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BREAD BAKING PROCESS ENERGY REQUIREMENTS AS
AFFECTED BY OVEN BELT SPEED AND TYPE OF BREADS
Islam F. El-Adly*, Adel H. Bahnasawy**,
Samir A. Ali** and El-Sayed G. Khater***
Agricultural and Biosystems Engineering Department – Faculty of
Agriculture – Benha University, P.O. Box 13736, Egypt
ABSTRACT
The main aim of the present work is to study and evaluate
baking process energy requirements which considered the most
consumable energy of bread baking stages. This was achieved by
determining the moisture content, baking time, productivity and three
types of energy (electrical, human and thermal) at four different belt
speeds for two different types of baladi-breads, namely Magr and
Mawi. Those four speedswere1.18, 1.97, 2.40 and3.55 ms-1. The
results show that initial moisture content of dough was 42.12% for
Magr but 62.02% for Mawi, while after baking it were 24.32, 24.61,
26.09 and 29.25% for Magr and 34.25, 39.50, 40.98 and 41.66% for
Mawi at each speed, respectively. The results also indicated that the
average baking time were 1.65, 1.10, 0.86 and 0.81minkg-
1andproductivity were 36.54, 54.63, 70.11and73.80 kghr-1 for Magr
Baladi bread while the average baking time were 1.87, 1.13, 0.89and
0.84minkg-1andproductivity were 32.62, 53.10, 67.48and 71.33 kghr-1
for Mawi baladi-bread at each speed, respectively. The specific energy
requirements consumed were3.57, 2.92, 2.54 and 1.93 kWhkg-1 for
Magr, while it were 4.35, 3.54, 3.11 and 2.53 kWhkg-1 for Mawi bread
at speeds 1.18,1.97, 2.40 and 3.55 ms-1, respectively. The results also
indicated that the total costs of baking stage per 1kg of bread baking
stage were 1.14, 0.86, 0.71 and 0.59 LE kg-1 for Magr while it were
1.34, 0.98, 0.82and 0.71LE kg-1 for Mawi bread, respectively at the
same speeds.
Keywords: Energy Requirements, Baking time, Productivity, Moisture Content,
Baladi-Bread
* Administrator of Agric. Eng. Dep., Fac. of Agric., Benha Univ., Egypt
** Professor. of Agric. Eng., Fac. of Agric., Benha Univ., Egypt
*** Lecturer. of Agric. Eng., Fac. of Agric., Benha Univ., Egypt
1. INTRODUCTION
nergy sources include oil, electricity and woodchip burners.
Previous studies in the baking industry estimate that the
specific energy consumption of a bread oven is typically
anywhere between 0.5and 7.3 MJkg-1 production depending on
specific products and operating conditions. In this sense, baking is
similar to (conventional) drying, both demanding a high amount of
energy in comparison with chilling, freezing, and canning, which need
less than 1 MJkg-1(Le Bail et al., 2010 and Purlis, 2012).
Thermal treatment of food targets two key objectives: cooking
and safety. Bread baking and drying are similar in terms of energy
demand with around 5 MJkg-1 in the case of bread baking (Dinçer,
1997; Fellows, 1996).
An experiment was carried out to evaluate energy consumption
in different types of bread baking (thermal, human and electrical). The
thermal energy represented the most energy consumed, where, it
represented 98.38-98.54% of the total energy consumed in bread
baking stages. Human energy represented from 0.18–0.22% and
electrical energy represented from 1.24-1.42% of the total energy
consumed in bread baking. The total costs of different types of baked
bread were 2.32, 1.76 and 4.80 LE kg-1 for Magr baladi, Mawi baladi
and French breads, respectively(El-Adlyet al., 2015; Khater and
Bahnasawy, 2014).
Jekayinfa (2007) revealed that bread-baking with wood as
energy source required the highest energy (6.15 kJmin-1) compared
with 3.37 kJmin-1 and 1.52 kJmin-1 obtained with gas and electricity as
sources of energy respectively. The cost of energy per kg of baked
bread was N7.58 ($ 0.059) with cooking gas as the energy source
followed by N6.05 ($ 0.047) for electricity and N5.05 ($ 0.04) for
wood in that order. The average baking rate using firewood, gas and
electricity as energy sources were 11.92 kgh-1, 17.97 kgh-1 and 20.58
kgh-1 respectively.
