Characterization of peanuts after dry roasting, oil roasting, and blister frying

Abstract

Peanuts were systematically deep fried, blister fried, or dry roasted at 177 °C to Hunter L-values of 53.0 ± 1.0, 48.5 ± 1.0, and 43.0 ± 1.0, corresponding to light, medium, and dark roasting, respectively. Thermal modifications of the epidermal and parenchyma cells were observed in the scanning electron microscopic images for processed peanuts, compared to raw peanuts. Peanut microstructure was most extensively damaged by blister frying, followed by deep frying, and then dry roasting. The moisture content decreased with increased surface color, due to more moisture loss with longer heat processing time. For light roasting, blister fried peanuts had significantly higher moisture contents than the deep fried and dry roasted peanuts, while for medium and dark roasting, blister fried had lower moistures than the other two. Descriptive sensory analysis was able to distinguish the flavor and texture profiles of peanuts prepared by different roasting methods. In storage testing throughout 16 weeks, peroxide value measurements indicated the blister fried peanuts had the longest shelf life, followed by the dry roasted, and then the deep fried. Descriptive sensory analysis proved that the rate of the loss of roast peanut flavor during storage was faster in dry roasted peanuts followed by blister fried and deep fried.

Full text

Characterization of peanuts after dry roasting, oil roasting, and blister
frying
Xiaolei Shi
a
, Jack P. Davis
b
,
1
, Zhoutong Xia
a
, K.P. Sandeep
a
, Timothy H. Sanders
b
,
Lisa O. Dean
b
,
*
a
Department of Food, Bioprocessing and Nutrition Sciences, North Carolina State University, Schaub Hall, Raleigh, NC 27695, USA
b
USDA, Agricultural Research Service, Market Quality and Handling Research Unit, 236 Schaub Hall, North Carolina State University, Raleigh, NC 27695,
USA
article info
Article history:
Received 27 June 2016
Received in revised form
3 September 2016
Accepted 27 September 2016
Available online 28 September 2016
Keywords:
Peanut
Oil roasting
Dry roasting
Microstructure
Shelf life
abstract
Peanuts were systematically deep fried, blister fried, or dry roasted at 177

C to Hunter L-values of
53.0 ±1.0, 48.5 ±1.0, and 43.0 ±1.0, corresponding to light, medium, and dark roasting, respectively.
Thermal modifications of the epidermal and parenchyma cells were observed in the scanning electron
microscopic images for processed peanuts, compared to raw peanuts. Peanut microstructure was most
extensively damaged by blister frying, followed by deep frying, and then dry roasting. The moisture
content decreased with increased surface color, due to more moisture loss with longer heat processing
time. For light roasting, blister fried peanuts had significantly higher moisture contents than the deep
fried and dry roasted peanuts, while for medium and dark roasting, blister fried had lower moistures
than the other two. Descriptive sensory analysis was able to distinguish the flavor and texture profiles of
peanuts prepared by different roasting methods. In storage testing throughout 16 weeks, peroxide value
measurements indicated the blister fried peanuts had the longest shelf life, followed by the dry roasted,
and then the deep fried. Descriptive sensory analysis proved that the rate of the loss of roast peanut
flavor during storage was faster in dry roasted peanuts followed by blister fried and deep fried.
Published by Elsevier Ltd.
1. Introduction
In the U.S.A., the majority of peanuts grown are converted into
value-added products utilizing the entire seed, such as peanut
butter, confections, and snack products. For such purposes, peanut
kernels are usually thermally processed as the first step in the
manufacture of the final products to achieve a specificflavor, color,
and texture (Perren &Escher, 2013). Peanuts are processed typi-
cally by dry roasting or oil roasting, and to a lower extent by boiling,
microwave heating, or a combination of multiple processing
methods (Woodroof, 1983; Young, Schadel, &Heertje, 1993). Dry
roasting is performed by heating the food material using hot air
without the use of oil or water as a carrier. The most commonly
used oil roasting methods for peanuts are deep frying and blister
frying. Blister frying has not been scientifically defined but ac-
cording to cooking instructions, this process involves the steps of
boiling blanched peanuts in water for a certain time, draining the
excess water, and then deep frying the soaked kernels in vegetable
oil, resulting in a highly crispy, highly crunchy snack with blisters
on the kernel surface (Miyagi, 2013). Although there are several
commercial products prepared by dry roasting, deep frying, and
blister frying, the scientific comparison of different roasting
methods has not been reported.
Roasting is defined as the heat treatment at temperatures above
125

