The influences of hot air drying (AD), medium- and short-wave infrared drying (IR), instant controlled pressure drop drying (DIC), and vacuum freeze drying (FD) on cell wall polysaccharide modification were studied, and the relationship between the modifications and texture properties was analyzed. The results showed that the DIC treated apple chips exhibited the highest crispness (92) and excellent honeycomb-like structure among all the dried samples, whereas the FD dried apple chips had low crispness (10), the minimum hardness (17.4 N), and the highest volume ratio (0.76) and rehydration ratio (7.55). Remarkable decreases in the contents of total galacturonic acid and the amounts of water extractable pectin (WEP) were found in all the dried apple chips as compared with the fresh materials. The highest retention of WEP fraction (102.7 mg/g AIR) was observed in the FD dried apple chips, which may lead to a low structural rigidity and may be partially responsible for the lower hardness of the FD apple chips. In addition, the crispness of the apple chips obtained by DIC treatment, as well as AD and IR at 90°C, was higher than that of the samples obtained from the other drying processes, which might be due to the severe degradation of pectic polysaccharides, considering the results of the amounts of pectic fractions, the molar mass distribution, and concentrations of the WEP fractions. Overall, the data suggested that the modifications of pectic polysaccharides of apple chips, including the amount of the pectic fractions and their structural characteristics and the extent of degradation, significantly affect the texture of apple chips.
Apple, one of the most cultivated and consumed fruits in China, is a significant part of the human diet. It has been identified as one of the main dietary sources of food antioxidants, mainly due to the phenolic compounds such as flavonoids and phenolic acids. These functional substances may contribute to the nutritional effects; for example, they could reduce risk of cancer, heart disease, and asthma [
Since drying has been used as an effective method to process apples, a large number of studies have been focused on the qualities of dehydrated apple, such as color, flavor, taste, and texture, as well as nutrition and functionality. Among these quality aspects, texture is one of the vital organoleptic properties which is closely related to consumer acceptability. Complex physicochemical and biological reactions occur during drying process which could greatly affect the microstructure and texture of material tissues. To be specific, the volume of a material would decline continuously due to the loss of osmotic pressure caused by the evaporation of inner moisture. Additionally, the shrinkage of tissues is also related to the loss of vacuolar pressure and further damage to the integrity of cell wall, thus resulting in textural changes that could play an important role in the quality of dried fruit and vegetable products [
Texture is the result of complex interaction among food components relating to molecular, supramolecular, and the microstructural levels [
Recently, the structural changes on cell wall polysaccharides during drying have also been reported for several fruits. The rehydration property of air-dried broccoli substantially affected by the amount and structure of cell wall pectin polysaccharides was reported [
The objective of this study was to investigate the effects of four different drying methods on the characteristics of cell wall polysaccharides of apple chips and to study the relationship between cell wall polysaccharides and texture of apple chips.
Apples (
Hot air drying (AD) was carried out using a convective dryer (DHG-9203, Yiheng Technical Co. Ltd., Shanghai, China). The dryer was loaded with 750 g (6.79 kg/m2) of apple slices that were spread on a tray in single layer. The samples were flipped over during drying to avoid sticking to the tray and allow equal dehydration from all sides. Fresh apple slices were dried at 60°C, 75°C, and 90°C, respectively.
Infrared drying (IR) was operated using a laboratory medium- and short-wave infrared dryer (STC-5, Senttech Infrared Technology Co. Ltd., Jiangsu, China). The dryer consists of three infrared lights with powers of 0.48, 0.60, and 0.90 kW and wavelengths of 3.15, 3.10, and 1.40
Freeze drying (FD) was conducted using an experimental freezing dryer (Alphal-4L plus, Christ Col, Osterode am Harz, German) with a drying area of 0.42 m2. Before freeze drying, the apple samples were prefrozen at −80°C for 12 h and then freeze-dried for 15 h [
Instant controlled pressure drop (French: Détente Instantannée Contrôlée, DIC), also known as explosion puffing drying (EPD), was developed since 1988 [
Moisture content was determined by drying the samples at 105°C until reaching constant weight [
The hardness and crispness of apple chips were measured by a TA-XT2i/50 Texture Analyzer (Stable Micro Systems Ltd., Surry, UK). A cylinder penetrometer probe (5 mm diameter) was used and the test parameters were set as follows: 2 mm/s of the prespeed and postspeed, 1 mm/s of the test speed, and 100 g trigger. In the test, hardness is the maximum force required to break the sample [
The VR was measured using quartz sand displacement method [
The RR of dried products is one of important indications for the occurrence of physical and chemical changes during drying process due to drying conditions, pretreatment, and sample composition [
Microstructure characterization was performed using a scanning electron microscope (SEM S-570, Hitachi Ltd., Tokyo, Japan) at 150 kV accelerated voltage and 10–15 mm working distance. The microstructure of the samples was magnified 50 times.