Baking is an energy-intensive process due to water evaporation
occurring in the product (latent heat of water vaporization is 2.257
MJkg-1 at100 °C). The energy demand for a conventional baking
E
process is around 3.7 MJkg-1, though it can be higher (up to 7 MJkg-1)
depending on specific products and operating conditions. In this sense,
baking is similar to (conventional) drying, both demanding a high
amount of energy in comparison with chilling, freezing, and canning,
which need less than 1 MJkg-1(Le Bail et al., 2010).
There are approximately 18,000 baladi bakeries in Egypt. The
Egyptian Ministry of Finance estimates that approximately 12-13
billion LE is spent per year on subsidizing baladi bread and flour
(World Bank, 2010).
Most ovens use gas or diesel. There is a two-part fuel subsidy
system: a standard subsidy and a special bakery subsidy. The latter is
used to help ensure that the bakeries can make a profit, given that the
price of a loaf of baladi bread has remained fixed at 5 piasters for 17
years. If the second subsidy was not in place, the government would
need to either increase the price of bread or decrease the price of flour
to achieve the same result (World Bank, 2010).
Studying the energy requirements of baladi–bread is very vital
in baking industry, which suffers of the imprecision determinations of
the costs required for bread-production. Moreover, there was an
increase in energy costs which requires searching on the best way to
save energy during bread-baking process stage.Also to obtain a
reliable database regarding the baking energy at different oven
operational parameters.Based on the results obtained by El-Adlyet
al.(2015) which concluded that baking stage was the most consumable
energy in bread baking,therefore, the main aim of the present work is
to study and evaluate energy requirementsbaking stage at different
oven belt speeds anddifferent types ofbaladi-breads which considered
the common types in Egypt especially in villages.
2. MATERIALS AND METHODS.
The experiment was carried out at a local bakery oven,
Moshtohor, Toukh, Kalubia Governorate, Egypt, during the season of
2016 to determine the energy requirements of baking stage of two
different types of baladi-breads, namely, Magr and Mawi at different
belt speeds.
2.1. Materials:
2.1.1. Ingredients used in baladi-bread
The bread ingredients of these types are shown in table (1).
Table (1):Bread ingredients of two types of baladi-breads.
Ingredients
Baladi bread
Magr
Mawi
Flourkg)
50
50
Water(litter)
33
75
Yeast(g)
400
400
Salt(g)
400
400
Samples were prepared using a standard recipe for French
bread: wheat flour (100%), water (54.1%), salt (1.6%), sugar (1.6%),
margarine (1.6%), and dry yeast (1.2%). Dough was made by mixing
the ingredients for 10 min in a home multi-function food processor at
constant speed. Then individual samples of 100–150 g (cylindrical
shape, ca. 0.15 m length, 0.04 m diameter) were formed and placed in
a perforated tray. After 1.5 h proving at ambient temperature, samples
duplicated their volume (Purliset al., 2009) .
Water and flour are the most significant ingredients in a bread
recipe, as they affect texture and crumb the most. Flour (14.5%
moisture, 13% protein, 0.55% ash, pH 5.7– 6.1, Zanoniet al.,1993) is
always 100%, and the rest of the ingredients are a percent of that
amount by weight. Approximately 50% water results in a finely
textured, light bread. Most artisan bread formulas contain anywhere
from 60% to 75% water. In yeast breads, the higher water percentages
result in more CO2 bubbles, and a coarser bread crumb. According to
100% flour rest of the ingredients will be in following measurements
like leavening agent yeast2%, sugar 4%, salt 2% and shortening agent
(ghee or mar-garine) 3% (Mondal and Datta,2008).
2.1.2. Description of oven components of baking stage:
Fig. () shows the oven components which consists of the belt,
motor, inverter, frontal fuel tank, frontal nozzel and burner, chimney,
oven wall, rearward fuel tank, and rearward nozzel and burner. The
belt is driven by 1.5hp motor and having a gear box to control the belt
speed depending on the load and the output of the inverter. The belt
feeding rate was 50 loaves. The dimensions of the belt are 5*0.83 m
for length and width. The oven dimensions are 5*2*1.77 m for length,
height and width.
