Original article Influence of the glass transition and storage temperature of frozen peas on the loss of quality attributes Miang Lim,* Hongbing Wu, Michael Breckell & John Birch Department of Food Science, University of Otago, PO Box 56, Dunedin, New Zealand (Received 11 March 2004; Accepted in revised form 27 July 2005) Summary Green peas of two different maturities were used to evaluate the influence of the glass transition temperature (Tg¢) and storage temperatures on quality loss during frozen storage. Young peas which have a lower tenderness rating (TR) value, possibly with less starch and higher sugar content than old peas, gave a lower Tg¢ value. Quality attributes, including green colour and vitamin C, were measured over time when the peas were stored at )5, )12, )20, )25 and )30 °C, respectively. The results indicated that these qualities were maintained during frozen storage as long as the peas remained below or close to their Tg¢. The loss of quality at different storage temperatures is relative to the difference between the storage temperature and the Tg¢, where the bigger the difference, the greater the loss. Keywords Differential scanning calorimeter, green colour, maturity, molecular mobility, Tg¢, vitamin C. Introduction Freezing has long been established as a method to preserve flavour, texture and the nutritional qual- ity of food. However, changes in colour, flavour and loss of ascorbic acid have been found in frozen peas during storage (Canet, 1989; Velasco et al., 1989) and the pH of green peas decreases during frozen storage (Berg, 1964). During freezing, water is removed from the food system as ice crystals, while dissolved constituents are concentrated in the unfrozen matrix (Franks, 1982; Roos, 1995). Water in plant tissues that is not completely frozen at the storage temperature plasticizes the unfrozen matrix thus allowing molecular mobility, with consequential physical and chemical changes, to persist (Mergentine & Wiegand, 1946; Canet, 1989; Slade & Levine, 1995). At lower temperatures, as no more water can be frozen out, the unfrozen matrix becomes extremely viscous and forms an amorphous glassy state (Franks, 1982). The temperature associated with this transition is called the glass transition temperature (Tg¢), where food materials are at their maximum-freeze concentration state, and the system is a mechanical solid (Lim & Reid, 1991; Slade & Levine, 1995). Hence if foods are stored at temperatures below Tg¢, physical and chemical reaction should be minimized. The rates of diffusion controlled reac- tions depend on the relative difference between the storage temperature and the Tg¢. The higher the storage temperature is above the Tg¢, the less viscous the unfrozen matrix becomes, allowing faster diffusion to occur (Slade & Levine, 1995). Tg¢ in foods can be determined using differential scanning calorimetry (DSC) by heating a frozen sample through the glass transition, which is observed by a change in heat capacity on the thermogram. The transition occurs over a range rather than at a fixed temperature. The Tg¢ is strongly dependent on the nature of the solute where components display translational mobility differences based on structure and molecular weight (Lim & Reid, 1991; Fennema, 1996). When the Tg¢ is measured without an annealing step (holding at an isothermal temperature just above the Tg¢) it is noted by some authors as the *Correspondent: Fax: 643 479 7567; e-mail: miang.lim@stonebow.otago.ac.nz International Journal of Food Science and Technology 2006, 41, 507–512 507 doi:10.1111/j.1365-2621.2005.01096.x Ó 2006 Institute of Food Science and Technology Trust Fund