Food Sci Biotechnol (2019) 28(5):1465–1474
https://doi.org/10.1007/s10068-019-00588-7
Effects of hot air, microwave and vacuum drying on drying
characteristics and in vitro bioaccessibility of medlar fruit leather
(pestil)
Senem Suna1
Received: 13 November 2018 / Revised: 11 February 2019 / Accepted: 18 February 2019 / Published online: 4 March 2019
 The Korean Society of Food Science and Technology 2019
Abstract The effects of microwave (90 W and 180 W), northern parts of Iraq and Iran. In Turkey, 4352 tons of
hot air (60 and 70 C) and vacuum (60 and 70 C with 200 medlar were harvested in 2017 according to Turkish Sta-
and 300 mbar) drying methods on drying characteristics, tistical Institute (Akbulut et al., 2016, TUIK, 2018). Medlar
total phenolic content, antioxidant capacity, color and is a potential source of ascorbic acid and natural antioxi-
in vitro gastrointestinal digestion of medlar pestil were dants as well as including phenolics like chlorogenic acid,
investigated. Medlar showed a good potential for pestil rutin and p-coumaric acid. Medlar fruits could be eaten raw
production while being the most applicable in microwave or consumed in the form of jam, jelly, marmalade, wine,
treatments. For drying kinetics, five thin-layer drying liquer and pickle. Fruits are likewise utilized in the therapy
models were applied and the Page and Modified Page were of constipation and evacuation of kidney and bladder
the best fitted models. L*, b*, chroma and hue angle stones. Fruit leather (pestil) is a traditional product formed
decreased while a* generally increased in dried pestils. by the addition of sucrose and starch into the pulp and the
Dried samples showed a general decrement in phenolics dehydration of fruit pulp into leathery sheets. As a result of
and antioxidant capacity. According to in vitro gastroin- the increasing awareness of consumers in healthy food
testinal digestion, intestinal phase of all the samples consumption, demand for value-added products has raised
resulted with an increment in phenolics, FRAP and DPPH and pestil gained importance as being an economic source
compared to undigested extracts. In conclusion, different of natural fruits with several nutrients (Chen and Marty-
drying methods may affect the release of phenolics and nenko, 2018; Jaturonglumlert and Kiatsiriroat, 2010; Lee
antioxidant capacity, while leading to increased bioacces- and Hsieh, 2008; Ruiz et al., 2014; Tontul and Topuz,
sibility during intestinal digestion. 2017). Polyphenols are of great importance being in this
group not only with health beneficial effects but also their
Keywords Medlar  Pestil  Drying kinetics  In vitro intake and bioaccessibility. Bioaccessibility is defined as
gastro intestinal digestion the quantity of nutrient delivered from food matrix. In this
topic, the bioaccessibility of polyphenols are affected from
molecular interactions between phenolics and other com-
Introduction ponents, release from the food matrix and the chemical
state of the compound as they are ingested as complex
Mespilus germanica L., which is a member of Rosaceae mixtures. Besides, mechanical and thermal treatments may
family, is commonly known as medlar. It is indigenous in result with the enhancement in digestibility and bioacces-
southeastern Europe, Anatolia, Crimea, Caucasia and the sibility of nutrients (Lemmens et al., 2010).
Drying of food is a complex thermal process including
simultaneous heat and mass transfer in the product.
& Senem Suna Advantages of thermal processing incorporate enzyme and
syonak@uludag.edu.tr
microbial inactivation, shelf life prolongation, enhanced
1 Department of Food Engineering, Faculty of Agriculture, digestibility and bioavailability of nutrients and antioxi-
Bursa Uludag University, 16059 Nilüfer, Bursa, Turkey dants as well as the negative effects like loss of desirable
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1466 S. Suna
nutrients. To obtain a high quality dried product, drying Preparation of pestil
method and time comes as the most important factors. In
this shed of light, thin layer drying models are used for the Medlar fruits were washed, peeled, pitted and homoge-
development of the system performance as well as allow- nized with a domestic blender (Beko, Turkey). Soluble
ing the determination of the best operational conditions solid content (Brix) of the puree was 23 Bx. Freshly
specific to different food or products (Sampaio et al., prepared puree (51%), distilled water (37%) and sucrose
2017). (10%) were boiled in an open kettle at constant stirring
Evaluation of drying methods used in food industry then wheat starch (1.6%) and ascorbic acid (0.4%) were
displays that, hot air drying is the quite common and added to the paste to obtain 42 Bx. Boiling was carried
effective method, even though it has some disadvantages out for 15 min on average. Final brix of the mixture was
such as the drying time continuation and low energy arranged according to similar studies like 40–50 Bx and
effectiveness (Arslan and Ozcan, 2010). As an elective 50–55 Bx in pomegranate (Tontul and Topuz, 2017) and
technique, vacuum drying is utilized at decreased pres- mango fruit leathers (Pushpa et al., 2006) respectively.
