Xiao Hong-Wei, Lin Hai, Yao Xue-Dong, Du Zhi-Long, Lou Zheng, Gao Zhen-Jiang. Effects of different pretreatments on drying kinetics and quality of sweet potato bars undergoing air

International Journal of FoodEngineering

Volume 5, Issue 5 2009 Article 5

Effects of Different Pretreatments on DryingKinetics and Quality of Sweet Potato Bars

Undergoing Air Impingement Drying

Hong-Wei Xiao∗ Hai Lin† Xue-Dong Yao‡

Zhi-Long Du∗∗ Zheng Lou†† Zhen-jiang Gao‡‡

∗China Agricultural University, [email protected]†Shihezi University, [email protected]‡Shihezi University, [email protected]

∗∗Chinese Academy of Agricultural Mechanization Sciences, [email protected]††China Agricultural University, [email protected]‡‡China Agricultural University, [email protected]

Copyright c©2009 The Berkeley Electronic Press. All rights reserved.

Effects of Different Pretreatments on DryingKinetics and Quality of Sweet Potato Bars

Undergoing Air Impingement Drying∗

Hong-Wei Xiao, Hai Lin, Xue-Dong Yao, Zhi-Long Du, Zheng Lou, andZhen-jiang Gao

Abstract

The effects of hot water blanching (HWB), superheated steam blanching (SSB) and citricacid pretreatment (CAP) on drying kinetics and quality of sweet potato bars undergoing air im-pingement drying were examined in this investigation. It was found that CAP could significantlyimprove the drying rate of the samples, whereas HWB and SSB could obviously decrease thedrying rate. In terms of quality, hardness, microstructure and color of the dried sweet potato barssubjected to different pretreatments were studied. Results illustrated that the pretreatments hadsignificant effects on the texture, microstructure and color of the samples. Considering the dryingkinetics and quality attributes, SSB is more suitable than HWB and CAP pretreatment for dryingsweet potato.

KEYWORDS: sweet potato bars, pretreatments, drying kinetics, quality, air impingement drying

∗Research support was provided by the National High Technology Research and DevelopmentProgram of China (863 Program) under Grant No. (2007AA100406-04), the Science and Tech-nology Support Project of Xinjiang Production and Construction Corps, China (No.2008ZJ28)and the Funding System for Scientific Research Projects of Doctor Subject of Chinese AdvancedUniversity (No.20060019011).

1. INTRODUCTION

Sweet potato is a nutritional, healthy and economical plant, which originated from Central America (Ishida et al., 2000). The dry matter in sweet potato consists of average 70% starch, 10% total sugars, 10% total fiber, 5% total protein, 3% ash, 1% lipid and rest 1% vitamins, organic acids (e.g. folic acid and pantothenic acid) and other components (Sablani and Mujumdar, 2006). It ranks as fifth most important food crop in developing countries (Zhang et al., 2000). Worldwide and China sweet potato productions were 126,299,661 tons and 102,240,110 tons in 2007, respectively (FAO, 2008).

Sweet potato is a seasonal plant harvested during October to November in China. Fresh sweet potatoes having relatively high moisture contents are very sensitive to microbial spoilage, even at refrigerated conditions. Thus, it must be consumed within a few weeks after harvest or be processed into various products which have lower moisture content for long time storage (Chen, 2002). Recently, in China more and more sweet potato is processed into oil fried chips and crisps. However, medical authorities have pointed out that a high fat diet was one of the major factors causing increased incidence of cardiovascular disease (USDA, 1990). A technique to produce low fat content sweet potato products is greatly needed. Drying is one of the techniques that could be used to prepare fat-free potato chips instead of traditional deep-fat frying process (Pimpaporn et al., 2007). Pretreatment is an essential step before processing of food materials (Senadeera et al., 2000). It has been reported that pretreatment can not only accelerate drying rate but also improve quality of dehydrated product by expelling intercellular air from the tissues, softening the texture, destroying the enzymes and microorganisms, or by dissociating the wax on the products skin and forming fine cracks in the skin (Jayaraman and Gupta, 2006). Many researchers have investigated the effect of different pretreatments on various fruits, vegetables and other food drying. Alvarez et al. (1995) experimentally studied the effect of blanching on the rate of moisture movement during drying of strawberries and found that blanching enhanced effective moisture diffusivity remarkably. Doymaz (2004) observed that the drying time of pretreated plums using chemical pretreatment was 29.4% shorter than that of untreated samples. Gazanfer and Sefa (2006) showed that chemical pretreatment could significantly accelerate the drying process and remarkably improve the quality of red peppers undergoing

