Investigation of the retaining effect of a pectin-containing edible film upon the crumb ageing of dietetic sucrose-free sponge cake

Abstract

The crumb ageing of dietetic sucrose-free sponge cake (DC) was assessed by scanning electron microscopy (SEM) observations carried out on the first and the sixth day of DC storage. The storage duration was selected according to the standard requirements for DC storage in Bulgaria. No visible changes in the microstructure had occurred on the sixth day of storage in the crumb of the DC covered with pectin film, in contrast to that without pectin film (control sample). The matrices of the partly gelatinized starch and coagulated proteins (light zones) in the DC covered with pectin film kept their fine structure and were not disposed in parallel to the air-pocket walls. Therefore, the extent of retrogradation of starch granules in the control sample was higher, and the ageing, too. This was the reason why the main factor of this process, i.e. the bound water states, was investigated. The quantitative changes in both bound water states (strongly and slightly bound water) were registered by combined dynamic analysis (differential thermal analysis (DTA) and thermogravimetry analysis (TGA)). A significantly bigger (1.5 times) amount of strongly bound water in the crumb of the DC covered with pectin film than in the control sample was proved on the sixth day of storage. The total amount of bound water in both cakes reduced during the storage process. This reduction was considerably more slightly expressed in the DC covered with pectin film as it had reached 4.75% towards 16% in the control sample. It was proved that, simultaneously with the changes in the bound water states, the qualitative characteristics (structural and mechanical properties, and colour) of the crumb of DC had also changed. Higher values of shrinkage and springiness of the DC covered with pectin film were read on the fifth day. Smaller changes in the crumb colour of the DC covered with pectin film were ascertained by spectrophotometry. The investigations demonstrated the prevention of crumb ageing of dietetic sucrose-free sponge cake when a pectin-containing edible film was used. This sponge cake had better preserved freshness, especially up to the fifth day of storage.