Dimensionsin mm
Figure (1): Schematic diagram of the oven components of the bakery
2.1.3.Measuring devices and tools:
The following measuring devices were used in this study:
Inverter was used to control the electricity input of the belt
motor (model IP65 (IEC-60529) NEMA-4 and 230v 50 Hz
phase output 0- v 3phase 5hp/4kW – KWAIT).
The clamp meter was used to determine the power
requirement (kW) by recording the voltage and current
strength (Model DT266 - Measuring range 200/1000A and
750/1000V with an accuracy of ± 0.01, China)to measure the
line current strength (I) and the potential difference value (V).
Mobile stopwatch with 1/100 s accuracy was used to record
the time spent during baking process.
A measuring cylinder for quantifying the amount of fuel
consumed during bread-baking operations.
Graduated flask to make calibration for fuel.
Digital balance.
2.2. Methods:
2.2.1. Power requirements determination:
The energy requirements of two types of baladi-bread baking
namely, Magr and Mawi bread were determined at belt speeds of 1.18,
1.97, 2.40 and 3.55 ms-1.There are three types of power requirements,
namely thermal energy, electrical energy and human energy. Fuel
consumption was determined by using a graduated flask at speed
treatments. Each treatment required 50 kg flour to obtain around 600
loaves. Recording the voltage and current strength to measure
electrical energy. Recording number of persons to measure human
energy. Moisture content, time, productivity of bread and power
requirements were determined for each treatment. Each treatment was
replicated 3 times and the average was taken.
2.3. Measurements and determinations:
2.3.1. Machine productivity:
It is the product mass/time, kg/h.
2.3.2. Power requirements:
The total power requirement (electrical, human and thermal)
for oven component was calculated for the production of finished
bread baking for two types of baladi-bread baking. The procedures
used could be explained as follows:
Electrical power requirement was estimated from the measured
electric current and voltage values and estimated according to Kurt
(l979) as follows:
(1)
1000 cos3
VI
Ep
Where:
Ep is the electrical energy, kW
I is the electric current, Amperes.
is the mechanical efficiency assumed to be
0.95(Metwally, 2010).
V is the electrical voltage, V
cosφ is the power factor being equal to 0.84
According to Odigboh (1997), at the maximum continuous
energy consumption rate of 0.30 kW and conversion efficiency of
25%, the physical power output of a normal human labor in tropical
climates is approximately 0.075 kW sustained for an 8–10 h workday.
This was calculated mathematically as:
(2) 075.0 NEm
Where:
Em is the human power, kW
N is the number of persons involved in an operation.
Thermal power requirement was estimated from equation:
(3) hvmET
Where: Et is the thermal energy, kW
m is the mass flow rate, kg s-1
hv is the heating value of disel, 42000 kJ kg-1(Shahinet
al., 2008)
The specific energy consumption was estimated by using the
following equation:
2.3.3. Statistical analysis:
The statistical analysis for the data obtained was done
according to Snedecor and Cochran (1980) and the treatments were
compared using Least Significant Differences (LSD) test at 95%
confidence level (Gomez, 1984).
2.3.4. Total operation costs:
Hourly cost is calculated according to the equation that is
given by Awady (1978) as follows:
) (5
k
m
ew
2
1
rt
i
ah
p
C
Where:
C is the hourly cost, LE h-1
p is the price of the equipment, LE
h is the year by working hours, h
a is the life expected of the machine, year
I is the Interest rate, %
t is the taxes and over heads ratio, %
r is the repair and maintenance ratio, %
w is the power of motor in, kW
e is the electricity cost, LEkW-1 h-1
m is the operator monthly salary, LE.
K is the monthly average working hours.
Cost inputs are listed in table (2).
Table (2): Cost inputs.
Items
Oven baking
Price of equipment, LE.
24000
Motor, kW
1.5
Life expected, year
10
Taxes, %
3
Repair, %
10
Interest, %
10
Labors, LE h-1
10
3. RESULTS AND DISCUSSIONS
This work focus on the effect of oven belt speeds on the
baking energy requirements of two types of baladi-bread baking.
Moisture content of bread, baking time, productivity and power
requirement were determined at different belt speeds.
