The effect of osmotic dehydration pretreatment with sweetenerson the quality of dried aronia berries

Aronia (A. melanocarpa) was cultivated in a farm in Sangju-si, Kyungsangbuk-do, Korea, and was still frozen upon purchase. After removing the stems, aronia berries were sorted by weight (1.20 and 0.20 g) and thawed for 90 min before pretreatment. Sucrose (100%; CJ, Incheon, Korea), glucose (99.5% with 0.5% dextrin; Hwami, Incheon, Korea), and xylitol (100%; Roquette, Lestrem, France) were used as sweeteners. These sweeteners are classified as a disaccharide, monosaccharide, and sugar alcohol, respectively; hence, they have different structures.

Sucrose, glucose, and xylitol were used to prepare highly concentrated osmotic solutions (60 °Brix) to achieve the same sweetness values. Aronia berries were placed in each solution in a ratio of 1:4 (w:w) and then subjected to osmotic dehydration at 40°C for 3 h by applying 40-kHz ultrasonic waves using an ultrasonic cleaner (DH.WUC. D22H, Daihan-Sci, Wonju, Korea). After pretreatment, osmo-dehydrated aronia berries were washed with distilled water (DW) for 10 sec and wiped with a tissue paper to remove any water from the surface.

Hot-air drying was conducted at 50°C for 57 h using a dryer (BL950903, Gumbok Stoke Co., Ltd., Seoul, Korea). Freeze-drying was conducted for 4 days using a lyophilizer (LP20-XX, Ilshin Biobase Co., Ltd., Dongducheon, Korea). To determine the optimum drying time for aronia berries, the conditions with the highest overall preference were selected through a preliminary experiment, and the drying time was set. The samples were grouped as follows: raw aronia (CON), freeze-dried aronia (FD), aronia osmodehydrated using sucrose and FDs, aronia osmo-dehydrated using glucose and freeze-dried (FDG), osmo-dehydrated using xylitol and freeze-dried (FDX), hot-air-dried aronia (HD), aronia osmo-dehydrated using sucrose and hot-airdried (HDS), aronia osmo-dehydrated using glucose and hot-air-dried (HDG), and aronia osmo-dehydrated using xylitol and hot-air-dried (HDX).

The moisture content (MC) of dried aronia was measured gravimetrically by drying in an oven (JSOF-150, JS Research, Inc., Gongju, Korea) at 103°C for 40 h, which was calculated by the following equation:

where w0 is the initial weight of aronia and wS is the weight of oven-dried aronia.

Here, 5 g of the aronia product was blended with 45 mL of DW using a hand blender (HR1604, Artreal Huiyang Manufacturing Ltd., Hui Zhou, China) for 30 sec and then filtered using Whatman No.4 filter paper. The total soluble solid content was measured using a refractometer (Master-a, Atago Co., Tokyo, Japan). The pH was measured using a pH meter (Orion 3 Star, Thermo Electron Co., Waltham, MA, USA).

After pretreatment in the same manner as before pH was measured, the sample was refiltered using a 0.45 m membrane filter, and the obtained filtrate was used for free-sugar analysis. The free-sugar content was measured by high-performance liquid chromatography (HPLC; 2695, Alliance, Waters Co., Milford, MA, USA), and a Sugar-Pak I column (6.5 mm×300 mm; Waters Co.) was used. The operating conditions were as follows: mobile phase, 0.01 M CaEDTA (50 mg/L DW); injection volume, 20 μL; column temperature, 90°C; flow rate, 0.5 mL/min; detection mode, refractive index.

Aronia methanol extract (AME) was prepared using a previously described method (Kim et al., 2016) with slight modifications. First, 5 g of aronia was cut into pieces, and 45 mL of 80% methanol solution was added. This mixture was shaken at 120 rpm at 30°C for 26 h in a shaking incubator (JSSI-300C, JSR, Gongju, Korea), and then the mixture was filtered (Whatman No.4 filter paper). The filtrate was concentrated using a rotary vacuum concentrator, methanol was completely removed using an oven at 70°C, and the concentrate was diluted with 80% methanol at 100 mg/mL of the concentrate.