C, at which non-enzymatic reactions occur to form pigments
with specific yellow-brown color (Kleinert, 1966). During roasting,
color has been extensively used as a quick and non-destructive
indicator of food quality for certain foods, especially for roasted
coffee beans, hazelnuts, almonds, and peanuts (Baggenstoss,
Poisson, Kaegi, Perren, &Escher, 2008; Kaftan, 2012; Ozdemir
et al., 2001; Pattee, Giesbrecht, &Young, 1991). Different temper-
ature and roasting time combinations were able to achieve the
equivalent peanut surface colors (McDaniel, White, Dean, Sanders,
&Davis, 2012; Smith, Perry, Marshall, Yousef, &Barringer, 2014).
McDaniel et al. (2012) found that, at a given color, moisture con-
tents decreased with decreasing roast temperatures due to the
*Corresponding author.
E-mail address: Lisa.Dean@ars.usda.gov (L.O. Dean).
1
Current address: JLA Global International, Albany, GA, 31721, USA.
Contents lists available at ScienceDirect
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journal homepage: www.elsevier.com/locate/lwt
http://dx.doi.org/10.1016/j.lwt.2016.09.030
0023-6438/Published by Elsevier Ltd.
LWT - Food Science and Technology 75 (2017) 520e528

longer roast times required for the same color formation. It was also
found that peanuts roasted at lower temperatures had higher
tocopherol contents. Another study investigating oven, microwave,
and combination roasting suggested that significant differences
were observed with flavor attributes for peanuts roasted to
equivalent colors by different roasting methods; however, no sig-
nificant differences were found in free fatty acid or peroxide values
(Smith et al., 2014). Also, different roasting methods may cause
different types of and/or alter the extent of thermal modifications
(e.g. cell wall rupture, protein body distension, and cytoplasmic
network disruption) to the microstructure of peanut kernels
(Young et al., 1993).
The method of roasting is known to affect the physical, chemical,
sensory, as well as the storing properties of roasted peanuts. As oil
roasting is a faster roasting process than dry roasting, the best way
to compare different roasting methods should be based on the
concept of equivalent color roasting instead of equivalent roasting
time. The process of blister frying involves an initial soak of the raw
peanuts in water which could result in loss of soluble protein and
sugars which are involved in peanut texture and roasted flavor. The
objective of this study was to systematically compare the effects of
different roasting methods on the peanut quality related proper-
ties, including moisture content, nutritional content, microstruc-
ture, and sensory properties, as well as storability, at equivalent
surface colors in order to identify what qualities can be controlled
in order to produce high quality peanut products.
2. Materials and methods
2.1. Materials
Jumbo grade size peanuts (>21/64 on a slotted screen) of the
Georgia 06G cultivar, a high-yielding, large-seed, runner-type va-
riety, were obtained during the 2013 harvest from the USDA ARS
National Peanut Laboratory (Dawson, GA, USA). The peanuts had
been cured, shelled, sized using standard screens, blanched and
stored utilizing standard industry practices prior to delivery to the
USDA ARS Market Quality &Handling Research Unit at North Car-
olina State University (Raleigh, NC, USA). A pilot plant scale roaster
previously described (Poirier, Sanders, &Davis, 2014) built by the
Bühler Aeroglide Corporation (Cary, NC, USA) with an adjustable air
flow rate (up to 1.27 m/s), air flow direction, bed depth, and tem-
perature control (up to 204

C) was used for the dry roasting. The
air flow direction was changed from up-flow to down-flow at the
half point of the roasting time. Following roasting, the roasting tray
containing the peanuts was placed onto a forced air blower for
cooling to ambient temperature. A pilot plant scale fryer (Vulcan-
Hart, Baltimore, MD, USA) holding 16 L of pure peanut oil (Ventura,
Brea, CA, USA) was used for deep frying and blister frying. Following
frying, the peanuts were spread onto a fine mesh steel screen with a
cooling fan installed above for cooling to ambient temperature. The
batch size for deep frying and blister frying was 2 kg of peanuts per
batch.
The peanuts were dry roasted, deep fried, or blister fried at
177

