison and evaluation of drying methods has been based on preservation of AO activity and total phenolic content. Obtained results confirmed superiority of freeze-drying; convective drying caused slight changes while osmotic dehydration showed a significant decrease of phenolic compounds and AO activity.     

Phenolic Analyses of Haskap Berries (Lonicera caerulea L.): Spectrophotometry Versus High Performance Liquid Chromatography

Haskap berries (Lonicera caerulea L.) are known for their high phenolics, anthocyanins, and antioxidant potential. The data on the phenolic profile of these fruits are lacking. In this study, the phenolic profiles of three haskap varieties; tundra, berry blue, and indigo gem grown in Nova Scotia, Canada were investigated for the first time using spectrophotometery and high-performance liquid chromatography. Berries were analyzed for total phenolic content/total reducing capacity, total anthocyanin content, and antioxidant potential (2,2-diphenyl-1-picrylhydrazyl radical scavenging activity). The total reducing capacity, total anthocyanin content, and 2,2-diphenyl-1-picrylhydrazyl values were 6.17–8.42 mg gallic acid equivalents/gram fresh weight, 4.49–6.97 mg cyanidin-3-glucoside equivalents/gram fresh weight, and 78.70–89.55%, respectively. The extracts were analyzed by reversed-phase diode array detector–high-performance liquid chromatography through a gradient elution using Synergi 4 µm Max-RP C12 column and the chromatograms were acquired at 520, 360, and 320 nm for athocyanins, flavonoids, and free phenolic acids, respectively. The identified anthocyanins were cyanidin-3-glucoside (82.81–91.99% of the total anthocyanins), cyanidin 3,5-di-glucoside (2.31–4.27%), cyanidin-3-rutinoside (1.54–9.20%), peonidin-3-O-glucoside (0.75–3.44%), and pelargonidin-3-glucoside (0.77–2.98%). Other flavonoids (quercetin-3-β-D-glucoside and quercetin-3-rutinoside) and free phenolic acids (chlorogenic and neochlorogenic) were also quantified.     

Effects of infrared heating on drying kinetics,antioxidant activity,phenolic content,and color of jujube fruit

This study investigated the effects of different infrared power (IP) levels (62, 88, and 125 W) and a pretreatment (soaking in a solution of 5 % potassium carbonate and 0.5 % olive oil) on the drying kinetics and some quality parameters of jujube fruit. The drying characteristics of jujube were greatly influenced by the pretreatment and IP level. The models of Lewis, Logarithmic, Page, and Aghbashlo et al. were fitted to the obtained experimental data using nonlinear regression analysis. The Page model showed a better fit to the experimental drying data when compared to the other models. The effective moisture diffusivity, calculated using Fick’s second law, ranged from 4.75 × 10^?10 to 4.17 × 10^?9 m²/s. Significantly, higher total phenolic content (TPC) and antioxidant capacity values were obtained for the pretreated samples subjected to higher IP levels. The total color change (ΔE) of the dried samples significantly increased with increasing IP level. Jujube fruits should be pretreated and then dried at 88 W IP to reduce phenolic degradation and undesired color changes and to increase the quality of the dried product.     

Chemical Composition, Antioxidant Capacity, and Sensory Quality of Pomegranate (Punica granatum L.) Arils and Rind as Affected by Drying Method

The objective of this study was to evaluate the application of: (1) freeze drying, (2) convective drying (50, 60, or 70 °C), (3) vacuum–microwave drying (240, 360, or 480 W), and (4) a combined method of convective pre-drying and vacuum–microwave finish drying in the processing of pomegranate arils and rind. The quality parameters under study included sugars and organic acids, punicalagins and ellagic acid, total polyphenols, total antioxidant activity, and sensory quality. In general, drying led to a reduction in all studied parameters; however, the behavior of arils and rind was different. Vacuum–microwave drying at 240 or 360 W was the best drying treatment for arils, while rind required freeze drying or soft conditions of convective drying (50 °C). Further research is needed to obtain proper results with convective pre-drying and vacuum–microwave finish drying of arils and rind. With proper selection of the drying protocol, high-quality dried arils will be available for consumers; these arils will be characterized by high contents of fructose (25 g 100 g^?1), phytic acid (2.2 g 100 g^?1), punicalagins (0.57 mg g^?1), total polyphenols (1.6 mg eq gallic acid g^?1), high antioxidant capacity (0.6 mg eq Trolox g^?1), and high intensities of garnet color, sweetness, sourness, and fresh pomegranate aroma. Besides, dried rind with very high contents of active compounds (123 mg g^?1 of punicalagins and 108 mg eq gallic acid g^?1) and high antioxidant capacity (26 mg eq Trolox g^?1) will be also available as functional material.     

Changes in bioactive compounds and antioxidant activity during convective drying of murta (Ugni molinae T.) berries

Murta (Ugni molinae T.) berries were air‐dried at five temperatures (40, 50, 60, 70 and 80 °C), and the changes in β‐carotene, phenolic acids, total phenolic and flavonoid contents and antioxidant capacities (DPPH and ORAC) were investigated. The berries showed a high content of β‐carotene, which decreased during drying temperature between 40 °C and 80 °C. Free and bound phenolic acids were also determined, showing gallic acid to be the prevalent phenolic acid. Total phenolic and flavonoid contents in the dried berries showed a higher decrease at lower temperature due to longer drying time. The radical‐scavenging activity also showed higher antioxidant activity at higher drying temperatures (70–80 °C) than at lower drying temperatures (40–50 °C). Total phenolic content (TPC) and flavonoids showed good correlation with antioxidant capacity. Murta berries proved to be an excellent source of antioxidants and bioactive compounds and are therefore a potential ingredient for new functional food products.     

Combined Drying of Apple Cubes by Using of Heat Pump,Vacuum-Microwave,and Intermittent Techniques

Apple cubes of 15 mm were dried naturally without adding any chemical preservative using various drying methods namely intermittent hot air–dehumidified air drying with cyclic temperature profile and step-up temperature profile, heat-pump-assisted (HP) drying, convective vacuum-microwave (C/VM) drying, and heat pump vacuum-microwave (HP/VM) drying. The drying kinetics of apple samples dehydrated by different methods was divided into characteristic drying periods and fitted with empirical models, which gave high value of determination coefficient. The application of C/VM in drying of fruits gave the shortest drying time compared to other drying methods (about 50 % of the total drying time). The drying time was affected by effective diffusivity ranging from 3.522?×?10^?8 to 1.431?×?10^?6 m²/min depending on the drying technique used. It was found that combined drying which apply vacuum microwave (C/VM, HP/VM) gave the lowest values in hardness and chewiness. In addition, HP/VM drying gave the highest retained total polyphenol content, antioxidant activity, and the best appearance quality.