'Schley', 'Desirable', 'Mohawk' and 'Shawnee' derived little benefit from storage in the unshelled state with regard to the inhibition of kemel color alteration.
Reprinted from Vol. 103(3), January 1978 Journal of the American Society for Horticultural Science
Mount Vernon, Virginia 22121, USA
J. Amer. Soc. Hort. Set 103(1): 137-141. 1978.
Genotype Variation in Pecan Kernel Color and Color Stability during Storage i S. J. Kays^ and D. M. Wilson
Departments of Horticulture and Plant Pathology, University of Georgia. Coastal Plain Experiment Station, Tifton, OA 31794 Additional index words. Carya illinoensis, pigmentation
Abstract. Substantial quantitative differences in kernel pigmentation of pecan {Carya illinoensis (Wang) K. Koch) were measured in 8 cultivars and 1 selection. These pecans exhibit a wide range in color stability with storage
and reversibUity of detrimental color changes using surface pH alteration. Major differences were found between genotypes in the positive benefit from storage in the unshelled versus the shelled state. Shelled pecans in the U.S. are grouped into various USDA
nounced effect on yield and net profit. While a number of
Cultivar is one of the more obvious of the factors affecting
factors (e.g. yield, kernel size, percent shell-out, precociousness, disease resistance, etc.), are normally considered when selecting a cultivar, kernel pigmentation is generally not a major consid eration due, primarily, to the absence of sufficient information
color. The wide variation in what is visually perceived as kernel color of the many commercially grown cultivars has been
on either the possible qualitative variation between cultivars or
color grades (4) which are used in part to characterize the
kernels general condition. A number of both pre- and post-
harvest parameters can alter the color of the pecan kernel (3).
known
for
some
time.
'
Cultivar selection for new plantings or for top-working
existing plantings is of critical importance, because of the large capital investment. Proper cultivar selection has a pro 1
Received
for
publication
June
on the cultivars currently available. Little has been documented
the relative resistance of these cultivars of deleterious color
changes during storage. As a consequence, the following in vestigation was undertaken to determine the relative suscepti bility of several pecan genotypes to postharvest changes in color quality. 3,1977.
,
Materials and Methods
.r,
11
>
toi.
2present address: Department of Horticulture, University of Georgia, Samples of Desirable, Sc ®y • 'Cherokee' Athens
GA 30602
'Mohawk',
48-15-3,
J. Amer. Soc. Hort. Sci. 103(1):137-141. 1978.
'Chickasaw,
Caddo,
and
Cherokee
137
pecans were harvested when the shuck (involucre) had dried sufficiently to allow the nut to be shaken free. Harvested
nuts were dried in a forced air drier (360C) for 24 hr and then
stored in open trays either shelled or in the shell (ovary wall) at ambient room temp (24—26°). Kernel color was visually rated initially and after 2, 4, 8, and 12 weeks of storage on replicated samples containing a miminum of 12 kemels using a 1 to 10 color rating system (1 = bright yellow; 10 = near black). A sample representing each numerical classification was selected and used to grade each experiment. The reversi
bility of color changes occurring during storage was measured
by treating kemels held 12 weeks with 0.5% SO2 for 6 hr
following wetting. The kemels were then dried (36°, 24 hr) and
ranking at harvest. These cultivars were followed closely by 'Shawnee' and 'Caddo'. 'Mohawk' had the darkest kemels
at harvest and the lowest color quality ranking after 12 weeks storage unshelled. 'Mohawk', '48-15-2' and 'Shawnee' displayed the lowest
change in color over 12 weeks of storage for both the shelled
and unshelled kemels (Fig. 1). 'Schley', 'Cherokee', 'Caddo', and 'Stuart' exhibited relatively high rates of color change, with 'Chickasaw' and 'Desirable' in the intermediate range. Measurements of relative concn of extractables using peak ht/g conversions of peaks from the liquid chromatograph detected at 254 nm, 270 nm, and with a broad bandpass filter from 325-385 nm showed few distinct differences between
regraded on a 1 to 10 color scale.
genotypes in relation to changes occurring during storage (Table 1). 'Cherokee' followed by 'Desirable' and 'Mohawk'
weeks storage was coarsely ground and defatted with ethyl ether by covering and stirring for 30 min, filtering, then cover
displayed the greatest changes in total (composite of 3
A 50g sample of each genotype taken initially and after 12
ing and stirring for an additional 30 min with hexane. The above samples were filtered and dried, finely ground and extracted twice with ether and hexane, after which they were stirred with 150 ml of methanol for 2 hr and filtered. These samples were then stirred for 2 hr with 150 ml of methanol containing
1% H3PO4, and filtered, followed by stirring for 2 hr with methanol + 1.5 n aqueous H3PO4 (85 + 15) and again filtering. The filtrates were stored at 0°C under N2 before injection
into the liquid chromatograph. Ten n\ portions of the filtrates were passed through a polyvic 0.6 /um millipore filter before injection into the liquid chromatograph. Dilutions in the ex
solvents) absorption and fluorescence with storage; however, these changes, when considered in relation to all genotypes, did not consistently reflect the degree of color change with visual rating. The selection 48-15-3, which produced the lowest visual color change between initial reading and storage 12 weeks unshelled, displayed a virtual absence of change in absorbance at 254 and 270 nm. On the other hand, 'Schley' and 'Chicka saw', which also had only small changes in absorbance at 254 and 270 nm with storage, underwent a substantial change in visual rating.