C to three equivalent surface colors (Light, L ¼53.0 ±1.0,
Medium, L ¼48.5 ±1.0, and Dark, L ¼43.0 ±1.0) as determined by a
Hunter Lab Model D25 colorimeter (Hunter Lab Associates, Reston,
VA, USA). Processing times were determined using the linear
regression on the preliminary curves of the Hunter L-values versus
roasting time. The time used for each treatment was summarized in
Table 1. Blister frying was conducted by immersing the peanuts in
boiling water for 10 min, followed by deep frying. After cooling, the
roasted peanuts were placed into glass jars, and stored frozen
(26

C) until further analysis.
2.2. Color distribution
Single seed color of 100 blanched kernels for each treatment
was measured using the Hunter Lab scale with a Model DP-301
Chroma Meter (Minolta Camera Co., Ltd. Japan). The percentage
of the single seed color within an interval of L ¼2.5 was calculated
by JMP Pro 10.0 (SAS, Cary, NC, USA) and plotted in lightness (L)
distribution charts (Fig. 1).
2.3. Protein content of rinse water
A CE Instruments NA 2100 Protein Analyzer (Thermo Finnigan,
Milan, Italy) was used for analysis of total nitrogen using combus-
tion analysis and thermal conductivity detection (TCD). The appa-
ratus was equipped with a Model AS128 auto-sampler, a
combustion oven containing the oxidation catalyst and a reduction
oven containing copper, traps for carbon dioxide and water con-
taining soda lime and anhydrous magnesium perchlorate, respec-
tively, and a GC column packed with active carbon and a thermal
conductivity detector. The TCD temperature was set at 60

C and
the pressure was 1200 Pa. An analytical portion (100 mg) of
phenylalanine was used for calibration. A portion of the rinse water
(500
m
L) used to soak the peanuts prior to blister frying was added
to metal capsules and loaded into the autosampler for nitrogen
measurement.
2.4. Sugar content of rinse water
The rinse water used to soak the peanuts prior to blister frying
was analyzed for sugar content using a Dionex BioLC HPLC system
(Dionex Corporation, Sunnyvale, CA, USA) at a controlled temper-
ature of 25

C(Pattee, Isleib, Giesbrecht, &McFeeters, 2000). The
system consisted of a gradient pump, an autosampler, and a Pulsed
Amperometric Detector (PAD). The column used was a Dionex PA-1,
250 mm length and 4 mm i.d., fitted with a Dionex PA-1 Guard
column. Aliquots of the rinse water were spiked with an internal
standard solution containing lactose and cellobiose (Sigma-Aldrich,
St. Louis, MO, USA). The solutions were passed through a syringe
fitted with a Dionex OnGuard
®
II H Filter into autosampler vials. An
external standard solution was prepared containing myo-inositol,
glucose, fructose, sucrose, raffinose, stachyose and the internal
standards. Sugars were identified through comparisons of retention
time of unknown samples to known standards. Sugar contents
were calculated from the chromatogram peak heights relative to
the internal standards. All sugar standards were purchased from
Sigma-Aldrich (St. Louis, MO, USA).
2.5. Moisture content
Whole peanut seeds from each treatment were analyzed in
triplicate using a forced air oven at 130

C for 6 h to determine the
moisture content (MC) (Young et al., 1982). The weight differences
before and after oven incubation were compared to dried mass for
MC.
Table 1
Roasting time used for each treatment to achieve equivalent color roasting of light,
medium, and dark at average surface Hunter L values of 53.0 ±1.0, 48.5 ±1.0, and
43.0 ±1.0, respectively.
Deep fry Blister fry Dry roast
Light 1.3 3.0 11.9
Medium 1.6 3.5 14.0
Dark 2.0 4.3 17.0
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528 521

2.6. Microstructure
Individual peanut seeds were cut with a clean razor blade to
create either a 1e1.5 mm thick cross-section profile of 1/4 of the
seed or trimmed to provide a 2e3mm
2
surface profile. Approxi-
mately 8 pieces per sample were placed into a solution of cold 3
mL/100 mL glutaraldehyde in 0.05 mol/L KPO
4
buffer (pH 7.0) and
stored for approximately 5 d at 4

