Evaluation of drying technologies for retention of physical quality and antioxidants in asparagus (Asparagus officinalis, L.)

Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
Evaluation of drying technologies for retention of physical quality
andantioxidants in asparagus (Asparagus of?cinalis, L.)
C.I. Nindoa, T. Sunb, S.W. Wangb, J. Tanga,*, J.R. Powersb
a Department of Biological Systems Engineering, Washington State University, 213 L J Smith Hall, Pullman, WA 99164 6120, USA
b Department of Food Science and Human Nutrition, Washington State University, Pullman, WA 99164, USA
Received30 September 2002; accepted21 January 2003
Abstract
The objective of this research is to evaluate drying methods that have the potential of adding value to green asparagus especially
for use as ingredient in instant foods or as a nutraceutical product. Five drying methods were used: namely, tray drying (TD),
spouted bed (SB) drying, combined microwave and spouted bed drying (MWSB), Refractance Window (RW) drying and freeze-
drying. Asparagus spears with diameters between 9 and 12 mm were blanched in 85C water-bath for 3 min, slicedinto 2–4 mm
thickness (or pureed for RW drying), then dried to moisture content less than 0.1 db. MWSB drying produced asparagus particles
with good rehydration and color characteristics, and was the fastest among the methods where heated air was used. When using
MWSB drying, the power level of 2 W/g and 60C heatedair resultedin highest retention of total antioxidant activity (TAA). TAA
of asparagus was enhanced after RW and freeze-drying, with the TAA values being signi?cantly higher than for heated air-drying
methods. In all cases, the tip portion of asparagus retained more TAA after drying than either middle or basal parts. The highest
amount of ascorbic acid was retained in the product after RW drying, followed by freeze-drying, MWSB and SB drying. TD resulted
in the least retention of ascorbic acid.
r 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.
Keywords: Drying technologies; Asparagus; Rehydration ratio; Ascorbic acid; Antioxidants
1. Introduction
low quality. Some processing plants make an effort at
increasing recovery rate by sorting the trimmedportions
Most of the asparagus (Asparagus of?cinalis, L.)
for canning as cuts. Apart from the low recovery rate
produced in the state of Washington is canned or fresh
problem, the Washington asparagus industry is facing
packedbefore shipping to grocery stores. About 2–3%
major competition due to low tariff imports of fresh
of total production is processed into pickled products.
asparagus (Calvin & Cook, 1997; USGAO, 2001).
Some small andmed
ium enterprises engage in other
Asparagus contains ?avonoids (mainly rutin) and
conversion processes, such as juice extraction. Thou-
other phenolic compounds, which possess strong anti-
sands of tons of high-quality asparagus parts are usually
oxidant properties (Makris & Rossiter, 2001). Vinson,
rejectedas trims d
uring canning andfresh packing
Hao, Su, andZubik (1998) analysed23 vegetables
operations. The large amount of byproducts from the
commonly consumedin the UnitedStates andfound
canning process is partly attributedto the ind
ustry’s
that asparagus is the ?rst in terms of total quality and
requirement that asparagus spears be trimmedto ?t
quantity of antioxidants. They also ranked asparagus
certain can sizes andnot necessarily that the trims are of
fourth in terms of total phenols content. Although
asparagus has this immense health bene?t, the results
Abbreviations: AA, ascorbic acid; FD, freeze-drying; MC, moisture
given by Vinson et al. (1998) show that per capita
content; MW, microwave; MWSB, microwave spoutedbed
; RW,
consumption of asparagus is still small comparedto the
Refractance Windowt; SB, spouted bed; TAA, total antioxidant
other vegetables. To increase its consumption andthe
activity; TD, tray drying.
returns to farmers andprocessors, it is important to take
*Corresponding author. Tel.: +1-509-335-2140; fax: +1-509-335-
advantage of the health bene?ts of asparagus by
2722.
E-mail address: jtang@mail.wsu.edu (J. Tang).
investigating some alternative value-adding processing
0023-6438/03/$30.00 r 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0023-6438(03)00046-X

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C.I. Nindo et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
methods. With novel drying methods like Refractance
porous product (Feng, Tang, Cavalieri, & Plump, 2001).