The total phenolic content (TPC) was determined using the Folin-Ciocalteu method (Benvenuti et al., 2004) with slight modifications. A mixture of 1 mL of AME and 1 mL of 50% Folin-Ciocalteu reagent was prepared and left in the dark for 15 min. Afterward, 1 mL of 10% sodium carbonate solution was added, and the mixture was left for 1 h before reading the absorbance values at 750 nm using an Evolution 201 UV-visible spectrophotometer (Thermo Fisher Scientific, Brooklyn, NY, USA). Gallic acid was used as the standard, and the results were expressed as mg of gallic acid equivalents (GAE)/100 g of aronia dry weight (dw).

The anthocyanin content (AC) was assayed using the pH differential method (Giusti and Re, 2001). The AME was diluted with 0.025 M potassium chloride buffer (pH 1.0), incubated for 15 min, and then the spectrophotometric absorbance at 515 and 700 nm was recorded. The dilution factor was set so that the absorbance value was 8.0-1.0 at 515 nm. Likewise, 0.4 M sodium acetate buffer (pH 4.5) was used in the same way as the pH buffer, and the AC was calculated as follows:

where A₅₁₅ nm = absorbanceat 515 nm;

A₇₀₀ nm = absorbanceat 700 nm.

The proanthocyanidin content (PC) was evaluated using Broadhurst and Jones’ method (1978) with slight modifications. An aliquot (0.3 mL) of AME was mixed with 1.8 mL of 4% vanillin-methanol solution and 0.9 mL of 32% HCl, and the mixture was incubated in the dark for 15 min. The absorbance was measured at 500 nm. The standard was (+)-catechin (CAT), and the results were expressed as mg of CAT/100 g dw.

The fluorescence recovery after photobleaching (FRAP) assay was conducted by using Benzie and Strain’s (1996) method. Acetate buffer (300 mM, pH 3.6), 10 mM 2,4,6-Tri(2-pyridyl)-s-triazine, and 20 mM FeCl3․6H2O (10:1:1,v:v:v)were mixed immediately before the experimentand incubated at 35°C for 10 min. An aliquot (1.8 mL) of this mixture was added to 0.2 mL AME. After incubating for 30 min, the absorbance was measured at 590 nm. The ABTS was measured as previously described by Re et al. (1999). A mixture of 2.45 mM potassium persulfate solution and 7 mM ABTS solution (1:2, v:v) was left for 16 h, and then diluted with ethanol to obtain absorbance of 0.70±0.02 at 734 nm. The AME (20 L) was mixed with 2 mL of the activated solution and incubated in the dark for 6 min before measuring the absorbance at 734 nm. For both AA assays, Trolox served as the standard, and the results were expressed as mmoL of Trolox equivalents (TE)/100 g dw.

Strength and hardness were measured by a rheometer (Compac-100II, Sun Scientific Co., Tokyo, Japan). The probe was set to penetrate 8% of the sample’s height at a speed of 100 mm/min. The test was repeated five times per sample.

The aronia product was blended using a hand blender (SU07843-12003, KCC-REI-PCE-HR1600, Philips, Amsterdam, the Netherlands). Color parameters, including L* (lightness), a* (redness), and b* (yellowness), were directly recorded using a colorimeter (CR-400, Konica Minolta, Tokyo, Japan). Before measuring, the colorimeter was calibrated against a standard white tile. Measurements were repeated 15 times per sample.

Fifteen trained panel members evaluated the appearance, color, texture, chewiness, sweetness, sourness, astringency, and overall preference on a 7-point scale. Astringency was measured through an objective evaluation (1 = little astringency; 7 = extreme astringency), while the other attributes were measured through preference ratings (1 = very bad; 7 = very good). A product that received a score of 3 points or less was considered commercially unviable. This sensory evaluation was performed with the approval of the Kyungpook National University Industry Foundation (Approval number: 2019-0052).