sures, empowering food to be dried at lower temperatures. Afterwards, cooked medlar mixture (approximately 100 g)
With this strategy, less oxidation responses happen because was evenly spread on a 10 9 10 cm square plastic mold
of the nonappearance of air, while the sensorial properties stated on the greaseproof paper with a thickness of 4 mm
of the dried foods are kept up. Microwave drying is another and dried under several conditions in hot air, microwave
option with different focal points, such as a higher drying and vacuum drying techniques. After the each treatment,
rate, homogeneous energy conveyance on the material and pestil samples were pulled from the surface, packaged with
preferable process control. On the contrary, it may lead to a low density polyethylene film and stored at room tem-
unequal warming and conceivable textural harm besides its perature until analyzed.
high installation costs (Arslan and Ozcan, 2010; Maskan,
2001). Drying procedure
There are so many literature about pestil production
studied with diverse kinds of fruits, however, medlar has Three different methods (hot air, microwave and vacuum
been evaluated with using advanced drying techniques for drying) were applied to the pestils and the drying experi-
the first time. Besides, there are several scientific reports ments were carried out in triplicate. Hot air drying treat-
about mathematical modelling of drying of several fruit ments were performed with a cabinet type laboratory dryer
and vegetables and revealing the phenolic content and (Yucebas Machine Analytical Equipment Industry Y35,
antioxidant capacity. To the best of the knowledge, this Izmir, Turkey) with the technical features of 220 V,
research was the first in which drying kinetics of medlar 50–60 Hz, 200 W. The temperature and relative humidity
fruit leather were studied with thin layer models and both in the dryer was measured by temperature sensor (± 2 C)
the bioaccessibility of phenolic content and antioxidants and relative humidity sensor (± 2%). Drying was per-
were reported via an in vitro digestion model. The first aim formed at temperatures of 60 and 70 C and a constant
of this research was to produce a natural high quality 20% relative humidity. 50 g of pestil (10 9 10 cm square)
medlar pestil and reveal the best drying parameters via with a thickness of 4 mm was placed on the greaseproof
mathematical modelling of drying kinetics. The second aim paper as a thin layer. The temperatures were applied in
was to determine the effects of different drying processes accordance with previous reports (Jaturonglumlert and
on total phenolic content and antioxidant capacity and Kiatsiriroat, 2010; Pushpa et al., 2006; Tontul and Topuz,
monitore their changes in the in vitro gastrointestinal track. 2017). All along drying, the samples were removed at
intervals and weighed ahead being returned to the dryer.
Pestil samples were weighed at 15 min intervals for 3 h
Materials and methods and the loss of moisture was determined by weighing the
plate using a digital balance (Mettler Toledo, MS3002S)
Materials measuring to the accuracy of 0.01 g. Drying experiments
were completed between 115 and 175 min depending on
Mature medlar fruits were harvested on January 15, 2018 in the temperature.
Bursa Turkey and left to storage at 4 ± C for a week to Microwave drying experiments were conducted in a
get ripened and softened. Width and length of the fruits domestic microwave oven (Bosch, HMT72G420, Munich,
were measured respectively as 3.10 ± 0.23 and Germany) with the technical features of 230 V–50 Hz and
3.58 ± 0.19 cm with a vernier caliper. maximum output of 800 W. The dimensions of the
microwave cavity were 520 9 479 9 341 cm in size and it
was consisted of a rotating glass plate of 315 mm diameter
123
Influence of drying methods on drying kinetics and bio accessibility of medlar pestils 1467
at the base of the oven. Drying treatments were performed
where Mt and Mt?dt are the moisture contents at t and
at 90 and 180 W microwave power levels. 50 g of pestil
t ? dt (g water/g dry base) respectively, and t is drying
samples stated on greaseproof paper were put on a rotating
time (min).
glass plate in the microwave oven. Drying was applied
Root mean square error (RMSE) gives deviation
between 16 and 60 min depending on the microwave
between the estimated and experimental amounts for the
power level. During drying, the rotating glass plate was
models. The higher correlation coefficient (R2), and
removed from the microwave oven at 5 and 1 min intervals
reduced RMSE, Chi square (v2), were used to determine
respectively at 90 and 180 W and weighed using a digital
the goodness of fit model in the hot air, microwave and
balance (Mettler Toledo, MS3002S) measuring to accuracy
vacuum drying curves of pestil samples. These parameters
of 0.01 g.
can be determined with the subsequent equations:
The vacuum drying experiments were carried out in a " #
vacuum dryer (Memmert VO400, Germany, 49 L volume) XN   1=21 2
at 60 and 70 C; 200 and 300 mbar. The temperature and RMSE ¼ MRexp;i MRpre;i ð3ÞN
vacuum levels were determined according to the literature i¼1
 
then 50 g of samples stated on the greaseproof paper were PN 2
2 i¼1 MRexp;i MRpre;i
used (Paul and Das, 2018). The moisture loss of samples v ¼ ð4Þ
N  n
during drying was recorded at 15 min intervals for 4 h.
