Polyphenols, Polyphenol Oxidase, and Discoloration of Barley-Based

Chapter 27

Polyphenols, Polyphenol Oxidase, and Discoloration of Barley-Based Food Products 1

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B.-K. Baik , S. E. Ullrich , and Z. Quinde-Axtell

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Downloaded by TUFTS UNIV on December 6, 2015 | http://pubs.acs.org Publication Date: June 13, 2008 | doi: 10.1021/bk-2008-0983.ch027

1

2

Department of Crop and Soil Sciences, Washington State University, Pullman, WA 99164-6420 Department of Food Science and Human Nutrition, Washington State University, Pullman, WA 99164-6376

The health benefits of eating barley have renewed our interest in its food uses. Dark-gray color appearing in barley foods, however, limits its prevalent use by food processors and consumers. We observed a large variation in the discoloration potential among barley genotypes, as determined by the brightness of flour dough and gel, polyphenol content and polyphenol oxidase (PPO) activity. Genotypic influence was greater than environmental factors on polyphenol content, PPO activity and discoloration potential of barley. Removal of the outer layer of barley grain, heat treatment, exclusion of oxygen and use of ascorbic acid effectively retarded the discoloration in flour dough and gel. Barley grain contained phenolic acids, catechin, and six dimeric and trimeric proanthocyanidins (PAs). Significant increase in discoloration of dough by addition of PAs and considerable reduction in discoloration of dough by heat treatment of flour further indicate that both polyphenols and PPO are responsible for the discoloration of dough. Despite its low concentration, monomeric PA fraction composed mainly of catechin was most capable of causing the discoloration of barley flour dough.

388

© 2008 American Chemical Society

In Color Quality of Fresh and Processed Foods; Culver, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


389 Barley ranks fourth in total world production among cereal grains. However, human consumption of barley in the form of food products is extremely limited in many western countries, including the U.S.A. About 5 1 % of barley we produce in the U.S.A. is used for feed, 42% for malt and less than 2% for food (7). The limited food uses of barley can be attributed to low consumer acceptability for barley-based food products due to cultural eating practices and undesirable color and taste of products; many consumers are not accustomed to eating barley, and so they do not consider it in terms of food products. The undesirable processing characteristics of barley as compared to wheat or rice might further contribute negatively to the food use of barley. Also, as economic conditions improve, consumers prefer to eat wheat or rice-based products rather than barley (2). Consequently, most consumers today are not familiar with the flavor and taste of barley-based food products and are hesitant to incorporate barley into their daily diets. On the other hand, there has been growing interest in eating barley, mainly due to the awareness of the health benefits of barley consumption. It is widely known that barley is a rich source of the soluble dietary fiber, β-glucan, which has been considered to have hypocholesterolemic and hypoglycemic properties. Based on the clinical evidence of β-glucan for lowering blood cholesterol levels, as well as other reports on the nutritional benefits of barley grain, the FDA recently approved the health claim for barley β-glucan, which allows food processors to label food products containing more than 0.75 g of β-glucan per serving as being heart healthy. Easy availability and inexpensive price could be other advantages of using barley in food products. For food uses, barley grain is first pearled to remove the hull and bran layers. Pearled barley grain is used as a rice extender, added in barley soups and prepared into a variety of dishes. Pearled barley grain can be further ground into flour and easily added to numerous wheat-based bakery and non-bakery products, including bread (3), muffins (4), biscuits (5), extruded snacks (6), noodles and pasta (7) without significantly affecting processing and product quality except for the gray color development. The undesirable dark discoloration reported in these products could result from enzymatic or nonenzymatic reactions. Barley grains contain an unusually high amount of phenolic compounds (0.2 - 0.4%) distributed mainly in the outer layers of the grain (hull, seed coat and aleurone layer) (8, P). Polyphenol oxidase is also present in barley and can be responsible for the oxidation of phenolic compounds, inducing darkgray color development in barley food products. We determined the relationships between the chemical constituents of grain and the discoloration potential of barley in food products, analyzed the significance of both genetic and environmental factors on the discoloration potential of barley, explored various processing methods for the retardation of discoloration, and identified the composition of barley phenolic compounds and their role in the discoloration potential of barley.


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Chemical Composition and Discoloration of Barley Foods


Chemical Analyses and Determination of Discoloration Potential Various types of barley, including hulled or hulless, proanthocyanidincontaining (PAC) or proanthocyanidin-free (PAF), and waxy or regular endosperm barley, were evaluated for their chemical composition and their discoloration potential. Barley grains were pearled to remove 30% and 15% by weight in hulled and hulless barley, respectively. Total polyphenol content and polyphenol oxidase (PPO) activity were determined using spectrophotometric methods (10). Polyphenols were extracted from barley flours with a solution of dimethylformamide, ammonium hydroxide, 4-aminoantipyrine and potassium ferrocyanide. The absorbance at 505 nm was measured and the polyphenol content was reported as gallic acid percentage. Polyphenol oxidase was extracted from barley flour using a phosphate buffer of pH 6.5, and the enzyme extract was added to a solution of L-dopa. The increase in absorbance over time was used to calculate enzyme activity. One unit of PPO activity was defined as the amount of enzyme giving a change in absorbance of 0.001/min, and was expressed in units/g. Pearled grains were milled into flour, which was subsequently used to prepare paste (10%, w/v) and dough of 64% water absorption for the determination of discoloration potential. Brightness (L*) of cooked grains, gel and dough was measured using a Minolta colorimeter (Minolta Camera Co., Ltd., Osaka, Japan).

Variation in Discoloration Potential of Barley Genotypes Polyphenol content, PPO activity and brightness (L*) of cooked grains, gel and dough as the measurement of discoloration potential are summarized in Table I. Differences in the brightness of cooked grain and gel were evident among all types of barley. Hulled PAF genotypes generally produced brighter (greater L*) cooked grains and gels than PAC and hulless genotypes. The brightness of flour dough gave the best differentiation of barley types in their discoloration potentials (Figure 1). Hulled barley genotypes produced brighter dough than hulless genotypes. In hulled barley, PAF genotypes yielded brighter dough than PAC genotypes. In hulless barley, brighter dough was produced from genotypes of regular endosperm starch than those of waxy endosperm starch. There were large variations in both total polyphenol content and PPO activity among barley genotypes. Total polyphenol content was about 0.04% in hulled PAF genotypes, while it ranged from 0.13 to 0.22% in hulled PAC and hulless genotypes. PAF genotypes exhibited much higher PPO activity than other


In Color Quality of Fresh and Processed Foods; Culver, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 0.047

0.198 0.183

0.177 0.184

0.046 0.053 0.050 0.054

0.144 0.131 0.175 0.220

14.0

71.3 68.2

82.9 55.4

121.0 163.7 204.8 192.7

59.8 96.2 57.3 60.6

1.4

60.5 58.7

59.1 57.2

62.2 62.5 61.8 60.3

59.9 59.1 60.6 59.6

Cooked Grain

1.1

54.1 53.0

55.0 54.6

59.3 56.8 57.5 57.7

55.0 53.7 56.0 55.1

Gel

Brightness (L*)

2.4

59.9 59.8

62.9 62.6

75.6 75.4 76.9 74.3

68.6 66.5 67.6 67.9

Dough

Least significant difference (P

Polyphenols, Polyphenol Oxidase, and Discoloration of Barley-Based

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