The cell wall polysaccharides of dried apple slices, namely, alcohol insoluble residue (AIR), were prepared following the procedure described by Gwanpua et al. [
The AIRs and the corresponding fractions obtained thereof (WEP, CEP, and NEP) were first hydrolyzed using concentrated sulfuric acid (95–98%) according to the method described by Ahmed and Labavitch [
The DM of the fractions of the AIR fractions were calculated as the ratio of the molar amount of methoxy groups to the molar amount of GalA content and expressed as a percentage. Before the measurement of the concentration of methanol, 20 mg of each dried AIR faction was weighed and was first hydrolyzed according to the description of Ng and Waldron [
Analyses of neutral sugars including fucose (Fuc), rhamnose (Rha), arabinose (Ara), galactose (Gal), and xylose (Xyl) were performed using the method described by Njoroge et al. [
Molar mass analysis was performed according to the method described by Yang et al. [
Statistical analysis of the experimental data was conducted by using SPSS Statistics (Version 17.0, SPSS Inc., Chicago, USA), applying one-way analysis of variance (ANOVA) and Duncan’s multiple range tests. Significant differences were defined at
The effects of drying methods and drying temperature on hardness, crispness, water content, volume ratio (VR), and rehydration ratio (RR) of dried apple chips are presented in Table
Moisture content, hardness, crispness, volume ratio, and rehydration ratio of the dried apple chips obtained by different drying methods.
Drying method | Drying condition | | Hardness (N) | Crispness | Volume ratio | Rehydration ratio |
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AD | 60°C | 8.30 ± 0.01 | 86.9 ± 8.1 | 4 ± 1 | 0.19 ± 0.00 | 4.56 ± 0.18 |
75°C | 6.50 ± 0.01 | 43.7 ± 4.7 | 38 ± 3 | 0.21 ± 0.01 | 4.95 ± 0.11 | |
90°C | 5.76 ± 0.50 | 43.4 ± 5.6 | 74 ± 10 | 0.21 ± 0.03 | 4.99 ± 0.10 | |
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IR | 60°C | 7.03 ± 0.11 | 57.7 ± 3.6 | 5 ± 1 | 0.19 ± 0.00 | 4.82 ± 0.10 |
75°C | 5.95 ± 0.13 | 50.3 ± 2.5 | 69 ± 7 | 0.21 ± 0.03 | 5.08 ± 0.13 | |
90°C | 5.72 ± 0.21 | 53.5 ± 2.0 | 74 ± 6 | 0.25 ± 0.01 | 5.04 ± 0.18 | |
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FD | 5.29 ± 0.23 | 17.4 ± 1.8 | 10 ± 3 | 0.76 ± 0.01 | 7.55 ± 0.09 | |
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DIC | 5.03 ± 0.51 | 44.4 ± 2.3 | 92 ± 4 | 0.31 ± 0.01 | 5.48 ± 0.35 |
Results are presented as mean values ± standard deviation of triplicate tests. Samples in the same column with different letters differ significantly at
The rehydration ratio (RR) of the FD dried apple chips was 7.55 (Table
The microstructure of dried apples produced by different drying methods was observed by scanning electron microscope to further analyze the texture characteristic of the dried apples chips (Figure
Microstructures of the apple chips dried by hot air drying (AD), medium- and short-wave infrared drying (IR), instant controlled pressure drop drying (DIC), and freeze drying (FD) (×50). (a) AD 60°C, (b) AD 75°C, (c) AD 90°C, (d) IR 60°C, (e) IR 75°C, (f) IR 90°C, (g) FD, and (h) DIC. The white circles added in (a), (b), and (c) were used to point the microstructure changes of apple chips, as well as in (d), (e), and (f).