3.1. Moisture content
Table (3) and fig. (2) show the average moisture content of
two types of baladi-bread (Magr and Mawi) that baked at different
belt speeds. The results indicated that the average moisture content of
bread increased with increasing speed of belt, where it increased from
24.32 to 29.25% when the belt speed increased from 1.18 to 3.55 ms-1
for Magr bread. Meanwhile, it increased from 34.25to 41.66%when
the belt speed increased from 1.18 to 3.55 ms-1 for Mawi bread.
The results indicated that the Mawi bread had more moisture
content compared to Magr bread, where it was 34.25% compared to
24.32% at 1.18 ms-1 belt speed, 39.50%compared to 24.61% at
1.97ms-1 belt speed, 40.98% compared to 26.09% at 2.40ms-1 belt
speed and 41.66% compared to 29.25% at 3.55ms-1 belt speed which
could be attributed to the initial moisture content of dough was
42.12% for Magr bread but was 62.02% for Mawi bread.
The results indicated that the average moisture content ranged
from24.32 to 29.25 and 34.25 to 41.66% for Magr and Mawi breads,
respectively.
Table (3): Effect of belt speed on the average moisture content of two
different types of baladi-breads.
Speed of belt,
m s-1
Average moisture content after baking,(%)
Magr
Mawi
1.18
24.32
34.25
1.97
24.61
39.50
2.40
26.09
40.98
3.55
29.25
41.66
Figure (2): Effect of speed of belt on moisture content of two different
types of baladi-breads.
Regression analysis was carried out to find a relationship
between belt speed (1.18-3.55ms-1) and moisture content of two
different types of bread. The most appropriate forms obtained were as
follows:
M.C1 = 22.96(SB) 0.165 for Magr bread R² = 0.808 (6)
M.C2 = 33.99(SB) 0.183 for Mawi bread R² = 0.886 (7)
Where M.C1is the moisture content of Magr bread,%
M.C2 is the moisture content of Mawi bread, %
SB is the baking belt speed (1.18-3.55), m/s
-
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
45.00
- 1.00 2.00 3.00 4.00
Moisture content, %
Belt Speed, m s-1
Magr Mawi
3.2. Baking time:
Table (4) and fig (3) show the baking time of two types of
baladi-bread (Magr and Mawi) at different ovens belt speeds. It could
be seen that the time of baking for one kg of bread decreased with
increasing speed of belt, where it decreased from 1.65 to 0.81min and
1.87 to 0.84minfor Magr and Mawi bread, respectively when the belt
speed increased from 1.18 to 3.55 ms-1.
The results indicated that the Mawi bread recorded higher time
for baking compared to Magr bread, where it takes 1.87 minkg-1
compared to 1.65 minkg-1 at 1.18 ms-1 belt speed, which could be
attributed to the higher initial moisture content of Mawi bread
meanwhile, it takes nearly the same time at 3.55 ms-1 belt speed.
The results show that the time of baking at lower speed (1.18
ms-1) was 2 times of that required at the higher speed (2.40 and 3.55
ms-1),which means using the higher speed could increase the bread
productivity.
Table (4): Effect of speed of belt on baking time of two different types
of baladi-bread
The statistical analysis showed that there were non-significant
differences between speed 1.18 and 1.97 ms-1 treatments and the
differences between 2.40 and 3.55 ms-1 were non-significant, while
differences between speed 1.18, 1.97 ms-1 and 2.40, 3.55 ms-1 were
significant for Magr, but there were significant differences between
speed (1.18 and 1.97 ms-1), (1.18 and 2.40 ms-1), (1.97and 2.40 ms-1)
and (1.18 and 3.55 ms-1), while it there were non-significant
differences between speed ( 2.40 and 3.55ms-1) for Mawi bread.
Belt speed,
m s-1
Average baking time,minkg-1
Magr
Mawi
1.18
1.65a
1.87 a
1.97
1.10a
1.13 b
2.40
0.86b
0.89c
3.55
0.81b
0.84 c
LSD at 0.05
0.06960
0.1476
It is worthy to mention that using 2.40 ms-1 belt speed gave
almost the same result of using 3.55 ms-1belt speed in terms of baking
time where indicate that using 2.40 ms-1 belt speed is preferred to save
energy.
Figure (3): Effect of speed of belt on baking time of two different
types of baladi-breads.
