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United States Department of Agriculture

Agricultural Research Service

Title: Collaborative Study on L-Dopa - Wheat Polyphenol Oxidase Assay

Author
item Bettge, Arthur

Submitted to: Cereal Foods World
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 30, 2004
Publication Date: December 1, 2004
Citation: Bettge, A.D. 2004. Collaborative study on l-dopa - wheat polyphenol oxidase assay. Cereal Foods World. 49:338-342.

Interpretive Summary: Among quality attributes associated with Asian noodles, color, along with eating texture are the two most important. A bright clear color that remains stable over time is desired. Discoloration, in the form of grey, brown or green shades, or general "dullness", has long been attributed to the enzyme, polyphenol oxidase (PPO). Discolored noodles occur frequently, incurring consumer dissatisfaction. Enzymatic discoloration of batters, pie crusts and refrigerated doughs has also been reported. Due to the elevated pH of chemically leavened doughs and batters, favorable conditions exist for PPO-caused discoloration. Further, extended holding time for batters or lengthy storage of refrigerated doughs allows for development of discoloration. PPO is located in the bran coat of wheat and, depending on milling extraction rates, is variably incorporated into flour. Within common wheats, variation in the PPO gene(s), expression of the gene(s), or PPO enzyme activation levels lead to variable levels of discoloration. Elimination of this discoloration process would lead to greater consumer acceptance of the wheat used to make flour, enhancing the marketability of low PPO-containing wheat. Several techniques have been employed in the past to quantify relative PPO activity in wheat. Generally, these have involved immersing small numbers (1 ' 10 kernels) of whole wheat grain in an excess of buffered aqueous substrate, followed by spectrophotometric or oxygen electrode assessment. A relatively rapid, small-scale, non-destructive test for PPO would be advantageous for both breeding and commercial applications. A method was developed that utilizes L-DOPA as a substrate, and meets the criteria for a good breeding test. The test was accepted as a first approval method by the American Association of Cereal Chemists as method 22-85. The L-DOPA method can be adapted for use on flour or ground wheat samples. This manuscript reports the results of a large interlaboratory collaborative evaluation of the test.

Technical Abstract: Among quality attributes associated with Asian noodles, color is very important. A bright, clear color that remains stable over time is desired. Discoloration, in the form of grey, brown or green shades, or general "dullness", has long been attributed to polyphenol oxidase (PPO). Enzymatic discoloration of batters, pie crusts and refrigerated doughs has also been reported. Several techniques have been employed in the past to quantify relative PPO activity in wheat. These have involved immersing small numbers (1 ' 10 kernels) of whole wheat grain in an excess of buffered aqueous substrate, followed by spectrophotometric or oxygen electrode assessment. Substrates that have been used include: phenol, catechol, tyrosine and L-dihydroxyphenylalanine (L-DOPA). Each method has its own advantages and disadvantages. Some are destructive to seed viability (phenol), others are less reactive substrates, requiring longer incubation times (catechol) and others have limited aqueous solubility (tyrosine and L-DOPA). A relatively rapid, small-scale, non-destructive test for PPO would be advantageous for both breeding and commercial applications. This report discusses the results of a large-scale collaborative test of the L-DOPA-based method that was developed and meets the criteria for a good breeding test. The L-DOPA method can be adapted for use on flour or ground wheat samples. The method was accepted as a first approval method by the American Association of Cereal Chemists as method 22-85.

Last Modified: 9/10/2014
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