The application of Differential Scanning Calorimetry to the gelatinisation of concentrated starch/water suspensions produced a well‐defined endotherm under suitable conditions. Measurement of the peak area enabled the heat of gelatinisation to be calculated. The method has been applied to the following aspects of gelatinisation: — 1 Intercomparison of different starches, 2) Effect of granule size, 3 Application to wheat flour, 4 Effect of starch damage, 5 Staling of starch gels, 6 Amylose and amylopectin.

A method was developed for the study of starch retrogradation by differential thermal analysis. The kinetics of the crystallisation process were studied using the expression 8 = exp (-ktn), where 8 = the fraction of crystallisable starch remaining uncrystallised at time t. The values for the Avrami exponent (n) and the rate constant ( k ) were found to coincide with values previously obtained for the staling of bread, as followed by measurement of crumb elastic modulus. No difference between the time constants of gels cooled slowly and rapidly after gelatinisation was found, indicating that the nucleation process in starch crystallisation is instantaneous in the systems studied. The fact that the Avrami exponent is unity also suggests that the nucleation process is instantaneous, and that it is followed by rod-like growth of crystals.

The quantitative relationships between loaf specific volume and the rate and extent of staling in bread as measured by changes in crumb elastic modulus have been examined. The results show that loaf specific volume is a major factor in determining both the rate and extent of staling, both of which decrease in a linear manner, over the range studied, as loaf volume increases. Only two factors have been found that have a significant influence on these curves, the basic breadmaking process and the storage temperature. The influence of changes in loaf specific volume on staling characteristics is greater in bread prepared by bulk fermentation than in bread prepared by the Chorleywood Bread Process. Bread made by the Chorleywood Bread Process stales less rapidly than bread made by the conventional bulk fermentation process. The effect of loaf specific volume on the rate of staling IS more marked as the storage temperature is lowered.

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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