Content area
Full text
Among quality attributes associated with Asian noodles, color and eating texture are the two most important (10). A bright clear color that remains stable over time is desired. Discoloration, in the form of gray, brown, or green shades, or general "dullness," has been attributed to polyphenol oxidase (PPO) (7).
There are two basic Asian noodle formulations: white salted and alkaline. White salted (udon) noodles are made from blends of soft wheat flours, and color tends to be slightly less important than eating texture as a quality factor, although a bright white to creamy yellow color is favored. Alkaline noodles, made from varying blends of soft and hard wheat flours, are much more prone to discoloration caused by PPO due to their formulation, which incorporates carbonate or hydroxide to increase the pH to near 10. Some alkaline noodles (Cantonese style) are sold fresh, while others are precooked or steamed (Hokkien and instant styles). A bright yellow color is favored in alkaline noodles. However, shades of gray, green, or brown can develop in as few as 2-4 hr (11) due to PPO. As a result, discolored noodles occur frequently, incurring consumer dissatisfaction.
PPO is located in the bran coat of wheat (2,3) and, depending on milling extraction rates, is variably incorporated into flour. PPO content and activity are primarily controlled by gene(s) located on chromosome 2D(1). As such, tetraploid (AABB) durum wheat grain has no detectable kernel PPO and, therefore, possesses excellent color and color stability in pasta production. Discoloration in durum wheat products results from lipid oxidation, either auto-oxidativeor lipoxygenase-driven systems. Within hexaploid (AABBDD) wheats, variation in the PPO gene(s), expression of the gene(s), and PPO activation levels lead to variable levels of discoloration.
Enzymatic discoloration of batters, pie crusts, and refrigerated doughs has also been reported, although these reports are primarily anecdotal (4). There is agreement, however, that PPO is the most probable cause. Due to the elevated pH of chemically leavened doughs and batters (5), conditions are favorable for discoloration caused by PPO. Further, extended holding times for batters and lengthy storage times for refrigerated doughs allow for development of discoloration.
In the past, several techniques have been used to quantify relative PPO activity in wheat. Generally, these have involved immersing small numbers...