Fig. 2 illustrates linear regressions between the initial visual
kemel color rating and initial absorbance per cm2 at 254 and
traction solvent were made when necessary.
270 nm and after 12 weeks storage. After 12 weeks of storage,
Liquid chromatography was accomplished using a Waters 202/401 liquid chromatograph equipped with a 254 nm detector and a M-6000 pump. The detectors included a Dupont model
the regression plot shifts upward, indicative of darkened kemel color. Correlation values (254 nm = .93 and 270 nm = .90) decreased to .78 (both wave lengths) after 12 weeks storage.
836 fluorescence/absorbance detector with excitation/absorb-
Adequate correlations were not found between the final visual rating and absorbance or absorbance or fluorescence at the
ance from 325-385 nm and emission for fluorescence above 541
nm, and a Varian Variscan detector set a 270 nm near an
absorption maximum (Band 11) of many flavanoid compounds (1). The flow sequence from the column was to the model 836,
then to the Variscan, then to the 254 nm detector. All values were normalized on a peak ht/g basis with the same aufs or units of fluorescence for each detector. The column, 2 mm ID x 61 cm, was packed with Corasil II, 37-50 fim. The solvent was 0.5%
H3PO4 in methanfol (5 ml H3PO4 + 1000 ml methanol) with
a flow rate of 1 ml/min. This solvent was selected to give a single peak at the void volume. The peak represents the ab-
other wave lengths measured. The partial reversal of detrimental color changes occurring
during storage (12 weeks) of shelled and unshelled kemels of each genotype is shown in Table 2. 'Schley' and 'Caddo' produced the greatest changes in their visual color rating than did the other genotypes tested. 'Chickasaw', 'Desirable', and 'Mohawk' produced the lowest total change. Storage conditions
(shelled or unshelled) did not appear to consistently affect the degree of reversibility. Unshelled 'Stuart' kemels exhibited a greater color reversal than did shelled, while shelled 'Cherokee'
sorbance or fluorescence of the mixture that eluted at the void
kemels were substantially higher in their degree of reversibility
volume; there was little separation using this system. Absorbance or fluorescence values (peak height/gm) were
than unshelled.
also expressed as peak ht/cm2 of surface area. Representative samples of each genotype had the pigmented surface tissue surgically, removed with a razor blade and photo-copied on
preweighed paper. The area was then measured both with a
planimeter and by weight. Surface area Was then plotted against kemel length x width x thickness measured with a micrometer. This allowed for the calculation of the mean surface area/kernel based on a relatively large sample size, i.e., 25 kernels.
Results
'Stuart', 'Cherokee', 'Chickasaw', 48-15-3 and 'Caddo' pecans stored as unshelled nuts, had a subltantially less darken ing of the testa than when stored in the shelled form (Fig. 1). The rate of color change of shelled kemels was typically greater for these cultivars during the first 4 weeks of storage, after which (4 through 12 weeks) the rate of detrimental change declined. 'Schley', 'Desirable', 'Mohawk' and 'Shawnee' derived little benefit from storage in the unshelled state with regard to the inhibition of kemel color alteration. Of the genotypes studied, 'Schley' and 'Stuart' had the highest initial color 138
Discussion
The results presented illustrate a relatively wide range be
tween pecan genotypes in initial color and stability of this color to detrimental changes with storage. Significant quantita tive and possible qualitative differences in pigmentation be tween genotypes were observed. This is based on the extremely wide range in color reversibility from surface pH alteration and
quantitative differences in absorbance. In addition, the geno types displayed major differences in the positive benefit, with regard to detrimental color changes, derived from storage in the shelled versus the unshelled state.
