| | Introduction | Results | Literature cited
Introduction
Color is one of the most important sensory attributes of food products. A food product with the unacceptable color is not likely to be chosen and eaten by consumers, even it is highly nutritious, flavorful and well texture. While acceptable color of a food varies depending on cultural, geographic and sociological aspects of a given population, certain food groups are acceptable only if they fall within a certain color range. Also, in many cases quality and value of raw materials are judged by their color.
White color of flours from cereal grain is generally preferred to dark color. Moreover, most of the food products prepared from cereal grains should be of bright light color to command high quality. Dark discoloration of abraded barley kernels when used as a rice extender, in soups or in preparation of baby foods has been a serious concern of food industries and a significant factor preventing use of barley in food formulation.
Barley grains contain numerous polyphenols, proanthocyanidins and catechins, which are distributed in the hull, seed coat and aleurone layer. Total polyphenol content of barley, expressed as gallic acid, ranges from 0.2 to 0.4% of grain (Bendelow and LaBerge 1979). PPO has high activity in raw barley (Clarkson et al 1992). The role of polyphenols in brewing, especially their implication in haze formation in beer, was reviewed by Gardner and McGuinness (1977). However, the relationship between polyphenols and discoloration of food products prepared from barley, and the role of PPO on discoloration of barley based products has not been investigated, mainly due to the insignificance of barley as a food.
Barley is increasingly incorporated into the human diet, because of human health benefits, easy availability and inexpensive price. To maintain or even increase consumer’s interest in barley foods, and to improve the willingness of food processors to use barley in food product formulations, it is crucial to control discoloration of barley-based food products. Discoloration of barley-based food products may be controlled by the proper selection of raw materials, appropriate processing and use of chemicals. We explored the grain components responsible for the barley food product discoloration, genotypic variation in discoloration as well as phenolics content and PPO activity, and environment effects on the grain components responsible for discoloration of barley food products.
Results
Total polyphenol content and PPO activity
Total polyphenol content of abraded barley kernels was lowest in hulled proanthocyanidin-free barley, ranging from 0.02 to 0.04% (Table I). Hulled proanthocyanidin-containing abraded barley had lower total polyphenol content (0.11-0.18%) than hulless abraded barley (0.19-0.26%). Although proanthocyanidin-containing barleys were abraded by 30% for hulled and 15% for hulless genotypes, abrasion does not necessarily remove the same layers or components of the kernel because of differences in thickness of outer layers, kernel size and shape among genotypes.
PPO activity of abraded barley grains varied among genotypes, while differences in PPO activity between barley classes were not evident. PPO activity of abraded barley grains ranged from 62.2 to 116.5 units/g in hulled barley and from 63.1 to 106.6 in hulless barley genotypes.
Table I. Total Polyphenol Content and PPO Activity of Abraded Grains of Different Barley Genotypes
| Barley Class | Protein (%) | Ash (%) | Total Polyphenol
(gallic acid %) | PPO Activity
(units/g) |
Hulled
PA + (n=10)
PA - (n=4)
Hulless
Regular (n=5)
Waxy (n=3) | .
8.0-11.1
8.8-10.1
10.4-15.0
11.7-13.1 | .
0.74-0.96
0.84-0.99
0.85-1.16
0.99-1.15 | .
0.11-0.18
0.02-0.04
0.22-0.26
0.19-0.21 | .
62.2-94.7
84.1-116.5
63.1-106.6
68.7-79.9 |
PA +: proanthocyanidin-containing
PA -: proanthocyanidin-free
Brightness of barley dough sheets
The L* values of barley flour dough sheets measured over time are summarized in Table II. Immediately after preparation, L* value of dough sheets exhibited relatively small differences between classes and genotypes of barley. The rate of L* value decrease during storage was highest in hulless barley genotypes, lower in hulled proanthocyanidin-containing and lowest in hulled proanthocyanidin-free genotypes. Accordingly, differences in L* values of the dough sheets among barley classes and individual genotypes were much more evident at 24 hr after preparation than immediately after preparation. Hulled proanthocyanidin-free barley exhibited the highest L* values (72.2-78.1), followed by hulled proanthocyanidin-containing barley (65.3-69.6) and hulless barley (59.0-63.9). There were also large variations in L* values among genotypes of the same barley class, indicating the complexity of discoloration in processed barley.