As GalA is dominant pectic saccharide in the AIR of apple; the amount of GalA found in AIR was estimated to evaluate the amount of pectin in each fraction of AIR. Table
Effect of drying methods on the galacturonan acid contents of water extractable pectin (WEP), CDTA extractable pectin (CEP), and Na2CO3 extractable soluble pectin (NEP) of the apple chips obtained by different drying methods.
Drying method | Drying condition | Galacturonan acid (mg/g AIR) | |||
---|---|---|---|---|---|
WEP | CEP | NEP | Total GalA | ||
AD | 60°C | 86.9 ± 2.8 | 41.5 ± 0.1 | 75.0 ± 6.0 | 203.4 |
75°C | 74.6 ± 1.8 | 40.3 ± 1.7 | 65.1 ± 5.2 | 180.0 | |
90°C | 33.8 ± 5.8 | 47.3 ± 4.2 | 45.8 ± 5.2 | 126.9 | |
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IR | 60°C | 85.0 ± 0.9 | 37.9 ± 1.2 | 53.4 ± 2.0 | 176.3 |
75°C | 75.3 ± 3.2 | 36.0 ± 0.1 | 61.0 ± 1.6 | 172.3 | |
90°C | 65.4 ± 2.1 | 71.0 ± 4.2 | 61.5 ± 3.0 | 197.9 | |
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FD | 102.7 ± 1.8 | 38.8 ± 1.1 | 33.6 ± 7.3 | 174.4 | |
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DIC | 64.2 ± 0.8 | 56.1 ± 2.9 | 51.8 ± 1.9 | 171.9 | |
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Fresh | 112.7 ± 0.9 | 32.0 ± 2.8 | 73.4 ± 1.4 | 218.1 |
Results are presented as mean values ± standard deviation of triplicate tests. Samples in the same column with different letters differ significantly at
The amount of WEP for AD and IR dried samples decreased with increasing of drying temperature, while the CEP contents increased. This might be explained by the fact that leaching of WEP occurred with certain amount of fluid flowing out of the tissue during the drying process or partially converted into CEP fraction. The amount of WEP of the apple chips dried by AD at 90°C, IR at 90°C, and DIC significantly decreased compared with the fresh samples. This was consistent with the result that degradation of WSP and leaching occurred in the drying process with high temperature [
The DM of pectin, a key functional parameter, was estimated as the ratio of the molar amount of methanol groups to the molar amount of GalA. The DM affects the hydrogen bonding between pectin molecular interactions and might also influence the texture of dried fruits and vegetable. The DM of the WEP, CEP, and NEP fractions are represented in Table
Effect of drying methods on the degree of methoxylation of water extractable pectin (WEP), CDTA extractable pectin (CEP), and Na2CO3 extractable pectin (NEP) from dehydrated apple chips obtained by different drying methods.
Drying method | Drying condition | Degree of methoxylation (%) | ||
---|---|---|---|---|
WEP | CEP | NEP | ||
AD | 60°C | 59.7 ± 3.2 | 37.6 ± 3.6 | 1.3 ± 0.0 |
75°C | 60.5 ± 0.7 | 34.6 ± 0.1 | 1.2 ± 0.3 | |
90°C | 71.8 ± 0.7 | 24.86 ± 1.2 | 0.7 ± 0.0 | |
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IR | 60°C | 66.2 ± 1.8 | 43.4 ± 0.9 | 2.0 ± 0.1 |
75°C | 72.4 ± 1.8 | 40.2 ± 1.2 | 0.9 ± 0.1 | |
90°C | 73.2 ± 6.2 | 17.5 ± 5.1 | 0.1 ± 0.0 | |
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FD | 80.8 ± 0.7 | 33.8 ± 0.1 | 5.0 ± 0.8 | |
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DIC | 77.5 ± 2.0 | 23.3 ± 0.4 | 2.3 ± 0.2 | |
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Fresh | 69.8 ± 0.6 | 36.4 ± 5.8 | 1.7 ± 0.3 |
Results are presented as mean values ± standard deviation of triplicate tests. Samples in the same column with different letters differ significantly at
Pectin is a kind of cell wall polysaccharide that mainly consists of a linear chain of covalently linked galacturonic acid. Various amounts of neutral sugars are attached to these regions as side chains, including fucose, rhamnose, arabinose, galactose, and xylose. Based on the amount and linkage types of side chains, pectin is generally described as homogalacturonan (HG), rhamnogalacturonan-I (RG-I), and rhamnogalacturonan-II (RG-II) [
Effect of drying methods on sugar ratios of water extractable pectin (WEP), CDTA extractable pectin (CEP), and Na2CO3 extractable pectin (NEP) from dehydrated apple chips obtained by different drying methods.
