Investigation of the factors affecting the staling of Arabic flat bread
2011-09-01T12:01:39Z (GMT) by
The focus of this thesis is the area of staling of Arabic flat bread (or pita bread). The reductions in shelf life due to bread staling cause significant economic losses in the Middle East where preservatives are not permitted. The aim was to investigate whether processing solutions could solve this problem. A commercial bread produced in Kuwait was initially studied. The industrial baking process was monitored closely, and the products analyzed for moisture content in Kuwait after storage at 4 and 20 °C. Representative samples were ground and freezedried in Kuwait, and transported to the UK for analysis by Differential Scanning Calorimetry (DSC). The findings from this study showed moisture content variations between different bread formulations after baking, however the DSC analysis was inconclusive due to possible changes in the freeze dried bread. The industrial process was thus replicated as closely as possible at Loughborough University so that the bread could be analysed immediately after baking and after up to 3 days storage. It was also found that high pressure stainless steel pans produced better DSC baseline stability compared to aluminium pans due to suspected leakage of the latter (used in literature studies), and were thus used for the rest of the study. Attention was also paid to the possibility that different parts of the bread receive different radiant heating intensities during baking as evidenced by the different degrees of brown coloration. This showed that whilst almost complete gelatinisation initially occurs, the highest levels of subsequent retrogradation occurred in an area intermediate between the centre and outside of the pita bread. This coincided with the region with the highest moisture content immediately after processing (and likely to receive the least amount of heat). A parallel study using DSC which subjected dough samples to a temperature profile similar to that found in baking also found that lower heating rates (with albeit higher heat exposure) produced greater amounts of retrogradation. In each case moisture contents during storage were comparable between samples, thus indicating that heating rates during processing is a key parameter governing subsequent retrogradation, and also that future studies should identify the sampling position within the bread when performing analyses. Thermocouple studies showed that the temperatures in the steam pocket that develops during puffing were close to that of the boiling point of water. Consequently, a further study was also performed in which Arabic flat bread samples were baked at different pressures (up to 2 bar) in a novel high pressure oven. The motivation was that changing pressure to increase baking temperatures could be a way of using processing rather than formulation methods to extend shelf life. Whilst applying pressure was found to retard subsequent retrogradation, it did result in breads becoming firmer during storage which was not the desired effect. However, this may be a consequence of the slightly lower moisture contents found in the bread baked at higher pressure.