Impact of protein on darkening in yellow alkaline noodles

Darkening in yellow alkaline noodles (YAN) was examined over a 24 h period in noodles made from 4 wheat varieties, including varieties with different levels of polyphenol oxidase (PPO) activity, selected to cover a range of protein levels. Noodles were made in the presence and absence of the PPO inhibitor, tropolone. The darkening was divided into two time periods: 0-4 h and 4-24 h. The first four hours was described by a composite rate equation, and this period was subdivided into two stages. The rate of darkening in the first stage was independent of both protein concentration and PPO activity. The amount of darkening (c), however, was highly dependent on protein concentration during this stage (-tropolone, r = 0.902; +tropolone, r = 0.905), but independent of PPO activity. The first stage darkening was a zero order reaction where additional protein does not increase the reaction rate, but when the protein supply has been depleted, the reaction stops. The rate of darkening during the first stage (k'(1) = 5.6 +/- 1.0) was similar to the rate of change in the protein structure (k'(1) = 6.5 +/- 1.3) as measured using the amide II band by infrared spectroscopy. This suggested that the first stage of darkening represents changes in light reflectance and absorbance caused by changes in hydrogen bonding rather than changes in covalent bonding. During the second stage of darkening, both the rate (k'(2)) and amount of darkening (DeltaL*(4h-c)) were significantly correlated with protein concentration (-tropolone, r = 0.465; +tropolone, r = 0.813), and in the absence of tropolone the amount of darkening was increased by PPO activity. The amount of darkening (DeltaL*(24h-4h)) during the second time period (4-24 h) (or third stage) was significantly correlated in the presence of tropolone (r = 0.375) and in the absence of tropolone (r = 0.428) with protein concentration. However, compared with earlier stages the response of non-PPO darkening during the third stage to change in protein concentration was smaller. Protein oxidation, or more specifically oxidation of tyrosine groups within the protein, appears to be the main mechanism involved in non-PPO darkening in YAN during the second and third stages with glutenin being the main reactant. Albumin and globulin are important substrates for PPO. No differences in darkening were detected in YAN made from the four varieties in the presence of tropolone; however, differences in YAN darkening were observed for the second and third stages due to site and year variation.