Table II. Brightness (L*) of Dough Sheets Prepared from Barley Flours
 | Brightness (L*) |
| Barley Class | 0 Hr | 24 Hr |
Hulled
PA + (n=10)
PA - (n=4)
Hulless
Regular (n=5)
Waxy (n=3) | . 78.0-80.9
79.8-82.6 .74.3-79.3
75.4-77.7 | . 65.3-69.6
72.2-78.1 .59.7-63.9
59.0-61.1 |
PA +: proanthocyanidin-containing
PA -: proanthocyanidin-free
Relationships between composition and discoloration potential of barley
Correlation analyses between the composition of barley kernels and L* values of dough sheets are summarized in Table III. Total polyphenol content significantly correlated with the L* values of dough sheets. A large variation in L* values of dough sheets among barley genotypes of similar total polyphenol content (Table I) may indicate that, in addition to polyphenols, other factors, including PPO activity and metal ions, may contribute to the discoloration of dough sheets.
Relationships between PPO activity and L* values of dough sheets were not significant. However, protein content and ash content exhibited significant negative correlations with L* values of barley dough sheets in barley genotypes. Protein content may be correlated with an unknown component that affects hardness or the rate of water binding during dough processing. Highly negative correlation between wheat ash content and spaghetti brightness was also reported by Matsuo et al (1982).
Table III. Simple Pearson Coefficients (r)a Between Composition and Brightness of Barley Flour Dough
Composition | Brightness of Dough (L*) |
| Total Polyphenols | -0.910*** |
| Polyphenol Oxidase Activity | 0.270 |
| Protein | -0.714** |
| Ash | -0.469* |
a*, **, *** = significant at P<0.05, P<0.01, P<0.001, respectively.
Genotypic and environmental effects on discoloration potential of barley
Twelve genotypes of barley grown in five environments (location-year combination) were analyzed to determine the relative contribution of genotype and environment on quality traits associated with discoloration potential of barley. Both genotype (G) and environment (E) contributed to significant variation for protein, ash, total polyphenol content, PPO activity and brightness of dough sheets (Table IV). G x E interactions were also significant for all parameters. Analysis of profile plots for all parameters with significant G x E interactions indicated that total polyphenol content, PPO activity and brightness of dough sheets had non-crossover interactions (Figure 1). Non-crossover interaction indicated that the rank of the means for genotypes was unchanged although the magnitude of the differences between genotypes changed among environments.
The ratios of genetic to environmental (G/E) variances showed that genetic factors had a larger influence than environmental factors on total polyphenol content, PPO activity and brightness of dough sheets (Table V). G/E variance ratios were similar in magnitude and nearly evenly balanced for protein and ash content. These results indicate that genetic factors were more important than environmental factors or G x E interactions in determining total polyphenol content, PPO activity and brightness of dough. Therefore, the selection of barley genotypes during the breeding process based on polyphenol content and PPO activity as well as dough brightness, could be the effective way to control discoloration of barley-based food products.
Table IV. Mean Squares for the ANOVA of Chemical Composition and Discoloration Potential of Abraded Barleya
Source of
Variation | df | Chemical Composition | | Brightness (L*)
of Dough |
Protein | Ash | Total Polyphenol | PPO Activity |
Genotype (G)
Environment (E)
G x E | 11
4
44 | 16.2***
40.9***
0.7** | 0.20***
0.39***
0.01*** | 0.049***
0.016***
0.001*** | 28592***
2823***
339*** | | 336.9***
69.6***
2.5*** |
a ** and *** = P<0.01 and 0.0001, respectively.
Table V. Ratiosa of Variances Estimated for Genotype and Environment Main Effects and Their Interactions for Chemical Composition and Discoloration Potential of Abraded Barley
| Chemical Composition | | Brightness (L*)
of Dough |
| Protein | Ash | Total Phenolic | PPO Activity |
G/E
G/E x E | 0.9
32.7 | 1.2
18.8 | 7.2
10.6 | 24.5
3.0 | | 11.7
12.1 |
aRatios of genetic to environmental variance components (G/E) and environmental variance to genotype by environment interaction components (E/G E).
Literature Cited
Bendelow, V.M., and LaBerge, D.E. 1979. Relationship among barley, malt, and beer phenolics. J. Am. Soc. Brew. Chem. 37:89-90.
Clarkson, S.P., Large P.J., and Bamforth, C.W. 1992. Oxygen-scavenging enzymes in barley and malt and their effects during mashing. J. Inst. Brew. 98:111-115.
Gardner, R. J., and McGuinness, J. D. 1977. Complex phenols in brewing: A critical survey. Master Brew. Assoc. Am. Tech. Quar. 14:250-261.
Matsuo, R. R., Dexter, J. E., Kosmolak, F. G., and Leisle, D. 1982. Statistical evaluation of test for assessing spaghetti-making quality of durum wheat. Cereal Chem. 59:222-228.
Baik, B.-K. (1), Z. Quinde (2), and S. E. Ullrich (1)
(1) Assistant Professor and Professor, respectively, Department of Crop and Soil Sciences;
(2) Graduate research assistant, Department of Food Science & Human Nutrition, Washington State University, Pullman, WA 99164-6420
Presented at the North American Barley Researchers Workshop, July 17-20, 2005 |
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