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Xiao et al.: Effects of Different Pretreatments on Drying of Sweet Potato Bars

Published by The Berkeley Electronic Press, 2009

greenhouse and open sun drying. González-Fésler et al. (2008) proved that blanching pretreatment strongly improved the drying rate of apple slices. However, few research results are available on the effect of different pretreatments on the drying kinetics and quality of dried sweet potatoes. Therefore, the objective of the current work was to determine the effect of several different pre-treatments including hot water blanching (HWB), superheated steam blanching (SSB) and citric acid pretreatment (CAP) on drying kinetics and some quality ttributes of dried sweet potato bars evaluated in terms of texture (hardness), microstructure and color.

2. MATERIALS AND METHODS

2.1 Materials

Good quality fresh sweet potato (Ipomoea batatas Lam.) was purchased from a local supermarket in Beijing, China, which was free from rotting and insect damage. The average initial moisture content of the sweet potato was 74.04% in wet basis or 2.852kg/kg in dry basis, as determined by vacuum drying at 70oC for 24h (AOAC, 1990). Prior to the experiments sweet potato was washed with tap water, peeled and cut into bars having the dimensions of 1cm×1cm×5cm using a type FL-022 kitchen cutter (Good idea, China). All the sweet potato bars were put in polyethylene bags stored in a refrigerator at 3±1oC and 90% relative humidity for 2 hours prior to the experiments.

2.2 SSB and air impingement drying equipment

The schematic diagram of equipment used for SSB and impingement drying is shown in Figure 1, which was installed in the College of Engineering of China Agricultural University, Beijing, China. This apparatus basically consist of a steam generator to produce superheated steam, series of round nozzles in lines, an electric heater to heat the air, a centrifugal fan to supply the air flow and circulate the air flow, and a Proportional-Integral-Derivative (PID) controller (Omron, model E5CN, Tokyo, Japan) to control drying temperature. The distance between the round impingement nozzles and the up surface of sweet potato bars is about 8cm. Air velocity was measured with Founder Probe Anemometer (Founder, China) having an accuracy of ±0.1m/s. During all experiments the air velocity was kept at 10.0 m/s. When doing SSB experiments, the steam generator was opened, while during impingement drying experiments it was turned off.

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International Journal of Food Engineering, Vol. 5 [2009], Iss. 5, Art. 5

http://www.bepress.com/ijfe/vol5/iss5/art5DOI: 10.2202/1556-3758.1758

Figure 1. Schematic diagram of equipment used for superheated steam blanching and impingement drying

1, steam generator; 2, Proportional-Integral-Derivative (PID) controller; 3, steam pipe; 4, open valve; 5, closed valve; 6, drying air recycle channel; 7, centrifugal fan; 8, electric heater; 9, drying air channel; 10, drying air distribute chamber; 11, temperature sensor of the controller; 12, temperature and moisture sensor; 13, impingement drying chamber with series of round nozzles; 14, material to be dried; 15, drying tray

2.3 Pretreatment and impingement drying experiments

The pretreatment experiments were carried out according to Table 1. The sample weight was kept at 300.0±0.5g for all runs. For HWB, the sliced sweet potato bars were immersed in a hot water bath (40:1 ratio of water to sweet potato bars) at 95±2oC for 1 and 3 min. The hot water treated bars were then immediately cooled down in cold water at 4oC to remove excess heat, and placed on a paper towel to remove excess water prior to impingement drying. For SSB, the samples were spread in a single layer on stainless steel wire grid in the drying chamber of

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impingement dryer. Then they were treated by the superheated steam generated from steam generator at 120oC and 35% relative humidity for 3 and 5 min separately. Then the pretreated sweet potato bars were cooled under ambient conditions at about 20oC. For CAP, the sliced sweet potato bars were dipped in 0.2% and 0.4% citric acid solution for 30 min, respectively. Then pretreated sweet potato bars were taken out and blotted with paper towel to eliminate excess water on its surface. Table 1. Experimental design for pretreatment sweet potato bars

Code Pretreatment NP No pretreated SSB3 min Superheated steam blanching for 3 min SSB5 min Superheated steam blanching for 5 min HWB1 min Hot water blanching for 1 min HWB3 min Hot water blanching for 3 min 0.2% CAP 0.2% citric acid pretreatment for 30 min 0.4% CAP 0.4% citric acid pretreatment for 30 min