C. Samples were rinsed in three
1 h changes of 0.05 mol/L KPO
4
buffer (pH 7.0) followed by 24 h
changes of 30, 50, 70, 95 and 100 mL/100 mL ethanol/water, while
cooling in an ice bath. The samples were then warmed to room
temperature, and dehydration was completed using two additional
24 h room temperature changes of pure ethanol. All samples were
critical point dried for 15 min in liquid CO
2
(Tousimis Samdri-795,
Tousimis Research Corporation, Rockville, MD, USA). Samples were
mounted on stubs with double-stick tape and silver paint, sputter
coated with approximately 50 Ågold-palladium (Hummer 6.2
sputtering system, Anatech U.S.A., Union City CA, USA) and stored
in a vacuum desiccator. Samples were viewed at 15 kV using a JEOL
JSM-5900LV scanning electron microscope (JEOL U.S.A., Peabody,
MA, USA).
2.7. Descriptive sensory analysis
Descriptive sensory panels were conducted at the USDA ARS
Market Quality and Handling Research Unit (Raleigh, NC, USA).
Sensory analysis was conducted every 4 wk until 16 wk after
roasting. Prior to testing, samples were equilibrated to ambient
room temperature. Duplicates of the roasted samples were evalu-
ated by well-trained panelists (n >6) in randomized order. A
peanut lexicon based on Johnsen, Civille, Vercellotti, Sanders, and
Dus (1988) with modifications by Sanders, Vercellotti, Crippen,
and Civille (1989) and Schirack, Drake, Sanders, and Sandeep
(2006) was used. The texture lexicon, containing terms of crispy,
crunchy, hardness, and breakdown was built by the panel with
terms commonly used to describe the texture of oilseed and nuts
(Civille, Lapsley, Huang, Yada, &Seltsam, 2010). Crispy was
described as the amplitude of the high-pitched sound associated
with mastication with the incisors. Crunchy was the amplitude of
the low-pitched sound associated with mastication with the mo-
lars. Hardness was the amount of force required to break through
the sample with the molars. Breakdown was the ease of which the
sample was broken as it was chewed with the molars. The Spec-
trum™Method (Sensory Spectrum, Inc., Chatham, NJ, USA) was
used for the training and evaluation sessions.
2.8. Peroxide value
Peroxide values (PV) were determined using AOAC method
965.33 (AOAC., 1990). In brief, peanuts were coarsely ground and
compressed using a Carver Model 3912 hydraulic press (Carver, Inc.,
Wabash, IN, USA) to express the oil. Oil aliquots of 5.0 0 ±0.05 g
were weighed into 250 mL flasks and dissolved using 30 mL of
acetic acid-chloroform solvent mixture (3:2 v/v). Then, 0.5 mL
saturated KI was added into the flask. After 1 min of shaking, 30 mL
H
2
O was added, followed by the addition of 0.5 mL starch solution
(1 g/100 mL water) as indicator. The solution was titrated by 0.01 or
0.001 N Na
2
S
2
O
3
until blue color just disappeared. The PVs were
calculated by the equation of PV ¼SN1000/W, where S is the
volume of Na
2
S
2
O
3
used for titration, N is the normality of Na
2
S
2
O
3
,
W is the oil mass in g. All chemicals and reagents used were pur-
chased from Thermo Fisher Scientific (Fairlawn, NJ, USA).
Fig. 1. Single seed color distribution of deep fried, blister fried, and dry roasted peanuts at equivalent light, medium, and dark surface colors.
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528522

