Hot Air Drying of Green Table Olives

G. ÖNGEN et al.: Drying of Green Table Olives, Food Technol. Biotechnol. 43 (2) 181–187 (2005)
181
UDC 66.047:582.931
preliminary communication
ISSN 1330-9862
(FTB-1354)
Hot Air Drying of Green Table Olives
Gaye Öngen1*, Sayit Sargi·n1, Derya Tetik2 and Timur Köse3
1Department of Bioengineering, Faculty of Engineering, Ege University,
×
35100 Bornova-Izmir, Turkey
2
×
Olive Culture Research Institute, Üniversite Caddesi 43, 35100 Bornova-Izmir, Turkey
3Department of Computer Engineering, Faculty of Engineering, Ege University,
×
35100 Bornova-Izmir, Turkey
Received: July 20, 2004
Revised version: January 17, 2005
Accepted: February 28, 2005
Summary
The characteristics of hot air-drying of green table olives (Domat variety) by using a
tray dryer were studied. Air temperature varied from 40 to 70 °C with an air velocity of 1
m/s. Drying rate curves were determined and quality of dried green olives was evaluated
by instrumental analysis (bulk density, particle density, porosity, shrinkage, moisture con-
tent, water activity, colour value, protein content, oil content, peroxide value and acidity).
Consumers’ acceptance test and microbiological analysis were also applied.
Key words: table olives, drying, dehydration, water activity, tray drier, consumers’ acceptibility
Introduction
the development of new alternative products and tastes
for table olives is under way. By introducing the drying
The world production of table olives is around 1
techniques green table olives can be consumed as a
million tons, with approximately 80 % coming from
snack food with a longer shelf-life. Water, being the
countries of the Mediterranean area. From the available
main component of foods, has a direct and decisive in-
data it can be estimated that approximately 45 % of the
fluence on their quality and shelf life through its effect
production is made up of green table olives. Table ol-
on several physicochemical and biological changes. Hot
ives are consumed on a large scale all over the world,
air drying is the conventional and most widely used
and their consumption is expanding owing to the in-
technique for the production of dehydrated fruits and
creasing popularity of the Mediterranean diet (1). They
vegetables (3). The removal of moisture prevents the
can be used as flavouring, an ingredient or simply as a
growth and reproduction of microorganisms that cause
snack or appetiser. They are especially used in Mediter-
decay and minimizes many of the moisture-mediated
ranean dishes including pizza, relishes, salads, sauces or
deteriorative reactions. It brings about substantial reduc-
antipasto platters.
tion in mass and volume, minimising packaging, storage
The increasing interest of the consumers in natural
and transportation costs and makes possible storage of
products involves the use and diffusion of technologies
the product under ambient temperatures (4).
which can offer a guarantee of preservation, hygiene
and genuineness of food products (1). Especially in the
In this study, the characteristics of air-drying of
developed world, there is now a great demand for a
green table olives were studied under different operat-
wide variety of high quality dried products, with em-
ing temperatures and the changes in physical and qual-
phasis on freshness and convenience (2). In this respect
ity characteristics were determined. In addition, the con-
* Corresponding author; Fax: ++90 232 388 49 55; E-mail: gongen@eng.ege.edu.tr

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182
G. ÖNGEN et al.: Drying of Green Table Olives, Food Technol. Biotechnol. 43 (2) 181–187 (2005)
sumers’ acceptability of dried green table olives as a
it gradully increased to 7 %. Fermentation period was 45
snack food was investigated.
days and acidity of fermentation medium was in the
range of w(lactic acid)=0.75–0.90 %. When the fermenta-
tion was completed acidity and brine concentration were
Materials and Methods
kept constant during storage. Before drying, fermented
Olive samples (Domat variety) were obtained lo-
green table olives were pitted and dipped into 0.4 % of
cally. They were calibrated (140–180 particles/kg) and
lactic acid to decrease salt concentration and acidity.
stored overnight at T=(10±2) °C before processing. The
flow sheet of drying process is given in Fig. 1.