After pretreatments, the sweet potato bars were dried in the air impingement drying equipment described as above at constant temperature of 60oC and velocity of 10.0 m/s. During drying experiments, the temperature and relative humidity of surroundings was at the range of from 20oC to 24oC and from 40% to 46%, respectively. After the dryer reached steady state conditions for the set point, the samples were spread in a single layer on stainless wire grid in the drying chamber. Each sample bars was spaced not to touch the adjacent ones. Weight loss of samples was measured by means of removing the drying tray from the chamber and weighing on a load cell having sensitivity of ±0.01g and was recorded at 30 min intervals during drying. It took less than 15 seconds to weigh the sample. The samples were dried until they reached the desired final moisture content of 13.0% (w. b) or 0.15kg/kg (d. b.). The product was cooled for 5 minutes after drying, and packed in low density polyethylene bags, which were then heat-sealed. All the drying experiments were conducted in triplicate and the average of the moisture ratio at each value was used for drawing the drying curves.

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2.4 Mathematical Modeling of Drying Curves

The moisture ratio (MR) of sweet potato bars during drying experiments was cal

calculated using the following equations: MR=e

et

MMMM

−−

0

(1)

Where Mt is the moisture content at time t, M0 is the initial moisture content, which value is 2.852 kg/kg (d. b), and Me is the equilibrium moisture content. The values of the equilibrium moisture content, Me are relatively small compared to Mt or M0. Thus the Eq. (1) can be written in a more simplified form as follows (Ramesh et al., 2001; Doymaz, 2009; Xiao et al., 2009).

MR=0M

M t (2)

2.5 Textural properties

The textural properties of dried sweet potato bars was evaluated by a compressive test using a texture analyzer (Instron 4301, Buckinghamshire, England), following the methodology cited by Nourian et al. (2003) with slight modifications. A dried sweet potato sample was placed on a hollow planar base. The force was then applied to the sample by a 2mm spherical probe at a constant speed of 0.5mm/s until the sample was cracked. The maximum compression fore of a rupture test of each sample was used to describe the sample texture in terms of hardness. All tests were performed in duplicate and the average values were reported.

2.6 Microstructural evaluation

The suface microstructure of dried sweet potato bars processed by different pretreatments was observed using scanning electron microscopy (SEM) following the procedure described by Kawas and Moreira (2001). Prior to SEM observations, the sweet potato samples were coated with gold in a vacuum evaporator (IB-3, Eiko, Japan) in order to stabilize the structure. Then they were mounted on aluminum stubs with conductive adhesive and viewed with a scanning electron microscope KYKY-2800 (KYKY Corp., Beijing, China) at an accelerating voltage of 20KV. At least duplicate specimens were viewed at different magnifications and images of representative areas saved for further analysis.

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2.7 Color Measurement

Colors of fresh and dried sweet potato bars underwent different pretreatments were measured with a colorimeter (Lovibond RT100, Germany).The calibration needs to be performed before measuring by placing the tip of measuring heat flat against the surface of the white calibration place. After standardization L* (Lightness), a* (redness/greenness), b* (yellowness/blueness) values were measured for fresh and dried sweet potato samples. L* represents light-dark spectrum with a range from 0 (black) to 100 (white); while a* shows red-green spectrum with a range from -60 (green) to +60 (red), and b* indicates yellow-blue spectrum with a range from -60 (blue) to +60 (yellow). For each sample at least five measurements were made at different positions of the sample and the measured values were compared with those of the fresh sample.

3. RESULTS AND DISCUSSION

3.1 Drying kinetics

To compare the effect of different pretreatments on the drying kinetics of sweet potato bars, the air impingement drying kinetics of samples underwent different pretreatments are shown in Figure 2. It shows that moisture ratio of sweet potato samples decreased with the increase of drying time in all cases. It also illustrates that the drying time to reach the equilibrium moisture content was approximately 360 and 480 min for 1 and 3 min HWB pretreatment samples, respectively. Whereas for 3 and 5 min SSB pretreatment ones the drying time was around 300 and 390 min, respectively. For immersing in 0.2%, 0.4% CAP for 30 min and the no-pretreated samples, it was 210, 180 and 270 min respectively. Based on these comparisons it was found that blanching pretreatment both HWB and SSB could obviously decrease the drying rate, whereas the chemical pretreatment of CAP had significant effect on improving the drying rate of the samples. In general, blanching can expel the air entrapped intercellularly inside the sample tissues and elimination the resistance of cell membranes and cell walls to water diffusion by structure softening (Alvarez et al., 1995; Mukherjee and Chattopadhyay, 2007). However, the result of the experiment is contrary to this general knowledge. This phenomenon was probably due to the fact that when the sweet potato samples were suffered from HWB and SSB pretreatments starch gelatinization occurred