The experiments were repeated three or more times, and the results were expressed as the mean standard deviation. ANOVA was performed using the SPSS software package (version 25.0; SPSS, Inc., Chicago, IL, USA). Significant differences were determined using Duncan’s multiple range test (p<0.05).

The MC, TSS, and pH data are presented in Table 1. The MC of raw aronia (CON) was 83.22%. The MC range of the FD was 3.83%, 7.45%, which was noticeably lower than that of HD. Mishra et al. (2015) prepared dried papaya cubes by infrared drying at 2 kGy, followed by their blanching and osmotic dehydration pretreatment. The dried cubes had an MC of 38%. Furthermore, no bacteria, yeast, mold, or Staphylococcus were detected after the cubes were stored for 60 days. We followed the same method for drying the aronia berries. We noticed that except for CON, HD, and HDS aronia berries, the other six products (FD, FDS, FDG, FDX, HDG, and HDX) seem to be safe from microorganisms. The MC of the pretreated aronia berries decreased in the order of CON, followed by the berries osmo-dehydrated with sucrose, glucose, and xylitol for both drying methods. This tendency may be explained by the proportion of sweetener infiltration and loss of moisture in the aronia berries during osmotic dehydration. The TSS range was 6,908.47 °Brix for the FD samples and 3.705.87 °Brix for the HD samples. Moreover, the FD samples showed generally higher pH than the HD samples; CON had the highest pH. This trend of pH is thought to result from freeze-drying that maintains the characteristics of the samples better than hot-air-drying. Glucose-treated products showed the lowest pH, irrespective of the drying method. As shown by Lee et al. (1990), the pH of glucose was 6.65, whereas the pH of sucrose, sorbitol, and maltitol were 7.30, 7.35, and 7.25, respectively.

The free sugar content of the dried aronia samples is presented in Fig. 1. The FDG group had the highest glucose content (617.34 mg/mL; Fig. 1A), followed by HDG (395.59 mg/mL). This may be due to osmotic dehydration performed using glucose solution, because the solute (glucose) extensively infiltrated the aronia berries. In addition, the sucrose products had a higher free sugar content than the xylitol samples probably because of sucrose degradation, which causes enhanced penetration of the sugar into the aronia berries during pretreatment. Fructose content (Fig. 1B) was the highest in FDS (308.99 mg/mL), followed by HDS (200.88 mg/mL). It is believed that the sucrose molecules that penetrated aronia berries in the FDS and HDS groups were decomposed to fructose and glucose by heat-drying. Sorbitol can be detected in aronia berries. Figure 1C presents the sorbitol content, which was highest in FDX (570.50 mg/mL), followed by HDX (421.35 mg/mL). Xylitol and sorbitol are sugar alcohols with five and six carbons each, respectively, and share a similar structure. These results suggest that the ratio of free sugar content is affected by the pretreatment solution.

Fig. 1. Free sugar content of dried aronia berries. (A), glucose; (B), fructose; (C), sorbitol . a-hValues with different letters on the bars are significantly different (p<0.05). CON, raw aronia; FD, freeze-dried; FDS, osmo-dehydrated using sucrose and freeze-dried; FDG, osmo-dehydrated using glucose and freeze-dried; FDX, osmo-dehydrated using xylitol and freeze-dried; HD, hot-air-dried; HDS, osmo-dehydrated using sucrose and hot-air-dried; HDG, osmo-dehydrated using glucose and hot-air-dried; HDX, osmo-dehydrated using xylitol and hot-air-dried.