Hot air drying
Olive samples were dehydrated by using the tray
Harvesting
dryer. The schematic diagram of the drying equipment
is given in Fig. 2. In order to evaluate the effect of air
Sorting-Grading
temperature on the drying process, four temperatures
(40, 50, 60 and 70 °C) were used. The air velocity was
Dipping in 2 % NaOH
kept constant at 1 m/s and the relative humidity was
maintained at 15 %. Every 30 min the samples were
Washing
taken out, weighed and returned to the dryer. Drying
was stopped when the mass of the samples reached a
constant value.
Fermentation (7 % NaCl)
Pitting
Moisture content
Samples were dried at 70 °C and 400 mmHg in a
Dipping in lactic acid volume fraction=0.4 %, 48 h
vacuum oven overnight according to the AOAC method
(5). Dry matter content was calculated on the basis of
Drying
fresh mass.
Fig. 1. Drying process of green table olives
Olive oil content
Fermentation was carried out in 700-L tank, which
The oil content of samples was analysed by using
contained 950 kg of green olives. During fermentation,
Abencor system and calculated as mass fraction of oil
the mass fraction of NaCl in brine was initially 5 % and
in %.
Fig. 2. Schematic diagram of the drying equipment. 1, centrifugal fan; 2, orifice plate; 3, differential manometer; 4, cooling and saturat-
ing tower; 5, cold water tank and evaporator; 6, 9, 11 and 15, thermocouples (T type); 7, circulation pump; 8, cold water shower; 10, elec-
tric heaters; 12, mixing chamber and air channels; 13, steam tank; 14, electric water heater; 16, injector and solenoid valve; 17, tempera-
ture and humidity sensor; 18, balance; 19, computer with data acquisition and control cards; 20, olives

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G. ÖNGEN et al.: Drying of Green Table Olives, Food Technol. Biotechnol. 43 (2) 181–187 (2005)
183
Protein content
For loose and tapped bulk density calculation, V1 or V2
must be used instead of V*.
Protein (N´6.25) was determined as total nitrogen
according to the Kjeldahl method with the addition of
The degree of shrinkage (sb/%) can be calculated
Kjeltabs ST as catalyst. Samples (1.5 g) were digested
from Eq. /6/ where Vi is the final volume of the fer-
with sulphuric acid using the Kjeldatherm digestion and
mented green table olive samples immersed in distilled
Vapodest 30 distillation systems (Gerhardt GmbH & Co.
water and Vf is the final volume of the dried olive sam-
KG, Bonn, Germany) equipped with an end-point titra-
ples immersed in carosen solution. Vf was calculated by
tor (Gerchard, Vap. 5).
considering the ratio of carosen and distilled water den-
sities:
Salt content
V -
i
V
s =
f ×100
/6/
b
The salt content of samples was analysed by using
Vi
the Mohr method (6).
Particle density
Caloric value
Known mass of sample (mp) was immersed into a
The caloric value of dried green table olives was deter-
known volume of carosen solution (Vc) in a measuring
mined according to procedure given by Balatsouras (7).
cylinder and the final volume (Vf) was read by using the
scale of the cylinder. Particle density (rp/(g/mL)) of the
Water activity (aw)
sample was found by Eq. /7/ (15):
Water activity was measured using a Testo 610 rela-
m
r
p
=
/7/
tive humidity and temperature measurement device (8)
p
V - V
f
c
and the results were calculated according to the Eq. /1/,
where ERH is equilibrium relative humidity:
Porosity
ERH
a =
/1/
Porosity was calculated by using Eq. /8/ (15), in
w
100
which for loose and tapped porosity calculation rbl or rbt
must be used instead of r *
b :
Colour
r *
L, a and b colour values of the samples were mea-
Porosity = 1 – b
/8/
sured using a spectral photometer (Datacolor, Textflash,
rp
USA). After standardisation, L, a and b values were
measured on fresh and dehydrated products. Colour
Sensory evaluation
values (DE), colour intensity (chroma, DC) and hue angle
were calculated according to Eqs. /2–4/ (9); L
Dehydrated samples were evaluated by a consumer
ref, aref and
b
acceptance test (N=110) (16). Panelists were scientists,
ref values belong to green table olive samples before de-
hydration:
research scholars of the department and undergraduate
students within the age group of 18–55 years.
