Effect of Drying Methods on Quality of Pistachio Nuts

Paper No: MBSK 02-213
An ASAE /CSAE Meeting Presentation


Effect of Drying Methods on Quality of Pistachio Nuts

M. Kashani Nejad1 L. G. Tabil1 A. Mortazavi2
Visiting Scholar Assistant Professor Professor

A. Safe Kordi3 M. Nakhaei4 M. Nikkho2
Professor Director Research Engineer

1 Department of Agricultural and Bioresource Engineering, University of Saskatchewan,
57 campus Drive, Saskatoon, SK, CANADA S7N 5A9
2 Department of Food Science & Technology, Ferdowsi University of Mashad, Mashad, 91775-1163, IRAN
3 Chemical Engineering Faculty, Sharif University of Technology, Tehran, IRAN
4 Momtazan Industrial Co., 5th km., Zangiabad Road, Kerman, IRAN

Written for presentation at the
2002 ASAE/CSAE North-Central Intersectional Meeting
Sponsored by ASAE and CSAE
Parktown Hotel
Saskatoon, Saskatchewan, CANADA
September 27-28, 2002

Summary: The effect of various methods of drying (sun drying, bin drying, vertical continuous
drying, vertical cylindrical drying and funnel cylindrical drying) on moisture content, lipid quality
(peroxide value, thiobarbituric acid value and free fatty acids), sensory attributes (firmness, sweetness,
rancidity, roasted flavor, split shell, shell appearance and over palatability), and percent split shell of
pistachio nuts (Pistacia vera L.) were studied.
Sun drying and bin drying resulted in higher split shell percent on pistachio nuts than other drying
methods. The different drying methods used in this study did not have any significant influence on the
free fatty acids, peroxide value and thiobarbituric acid of lipids in pistachio nuts. Drying significantly
affected shell appearance and split shell. Overall, the bin drying method produced pistachio nuts with the
best quality.

Keywords: Pistachio, Drying, Quality

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Effect of Drying Methods on Quality of Pistachio Nuts
M. Kashani Nejad, L. G. Tabil, A. Mortazavi, A. Safe Kordi, M. Nakhaei and M. Nikkho

Introduction

The pistachio nut (Pistacia vera L.) is one of the popular tree nuts of the world. Several
species of the genus Pistacia are referred to as pistachio, but only the fruits of Pistacia vera
attain sufficiently large size to be acceptable to consumers as edible nuts (Shokraii and Esen,
1988). Due to its high nutritional value and split shell, pistachio is an increasingly important nut
crop consumed as raw, salted or roasted. The kernels are a good source of fat (50-60%) and
contain unsaturated fatty acids (linoleic, linolenic and oleic acid), essential for human diet
(Shokraii, 1977; Maskan and Karatas, 1998). It is consumed in confectionery and snack foods.
Because of the deep green color of the pistachio kernels, it is favored in the ice cream and pastry
industries (Woodroof, 1967).
There has been a dramatic increase in production of pistachio in different regions of the
world during the past 20 years. Pistachio is cultivated in the Middle East, United States and
Mediterranean countries. It contributes substantially to the agricultural exports of some of these
countries. Iran produces more than 200,000 tonnes of pistachio annually and 101,235 tonnes was
exported in 1999 (Data and Information Administration, 2001).
Proper harvesting and postharvest handling are key parts in achieving maximum yield of
good quality nuts that determine marketability and profit. Processing right after harvest is very
important on pistachio quality. When pistachios arrive at the processing plant, the following
procedures are conducted: a) dehulling, to separate soft hull from nuts; b) trash and blank
separation, to remove blank pistachios and trashes such as small branches, remaining shells and
leaves; c) unpeeled pistachios separation, to remove unpeeled and unripe nuts; d) washing,
which involves spraying water at high pressures on the pistachios to clean the nuts; e) drying, to
decrease moisture content of pistachios from 37-40% to the appropriate level; f) split nuts
separation, to separate split nuts from non split ones; g) salting; h) roasting; and i) packaging
(Salajegh, 1996; Nakhaei Nejad, 1998).
During the drying process, nuts can undergo undesirable reactions (especially rancidity)
which cause degradation of quality, because of the odd colors and flavors formed (Fennema,