Sugar ratio | AD | IR | FD | DIC | Fresh | |||||
---|---|---|---|---|---|---|---|---|---|---|
60°C | 75°C | 90°C | 60°C | 75°C | 90°C | |||||
WEP | 1 | 11.83 | 10.35 | 6.05 | 12.35 | 9.24 | 7.02 | 7.19 | 5.64 | 13.62 |
2 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.01 | 0.02 | 0.02 | 0.01 | |
3 | 10.31 | 8.50 | 9.78 | 9.05 | 8.79 | 9.80 | 6.87 | 6.92 | 7.18 | |
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CEP | 1 | 37.05 | 35.02 | 44.27 | 30.76 | 31.27 | 49.42 | 27.03 | 33.55 | 26.63 |
2 | 0.00 | 0.01 | 0.00 | 0.00 | 0.01 | 0.00 | 0.01 | 0.00 | 0.01 | |
3 | 5.61 | 4.42 | 5.90 | 5.62 | 4.28 | 4.31 | 4.21 | 3.98 | 5.12 | |
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NEP | 1 | 13.54 | 8.67 | 6.04 | 10.78 | 9.86 | 6.76 | 3.79 | 6.56 | 5.65 |
2 | 0.01 | 0.02 | 0.02 | 0.01 | 0.01 | 0.02 | 0.04 | 0.03 | 0.03 | |
3 | 3.84 | 3.88 | 4.90 | 3.35 | 4.87 | 4.97 | 3.74 | 3.21 | 4.65 |
Figure
Molar mass distribution of the water extractable pectins of the apple chips dried by hot air drying (AD), medium- and short-wave infrared drying (IR), instant controlled pressure drop drying (DIC), and vacuum freeze drying (FD). Solid lines indicate molecular weight of the WEP fraction, and dash-dotted lines indicate the concentration of the WEP fraction.
The modifications in cell wall polysaccharides could affect the physicochemical and physical properties of the apple chips. Firstly, the occurrence of polysaccharide depolymerization and the modification of cell wall polysaccharide intermolecular interactions (Table
Apple chips were produced by AD, IR, FD, and DIC, respectively. The influences of the modification in the extractability, composition, and structure of cell wall polysaccharide induced by various drying processes on the volume expansion, microstructure, rehydration behavior, and so on suggested that cell wall polysaccharides modification played a significant role in the texture properties of the apple chips. The amounts and structural properties of the WEP and CEP fractions obviously related to the texture properties of the dried samples. Based on the data, the apple chips exhibited higher crispness and better microstructure when there was less amount of WEP fraction, which might be partially attributed to depolymerization and leaching of the pectic polysaccharides. Cell wall polysaccharide degradation was in favor of volume expansion during instant pressure drop treatment, as well as AD and IR drying at elevated temperatures, consequently, contributing to a superior porous structure and crispier texture. Since cell wall polysaccharides are a nonnegligible factor affecting the formation and hardening of the porous structure for dried products, the influences of cell wall polysaccharide modification during different stages of drying process, for example, pretreatment, predrying, DIC, and final drying, on texture evolution can be better understood by further study using model system.
The authors declare that they have no conflicts of interest.
The authors acknowledge the National Key R&D Program of China (2016YFD0400700 and 2016YFD0400704).