Duration of drying process was recorded between 85 and where MRexp,i is the experimentally dimensionless mois-
230 min depending on the vacuum pressure. ture ratio for test i, MRest,i is the estimated dimensionless
Moisture analyzer (Sartorius MA150, Germany) was moisture ratio for test i, N is the number of observation and
used for the determination of moisture content of pestils. n is the number of constants in the model (Arslan and
Drying experiments were all continued until the moisture Ozcan, 2010).
content decreased to almost 0.08 g water/g dry base (initial
moisture 1.49 g water/g dry base) and during drying Physicochemical analyses
treatments all of the samples were weighed for a maximum
duration of 10 s. Color analysis
Mathematical modelling of drying curve Color of medlar pestils were determined by using a chroma
meter (Konica Minolta CR-5, Bench-top, Japan). L*, a*, b*
Five thin-layer mathematical drying models were utilized values were displayed as lightness, redness and yellowness.
to choose the best model for portraying the drying curve of Chroma and hue angle were calculated from these values
pestils. This models name and references were given (Mujumdar, 2000).
respectively as Page MR = exp(-ktn) (Sarsavadiva et al.,
1999), Modified Page MR = exp [(-kt)n] (Overhults Extraction of polyphenols
et al., 1973), Logarithmic MR = a exp(-kt) ? c (Arslan
and Ozcan, 2010), Lewis MR = exp(-kt) (Doymaz, 2006) Initial (undigested) extracts of medlar pestils were prepared
and Handerson and Pabis MR = a exp(-kt) (Westerman according to Vitali et al. (2009).
et al., 1973).
The moisture ratio (MR) and drying rate of pestils In vitro bioaccessibility
throughout drying were calculated using the consequent
equations: In vitro digestion of medlar pestils was performed
 according to Minekus et al. (2014). Samples were passed¼ M MeMR ð1Þ through a two-step gastrointestinal digestion stages as
Mi Me stomach and intestinal phases.
where MR is moisture ratio, M is the moisture content at a
specific time (g water/g dry base), Mi corresponds to initial Total phenolic content
moisture content (g water/g dry base), Me is the equilib-
rium moisture content (g water/g dry base). Total phenolic content was determined by Folin–Ciocalteu
 spectrophotometric method (Spanos and Wrolstad, 1990)Mtþdt Mt
Drying rate ¼ ð2Þ and the results were expressed as mg gallic acid equiva-
dt
lents per 100 g dry weight (R2 = 0.9835).
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1468 S. Suna
Antioxidant capacity 180 W had the shortest one with 15 min. Microwave dry-
ing spared the time by causing fast dissipation of water
Antioxidant capacity of the medlar pestils was measured (Arslan and Ozcan, 2010; Zheng et al., 2012). In hot air
with 2-diphenyl-1-picrylhydrazyl (DPPH) (Katalinic et al., drying, the time required to reduce the moisture was found
2006) Copper(II) reducing antioxidant capacity (CUPRAC) higher at 60 C with 175 min than 70 C with 115 min at a
(Apak et al., 2008) and Ferric Reducing Antioxidant Power constant drying air with 20% relative humidity. Several
(FRAP) (Benzie and Strain, 1996) methods. Trolox was authors reported that, increasing drying temperature speeds
used as the calibration of the standard curves respectively up the drying process, thus shortens the drying time such as
as R2 = 0.9929, R2 = 0.9987, R2 = 0.9993. The results in pomegranate (Tontul and Topuz, 2017) and rose hip
were given as lmol Trolox equivalent (TE) per g dry leathers (Ruiz et al., 2014). Drying durations at 90 W and
weight in all assays. 180 W were recorded as 60 min and 15 min respectively
(Fig. 1A). These results explained that drying duration at
Sensorial analysis 180 W was 75% less than the samples dried at 90 W.