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The results on TPC, AC, PC, and AA (FRAP and ABTS) are presented in Table 2. The TPC, AC, and PC displayed a similar tendency, being highest in the FD group. Among the osmo-dehydrated products, FDS and FDX had relatively high TPC, AC, PC and AA values, whereas HDX had the lowest value. The FD products had higher values for these variables than the HD products. FRAR and ABTS activity exhibited a similar tendency to the phenolic compound contents. For FD, the FRAP and ABTS values of AA were 38.70 and 46.06 mM TE/100 g dw, respectively, which were the highest values observed among all the samples. Among the osmo-dehydrated products, the FRAP activities for FDS (21.12 mM TE/100 g dw) and FDX (20.91 mM TE/100 g dw) remained higher than those of HDX (10.17 mM TE/100 g dw), which had the lowest value. ABTS activity also showed that FDS (26.92 mM TE/100 g dw) had the third-highest value, followed by FDX, whereas HDX had the lowest value. Overall, the ABTS activity was slightly higher than the FRAP activity.

The texture and color of the dried aronia products are presented in Table 3. The FD samples tended to have higher texture values than the HD samples, and pretreatment with osmotic dehydration further raised these values. In the strength test, FDG had the highest strength at 22.79 kg/cm2. Hardness showed the same tendency as strength, but the hardness values were comparably higher than the strength values. FDG had the highest hardness valueat 288.31kg/cm2, followed by FDS at 203.06 kg/cm2. Zhao et al. (2014) revealed that mangoes pretreated by osmotic dehydration with glucose solution had higher hardness than those osmo-dehydrated with sucrose or maltose solution, because the lower water content and higher freezing rate of the glucose-pretreated samples can reduce the cell structure damage, resulting in a firmer texture. The lightness (L*) value was the highest for FD (25.28). Redness (a*) is the most important color parameter for aronia products as it greatly appeals to consumers. FDG and FDS recorded the highest a* values among the products, which were 17.90 and 17.43, respectively. Redness (a*) was increased by lyophilization and decreased by hot-air-drying, except for HDG. For the same drying method, aronia berries osmodehydrated with glucose solution presented a higher a* value, whereas xylitol solution tended toward a lower a* value. In the case of yellowness (b*), the FD samples had greater yellow intensity than CON and HD samples. FD had the highest yellowness at 7.29 (b*). When samples that underwent the same drying process but different pretreatments were compared, FDX and HDX showed the lowest b* values at 5.71 and 1.26, respectively. This is because polyols such as xylitol do not caramelize, in contrast to other sweeteners (Bar, 1985).

The sensory test scores are shown in Table 4. In terms of appearance, the HD samples showed relatively low preference, which may be attributed to their shattered appearance caused by hot-air-drying. In addition, HDG received the lowest score for color. There was no noticeable difference in texture, but FDX scored the lowest with 3.53 points. Unlike the other freeze-dried products that had a crispy texture, FDX was soft and sticky, as also evidenced by the relatively low instrumentally determined strength and hardness (Table 4), as well as chewiness. Chewiness score was highest for FDG with 5.20 points, followed by FDS, which could be a reflection of the relatively high strength and hardness values obtained for these products, as shown in Table 4. According to Joo et al. (2013), acorn powder cookies with higher hardness had greater texture sensory preference. Regarding sweetness, FDS had the highest acceptability, whereas HD (2.60 points) was commercially unviable. Because of its perceived sourness, CON is also commercial unviable. Astringency was most severe for HD (4.93 points). For the overall preference, FDS scored the highest with 5.13 points, followed by FDG with 4.80 points. In comparison, CON, FD, and HD scored low, possibly because these samples did not undergo pretreatment. Therefore, osmotic dehydration, especially using sucrose or glucose solution, may increase the acceptability of dried aronia products, whereas non-pretreated products were considered commercially unacceptable.

In conclusion, based on the results of MC and TSS, it is recommended that freeze-drying should be used rather than hot-air-drying to prepare dried aronia products. Phenolic compound contents and AA were higher in the order of FD, CON, and FDS or FDX. The sensory test showed that FDS scored the highest value of sweetness and overall preference, whereas CON, FD, and HD are considered of no commercial value. FDG and FDS showed high texture and redness values. Therefore, FDS, which has high scores in sensory tests and physical property values, with relatively low reduction of phenolic contents and AA, seems to be the most appropriate preservation method to obtain new dried aronia products.