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1985). The major oxidative reactions in dried foods are due to peroxidation of lipids. Lipid
oxidation in foods is associated almost exclusively with unsaturated fatty acids and it is often
autocatalytic, with oxidation products themselves catalyzing the reaction so that the rate
increases with time (Karel, 1985). The pistachio is a nut with a high lipid content and very rich in
unsaturated fatty acids; this makes pistachio nuts very sensitive product owing to rancidity.
The hydrolysis of lipids results in a progressive increase of food acidity, caused by the
formation of fatty acids. Therefore, lipid hydrolysis favors the lipid oxidation because the fatty
acids formed can be substrates of the oxidation reaction (Richardson, 1984).
Hydroperoxides, which are the major products of lipid oxidation, can break down the
secondary products such as aldehydes, alcohols, ketones or acids and cause off-odors and off-
flavors. These secondary products, as well as peroxides and lipid free radicals can react with
proteins and vitamins, causing losses in nutritional value and functionality properties of food
constituents (Karel, 1985).
The quality of a dried food product and its cost are greatly influenced by the drying
operation. Products that have been dried at lower temperatures exhibit good storage stability but
require longer processing times. Low water activity retards or eliminates the growth of
microorganisms, but results in higher lipid oxidation rates (Fennema, 1985). Therefore, it is
important to establish the optimal drying conditions to avoid damage in pistachio quality.
In comparison with other food products, studies on the drying of pistachio nuts and its effect
on pistachio quality are very limited. Drying temperature affects the sensory attributes of
pistachio nuts and its roasted flavor increases during high temperatures drying (116-138ºC)
(Kader et al., 1979). Drying to appropriate moisture content (4-6% wet basis (w.b.)) is an
important factor insuring good quality. Nuts dried to 4% (w.b.) moisture are rated higher in
crispness, and sweetness and lower in bitterness and rancidity than those dried to 6 or 11% (w.b.)
moisture. Nuts at 6% (w.b.) also scored higher in sweetness and lower in bitterness and rancidity
than those at 11% (w.b.) (Kader et al., 1982). Drying affects the constituents of pistachio but its
influence is less than blanching and roasting. Unsaturated fatty acids are more susceptible to
changes during processing (Luh et al., 1981). Studies have shown that there is little or no effect
of drying temperature on pistachio quality as measured by percent closed shell edible nuts or
percent loose kernels in a sample (Thompson et al., 1996).

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Several studies on drying of other nuts are found in literature. It is not advisable to dry
hazelnut at temperatures higher than 50°C because the rate of rancidity reactions increases
resulting in degradation of hazelnut quality. The optimal drying air temperature for hazelnuts is
40-50ºC, because lower temperatures require longer processing times. Temperatures higher than
50ºC favor lipid oxidation in hazelnuts, especially in shelled hazelnuts, with an increasing trend
of the K232 and K270 indices and a decrease of oxidation stability values (Lopez et al, 1997a).
The drying of hazelnut at temperatures between 30 and 70ºC reduces the initial activity of
the enzymes lipase, peroxidase and polyphenoloxidase. Reduction of enzymatic activity depends
on the enzyme characteristics and the drying conditions. Higher enzymatic activity is observed in
shelled hazelnuts than in unshelled hazelnuts, possibly related to the higher water activity of
dried shelled hazelnuts than dried unshelled hazelnuts at the same moisture content (Lopez et al,
1997b).
Bostan (2000) reported that shell and kernel color of hazelnuts are affected by drying method
(drying in lawn, concrete, plastic and board). The best method is drying in concrete, as nuts in
this treatment dry in a short period, exhibit high pellicle removal, and have good shell and kernel
color.
Sun drying of hazelnuts is considered best as artificial heat can cause rancidity (Rosa, 1979).
Unshelled nuts can be stored for a year and shelled kernels for 4-5 months in winter but only for
one month in warm conditions.
Around the 1990’s, different commercial dryers (bin dryer, vertical continuous dryer,
vertical cylindrical dryer and funnel vertical dryer) were used in Iran to dry pistachio nuts, but
their effect on pistachio nuts quality has not been studied yet. The objective of this investigation
was to study the effect of various drying methods on the different quality parameters of pistachio
nuts.

Materials and Methods

Sample preparation
Ohadi is the major pistachio nut cultivar grown in Iran, therefore the Ohadi variety was used
in this study. After harvest, the pistachio nuts were immediately transported to the Rafsanjan
Pistachio Cooperative Company (RPCC) processing plant in Rafsanjan, Iran.