Furthermore in vacuum drying, treatments at 60 C
Sensorial properties like color, appearance, taste, chewi- (125 min for 200 mbar and 230 min for 300 mbar) resulted
ness and overall acceptability were analyzed. A 9-point with longer durations than 70 C (85 min for 200 mbar and
hedonic scale varying from ‘‘like extremely (9)’’ to ‘‘dis- 115 min for 300 mbar). In the light of these data, incre-
like extremely (1)’’ was used for the evaluation of the ment of the microwave power and temperature prompted
samples (Lim, 2011). an impressive decrease in the drying durations. In vacuum
drying technique, drying time decreased as the vacuum
Statistical analysis increased (Fig. 2C). Furthermore, increase in both vacuum
and temperature allowed decrease in the drying time by
JMP software package version 8.0 (SAS Institute Inc. NC, accelerating moisture migration from the center to the
27513) was used for statistical evaluation. When significant outside. Identical behaviour was reported at vacuum drying
differences were observed (p\ 0.05), the least significant of potato slices (Song et al., 2009) and hot air drying of and
difference (LSD) test was used to determine the differences apples (Vega-Gálvez et al., 2012).
among means in triplicate. The calculated drying rates of pestils were depicted in
Fig. 2. Drying rates ranged from 0.01 [g water/g dry matter
(min)] for experiments at 60 C–200 mbar vacuum to 0.15
Results and discussion [g water/g dry matter (min)] for the experiments at a
microwave power level of 180 W. The drying rate of
Drying characteristics of medlar pestils pestils raised with increasing temperature and power for
both in hot air and microwave drying. Besides, drying rate
Moisture ratio of the pestil samples obtained by Page’s accelerated with the increasing vacuum pressure and tem-
equation as a function of drying time was shown in Fig. 1. perature. In microwave drying, a constant-rate period was
The moisture ratio decreased considerably with increasing observed and constant rate varied from 0.6 to 1.4 (g water/g
drying time as expected. Drying technique significantly dry matter) for a microwave power level of 90 W. For the
affected total drying duration in order to obtain the final power of 180 W, a decrement was seen after a very short
moisture content. Vacuum drying at 60 C–300 mbar had acceleration at the beginning. The rate of pestil drying with
the longest duration with 230 min while microwave of hot air and vacuum methods generally decreased, and the
Fig. 1 Moisture ratio of pestil samples versus drying time at microwave (A), hot air drying (B) and vacuum drying (C) conditions
123
Influence of drying methods on drying kinetics and bio accessibility of medlar pestils 1469
Fig. 2 Drying rate curves for pestils versus the moisture content at different microwave (A) hot air drying (B) and vacuum drying (C)
drying process took place in a falling rate period (Wang (Table 2). While vacuum-70 C–200 mbar treated pestils
et al., 2007) (Fig. 2B, C). Our results were in agreement had the nearest yellow color compared to non-dried sample
with the study of strawberry fruit leather (Lee and Hsieh, (30.49 ± 0.22), hot air-60 C dried pestils were deter-
2008). mined as the least yellow samples (16.11 ± 0.98). Micro-
wave dried samples showed lighter and yellower color
Modelling of drying curves when compared to hot air, as a result of short drying period.
Besides, in microwave drying, samples showed higher
The statistical results of dried pestils obtained from dif- lightness and yellowness value with higher power, because
ferent models including drying model coefficients, R2, of the lesser exposure to maillard and nonenzymatic
RMSE and v2, were illustrated in Table 1. The R2, RMSE browning reactions amid drying. The red and yellow color
and v2 values varied from 0.8820 to 0.9995, 0.001676 to of pestils is ascribed to the nearness of carotenes. In
0.111869 and 0.000048 to 0.155179, respectively. The addition to maillard reaction, the expansion of a value may
suitable drying methods with the highest value of R2 and be because of the decomposition of pigments (Maskan,
the lowest values of RMSE and v2 were determined from 2001).
Page and Modified page models in all cases. Chroma value, which was used to perceive the color
intensity, was significantly (p\ 0.05) the highest in non-
Color properties dried sample. Vacuum 70 C–200 mbar treatment showed
the second highest result (35.82 ± 0.45) while the lowest
General color of pestils was depicted as yellowish orange value (24.48 ± 1.19) was determined from hot air-60 C.
and L* was found statistically significant (p\ 0.05) In compliance with chroma, non-dried mixture resulted
(Table 2). Non-dried mixture showed the highest L* and with the highest amount, and the second highest and the
stated in the same group with 200 mbar vacuum treatments lowest values of hue angle were calculated from the same
at 70 C and 60 C. When compared to non-dried mixture, treatments (Table 2). Consequently, vacuum drying had the
L* value was significantly affected by different drying best color properties. This was associated with vacuum
treatments (p\ 0.05) and resulted with a 1.51–30.83 77% conditions, in which the damp material is dried under sub-
decrease. Additionally, L* of pestils were raised with atmospheric pressures, counteracted color damages and
increasing microwave power resulting with a lighter color. permitted to be in higher quality than conventional air
At the point when high powers were utilized, drying was process at atmospheric pressure (Krokida and Maroulis,
performed in a shorter time so color was more substantially 1999).