Window (RW) technology and combined microwave
Such a porous product reconstitutes much faster than
andspoutedbed(MWSB), increasedretention of the
the ones obtainedby using conventional hot-air-drying.
bene?cial bioactive compounds in the dried asparagus
Feng andTang (1998) reportedlow bulk d
ensities
can be achieved. In most Asian countries, including
resulting from the puf?ng effect when diced apples were
Japan and China, dehydrated vegetables are popularly
dried in a microwave spouted bed. In a microwave
used as ingredients in soups and are usually included in
puf?ng study using carrots Torringa, Van Dijk, and
foods such as instant noodles. This means that a large
Bartels (1996) concluded that drying at higher micro-
market exists for dehydrated asparagus.
wave power densities gives much shorter drying times,
Similar to other vegetables, changes in color, chemical
reduced shrinkage, higher open-pore porosity and
andtextural attributes of asparagus occur d
uring
improvedrehyd
ration characteristics in comparison
thermal treatments such as canning, pasteurization and
with a standard convective drying process. Nijhuis
drying. For example, the degradation of ascorbic acid
et al. (1998) also observedthat the more homogeneous
andchange of the green color of asparagus spear surface
dehydration and the developed internal pressure with
during thermal treatments have been shown to follow
microwaves result in less reduction in the volume of
?rst-order reaction kinetics (Esteve, Frigola, Martoreli,
microwave-dried products.
& Rodrigo, 1998; Lau, Tang, & Swanson, 2000). Strahm
Ascorbic acid(Vitamin C) is an important nutrient in
andFlores (1994) and Strahm, Flores, andChung
vegetables. It is more sensitive to heat, oxygen andlight
(1995) also studied drying and color kinetics of low-
than most other components in asparagus such as
grade green asparagus during heated air-drying and
vitamin A, E or some phenolic compounds. Ascorbic
observedsimilar trend
s. More recently, Krokida,
acidcan also act as a synergist with tocopherols by
Maroulis, andSaravakos (2001) reviewedthe effect of
regenerating or restoring their antioxidant properties
hot air, vacuum, microwave, freeze- andosmotic-drying
(Yanishlieva-Maslarova,
2001).
Depending
on
the
on the color of dehydrated agricultural commodities and
type of process, product physical properties, and the
statedthat the browning of fruits andvegetables during
time–temperature regimes used, thermal energy can
drying is due to both enzymatic and nonenzymatic
cause varying degrees of loss of vitamin C during
browning reactions. Browning is usually a negative
drying of green vegetables. Generally, if a process for
quality attribute, but other studies show that overall
drying vegetables takes place at a low temperature
antioxidant properties of certain foods may be enhanced
andwithin a short time, relatively high retention of the
due to formation of melanoidins during the advanced
heat-labile vitamin C is expected. This is the reason we
steps of the Maillardreaction (Anese, Manzocco, Nicoli,
usedascorbic content in asparagus as a marker to ?nd
& Lerici, 1999a; Anese, Nicoli, Massini, & Lerici, 1999b;
out the best drying method for retaining nutritional
Nicoli, Anese, Parpinel, Franceschi, & Lerici, 1997).
quality of dried asparagus. Although antioxidants are
The time it takes to reconstitute a dried vegetable and
usually not damaged during drying (Pokorny &
appearance of the vegetable are two important physical
Schmidt, 2001), it is important to know the retention
factors that needspecial attention when d
esigning,
of total antioxidant activity (TAA) after a particular
selecting or evaluating a given drying process. The
drying operation.
ability of foodproducts to reconstitute in piece form,
The objective of this research is to evaluate effective
such as sliced or diced vegetables, depends primarily on
drying methods that might produce high-quality aspar-
the internal structure of the dried pieces and the extent
agus. Drying rate, rehydration ratio, color, and reten-
to which the water-holding components (e.g. proteins
tion of TAA andascorbic acidwere usedas indices for
and starch) have been damaged during drying (Brennan,
comparison. We compared ?ve different methods:
Butters, Cowell, & Lilly, 1990). With hot-air-drying,
namely, tray, spoutedbed(SB), MWSB, RW and
higher drying rate at the initial stage may cause case
freeze-drying. Tray drying (TD) is commonly used for
hardening of the product surface and result in loss of
drying of vegetables, and it was chosen because of its
ability for fast reconstitution. Somogyi andLuh (1986)
simplicity andlow cost. TD is also often usedin R&D
reported a study on the effect of drying methods on
laboratories to simulate industrial tunnel or conveyor
quality of dehydrated green asparagus in which it was
dryers. Freeze-drying on the other hand is expensive,
observedthat freeze-d
riedgreen asparagus with hot
but quality of product is usually high. SB and MWSB
water blanching was faster in reconstitution andmore
drying (for food particles), and RW drying (for food
tender in texture than the hot-air-dried product. Drying
?akes or powders) fall in between and have been shown
of vegetables with hot air usually results in considerable
to produce high-quality dried foods (Feng, Tang,
shrinkage andformation of d
ense structure. Since
Mattinson, & Fellman, 1999; Abonyi et al., 2002). Our
microwave heating targets the water at the core of
ultimate intention is to use the dried asparagus either as
particulate products that generates relatively high
ingredient in instant soups or for further processing into
internal vapor pressure, it is possible to produce a
a nutraceutical product.