2.9. Statistical analysis
Results from triplicate experiments were analyzed using JMP
PRO 10.0 (SAS, Cary, NC, USA). Factorial analysis of variance
(ANOVA) and least-squares means (LSMEANs) student's t-test was
conducted on the color distribution, MC, and sugars. One-way
ANOVA was performed on the results of the sensory analysis. A
significance standard of
a
¼0.05 was used.
3. Result and discussion
3.1. Lightness (L) distribution
The color distribution of 100 blanched kernels prepared by each
roasting method at equivalent light medium and dark surface
colors is represented in Fig. 1. Each block in this figure represents
the probability that the surface color of a single seed is within the
corresponding color range. The probability distribution was fitted
as a normal distribution. Statistical analysis indicated that there
was no significant evidence of an interaction between roasting
method and the surface color. Under the experimental conditions
presented here, the degree of roast significantly contributed to the
different Hunter L-values of blanched peanut kernels (p <0.001).
For each roast degree, there was no evidence that one roasting
method was significantly different in color from another roasting
method. Also, the distribution dispersion (
s
) was close for all of the
roasts, indicating the uniformity of roasting could not be
differentiated.
3.2. Microstructure
The scanning electron microscopic (SEM) images of the raw
peanut outer rounded surface showed that it is composed of well
aligned rectangular epidermal cells (Fig. 2. A). This image was very
similar to the one previously published (Young &Schadel, 1990).
The outer region of the cross section of the raw peanut kernel was
seen to be composed of a single layer of epidermal cells [E] and
parenchyma cells [P] (Fig. 2. B). The granules inside the parenchyma
cells are lipid bodies, protein bodies, and starch grains. The size of
protein bodies has been reported to be 5e12
m
g in diameter and the
starch grains, 4e15
m
g in diameter (Young &Schadel, 1990). The
lipid bodies are reported to be smaller with a diameter about
1e2
m
g(Yatsu, 1981). The membrane of the granules formed a
continuous, dense cytoplasmic network. In this study, most of the
oil was removed during the fixation step, therefore, the large
granules seen were assumed to be starch and protein bodies,
although they were not distinguishable by SEM (Young &Schadel,
1991).
The SEM images of the outer surfaces of the roasted peanut
kernels are compared in Fig. 3. Damage to the kernel surface was
observed in the images of the deep fried peanuts. The damaged
region exposed the inner parenchyma cells. With the increase in
color intensity due to longer roasting times, almost all cytoplasm
was lost from the cells. For the blister fried peanuts, there was more
extensive damage to the layer of epidermal cells. In those damaged
regions, almost all cells were lost from the cytoplasm. The rinse
water from soaking the peanuts prior to blister frying was found to
have a protein content of 2.52 ±0.03 mg/mL and a total sugar
content of 3.98 ±0.26 mg/mL. This represented losses of 2.5 g of the
protein in 100 g of the whole raw peanuts and 4.0 g of sugars from
100 g of raw peanuts to the water used to presoak the peanuts.
Although the protein loss was minimal (<10%), most of the free
sugars were lost which was expected to have some effect on the
flavor. In the dry roasted peanuts, no obvious epidermal damage
was observed, but with the increase in the color intensity due to the
longer roasting time, there was a higher degree of deformation of
the epidermal cells from the well-aligned rectangle to an irregular
shape. As for the outer region of the cross section of peanut kernels
(Fig. 4), there was no obvious difference among the different
roasting colors for each roasting method, although the darker dry
roasted peanuts showed a higher degree of cell wall rupture. When
compared across the different roasting methods, the deep fried and
the dry roasted kernels showed comparable changes to the cell
structures, mainly in cytoplasmic network disruption. Comparable
changes in the peanut microstructure during oil and dry roasting
have been reported (Young &Schadel, 1990; Young et al., 1993). An
extensively damaged cell structure was observed for blister frying,
resulting in a considerable number of empty cells depleted of
granules. The blister fried peanuts also showed cell wall rupture
and disruption of the cytoplasmic network.
3.3. Moisture effects
The moisture contents (MC) of the raw and roasted peanuts are
displayed in Fig. 5. The MC decreased from the initial value of
5.66 g/100 g based on dry weight in the raw peanuts to final values
of 0.85e1.64 g/100 g in the roasted peanuts. The MC decreased with
the increased surface color, due to the increasing moisture losses
with longer heat processing time (McDaniel et al., 2012). The initial
MC of peanuts prepared for blister frying was the highest as the
peanuts were immersed in boiling water prior to deep frying. This
also resulted in the blister fried peanuts of the light roasting
Fig. 2. Scanning electron micrograph of unroasted peanut. Figure (A) is the scanning electron micrograph of epidermal cells on the raised outer rounded surface of a raw peanut
cotyledon. Figure (B) represents the scanning electron micrograph of the cross section of outer surface of epidermal cells (Arrow E) and subtending parenchyma cells (Arrow P) of a
raw peanut cotyledon.
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528 523