Regression analysis was carried out to find a relationship
between belt speed (1.18-3.55ms-1) and time of baking of two
different types of breads. The most appropriate forms obtained were
as follows:
TB1 = 1.753(SB)-0.67 for Magr bread R² = 0.915 (8)
TB2 = 1.979(SB)-0.76 for Mawi bread R² = 0.908 (9)
Where:
TB1 is the baking time of Magr bread, minkg-1
TB2 is the baking time of Mawi bread, minkg-1
SB is the baking belt speed (1.18-3.55), ms-1
3.3. Productivity
Table (5) and fig. (4) show the average oven productivity of
two types of baladi-breads (Magr and Mawi) at different belt speeds.
It could be seen, the average productivity of the oven increased with
-
0.50
1.00
1.50
2.00
- 1.00 2.00 3.00 4.00
Baking Time, min kg-1
Belt Speed, m s-1
Magr Mawi
increasing belt speed, where it increased from 36.54 to 73.80 kgh-
1and32.62to 71.33 kgh-1when the belt speed increased from 1.18 to
3.55 ms-1 for Magr and Mawi breads, respectively.
The results indicated that the oven productivity of Magr bread
was higher than that of the Mawi productivity, where it was 36.54,
54.63, 70.11 and 73.80 kgh-1 for Magr compared to 32.62, 53.10,
67.48 and 71.33 kgh-1 for Mawi at 1.18, 1.97, 2.40 and 3.55 ms-1 belt
speed, respectively.
The results show that productivity at higher speed (3.55 ms-1)
was twice of that produced at the lower speed (1.18 ms-1),which
means using the higher speed could increase the bread productivity.
Table (5): Effect of speed of belt on the average oven productivity of two
different types of baladi-breads.
The statistical analysis showed that there were significant
differences between all speeds for both Magr and Mawi bread. It
could be concluded that using the highest speed gave the highest
production where indicate that using 3.55 ms-1 belt speed is preferred
to get on save energy high production.
Belt speed ,
m s-1
Average productivity,kgh-1
Magr
Mawi
1.18
36.54d
32.62 d
1.97
54.63 c
53.10 c
2.40
70.11b
67.48 b
3.55
73.80a
71.33 a
LSD at 0.05
1.968
2.771
Figure (4): Effect of speed of belt on average productivity of two
different types of baladi-bread.
Regression analysis was carried out to find a relationship
between belt speed (1.18-3.55ms-1) and average productivity of two
different types of bread. The most appropriate forms obtained were as
follows:
Pro1= 34.32 (SB) 0.672 for Magr bread R² = 0.917(10)
Pro2 = 30.83 (SB) 0.743 for Mawi bread R² = 0.910(11)
Where:
Pro1 is the productivity of Magr bread, kgh-1
Pro2 is the productivity of Mawi bread, kgh-1
SB is the baking belt speed (1.18-3.55), ms-1
3.4. Energy requirements for baking stage:
There are three types of energy requirements which included
electrical, human and thermal energies as shown in table (6) and fig
(5), where it shows the specific energy requirements in baking stage of
two different types of baladi bread at different belt speeds. It could be
seen that the specific energy consumed in bread baking stage were
3.57, 2.92, 2.54 and 1.93 kWhkg-1 for Magr at speeds 1.18, 1.97, 2.40
and 3.55 ms-1, while they were4.35, 3.54, 3.11 and 2.53 kWhkg-1for
Mawi bread at the same speeds, respectively. The highest values of
-
10.00
20.00
30.00
40.00
50.00
60.00
70.00
80.00
- 1.00 2.00 3.00 4.00
Productivity, kg h-1
Belt Speed, m s-1
Magr Mawi
specific energy consumed (3.57 and4.35 kWhkg-1) were found at1.18
ms-1 belt speed for Magr and Mawi bread .These results agreed with
those obtained by Grönroos et al. (2006) whose found that the energy
consumption for rye bread (organic and conventional) were 3.72and
4.28 kWh kg-1of bread, respectively, meanwhile the lowest values of
energy consumed (1.93 and 2.53 kWhkg-1) were found at 3.55 ms-
1belt speed for Magr and Mawi, respectively. These results also agreed
with those obtained by Le Bail et al. (2010)whose found that the
energy consumption of a bread is typically anywhere between 0.5 and
7.3 MJ kg-1 production (0.14 and 2.044 kWh kg-1) and it can be higher
up to 7 MJ kg-1(2.044 kWh kg-1) depending on specific products and
operating conditions.