Comparison of differences between genotypes with a visual grading system assumes a linear gradient between each numeric
level, e.g. the degree of color difference between rating 1 and
2 is equivalent to the degree of difference between grade 6 and
7. It is our opinion that this is not a serious factor in this study since virtually all of genotypes underwent color changes over predominently the same numerical range. In addition, absorb ance of extractables at 254 and 270 nm correlated well with
the initial visual rating of the kemels. The decreased correla-
J. Amer. Soc. Hort. Sci. 103(1):137—141. 1978.
STUART
CHEROKEE
CHICKASAW
SCHLEY
DESIRABLE
48-15-3
/
SHAWNEE
MOHAWK
CADDO
6.0 h
024
8
12
024
8
12
024
8
12
LENGTH OF STORAGE Shelled Unshelled
(WEEKS)
Fig. 1. The effect of pecan genotype and storage of nuts shelled versus unshelled in the change in visual color rating over a 12 week storage period at 23.9 - 26.7°C (75 - 80°F).
tion of absorbance with the visual ratings after storage may be
vs. 5 to 4 also assumes linearity. The color reversal by SO2
due to qualitative pigment changes occurring and/or non-
may be indicative of: (a) the degree of surface pH shift during storage, (b) qualitative and/or quantitative differences in the levels of pH sensitive pigment(s), (c) interference from adjacent pigments (e.g. masking), or (d) combination of any or all of
linearity at the darker end of the visual color scale.
Fluorescence or absorption of extracts containing some of the flavonoid pigments represent only a portion of the total pigmentation of the kernels; also, the extracts contain other compounds that are not pigments that may change with storage,.
these factors. With these possible limitations in mind, based on the rather large magnitude of difference in color reversibility
Comparison of the degree of color reversibility between cultivars is open to greater criticism since a color shift from 8 to 7
substantial difference among genotypes.
J. Amer. Soc. Hort. Sci. 103(1): 137-141. 1978.
between genotypes, we feel that the data are indicative of a
139
Table 1. Fluorescence (excitation 325-385 nm, emission above 451 nm) and absorbance at various wave lengths of pecan kernel extractables at harvest and after 12 week storage. Absorbance
Cultivar
Treatment
MEOH
Solvent^ 1%
nm
270 nm
Solvent 1%
Solvent 1%
254
Fluorescence Solvent 15-85
MEOH
1%
15-85
0.60
5.29
15-85
Peak wtig 3.89 5.91
4.09 4.31
3.50 6.31
5.61
1.54
3.45
9.51
2.55
5.32
1.20
4 . 3 2
1.60
3.50
4.02 4.70
0.95
6.51
6.92 7.39
10.19 8.58
1.86 2.19
4.55 5.59
7.19 6.09
1.36 1.65
6.89 7.48
8.10
10.17
1.92 3.09
4.73 4.99
5.77
1.84
1.48 2.19
6.27 1.89
0.55 7.47
10.97 10.76
2.53 2.24
6.54 6.96
7.56
10.46
7.27
1.75 1.64
4.98 4.07
3.77 3.29
2.06 8i07
14.26
11 . 3 2 13.66
2.91 3.29
10.04 10.97
8.32 9.77
2.23 2.49
1.64 1.68
1.87 2.47
1.40 5.14
7.30 10.09
6.08 6.43
7.39 8.91
2.10 2.72
4.49 4.65
5.42 6.43
1.59 1.98
5.20 5.77
1.16 1.54
2.24 2.19
1.43 1.89
3.63 0.94
6.19 6.56
6.55 6.77
1.57 1.64
4.66 5.17
5.02 4.98
1.17 1.24
0.67 1.07
0.86 0.81
0.19 0.30
0.37 0.49
0.27 0.25
0.10 0.21
0.84
1.96
1.02 0.96
0.36 0.25
0.62 0.94
0.65 0.69
0.25 0.19
0.75 I.3I
1.23 1.74
0.31 0.15
0.59 0.64
0.37 0.82
0.09 0.08
0.89 1.67
1.42 2.44
0.36 0.63
0.64 1.15
1.02 1.73
0.28 0.46
0.87 0.81
0.78 I.I4
0.24 0.28
0.33 0.39
0.22 0.31
0.09 0.17
1.07
Stored
0.88
0.98 1.20
0.22 0.29
0.79 0.64
0.74 0.86
0.17 0.23
Initial Stored
0.75 0.80
1.06 0.96
0.20 0.21
0.28 0.37
0.48 0.32
0.09 0.09
1.01 1.08
1.49
0.27 0.32
0.67 0.82
1.05 0.C2
0.20 0.24
Initial Stored
0.81 I.II
0.93 1.57
0.33 0.25
0.27 0.41
0.21 0.34
0.12 0.30
1.13
1.33 1.66
0.31 0.51
0.77 0.82
0.94 1.16
0.24
1.12
Initial Stored
1.10
1.25 1.05
0.20 0.36
0.52
I.2I
0.48
1.05 0.32
0.09 1.25
1.63 1.75
1.83 1.80
0.42 0.37
1.09 1.16
1.26 1.21
0.29 0.27
Initial
1.52
1.38
0.68 0.59
0.53 0.59
1.50
1.46
2.29 2.57
1.81
2.00
0.90 0.73
2.04
1.85
0.37 0.27
0.37
Stored
1.98
1.76
0.40 0.45
Initial
0.70 0.84
0.83 1.09
0.27 0.28
0.31
0.23
1.21
1.01
1.23
0.41
0.86
1.68
1.07
1.48
0.35 0.45