intensity having a higher MC than the deep fried and the dry
roasted peanuts. With the extended processing time of blister
frying, the water release from the kernels was seen to cause more
extensive damage to the cell structures (Fig. 3). The damaged areas
of the epidermal cells was attributed to this additional moisture
loss and resulted in a lower MC in both the medium and dark
colored blister fried peanuts.
3.4. Descriptive sensory analysis
Radar plots were used to describe the sensory properties of the
processed peanuts (Figs. 6 and 7). The flavor profiles included the
attributes of roasted peanutty (RP), sweet aromatic, dark roast, raw
beany, woody, sweet, bitter, and astringency. The panelists were not
able to distinguish differences in the astringency of the samples. For
the other attributes, when the peanuts were dry roasted or fried to
the light color intensity, the deep fried and the blister fried peanuts
had similar flavor profiles, though the blister fried peanuts had
higher RP and dark roast flavors. The dry roasted peanuts had
higher dark roast flavor intensities, lower raw beany, and higher
woody and bitterness. When the peanuts were medium roasted or
fried, the dry roasted peanuts also showed higher dark roast in-
tensity and lower raw beany flavors, as well as higher bitterness.
When the peanuts were dark roasted or fried, the flavor profiles for
all three roasting methods were similar to each other. Across the
three roasting colors, RP tended to increase with the increase of
roast color intensity from light to dark, but the positive attribute of
sweet aromatic had no significant increase.
The texture profiles were composed of the attributes of crispy,
crunchy, hard, and breakdown (Fig. 7). The texture profile patterns
were similar across the three roast intensities of light, medium, and
dark. In general, the blister fried peanuts were found to have the
highest crispiness, crunchiness, hardness, and the lowest break-
down. The deep fried peanuts were found to have higher crispiness
and hardness values than the dry roasted peanuts. The highest
breakdown was observed in the dry roasted peanuts at the light
roast intensity, but there were no significant differences in the
breakdown of the deep fried and dry roasted peanuts at the me-
dium and dark roast intensities.
3.5. Peroxide value
Peroxide value (PV) is the measure of the concentration of
peroxides and hydroperoxides formed in the initial stages of lipid
oxidation. The end of the shelf life was considered to be the time at
which the PV reached 10 meq/kg of oil. The raw peanuts stored
under the same conditions as the roasted samples had the lowest
PV throughout the 16 wk of storage and remained below 5 meq/kg
oil (data not shown). For each roasting method, there was a trend
that the lighter roasted or fried peanuts had higher PV than the
darker intensities, especially during the first 12 wk of storage
(Fig. 8). This indicated that the dark roasted peanuts were less
prone to lipid oxidation. This is explained by a hypothesis that there
may be more antioxidants formed during thermal processing for
Fig. 3. Scanning electron micrograph of deep fried, blister fried, and dry roasted peanut cotyledons of light, medium, and dark surface colors on the outer rounded surface. Images
were taken at locations with damaged epidermal cells for deep fried and blister fried peanuts. Arrow [B] is the bodies of protein or starch; arrow [cp] is the cytoplasm network;
arrow [v] is the emptied space; arrow [w] is the cell wall; arrow [G] is the guard cell.
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528524

darker samples that protected the samples from oxidation
(McDaniel et al., 2012). The PV increased steeply in the deep fried
peanuts for the first eight wk, then decreased at a similar rate from
wk 8 to wk 12, and then showed minor increases from wk 12 to wk
16. As PV measures only the primary products formed in the initial
stages of lipid oxidation, the decreases after 8 wk were attributed to
the decay of primary products in the latter stages of lipid oxidation.
As the first measurement during the storage test was made at wk 4
Fig. 4. Scanning electron micrograph of deep fried, blister fried, and dry roasted peanut cotyledons of light, medium, and dark surface colors at the cross section. Arrow [cp] is the
cytoplasm network; arrow [v] is the emptied space; arrow [E] is the epidermal cells; arrow [s] is the space between parenchyma and cell wall in parenchyma cells.
Fig. 5. Moisture content of blanched peanuts as a function of roasting method.
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528 525