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and during the subsequent drying process a resistant film layer was formed on the surface of the samples, which reduced water transfer and increased the drying time. Similar findings were reported by Maté et al. (1998) for drying blanched potato slices and Leeratanarak et al. (2006) for potato chips undergoing low-pressure superheated steam drying.

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600Drying time (min)

Moi

stur

e ra

tio (M

R)

HWB 3 minSSB 5 minHWB 1 minSSB 3 minNPCAP 0.2% 30 minCAP 0.4% 30 min

Figure 2. Air impingement drying kinetics of sweet potato samples underwent different pretreatments at drying temperature of 60oC and air velocity of 10m/s. Figure 2 also illustrates that the drying time of the sweet potato samples increased with increasing the HWB and SSB pretreatment time. This might be due to the fact that longer blanching time can cause higher degree gelatinization of sweet potato starch, which could affect the cell structure and increase the internal resistance to moisture movement (Leeratanarak et al., 2006).

3.2 Textural properties

Hardness is an important parameter used to investigate the quality of dried products, which is related to the strength of the structure under compression

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(Chong et al., 2008). Commonly, the hardness of the dried sweet potato bars is softer the quality of it is better. The texture of dried sweet potato subjected to different pretreatment is reported in terms of hardness, which is illustrated in Figure 3. It shows that the samples subjected to 3min SSB pretreatment and NP ones had the minimum and maximum hardness, with 4.416 N and 12.348 N as its value, respectively. It also shows that the hardness of the samples under SSB and HWB pretreatment was softer than CAP and NP ones. This phenomenon is probably due to the fact that blanching can cause starch gelatinization, softening of structure and led to less hardness of dried starchy product (Leeratanarak et al., 2006). From Figure 3, it can be also found that the hardness of dried samples was proportional to its pretreatment time and showed an increasing trend as the drying time increased for SSB and HWB ones. This might be due to the fact that as the drying time to reach the desired content increased, the case hardening during drying was more pronounced and it increased the hardness of the samples. However, there was no significant different was found between CAP ones.

Figure 3.Hardness of the dried sweet potato bars underwent different pretreatments before drying at temperature of 60oC and air velocity of 10m/s.

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3.3 Microstructural evaluation

Microstructural evaluation can help quantifying the product changes during drying process and may also improve the understanding of mechanisms and changes in quality factors, especially the changes in food texture (Aguilera and Stanley, 1999). Microstructural observation of the surface of dried sweet potato bars was performed to evaluate the effects of different pretreatments on the microstructural changes of the dried samples (Figure 4). It is clearly seen from Figure 4 A-D that when the sweet potato bars were subjected to HWB and SSB pretreatments, the dried samples had a homogeneous compact structure. In addition, no pores and starch granules were found on the surface of the samples. As a result, the structure would slower water transfer or penetration during drying or rehydration process. Contrary to SSB and HWB pretreatments, the samples subjected to CAP had large with non uniform pores and lots of starch granules on its surface as shown in Figure 4 E-F. It is a common sense that structure determines its function. Absolutely, such porous structure would facilitate rapid water migration during drying. In terms of NP ones, the dried sweet potato tissues showed more numerous starch granules and fewer pores than CAP samples on its surface as illustrated in Figure 4 G. Therefore it is interesting to note that different pretreatments cause various changes of microstructure of the samples and lead to product properties varied differently. Similar results were reported by Nieto et al. (2001) and Kingcam et al. (2008).

A B

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Figure 4. Scanning electron micrograph of the surface of dried sweet potato bars underwent different pretreatments. A: samples of HWB 1 min; B: samples of HWB 3 min; C: samples of SSB 3 min; D: samples of SSB 5 min; E: samples of 0.2% CAP 30 min; F: samples of 0.4% CAP 30 min; G: samples of NP.

