Original article The glass transition approach to determination of drying protocols for colour stability in dehydrated pear slices Hang-Ing Ling, John Birch* & Miang Lim Department of Food Science, University of Otago, Dunedin, New Zealand (Received 19 April 2004; Accepted in revised form 23 December 2004) Summary Colour stability in dehydrated fruit is often attained by use of sulphite treatment to inhibit browning during processing and storage. However, colour stability may be able to be conferred on products by ensuring that the molecular mobility is restricted in dried fruit by drying to sufficiently low moisture to raise the glass transition temperature (under mild conditions). Pear slices were dehydrated at a low temperature (40 °C) to a low final moisture content (below 5%) after it was shown that, for a moisture content of 22% (which is the moisture content of commercial dehydrated pears) pretreatment with sulphite would be necessary to stabilise the colour. The use of longer drying regimes resulted in a moisture content where the food matrix would be closer to the glassy state and conferred colour stability on the dehydrated product compared to a product containing more moisture. For these low-moisture products, pretreatment with sulphite would not be necessary to preserve colour stability. Keywords Enzymatic browning, water activity. Introduction Pears (Pyrus Communis L.) are climacteric fruit and their dehydrated products have a longer storage life, decreased weight and a saving in the cost of transportation, thus providing a possible outlet for surplus pears and grades that are not acceptable to canneries. However, there is often a perceived loss of qualities such as colour, flavour and texture, and there are poor rehydration properties associated with dehydration. Enzymatic and non-enzymatic browning are two major concerns during dehydration and storage (Sullivan, 1981). Enzymatic browning, as a result of polyphenol oxidase (PPO) action, has been widely studied in various fruit including pears (Mayer & Harel, 1979; McEvily et al., 1992; Asaka et al., 1994; Siddiq et al., 1994; Lambrecht, 1995; Osuga & Whitaker, 1995; Va´mos-Vigya´zo´, 1995). PPO is relatively heat labile, being active at 40– 45 °C (Hansmann et al., 1998) but short exposure at 70–90 °C causes partial or total inactivation (Macheix et al., 1990; Va´mos-Vigya´zo´, 1995) and it can also be suppressed at reduced water activity (a w ) values (Hansmann et al., 1998). The Maillard, or non-enzymatic browning reac- tion results in the degradation of nutrients such as sugars, essential amino acids and ascorbic acid. It also reduces protein digestibility and inhibits digestive enzymes (Sapers, 1993; BeMiller & Whistler, 1996), The Maillard reaction is most likely to occur during thermal processing and in low-moisture systems (a w ¼ 0.3)0.7; Eichner & Wolf, 1983; Hansmann et al., 1998), as removal of moisture allows the solid content to be concentra- ted and hence leads to an increase in the interac- tions between the reducing sugars and amino acids in the fruit. Sulphite and its derivatives have been used for many years to inhibit non-enzymatic and enzy- matic browning reactions during fruit preparation, *Correspondent: Fax: +64 3479 7567; e-mail: john.birch@stonebow.otago.ac.nz International Journal of Food Science and Technology 2005, 40, 921–927 921 doi:10.1111/j.1365-2621.2005.00996.x Ó 2005 Institute of Food Science and Technology Trust Fund