saved (Pushpa et al., 2006). Furthermore, pestils dried with
hot air presented darker color as a result of the exposure to Effects of different drying methods on total
high temperature all along the time of drying. polyphenolic content and antioxidant capacity
The highest a* values were found to be significantly the in the in vitro gastrointestinal tract
same in vacuum drying (300 mbar) at 70 and 60 C with
20.71 ± 0.17 and 20.38 ± 0.36 respectively. Compared to The initial total phenolic content of the mixture before
the fresh sample, a* increased with vacuum 300 mbar drying was determined as being the highest with
treatments at 60 and 70 C whereas this value reduced in 396.62 mg GAE/100 g dw (p\ 0.05). Phenolic content
hot air (60 C) and vacuum 60 C–200 mbar treatments. after drying was affected by the method (p\ 0.05) with a
b* value was significantly the highest in non-dried mixture decrease ranging from 50.59% (hot air-60 C) to 60.65%
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1470 S. Suna
Table 1 Statistical results obtained from the modeling of medlar pestils
Model name Applications Model coefficients R2 RMSE v2
Page Hot air 60 C n = 1.2299, k = 0.0054 0.9918 0.002510 0.000968
Hot air 70 C n = 1.1233, k = 0.0132 0.9970 0.003635 0.000152
Vacuum 60 C–200 mbar n = 1.2667, k = 0.0058 0.9968 0.003588 0.000173
Vacuum 70 C–200 mbar n = 1.2124, k = 0.0117 0.9887 0.007801 0.000596
Vacuum 60 C–300 mbar n = 0.793, k = 0.0215 0.9953 0.035260 0.026401
Vacuum 70 C–300 mbar n = 1.1606, k = 0.0120 0.9983 0.002785 0.000089
Microwave-90 W n = 1.5652, k = 0.0056 0.9967 0.004170 0.000267
Microwave-180 W n = 1.7027, k = 0.031 0.9995 0.001775 0.000054
Modified page Hot air 60 C n = 1.2299, k = 0.0143 0.9981 0.002510 0.000096
Hot air 70 C n = 1.1233, k = 0.0212 0.9970 0.003635 0.000152
Vacuum 60 C–200 mbar n = 1.2667, k = 0.0171 0.9968 0.003588 0.000173
Vacuum 70 C–200 mbar n = 1.2124, k = 0.0256 0.9887 0.007801 0.000596
Vacuum 60 C–300 mbar n = 0.7930, k = 0.0079 0.9952 0.035260 0.026401
Vacuum 70 C–300 mbar n = 1.1606, k = 0.0638 0.9983 0.081725 0.077285
Microwave-90 W n = 1.5652. k = 0.0364 0.9967 0.004170 0.000267
Microwave-180 W n = 1.6930, k = 0.1296 0.9994 0.001676 0.000048
Logarithmic Hot air 60 C k = 0.0265, a = 1.4861 0.9541 0.045217 0.034554
Hot air 70 C k = 0.0347, a = 1.2778 0.9581 0.040614 0.022268
Vacuum 60 C–200 mbar k = 0.0321, a = 1.5084 0.8927 0.055795 0.047086
Vacuum 70 C–200 mbar k = 0.0394, a = 1.213 0.9365 0.042717 0.022354
Vacuum 60 C–300 mbar k = 0.0220, a = 1.3341 0.9662 0.024239 0.013257
Vacuum 70 C–300 mbar k = 0.0355, a = 1.3150 0.9536 0.042756 0.024679
Microwave-90 W k = 0.0803, a = 1.7878 0.9303 0.071605 0.086652
Microwave-180 W k = 0.3085, a = 2.2436 0.8820 0.090909 0.155179
Lewis Hot air 60 C k = 0.0165 0.9909 0.012723 0.002279
Hot air 70 C k = 0.0233 0.9922 0.111869 0.001267
Vacuum 60 C–200 mbar k = 0.0202 0.9643 0.017957 0.003901
Vacuum 70 C–200 mbar k = 0.0297 0.9619 0.021268 0.003694
Vacuum 60 C–300 mbar k = 0.0072 0.9858 0.041929 0.035259
Vacuum 70 C–300 mbar k = 0.0251 0.9875 0.013446 0.001830
Microwave-90 W k = 0.0480 0.9528 0.030132 0.012787
Microwave-180 W k = 0.1730 0.9259 0.028986 0.013503