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Drying experiments
After dehulling, blank separation, unpeeled pistachios separation, washing and sorting,
samples were dried using the following methods:
a. Sun drying: Pistachio nuts were spread out in a thin layer 2 to 3 cm thick on a concrete
floor under the sun for 2 days at an average temperature of 26.5º C and average relative
humidity of (RH) of 18%.
b. Bin drying: Samples were dried in a 600-kg capacity batch bin dryer (Figure 1) in 50 cm
depth for 8 hours at an average drying temperature of 65±2ºC.
c. Vertical continuous drying: Pistachio nuts were dried in a 5000-kg capacity two-stage
vertical continuous dryer (Figure 2) with for 10 hours. Drying temperatures for the first
(top) and second stage (bottom) were 45º and 40ºC, respectively.
d. Vertical cylindrical drying: Pistachio nuts were dried in a 4500 kg capacity vertical
cylindrical dryer (Figure 3) at a drying temperature of 55±2ºC for 8 hours.
e. Funnel cylindrical drying: Samples were dried in a funnel cylindrical dryer (Figure 4),
which was equipped with some perforated funnels to adjust the movement and holding
time of pistachio nuts in the dryer. Dryer capacity was about 6000 kg and the dryer was
operated at a temperature of 80ºC for 5.5 hours.
After each drying, 5 kg samples were taken in triplicate and put into the sealed plastic bags
and kept in storage at 0ºC until quality parameters were measured.

Chemical analysis
Moisture content was determined by drying of 5 g of ground sample in an air
convection oven at 103±2ºC until a constant weight was reached (AOAC 1995). For
determination of pistachio oil characteristics, 100 g kernels of each sample were ground and
cold extracted using hexane as solvent. The solvent was evaporated at low temperature. Free
fatty acids were determined by titration method of AOAC (1995) as percent oleic acid. The
peroxide value was measured by using the iodometric titration method recommended by
AOAC (1995). Thiobarbituric acid values were determined by absorbance at 532 nm (Farag
et al, 1989). All chemicals used in this study were supplied by the Merck Company.


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Percent split shell nuts
Five hundred grams of whole pistachio nuts were taken and the number of nuts with split
shell was counted. An indicator with 0.6 mm thickness and 6 mm width was used for
determining of split shells (Standard and Industrial Research Institute of Iran, 1999).
Percent split shells was calculated from the following equation:
number

of

g

500

in

shell

split

with
nuts


shell

split

%
=
%
100

*

number

total
of

g

500

in

nuts

Quality evaluation
A taste panel of 16 trained and experienced individuals participated in the sensory
evaluation of pistachio nuts. The sensory evaluation was done on the basis of 5-point
Hedonic scale (table 1). The panelists were trained on the use of Hedonic scale and what
they need to consider during the evaluation. Samples were put at room temperature 12 hours
before evaluation to reach the ambient temperature. The panelists received 3 nuts per sample
and all sensory evaluation were replicated 3 times. Samples were labeled with three-digit
random number and a randomized order of presentation was used. The quality attributes
evaluated were: firmness, sweetness, rancidity, roasted flavor, shell appearance, split shells
and overall palatability (Kader et al., 1982).

Data analysis
The effect of drying methods on the different parameters of pistachio nuts was
determined using a completely randomized block design (RCBD) following the analysis of
variance (ANOVA) method. Significant differences of treatment means were compared
using the least significant difference (LSD) test at 5% significance level using MSTATC
statistical version 1.4 software program (Michigan State University, East Lansing, MI).

Results and Discussion
Moisture content: Significant differences in moisture content of dried pistachio nuts were
found between drying methods used in this study (Figure 5). Moisture content of nuts was
less than 4% (w.b.) in all drying methods. Nuts dried with the vertical cylindrical and
vertical continuous driers had lower moisture content due probably to longer holding time.
Nuts dried in all methods had lower moisture content than the standard moisture content

