Biosystems Engineering (2005) 92 (2), 183–195 doi:10.1016/j.biosystemseng.2005.06.004 PH—Postharvest Technology High-temperature Treatment for Efficient Drying of Bread Rye and Reduction of Fungal Contaminants E.F. Kristensen 1 ; S. Elmholt 2 ; U. Thrane 3 1 Department of Agricultural Engineering, Research Centre Bygholm, Danish Institute of Agricultural Sciences, DK-8700 Horsens, Denmark; e-mail of corresponding author: erikf.kristensen@agrsci.dk 2 Department of Agroecology, Research Centre Foulum, Danish Institute of Agricultural Sciences, DK-8830 Tjele, Denmark 3 Technical University of Denmark, BioCentrum-DTU, Centre for Microbial Biotechnology, Søltofts Plads 221, DK-2800 Kgs. Lyngby, Denmark (Received 9 July 2004; accepted in revised form 8 June 2005; Published online 15 August 2005) Mycotoxin-producing fungi are natural contaminants of cereals and their toxins are harmful to humans and animals. Ochratoxin A (OTA) is among the most important. Heat treatment by drum drying does not eliminate already formed mycotoxins but the technique can reduce the number of viable fungi on the grain. The aim of this study was to establish a drying regime that kills fungal propagules on rye without reducing its quality for baking. Special attention was paid to some important mycotoxin-producing species. As drying temperatures and retention time in the drum are essential, the drum drier must be equipped with an effective control unit. Two different control systems were tested. The results showed that fungi colonising the grain can be much more efficiently and precisely reduced by controlling the maximum grain temperature (MGT) than the fixed constant drying air temperature (FAT). Drum drying very efficiently reduced the fungal propagules colonising the grain, including the mycotoxin-producing Penicillium verrucosum, Fusarium avenaceum, F. culmorum, F. poae, F. sporotrichioides and F. tricinctum. Both temperature and retention time in the drum affected survival rate of the fungi. Using MGT, a retention time of 105 min and a temperature of 64 1C killed 99% of the yeast propagules and 98% of the filamentous fungi. Moisture contents were reduced to about 12%. A similar drying regime in a supplementary trial reduced the number of P. verrucosum contaminated kernels from more than 70% to 12% but confirmed that drum drying did not destroy already formed OTA. The combination of a high drying capacity and a short but efficient heat treatment was obtained by drum drying as opposed to on-floor, batch and ordinary continuous flow drying, and it reduced the risk of mould deterioration to almost zero when the grain was properly stored afterwards. At the same time a high quality for baking was maintained. The highest baking quality in rye was obtained at grain temperatures of about 62 1C and only at grain temperatures above 70 1C visual quality changes were detected. r 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd 1. Introduction Mycotoxin-producing fungi are natural contaminants of cereals and their toxins are harmful to humans and animals (Smith et al., 1994; Frisvad & Thrane, 2002). Ochratoxin A (OTA) is regarded a major health problem in food and feed production due to its nephrotoxic, carcinogenic, teratogenic and immunosup- pressive properties (Kuiper-Goodman, 1996; Walker, 1999; Jørgensen & Jacobsen, 2002; Schwartz, 2002). In countries with a temperate climate, Penicillium verruco- sum is regarded the sole producer of OTA in cereals (Larsen et al., 2001; Lund & Frisvad, 2003) and rye is particularly sensitive to contamination by this fungus (Czerwiecki et al., 2002; Elmholt, 2003) and its toxic metabolites (Jørgensen et al., 1996; Jørgensen & Jacobsen, 2002; Schwartz, 2002). Contamination by Fusarium is another important problem in cereals as species of this genus can invade the cereal plant and reduce the yield or even cause plant death. In addition, Fusarium species produce a number of potent mycotox- ins, including deoxynivalenol, nivalenol, T-2 toxin and ARTICLE IN PRESS 1537-5110/$30.00 183 r 2005 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd