Chapter 19
Effect of Storage on Roasted Peanut Quality Descriptive Sensory Analysis and Gas Chromatographic Techniques 1
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Karen L. Bett and T. D. Boylston
Agricultural Research Service, U.S. Department of Agriculture, Southern Regional Research Center, 1100 Robert E. Lee Boulevard, New Orleans, LA 70124
Flavor of roasted peanuts, as well as the profile of volatile compounds change during storage. Florunner peanuts from two crop years were sorted by commercial grade sizes and roasted. After roasting, peanuts were stored in open containers at 37°C until sampling for a maximum of 12 weeks. Six flavor descriptors (roasted peanutty, sweet aromatic, cardboardy, painty, fermented/fruity, and woody/hulls/skins) were evaluated for intensity. Volatile compounds were analyzed during the 12 weeks using headspace analysis by gas chromatography with trapping on Tenax adsorbent. Lipid oxidation descriptors, such as painty and cardboardy flavor intensities, as well as lipid oxidation products, such as hexanal, octanal and 2-octanone increased during storage. In contrast, roasted peanutty flavor intensity decreased during storage along with the alkylpyrazines. Commercial seed size affected the concentration of carbonyl compounds, but not the alkylpyrazines. Roasted peanut flavor is composed of a complex blend of heterocyclic and other volatile compounds formed during roasting through thermal degradation reactions, including Maillard reactions between carbohydrate, free amino acid and protein (7,2). The alkylpyrazines, which have nutty flavor characteristics, are predominant compounds in the volatile profile of roasted peanuts. Lipid oxidation during the storage of peanuts and other lipid-containing foods has long been recognized as contributing to the development of undesirable flavors in these foods (3). These reactions lead indirectly to the formation of numerous aliphatic aldehydes, ketones, and alcohols with "cardboardy" or "painty" flavor characteristics (4). The free-radicals and hydroperoxides formed during the autoxidation of lipids may also interact with other components of the food system, 1
Current address: Washington State University, Department of Food Science and Nutrition, Pullman, WA 99164
This chapter not subject to U.S. copyright Published 1992 American Chemical Society St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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19. BETT & BOYLSTON
Effect of Storage on Roasted Peanut Quality
323
including amino acids, proteins, and other nitrogen-containing compounds, and further affect overall flavor quality (5). Differences in the seed size and maturity of peanuts have contributed to variability in the content and composition of free amino acids, sugars (6,7), lipids (£), roasting characteristics and flavor quality (7,9). Seed size and maturity are related based on work of Sanders et al. (9) that showed a decrease in the percentage of immature peanuts in a commercial size class as the size of the peanuts increases. The potential for desirable roasted peanutty flavor increases and the formation of undesirable off-flavors decreases as seed size increases and/or peanuts mature (9). The objectives of this study were to determine the effect of elevated storage temperature on the flavor and volatile compounds, determined through sensory evaluation and instrumental analysis, of roasted peanuts. This study was designed as a model to further understand the relationships between the sensory and instrumental analysis of flavor quality of roasted peanuts. Methods for Evaluating Flavor and Flavor Compounds Preparation of Samples. Florunner peanuts from two crop years (1988 and 1989) were sorted by commercial grade sizes into #l's which rode a 5.95mm screen and fell through a 7.14mm screen and medium/jumbos which rode a 7.15mm diameter screen. The jumbos and mediums were not separated because Sanders et al. (9) did not find significant differences between the two commercial seed sizes, and combining the two resulted in a larger sample to work with. The four different treatment combinations (two crop years and two commercial grade sizes) of peanuts were placed uncovered in separate glass 190 x 100 evaporation dishes and placed in a 37°C oven for storage. Peanuts were stirred every two weeks. Samples were taken at 0, 4, 8, and 12 weeks for sensory analysis and 0, 2, 4, 6, 8, and 12 weeks for gas chromatographic (GC) analysis. The peanuts were reduced to a paste according to Sanders et al. (10) to assure a uniform sample for G C and sensory purposes. Volatile Flavor Compound Analysis. The volatile flavor compounds were isolated from the peanut butter samples using headspace analysis techniques with trapping on Tenax-GC adsorbent (Boylston, T.D. and Vinyard, B.T. / . Food ScL, in press.). The purge (water bath) temperature was 90°C and volatiles were collected for 4 nr. Tetradecane (25 ug) was included as quantification standards with the sample prior to purging and pentadecane (5 ug) was placed on the top of the adsorbent prior to elution of volatiles. The volatiles were eluted from the adsorbent with 15 ml H P L C grade hexane and concentrated to 100 pi. The volatile flavor compounds were separated on a cross-linked, 5% phenylmethyl silicone fused silica capillary column (HP-5, 50 m, 0.32 mm od, 0.52 u film thickness, Hewlett-Packard) installed in a gas chromatograph equipped with a flame ionization detector (Model 5890A, Hewlett-Packard). The G C oven temperature was initially held at 35°C for 15 min, then increased at a rate of 2°C/min to a final temperature of 250°C and held for 45 min. Injector and detector temperatures were set at 200°C and 250°C, respectively. The extracts (2.5 pi) were injected using a splitless injection, with a column flow rate of 1.1 ml/min and a purge flow rate of 2.0 ml/min. Contents of the volatile compounds were quantified based on standard curves for the quantification standard, pentadecane (20 to 500 ng).
St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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LIPID OXIDATION IN FOOD
Identification of the volatile compounds in the samples was based on the comparison to retention times and mass spectra of standards and mass spectra published in the literature (Boylston, T.D. and Vinyard, B.T. / . Food Sci., in press.). A gas chromatograph-quadrupole mass spectrometer (Model 4500, Finnigan-MAT) interfaced with an Incos data system was used for confirmation of the identity of the volatile compounds in the extracts. G C conditions were as for the chromatographic analysis. The conditions for the mass spectrometer were set as follows: ionizing voltage, 70 eV; emission current, 0.3 mA; electron multiplier voltage, 1800 kV; ion source temperature, 150°C; ionization chamber pressure, 6.0 x 10' atm; and scan range 33 to 250 m/z in 0.95 sec with a 0.05 sec hold. Downloaded by NANYANG TECHNOLOGICAL UNIV on June 8, 2016 | http://pubs.acs.org Publication Date: August 5, 1992 | doi: 10.1021/bk-1992-0500.ch019
6
Descriptive Sensory Analysis. Descriptive flavor analysis was accomplished using the Spectrum Method described by Meilgaard et al. (11). The descriptive panel consisted of 12 persons selected for availability and normal abilities to taste and smell. The six descriptors monitored were roasted peanutty (RPT), sweet aromatic (SAC), cardboardy (CBD), painty (PTY), woody/hulls/skins (WHS), and fruity/fermented (FFY) (10,12). Each descriptor was evaluated for intensity using a 15-point scale (11). Since there were 16 samples and only eight samples were presented at each session, year effect was confounded with session effect. Year effect was considered replication by the authors and was not as important a comparison as storage time or peanut size. Samples were served in 1 oz. plastic cups identified with 3-digit random numbers under red lights to mask color differences. Each sample was presented twice to the panel as a repeated measure. The data was checked for outliers and faulty panelists using the methods of Crippen et al. (Crippen, K . L . , Shaffer, G.P., Vercellotti, J.R., Sanders, T.H., Blankenship, P.D. /. Sensory Stud., in press.). The means of panelists were calculated for each treatment combination/repeated measure sample, and used as the data set for the statistical analysis. Statistical Analysis. Peanuts from the two crop years provided the replication for the experiment. Duplicate analyses for each storage time, seed size, and crop year treatment were conducted for the volatile analyses. The data for each sensory attribute and G C peak was subjected to a repeated measures analysis of variance (13) to identify significant size, crop year, storage, and/or interaction effects. A l l sensory data exhibited a spherical error structure (14) required for the further use of a repeated measures analysis to identify storage trends. Storage trends respective to each sensory attribute were identified for each level of a significant effect by using polynomial, profile, and 0-day to each subsequent day, contrasts. Some of the data collected from volatile compounds did not possess this required structure. Hence, storage trends for the G C data were identified by fitting individual regressions to data from each level of a significant effect (13). Results of Peanut Storage Peanut flavor descriptors were significantly affected by the experimental treatments. Means across storage were significantly different for cardboardy, painty, roasted
St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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19. BETT & BOYLSTON
Effect of Storage on Roasted Peanut Quality
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peanutty, sweet aromatic, and woody/hulls/skins (Table I). Roasted peanutty, sweet aromatic, and woody/hulls/skins had significant interaction effects as well as significant main effect. Volatile flavor compounds representative of desirable and undesirable flavor characteristics of roasted peanuts were selected to evaluate the influence of storage time and seed size on flavor composition. These compounds were the major peaks on the G C trace of each treatment. The effect of the storage time and peanut seed size treatments on the content of the volatile flavor compounds varied depending on the nature of the flavor compounds (Tables II-IV). Storage time had a significant effect (p0.05) for the heterocyclic compounds. For the lipid oxidation products, storage time had a significant effect on 15 of the 18 compounds analyzed in this study. However, peanut seed size, crop year, and interactions between storage time and seed size, and storage time, seed size, and crop year had a significant effect (p0.05)
Size
Time
Flavor Descriptor
Main Effects
NS NS NS 0.0584 NS 0.0435
Time x Size 0.0073 0.0103 NS 0.0527 NS NS
Time x Year
Interactions
NS NS NS NS NS 0.0049
Size x Year
NS NS 0.0384 NS NS NS
Time x Size x Year
Table I. Significance of treatment effects on sensory attributes, as determined by repeated measures analysis of variance
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8
9\
St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
0.0001 0.0001 0.0001 NS 0.0071 0.0001 0.0002 NS 0.0320 0.0095 NS 0.0097
2,5-Dimethylpyrazine Ethylpyrazine 2,3-Dimethylpyrazine 2-Ethyl-6-methylpyrazine 2-Ethyl-5-methylpyrazine Trimethylpyrazine 2-Ethyl-3,6-dimethylpyrazine 2-Emyl-3,5-dimethylpyrazine 2,5-Diethylpyrazine 2-Isobutyl-3-methylpyrazine 2,3-Diethyl-5-methylpyrazine 2-Methylfurfural
Year NS NS NS NS NS NS NS NS NS NS NS NS
Size NS NS NS NS NS NS NS NS NS NS NS NS
NS = mean peak contents were not significantly different (p>0.05)
Time
Compound
Main Effects Time x Year NS NS NS NS NS NS NS NS NS NS NS NS
Time x Size NS NS NS NS NS NS NS NS NS NS NS NS
Interactions
NS NS NS NS NS NS NS NS NS NS NS NS
Time
Table II. Significance of treatment effects on heterocyclic compounds, as determined by repeated measures analysis of variance
Downloaded by NANYANG TECHNOLOGICAL UNIV on June 8, 2016 | http://pubs.acs.org Publication Date: August 5, 1992 | doi: 10.1021/bk-1992-0500.ch019
St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
0.0001 0.0010 0.0013 0.0001 0.0001 0.0007 0.0001 0.0001 0.0065 0.0001 0.0012 NS 0.0005 0.0002 NS 0.0015 0.0001 0.0001
Hexanal Heptanal 2-Heptenal Octanal 2-Octenal Nonanal Decanal 2-Decenal t,c-2,4-Decadienal t,t-2,4-Decadienal 2-Hexen-l-ol l-Octen-3-ol 2-Heptanone 2-Octanone 2,3-Octanedione 3-Octen-2-one 2-Nonanone 2-Pentylfuran NS 0.0015 0.0031 0.0106 0.0009 0.0084 0.0114 0.0048 NS NS NS 0.0337 NS NS 0.0075 NS 0.0005 NS
Size NS 0.0206 NS NS NS NS NS NS NS NS NS NS 0.0342 NS NS NS NS NS
Year
NS = Mean peak contents were not significantly differendy (p>0.05)
Time
Compound
Main Effects
0.0257 0.0056 NS 0.0001 0.0003 0.0120 0.0001 0.0002 NS NS NS NS NS NS NS NS 0.0001 0.0036
Time x Size
Interactions
NS NS NS NS 0.0194 0.0389 0.0059 NS NS NS NS NS NS NS NS NS 0.0010 NS
Time x Year NS NS NS NS NS 0.0476 0.0048 NS NS NS NS NS NS NS NS NS 0.0025 NS
Time x
Table III. Significance of treatment effects on lipid oxidation products, as determined by repeated measures analysis of variance
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as
1o
H
0
I3
00
St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
0.0395 0.0001 NS NS NS
Benzaldehyde Limonene Phenylacetaldehyde Naphthalene Vinylphenol 0.0090 NS NS NS NS
Size 0.0348 NS NS 0.0400 NS
Year 0.0261 NS NS NS NS
NS NS NS NS NS
NS NS NS NS 0.0974
Time x Size Time x Year Time x Size x Year
Interactions
NS = Mean peak contents were not significandy different (p>0.05)
Time
Compound
Main Effects
Table IV. Significance of treatment effects on contents of miscellaneous compounds, as determined by repeated measures analysis of variance
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St. Angelo; Lipid Oxidation in Food ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
4.76 4.27
1157
965
2-Methylfurfural
b
2-Isobutyl-3-methylpyrazine
7.31
17.11
2,3-DiethyI-5-methylpyrazine
1087
9.15
102.53
1080
1085
115.76
1001
129.50
26.71
1140
b
1000
997
b
2,5-Diethylpyrazine
b
2-Ethyl-3,5-Dimethylpyrazine
2-Emyl-3,6-Dimethylpyrazine
Trimethylpyrazine
b
2-Ethyl-5-methylpyrazine
b
8.86
12.76
914
917
300.86
0
909
RI
3.53
4.48
4.39
15.57
7.64
84.29
2.25
4.08
3.76
15.10
6.82
72.70
57.56
66.85
88.62 76.70
22.27
2.08
3.22
84.80
4
16.55
3.91
5.73
135.98
2
1.86
3.38
3.82
12.98
5.26
60.94
44.44
54.88
12.44
0.73
1.33
49.79
6
Storage Time (weeks)
a
1.77
3.17
3.52
11.99
5.02
55.43
39.85
48.19
27.08
0.59
1.25
42.18
8
7.26
1.56
4.24
8.42
7.33
33.51
21.25
58.88
19.30
0.00
0.11
26.09
12
Cubic
NS
Quadratic
Linear
NS
Linear
Quadratic
Quadratic
NS
Cubic
Cubic
Cubic
Model
* Data for 2 seed sizes (#1, Jumbo/medium (JM)), 2 crop years (1988, 1989), and 2 replications have been pooled Interactions between storage time, seed size, and crop year were not significant (p>0.05). Storage time had a significant effect (p