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(6% w.b.) suited to dry and hot region. Our data agree with those reported by Kamangar and
Farsam (1977) who reported that the moisture content of dried pistachio nuts in Kerman
province was less than 6% ( w.b.).
Percent split shells: Significant differences were observed among the drying methods in
percent split shell nuts (Figure 6). Although split shells increased with drying because of
moisture loss, nuts dried using sun drying and with the bin dryer had the highest percent
splits and those dried with the vertical continuous dryer had the lowest percent splits. This
difference is due to bed depth. Pistachio nuts were spread in a thin layer (2-3 cm) during sun
drying and maximum 50 cm depth during bin drying. In the other drying methods the
pressure exerted by nuts in the upper layers due to high depths, may have prevented the
splitting of the shells of nuts in the lower layers.
Lipid quality: It is seen from Figure 7 that differences in free fatty acids content due to
drying methods were not significantly different. Free fatty acids content is often used as an
indicator for fat hydrolysis and some processes such as drying may increase fat hydrolysis.
Since the total amount of free fatty acids in all drying methods is less than 5% (maximum
allowable level), the pistachio nuts are acceptable from this quality parameter. Similar
results were obtained by Shokraii (1977) but our data had a small difference with the value
reported by Maskan and Karatas (1999). These differences are due to compositional
differences between varieties, Ohadi versus Gaziantep used by Maskan and Karatas (1999).
Since hydroperoxides are the major products of lipid oxidation, their content, i.e. peroxide
value, is used to determine the initial stages of oxidation. Peroxide value of all samples was
less than the standard level (1 meq/kg) (Figure 8). Nuts dried with funnel cylindrical dryer
had the highest peroxide value which is due to high temperature (80ºC) of the dryer.
Thiobarbituric acid value is sometimes used as an auxiliary method to determine lipid
oxidation. No significant differences were found in thiobarbituric acid value of nuts dried
with various methods. The nuts dried in funnel cylindrical dryer had the highest value and
sun drying the lowest value.
Sensory attributes: Significant differences were found in firmness of pistachio nuts dried
using various methods (Table 2). The lower is the moisture content of pistachio, the higher
is its firmness. The higher firmness of nuts dried in vertical cylindrical dryer was due to
their lower moisture content (2.427%) and the lower firmness of nuts dried in bin dryer was

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due to their higher moisture content (3.272%) than those dried with other methods. These
results agree with those reported by Kader (1982).
Few significant differences were observed in sweetness scores (Table 2) and panelists
detected slightly higher sweetness in nuts dried in the funnel cylindrical dryer and slightly
lower in samples dried in vertical continuous dryer.
The scores by panelists showed that differences in rancidity were not significant (Table
2). These results support our previous findings of peroxide values. Panelists detected a few
differences in roasted flavor of pistachio nuts. Kader (1982) reported that drying at high
temperatures improve roasted flavor among pistachio nuts.
Results showed that differences in shell appearance of nuts dried with various methods
were significant (Table 2). The nuts dried in the funnel cylindrical dryer got the highest
score and those dried in vertical continuous dryer got the lowest score. Shell appearance is
one of the most important factors in pricing of raw pistachio for export. Shell staining as an
appearance quality defect should be minimized by avoiding delays in harvesting and delays
between harvest and hull removal. The cause of shell staining is not yet known, but the high
content of phenolic compounds in the hull may be a factor (Labavitch et al., 1982).
Significant differences were found in split shell of nuts dried using various methods.
These results were the same as our previous findings of percent split shell of nuts. Split shell
of pistachio nuts is also another important factor affecting price.

Conclusion
The study of the effects of various drying methods on quality of pistachio nuts indicated
that different drying methods used in this study do not have any significant influence on
lipid quality of pistachio nuts. Shell appearance and split shell significantly are affected by
drying methods. It was found that the bin drying is the best commercial method and
produces nuts with good quality.

Acknowledgments
The authors gratefully acknowledge the Rafsanjan Pistachio Research Institute and
Rafsanjan Pistachio Cooperative Company of Rafsanjan, Iran for providing the pistachio
samples and facilities.

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References

1. AOAC. 1995. Official Methods of Analysis, Association of Official Analytical Chemists,
Washington, DC, USA.

2. Bostan, S. Z. 2000. Research on some important quality characteristics of hazelnut
cultivars in different drying grounds. Bahce. 28:1-2,73-78.

3. Data and Information Administration. 2001. Agricultural Statistic Collection, Ministry of
Agriculture of Iran, Dept. of Budget and Programming, Tehran, Iran, 272 pp.