2
2
2
DE =
(
[ L - L ) + (a - a ) + (b - b ) ]
/2/
ref
ref
ref
Microbiological analysis
2
2
DC =
(
[ a - a ) + (b - b ) ]
/3/
To determine microbiological quality of dried green
ref
ref
table olives (stored in plastic bags at 65 % relative hu-
hue angle = 1/tan (b/a)
/4/
midity and 10 °C for 12 months), samples were taken
under aseptic conditions, then transferred in 0.1 % pep-
Peroxide value and acid value
tone water and homogenised with blender. Appropriate
10-fold dilutions of the samples were prepared in pep-
Peroxide value (meq/kg oil) and acid value (% acid-
tone water and plated in duplicate on the selective
ity in terms of oleic acid) were determined according to
growth media to estimate microbial burden. Aerobic
standard methods (10).
mesophilic bacteria counts, mould counts, Escherichia
coli, and Salmonella spp. analysis were done according to
Bulk density and shrinkage
standard methods (17–20).
A glass cylinder (500 mL) was used for bulk density
measurements. A known mass of sample (m) was poured
Statistical analysis
into the cylinder and the volume was evaluated by
All treatments were done in triplicates and one way
reading the scale of cylinder (V1) (11,12). Loose bulk
analysis of variance (ANOVA) and Duncan’s post hoc
density (rbl) was found by Eq. /5/. The cylinder with
tests were used. All hypotheses were tested at a=0.05
the same sample was tapped 20 times on the smooth
significance level. All data were analyzed using SPSS
and soft surface from 10-cm height and the volume of
(10.0) for Windows.
the sample was evaluated by reading the scale (V2)
(13,14). Tapped bulk density (rbt/(g/mL)) was calcula-
ted by Eq. /5/:
Results and Discussion
r
m
=
/5/
The typical drying rate curves are shown in Fig 3.
bt
V*
As expected, increasing drying temperature leads to

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184
G. ÖNGEN et al.: Drying of Green Table Olives, Food Technol. Biotechnol. 43 (2) 181–187 (2005)
)) 1.4
h
1.6
))
t
er·
r
·
h
1.2
40 °C
e 1.4
40°C
tt
mat
50 °C
ry 1.0
ma
50°C
d
1.2
g
60 °C
ry
d
60°C
r/(k 0.8
g
0
1.
e
70 °C
(k
70°C
r/
wat 0.6
t
e 0.8
a
(
k
g
w
/ 0.4
g
e
0.6
(
k
/
rat 0.2
t
e 0.4
ra
0
g
Drying
i
n 0.2
0
5
10
15
20
25
30
35
40
r
y
D
0
t / h
0
0.5
1
1.5
2
2.5
3
3.5
Fig. 3. Drying rate versus time curve of green table olive samples
Free moisture content / (kg water/kg dry matter)
Fig. 4. Drying rate versus free moisture content of green table ol-
lower drying time. Drying at 70 °C required 14 hours
ive samples
and 30 minutes, whereas 39 hours were needed to ob-
tain the same moisture content at 40 °C. Dehydration
process was stopped when the moisture content of sam-
tion of spores, and participation in several types of che-
ples reached 5.05–4.95 %, which is the equilibrium mois-
mical reactions is an important issue. This availability,
which is defined as water activity (a
ture content. Drying rate is defined as mass of water re-
w), has a direct rela-
tionship with the equilibrium moisture content. As shown
moved per mass of dry matter and time (kg/(kg·h)).
in Table 1, in all experimental sets the average water ac-
Under the experimental conditions, the samples did not
tivities were not significantly different, with an average
show any constant drying rate, suggesting that diffusion
value of a
was the most dominant mechanism governing moisture
w=0.69, which is below the minimum aw val-
ues for the growth of most bacteria, yeast and moulds
movement in olive samples. For all drying conditions
(23).
