PROPERTIES OF THE FREEZE DRYING “SCORCH” TEMPERATURE

IN RECENT YEARS a number of studies have been reported on mathematical simulation of the freeze drying process (Sandal1 et al., 1967; Dyer and Sunderland, 1968; Cho and Sunderland, 1970; Aguilera, 1973). An integral part of most of these studies has been the inclusion of a maximum temperature value for the dry region, generally called the “scorch” temperature, maximum allowable dry layer temperature, maximum surface temperature and so forth. In all cases, the attempt is to define a product temperature which marks the transition from an acceptable to an unacceptable product, with product quality generally related to the formation of dark color (hence, “scorch” temperature). While it is well recognized that browning is time and moisture, as well as temperature dependent, it is presumably the ease of measuring and controlling temperature that promotes considering browning from a temperature-only approach. The kinetics of browning reactions have been widely studied for many model systems and food materials. However, these studies have tended to concentrate on determination of reaction mechanisms or storage stability of foods, including dehydrated products. Indeed, it seems likely that most reported values of the “scorch” temperature are developed from storage studies, and thus are very low, having severe economic implications on the cost of freeze drying (Flink and Fosbql, 1972). There &pear to be very few studies on brown&g occuring during dehydration processes, especially freeze drying. One notable exception for freeze drying is a study on browning kinetics using a glucose/glycine mixture deposited upon cellulose (Kluge and Heiss, 1967). In these studies, browning kinetics and drying behavior were combined to develop times at which a level of browning, arbitrarily chosen to be the limit of acceptability, was achieved. In recent work on computer simulation of continuous freeze drying (Aguilera, 1973 ; Aguilera and Flink, 1974a, b), methods have been developed to obtain local values of temperature and moisture. This information can be used with browning kinetics to develop integrated levels of browning during the freeze drying process for inclusion into any process optimization procedure. This paper reports on a study of browning kinetics at high temperatures and low moisture contents, as might be found in the dry layer during a freeze drying process. Additionally, a visual organoleptic assessment of these browned materials is presented.