and at this time point the PV was already much higher than
10 meq/kg oil, the shelf life for the deep fried peanuts was deter-
mined to be 1 wk using the linear regression of PV from the data
achieved from wk 0 to wk 4. For the blister fried peanuts, PV
remained lower than 20 meq/kg oil for the entire testing period,
indicating they were less prone to lipid oxidation than the deep
fried and dry roasted peanuts. The PV of the light and medium
blister fried peanuts exceeded 10 meq/kg oil at wk 4, but the dark
blister fried peanuts remained less than 10 meq/kg oil for the first
12 wk. For the dry roasted peanuts, the PV increased progressively
for the initial 4 wk, but decreased slightly for medium and dark dry
roasted samples. As with the deep fried peanuts, the slight decrease
was attributed to the decay of primary products during the latter
stages of lipid oxidation.
3.6. Flavor stability
The flavor of the roasted peanut samples was not found to be as
sensitive as PV as a measure of storage life. The flavor changes were
not reported until the PV reached 30e40 meq/kg oil. For the
roasted peanutty (RP) flavor attribute, all the samples showed
progressive decreases throughout the 16 wk, regardless of the
roasting method and color intensities (Fig. 9). Although the PV of
the blister fried peanuts remained at a low level, the RP decreased
significantly for the light and medium color intensity blister fried
peanuts. The decrease in the RP of the dark blister fried peanuts was
minor until 12 wk. Another positive attribute, sweet aromatic
remained stable throughout the 16 wk of storage and showed a
small increase for the blister fried peanuts from 4 wk to 12 wk
(Fig. 9). Two negative attributes highly associated with the lipid
Fig. 6. Flavor profile analysis of light roasted peanuts prepared by deep frying, blister frying, and dry roasting.
Fig. 7. Texture profile analysis of dark roasted peanuts prepared by deep frying, blister frying, and dry roasting.
Fig. 8. Peroxide values of deep fried, blister fried, and dry roasted peanuts at light, medium, and dark surface colors during storage at ambient room temperature.
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528526

oxidation during storage are cardboardy and painty. Similar pat-
terns were observed from the curves of the cardboardy and painty
attributes as a function of storage time (Fig. 10). The general in-
tensity ranking of the cardboardy attribute for each time point was
dry roast >blister fry >deep fry. It was not until 12 wk that there
was a significant increase in the cardboardy intensity. As for the
painty attribute, the intensity remained low for the blister fried and
the deep fried peanuts. For the dry roasted peanuts, the intensity of
the painty attribute remained at a low level for the first 8 wk of
storage. The light intensity dry roasted peanuts showed the largest
increase of the painty attribute from the 8 wk point, while the
medium and dark intensity of the dry roasted peanuts had the
largest increase in this attribute at 12 wk of storage.
Fig. 9. Sensory intensities of roasted peanutty and sweet aromatic flavor attributes in deep fried, blister fried, and dry roasted peanuts during storage at ambient room temperature.
Fig. 10. Sensory intensities of cardboardy and painty flavor attributes in deep fried, blister fried, and dry roasted peanuts during storage at ambient room temperatures.
X. Shi et al. / LWT - Food Science and Technology 75 (2017) 520e528 527

4. Conclusion
Peanut roasting for consumer products is most commonly done
by using either a dry roasting or an oil roasting process. These
methods can achieve similar color in the final products by adjusting
time and/or temperature. Dry roasting produced the least surface
damage and blister frying resulted in the greatest damage with
subsequent removal of nutrients, especially sugars. Flavors, tex-
tures and shelf life, as defined by the onset of off flavors or flavor
fade of roasted peanut flavor, will vary with the roasting method
and with the final roast color. As the roast color became darker,
there was less loss of RP flavor with storage time. Oil roasted pea-
nuts were found to have faster onset of rancidity measured
chemically (PV) due to the increased surface oil. The dry roasted
were found to have higher levels of off flavors as the MC were
higher which plays a role in other types of lipid oxidation. The
textures of the roasted products were also affected by MC with the
oil roasted types having harder textures and lower MC. Type of
roasting, final moisture, degree of roast and cellular damage were
all found to play a role in the final characteristics of roasted peanuts.
Acknowledgements
Commercial, or not-for-profit sectors.
Mention of trade names or commercial products is solely for the
purpose of providing specific information and does not imply
recommendation or endorsement by the U.S. Department of
Agriculture.
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