there was a fast falling rate period at the start of drying,
followed by a more slowly falling rate region. This has
No significant differences were found in terms of
also been reported in previous papers (3,4,21,22). The
loose bulk density, tapped bulk density, particle density,
change in the drying rate as a result of the reduction of
shrinkage, loose porosity and tapped porosity of the
the free moisture content of the samples during drying
dried green table olives in all applied drying tempera-
is given in Fig. 4.
tures (Table 2). This is because of the uniform particle
size of the olive samples, which were calibrated before
The effect of hot air drying on the quality character-
drying process. At the end of the drying period all the
istics of green table olives is shown in Table 1. Avail-
samples became spherical and their moisture contents
ability of water for growth of microorganisms, germina-
had no significant differences (Table 1).
Table 1. The effects of hot air drying on the quality of green table olive samples
Green table
Dried green table olives
olives
40 °C
50 °C
60 °C
70 °C
w(moisture)/%
76.29± 0.57
5.05± 0.25a
4.95± 0.21a
4.98± 0.17a
4.95± 0.22a
w(olive oil)/%
14.67± 0.26
64.00± 0.90a
65.30± 0.87a
67.60± 0.36b
67.50± 0.46b
w(protein)/%
1.13± 0.18
4.06± 0.23a
4.15± 0.03a
4.12± 0.06a
3.75± 0.01b
w(salt)/%
0.50± 0.09
2.00± 0.10a
2.00± 0.10a
2.00± 0.30a
2.10± 0.10a
Caloric value
137.12± 3.15
584.76± 11.67a
606.33± 7.90b
627.01± 3.51c
624.34± 4.14c
Water activity
0.99± 0.00
0.69± 0.01a
0.69± 0.01a
0.69± 0.01a
0.69± 0.01a
Colour
L
55.53± 9.67
36.02± 3.44a
34.70± 3.26b
36.38± 3.06a
31.52± 1.94c
a
–0.98± 0.98
2.41± 1.22a
3.14± 0.94b
3.94± 1.05c
3.29± 0.76b
b
35.48± 6.80
19.62± 2.52a
18.87± 3.12a
19.85± 3.13a
13.67± 2.64b
hue angle
*
82.93± 3.70a
80.37± 3.28b
82.06± 5.66a,b
76.12± 4.06c
DE
*
27.11± 5.24a
28.76± 5.18a
27.32± 4.32a
34.82± 5.99b
DC
*
16.32± 4.15a
17.13± 3.57a
16.30± 3.50a
22.23± 4.08b
Peroxide value
8.61± 0.03
45.00± 0.02a
29.00± 0.04b
35.00± 0.05c
62.00± 0.01d
Acidity/%
1.28± 0.00
3.97± 0.01a
2.96± 0.01b
3.02± 0.01c
5.90± 0.01d
a-dmean values with different supercript(s) in the same row are significantly different (p<0.05)
*used as a reference for related equations

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G. ÖNGEN et al.: Drying of Green Table Olives, Food Technol. Biotechnol. 43 (2) 181–187 (2005)
185
Table 2. Physical properties of dehydrated samples
Green table
Dried green table olives
olives
40 °C
50 °C
60 °C
70 °C
rbl/(g/mL)
0.47± 0.01
0.38± 0.02a
0.40± 0.01a
0.40± 0.02a
0.39± 0.02a
rbt/(g/mL)
0.55± 0.01
0.41± 0.01a
0.41± 0.02a
0.44± 0.01a
0.41± 0.02a
rp/(g/cm3)
1.10± 0.12
1.10± 0.32a
1.08± 0.03a
1.09± 0.18a
1.06± 0.04a
sb/%
0.00± 0.00
76.57± 3.70a
76.51± 2.73a
77.45± 2.81a
74.18± 2.72a
Loose porosity
0.52± 0.10
0.64± 0.09a
0.63± 0.00a
0.63± 0.05a
0.63± 0.01a
Tapped porosity
0.49± 0.08
0.61± 0.11a
0.62± 0.01a
0.59± 0.06a
0.61± 0.01a
amean values with different supercript(s) in the same row are significantly different (p<0.05)
For the calculation of caloric value chemical analyses
Since dried green olives contain more than 65 % of
were performed to measure the protein and oil content of
oil in dry basis, the quality characteristics of the oil after
the olive samples. The carbohydrate and fiber content
drying process are of importance. Peroxide value (PV,
were omitted because all the sugars were fermented and
meq/kg oil) is a measure of oxidative rancidity and
the fiber content is not digestible for the humans. The