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509
2. Materials and methods
description of this system is provided in Feng et al.
(2001). Microwave energy, suppliedby a 1.5 kW
Fresh green asparagus usedfor the tests were
magnetron operating at 2450 MHz, was appliedto the
obtainedfrom farmers’ ?eld
s in Central Washington.
asparagus at 0, 2, and4 W/g of wet material. The power
The spears were storedin a cool box with ice and
level of 0 W/g representedSB drying. These experiments
transportedon the day of harvest to Washington State
were repeatedthree times. By using known moisture
University. The spears were blanchedin a water-bath
content andweight of asparagus at the start of each
maintainedat 85C for 3 min andimmediately cooledin
experiment andperiodically taking the sample weight,
ice water (0–2C) for equal time. Spot test for
the drying end point was determined when moisture
peroxidase activity was conducted to determine ade-
content hadfallen to less than 0.1 (db).
quacy of the blanching process. After blanching, the
Asparagus was also dried from a puree using a new
water adhering on the surfaces was removed with paper
thin-?lm drying method called Refractance Windowt
towel after which the portions selectedfor drying were
drying. This drying system utilizes circulating water at
cut into 2–4 mm slices. These were then dried using a
95–97C as a means to carry thermal energy to materials
laboratory microwave spoutedbeddryer (Fig. 1), a pilot
to be dehydrated. In operation, a pureed product is
scale tray dryer (Arm?eld Limited, Ringwood, Hamp-
spreadas a thin ?lm on a transparent conveyor belt that
shire, UK) similar to that described by Maskan (2000),
moves over hot water circulating in a trough (Fig. 2).
or a freeze dryer (Freeze Mobile 24, Vertis Company,
Thermal energy from the circulating hot water is
Inc., Gardiner, NY, USA). In each experiment 100 g of
transferredthrough the belt to remove moisture in the
blanchedasparagus was used
; andfor TD the 100 g
product. The actual product temperature is usually
of
slicedasparagus
was
placedon
four
trays
between 70C and80C (Abonyi et al., 2002). Detailed
(28 cm  18 cm) arrangedin a vertical stack. Moisture
information about the general engineering performance
content of asparagus before andafter d
rying was
of this system is provided in Abonyi et al. (2002). In our
determined by the vacuum oven method (AOAC, 1995).
study, prior to RW drying, fresh asparagus spears were
During MWSB drying, asparagus pieces were placed
washed, blanched, divided into tip, middle and bottom
in a spout funnel made of a special microwave-
portions andshippedin a cooler box by priority
transparent plastic material, andthen ?uid
izedwith
overnight delivery to MCD Technologies Inc. (Tacoma,
heatedair at 50C, 60C, and70C. A detailed
WA). The fresh samples were pureedin a blender (Oster
Power meters
Forward
Reflected
power
power
Circulator
Waveguide
Magnetron
1.5kW, 2450MHz
Tub
Tuners
MW Power controller
Directional coupler
80mm
Multi-mode
Air
microwave cavity
Sample
Out
80mm
Spouted
bed
Temperature
controller
80mm
Hot air
30mm
Heater
Ambient
Valve
air
Dimensions of
spouted bed
Blower
Fig. 1. Schematic of combined microwave and spouted bed drying (MWSB) drying system.

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C.I. Nindo et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
Stainless steel hood
Exhaust
Product application
Product removal
Vapor + Air
T
Water flumes
T
Cooling water
Plastic conveyor belt
Hot water pump Water tank & heater
Fig. 2. Schematic of pilot scale Refractance Windowt drying system (adapted from Abonyi et al., 2002).
