560 JOURNAL OF FOOD SCIENCE—Vol. 67, No. 2, 2002 © 2002 Institute of Food Technologists Food Engineering and Physical Properties van 1979; Biliaderis and others 1986; Liu and others 1991; Eliasson 1992; Cooke and Gidley 1992). Water acts as a plasti- cizer in the starch-water mixture (Slade and Levine 1995). Starch is completely gelatinized in the presence of excess water upon heating. When gelatinized starch is cooled to low temperatures, starch retrogradation takes place to varying degrees. Retrogradation is the process that occurs when starch molecules reassociate and form an ordered structure during storage (Atwell and others 1988). DSC has been widely employed in the study of starch retrogradation. Retrograda- tion of starch is influenced by the ratio of amylose to amy- lopectin (Gudmundsson 1994), presence of other compo- nents in the starch, moisture content, cooking temperatures of the starch-water system, and storage conditions of gelati- nized starches (Longton and LeGrys 1981; Zeleznak and Hoseney 1986; Russell 1987; Fisher and Thompson 1997; Liu and Thompson 1998). Retrogradation of starch is an impor- tant physical process influencing the product quality and shelf life of starch-based foods. The objective of this work was to gain an understanding of the thermal properties of potato dry matter that would help control the quality of potato products. In this study, we in- vestigated the effect of moisture content on the thermal properties of potato dry matter in comparison to isolated potato starch. Materials and Methods Potatoes Potatoes grown on Prince Edward Island in Canada were purchased at a local market and were stored at 5 °C. Prior to isolation of starch, the tubers were conditioned at 23 °C for 2 d. Dry matter and its content in potato Potato dry matter was obtained by freeze-drying. Dry matter content was determined from the difference in the weight of potato samples before and after freeze-drying. Po- tato sections (about 5 g, 4 cm  2 cm  1 cm) were prepared from the center of the tubers and lyophilized in a freeze dry- er (Model 8, Labconco®, Kansas City, Mo., U.S.A.). A typical JFS: Food Engineering and Physical Properties Characterization of Thermal Properties of Potato Dry Matter–Water and Starch–Water Systems Q. LIU, R. YADA, AND J. ARUL ABSTRACT: Potato dry matter samples of varying moisture contents were analyzed by differential scanning calo- rimetry (DSC). One endothermic and one exothermic transition were observed when the moisture content was < 50% (w/w) in the dry matter–water system. Both endothermic and exothermic transitions of potato dry matter were influenced by moisture content. One endothermic transition at about 66 °C was observed when the samples were reheated after 2 wk of storage at 5 °C. Gelatinization and retrogradation of starch are chiefly responsible for the endothermic transitions in the potato dry matter. The exothermic transition may originate from chemical reactions such as Maillard and caramelization during heating of a potato dry matter–water system when water is limited (< 50%, w/w) and at temperatures > 120 °C Key Words: potato dry matter, thermal behavior, moisture content, gelatinization, retrogradation Introduction T HE YIELD OF POTATO CHIPS AND FRENCH FRIES AND THE TEX- ture of French fries and canned and reconstituted dehy- drated potatoes are directly related to the dry matter con- tent of the potatoes. However, the dry matter content and composition of potatoes vary among varieties (Smith 1975). The fertilizer application, cultivation and environmental con- ditions, potato storage history and transgenic transformation influence dry matter content and chemical composition of potatoes (Liu 1997). The potato tuber contains 13% to 37% dry matter; 13% to 30% carbohydrates, 0.7% to 4.6% proteins, 0.02% to 0.96% lipids, and about 0.44% ash. In addition, ascorbic acid and other vitamins, phenolic substances, and nucleic acids are also present (Kadam and others 1991). Chemical composi- tion and structure of components, such as starch, nonstarch polysaccharide, sugar and organic and inorganic com- pounds, and proteins influence the properties of potatoes and potato products (O’Donoghue and others 1996; Rodrigu- ez-Saona and Wrolstad 1997; Rodriguez-Saona and others 1997). Since starch is the major component of the dry matter of potato, its molecular organization and interactions with nonstarch polysaccharides and sugars are important factors influencing sensory attributes and shelf life of potato prod- ucts such as mashed potatoes, French fries and potato chips (Lisinska and Leszczynski 1989). During potato processing such as boiling, baking and fry- ing, heat is applied to different degrees. Heat treatment al- ters the functional properties of potato products. Thus, an understanding of the thermal properties of potato tubers be- comes important in potato processing and in controlling product quality. The physicochemical properties of the dry matter can be used to assess the characteristics of different potatoes and to predict the functional properties of their starches. Although dry matter content is important in the se- lection of potatoes for processing and other applications (Li- sinska and Leszczynski 1989), characterization of the potato dry matter has not yet been done. Differential scanning calorimetry (DSC) is a very useful tool for monitoring the thermal properties of starch (Dono-