Henderson and Pabis Hot air 60 C k = 0.0176, a = 1.1388 0.9958 0.011970 0.002201
Hot air 70 C k = 0.0245, a = 1.1100 0.9957 0.013836 0.002215
Vacuum 60 C–200 mbar k = 0.0220, a = 1.2462 0.9784 0.024603 0.008138
Vacuum 70 C–200 mbar k = 0.0327, a = 1.2045 0.9732 0.032325 0.010240
Vacuum 60 C–300 mbar k = 0.0059, a = 1.2278 0.9893 0.081752 0.141925
Vacuum 70 C–300 mbar k = 0.0268, a = 1.1539 0.9932 0.018471 0.003948
Microwave-90 W k = 0.0555, a = 1.3710 0.9781 0.032637 0.016365
Microwave-180 W k = 0.2127, a = 1.5595 0.9691 0.039872 0.027515
RMSE root mean square error, R2 correlation coefficient
(microwave-90 W) (Table 3). The lowering effect of dry- to the results of total antioxidant capacity, a general
ing on phenolics can be ascribed to the degradation and decrement both in CUPRAC (0.31–57.34%), DPPH
oxidation during heat treatment (Qu et al., 2010). Similar (53.39–57.32%) and FRAP (54.37–56.83%) assays was
decrement was reported in blueberry and rose hip leathers determined in undigested extracts. Microwave-180 W
(Chen and Martynenko, 2018; Ruiz et al., 2014). According (125.77 ± 6.00) was found significantly (p\ 0.05) higher
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Influence of drying methods on drying kinetics and bio accessibility of medlar pestils 1471
Table 2 Color values of medlar pestil samples
Drying processes L* a* b* Chroma Hue angle
Non-dried mixture 49.52 ± 0.00a 18.52 ± 0.01de 34.14 ± 0.01a 38.84 ± 0.02a 61.52 ± 0.02a
Hot air-60 C 34.25 ± 1.27c 18.42 ± 0.73e 16.11 ± 0.98f 24.48 ± 1.19f 41.15 ± 0.61e
Hot air-70 C 35.36 ± 0.25c 19.34 ± 0.25cd 18.92 ± 0.27f 27.06 ± 0.36e 44.37 ± 0.20d
Microwave-90 W 35.51 ± 0.96c 18.94 ± 0.60cde 19.01 ± 0.51f 26.84 ± 0.47e 45.12 ± 1.35d
Microwave-180 W 42.03 ± 0.36b 19.61 ± 0.72bc 24.54 ± 0.78g 31.42 ± 1.06d 51.38 ± 0.15c
Vacuum 60 C–200 mbar 48.28 ± 1.28a 18.27 ± 0.47e 28.87 ± 0.19c 34.12 ± 0.29c 57.76 ± 0.51b
Vacuum 70 C–200 mbar 48.77 ± 0.63a 18.80 ± 0.49cde 30.49 ± 0.22b 35.82 ± 0.45b 58.34 ± 0.48b
Vacuum 60 C–300 mbar 42.75 ± 0.44b 20.38 ± 0.36ab 26.07 ± 0.28d 33.10 ± 0.19c 51.98 ± 0.72c
Vacuum 70 C–300 mbar 41.93 ± 0.05b 20.71 ± 0.17a 26.18 ± 0.16d 33.38 ± 0.21c 51.65 ± 0.19c
a–gDifferent letters in the same column display significant difference (p\ 0.05)
than other samples in CUPRAC assay with the nearest (65.15–85.24%) and DPPH (10.19–18.71%) compared to
result to non-dried mixture. DPPH analysis of the pestils undigested extracts. In agreement with current data, greater
were found statistically significant (p\ 0.05). Microwave- total phenolic content from intestinal phase than stomach
90 W (6.87 ± 0.01), vacuum 70 C 300 mbar phase and initial extracts in tomato sauce using the same
(6.81 ± 0.07) and vacuum 60 C 300 mbar (6.78 ± 0.01) in vitro digestion method was reported (Tomas et al.,
revealed the highest results as well as showing no signifi- 2018). Moreover, Oliveira and Pintado (2015) determined
cant differences (p[ 0.05). In FRAP analysis, microwave- an increase in antioxidant capacity of strawberry and peach
90 W (23.19 ± 0.54) was significantly (p\ 0.05) higher yoghurt in the intestinal phase compared to non-digested
than those of the other samples with the lowest decrement extracts, which is consistent with current DPPH analysis.