Model SV, John Oster Mfg. Co., Milwaukee, WI, USA)
measuredusing Minolta Chroma CR-200 color meter
that was nitrogen ?ushed during each use. The blender
(Minolta Co., Osaka, Japan). The color parameter
was modi?ed by drilling a hole in the cover for inserting
values of lightness (L), greenness/redness (À/+a*),
a hose from a regulatednitrogen bottle thereby ?ushing
andblueness/yellowness (À/+b*) were recorded for
most oxygen from the blender jar while the samples were
each product. The ratio b*/a*, which is a measure of
pureed. To facilitate spraying of the puree onto the
hue, was calculatedandplottedagainst the lightness L
conveyor belt for drying, a predetermined amount of
for easy comparison. The deviation from the raw
distilled water was added during blending thereby
material color was representedas DEÃ andcalculated
q???????????????????????????????????????????????????????????????????????
increasing the water content from 11.5 to about 21.0
as
fðL À L
; where L
(db). The residence time for the puree on the belt was
0Þ2þðan À a0Þ2þðbn À b0Þ2g
0, a0
4.5 min. The dried product (o0.1 MC db) was packaged
andb0 are the color parameters of blanchedasparagus
in sealedaluminum bags, ?ushedwith a drop of liquid
before drying.
nitrogen andshippedto Washington State University
for analysis.
2.2. Analysis of total antioxidant and ascorbic acid
In summary, the drying methods and test conditions
were:
To evaluate the effect of the drying methods on
retention of ascorbic acidandtotal antioxidants, two
*
SB using air temperatures at 50C, 60C and 70C.
sets of drying experiments were conducted: one with
*
Combination of MWSB drying using air tempera-
slicedwhole spears, andanother after cutting the spears
tures at 50C, 60C and70C. Microwave (MW)
into three sections (apical, middle and basal parts). The
energy was appliedat 2 and4 W/g (wet basis).
three sections were cut as follows: apical portion (0–
*
TD at air temperatures of 50C, 60C, and70C.
7 cm), middle (7–14 cm), and basal (7–21 cm). The
*
Freeze-drying (heating plate temp. 20C; condenser
sections were then dried by various methods as
temp. À64C; abs. pressure 3.3 kPa).
previously described and ground before extraction.
*
RW drying from asparagus puree (heating water
TAA was measuredby ABTS/HRP d
ecoloration
temperature, 95C).
method(Miller, Diplock, & Rice-evans, 1995; Cano,
Hern!andez-Ru!?z, Garc!?a-C!arnovas, Acosta, & Arnao,
The following parameters were usedfor evaluating
1998). The assay estimates the relative ability of the
the drying results:
antioxidant substance to scavenge the radical cation of
(1) Drying rate, (2) L, a*, b* color values, (3) rehy-
3-ethylbenzothiazoline-6-sulfonate
(ABTSd+)
com-
dration ratio, (4) TAA, and (5) ascorbic acid content.
pared to a standard antioxidant (Trolox) using a dose–
response curve. The hydrophilic antioxidant activity of
2.1. Physical analysis
dry asparagus and fresh asparagus was measured. Dried
andfresh asparagus were groundfor 50 s using a coffee
Rehydration ratio of slices was determined using 2 g
grinder before extraction. Ground asparagus (0.20 g for
samples dipped into water maintained at 70C in a
dried asparagus and 2.00 g for fresh asparagus) were
water-bath andthe weight of asparagus was measured
added to 28 ml 50 mM MES (pH 6.0) buffer, mixed by
after soaking for 5, 10, 15, 20, and30 min. The amount
vortexing for 20 s, andcentrifugedat 35,000 g for 20 min
of water absorbed during soaking divided by the mass of
at 4C. The MES buffer supernatant was used
dry solids gave the rehydration ratio. Color was
for measuring hydrophilic antioxidant activity. For

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C.I. Nindo et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
511
hydrophilic antioxidant activity, the reaction mixture
indophenol solution until light distinct rose pink color
contained200 mL 15 mM aqueous ABTS, 80 mL 1.0 mM
persistedfor more than 5 s. Three replicates of each
H2O2, 10 mL 3.3 unit/ml HRP and1660 mL 50 mM
sample were analysed.
