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Pistachio nuts of the Ohadi
cultivar used in this study were sourced from the Iran Pistachio Research Center.
Unsplit nuts were separated from the split nuts before drying in a batch
cylindrical dryer (Rezaei Model, Rafsanjan, Iran) to decrease the moisture
content to around 5% (dry basis). The raw nuts were stored in a freezer at
-18±2°C before analysis.

 

 2.2.
Determining the optimum roasting conditions

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The parameters used to
determine the optimum roasting conditions were temperature and time. Fifty gr
of kernels was placed in a single layer in Pyrex Petri dishes (9 cm in
diameter) and were roasted in a convection Memmert oven (model UNB 500,
Germany) between 90 to 190°C for 5 to 65 min based on the experimental design
shown in Table 1. Three replications were carried out for each
run. After roasting, the whole kernels were allowed to cool at room temperature
(20±2°C).
About 25 gr of the kernels was ground for 1 min in a Waring laboratory blender
(Model HGBTWT, Torrington, USA) at low speed to produce pistachio
powder (ground kernel). Both whole and ground kernels were stored in
a freezer at -18±2°C prior to analysis for less than 3 days.

 

2.3.
Determining the hardness and
fracturability

The hardness and
fracturability of the whole kernels were measured using a texture analyzer
(TA.HD. Plus Texture Analyser, UK). A kernel was placed horizontally in the
center of the analyzer plate and double compression was applied using a
cylindrical probe (75.0 mm in diameter instead of 5mm) at a test speed of 1.0
mm/s and a deformation of 1.5 mm. The test was carried out in ten replicates.
Textural properties were derived from the force–time curves. The two textural
parameters fracturability (in Newton) (first peak of first compression) and
hardness (in Newton) (maximum peak of first compression) were considered to
evaluate textural properties (15, 16).

 

 

 

 2.4. Determining the moisture content

The moisture content of
whole kernels was determined according to the AOAC Method 925.09 (17). The measurements were performed in
triplicate.

 

2.5. Preparation of pistachio paste using
colloid mill

The nut shells were manually removed to obtain the kernels. The kernels
were roasted in the convection Memmert oven (model UNB 500, Germany) at 134±1°C
for 30 min before milling at 1000 rpm into a paste using a Supermass colloider
mill (Masuko, Model MKZA6-5, Japan). The nut kernels were placed in a single
layer in glass petri dishes (9 cm in diameter) in which each petri dish
contained 50 gr of the kernel. Four milling gap sizes (20, 40, 60, and 80 ?m)
were used in order to produce pastes of different particle sizes. One kilogram
of pistachio kernel was milled for each gap size. Three batches of paste were
prepared for each gap size and stored at -18±2°C in polyethylene containers (each containing
100.0 g) for less than 3 days. Prior to analysis, the pastes were removed from
the freezer and allowed to stand at 20±2°C
for 1h.

 

2.6. Determining the pistachio paste particle
size

The particle size of the pistachio paste samples was determined using a
particle size analyzer (Mastersizer 2000, UK) according to the Malvern method
(2007)(18). The samples (10.0 gr) were obtained from the
container using a stainless steel spatula
suspended in water (1:10 w/v) and homogenized
using an Ultra-Turrax T25 homogenizer (Jank and Kunkel, IKA-Labortechnik,
Breisgau, Germany) at 2500 rpm for 3 min. The data were analysed using the
Malvern software Version 5.60. Five replicate analyses were carried out for
each sample, and the mean was reported.

 

2.7. Determining the pistachio paste stability

Sixteen grams of pistachio paste of different particle sizes were placed
in separate 15 ml rounded bottom centrifuge tubes (Pyrex, 1.5 cm in diameter,
11cm height) using a stainless steel spatula, heated at 80?C water bath for 30
min, cooled in water at room temperature for 15 min, and then centrifuged at
4000 ×g at 20°C for 10 min. The height of the oil layer that
separated from the pastes was determined using a dial micrometer. The colloidal
stability (CS) was determined using the following equation, where H0 is
the height of the separated oil, and Ht is the total height of
the pistachio paste in the test tube at the begining of the experiment (19, 20):                            

CS = (Ht-Ho/Ht) ×100                                                   

 

 2.8. Determining
the color of pistachio kernel and paste

The color
of roasted whole kernel, ground kernel, and pistachio paste were measured using
an Ultrascan PRO Spectrocolourimeter (Hunter
Lab A60-1012-402 Model, Reston, VA, USA).
Twenty
grams of the sample was placed in the glass container of the Hunter Lab before
the measurements were made. The paste was spread in the glass container. The parameters
determined were the L value (whiteness/darkness), ‘a’ value (greenness/
redness), b value (blueness/yellowness), and YI (yellowness index) for roasted
whole kernel and ground kernel (21), and
L, a, and b values for pistachio paste. The analysis was replicated three times.

 

 

2.9. Response Surface
Methodology (RSM)

RSM was performed using two
factors (x1 and x2) with five levels (70, 90, 140,190,
and 211°C) central composite design for the responses hardness, moisture
content, L value, ‘a’ value, b value, and YI of the kernels, and L value, ‘a’
value, b value, and YI of the ground kernel. The x1 and x2
values reflected the air temperature and roasting time, respectively. The first
or second order polynomial expressions for a selected experimental range
(90-190°C and 5-65 min) was used to estimate the actual response surfaces (22). Analysis of variance (ANOVA) was applied to determine the effect of
particle size on the colloidal stability and the color of pistachio paste
samples. The correlation coefficients (R2) were determined using the
Rheowin Data Manager Version 3.30.0000. A Minitab software version
16.1.0.0 (Minitab Inc., USA) was
used for running the response surface and ANOVA tests. Tukey’s test was used for comparing the means
of the pistachio paste samples (P 0.85).

Although the effects of
roasting conditions on the quality of the pistachio nuts have been studied by
Kahyaoglu in 2008, he focused on modeling and optimizing roasting of the Kirmizi
Turkish variety of the pistachio nuts for direct consumption of whole roasted
nuts. The aim of this study was to find the optimum roasting condition for the production
of pistachio paste from the Ohadi variety of pistachio nuts, which is more popular
in the world. In addition, this study recommends the best roasting temperature
and time for the production of pistachio paste, while Kahyaoglu did not suggest
the optimal roasting conditions.

 

4.5. Effect of particle size on colloidal
stability of pistachio paste

In 1994, Barnes reported that decreasing particle size improves the
dispersion of solid phase in oil phase and subsequently increases the
cohesiveness of the final product and finally resulted in a more stable
colloid. On the other hand, no significant difference (p >
0.05) in colloidal stability was observed when the mill gap size was above 20
µm (40, 60 and 80 µm),
due to the narrow range of size among the paste particles. In 1987, Matsunobu et
al. claimed that the instability of almond paste was observed only when the particle
size was larger than 105 µm (). The
findings obtained in the present study clearly indicated that the particle size
of pistachio paste influenced the colloid stability.

 

 4.6
Effect of particle size on the color of pistachio paste

In 2008, The Ciftci Group reported no significant effect of particle size
on the color of sesame paste. However, other parameters such as the presence of
pigments such as green pigments and chlorophyll can influence the color of
pistachio paste. The paste color can also be affected by the roasting process (15).
However, in this study, the roasting conditions of all samples were the same,
and no significant difference (p

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