An article published this month in the Journal of Agricultural and Food Chemistry by researchers from the Netherlands show that both water activity, which determines the direction of water migration, and water content have an effect on the perceived crispiness of bread.
However, the study was able to investigate the water content and water activity separately and found that water content is perhaps more significant than water content.
“The water content of the crust was found to be decisive for the transition point,” wrote the study’s authors.
“The distribution of the water in samples with a history of high water content is more inhomogeneous, which results in crispy and less crispy regions, thus making them overall more crispy than samples with the same water content but higher water activity.”
Discovering techniques to maintain a crust’s crispiness could help extend the shelf life. By modifying these factors, the researchers said, bakers can improve bread ingredients to produce crisper, longer-lasting crusts for bread products, and this could also mean less need for introducing artificial preservatives, which are increasingly unwanted my consumers.
It is already known that a product’s crispiness suffers when moisture is introduced. The article explained: “Water causes hydration, which causes a glass to rubber transition of the amorphous regions of the macromolecules present that were initially in the glassy state.”
The aim of the study was to investigate how water content and water activity contributed to the loss of sensorial crispiness in a bread crust model.
Bread crust samples were tested at different conditions, altering the relative humidity (RH) trajectory (hysteresis effect) to identify the effect water has on the products.
Varying water activities with the same water content, and vice versa, were achieved by either exposing a dry sample to different water vapour pressures or exposing it to 90 per cent air humidity before drying it to meet the desired pressure of water vapour.
The results found that sensorial crispiness could be determined by both the water content and the activity.
Samples with a different water activity but equal water content showed the same amount of crispiness, whereas high water content resulted in a mix of crispy and non-crispy sections, and was therefore perceived as being crispier.
Source: Journal of Agricultural and Food Chemistry
August 2008, doi: 10.1021/jf800522c
“Water Content or Water Activity: What Rules Crispy Behavior in Bread Crust”
Authors: N H van Nieuwenhuijzen, C Primo-Martin, M B J Meinders, R H Tromp, R J Hamer, T van Vliet.