sodium phosphate buffer (pH 7.5). The reaction was
monitoredat 734 nm until stable absorbance was
obtained. Fifty microliters of extract from the MES
3. Results and discussion
phase was added to the reaction medium and the
decrease in absorbance, which is proportional to the
3.1. Drying kinetics
ABTSd+ quenched, was determined after 5 min. The
standard for TAA was 1 mM Trolox in MES buffer for
Fig. 3a shows the change of moisture content versus
hydrophilic antioxidant activity (we do not report any
residence time during TD at 60C air temperature, SB
hydrophobic antioxidant data).
drying at 60C, andMWSB drying at 2 and4 W/g of
Ascorbic acid in asparagus was determined accord-
power input levels with a ?uidization temperature of
ing to AOAC of?cial method(AOAC, 1995). Half a
60C. To reduce the moisture content from 11.5 to just
gram of dried asparagus (5 g for fresh) was blended with
below 0.1 db, TD took 3.5 h, while combined MWSB
50 mL of metaphosphoric-acetic acidsolution to extract
drying at a power level of 4 W/g took only 0.6 h. At a
ascorbic acid. The mixture was centrifuged and then
microwave power level of 4 W/g, the drying rate was 6.0,
the supernatant was taken andtransferredto a
2.8, and1.7 times faster than TD, SB, andMWSB
volumetric
?ask.
This
was
rapidly
titrated
with
drying at 2 W/g, respectively (Fig. 3b). This was due to
12
10
db] g
8
MWSB, 4W/g
MWSB, 2W/g
6
SB
Moisture, [glCO
4
TD
2
0
0
50
100
150
200
250
(a)
Residence time, min
1.0
MWSB,
4W/g
0.8
MWSB,
0.6
2W/g
0.4
SB
Drying rate, db/min
0.2
TD
0.0
0
2
4
6
8
10
12
14
(b)
Moisture, db
Fig. 3. Representative curves for microwave andspoutedbed(MWSB), spoutedbed(SB) andtray (TD) drying of asparagus slices at an air
temperature of 60C: (a) moisture loss curves and(b) drying rate curves.

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C.I. Nindo et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
Table 1
fast drying is the result of high heat and mass transfer
Average drying rate of asparagus by ?ve different drying methods
that takes place in the thin-?lm RW drying system. The
continuous circulation of hot water below the thin
Drying methodMW power
Temp. Hours to dry to Drying rate
level (W/g)a (oC)
p0.1 MC (db)
(% db/min)
conveyor belt (B0.2 mm) andthe thin spread
ing of
puree on a large surface coupledwith forceddraft above
Freeze-drying
—
20
18–24
1.0–0.8
the heatedpuree facilitatedthe drying process. As the
Tray drying
—
50
5.5
3.4
puree dries, evaporative cooling takes place and
60
3.5
5.3
prevents the puree from reaching the heating water
70
2.5
10.4
temperature (Abonyi et al., 2002).
Spoutedbed
0
50
2.3
8.2
3.2. Color and rehydration characteristics
60
1.7
11.5
70
1.2
16.3
Among the drying methods studied, freeze-drying
MWSB
2
50
1.6
12.0
produced asparagus with the greatest lightness (L),
60
1.0
19.1
greenness (Àa*), andyellowness (+b*) comparedto
70
0.8
22.9
asparagus dried by the other four methods (Table 2).
4
50
1.0
19.1
60
0.6
32.7
The RW-dried product was ?aky and sheet-like with a
70
0.5
38.1
bright green color, suggesting that most chlorophyll was
retainedin the process. Whole pureedasparagus that
RW
—
70–80
0.074
1301
was dried by RW was closest in greenness to the freeze-
a W/g: Watts/g of fresh asparagus; RW: Refractance Windowt.
dried product. Particles of asparagus tips that were tray
Under freeze-drying, time is for complete dryness and 20C is the
dried at 60C were the least green andhadthe lowest L-
heating plate temperature.
value (i.e. darkest). Asparagus tips that were tray dried
at 60C hadthe largest deviation (DEÃ value of 26.0)
the high rate of energy transfer into asparagus pieces
from that of the fresh material, followedby MWSB-
andinternal vapor pressure generatedby the microwave
dried whole asparagus and RW-dried products (Table
?eld(Feng et al., 2001). Due to continuous mixing of
2). The color of freeze-dried asparagus deviated the least
particles andbetter heat andmass transfer at product
(DEÃ value of 9.6). The color data of freeze-dried (FD)
surface, the SB drying is much faster than TD process.
asparagus is included here for comparison, because
The drying rate also increased with drying air
freeze-drying often produces the highest possible quality
temperature (Table 1). However, when asparagus was
among all industrial drying methods.
