DRYING TECHNOLOGY
Vol. 22, No. 9, pp. 2201 2209, 2004
TECHNICAL NOTE
Studies on Hot Air and Microwave Vacuum
Drying of Wild Cabbage
Xu Yanyang,1 Zhang Min,1,* Arun S. Mujumdar,2
Zhou Le-qun,3 and Sun Jin-cai3
1
School of Food Science and Technology,
Southern Yangzte University, Wuxi, China
2
Department of Mechanical Engineering,
National University of Singapore, Singapore
3
Haitong Food Group Ltd. Co, Zhejiang, Cixi, China
ABSTRACT
Wild cabbage was dehydrated by a combination of hot-air drying
and microwave vacuum drying. It showed that the total drying
time was reduced by about half while the retention values of the
nutrient components and chlorophyll were improved significantly.
Microwave/vacuum-dried products had a more porous texture.
Finally, microwave drying showed effective bactericidal action in
the product with acceptable quality of dried product.
Key Words: Combination drying; Convection; Microwave;
Vacuum; Wild cabbage.
*Correspondence: Zhang Min, School of Food Science and Technology,
Southern Yangzte University, Wuxi 214036, China; E-mail: [e-mail].
2201
DOI: 10.1081/LDRT-200034275 0737-3937 (Print); 1532-2300 (Online)
Copyright & 2004 by Marcel Dekker, Inc. www.dekker.com
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1. INTRODUCTION
Combination or hybrid drying refers to drying technologies which
employ different drying methods at different stages of the drying cycle.
Such technologies include such combinations as hot air drying followed
by microwave or freeze drying, osmotic dehydration followed by hot
air drying etc. Using two-stage drying can result in better thermal
performance but also, more importantly, quality changes which may be
desirable.
Currently over 90% of vegetable drying processors in China use
just hot-air drying technology. Although this operation is simple and
inexpensive, the product quality is often low and hence the economic
return is poor.[1] Common hot-air drying typically requires about two-
thirds of the total drying time for removing the final one-third of the
moisture content. Moreover, it can destroy thermo-labile components
and cause solute migration and formation of a crust.[2] On the other hand
the microwave vacuum drying technique, not only lowers the drying
temperature, but also accelerates drying kinetics. This technique was
applied to dry wild cabbage.
Wild cabbage (Brassica olerace) is cultivated all over China. It has
nutrients such as ascorbic acid as well as calcium (three times that in
cucumber and five times their content in tomato). Also, it contains
elements such as molybdenum, manganese, selenium, etc.[3,4] There is
important export market for dried wild cabbage.
The objective of this project was to examine the feasibility of using
microwave-vacuum drying to dry wild cabbage efficiently to produce a
high quality dried product.
2. MATERIALS AND PROCEDURE
2.1. Materials and Experimental Apparatus
The material tested was the   Jingfeng   cabbage supplied by Haitong
Food Group Co., Zhejiang, China. The samples for testing were cut into
slices of dimensions (20 mm 25 mm). Specific process flow employed is
shown as follows:
Raw material selection ! Pretreatment (washing, cutting, blanch-
ing, cooling, centrifuge, osmotic dehydration) ! Hot-air drying !
Microwave vacuum drying ! selection, packaging ! Storage.
The experimental apparatus is shown in Figure 1.
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Figure 1. Schematic of the experimental apparatus. (a) STJ-1 vegetable direct
fired dryer. (b) WZD4S-01 microwave vacuum dryer.
2.2. Procedure
Moisture content was measured by the gravimetric method,[5] using
an electric convection oven and an electronic balance. Precision of the
electronic balance was 0.0001 g. Ascorbic acid determinations were
made using 2,6-dichlone indophenol titration.[6] Chlorophyll determina-
tions were adopted from Arnon,[7,8] using a VIS-723 spectrophotometer.
Color determinations were made with a KangguangÕ SC-80C pan-
automatic chromatis-meter. Color and luster, texture, flavor, and mouth
sense determinations were made using sensory panel evaluations.
Microbial analysis included total plate count, coliforms, pathogenic
bacteria according to Chinese standard ZB X 09001-86, ZB X 09002-86,
ZB 8-83.
3. RESULTS AND DISCUSSION
3.1. Effect of Different Microwave Power on Drying Time
Cabbage pieces, dried by hot-air to different intermediate moisture
contents were finish-dried to below 5% dry basis using the microwave
vacuum drying technique at different microwave power levels. The drying
time to reach the desired final moisture content was recorded. The results
are shown in Table 1.
Table 1 shows that the higher the microwave power level, the shorter
the drying time. When the microwave power was varied from 1400 W to
Table 1. The result of sensory evaluation for cabbage dried by different microwave power.
Initial Degree
moisture Microwave of Drying
content power vacuum time
Sample (%, w.b.) (W) (kPa) (min) The result of sensory evaluation
Y1 49.3 1900 2 30 Damp, paste, normal color and luster, flavour, several bubbles in
the sample
Y2 15.5 1900 2 8 A little burnt, several bubble in the sample , loosen texture
Y3 15.8 3800 2 4 A lot of burnt, most loosen texture , burnt odour, several bubble in
the sample
Y4 15.0 1400 2.5 10 Damp, more loose texture, normal color and luster, flavour,
several bubble in the sample
Y5 15.4 2800 2.5 6 A lot of burnt, most loosen texture , burnt odour, several bubble
in the sample
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1900 W, the drying time was reduced and the quality of dried product was
better, but in that power range vapor bubbles were developed in the
sample, which affected the final quality. Since the initial moisture content
of sample Y1 was large i.e. 49.3%, the drying time was longer. The initial
moisture contents of the other samples (Y2 Y5) were about 15%, the
results showed that drying time was shorter than that of Y1. Therefore,
microwave vacuum drying appears to be more suitable for final drying
stage. The reason for the formation of bubbles in the samples is not
known yet. It could be due to the development of hot spots. There is
possibility of caramelization of sugar in the product due to the hot spots.