(54.37%) compared to non-dried mixture. This study supported that the bioaccessible phenolics
Ruiz et al. (2014) reported trolox equivalent antioxidant increased during intestinal digestion. This situation was
capacity (TEAC) of hot-air dried rose hip leathers in explained previously that, by the entrance in the colon
average as 24 and 17 lmol TE/g dm (dry matter) respec- phenolics could be catabolized by human gut microbiata
tively in 60 C and 70 C treatments. Additionally TEAC into low molecular compounds allowing them to be
of rose hip leathers were presented as 33 and 21 lmol TE/g absorbed throughly (González-Sarrı́as et al., 2017). In
dry matter in vacuum drying at 60 C and 70 C. These general, current data showed that applied drying methods
results were in accordance with CUPRAC analysis being and treatments (microwave: 90 W and 180 W, hot air: 60
higher in hot air drying at 60 C than 70 C (Table 3) and and 70 C, vacuum: 60 and 70 C with 200 and 300 mbar)
the mean interval of current data. Chen and Martynenko may have an influence on the release of total phenols and
(2018) reported DPPH results of dried blueberry leather antioxidant capacity, therefore, on the bioaccessible frac-
between 59.19 and 71.92 lmol TE/g dm in freeze and tion. Moreover, both undigested (initial) and in vitro
forced-air dried samples. digested pestil samples measured by CUPRAC and FRAP
The effect of in vitro digestion on total phenolic content methods were found to be higher than DPPH assay. Results
and total antioxidant capacity of the pestils was presented of CUPRAC could be attributed to the properties of the
in Table 3. During stomach phase, changes in phenolics of reagent being selective and stable as well as the Cu(I) ion
all the samples were statistically significant (p\ 0.05). emerging as a product of the CUPRAC cannot then act as a
Generally for all the treatments except for microwave prooxidant (Apak et al., 2008). Besides, lower results of
(90 W and 180 W) and vacuum 200 mbar (60 C and DPPH can be explained with the method’s characteristics,
70 C), results were decreased in stomach phase compared which targets sterically hindered radicals than biologically
to initial values. Additionally a declining trend was active short-lived ones (Schaich et al., 2015).
observed after simulated gastric digestion in CUPRAC,
DPPH and FRAP (p\ 0.05) in accordance with Tomas Sensorial properties
et al. (2018) both in these three assays. Remarkably, total
phenolic content from intestinal phase was higher than Pestils dried at 70 C with a vacuum of 200 mbar had the
stomach phase and initial extracts in all of the samples. highest color scores (Table 4). 60 C–200 mbar resulted
Intestinal phase of all the samples resulted with an incre- with the highest values of appearance and taste criterias.
ment in total phenolic content (49.42–97.15%) FRAP While the chewiness of pestils were found the highest in
123
1472 S. Suna
Table 3 Changes in the total
Analysis Initial Stomach phase Intestinal phase
phenolic content and total
antioxidant capacity of pestil TPC
samples during simulated a a a
in vitro digestion Non-dried mixture 396.62 ± 3.84 366.83 ± 4.53 672.79 ± 21.03
Hot air-60 C 195.94 ± 1.07b 168.38 ± 4.56d 292.78 ± 7.50d
Hot air-70 C 186.73 ± 7.61bc 172.81 ± 6.16cd 318.35 ± 7.98bc
Microwave-90 W 156.06 ± 9.31g 167.26 ± 6.15d 307.68 ± 5.25bcd
Microwave-180 W 159.25 ± 2.07fg 174.96 ± 4.78cd 309.84 ± 4.43bc
Vacuum 60 C–200 mbar 168.11 ± 2.93ef 180.30 ± 6.64bc 293.27 ± 7.49d
Vacuum 70 C–200 mbar 181.37 ± 9.42cf 188.23 ± 6.48b 304.46 ± 7.26cd