guide to olive oil quality. Along with the free fatty acid
differences in caloric values of samples dried at different
content (AV, acid value) it is a measure of hydrolytic
temperatures were due to the oil and protein contents,
rancidity (27). As shown in Table 1, the mean values of
which were found to be significantly different (p<0.05)
PV and AV were found significantly different for each
(Table 1). The caloric value of the dried olive samples
drying air temperature (p<0.05). Both PV and AV of
was found to be similar to those of snack foods such as
samples dried at 50 °C have the lowest values. The
peanut, hazelnut and pistachio (24). The salt content of
highest peroxide value was obtained at 70 °C because of
the dried samples was not significantly different at the
the higher temperature as compared to other applica-
applied drying temperatures. Average final salt content
tions. At 40 °C the peroxide value and acidity were also
was 2 % in all dried samples (Table 1). This salt content
found to be high due to longer drying period. Similar
is preferable for dietetic table olive products.
trend was observed at 60 °C, which had a higher perox-
The colour of products can be specified by three co-
ide value and acidity as compared to drying at 50 °C.
ordinates in the colour space, which can be obtained di-
rectly with a tristimulus colorimeter. The L, a, b system
The acceptability of dried green table olives as a
is the most frequently used scale for measuring the col-
snack food was evaluated by sensory analysis. The re-
our of food products (25). The L, a, and b values were
sults of the consumers’ acceptability test showed that 82
found significantly different for each drying tempera-
% of the panelists prefer the samples dried at 50 °C in
tures. However, these values do not represent the colour
terms of overall acceptability (texture, taste, colour,
change on inhomogeneous materials (9). Thus, the dif-
chewiness). This result overlaps with the experimentally
ferent combinations of tristimulus L, a and b values are
determined quality characteristics of the dried green ol-
used to represent the colour change in foodstuffs (26).
ives at 50 °C, in which the peroxide and acid values are
Browning was expressed as hue angles because this pa-
at the lowest level. After the evaluation of physical, che-
rameter showed a better correlation with visual estima-
mical and sensory analyses performed on dried green
tion than did chroma and total colour difference (9). As
table olives, the samples dried at 50 °C were analysed in
shown in Table 1, hue angle, DE and DC values of the
terms of microbiological quality. For this purpose Turk-
samples dried at 70 °C are significantly different (p<0.05),
ish Food Codex microbiological criteria for snack foods
as compared to other temperature applications. This dif-
were taken into consideration. The results of microbio-
ference represents the increase in the degree of brown-
logical analysis performed on green table olives and
ing.
dried green table olive samples are given in Table 3.
Table 3. Microbiological analysis of fresh and dried green table olive samples
Green table olives
Dried green table olives*
Analysis
N
Mean
Range
Mean
Range
value
(min-max)
value
(min-max)
Aerobic mesophilic bacteria at 25 °C/(CFU/g)
5
2.9·106
2.2·106– 3.8·106
1.6·104
1.0·104– 2.4·104
Aerobic mesophilic bacteria at 37 °C/(CFU/g)
5
4.1·106
3.4·106– 4.7·106
1.5·104
1.1·104– 2.2·104
E. coli
5
–
–
–
–
Salmonella spp.
