EFFECT OF BAKING AND TOASTING ON THE PROTEIN QUALITY AND LYSINE AVAILABILITY OF BREAD

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Abstract
The protein efficiency ratio (PER) and net protein ratio (NPR), the protein digestibility in vivo (in rats) and in vitro (multienzyme digestion), and the lysine availability in vivo (in rats) and in vitro (fluorodinitrobenzene method) of unbaked bread mix, bread, crust, crumb and bread toasted at 2 levels of darkening were determined. Compared to an adjusted PER of 0.75 for the unbaked bread ingredients, the PER of the entire loaf was 0.46, but that of the crumb was 0.91, while the crust showed a negative PER, -0.23. The corresponding NPR were 1.51, 1.22, 1.62 and 0.69. Light toasting reduced the PER of the bread to 0.40 (NPR 1.11) and dark toasting to 0.16 (NPR 0.95). The protein digestibility decreased as the bread ingredients were exposed to greater amounts of heat (baking, light and dark toasting), but the crumb protein of untoasted bread was slightly more digestible in vivo than the protein of the raw bread ingredients. The total lysine content also decreased with increasing exposure to heat, the crumb lysine content being an exception. The available lysine content, as measured by the biological or the chemical methods, suffered significant decrease as the heat exposure increased and correlated very highly with PER (available lysine in vivo vs. PER, r = 0.988; available lysine in vitro vs. PER, r = 0.979).