American Journal of Food Technology1557-45711557-458XAcademic Journals Inc.10.3923/ajft.2011.215.225ChutichudetB. ChutichudetP. KaewsitS. 3201163An analyzing study to evaluate the relationship between Polyphenol Oxidase (PPO) activity, internal qualities and degree of browning was conducted on lettuce var. Grand Rapids in order to understand the basis changes relating to the appearance of leaf browning and some quality characteristics. For PPO activity, Factorial experiment in completely randomized design was arranged with two factors: developmental stage (Factor A) at four levels (28, 42, 59 and 73 days after planting, DAP) with three different parts of plant (Factor B) (stem, leaf and root), while Completely Randomized Design (CRD) was applied for studying the internal qualities and browning appearance in leaves at 28, 42, 59 and 73 DAP. The experiment was carried out during May-July, 2009, with four replications, ten plants per replication. 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