4. Farag, R. S., A. Z. M. A. Badei, F. M. Hewedi, and G. S. A. El-Baroty. 1989. Antioxidant
activity of some spice essential oils on linoleic acid oxidation in aqueous media. J. Am.
Oil Chem. Soc., 66: 800-804.

5. Fennema, O. 1985. Chemical changes in food during processing. An Overview. In:
Chemical changes in food during processing. T. Richardson & J. W. Fineley (ed.), AVI
Publishing Company Inc., Westport, CT., pp.1-16.

6. Kader, A. A., C. M. Heintz, J. M. Labavitch and H. L. Rae. 1982. Studies related to the
description and evaluation of pistachio nut quality. J. Amer. Soc. Hort. Sci., 107(5):812-
816.

7. Kader, A. A., J. M. Labivitch, F. G. Mitchell and N. F. Sommer. 1979. Quality and safety
of pistachio nuts as influenced by post harvest handling procedure. The Pistachio
Association Annual Report, pp. 45-56.

8. Kamangar, T. and H. Farsam. 1977. Composition of pistachio kernels of various Iranian
origins. J. Food Sci., 42:1135-1136.

9. Karel, M. 1985. Control of lipid oxidation in dried foods. In: Concentration and drying of
foods. D. MacCarthy (ed.), Elsevier Applied Science Publishers, New York, pp.37-51.

10. Labavitch, J. M., C. M. Heintz, H. L. Rae and A. A. Kader. 1982. Physiological and
compositional changes associated with maturation of ‘Kerman’ pistachio nuts. J. Amer.
Soc. Hort. Sci., 107: 688-692.

11. Lopez, A., M. T. Pique, J. Boatella, J. Parcerisa, A. Romero, A. Ferran and J. Garcia.
1997a. Influence of drying on the hazelnut quality. ?. Lipid oxidation. Dry
Technolnology, 15:965-977.


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---

12. Lopez, A., M. T. Pique, A. Ferran, A. Romero, J. Boatella and J. Garcia. 1997b.
Influence of drying on the hazelnut quality. ?. Enzymatic activity. Dry Technology, 15:
979-988.

13. Luh, B. S., V. S. Wong and N.E. El-shimi. 1981. Effect of processing on some chemical
constituents of pistachio nuts. J. of Food Quality, 5:33-41.

14. Maskan, M. and S. Karatas. 1998. Fatty acid oxidation of pistachio nuts stored under
various atmospheric conditions and different temperatures. Journal of the Science of
Food and Agriculture, 77, 334-340.

15. Maskan, M. and S. Karatas. 1999. Storage stability of whole-split pistachio nuts (Pistacia
Vera L.) at various conditions. Food Chemistry, 66:227-233.

16. Nakhaei Nejad, M. 1998. Pistachio hulling and processing in Iran. Momtazan Industrial
Co., Kerman, Iran.

17. Richardson, T., 1984. Controlling acyl transfer reactions of hydrolyses to alter food
constituents. In: Chemical changes in food during processing. T. Richardson & J. W.
Finley (ed.), AVI Publishing Company Inc., Westport, CT., pp. 219-254.

18. Rosa, M. D. 1979. Good hazelnut storage. Frutticoltura. 41: 2, 28-30.


19. Salajegh, F. 1996. Pistachio processing. Ministry of Agriculture, Tehran,Iran.

20. Shokraii, E. H. and A. Esen, 1988. Composition, Solubility and electrophoretic patterns
of protein isolated from kerman pistachio nuts (Pistacia vera L.). Journal of Agricultural
and Food Chemistry, 36, 425-429.

21. Shokraii, E. H. 1977. Chemical composition of the pistachio nuts (Pistacia vera L.) of
Kerman, Iran. J. Food Sci., 42, 244-245.

22. Standard and Industrial Research Institute of Iran. 1999 . Pistachio test methods. National
Standard No. 4920.

23. Thompson, J, T. Rumsey, and J. Knutson, 1996. Effect of sample drying temperature on
shell splitting. California Pistachio Industry Annual Report, pp.77-78.

24. Thompson, J., T. Rumsey, and J. Knutson, 1996. Pistachio quality during processing.
California Pistachio Industry Annual Report, 96, pp.104-106.

25. Woodroof, J. G. 1967. Tree nuts-production, processing products (vol. ?). Westport,
Connecticut: The AVI Publishing Company, Inc.

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