0.75 0.77
0.90 1.07
0.26 0.33
Initial Stored
4.19 6.71
5.39 5.II
1.20 1.90
2.30 3 . 11
1.70
1.30
7.92
6.39 6.01
2.25
1.60
Initial Stored
4.15
6.75 9.61
1.70
3.25
2.02
0.49
1.99
3.51
4.51
0.45
4.88 9.21
7.80
0.85
13.42
Initial Stored
4.72
4.22 6.21
1.30
1.80
1.21
0 . 5 0
1.50
2.10
1.70
0.90
5.83 4.80
Initial Stored
5.10
7.28 6.59
1.36 1.44
1.91 2.50
3.28 2.20
0.59
5.49
Initial Stored
4.97 6.78
5.69 9.57
2.00 1.54
1.68 2.49
1.28 2.09
0.76
Initial Stored
6.64 7.27
7.46 6.27
1.20 2.14
3.13 2.88
Initial Stored
8.41 10.27
7.64 11 . 0 7
2.06 1.47
Shawnee
Initial Stored
4.21 5.05
4.96 6.53
Caddo
Initial Stored
4.75 4.97
Stuart
Initial Stored
Cherokee
Initial
Stuart
Cherokee
Schley
Chickasaw
Desirable
48-15-3
M o h a w k
7.21
4.40
0.60
9.77
12.70
1.60
7.08
1.60 1.20
1.25
Peak htlcrn^
Stored I
Schley
Chickasaw
Desirable
48-15-^
M o h a w k
Shawnee
Initial
Stored
1.25
2.47
0.36
ZRefer to text for solvent
The data indicate that certain distinct quality advantages stable pigment complex. The availability of relatively large
may be derived by tailoring the postharvest handling technique areas of specific cultivars in monoculture in the major proused for bulk lots of nuts of a specific cultivar. For example, duction regions lends itself to this possibility, cultivars with low color stability should be processed and The effect of the shell in relation to retention of optimum stored under refrigerated conditions prior to those with a more color may center around differences between genotypes in 140
J. Amer. Soc. Hort. Sci. 103(1): 137—141. 1978.
Table 2. Change in color rating after SO2 treatment of pecan kernels which had been stored 12 weeks either sheUed or unshelled.
=====
Treatment Stuart Cherokee Schley Chickasaw Desirable 48-15-3 Mohawk Shawnee Caddo SheUed UnsheUed
FINAL
diffusion resistance to oxygen. Data from our laboratory indicate that while oxygen partial pressure does not appear
/
to be a major factor in the induction of pigmentation (1),
r = 0.78
it is of paramount importance in post-harvest color changes (2). Variations in susceptibility to oxygen among genotypes
may as well be an important factor. It should be noted that this work was directed exclusively
at postharvest color and color changes, and this parameter alone may not accurately characterize the kernel quality when
A /final /initial /| / r=0.78/ / r»0.90 /
/
270
nm
//X
considered in toto. Oxidative changes in kernel lipids resulting
in off-flavor may not adequately correlate with kernel color
changes. This reiterates the question often posed: Is color alone an adequate measure of over-all biochemical quality of the
nut?
1
,
.
The magnitude of importance placed on kernel color m
ascertaining market value and the relatively wide range in color and color stability among genotypes we have demonstrated
points toward the need for a better understandmg of the chemistry of pecan pigmentation. Potential genetic exploi
tation of these differences in the gene pool for kernel color
' / / /
#
' ' V
and color stability await the identification and character
ization of the specific pigment molecules comprising kernel c o l o r.
Literature Cited
1. Kays, S. J. 1977. Effect of the nut's internal oxygen partial pressure
on the induction of pigmentation in the kernels of pecan, Carya
illinoensis (Wang.) K. Koch./, Amer. Soc. Hort. Sci. 102(5).531-533.
2. and D. M. WUson. 1977. Effect of subatmospheric Initial Visual Color Roting
Fig. 2. Linear regression and correlation between the initial visual color rating and total absorbance/cm^ at 254 and 270 nm (composite
of solvents) for aU pecan genotypes taken initially and after 12 weeks storage (unsheUed).
J. Amer. Soc. Hort. Sci. 103(1):137-141. 1978.
pressure and oxygen partial pressure on the color stability of stored pecan kernels, Lebensin.-Wiss. u.-Technol. 10:109-110.
3' South a n d(3(5):471-473. 1976. Pecan kernel color. Pecan
4. U. S. Dept. Agriculture. 1969. United States standards for grades of shelled pecans. Consumer & Mkt. Serv.
141