3.2. Effect of Different Drying Methods on
Drying Characteristic
After the cabbage was chosen, cut, washed, blanched and centri-
fuged, it was dried by a combination of hot-air and microwave vacuum.
Meanwhile, the control sample was dried by only hot-air. The initial
phase in the hybrid drying process was hot-air drying. When the material
moisture content dropped to 15 20%, it went into finish drying phase
with either hot-air drying (AD) again or by microwave vacuum drying
(MVD). The temperature of the hot air was 60 85 C. The velocity of the
air stream was 1.15 m/s, the input power for microwave vacuum drying
was 1900 W with the degree of vacuum of 2 kPa. A plot of moisture
content vs drying time is shown in Fig. 2.
As Fig. 2 shows, the needed time for AD was more than 360 min, the
moisture content of the final product reached 6.5%. The drying time for
AD þ MVD was only 248 min, when the moisture content of its final
products dropped to 4.1%. Compared to AD, the whole drying time for
Figure 2. The curve of moisture content and drying time.
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AD þ MVD was shorter and the moisture content of the final product
dropped below 5%. This corresponds well with the experimental results
of Bonafonte et al.[9]
3.3. Effect of Different Drying Methods on the
Dehydrated Cabbage Quality
The quality of the dehydrated vegetable product is mainly
determined by the following five parameters: (1) retention of pigment,
(2) retention of flavor substances, (3) retention of nutrients, (4) inhibition
of browning, and (5) control of surface.[10] Chlorophyll and ascorbic acid
contents were determined in this work, and relevant retention rates were
calculated. These results are shown in Table 2.
From Table 2, it can be seen that the retention rate of chlorophyll
and ascorbic acid for samples treated by AD þ MVD (air drying followed
by microwave-vacuum drying) are higher than those for AD alone. This
agrees with the findings of Zhang and Onayemi.[11,12] It is well recognized
that rapid drying can retain more chlorophyll and nutrient components
in the dried product.
3.4. Effect of Different Drying Methods on
Color of Dehydrated Cabbage
Color is an extremely significant factor for the quality evaluation of
dehydrated vegetables. CIE-L a b uniformity hue space system is a
frequent application system which represents color characteristics in
agricultural products. The value of L represents the trend of brightness
(black white), while the value of a represents the trend of red green and
the value of b represents the trend of yellow blue. The smaller L value,
Table 2. The effect of different drying methods on the retention rate of
chlorophyll and ascorbic acid in the dehydrated cabbage.
The retention The retention
Drying rate of rate of ascorbic
method chlorophyll (%) acid (%)
76.1 21.4
AD
78.0 27.1
AD þ MVD
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Table 3. Comparison of dried product color by different drying methods.
Color value
Samples L a b E L a b
71.92 9.08 34.54
AD
66.67 10.08 35.46 5.64 5.25 0.94 0.92
AD þ MVD
66.48 4.63 28.30 16.01 5.44 13.0 6.24
AD þ MVD (b)
82.92 3.59 18.07 20.55 11.00 5.5 16.46
Fresh
the darker the color of the sample. It shows that L value is relevant to
browning in fruits and vegetables.[13]
The authors applied chromatis-meter for the color determination of
dehydrated cabbage. The results are shown in Table 3. The values of L
were 82.92 in fresh sample and 66.48 71.92 in processed cabbages
respectively. Compared to the fresh one, the L value for processed
products decreased all. This showed that processing treatment did
reduce the vegetables brightness. In addition, samples treated by
AD þ MVD had worse brightness in L value than that treated by AD
alone. Sample treated by AD þ MVD was inclined to be more green.
The a value was smaller for fresh cabbage, which may be relevant to its
high moisture content (91%). The sample of AD þ MVD (b) was
treated by AD þ MVD with excessive browning, where the a value
was þ 4.63 and its L value was smaller. It means that it was inclined to
be more brown.
The value of E represents total chromatic aberration value.
Compared to AD, sample treated by AD þ MVD was the smallest in
E value, following the samples of AD þ MVD (b) and the fresh. The
results showed that the samples treated by AD þ MVD have better color
than by AD.
3.5. Effect of Different Drying Methods on the
Microbial Index
The microbial indexes were determined for the dehydrated cabbage
using the two drying methods. The results are shown in Table 4. The
results show that the microbial indexes of both products corresponded
with the national food hygiene acceptance standards without exception.
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Table 4. Microbial index of the products.
Microbial index
Drying method Total plate account Coliforms Pathogenic bacteria
400/100 g Negative Absent
AD
375/100 g Negative Absent
AD þ MVD
4. CONCLUSIONS
(1) Combination drying involving hot air drying followed by
microwave-vacuum drying shortens drying time and also greatly
improves the retention of chlorophyll and ascorbic acid in the
dried product.
(2) Microwave vacuum drying offers several favorable character-
istics such as effective sterilization, convenient control.
ACKNOWLEDGMENTS
Authors are grateful to leaders and staffs of Haitong Food Group
Co., Ltd, Zhejiang, China for supporting this experimental study.
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