tray dried at 70C we observedincreasing darkening of
CombinedMWSB d
rying of asparagus slices also
the samples as drying progressed. TD of product at
produced particles with good rehydration capacity
50C took the longest time. Among the drying methods
(Table 3). Asparagus dried under combined MWSB at
in which hot air was used(i.e. tray, SB andMWSB
a power level of 4 W/g hadthe highest rehydration ratio
drying), MWSB at a power level of 4 W/g and a
among the drying methods that used heated air. The
temperature of 70C was the fastest (Table 1). High
investigation by Feng et al. (1999) also showedthat
microwave power and high ?uidized-bed temperature
blueberries dried in MWSB had very good rehydration
resulted in larger drying rates. While drying blueberries,
characteristics. When the appearance of fresh, dried and
Feng andTang (1998) also foundthat combinedMWSB
rehydrated asparagus was measured, dried asparagus
drying is much faster than TD. But we noted some
tended to become darker than the fresh asparagus.
charring of the asparagus that was dried at a power level
Rehydrated asparagus appears lighter and the greenness
of 4 W/g. Therefore, the bene?t of increaseddrying rate
is reduced somewhat when compared to fresh asparagus
at higher microwave power densities may be reduced
(Fig. 4), but the product still looked excellent for use in
because of this charring. Comparedto the darkening of
instant soups.
product at 70C air temperature andthe charring
observedat 4 W/g of microwave power, goodcolor
3.3. TAA and ascorbic acid content of dried asparagus
was achievedat a lower temperature (60C) anda lower
microwave power (2 W/g).
The average TAA of the batch of raw material that
The residence time for drying of 800 g of pureed
was dried as whole spears was 65.775.2 Trolox
asparagus from an average moisture content of 21.0 to
equivalents (mmol/g dried sample). Of the three drying
0.04 db on the pilot scale RW system was 4.5 min.
methods involving the use of heated air and/or micro-
Similar drying residence times for this system were
wave energy, MWSB drying at a power level of 2 W/g
obtainedin a separate energy utilization study in which
andat 60C air temperature provided the highest
up to 145 kg of pumpkin puree was usedas the starting
retention of TAA (Table 4). All dried asparagus tended
material (Nindo, Feng, Shen, Tang, & Kang, 2003). This
to lose some of their TAA as comparedto the raw

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513
Table 2
Color parameters of asparagus dried by ?ve different methods
Drying methodSpear section
Drying time (h)
Tempa (oC)
L
a*
b*
Hue angle
DEÃ
Tray drying
Tips
4
60
14.471.7
À0.670.1
12.271.0
92.6
26.0
Base
5
50
24.872.1
À2.670.7
20.671.4
92.7
17.9
Base
4
60
19.073.3
À2.970.6
16.172.2
100.1
21.7
Spoutedbed
Tips
1.7
50
22.273.2
À2.570.3
14.371.7
100.0
18.2
Tips
1.2
60
22.771.9
À2.670.6
12.870.7
101.5
17.6
MWSB
Whole
1.1
50
17.973.8
À3.370.7
14.773.2
102.8
22.5
Base
1.2
60
26.872.0
À1.970.5
17.671.8
96.1
14.8
RW
Whole
o0.1
95a
18.373.4
À4.570.4
15.671.6
106.0
22.4
Freeze-drying
Whole
>12
20b
37.774.5
À4.970.7
20.771.4
103.4
9.6
Raw material
—
—
—
40.272.9
À3.170.7
11.772.6
104.7
—
a Circulating water temperature.
b Plate temperature. DEÃ is calculatedby using the color of raw material as reference.
Table 3
Rehydration ratio of asparagus after MW spouted bed, spouted bed, and tray drying
Soaking time (min)
50C
60C
70C
4 W/g
2 W/g
SB
TD
4 W/g
2 W/g
SB
TD
4 W/g
2 W/g
SB
TD
0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
5
1.4
1.2
0.8
0.9
1.3
1.3
1.0
1.4
1.5
1.3
1.3
1.0
10
2.3
2.1
1.4
1.3
2.1
1.9
1.8
2.0
2.4
2.2
2.0
1.6
15
3.1
2.7
2.1
1.9
2.7
2.5
2.5
2.7
3.1
2.9
2.7
2.4
20
3.7
3.3
2.8
2.5
3.2
3.2
2.9
3.4
3.9
3.5
3.6
3.2
30
4.9
4.3
4.0
3.5
4.0
4.1
3.6
4.3
4.8
4.4
4.5
4.4
Units: grams of water absorbedper gram of driedasparagus.
