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Using differential thermal analysis (d.t.a.) the progress of ageing of concentrated wheat starch gels stored at temperatures from -l o to 43" has been investigated.A very close relationship has been found between the ageing of starch gels as measured by d.t.a. and the staling of bread as measured by crumb firmness at storage temperatures of -lo, 10" and 21" but some differences have been found at 32" and 43". The results at -lo, 10" and 21" provide very strong confirmatory evidence that starch crystallisation is the chief factor in the firming of bread. At elevated storage temperatures (32" and 43") the rBle of starch crystallisation in the firming of bread apparently gradually diminishes. Analysis of the results indicates that the mechanism of crystallisation of the starch, instantaneous nucleation followed by rod-like growth of crystals, is the same over the whole range of storage temperatures -1 to 43". Evidence is also presented to show that there is a possibility that at higher storage temperatures a more symmetrically perfect crystal structure is being formed. IntroductionAn important feature of bread staling is that the rate has a negative temperature coefficient, i.e. the rate of staling increases as the storage temperature is lowered. This characteristic of staling has been studied in detail by Cornford, Axford & E1ton.l They measured the rate of crumb firming of bread prepared by the bulk fermentation process and stored at temperatures ranging from --I" to 32". They found that, independently of temperature, the loaves all tended to the same ultimate value of crumb firmness but that the rate of attainment of this value was dependent on storage temperature. Time constants, the reciprocal of rate constants, of from 1.39 days at -1" to 5.51 days at 32" demonstrated the negative temperature coefficient of the rate.Included in the studies of Axford, Colwell, Cornford & Elton2 was an extension of the above work to bread prepared by the Chorleywood Bread Process3 and stored at temperatures from -1 " to 66". Time constants increased steadily over the range of temperatures, varying from 1.44 days at -1 " to 23.3 days at 66".

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Flours milled from English ('weak'), Canadian ('strong') and mixed English and Canadian wheats ('medium') had different rates of lipolysis (weak > medium > strong) during prolonged storage at ambient temperatures (average about 12°C). Lipolysis was more rapid in the medium flour at 25"C, but was very slow in 'control' flours kept at -20°C in an inert atmosphere. Loss of baking quality in stored flours was assessed using the Chorleywood Bread Process (CBP), a long fermentation process (LFP) and the Activated Dough Development process (ADD). The loaf volume changes varied with the flour type and test method, especially depending on whether or not fat was included in the recipes. In tests with fat, the CBP was the most sensitive to deteriorative changes (strong= medium > weak); with the medium and strong flours, loaf volumes were nearly constant for 24 months before decreasing rapidly. Deterioration became apparent more gradually when using LFP and ADD tests with fat. When fat was omitted, short-term improvement (weak > medium > strong) occurred with all three baking methods, but there was subsequent loss of volume. To account for these changes in baking behaviour, it is suggested that the tests differ in their sensitivity to flour type and variations in dough composition, especially to the level of bakery fat, the level of fatty acids (generally deleterious), and to protein changes which were also detectable by rheological tests on stored flour doughs (flour 'maturation' ; generally beneficial).

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Commercial baking trials, designed to investigate the effects of a-amylase activity during baking are described. Increasing a-amylase activity changes both the physical and chemical properties of bread, the physical changes resulting in a loss of crumb mechanical strength, and the chemical changes resulting in an increase in the amount of starch degradation products in the bread, and thus an increase in crumb stickiness. Both types of change can cause problems at the bread slicing stage. A reduction in water added at doughmaking is a successful ameliorative measure, as also is special lubrication of the slicer blades.Results from the commercial trials have been corroborated and extended in the pilot scale bakery and the laboratory, the more controllable conditions furnishing results that implicate the high molecular weight degradation products of starch as the predominant cause of crumb stickiness. These pilot scale trials also provide evidence for the commercial usefulness of additions of acid calcium phosphate and extra fat as ameliorative measures.