Reducing microbial counts in home-grown wheat by storage at elevated temperatures

Abstract

A worthwhile reduction in numbers of micro-organisms without damaging baking quality can be expected by storing wheat at a temperature of about 50°C for 16-24 hours.

The work was divided into two separate phases. In the first phase the reductions in counts of total bacteria, enterobacteriaceae, mesophilic spores, moulds and yeasts in wheat adjusted to moisture contents of 13.1, 14.8, 16.0 and 17.8% were determined at storage temperatures of 21, 30, 37 and 44°C. At suitable intervals samples were taken for milling and the resultant flours tested for microbial content and baking quality characteristics. The count of all organisms, except mesophilic spores, declined during storage at a rate that increased with increase in temperature and wheat moisture content. The same trend occurred in flour but the counts were lower by a factor of about 100-fold than in the corresponding wheat. At 44°C, even with the highest moisture content wheat (17.8%), it took several days to obtain a 90% reduction in counts. Some loss in baking quality was observed in the wheats stored for prolonged periods at the highest temperatures and moisture contents. Of the various tests used to assess the baking quality of flours from the stored wheats the SDS sedimentation test and loaf crumb structure provided the best indication of loss in quality. Of particular interest was the unexplained finding that the Hagberg Falling Number increased during warm storage although the alpha-amylase activity was unaffected.

The second phase of the project was a study to determine the shortest heating time/temperature combination for obtaining a worthwhile reduction in bacterial numbers in wheat without causing gluten damage as measured by germination rate and the protein solubility test. The results indicate that storage at a temperature of about 50°C for 1 6-24h may be optimum although further research is needed to assess whether or not carrying out such a procedure is feasible commercially and to investigate how it might be achieved.