Vacuum 60 C–300 mbar 187.74 ± 7.39bc 166.19 ± 8.30d 305.15 ± 7.69cd
Vacuum 70 C–300 mbar 173.86 ± 1.27de 170.92 ± 5.68cd 323.52 ± 3.00b
CUPRAC
Non-dried mixture 126.16 ± 24.11a 46.55 ± 9.99f 57.73 ± 8.34c
Hot air-60 C 96.10 ± 9.72b 93.41 ± 5.24ab 64.85 ± 7.18c
Hot air-70 C 70.22 ± 8.58b 89.42±5.83bc 95.55 ± 4.84a
Microwave-90 W 69.14 ± 8.13c 48.80 ± 5.65ef 35.50 ± 3.69d
Microwave-180 W 125.77 ± 6.00a 101.93 ± 6.20a 82.28 ± 9.60b
Vacuum 60 C–200 mbar 98.19 ± 7.40b 62.77 ± 6.42d 84.19 ± 8.38ab
Vacuum 70 C–200 mbar 98.40 ± 8.30b 80.96 ± 6.87c 69.28 ± 9.98c
Vacuum 60 C–300 mbar 55.49 ± 9.19c 37.48 ± 7.55f 25.80 ± 6.68d
Vacuum 70 C–300 mbar 53.81 ± 5.94c 60.59 ± 6.36de 33.23 ± 3.75d
DPPH
Non-dried mixture 14.74 ± 0.03a 12.95 ± 0.11a 15.47 ± 0.23a
Hot air-60 C 6.51 ± 0.12d 5.82 ± 0.10c 7.43 ± 0.12bcde
Hot air-70 C 6.67 ± 0.07c 5.75 ± 0.06c 7.35 ± 0.15cde
Microwave-90 W 6.87 ± 0.01b 5.88 ± 0.11bc 7.76 ± 0.43b
Microwave-180 W 6.29 ± 0.01e 5.86 ± 0.13bc 7.27 ± 0.18de
Vacuum 60 C–200 mbar 6.30 ± 0.06e 5.55 ± 0.02d 7.18 ± 0.09e
Vacuum 70 C–200 mbar 6.47 ± 0.01d 6.04 ± 0.09b 7.68 ± 0.15bc
Vacuum 60 C–300 mbar 6.78 ± 0.01b 5.91 ± 0.20bc 7.56 ± 0.18bcd
Vacuum 70 C–300 mbar 6.81 ± 0.07b 5.85 ± 0.09bc 7.51 ± 0.09bcde
FRAP
Non-dried mixture 50.83 ± 0.40a 46.00 ± 0.65a 80.41 ± 0.85a
Hot air-60 C 22.93 ± 0.22bc 20.25 ± 0.04b 39.39 ± 0.52cde
Hot air-70 C 22.79 ± 0.50bcd 20.18 ± 0.57b 39.94 ± 1.13bcd
Microwave-90 W 23.19 ± 0.54b 20.44 ± 0.26b 38.30 ± 0.18e
Microwave-180 W 22.29 ± 0.46cd 20.92 ± 0.28b 41.29 ± 0.92b
Vacuum 60 C–200 mbar 21.94 ± 0.62d 20.14 ± 0.40b 39.23 ± 1.49de
Vacuum 70 C–200 mbar 22.31 ± 0.28bcd 20.92 ± 0.48b 40.71 ± 0.72bc
Vacuum 60 C–300 mbar 22.92 ± 0.85bc 19.95 ± 0.47b 39.76 ± 0.35cd
Vacuum 70 C–300 mbar 22.94 ± 0.45bc 20.30 ± 0.29b 39.41 ± 0.19cde
TPC (total phenolic content) and total antioxidant capacity (CUPRAC, DPPH, FRAP) is expressed as mg of
GAE (gallic acid equivalent) per 100 g dw and lmol of TE (Trolox equivalent) per g dw respectively
Different letters in the same column display significant difference (p\ 0.05)
hot air-60 C, the second highest scores were obtained had the lowest scores of all the criterias and the increased
from vacuum (200 mbar) treatments at 60 C and 70 C. microwave power resulted with lower sensorial scores
Similarly, overall acceptances of pestils were found as the (Pushpa et al., 2006). The results of sensorial analysis
best in vacuum (200 mbar) treatments at 60 C and 70 C. showed that, although microwave-180 W had the lowest
Additionally, drying at microwave with a power of 180 W
123
Influence of drying methods on drying kinetics and bio accessibility of medlar pestils 1473
Table 4 Sensorial properties of medlar pestils
Drying processes Color Appearance Taste Chewiness Overall acceptability
Hot air-60 C 6.50 ± 1.76b 7.50 ± 1.22a 7.83 ± 1.47a 8.33 ± 0.51a 7.66 ± 1.21a
Hot air-70 C 7.66 ± 0.51ab 7.33 ± 0.81a 7.00 ± 1.54ab 7.66 ± 0.51abc 7.50 ± 0.83a
Microwave-90 W 4.00 ± 2.00c 4.16 ± 1.83b 6.16 ± 0.98bc 7.00 ± 0.63cd 5.66 ± 0.81b
Microwave-180 W 3.66 ± 1.86c 3.33 ± 2.16b 4.66 ± 1.50c 6.50 ± 0.83d 5.16 ± 0.75b
Vacuum 60 C–200 mbar 8.00 ± 1.54ab 8.66 ± 0.81a 8.16 ± 0.98a 8.16 ± 1.16ab 8.16 ± 0.98a
Vacuum 70 C–200 mbar 8.50 ± 1.22a 8.00 ± 1.54a 8.00 ± 1.26a 8.16 ± 1.16ab 8.16 ± 1.32a
Vacuum 60 C–300 mbar 7.00 ± 0.89ab 7.33 ± 0.81a 7.00 ± 1.26ab 7.33 ± 0.81bcd 7.50 ± 0.54a
Vacuum 70 C–300 mbar 6.66 ± 1.03b 7.33 ± 0.81a 7.50 ± 1.51ab 7.66 ± 0.81abc 7.66 ± 1.03a
a–dDifferent letters in the same column display significant difference (p\ 0.05)
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