Table (6): The specific energy requirements for baking stage of two
different types of baladi-bread at different belt speeds.
Belt speed,
m s-1
The specific energy requirement for baking
stage,kWhkg-1
Magr
Mawi
.18
3.57
4.35
1.97
2.92
3.54
2.40
2.54
3.11
3.55
1.93
2.53
Fig. (5): The specific energy requirements for baking stage of two
different types of baladi-breads at different belt speeds.
Regression analysis was carried out to find a relationship
between belt speed (1.18-3.55ms-1) and energy requirements for
baking stage of two different types of baladi-breads. The most
appropriate forms obtained were as follows:
SER1= 4.049 (SB)-0.55 for Magr breadR²=0.973 (12)
SER2 = 4.798(SB)-0.49 for Mawi breadR² = 0.990(13)
Where:
SER1 is the specific energy requirements of Magr bread, kWhkg-1
SER2 is the specific energy requirements of Mawi bread, kWhkg-1
SB is the baking belt speed (1.18-3.55), ms-1
3.5. Total costs for bread baking:
Table (7) and fig. (6) show the estimated costs of two different
types of bread baking. It could be seen that the total costs for baking
stagewere1.14, 0.86, 0.71 and 0.59 LE kg-1 for Magr bread compared
with 1.34, 0.98, 0.82 and 0.71 LE kg-1 for Mawi baladi bread at speeds
1.18, 1.97, 2.40 and 3.55 ms-1, respectively.
The results indicated that the Mawi bread recorded higher cost
for baking stage compared to Magr bread, where it takes 1.34 LEkg-
1compared to 1.14 LEkg-1 at 1.18 ms-1 belt speed, which could be
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 1 2 3 4
Specific Energy
Requirements, kWhkg-1
Belt Speed, m s-1
Magr Mawi
attributed to the higher initial moisture content of Mawi bread which
took longer time.
The results show that the costs of baking stage at lower speed
(1.18 ms-1) was 2 times of that required at the higher speed (2.40 and
3.55 ms-1), which means using the higher speed could decrease the
baking costs.
Table (7): Comparison between total costs of baking stage of two
different types of baladi-bread.
Fig. (6): Comparison between total costs of baking stage of two
different types of baladi-bread.
Regression analysis was carried out to find a relationship
between belt speed (1.18-3.55ms-1) and costs of baking stage of two
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 1 2 3 4
Total Cost Of Baking Stage, LE kg-1
Belt Speed, m s
-1
Magr Mawi
Speed of belt,
ms-1
Costs of baking stage,LEkg-1
Magr
Mawi
1.18
1.14
1.34
1.97
0.86
0.98
2.40
0.71
0.82
3.55
0.59
0.71
different types of baladi-breads. The most appropriate forms obtained
were as follows:
T.C1= 1.262(SB)-0.61 for Magr bread R²=0.988(14)
T.C2 = 1.454(SB)-0.59 for Mawi bread R² = 0.980(15)
Where:
T.C1 is the total cost of baking stage of Magr bread, LE kg-1
T.C2 is the specific energy requirements of Mawi bread, LE kg-1
SB is the baking belt speed (1.18-3.55), m s-1
4. CONCLUSION
This study successfully investigated the energy requirements
of baking two types of baladi-breads, namely, Magr and Mawi at
different belt speeds (1.18, 1.97, 2.40 and 3.55 ms-1). The study results
concluded that the specific energy consumed in bread baking
were3.57, 2.92, 2.54 and 1.93 kWhkg-1 at belt speeds of 1.18,1.97,
2.40 and 3.55 ms-1, respectively for Magr bread, while it were 4.35,
3.54, 3.11 and 2.53 kWhkg-1 for Mawi bread at the same speeds,
respectively. Costs study revealed that baking stage costs per 1kg of
bread baking stage ranged from0.59-1.34LE kg-1 depending on type of
bread and belt speed.
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El-Adly, I.F., Khater, E.G., Bahnasawy, A.H., Ali,
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December 2015:pp 535-554.
Fellows, P.J. 1996.Food Processing Technology Principles and
Practice. Springer, Cambridge (Chapters 1:54–58, 15:314–
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Gomez, K.A. 1984. Statistical procedures for agricultural research, 2nd
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