10
–
–
–
–
Mould/ (CFU/g)
5
1.4·105
3.4·104– 4.5·105
–
–
* samples dried at 50 °C and stored for 12 months
N number of samples
– not detected

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186
G. ÖNGEN et al.: Drying of Green Table Olives, Food Technol. Biotechnol. 43 (2) 181–187 (2005)
E. coli and Salmonella spp. were not detected neither
10. Standard Methods for the Analysis of Oils, Fats and De-
in green table olive nor in dried green table olive sam-
rivatives, International Union of Pure and Applied Chem-
ples. Mould population was eliminated by the effect of
istry (IUPAC) (1979) 2.501, 2.201.
drying process and mould population at the end of 12
11. B.J. Crewdson, A.L Ormond, R.M. Redderman, Air impe-
months of storage was not detected. Aerobic mesophilic
ded discharge of fine particles from a hopper, Powder
Technol. 16 (1977) 197–207.
bacteria count of dried samples after 12 months of stor-
12. K. Sommer: Size Enlargement. In: Ullmann’s Encylopedia of
age was found to be in the acceptable range according
Industrial Chemistry, Vol. B2, 5th ed., G. Wolfgang, B. El-
to Turkish Food Codex for snack foods (28). Similarly,
vers, M. Ravenscraft, J.F. Rounsaville, G. Schultze (Eds.),
no rotting was reported after 1 year of storage of olive
WCH, Germany (1988) pp. 284–285.
samples dried at 50 °C when different pretreatments for
13. H. Egan, R.S. Kirk, R. Sawyer: Pearson’s Chemical Analysis
drying of green table olives were applied (29).
of Food, 8th ed., New York (1981) p. 591.
14. J. Pisecky: Standards, Specifications and Test Methods for
Dry Milk Products. In: Concentration and Drying of Foods, D.
Conclusion
McCarthy (Ed.), Elsevier Applied Science, London (1985) pp.
These results indicate that drying of green table ol-
203–209.
ives at 50 °C for 22 h in a tray dryer gives the acceptable
15. G.D. Hayes: Food Engineering Data Handbook, Longman Sci-
entific and Technical, New York (1987) p. 172.
final product which is proven by sensory analysis. Be-
16. M. Meilgaard, G.V. Civille, B.T. Carr: Sensory Evaluation
sides, they can be stored for 1 year without the risk of
Techniques, 3rd ed., CRC Press, New York (1999) pp. 242–
microbial deterioration. That is why widespread con-
245.
sumption of this valuable food stuff as a snack food is
17. Bacteriological Analytical Manual (Revision A, chapter 3), 8th
possible.
ed., US Food and Drug Administration Center for Food
Safety and Applied Nutrition (1998).
Acknowledgement
18. Turkish Standard of General Guidance for Enumeration of
The authors express their thanks to Prof. Abdülka-
Yeasts and Moulds-Colony Count Technique, TS 6580 (1989).
dir Yaghcioghlu, Asst. Prof. Vedat Demir and Res. Asst.
19. Turkish Standard of General Guidance for the Enumera-
tion of Coliforms-Most Probable Number Tecnique, TS
Tuncay Günhan of Ege University, Faculty of Agricul-
7725 (1996).
ture, for the support offered in the drying process.
20. Turkish Standard of General Guidance on Methods for
Detection of Salmonella, TS 7438 (1996).
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187
Su{enje zelenih stolnih maslina vru}im zrakom
Sa`etak
Ispitana su svojstva zelenih stolnih maslina (vrsta Domat) su{enih vru}im zrakom u
su{ioniku s policama. Temperatura zraka mijenjala se od 40 do 70 °C uz protok zraka od 1
m/s. Utvr|ene su krivulje brzine su{enja, a kakvo}a su{enih zelenih maslina ispitana je
instrumentalnom analizom (ukupna gusto}a, gusto}a ~estica, poroznost, sme`uranost, ko-
li~ina vlage, aktivitet vode, boja, udjel proteina i ulja, peroksidna vrijednost te kiselost),
ocjenom potro{a~a o prihvatljivosti proizvoda i mikrobiolo{kom analizom.

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