50
After
45
rehydration
40
Fresh,
blanched
35
30
*
After
drying
25
e
s
s
, L
n
20
ht
g
15
Li
10
5
0
-10
-8
-6
-4
-2
0
Hue (b*/a*)
Fig. 4. Hue and lightness values for raw, dried and rehydrated asparagus. MWSB-dried asparagus (power level (W/g)/temp. (C)): (n) 4/70;
(+) 4/60; (m) 4/50; (&) 2/70; (–) 2/60; (’) 2/50; SB-dried asparagus (Temp. (C)): (J) 70; ( Â ) 60; () 50.
asparagus. In a secondset of experiments in which the
andbasal portion (14–21 cm) was 65.572.1, 50.173.5,
raw asparagus was cut into three portions (Table 5),
and37.773.0 Trolox equivalents (mmol/g, dry weight
the distribution of water-soluble antioxidant activity in
basis), respectively. If the high standard deviation for tip
the apical portion (0–7 cm), middle portion (7–14 cm),
portion is taken into account, it can be seen that

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514
C.I. Nindo et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
Table 4
Total antioxidant activity of tray dried, spouted bed and MWSB asparagusa
Drying method/condition
Microwave power
Temp. (oC)
TAA (Trolox equiv.)
Retained
level (W/g)
(mmol/g, dry basis)
TAA (%)
Raw material
—
—
65.775.2
100.0
Tray drying
0
50
48.171.8
73.1
60
47.671.4
72.4
70
46.972.9
71.4
Spoutedbed
0
50
40.572.9
61.7
60
52.074.1
79.2
70
39.272.9
59.7
Microwave andspoutedbed
2
50
42.671.5
64.8
60
53.573.3
81.4
70
45.671.6
69.4
4
50
46.272.2
70.3
60
49.971.1
75.9
70
42.071.6
63.9
a The three listed drying methods are the ones that involved the use of heated air. The TAA values are for whole asparagus spears.
Table 5
Effect of drying method on TAA of tip, middle and basal portions of green asparagus
Drying treatment
MW power
Temp.
Tip
Middle
Basal
Whole
(W/g)
(C)
portion
portion
portion
spear
Raw material
—
NA
65.572.1
50.173.5
37.773.0
51.172.9
Freeze-drying
—
20 (plate)
98.473.7aa
74.574.1a
55.773.5a
76.273.8a
Refractance Window
—
95 (water)
88.276.2a
75.272.6a
56.271.4a
73.273.4a
Tray drying
—
60 (air)
67.174.8b
45.773.3b
40.671.7b
51.173.3b
Spoutedbed
—
60 (air)
49.172.1c
42.671.4b
40.972.7b
44.272.1d
Microwave andspoutedbed
2
60 (air)
55.874.1bc
42.472.9b
37.171.8b
45.172.9cd
4
60 (air)
56.976.1bc
55.273.2c
37.573.7b
49.974.3bc
a Tukey’s HSD test. Same letter in the same column means there is no difference between treatments. Units of TAA are in Trolox equiv., mmol/g
(dry weight basis).
Refractance Windowt drying method resulted in the
more available by the heating process. It has recently
maximum TAA in all the three portions. However, after
been shown that thermal processing of sweet corn
carrying out Tukey’s HSD test, the retention of TAA
causedantioxid
ant activity andtotal phenolics to
after RW andfreeze-d
rying was not signi?cantly
increase by 44% and54%, respectively, although 25%
different. On examining the effect of drying method on
loss of ascorbic acidwas observed(Dewanto, Wu,
retention of TAA in three different portions of blanched
Adom, & Liu, 2002). Other relatedstudies on thermally
asparagus, both RW andfreeze-d
rying which rely on
processedtomatoes, tomato juice andother prod
ucts
different heating mechanisms showed enhanced TAA
have shown that heating causedan increase in their
than the drying methods where heated air was used.
overall antioxidant potential due to production of
Heated air inherently exposes the products to oxidation,
nonnutrient antioxidants (Nicoli et al., 1997; Anese
thus reducing their TAA. In the RW drying process, the
et al., 1999a; Dewanto, Wu, & Liu, 2002). Nicoli et al.