C D

E F

G

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3.4 Color Evaluation

Color is one of the most important quality criterion of foods. Undesirable changes in color of food may lead to a decrease in its quality and marketing value. In order to evaluate the effect of different pretreatments on the color of dried sweet potato bars subjected to impingement drying, the colors of the samples were measured in terms of Hunter parameters (L*, a* and b*). The results are shown in Figure 5. In case of the lightness of dried sweet potato samples it was found that compared to the fresh ones the lightness of all pretreated samples was decreased. From Figure 5a, it was also found that the lightness of the dried samples suffered from SSB and HWB was lower than the CAP and NP ones. Moreover, the lightness decreased with increasing of the pretreated time of HWB and SSB. This phenomenon was most probably related to different degree of starch gelatinization occurred during pretreatments and drying process. Similar findings have been reported by Pimpaporn et al.(2007) that the lightness of starchy product could be decreased by the clarity-like characteristics of gelatinized starch. It is know that positive a* values represent redness of the product. The a* values of fresh and dried sweet potato samples subjected to different pretreatments are illustrated in Figure 5b. It can be found that all dried samples were significantly redder than the fresh ones. This was mainly due to Maillard reaction occurred during drying process. Furthermore, the redness of the sweet potato bars pretreated by SSB was obviously higher than the samples underwent HWB, CAP and NP. The results probably owing to the fact that during HWB and CAP process sugars and some soluble pigments leaked out of the samples, which were the substrates of Maillard reaction. Lee (1958) demonstrated that hot water blanching resulted in considerable loss of nutrients such as sugars, proteins, carbohydrates and water soluble minerals. Selman (1987) observed that about 8% of tissues and 3% of total solids were lost after 10 min of HWB of carrots at 70oC. Federico et al. (2005) reported that hot water blanching caused carrots tissue cell membrane disruption and nutrients loss. The b* values of the samples are shown in Figure 5c, which represents yellowness of the product. It illustrated that the yellowness of the dried sweet potato bars underwent pretreatments of SSB and HWB was obviously higher than fresh and any other ones. The yellowness was not significantly different between samples pretreated by SSB and HWB. However, compared with the fresh samples the yellowness of the dried ones subjected to CAP and NP was decreased markedly.

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(a)

(b)

12



(c) Figure 5. Effect of different pretreatments on the color values of dried sweet potato bars compared to fresh one. (a): Effect of different pretreatments on lightness of the sweet potato; (b): Effect of different pretreatments on a* value of the sweet potato; (c): Effect of different pretreatments on b* value of the sweet potato.

4. CONCLUSIONS

The effect of different pretreatments on drying kinetics and quality of sweet potato bars undergoing air impingement drying were examined in this investigation. In case of the drying kinetics it was found that blanching pretreatment both HWB and SSB could obviously decrease the drying rate, whereas the chemical pretreatment of CAP had significant effect on improving the drying rate of the samples. In terms of quality, the attributes such as texture, microstructure and color of the dried sweet potato bars subjected to different pretreatments were studied. It was found that the samples pretreated by SSB and HWB were softer than CAP and NP ones. The microstructure of the dried sweet potato samples pretreated by HWB and SSB was homogeneous, compact and no pores and starch granules were found on the surface of it. However, the samples subjected to CAP and NP

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had large and not uniform pores and lots of starch granules on its surface. The color evaluation illustrated that the lightness of dried sweet potato bars pretreated by SSB and HWB was lower than the samples suffered from CAP and NP. Whereas, the redness of the sweet potato bars pretreated by SSB was obviously higher than the samples underwent HWB, CAP and NP. Furthermore, the yellowness of the dried samples underwent SSB and HWB pretreatments was obviously higher than that of the CAP and NP ones. Considering the drying kinetics and quality attributes, SSB is more suitable than HWB and CAP pretreatment for drying sweet potato.

NOMENCLATURE

a* Redness/greenness b* Yellowness/blueness CAP Citric acid pretreatment d. b. Dry basis Deff Effective moisture diffusivity HWB Hot water blanching L Thickness of sweet potato bars L* Lightness Me Equilibrium moisture content M0 Initial moisture content MR Moisture ratio Mt Moisture content at time t n Positive integer NP No pretreated R2 Correlation coefficient SEM Scanning electron microscopy SSB Superheated steam blanching t Drying time w. b. Wet basis

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Documents

Xiao Hong-Wei, Lin Hai, Yao Xue-Dong, Du Zhi-Long, Lou Zheng, Gao Zhen-Jiang. Effects of different pretreatments on drying kinetics and quality of sweet potato bars undergoing air