The wounds and injuries incurred during minimal processing of lettuce stimulates phenolic metabolism, which leads to tissue browning. Phenylalanine ammonia-lyase (PAL, EC 188.8.131.52.) is the first committed enzyme in the phenylpropanoid pathway and regulates its overall activity. The maximum activity of wound-induced PAL occurs sooner as the temperature increases from 0 to 25 °C, but the maximums are lower. A heat shock at 50 °C for 90 s protects minimally processed lettuce tissue against browning, helps to retain greenness, and decreases the production of phenolics when applied either after or before wounding. Browning is reduced when the heat shock is applied up to 36 h after wounding, while the maximum effect occurs around 6 h before cutting. Like the heat-shock treatment, a number of protein synthesis inhibitors (e.g., chloramphenicol, cycloheximide, emetine, and puromycin) reduced wound-induced PAL activity, but they did not prevent browning itself. These inhibitors seemed to cause chemical damage that promoted tissue browning. When cycloheximide was applied in combination with heat-shock treatments browning did not occur. Heat shocks appear to control tissue browning by more than just interfering with protein synthesis.
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