pureedproduct heats up very fast. This causedincreased
(1997) concluded that although natural antioxidants are
release of phenolic compounds bound in the cell matrix.
lost during heating, the overall antioxidant properties of
Moisture loss is very intensive during the ?rst 1 min of
foods could be maintained or enhanced by the develop-
drying, and the partial pressure of oxygen near the
ment of new antioxidants.
product becomes very low due to the high local vapor
The retention of ascorbic acidwas highest in RW
pressure createdby moisture evaporation (Abonyi et al.,
drying, followed by freeze-drying, MWSB drying, SB
2002). This condition prevented the oxidation of
drying, and was least in TD (Fig. 5). After TD, less
phenolic antioxidants in asparagus that were made
ascorbic acid was retained in the tips than in the middle

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C.I. Nindo et al. / Lebensm.-Wiss. u.-Technol. 36 (2003) 507–516
515
Fig. 5. Ascorbic acidcontent of three portions of asparagus spear after microwave spoutedbeddrying (MWSB), spoutedbeddrying (SB), andtray
drying (TD) at 60C. Power level for MWSB is 2 W/g.
and basal parts. In absolute terms, the middle and basal
spoutedbedandMWSB d
rying at the 2 W/g level,
parts containedmore ascorbic acidthan the tip portion
respectively. However, the 4 W/g power level caused
of asparagus. Using ascorbic retention as an index for
some charring of the product. The reconstituted
comparison, the RW-dried product is almost indistin-
dried asparagus was lighter in color with a hue angle
guishable from the raw material. This result corrobo-
greater than both the fresh andthe driedcounterparts.
rates the ?ndings of Abonyi et al. (2002) who reported
MWSB-dried asparagus had the highest rehydra-
only 6% loss of ascorbic acid(AA) in strawberry puree
tion ratio after 30 min in 70C water. TAA of asparagus
and9.9% loss of b-carotene in carrot puree after RW
after RW andfreeze-d
rying was signi?cantly higher
drying. They attributed the low loss of AA and b-
than after tray drying, spouted bed and combined
carotene to the unique moisture loss characteristics (and
microwave spoutedbedd
rying. In all cases, the tip
the more moderate time–temperature combination)
portion retainedmore TAA after d
rying than either
associated with RW drying technology. According to
middle or basal parts. Among the methods involving
Jayaraman andGupta (1995), slow drying methods like
the use of heatedair, microwave spoutedbeddrying at
sun drying cause increased loss of ascorbic acid (AA).
2 W/g and60C resultedin highest retention of TAA
Freeze-drying resulted in lower retention of ascorbic
in asparagus. RW drying method resulted in the
acid for the tip and middle portion of asparagus
highest retention of ascorbic acidin asparagus as
compared to RW drying. This may be due to the long
compared to all the other drying methods studied.
drying time (18–24 h) and the large surface area or open-
These results are useful for evaluating the effect of the
pore structure of the dried asparagus that facilitates
selected drying methods on TAA and ascorbic acid and
oxidation of ascorbic acid. The combination of short
not necessarily for making a choice of particular process
drying times (less than 5 min) and product temperatures
over another.
between 70C and80C during RW drying is important
for retention of nutritional quality of dried asparagus.
Acknowledgements
4. Conclusion
We thank the USDA Cooperative State Research,
Combinedmicrowave andspoutedbedd
rying of
Education and Extension Service (CSREES) for provid-
asparagus slices at a power level of 4W/g was at least 5
ing the grant without which this research wouldnot
times faster than tray drying when air temperatures
have been conducted. Special thanks to Richard E.
between 50C and70C were used. At a drying air
Magoon andKarin M. Bollandof MCD Technologies,
temperature of 60C, the MWSB drying rate at 4 W/g
Inc. (Tacoma, WA) for their assistance with the RW
power level was 6.0, 2.8, and1.7 times faster than tray,
drying experiment.

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516
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Document Outline

• Evaluation of drying technologies for retention of physical quality and antioxidants in asparagus (Asparagus officinalis, L.)
  • Introduction
  • Materials and methods
    • Physical analysis
    • Analysis of total antioxidant and ascorbic acid
  • Results and discussion
    • Drying kinetics
    • Color and rehydration characteristics
    • TAA and ascorbic acid content of dried asparagus
  • Conclusion
  • Acknowledgements
  • References

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