This publication is the fourth of a series of seven supplementary investigative materials for use in secondary science classes providing up-to-date research-related investigations. This unit is structured for grades 9 through 12. It deals with physical factors that affect color changes in plant foods during processing and in the preparation of fresh and frozen green vegetables for consumers. In addition, this guide provides background information on plant pigments and the changes they undergo during food processing. The first part of this guide identifies: (1) materials needed; (2) supplementary information; (3) suggested reading; and (4) films and film loops. The second part provides students with background information and two investigations: (1) observations on color changes in selected plant material, and (2) factors that affect stability of chlorophyll in green vegetables. Each investigation consists of: (1) materials needed for a four-student team; (2) procedures; (3) questions for thought; (4) extending the investigation; and (5) suggested readings. (HM)